摘要:

Abstract. Methods of synthesis, physicochemical and structural characteristics, and reactivity of 1,8-bis(dialkylamino)naphthal± enes and some of their close analogues pertaining to the class of so- called `proton sponges' are considered. The bibliography includes 154 references. I. Introduction In 1968, Alder et al.1 reported on a discovery of unusual changes in the variations of basicity in the series of N-methylated 1,8- diaminonaphthalenes (Table 1).It has been found that the pKa values for the unsubstituted diamine 1 and its mono-, di-, and tri- methyl derivatives 2 ± 5 fall in the range typical of ordinary aryl amines, while that for 1,8-bis(dimethylamino)naphthalene 6 increases abruptly by nearly six orders. As a result, the basicity of compound 6 exceeds greatly that of both the known aromatic amines and practically all alkylamines.Further studies revealed that this feature is characteristic not only of aqueous solutions of the amine 6, but also of solutions in non-aqueous solvents (e.g. acetonitrile 2) and the gas phase.3 It is believed that the abnormally high basicity of the com- pound 6 is caused by three main factors: (1) destabilisation of the base due to strong repulsion of unshared electron pairs of nitrogen atoms, (2) formation of a strong intramolecular hydrogen bond (IHB) in the protonated form, the cation (6-H+), and (3) steric strain relief in the molecule upon the transition from a non-planar base to a planar cation.Another peculiar feature of the compound 6 is the slowness of proton addition ± detachment due to the shielding of internitrogen space by four methyl groups.Thus, the high thermodynamic basicity of 1,8-bis(dimethylamino)naphthalene is associated with its rather low kinetic basicity. This circumstance made it possible to establish a similarity in the behaviour of the diamine 6 and genuine sponges, which slowly absorb water and retain it very strongly so that water is difficult to squeeze out.For this particular reason, the compound 6 has been named `proton sponge',4 which is generally accepted now and is further extended to all other compounds possessing this type of properties. At present, we can speak about the concept of `proton sponges', which is based on the following principles: (1) the proper struc- tural organisation of their molecule which provides rigid fixation of two nitrogen atoms at a sufficiently close distance from each other; (2) the existence in the molecule of a base of a destabilising repulsion effect of unshared electron pairs of the nitrogen atoms; (3) the occurrence of a strong IHB in the cation, which relieves steric and electronic strains characteristic of a base, and (4) the presence of a hydrophobic environment at the nitrogen atoms, most often in the form of alkyl groups, which actually accounts for the `sponge' effect.H+ 7H+ Me H Me 6 6-H+ N N Me Me N + Me Me Me Me N A F Pozharskii Rostov State University, ul. Zorge 7, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 228 56 67. Tel. (7-863) 222 39 58. E-mail Pozharsk@pozhar.rnd.runnet.ru Received 7 April 1997 Uspekhi Khimii 67 (1) 3 ± 27 (1998); translated by R L Birnova UDC 547.654.1 Naphthalene `proton sponges' A F Pozharskii Contents I.Introduction 1 II. The types of naphthalene `proton sponges' 2 III. Synthesis of 1,8-bis(dialkylamino)naphthalenes and their analogues 2 IV. Physicochemical properties of naphthalene `proton sponges' 5 V. Reactivity of naphthalene `proton sponges' 16 VI.Applications of the `proton sponges' in organic syntheses 22 VII. Conclusion 22 Table 1. Basicity constants, pKa, of N-methyl derivatives of 1,8- diamino- naphthalene (25 8C). Compound pKa in water 1 in acetonitrile 2 1 4.61 10.99 2 7 11.64 3 5.61 11.95 4 7 12.87 5 6.43 12.91 6 12.34 18.18 R4R3N NR1R2 1 ± 6 Com- R1 R2 R3 R4 pound 1 H H H H 2 Me H H H 3 Me H Me H 4 Me Me H H 5 Me Me Me H 6 Me Me Me Me Russian Chemical Reviews 67 (1) 1 ± 24 (1998) #1998 Russian Academy of Sciences and Turpion LtdOn the basis of this concept, the synthesis of a series of non- naphthalene proton sponges has been carried out since the mid- 1980's.Thus, the research group headed by Staab et al. synthes- ised and analysed sponges of the fluorene 7,5 heterofluorene 8,6 and phenanthrene 9 7 series and their analogues 10 8, 9 and 11 10 with an sp2-hybridised nitrogen heteroatom.The vinamidine sponges 12, 13 possessing even stronger basicity than compounds 6 ± 11 were synthesised by Schwesinger et al.11 Until now, the main attention in the studies on `proton sponges' has been given to their structural and physicochemical characteristics and bases and cations were considered from some- what different points of view.Thus the following aspects have been studied for cations: (1) geometry of the intramolecular hydrogen bond (the distance between the amine nitrogen atoms, the N7H. . .N angle and, particularly, the position of the proton responsible for the IHB symmetry, (2) the 1H NMR spectra (primarily, the chemical shift of the NH-proton and the constants of its spin-spin coupling with the NMe2 groups), (3) kinetic NH- acidity (deprotonation rate).Bases were analysed as regards: (1) their molecular geometry (the distance between the nitrogen atoms, distortion from the planar structure of the rings, conformational phenomena), (2) basicity, (3) the nature of interactions of unshared electron pairs of the nitrogen atoms with one another and with the p-system of the ring, and (4) reactivity.Several reviews survey exclusively structural and physico- chemical aspects of the `proton sponges'.12 ± 15 At the same time, a vast range of interesting information has been accumulated concerning methods of their synthesis and, especially, their reactivity. This review has been written with the aim of general- ising the data on the methods of synthesis, reactivity, and physicochemical characteristics of `proton sponges' of the naph- thalene series. The literature cited herein includes the works published before 1997.II. The types of naphthalene `proton sponges' The types of naphthalene `proton sponges' known to date are represented by the structures 14 ± 30.These include 1,8-bis(dialkylamino)naphthalenes 14 (the substitu- ents R1 ±R4 can be identical or different), their analogues 15 ± 18 in which the amine nitrogen atoms are incorporated into five- or six-membered rings, compounds 19 ± 24 with bridgehead heter- oatoms, compounds 25 ± 27 with the nitrogen atoms incorporated in the fused rings, and the so-called double `proton sponge' 28.The properties of `proton sponges' have also been found in iminophosphoranes 29 and 1,8-bis(dimethylamino-methyl)naph- thalene 30. III. Synthesis of 1,8-bis(dialkylamino)- naphthalenes and their analogues 1. 1,8-Bis(dialkylamino)naphthalenes There are two general approaches to the synthesis of these compounds which successfully complement each other. The first of them is based on the alkylation of 1,8-diaminonaphthalene or its partially alkylated derivatives.In the second, 1,1,3-trialkyl-2,3- dihydroperimidinium salts serve as the starting compounds. a. Syntheses based on 1,8-diaminonaphthalenes The first representative of naphthalene `proton sponges' described in the literature was 1,8-bis(diethylamino)naphthalene 31, which was obtained by heating the diamine 1 with an excess of ethyl bromide at 135 8C in the presence of alkali 16 (see also Ref. 17). Later, 1,8-bis(dimethylamino)naphthalene 6 was synthesised by treating the diamine 1 with dimethyl sulfate (the yield was not specified).18 Initially, this compound was characterised as an oil, 7, 8 9 10 N N Me Me Me Me N N 7: X=CH2; 8: X=O, S, Se, Te. N Me Me N Me Me X R=H, Me. 12 11 N N N N N N R R R R N N N N N N N N 13 R4R3N NR1R2 14 N N 16 N N 15 17, 18a7c N N X X 17: X=CH2, R=H; 18a: X =O, R =H; 18b: X =O, R =OMe; 18c: X =O, R=OEt. R R n=1 (a), 2 (b), 3 (c), 4 (d), 5 (e). 19a7e N N Me Me (CH2)n N N Me Me O 20 N N Me Me 21 N N 22 N N 23 N N Me Me 24 N Me2N Me 25 N Me N Me 26 N N 27 Me2N NMe2 NMe2 Me2N 28 N N PPh3 Ph3P 29 Me2NH2C CH2NMe2 30 2 A F Pozharskiialthough it is now known as a crystalline substance with m.p. 47 ± 48 8C.1 Two convenient procedures have been elaborated for exhaus- tive alkylation of the diamine 1. Both of them entail the use of strong bases (sodium or potassium hydrides in anhydrous tetra- hydrofuran 19 or potassium hydroxide in dimethyl sulfoxide) for the ionisation of N7H-bonds.20 Obviously, under these conditions it is the N-anions of the original diamine and intermediate substitution products that undergo alkylation.The ionisation is favoured by the very high (in comparison with ordinary arylamines) NH-acidity of the diamine 1 (pKa=24.5, DMSO, 25 8C), which is ascribed to the stabilisation of the N-anion 32 through IHB.21 The yields of `proton sponges' vary from high to satisfactory.Thus in the alkylation of the compound 6 in the presence of KH±THF or KOH±DMSO systems, the yields exceed 90%. To obtain good yields of the compound 31, it is recommended to use hexametapol as a solvent instead of DMSO.20 However, the alkylation of 1,8- diamino-2,7-dimethoxynaphthalene 33 gives compounds 34 and 35 in low yields (40% and 21%, respectively).22 The yields of the `proton sponges' 37, 38 obtained by the alkylation of the diamine 36 are even lower.23 The yield of the `double sponge' 28 obtained by methylation of 4,5-diamino-1,8-bis(dimethylamino)naphthal- ene with dimethyl sulfate in an NaH±THF system was 31%.24 Depending on the length of the methylene chain, alkylation of the diamine 1 with a,o-dihalogenoalkanes gives various types of `proton sponges' (Table 2).If the halogens are separated by 4 or 5 atoms, compounds 15 ± 18 are formed in which the amine nitrogen atoms are incorporated into five- or six-membered rings. With a closer arrangement of the halogen atoms, bridged sponges 22 ± 24 are formed. Interestingly, the alkylation with 1,3-dibromo- propane gives, along with compound 23, a low yield of the pentacyclic compound 27.Apparently, its precursor is naph- tho[1,8-b,c]-1,5-diazacyclooctane (also isolated in low yield), which undergoes first N-, and then intramolecular C-alkylation with 1,3-dibromopropane. In some cases, 1,8-bis(methylamino)naphthalene and its derivatives served as the starting compounds in the synthesis of `proton sponges' by alkylation. Thus heating of 1,8-bis(methyla- mino)-4,5-dinitronaphthalene 39 with methyl iodide in DMF in the presence of potassium carbonate gave the compound 40 (yield 32%).24 Alder et al.22 have synthesised a series of `proton sponges' 19 ± 21 by treating 1,8-bis(methylamino)naphthalene 3 with a,o- dihalogenoalkanes (Table 3).The alkylation of N,N,N0-trialkyl-substituted 1,8-diamino- naphthalenes proceeds very smoothly.The reaction is usually performed by heating the latter compounds with an excess of alkyl halide in an appropriate solvent or without any solvent. The proton sponge salt formed is converted into the base by treatment with an aqueous alkali.25, 26 b. Syntheses based on 1,1,3-trialkyl-2,3-dihydroperimidinium salts Far from all of the naphthalene `proton sponges' can be obtained by alkylation of peri-diamines or by functionalisation of the unsubstituted compound 6.Therefore, the author of this review with his coworkers have developed an alternative and practically versatile procedure for the synthesis of 1,8-bis(dialkylamino)- naphthalenes from 1,1,3-trialkyl-2,3-dihydroperimidinium salts. First, quaternisation of readily available 1,3-dialkyl-2,3-dihydro- perimidines 41 gives the salts 42, which are subjected to reductive scission with lithium aluminium hydride, resulting in the corre- sponding `proton sponge' (in the ring-opening, the m-methylene group is converted into the methyl group, i.e., in this case R4=Me).However, sometimes this reaction does not proceed smoothly, e.g., if the molecule of a salt contains halogens or other easily reducible groups. Better results are achieved when the salts 42 are treated with an aqueous alkali, which gives high yields of the corresponding N,N,N0-trialkyl-substituted 1,8-diaminonaph- thalene 43.Its alkylation and subsequent treatment of the quater- nary salt with alkali give the target `proton sponge'. This approach was used, in particular, in the synthesis of 1,8-bis(dialkylamino)- R1 H2N NH2 R1 1, 33, 36 33: R1=MeO; 36: R1=EtO.KH THF R1 N R1 H H H 7 K+ 32 R2X (excess) R1 R22 N NR22 R1 6, 31, 34, 35, 37, 38 31: R1=H, R2=Et; 34: R1=MeO, R2=Me; 35: R1=MeO, R2=Et; 37: R1=EtO, R2=Me; 38: R1=EtO, R2=Et. N MeHN NHMe NO2 O2N MeI K2CO37DMF Me2N NMe2 NO2 O2N 39 40 Table 2. `Proton sponges' obtained by treatment of 1,8-diaminonaph- thalene with a,o-dihalogenoalkanes and their analogues.22 `Proton a,o-Dihalide Reaction conditions Yield (%) sponge' 15 Br(CH2)4Br Na2CO3, refulx 60 16 a,a 0-Dibromo- Na2CO3 ±DMF, 5 o-xylene refulx 17 Br(CH2)5Br Na2CO3, refulx 60 18a O(CH2CH2Cl)2 Na2CO3, 150 8C 10 22 BrCH2CH2Br Na2CO3 ±DMF, 42 refulx 24 Br(CH2)3Br Na2CO3 ± acetone, 34 reflux 23 Br(CH2)3Br DMF, diglyme 1 ± 5 27 Br(CH2)3Br or sulfolane, 150 ± 200 8C 1 ± 2 Table 3.`Proton sponges' obtained by treatment of 1,8-bis(methylamino)- naphthalene with a,o-dihalogenoalkanes and their analogues.22 `Proton a,o-Dihalide Reaction conditions Yield sponge' (%) 19b Br(CH2)2Br NaH±THF 80 19b Br(CH2)2Br NaHCO3 ±DMF 25 19c Br(CH2)3Br NaHCO3 ± MeO(C2H4O)2Me, 43 190 8C 19c Br(CH2)3Br NaH±THF 1 19d a Br(CH2)4Br NaHCO3 ± MeO(C2H4O)2Me, 58 reflux, 46 h 19e Br(CH2)5Br Na2CO3 ± MeO(C2H4O)2Me 25 20 O(CH2CH2Cl)2 Na2CO3 ± MeO(C2H4O)2Me, 77 reflux 21 a,a 0-Dibromo- Na2CO3 ±DMF, 13 o-xylene reflux a The compound was isolated and purified as a salt with the BF74 anion.Naphthalene `proton sponges' 3naphthalenes 6, 31, and 44 ± 47 with all possible combinations of the methyl and ethyl groups.25 Compound R1 R2 R3 R4 44 Me Me Me Et 45 Me Me Et Et 46 Me Et Me Et 47 Et Et Et Me In a similar way, the acenaphthene `sponges' 48, 49,25 4-hal- ogeno- and 4,5-dihalogeno-1,8-bis(dimethylamino)naph-thalenes 50 ± 53,27, 28 and partially hydrogenated derivatives of 10-dime- thylaminobenzo[h]quinoline 25 and quinolino[7,8:70,80]-quinoline 26 were synthesised.29 It should be noted that quaternisation of 6-halogeno-1,3- dimethyl-2,3-dihydroperimidines 54a,b gives a mixture of iso- meric salts 55a,b and 56a,b in comparable amounts.Separation of this mixture is not required, since methylation of the isomeric N,N,N0-trialkyl-substituted 1,8-diaminonaphthalenes formed upon alkaline treatment give the same final product.27, 28 In some cases, however, alkylation of non-symmetrical dihy- droperimidines occurs regioselectively.Thus if the nitrogen atoms have different substituents (e.g., Me and Et), quaternisation affects only the nitrogen atom carrying the smallest group.25 Analogously, but owing to electronic (rather than steric) factors, methylation of 1,3-dimethyl-6-nitro-2,3-dihydroperimidine 54c gives exclusively the salt 55c.30 The latter manifests an abnormal behaviour upon alkaline treatment.The main product of this reaction is the naphthol 57 formed in 65% yield. Dihydroperimidine 54c was formed in a small amount (5%), whereas nitronaphthylenediamine 58 was present in only trace amounts. 2. Analogues of `proton sponges' with N-aryl groups Two compounds of this type have been described thus far, viz., 1,8-bis(diphenylamino)- and 1,8-bis(methylphenylamino)- naphthalene,31 which do not resemble classical `proton sponges' in properties and molecular geometry.Fairly accessible 1,8-bis(phenylamino)naphthalene 59 served as the starting com- pound for their synthesis. Its sequential arylation (with dehydro- benzene generated from o-bromofluorobenzene and then with iodobenzene according to Ullman) has led, via the triphenyl- substituted compound 60, to 1,8-bis(diphenylamino)naphthalene 61.An attempted exhaustive Ullman arylation of the diamine 1 was unsuccessful. To synthesise 1,8-bis(methylphenylamino)- naphthalene 62, the diamine 59 was converted into the dilithium salt, which was further subjected to methylation. 3. Iminophosphorane `sponges' In addition to iminophosphorane 29, its analogues 63 and 64 were synthesised.32, 33 The general method for the synthesis of these compounds involves the reaction of a bromine ± triphenylphos- phine (diphenylmethylphosphine) complex with the correspond- ing amine in the presence of triethylamine. The yields are high, as a rule. 4. 1,8-Bis(dimethylaminomethyl)naphthalene Compound 30 was obtained in 90% yield by reduction of 1,8- bis(dimethylcarbamoyl)naphthalene with lithium aluminium hydride.34 In some features (see below), this can also be referred to as a `proton sponge'.LiAlH4 N N R3 R1 41 R2I N N R3 R1 R2 + I7 42 R3HN NR1R2 43 NR1R2 R4R3N 6, 31, 44747 KOH H2O 1. R4I 2. KOH Me2N NMeR Me2N NMe2 R2 R1 48, 49 50753 48: R=Me; 49: R=Et. 50: R1=Cl, R2=H; 51: R1=Br, R2=H; 52: R1=R2=Cl; 53: R1=R2=Br.R=Br (a), Cl (b), NO2 (c). N N Me Me R MeI + N N Me Me Me R I7 N Me Me R Me + I7 + 54a7c 55a7c 56a,b N 55c KOH H2O Me2N NO2 OH Me2N NO2 NHMe + 54c + 57 58 PhN NPh H H 59 1. MeLi 2. MeI Ph2N NPh H 60 F Br Mg PhI K2CO37CuI 61 Ph2N NPh2 62 PhN NPh Me Me 3 Ph3P . Br2 Et3N Me2N N PPh3 63 1 Ph2MeP. Br2 Et3N N PPh2Me MePh2P 64 N 4 A F PozharskiiIV. Physicochemical properties of naphthalene `proton sponges' 1.Basicity Unquestionably, high basicity is the main distinctive property of all the `proton sponges'. The analysis of factors influencing this parameter sheds more light on both structural peculiarities of these compounds and their reactivity. We have already mentioned the three main reasons for the high basicity of `sponges', namely, destabilisation of the base as a result of repulsion of unshared electron pairs of the proximal nitrogen atoms, steric strain relief upon the transition to the cation, and the formation of a strong IHB in the cation.The greatest importance is given to the latter factor, therefore the stability and geometry of IHB in cations of the `proton sponges' have become objects of intense studies using 1H NMR, IR spectroscopy, X-ray analysis, and other methods (see below).13 The contribution of IHB to the general increase in the basicity of a `proton sponge' can be roughly estimated from the pKa value of 2,20-bis(dimethylamino)diphenyl 65.This value is equal to 7.9, i.e., the basicity of this compound is nearly three orders of magnitude higher than that of N,N-dimethylaniline.35 Since the benzene rings in the compound 65 are turned at an angle of 135 8 relative to one another, this is free from both noticeable repulsion of unshared electron pairs of the nitrogen atoms and steric strain due to this repulsion.Thus this difference in basicity can be ascribed, with a fair degree of confidence, to the stabilising effect of the IBH in the cation 65-H+. Apparently, the energy contribu- tion of the IBH in case of the `proton sponge' is even higher, since the cation 6-H+ contains a six-membered ring, which is more stable than the seven-membered ring in the cation 65-H+.Of special note is the fact that the second ionisation constant of the `proton sponge' measured in water (pK2a =79.0)36 is much lower than the first one (pK1a =12.1). It is not surprising that in contrast to 1,8-diaminonaphthalene, which gives a stable crystal- line dihydrochloride, the corresponding salts of the `proton sponge' cannot be obtained.The reason is that the rupture of the IBH needed for the second N-protonation is energetically unfav- ourable even under the action of strong acids. Cations of other 1,8-bis(dialkylamino)naphthalenes behave in a similar way.At the same time, under the action of HClO4 or CF3SO3H, the N-methy- lated cation of the `proton sponge' 66 devoid of the IHB forms the dication 67, which is partly isomerised into the C-protonated forms 68 and 69.37, 38 For the same reason, the chelated cations 19d-H+and 19e-H+ do not undergo second N-protonation under analogous condi- tions, whereas the non-chelated cations 22-H+ and 23-H+ do form the N(1),N(8)-dications.37 Tables 4 and 5 present the values of the basicity constants, pKa, of the naphthalene `proton sponges' both without additional substituents in the naphthalene nucleus (6, 31, 44 ± 47), and with substituents at ortho- (25, 26, 70 ± 73) and para- (48 ± 53, 74 ± 86) positions, as well as for one 2,4-disubstituted 87 and polynuclear compounds 88 ± 93.Only a few of the pKa values have been measured in water or in mixtures of water with other solvents. Most of the data were obtained for acetonitrile; in some cases the measurements were performed in DMSO. The pKa values of the compound 6 in acetonitrile (18.2), water (12.1, 12.3), and DMSO (7.5) indicate that the basicity of the `sponge' drops by more than ten orders of magnitude as the proton-acceptor properties of a solvent increase.Me2N NMe2 H+ 7H+ Me2N NMe2 H + 65 65-H+ NMe2 + BF4 7 HClO4 + + 66 67 + CH2 + + C + H2 68 69 Me3N Me3N HNMe2 Me3N NMe2 NMe2 Me3N 19d-H+ 19e-H+ N H + + N N H 22-H+ 23-H+ N Me Me N Me Me N + H + N H N R2 R1 Me2N NMe2 70773 70: R1=Cl, R2=H; 71: R1=Br, R2=H; 72: R1=R2=Cl; 73: R1=R2=Br. Table 4. Basicity constants, pKa, of naphthalene `proton sponges' and some of their analogues in aqueous solvents.Compound Solvent a pKa Ref. 6 H2O 12.3; 12.1 1, 17 6 ±H2O (1 : 4) 11.5 17 31 DMSO±H2O (3 : 7) 13.0 39 31 ±H2O (1 : 4) 12.7 17 15 H2O 10.0 40 18a H2O 7.5 23 34 DMSO±H2O (3.5 : 6.5) 16.1 41 35 DMSO±H2O (3.5 : 6.5) 16.3 41 37 DMSO±H2O (3.5 : 6.5) 16.1 23 38 DMSO±H2O (3.5 : 6.5) 16.1 23 18b H2O 13.0 23 18c H2O 12.5 23 19a EtOH ±H2O (1 : 1) 3.8 42 19b H2O 4.6 40 19c H2O 10.3 40 19d DMSO±H2O (3 : 7) 13.6 40 19e DMSO±H2O (3 : 7) 13.0 40 20 DMSO±H2O (3 : 7) 12.9 43 a Ratio of components is given in brackets (u/u).O O O O Naphthalene `proton sponges' 5Measurements of the gas-phase basicity (that is the proton affinity) of the `proton sponge' and other 1,8-diaminonaphtha- lenes have been carried out.3 The corresponding values are given in Table 6 in comparison with analogous values for methylated derivatives of aniline and ammonia.The abrupt jump in basicity in both the gas phase and water is observed only for the transition from 1-dimethylamino-8-methylaminonaphthalene 5 to 1,8-bis- (dimethylamino)naphthalene 6. This indicates that the abnor- mally high basicity of the `proton sponge' in vapour and in water is due to the same factors.It is interesting to note that in the gas phase, all 1,8-diaminonaphthalenes are protonated at the nitrogen atom, whereas 1-aminonaphthalene and 1,3-diaminobenzene are protonated in the ring. It is believed that the stability of 1,8- diaminonaphthalene N-cations in the vapour is accounted for by the stabilising effect of the IHB.The dependence of gas-phase basicities of amines (including 1,8-diaminonaphthalenes) and core-bond energies of the s-back- bone has been studied.47 It is linear for the bases the geometry of which is not changed significantly upon protonation. However, such a correlation was not found for compounds 3, 5, and 6. It was suggested that for these compounds the degree of deviation from linearity can serve as a measure of steric strain relief upon the transition from the base to the cation.For the `proton sponge' 6, this value was estimated to be 62.8 kJ mol71, and for the amines 3 and 5, 37.7 and 33.5 kJ mol71, respectively. Let us now consider the effect of structural changes at the nitrogen atoms and in the ring on the basicity of `sponges'.Successive substitution of the N-methyl groups by ethyl groups slightly increases the basicity, as a result of which the pKa value of the `tetraethyl sponge' 31 is 0.8 ± 1.2 units higher than that of its tetramethyl analogue 6 (cf. Tables 4 and 5). More substantial changes in basicity are observed in those cases where the amine nitrogen atoms are incorporated into the rings.Thus, the basicity of 1,8-dipyrrolidino- and 1,8-dimorpholinonaphthalenes 15 and 18 is lower by 2.5 and 4.5 orders of magnitude, respectively, than that of the `proton sponge' 6 (Table 4). The same tendency, although less pronounced, is manifested in compounds 25 and 26 (Table 5) the nitrogen atoms of which are incorporated into fused ring systems.It may be assumed that their structure is not optimum for the formation of a sufficiently strong IHB in the cation, e.g., because the axes of unshared electron pairs of the nitrogen atoms can hardly be coplanar with the ring. This phenomenon was also noted for some non-naphthalene `proton sponges'. Thus the basicity of benzo[1,2-h : 4,3-h0]diquinoline 11 with a helicene structure is 2.5 orders of magnitude less10 than that of the planar quino[7,8-h]quinoline 10 (Ref. 8) (pKa=10.3 and 12.8, respectively). Table 5. Basicity constants, pKa, of naphthalene `proton sponges' in acetonitrile. Com- pKa a Ref. Com- pKa a Ref. pound pound 6 18.2; 18.5 2, 44 79 8.0 b 45 6 7.5 b 36 28 9.8 (4.9) b 24 31 18.95 25 50 17.4 44 44 18.5 25 51 17.3 44 45 18.7 25 52 16.1 44 46 18.7 25 53 16.4 44 47 18.9 25 80 14.9 44 48 18.3 25 81 12.3 44 49 18.6 25 82 16.8 44 25 17.5 29 83 14.9 44 26 17.6 29 84 14.1 46 70 18.35 44 85 15.3 46 71 18.20 44 86 11.8 46 72 17.8 44 87 15.5 46 73 17.45 44 88 18.6 44 74 17.8 44 89 14.6 44 75 14.45 44 90 18.1 (18.1) 44 76 9.8 b 45 91 18.3 (18.3) 44 77 18.35 44 92 18.4 (13.6) 44 78 10.1 b 45 93 18.5 (14.5) 44 79 19.15 (7.6) 44 a The values in brackets relate to the second ionisation constant, pK2 a .b The pKa values were measured in DMSO solution. Me2N NMe2 Me O 89 90 Me2N NMe2 Me2N NMe2 91 Me2N NMe2 CH2 NMe2 Me2N O NMe2 Me2N NMe2 92 O NMe2 NMe2 Me2N 93 Me2N NMe2 R2 R1 74784 74: R1=CH CH2, R2=H; 75: R1=CH C(CN)2, R2=H; 76: R1=NH2, R2=H; 77: R1=NHCOMe, R2=H; 78: R1=NHMe, R2=H; 79: R1=NMe2, R2=H; 80: R1=CHO, R2=H; 81: R1=R2=CHO; 82: R1=COMe, R2=H; 83: R1=COCF3, R2=H; 84: R1=NO2, R2=H.Et2N NEt2 R3 R2 R1 85787 85: R2=NO2, R1=R3=H; 86: R1=H, R2=R3=NO2; 87: R1=R2=NO2, R3=H. O Me2N NMe2 88 Table 6. Effect ofN-methylation on the gas-phase proton affinity (PA) and changes in free energy (DG10 ) of protonation of the succeeding base in comparison with the preceding base in the gas phase and in water.3, 47 Base PA /kJ mol71 DG10 /kJ mol71 gas phase water 1 954.4 7 7 3 982.8 728.5 75.9 5 1000.8 718.0 74.6 6 1030.1 729.3 733.9 PhNH2 895.0 7 7 PhNHMe 925.9 731.0 71.26 PhNMe2 953.5 727.6 71.7 NH3 866.1 7 7 MeNH2 907.9 74.2 77.9 Me2NH 939.3 731.4 0 Me3N 960.2 720.9 +4.6 6 A F PozharskiiAs can be inferred from Table 4, of a set of diamines 19, only compounds 19c ± e can be regarded as `proton sponges'.The basicity of the first two representatives of this series lies at a level characteristic of ordinary arylamines. It is understandable that the methylene and ethylene bridges in the molecules 19a,b provide rigid fixation of the nitrogen atoms in the configuration where unshared electron pairs largely acquire p-character without any noticeable reciprocal repulsion; in addition, the IHB cannot be formed in their cations.Apparently, for this reason the bridged diamines 22 ± 24, which form the non-chelated cations 22-H+ and 23-H+, are not `proton sponges'. The effect of substituents in the nucleus on the basicity of `proton sponges' is roughly as expected (Tables 4 and 5) for 2,7- dialkoxy derivatives 34, 35, 37, and 38 the basicity of which is four orders of magnitude higher than that of the non-substituted `sponges' (Table 4), and their salts are not significantly deproto- nated by strong aqueous alkalis.Formerly, these compounds were considered to be the strongest of the known neutral bases. It is believed that the molecules of compounds 34, 35, 37, and 38 manifest the so-called `supporting effect' [Buttress effect.Ed.], in which ortho-substituents favour closer approach of unshared electron pairs of both dialkylamino groups to each other, enhanc- ing electrostatic repulsion and, presumably, steric strain in the base. The `supporting effect' was also observed in 2,7-dichloro- and 2,7-dibromo-1,8-bis(dimethylamino)naphthalenes 72 and 73.44, 48 The attempts to separate the first and second ionisation constants for the binaphthyl 90 and binaphthylmethane 91 `sponges' were unsuccessul.44 Apparently, both their basic centers have little influence on each other.The basicity of compounds 29, 63, and 64 could not be determined experimentally due to their poor solubility, slow rates of protonation, and hydrolysis of the iminophosphorane group. However, using complex extrapolations, the pKa value for compound 29 in water was estimated to be equal to 15.6, which markedly exceeds the basicity of the classical `proton sponges'.49 The basicity of 1,8-bis(dimethylaminomethyl)naphthalene 30 (pK1a =18.3, pK2a =11.4 in acetonitrile) is at a level typical of the `naphthalene sponges'.50 However, the fact that this compound binds two protons instead of one (as is the case with the classical `proton sponges') testifies to the insufficiently strong IHB in the monocation; the latter finding was confirmed by 1H NMR spectroscopic data. 2. The mechanism and rates of deprotonation of cations As has already been mentioned, the high basicity of `proton sponges' is combined with low rates of their protonation ± deprotonation. The deprotonation rate constants for several `sponges' are given in Table 7, which also contains data for the 1-dimethylamino-8-methoxynaphthalene cation 94-H+ and 8-dimethylaminonaphthol 95 for comparison.Whereas the deprotonation rate for ordinary ammonium salts is limited by diffusion (k&361010 litre mol71 s71),51 in the case of `sponge' cations this process is much slower. Thus for the cation 6-H+ k=1.96105 litre mol71 s71, and for its tetraethyl ana- logue 31-H+ k is even one order of magnitude lower.Deprotona- tion of their 2,7-dimethoxy derivatives, 34-H+ and 35-H+, is especially slow, and in the latter case the reaction can even be followed spectrophotometrically. It is noteworthy that because of the high basicity of compounds 34 and 35, the deprotonation of their cations is observed only in alkaline solutions of DMSO.It is reasoned that the low rates of cation deprotonation are mainly caused by the necessity of cleavage of a very strong IHB. Presumably, a certain role is played by steric factors: in the cation, the proton resides in a sort of hydrophobic `pocket', therefore, access to a base must be hindered (especially in the case of tetraethyl `sponges').Compounds 94-H+ and 95 are also depro- tonated very slowly, but still faster than the `sponge' cations. It may be inferred from these data (Table 7) that the strength of the IHB decreases in the following order: 6-H+>95>94-H+. Vivid debates in the current literature concern the mechanism of deprotonation of chelated species of `proton sponge' cations.14 Virtually all the data indicate that this process occurs in two steps: first, the IHB is cleaved under the influence of a base, and then deprotonation of the unchelated cation takes place.Apparently, the limiting step in this process is its first stage, therefore, the equilibrium concentration of the unchelated cation at each moment of time is low. However, we have recently observed for the first time in our laboratory all the three particles (both cations and the deprotonated base) in equilibrium, with 4-nitro-1,8- bis(dimethylamino)naphthalene 84 as an example.55 It was found that the 1H NMR spectrum of the cation 84-H+ in [2H6]- DMSO contained peaks not only of the chelated cation 84-H+-a, but also those of the free base 84 and of a third species the characteristics of which corresponded to those of the non-chelated cation 84-H+-b.At an initial concentration of the dissolved perchlorate 84-H+ of 561072 mol litre71, the ratio 84-H+-a : 84-H+-b : 84 is 68 : 19 : 13, i.e., under these conditions the chelated cation is predominant. If the concentration of the + O NMe2 Me H O NMe2 94-H+ 95 H Table 7. Rate constants for the reaction of deprotonation of some `proton sponge' cations.Cation BH+ Solvent a k /litre mol71 s71 Ref. 6-H+ H2O 1.96105 17, 52 6-H+ Dioxane ±H2O (1 : 4) 4.66105 17 6-H+ DMSO±H2O (3 : 7) 6.16105 14 31-H+ Dioxane ±H2O (1 : 4) 1.66104 17 31-H+ DMSO±H2O (3 : 7) 1.66104 14 34-H+ DMSO±H2O (3 : 2) 4.46102 41 35-H+ DMSO±H2O (3 : 2) 3.3 41 18a-H+ (see b) H2O 1.26103 43, 53 20-H+ DMSO±H2O (3 : 7) 6.26103 43 94-H+ (see b) H2O 0.46107 54 95 DMSO±H2O (4 : 1) 3.06106 54 Note For more detailed information see Refs 23 and 40.a Ratio of components is given in brackets (u/u). bHPO27 4 was used as the base instead of OH7. k BH++OH7 B+H2O. + Me2N NMe2 NO2 +HO7SMe2+ClO¡4 84 Me2N H NMe2 NO2 + ClO4 7 + 7 Me2N H NMe2 NO2 + ClO4 7 84-H+-a 84-H+-b +O7SMe2 7 + O7SMe2 Naphthalene `proton sponges' 7original salt decreases by one order, this ratio changes drastically (26 : 5 : 69) and the deprotonated base becomes the predominant form.The cation 84-H+-a does not undergo any noticeable changes in acetonitrile. Similarly, the cation of the `proton sponge' 6-H+ is the only species that is present in [2H6]-DMSO. Apparently, there are three reasons for the abnormal behav- iour of the cation 84-H+: (1) considerable asymmetry of the IHB [according to X-ray and 1H NMR data, the proton resides predominantly at the N(8) atom], (2) decreased (four orders of magnitude in comparison with compound 6) basicity of the nitro- derivative 84, and (3) the optimum moderate basicity of dimethyl sulfoxide sufficient to induce partial scission of the IHB and subsequent deprotonation, but at the same time insufficient for the complete shift of the equilibrium between the non-chelated cation and the base to the right. 3. Molecular and crystalline structure a. Bases The most important peculiarity of `proton sponge' bases is a considerable distortion of the planar structure of their molecules due to the trend of the dialkylamino groups to maximally separate from one another.According to X-ray data, for compound 6,56 (1) the angle C(1)7C(9)7C(8) increases to 125.8 8; (2) the distance C(1)7C(8) increases to 2.56A against 2.45A in naphthalene; (3) the distance N. . .N is rather large (2.79A) (cf. 2.72 ± 2.74A in 1,8-diaminonaphthalene).57 The mean plane of the naphthalene system is formed by C(2), C(3), C(9), C(10), C(6), and C(7) atoms, whereas the C(1)7C(9)7C(10)7C(4) and C(8) ± C(9) ± C(10) ± C(5) fragments are symmetrically twisted relative to the central bond C(9) ± C(10); the torsion angles are equal to 8.9 8 and 10.5 8, respectively.Thus the two benzene rings begin to resemble two chairs put against each other so that the back of one chair is next to the legs of the other chair, and vice versa.As a conse- quence, the C(1) and C(8) atoms and, correspondingly, the nitro- gen atoms deviate (the latter, by 0.4A) in opposite directions from the mean plane of the fused system (Fig. 1a). The orientation of the methyl groups is also different (Fig. 1b) as if one pair is directed inward towards the cyclic system, while the other is directed outward, the latter lying nearly in the mean plane of the ring.Obviously, the axes of the unshared electron pairs at the nitrogen atoms are also oriented in opposite directions to form an angle of about 40 8 with the axes of the aromatic p-electrons. The latter circumstance deserves special notice, since with such angles the conjugation of the dimethylamino groups with the p-system of the ring is still considerable.An analogous pattern with some modification is also observed for other `proton sponges' (Table 8). Thus in the 4-nitro-deriva- tive 84, as well as in 4,5- 81 and 2,5-dialdehydes 96, the distance between the nitrogen atoms of the dimethylamino groups shows a tendency to increase, presumably, as a result of conjugation of the latter with p-acceptor substituents. This must be accompanied by flattening of bonds at the amine nitrogen atoms (due to the greater contribution of the structures of the type 84a and 84b), so that NMe2 groups require larger space.Accordingly, the C(1)7N(1) and C(8)7N(2) bonds in compounds 84 (1.3771.38A) and 81, 96 (1.36A) become somewhat shorter than those in the `proton sponge' 6 itself (1.40A). In compounds with electron-donor groups 28 and 35, the lengths of C(1)7N(1) and C(8)7N(2) bonds are the same as in compound 6 (1.40A). The N.. .N distance remains virtually unchanged, although the distortions in the naphthalene ring structure become more pronounced. As expected, the N. . .N distance increases in the `proton sponges' 61, 17, 18a, and 23 having bulkier or less flexible substituents at the amine nitrogen atoms.A characteristic feature of the molecular structure of 1,8- bis(dimethylaminomethyl)naphthalene 30 is the practically planar geometry of the naphthalene ring.62 b. Cations A great number of X-ray studies have been carried out with the salts of the `proton sponge' 6-H+ with various anions (Table 9). All these studies have demonstrated that the transition to the cation brings about drastic changes in the molecular structure.Owing to the formation of the hydrogen bridge, the dialkylamino groups rotate in such a way that the dihedral angle between them and the naphthalene system plane tends to reach 90 8. This results in steric strain relief, flattening of the naphthalene fragment, and considerable approach of the nitrogen atoms. TheN. ..Ndistance varies within the range of 2.55 to 2.65 A, being, on the average, 2.58A. In the majority of salts, the naphthalene ring is virtually flat and both N atoms lie in the same plane. For some salts (X7 = C6Cl5O7, BF74 , SCN7), the twisting (by no more than 4 ± 5 8) of the planes of C(1)7C(9)7C(10)7C(4) and C(8)7C(9)7C(10)7C(5) around the central bond C(9)7C(10) still takes place.As a result, the molecule acquires a propeller shape, and the nitrogen atoms diverge in different directions from the mean plane (up to 0.25 A). However, these deformations are much less than those observed in the `proton sponge' base. The C(1)7N(1) and C(8)7N(2) distances in the cation 6-H+ increase up to 1.45 A, which testifies to the lack of conjugation between the Me2N NMe2 CHO CHO 96 84 Me2N NO2 7 + NMe2 NO2 7 + 84a 84b NMe2 Me2N Me77N N77Me 40 8 40 8 Me Me b a 1 2 3 4 10 5 6 7 8 9 Figure 1.Structure of the `proton sponge' molecule 6: view from the top (a) and along the mean plane of the ring (b) (the empty and full circles belong to the atoms located on different sides of the mean plane). Table 8. The distance between the nitrogen atoms, r(N .. . N), in some 1,8-diaminonaphthalenes. Com- r(N . . . N) /AÊ Ref. Com- r(N . . . N) /AÊ Ref. pound pound 1 2.72; 2.74 a 57 28 2.75 24 6 2.79 56 34 2.76 60 17 2.89 58 84 2.86 55 18a 2.86 58 81 3.03 61 23 2.89 59 96 2.95 61 61 2.86 31 a For two independent molecules. 8 A F Pozharskiidimethylamino groups and the aromatic p-system. This conclu- sion is confirmed by a variety of other data.In contrast to 1,8-bis(dialkylamino)naphthalene cations, the naphthalene ring in cations of the iminophosphorane `sponges' 29, 63, and 64, is strongly distorted, but the distance between the nitrogen atoms is smaller (2.52 ± 2.60A).32, 33 The main attention in the studies of `proton sponge' cations has been paid to the symmetry and geometric characteristics of the hydrogen bridge.This problem was comprehensively discussed in a number of reviews,13, 86 therefore we shall confine ourselves to a mere statement of the central issues. Theoretically, four types of potential energy curves for the N. . .H7N system are possible (Fig. 2). The profile a having one minimum corresponds to a com- pletely symmetrical bridge, whereas the profile b, which has a low activation barrier, reflects the rapid tautomeric equilibrium between two equivalent asymmetric structures.It is the question as to which of these two profiles is realised in the cation of the `proton sponge' 6-H+ that has been discussed in most detail. Theoretical calculations of an [H3N. . .H. . .NH3]+ system have shown that when the distance between the nitrogen atoms is 2.75 A, the potential curve has two minima with a barrier of 10.9 kJ mol71; with a decrease in the distance down to 2.50 A, the barrier disappears.13 In terms of the data listed in Table 9, these results may indicate that the energy profile for the cation 6-H+ corresponds to type b with a low barrier of transition between the structures 6-H+-a and 6-H+-b.Me2N NMe2 H + NMe2 Me2N H + 6-H+-a 6-H+-b Table 9.Geometric parameters of the hydrogen bridge in `proton sponge' salts 6-H+X7. X7 T /K Distances /A Angle Ref. N_H7N N7H N_H N_N /degree [(hfac)¡3 Cu2+]7 (see a) 298 1.27 1.49 2.65 145 63, 64 [(hfac)¡3 Mg2+]7 (see a) 298 1.25 1.58 2.60 134 63, 64 Br7. 2H2O 298 1.30 1.30 2.55 153 65 BF¡4 298 1.30 1.31 2.56 159 66 SCN7 298 1.30 1.30 2.57 160 67 SCN7 188 1.32 1.32 2.58 156 67 Tetrazolide .H2O 298 1.31 1.31 2.57 157 68 2,4-Dinitroimidazolide 298 1.18 1.47 2.61 160 69 1,8-Bis(tosylamido)-2,4,5,7-tetra- 298 1.05 1.63 2.61 152 70 nitronaphthalene (monoanion) 1,8-Bis(trifluoroacetamido)- 298 1.22 1.42 2.59 156 71 naphthalene (monoanion) Pic2N7 (see b) 298 1.05 1.57 2.57 158 72 [O(Ph)C2B10H10]7 298 1.22 1.52 2.58 140 73 OTeF¡5 (triclinic form) 167 1.17 1.46 2.57 159 74 OTeF¡5 (orthorhombic form) 143 1.37 1.37 2.58 140 75 Squarate (monoanion) 150 1.08 1.55 2.58 157 76 Squarate (dianion) . 4H2O 100 0.94 1.69 2.57 156 77 The same c 100 0.97 1.66 2.59 162 77 C6Cl5O7. 2C6Cl5OH 100 1.11 1.47 2.56 162 78 C6F5O7. 2C6F5OH 100 1.07 1.56 2.57 154 79 The same c 100 0.86 1.84 2.57 141 79 Chloranilic acid dianion 298 1.14 1.51 2.59 155 80 150 1.07 1.59 2.59 152 80 D-Hydrogen tartrate .3H2O 100 0.91 1.75 2.61 157 81 The same c 100 0.84 1.86 2.61 149 81 3,4-Furandicarboxylate 298 1.06 1.62 2.62 155 82 (monoanion) Hemimellitate (monoanion) . 1 2H2O 100 0.90 1.72 2.60 164 83 The same c 100 0.94 1.72 2.60 155 83 Maleate (monoanion) 298 1.17 1.49 2.61 157 84 1,2-Dichloromaleate (monoanion) 100 1.11 1.61 2.64 153 85 a hfac is hexafluoroacetonate.b Pic is picryl. c Data for two independent molecules obtained by differential Fourier synthesis are given. c a b d r(N7H) E Figure 2. Types of potential energy curves for the IHB in `proton sponge' cations; for a ± d, see text. Naphthalene `proton sponges' 9Indeed, in some cases it was shown that the proton NH lies in the plane of symmetry of the cation 6-H+ not only at room temperature, but even at low temperatures; the length of theN7H bond is averaged (*1.3 A). In those salts (which constitute the majority), where the hydrogen bridge is asymmetrical, the N7H bond is much longer than the standard value (*0.9 A). In the case of the salt 6-H+ with the dianion of the chloranilic acid, a decrease in the temperature from 300 to 150 K results in the shortening of this bond from 1.14 to 1.07A.This is indirect evidence that the elongation of the N7H bond is due to the disordered position of the NH-proton oscillating between the nitrogen atoms and the plane of symmetry. Obviously, the anion influences the mode of these oscillations and, correspondingly, the geometry of the hydrogen bridge both through the electric field induced by it and due to the changes in the crystal lattice.The formation of a weak bifurcated hydrogen bond involving the anion and the NH-proton was recorded both for the 6-H+ salt with hydrosquarate and squarate anions. Thus at present the majority of researchers tend to accept that the cation 6-H+ exists in a position of fast tautomerism 6-H+-a 6-H+-b. An asymmetrical hydrogen bridge was observed in crystals of the `double sponge' dication 28-(2H+) as well as in cations of the bisiminophosphorane 29 and the compound 30.It is probable that the energy curve profile of the type c corresponds to these bridges. As expected, in cations of unsymmetrical bases, e.g., 84-H+ or 63-H+, the proton practically completely resides on the more basic of the two N atoms: on N(8) and the imine nitrogen, respectively (type d curve, Fig. 2) (Table 10). It should be mentioned in conclusion that in cations of all the `proton sponges' the hydrogen bridge is non-linear and the N7H. . .N angle lies within the range of 150 ± 160 8 with only few exceptions. The neutronographic method, which is considered to be more precise for establishing the molecular structure than X-ray structural analysis (as was shown with the 6-H+ salt with the dichloromaleic acid anion), gives basically the same results.85, 87 4.NMR spectra 1H, 13C, and 15N NMR spectra of the `proton sponges' and their cations were investigated both for solutions and solid state.NMR spectroscopy was used to establish structural characteristics, p-electron distribution, and the ease of proton exchange between the cations and bases. a.Bases As expected, the signals for all the aromatic protons in the 1H NMR spectrum of compound 6 (Table 11) are at higher field than those for naphthalene (d 7.46 and 7.81 ppm for a- and b- protons, respectively). The greatest shielding is of the protons at positions 2 and 7, then H3(6) and H4(5), the difference in the chemical shifts between meta- and para-protons being very small. If one assumes that the degree of proton shielding is propor- tional to the +M-effect of the substituents, the following con- clusion can be made: the electron donor capacity of the NMe2 groups in the molecule of compound 6 is somewhat lower than that of the peri-substituents in 1-dimethylamino-8-methoxy- 94 and 1,8-di-methoxynaphthalenes 97 (Table 12).Even in 1- methoxynaphthalene 99, the donor effect of the substituent seems to be higher than in 1-dimethylaminonaphthalene 98. This conclusion is confirmed by analysis of 13C NMR spectra (Table 13). Since in benzene derivatives the shielding effects of NMe2 and MeO groups are opposite and correspond to the Hammett s-constants, it is reasonable to conclude that the decreased electron donor capacity of the NMe2 groups in 1- dimethylamino- and 1,8-bis-(dimethylamino)naphthalenes is a R2 R1 1 2 3 4 4a 5 6 7 8 8a 94: R1=OMe, R2=NMe2; 97: R1=R2=OMe; 98: R1=NMe2, R2=H; 99: R1=OMe, R2=H. 94, 97 ± 99 Table 11. 1H NMR spectra of 1,8-bis(dimethylamino)naphthalene 6 and its cations 6-H+.91 Compounda Solvent d /ppm J /Hz H(2), H(7) H(3), H(6) H(4), H(5) CH3 (NHN)+ JH(2) ± H(3) JH(3) ± H(4) JH(2) ± H(4) JNH±CH3 6 CDCl3 92 6.96 7.33 7.39 2.82 7 7.3 8.2 1.5 7 6 CD2Cl2 6.92 7.27 7.32 2.77 7 7.2 8.1 1.5 7 6 CCl4 1 6.76 7.09 7.18 2.71 7 7.6 7.9 1.2 7 6 CD3CN 6.90 7.23 7.30 2.70 7 7.2 8.1 1.5 7 6 CD3NO2 6.91 7.24 7.30 2.73 7 7.2 8.1 1.5 7 6-H+ CF3CO2H1 8.04 7.71 7.83 3.21 19.51 8.3 7.6 0.9 2.0 6-H+Cl7 CD3CN 8.00 7.70 8.04 3.20 18.58 7.5 8.4 0.9 2.7 6-H+Br7.H2O CD3CN 7.96 7.71 8.05 3.17 18.66 7.8 8.4 0.9 2.7 6-H+NO¡3 CD3CN 7.94 7.70 8.04 3.14 18.92 7.8 8.4 0.9 2.7 6-H+BF¡4 CD3CN 7.92 7.71 8.05 3.12 18.67 7.5 8.4 0.9 2.7 6-H+ClO¡4 CD3CN 7.91 7.70 8.05 3.11 18.66 7.8 8.4 1.2 2.7 6-H+ClO¡4 CD3NO2 7.98 7.75 8.08 3.25 19.08 7.8 8.4 0.9 2.7 6-H+ClO¡4 (CD3)2SO 93 8.08 7.73 8.09 3.12 18.33 7.7 8.2 1.1 2.6 a The 1H NMR spectra of `proton sponge' salts with the following anions: I7, NCS7, PF76 ; Ph4B7 were also recorded in CD3CN.94 The values of chemical shifts in these salts are very close to those cited above.Table 10. Geometric parameters of the hydrogen bridge in cations of some `proton sponges' at 298 K. Cation Anion Distances /A Angle Ref. N_H7N, N_N N7H H_N /degree 84-H+ ClO¡4 2.57 0.99 1.64 153 55 28-(2H+) 2Br7 2.57 1.22 1.39 158 24 29-H+ Br7 2.58 0.85 1.78 159 32 63-H+ Br7 2.52 1.20 1.38 154 33 30-H+ NO¡3 2.64 1.28 1.39 164 34 30-H+ NO¡3 2.63 a 0.83 a 1.83 a 161 a 88 30-H+ NO¡3 2.63 a 1.04 a 1.61 a 165 a 88 30-H+ PicO7 2.72 1.07 1.68 162 89 30-H+ ClO¡4 2.68 1.18 1.51 167 90 a At 100 K. 10 A F Pozharskiiresult of their considerable non-coplanarity with the nucleus caused by peri-interactions.It is noteworthy in this respect that the chemical shift of the N-methyl groups (d 2.8 ppm) in the `proton sponge' is lower than that of 1-dimethylaminonaphtha- lene (d 3.0 ppm), which testifies to an increased contribution of the s-component in the n-orbital of the amine nitrogen. Evidence for the effects of substituents in the nucleus on the parameters of the 1H NMR spectra can be inferred from Table 14.As can be seen, ortho- (Cl, Br) and7M-substituents at position 4 (CHO, COCF3, NO2) cause a down-field shift of the N-methyl signals (by *0.2 ppm). In addition, 7M-substituents produce a strong deshielding effect on H(5) [to a lesser degree, on H(3)]. This is accompanied by reduction of the spin-spin coupling constant JH(5) ± H(7) and marked increase in the spin-spin coupling constant JH(2) ± H(3) (presumably, due to an increased contribution of structures of the type 84a,b).Recent measurements of MAS 1H NMR and 13C NMR spectra of a solid sample of the `proton sponge' 6 gave quite unexpected results:96 they revealed strong asymmetry of its molecule. It was found that all four methyl groups, like all ten ring carbon atoms, resonate as separate peaks, i.e., they are non- equivalent (Table 13).This has been interpreted in terms of electrostatic interactions between the neighbouring molecules in the crystal lattice and has led to partial revision of the previously established 56 results of X-ray studies of the `proton sponge'. 1H NMR spectra have been used to study the ease of the proton exchange between the bases of the `proton sponge' 6 and its tetraethyl analogue 34, on the one hand, and the corresponding cations, on the other.97 The 1H, 13C, and 15N NMR spectra of compound 30 have also been recorded.34, 98 Table 12. 1H NMR spectra of some close analogues of the `proton sponge' in CDCl3.92 Compound d /ppm H(2) H(3) H(4) H(5) H(6) H(7) H(8) CH3 94 6.89 7.45 7.38 7.38 7.45 7.02 7 4.01 (MeO); 2.87 (NMe2) 97 6.86 7.36 7.40 7.40 7.36 6.86 7 3.99 98 7.18 7.50 7.60 7.60 7.60 7.94 8.38 3.00 99 6.88 7.47 7.56 7.56 7.56 7.90 8.40 4.06 Table 13. 13C NMR spectra of the `proton sponge' and some of its close analogues. Com- Solvent d/ppm Ref. pound C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(8a) C(4a) CH3 6 CDCl3 150.70 112.70 125.40 121.70 121.70 125.40 112.70 150.70 120.60 137.80 44.40 92 6 CDCl3 151.26 113.47 126.02 122.49 122.49 126.02 113.47 151.26 121.37 138.66 7 95 6 CD3CN 151.33 113.57 126.30 122.28 122.28 126.30 113.57 151.33 121.13 138.58 44.68 91 6 CD3NO2 152.03 113.92 126.78 122.65 122.65 126.78 113.92 152.03 121.65 139.12 44.99 91 6 see a 119.80 112.10 124.60 120.90 122.10 123.90 113.30 121.80 149.70 136.50 41.0; 41.8 96 44.2; 45.1 1 CDCl3 144.20 111.30 126.00 119.40 119.40 126.00 111.30 144.20 116.80 136.70 7 57 94 CDCl3 156.10 106.40 126.00 122.00 121.60 125.60 113.30 150.70 120.30 137.60 56.3; 45.6 92 97 CDCl3 157.10 106.30 126.30 120.80 120.80 126.30 106.30 157.10 117.60 137.40 56.40 92 6-H+ClO¡4 CD3CN 145.12 122.36 127.86 130.04 130.04 127.86 122.36 145.12 120.02 136.19 46.73 91 6-H+BF¡4 see a 117.40 120.70 128.30 126.50 126.50 128.30 120.70 117.40 143.40 134.80 43.6; 46.4 96 a Spectrum for solid compound.Table 14. 1H NMR spectra of `proton sponge' derivatives in CDCl3 (300 MHz, 298 K). Com- d /ppm J /Hz Ref. pound 1-NMe2 8-NMe2 H(2) H(3) H(4) H(5) H(6) H(7) JH(2) ± H(3) JH(3) ± H(4) JH(5) ± H(6) JH(6) ± H(7) JH(5) ± H(7) 70 3.03 2.78 7 7.32 7.39 7.37 7.29 7.05 7 8.71 7.91 7.62 1.33 28 71 3.02 2.75 7 7.30 7.51 7.37 7.30 7.07 7 8.71 7.99 7.39 1.39 93 72 2.98 2.98 7 7.33 7.45 7.45 7.33 7 7 8.71 8.71 7 7 28 73 2.96 2.96 7 7.35 7.51 7.51 7.35 7 7 8.70 8.70 7 7 93 50 2.78 2.80 6.80 7.38 7 7.80 7.41 6.98 8.21 7 8.35 7.62 1.03 28 51 2.82 2.84 6.79 7.63 7 7.84 7.45 7.02 8.20 7 8.20 7.61 1.17 27 52 2.76 2.76 6.74 7.37 7 7 7.37 6.74 8.28 7 7 8.28 7 28 53 a 2.74 2.74 6.62 7.60 7 7 7.60 6.62 8.35 7 7 8.35 7 27 74 b 2.82 2.80 6.98 7.51 7 7.64 7.35 6.98 7.98 7 8.20 8.20 <1.0 109 79 2.79 2.79 6.86 6.97 7 7.85 7.30 6.90 8.13 7 8.35 7.62 1.02 45 80 2.97 2.78 6.84 7.72 7 8.89 7.48 6.97 8.20 7 8.21 7.62 0.88 131 83 c 3.00 2.72 6.84 7.90 7 8.64 7.45 6.94 9.08 7 8.43 7.77 0.88 110 84 2.99 2.78 6.70 8.30 7 8.43 7.49 6.95 9.08 7 8.49 7.62 0.88 55 a At755 8C.b In [2H6]-acetone. c In [2H6]-DMSO. Naphthalene `proton sponges' 11b. Cations On going from the base 6 to the cation 6-H+, the signals for all the protons are shifted down-field: H[2(7)], by +1.1 ppm, H[3(6)], by +0.4 ppm, H[4(5)], by +0.7 ppm, and CH3 , by +0.3 ppm. Although it would be logical to expect that the signals of ortho- protons were at the lowest field, it is not always the case (Table 11).For example, in a solution of the perchlorate 6-H+ in [2H6]-DMSO, the signals of ortho- and para-protons overlap; however, in CD3CN and CD3NO2, the doublet of doublets of H[4(5)] are at lower field. The signals of the both types of protons can be distinguished by larger values of the ortho-constants, JH(3) ± H(4) and JH(5) ± H(6) in comparison with JH(2) ± H(3) and JH(6) ± H(7).The nature of the anion in `proton sponge' salts has practically no effect on their NMR spectra (Table 11). Yet, the main peculiarity of the 1H NMR spectra of `proton sponge' cations is the unusually high chemical shift of the NH- proton (d 18 ± 20 ppm), which is indicative of strong chelation. For the cations of symmetrical `proton sponges' including 6-H+, the NH signal in high-resolution spectra has thirteen lines due to spin-spin coupling with the protons of the methyl groups.The signal of the latter is split into a doublet with an intensity of 12 proton units with a spin-spin coupling constant of about 2.5 Hz. Thus the NMR method has permitted recording of a symmetrical IHB for these cations at room temperature, which probably reflects rapid (on the NMR time scale) deviations of the NH- proton relative to the plane of symmetry.The same is observed in the spectra of the symmetrical iminophosphorane `sponges' 29 and 64.32, 49 Although, as has been shown earlier, the hydrogen bridge has an asymmetrical shape in crystals of their cations, according to NMR data both tautomeric forms are in fast equilibrium and the NH-proton is shared equally by both nitrogen atoms.In non-symmetrically substituted cations, the dimethylamino groups are non-equivalent, they are observed as two doublets with different spin-spin coupling constants (Table 15). It has been suggested to use the relationship between the coupling constants for estimating the degree of IHB asymmetry in a particular cation.93 The most asymmetrical is the hydrogen bridge in the cations of 2-halogeno-1,8-bis(dimethylamino)naphthalene 70-H+ and 71-H+; in [2H6]-DMSO, the NH-proton belongs to the N(8) atom by more than 80%.The asymmetry of IHB in the cations of 4-amino- 76-H+ and 4-methylamino derivatives 78-H+ is very strong [in these cases, the NH-proton is closer to the N(1) atom] and it is somewhat less in the cations having 7M-sub- stituents at position 4: 80-H+, 82-H+± 84-H+, and 100-H+. The least asymmetrical is the IHB in the cations of 4-vinyl- 74-H+, 4- acetamido- 77-H+, and 4-halogeno-derivatives 50-H+ and 51- H+.It was established that the transition from [2H6]-DMSO to a less proton-accepting solvent, acetonitrile, favours the symmetr- isation of the hydrogen bridge, which is especially pronounced in the cations of 2-halogeno- and 4-amino-1,8-bis(dimethylamino)- naphthalenes.Me2N NMe2 R2 R1 100 ± 102 100: R1=CN, R2=H; 101: R1=PhCO, R2=H; 102: R1=R2=PhCO. Table 15. Parameters of 1H NMR spectra of the groups involved in hydrogen bridge formation in `proton sponge' cations.93 Cation Solvent d /ppm J /Hz Proton localisation (%) a 1-NMe2 8-NMe2 NH+ JNH± 1-NMe2 JNH± 8-NMe2 1-NMe2 8-NMe2 6-H+ [2H6]-DMSO 3.12 3.12 18.33 2.63 2.63 50 50 6-H+ CD3CN 3.11 3.11 18.69 2.64 2.64 50 50 70-H+ [2H6]-DMSO 3.18 3.32 18.02 0.77 4.06 16 84 70-H+ CD3CN 3.25 3.27 18.80 1.54 3.84 29 71 71-H+ [2H6]-DMSO 3.21 3.33 18.09 <1 4.06 16 84 71-H+ CD3CN 3.27 3.26 18.87 1.65 3.84 30 70 72-H+ [2H6]-DMSO 3.37 3.37 19.91 2.41 2.41 50 50 73-H+ [2H6]-DMSO 3.40 3.40 20.09 2.41 2.41 50 50 73-H+ CD3CN 3.41 3.41 20.33 2.64 2.64 50 50 50-H+ [2H6]-DMSO 3.11 3.14 18.52 2.42 2.64 48 52 51-H+ [2H6]-DMSO 3.11 3.14 18.56 2.31 2.53 48 52 52-H+ [2H6]-DMSO 3.10 3.10 19.14 2.52 2.52 50 50 53-H+ [2H6]-DMSO 3.10 3.10 19.12 2.41 2.41 50 50 74-H+ [2H6]-DMSO 3.12 3.12 18.64 2.30 2.30 50 50 76-H+ [2H6]-DMSO 3.09 2.99 18.26 3.23 1.21 73 27 76-H+ CD3CN 3.10 3.02 18.64 3.41 1.87 65 35 77-H+ [2H6]-DMSO 3.12 3.11 18.55 2.86 2.24 56 44 77-H+ CD3CN 3.12 3.11 18.90 2.75 2.63 51 49 78-H+ [2H6]-DMSO 3.11 2.98 18.31 3.40 1.21 74 26 79-H+ [2H6]-DMSO 3.11 3.06 18.48 2.86 2.20 57 43 79-H+ CD3CN 3.10 3.07 18.84 2.97 2.31 56 44 80-H+ [2H6]-DMSO 3.10 3.21 18.43 1.75 3.08 36 64 80-H+ CD3CN 3.10 3.20 18.75 1.98 3.30 38 62 82-H+ [2H6]-DMSO 3.10 3.17 18.58 1.97 2.86 41 59 83-H+ [2H6]-DMSO 3.09 3.23 18.35 1.44 3.15 31 69 83-H+ CD3CN 3.10 3.22 18.67 1.86 3.41 35 65 84-H+ [2H6]-DMSO 3.17 3.28 18.50 1.72 3.19 35 65 84-H+ CD3CN 3.08 3.19 18.72 2.13 3.36 39 61 100-H+ [2H6]-DMSO 3.09 3.21 18.15 1.88 2.97 39 61 a Indices of proton localisation at each of the dimethylamino groups (PL) were calculated by the formula: PL=[JNH± 1(8)-NMe2 / (JNH± 1-NMe2+JNH± 8-NMe2)] . 100%. 12 A F PozharskiiFor a series of compounds of the same type, there is no apparent correlation between the stability of the IHB and the chemical shift of the NH-proton. It is therefore sufficient to compare the cation 6-H+ and its 4-nitro-derivative 84-H+. Although in the latter case, the signal of the NH-proton is shifted down-field by 0.4 ppm relative to the analogous signal in the spectrum of the cation 6-H+, its hydrogen bridge is partly cleaved by DMSO and other dipolar solvents (see section IV. 2), whereas in the cation 6-H+, no signs of its cleavage were observed. Apparently, the stability of the IHB is largely determined by the basicity of the corresponding `proton sponge', while the dNH value depends on the distance between the nitrogen atoms and the degree of NH-proton expulsion from the plane of the ring as a result of which it experiences the effect of the diamagnetic component of the magnetic field of the naphthalene p-system.The unusually high chemical shift of the NH-proton (d *20.0 ppm) in the spectra of cations of 2,7-dihalogeno-deriva- tives of the `proton sponge' is indirect evidence in favour of this explanation.It is unlikely that in these compounds the NH-proton lies in the plane of the ring. More probably, the nitrogen atoms become closer to each other pushed by the halogens, so that this proton is out of the plane and undergoes strong deshielding in the cation 11-H+ (dNH 24.0 ppm).10 On the whole, the factors influencing the magnitude of the chemical shift of the NH-proton in `proton sponge' cations have ben studied insufficiently.For example, it is still unclear why the signals of the NH-protons in the cations 25-H+ (d 16.56 ppm), 26-H+ (d 14.90 ppm),29 29-H+, 64-H+ (d * 16.5 ppm),32, 49 and 30-H+ (d *15.8 ppm) 34 are at much higher fields than those in the cations of 1,8-bis(dialkyl- amino)naphthalenes. 13Cand 15N NMRspectra of salts of 6-H+(with tetrazolide,99 1-methyltetrazole-5-thiolate, and hydrogen squarate 100 anions) were recorded for solutions in MeCN and in the solid state. As expected, the signals of C(2) and C(4) nuclei are more sensitive to salt formation: they shift 4 ± 5 ppm down-field. However, the largest changes in the chemical shifts (by 6 ppm for solutions in MeCN and by 10 ppm in solid state) are observed in the 15N NMR spectra upon transition from the base to the cation.It is 15N NMR spectroscopy that is recognised as being the most appropriate for the study of the structure of cations in solution and in the crystalline state. Recent 1Hand 13C NMRspectral data of solid samples of 6-H+ with thiocyanate and tetrafluoroborate revealed two types of N-methyl groups, presumably located close to the plane of the ring and remote from it.At the same time, unlike the base, the naphthalene ring of the cation 6-H+ itself did not reveal any signs of asymmetry.96 13C (Ref. 49) and 31P MAS (Ref. 101) NMR spectra of cati- ons of the iminophosphorane `proton sponges' 29, 63, and 64, and 13C and 15N NMR spectra of the salts 30-H+ have been ana- lysed.98 c.NMR spectra and molecular dynamics The majority of the naphthalene `proton sponges' are sterically strained, therefore, many of them exist in different conformations due to hindered rotation of dialkylamino groups around the CAr7N bonds, distorted ring shape, inversion of nitrogen, etc. The corresponding transitions can most effectively be followed by NMR spectroscopy.Sometimes, dynamic processes manifest themselves even at room temperature as a widening (sometimes, rather prominent) of peaks of the N-alkyl groups and aromatic protons. Although the solution 1H and 13C NMR spectra of the `proton sponge' 6 demonstrate the equivalence of all the four N-methyl groups at room temperature, this was shown to result from rapid reversible transitions that average the respective conformations on the NMR time-scale.26 It was found that the singlet of the methyl groups in the 1H NMR spectrum of compound 6 in CF2Cl2 splits into a doublet upon cooling down to 7120 8C.The free energy of activation (DG=) for the con- formational transition was estimated as 31.40.8 kJ mol71. It was assumed that the interconversion of the methyl groups is of a `narcissistic' type and occurs via a planar transition state with C2u symmetry (Scheme 1).Scheme 1 In the case of sterically more hindered 1-benzylmethylamino- 8-dimethylaminonaphthalene, the conversion of the NMe2 group singlet into the doublet occurs at 738 8C; simultaneously, the methylene group singlet splits into a quartet. In this case, the DG= value is equal to 57.31.7 kJ mol71.26 Similar dynamic processes were observed in the 1H NMR spectra of compounds 62,31 53,27 and 90.102 5. Nuclear quadrupole resonance spectra Contrary to the original interpretation of the 14NQR spectrum, with cross-relaxation, of the base 6,103 recent measurements revealed the presence in this spectrum of four (rather than two) resonance peaks from two nitrogen atoms.104 This finding unam- biguously testifies to their non-equivalence and thus correlates with NMR data on the asymmetry of the `proton sponge' molecule in crystals. 6. Electron spectroscopy for chemical analysis According to Hasselbach et al.,105 neither NMR nor X-ray diffraction analysis can give an unequivocal answer to the ques- tion about the IHB symmetry in the `proton sponge' cation.This can be done only with the help of `faster' methods, such as electron spectroscopy for chemical analysis (ESCA). Here, the ionisation of core electrons takes about 10716 s, therefore the non-equiv- alent atoms can be distinguished even at fast equilibrium. The ESCA spectrum recorded for the salt 6-H+BF74 in the N1s energy region revealed two maxima of identical height, which points to the non-equivalence of nitrogen atoms in the cation 6-H+.However, the splitting of the two peaks was relatively low. This points to the fact that the hydrogen bridge in the cation 6-H+ is nevertheless nearly symmetrical. ESCA studies of a number of other `proton sponge' salts have also been carried out.106, 107 7. Electronic absorption spectra, colour, and solvatochromism Perhaps the most valuable information about the electron inter- action of dialkylamino groups and the naphthalene p-system in `proton sponges' can be derived from UV spectra.The longwave absorption band in the UV spectra of 1,8-diaminonaphthalene and itsN-alkyl derivatives including compound 6 lies in the region 340 ± 350nm (Table 16). As in the case of other aryl amines, it is ascribed to the transfer of electron density from the n-orbital of the nitrogen atoms to the p-antibonding orbital of the naphtha- lene system (p ± p* transition).The intensity of this band strongly depends on steric factors influencing the rotation of the amino group around the CAr ±N bond. We used this circumstance 2 to determine the value of the dihedral angle (j) between the plane passing through theCAr7Nbond perpendicularly to the aromatic system and the plane passing through the same bond and the symmetry axis of the unshared electron pair of the nitrogen atom.The angle j was determined from the known ratio, e/e0=cos2j, where e is the extinction coefficient at the absorption maximum for the amine under study, and e0 is the same for a planar model, in which j=0, i.e., the conjugation is a maximum. 1,3-Dimethyl- 2,3-dihydroperimidine 19a was used as a model for which the longwave absorption band did have the maximum intensity. The experimental values of j are listed in Table 16. For the `proton N N Me Me Me Me C2 C2 C2u N Me Me N Me Me N Me Me N Me Me Naphthalene `proton sponges' 13sponge' 6, j=35 8, which agrees well with the X-ray data (j=40 8) 56 and other estimates.1 The not very high value of the angle j points to the possibility of significant conjugation of the dimethylamino groups with the naphthalene system in the base 6, which is confirmed by all its physicochemical properties and reactivity.The p-donor effect of the dimethylamino groups is especially prominent in the derivatives of the `proton sponge' containing 7M-substituents at position 4.In contrast to the colourless compound 6, these compounds give different colours from yellow to violet depending on the electron-acceptor properties of the substituent (Table 17). As expected, the longwave absorption band, which is determined by direct polar conjugation, is very sensitive to the solvent polarity.Thus in the case of the 4-nitro- derivative 84, the lmax in hexane, benzene, methanol, and DMSO is 411, 444, 463, and 484 nm.55 Such a solvatochromism may be due to the different solvation of the excited state, which can be represented by bipolar structures of the type 84a,b. Evidently, the more polar the solvent, the stronger the solvation, the smaller the electron transfer energy, and the deeper the colour.Since chelation of the NH-proton takes the dimethylamino groups completely out of conjugation with the ring, the absorp- tion spectra of `proton sponge' cations closely resemble the spectra of appropriate naphthalenes devoid of dimethylamino groups. Thus, the lmax of the longwave absorption band for the cations 6-H+ and 84-H+ are equal to 288 nm (lg e=3.85) 108 and 334 nm (lg e=3.50) 55 (in 0.1 M HCl), whereas the correspond- ing values for naphthalene and 1-nitronaphthalene are 297 nm (lg e=2.82) and 342 nm (lg e=3.59).An interesting variety of solvatochromism was observed for the cation 84-H+.55 When the colourless perchlorate is dissolved in DMSO, DMF, or pyridine, the solution immediately turns deep red or red-orange (for DMF); in the majority of other solvents, colourless, yellowish, or pink solutions are formed.The reason for solvatochromism is the decreased basicity of 1,8-bis(dimethyl- amino)-4-nitronaphthalene 84 and IHB asymmetry in its cation, so that the solvents with high proton-acceptor capacity disrupt the IHB, thereby releasing the dimethylamino groups for the con- jugation. Practical applications of solvatochromism of the per- chlorate 84-H+ are proposed.55 8.Infrared spectra The most important in the IR spectrum of the base 6 are the bands at 2780, 2830, 2870, and 1580 cm71 (Refs 86, 111). The first three of them, known as Bolman bands, are associated with stretching vibrations of the C7H bonds of the methyl groups trans-oriented with respect to the unshared electron pairs of the nitrogen atoms.The band at 1580 cm71 is associated with stretching vibrations of the ring. Its high intensity is the consequence of the asymmetry of the naphthalene system. Indeed, the intensity of this band sharply decreases upon protonation, and the band slightly shifts towards high frequencies. The Bolman bands also practically disappear upon transition to the cation or, in the case of partial protonation, decrease in intensity. These changes can be used for the quantita- tive estimation of the degree of protonation of the base 6 by different acids including phenols.However, the most interesting feature is the appearance of stretching vibration bands of the hydrogen bridge ns(NHN) in the IR spectra of 6-H+ salts. The dependence of the position of this band on pressure,112 counterion structure,113 deuteration [the isotope ratio, n(NHN)/n(NDN) was determined],114 and the change in the aggregate state 115 was studied.In solution (the measurements were performed in aceto- nitrile, dichloromethane, and dichloroethane), the band manifests itself as a very broad `continuum' at the background level extending from 3000 to about 300 cm71.In the crystalline state, the type of absorption changes drastically; the band shifts towards 800 ± 250 cm71 and acquires fine structure. This phenomenon is typical of very stable hydrogen bridges having a short N. . .N distance (*2.55 ± 2.65 A). It is assumed that the band structure can be explained in detail only by taking into account the super- position of d- and g-deformation oscillation, their overtones, and internal oscillations causing modulation of the hydrogen bridge geometry.111 These data are consistent with the abnormally high isotope ratio, which in some cases reaches 1.8 ± 2.05 (in the case of weak IHBs, it is less than 1.45).114 There is a tendency to broad- ening of the ns(NHN) band, and its shift towards higher frequen- cies with an increase in the proton-acceptor capacity of the anion.113, 114 Thus, in the case of the anions Ph4B7 and F7, the centre of the band is at 463 and 583 cm71, respectively.The effect of the anion was explained by its bifurcation interaction with the NH-proton of the cation. It should be recalled, however, that X-ray studies of numerous salts of the `proton sponge' allowed one to reveal reliably the bifurcation effect only in two cases (Section IV. 3. b). On the basis of the IR spectral data of proton sponge salts, most authors concluded that the potential energy curve for the hydrogen bridge corresponded either to a potential with two minima and a very low barrier, or to a potential with one minimum and a flat bottom (Fig. 2a, b). Because of the nearly planar structure of the naphthalene system in compound 30, its IR spectrum differs substantially from that of the `proton sponge' 6.62 9.Mass spectra Mass spectra of sterically hindered bis(dimethylamino)arenes including various types of `proton sponges' were studied in most detail.116, 117 In all cases, the fragmentation of the molecular ion which is the most intense, as a rule, is determined by the proximity of dimethylamino groups (the so-called `proximity effect') and at early stages involves practically only these groups.The fragmen- tation processes include isomerisation and elimination of the MeNH2 , Me2NH, and H . fragments giving rise to stable hetero- cyclic ions. Four main pathways of fragmentation of the molecular ion of the base 6 under electron impact are shown in Scheme 2.116 The first pathway (a) is a stepwise counterflow intramolecular transfer Table 16.The position of longwave absorption bands (lmax, e) and magnitudes of dihedral angles (j) in UV spectra of 1,8-diaminonaphtha- lenes.2 Compound lmax 1073 e j /deg /nm /litre mol71 cm71 1 337 10.40 42 2 339 8.96 46 3 351 6.07 55 4 341 8.72 90; 14 5 351 9.50 81; 0 6 341 12.40 35 19a 344 18.60 0 Table 17.Position of the longwave absorption band in UV spectra and colour of some `proton sponge' derivatives (in methanol). Compound lmax /nm lg e Colour Ref. 74 353 3.62 Pale yellow 109 75 516 4.02 Dark violet 110 80 407 3.84 Yellow 61 81 449 3.52 Brown 61 82 389 3.68 Yellow 110 83 431 3.68 Red 110 84 463 4.02 Dark red 55 89 521 4.00 Claret 110 96 403 3.67 Yellow 61 101 405 3.57 " 110 102 413 3.82 " 110 14 A F PozharskiiofH .and Me . species from one nitrogen atom to another resulting in the ions 6a+. , 6b+. , and 6c+. , respectively. Scheme 2 The latter eliminates MeNH2 , which accounts for the presence in the spectrum of an intense peak with m/z 183; this peak was assigned to the 1,2-dimethyl-1,2-dihydrobenzo[c,d]indolium ion.Its subsequent aromatisation gives the 1-methylbenzo[c,d]indol- ium ion (m/z 168). The peak of this ion is second in intensity after the peak of M+. and in some `proton sponge' derivatives it is the most abundant. The second pathway of fragmentation (b), which also gives the 1-methylbenzo[c,d]indolium ion, consists in the elimination of Me2NH from the 6a+.ion. The third (c) and the fourth (d) pathways represent direct fragmentation of the molec- ular ion 6+. consisting in the loss of Me2N . or Me . species. In the latter case, methyl derivatives of the benzo[c,d]indazolium ion with m/z 199 and 184 are formed. The above fragmentation features are observed in the mass spectra of other derivatives of the `proton sponge' and, which is the most interesting, in the spectrum of 1,8-bis(diphenyl-amino)- naphthalene 61.117 10.Dipole moments Dipole moments in benzene were measured for three compounds of the given series: the `proton sponge' 6, its 4-formyl- 80, and 4,5- diformyl derivatives 81.61 The dipole moment of compound 6 (m=1.19 D) is precisely equal to that of 1-dimethylamino- naphthalene. This is lower than the expected value, since the p-conjugation of both dimethylamino groups with the ring would result in the summation of the vectors of the corresponding dipole moments.Apparently, due to the high non-coplanarity of the molecule 6, both vectors are directed at a considerable angle to each other. In contrast, the dipole moments of the aldehyde 80 (m=5.44 D) and the dialdehyde 81 (m=9.21 D) appeared to be unexpectedly high.This suggests that the conjugation of the aldehyde and the NMe2 groups in them is very efficient. The additional moment of the p-interaction, mp, in compounds 80 and 81 is equal to *1.4 and 2.3 D, which is higher than in 4-di- methylaminonaphthalene-1-carbaldehyde (mp=0.35 D) and even in p-dimethylaminobenzaldehyde (mp&1 D). These data are in good agreement with the X-ray data for compound 81.The high dipole moment makes this compound closer to ionic substances; it is therefore not incidental that in contrast to many other derivatives of the `proton sponge', the dialdehyde 81 is rather soluble in water and has a high melting point (162 ± 164 8C). 11. Donor-acceptor properties a. Gas-phase ionisation potentials The gas-phase ionisation potentials, IP1, of the `proton sponge' 6 and its partially N-methylated precursors 1 ± 5 measured by electron impact were strikingly similar, from 7.38 to 7.47 eV (see Table 18).2 This could indicate that the first electron is ejected from the p-orbital of compounds 1 ± 6.Indeed, if these values characterised the ejection of an n-electron, then, taking into account significant differences in the basicity of these compounds, the ionisation potential of the `proton sponge' would be much lower in comparison with that of the diamines 1 ± 5.However, Maier,118 who used photoelectron spectroscopy to measure the ionisation potentials of compounds 1 and 6, came to a somewhat different conclusion. The IP1 value for the diamine 1 (7.10 eV) was also assigned to the p-ionisation potential, while for the `proton sponge' 6 (IP1=7.05 eV) it was ascribed to electron ejection from the n-orbital.This conclusion was made on the basis of calcu- lations using perturbation theory, which revealed that the highest occupied p-MO for compound 6 correlated better with the second ionisation potential, IP2=7.47 eV. It is hardly probable that this argument is self-explanatory. It will be recalled once again that according to UV spectroscopy data, the conjugation of the nitro- gen atoms with the naphthalene ring and, correspondingly, the p- donor capacity of compound 6 is at least no less than that of 1,8- diaminonaphthalene.b. Anodic oxidation potentials. Radical cations Electrochemical oxidation of the `proton sponge' 6 on a platinum disc electrode in MeCN gives two reversible one-electron waves with E1=2 ox =0.36 and 1.02 V.119 Presumably, the radical cation 6+.is formed at the fist step, and the dication 103, at the second; the structure with a s-bond between the nitrogen atoms was ascribed to the latter. The radical cation 6+. was also generated by oxidation of compound 6 with PbO2.Its EPR spectrum represents an unresolved singlet with a width DH of 23 ê (g=2.0043). On the basis of these data, it was concluded that the conjugation of the dimethylamino groups with the naphthalene ring in compound 6 is significant and according to its electron-donor properties it occupies a position between N,N-dimethylaniline (E1=2 ox =0.68 V) and N,N,N0,N0-tetramethyl-p-phenylenediamine (E1=2 ox = 0.015 V).The oxidation of compound 23 occurs in an analogous way, but more easily.120 Its radical cation can be conserved unchanged in acetonitrile over a period of many months. TheUVspectrum of this radical cation (lmax=480 nm, lg e=3.1) is very similar to that of compound 23 itself, which has led to a conclusion that the naphthalene fragment of the molecule does not take part in the oxidation.In contrast to compounds 6 and 23, the `double proton sponge' 28 is oxidised in one two-electron wave with E1=2 ox =70.50 V (relative to Fc/Fc+ = 0.0 V, Fc is ferrocene).24 Apparently, the driving force of this process is the formation of the resonance-stabilised dication 104, which was isolated in the form of black crystals and thoroughly characterised.24, 121 The radical cation 28+.giving the EPR spectrum with hyperfine structure can be obtained by chemical oxidation of compound 28.24 6+. a, b NMe2 N Me CH2 H + 6a+. c 7Me2N . Me CH2 + (m/z 170) NMe + (m/z 199) d Me2N . N 7Me . 72H . 7Me . 7Me2NH +. C N Me H Me (m/z 183) 7Me . N N Me CH Me Me H H + 6c+. 7MeNH2 . N Me CH2 Me H .+ 6b+. CH2 Me +. (m/z 169) CH Me + (m/z 168) NMe +.(m/z 184) N N MeN 7H . NMe 6 7e +e 7e +e Me2N +. + + 6+. 103 NMe2 Me2N NMe2 Naphthalene `proton sponges' 15c. The formation of p-complexes All 1,8-diaminonaphthalenes, including the `proton sponge' 6 can easily form molecular complexes (MC) in which they act as donors. An attempt has been made to use the complexes with 1,3,5-trinitrobenzene (1 : 1, the complex formation constantsKc lie in the range 2.0 ± 3.4 litre mol71) for estimating their electron- donor capacity.2 Presumably, stronger donors would produce MC with an absorption maximum of the charge transfer band shifted to longer wavelengths. However, measurements revealed (Table 18) the lack of correlation between the basicity of 1,8- diaminonaphthalenes and the ease of formation of MC by these compounds.Morever, the most basic compound (the `proton sponge' itself) gives a MC having the lowest value of lmax (500 nm). It was therefore concluded that compounds 1 ± 6 behave as p-donors in the formation of MC, whereas the differences in the ease of formation and stability of MC are determined predom- inantly by steric factors and inductive effects of the methyl groups.d. The formation of H-complexes In some cases, IR and NMRmethods were used to investigate the interaction of compound 6 with phenols,113, 122 ± 124 carboxylic acids,123 and SH-acids.125 An unexpected finding was that phenol (pKa=9.99) hardly protonated the `proton sponge', although its basicity exceeded that of the phenolate anion by more than two orders of magnitude.Distinct protonation is observed only with phenols having pKa<8.4. This reaction gives two types of salts having the composition of 1 : 1 and 1 : 2, the latter being much more stable. This is rationalised by formation of homoconjugated anions, ArOH. . .7OAr. Evidence for their stability can be derived from the fact that the salt of the base 6 with pentachloro- phenol (1 : 1) disproportionates in acetonitrile to give a 1 : 2 salt and a non-protonated `proton sponge':113 e.Electron acceptor properties. Radical anions There is only one study concerned with the ability of `proton sponges' to undergo one-electron reduction.126 It has been shown that treatment of compound 6 with sodium in 1,2-dimethoxy- ethane gives a stable radical anion; its EPR spectrum was recorded at 720 8C.The hyperfine splitting constants (HFS) for the unpaired electron with cyclic protons are: aH(4),H(5)=4.46, aH(3),H(6)=1.77, and aH(2),H(7)=1.39 Gs, i.e., they are somewhat smaller than the corresponding values for the naphthalene radical anion. At the same time, the HFS constants with the protons of the methyl groups and 14N nuclei are very small.This was interpreted as a result of non-coplanarity of the dimethylamino groups and the naphthalene ring. The magnitude of the dihedral angle between the axes of unshared electron pairs of the nitrogen and the 2pp-orbitals of cyclic C-atoms in the radical anion 67. was estimated to be 60 ± 70 8. 12. Quantum-mechanical calculations Using the ab initio method with optimisation of geometry, the optimum structures of the `proton sponge' 6 and its protonated form 6-H+ were calculated.127 The following main conclusions were made: (1) in the gas phase, the molecule of compound 6 has C2 symmetry and the fragment N(C10H6)N is essentially non- planar; (2) the degree of non-planarity in the isolated molecule of the base is lower than in the crystals; (3) increased orders of the N7CAr bonds attest to a significant conjugation of the nitrogen atoms with the p-system of the ring; (4) in protonation, the molecule becomes practically planar and its symmetry approx- imates the C2u type; (5) the hydrogen bridge in the isolated cation 6-H+ is symmetrical; its optimum parameters are as follows: r[N(1)7H]=1.05 A, r[N(2) .. . H]=1.64 A, the angle N(1)7H7N(2)= 156.8 8.Semi-empirical simulations of the optimum structures of the base 6 and the cation 6-H+ were made 13, 84 using the AM1 and PM3 methods; the enthalpies of protonation for compounds 1, 3, 5, and 6 and some hypothetical aza-derivatives of the `proton sponge' were also estimated.128 V. Reactivity of naphthalene `proton sponges' Naphthalene `proton sponges' are typical electron-rich com- pounds.In addition to high basicity, they are characterised by significant p-donor capacity and the presence of a negative p-charge on the cyclic carbon atoms. It is not surprising that electrophilic substitution and oxidative conversions are the most typical reactions for these compounds. A few examples of nucle- ophilic substitution are reported, only for compounds with electron-acceptor groups in the ring.It is probable that some reactions of the `proton sponge', e.g., chlorination with N-chlor- obenzotriazole (CBT) or bromination with N-bromosuccinimide (NBS) occur through a radical mechanism, as can be judged from their unusual orientation. However, convincing evidence for this mechanism is absent, and the corresponding transformations are discussed in Section V. 2 dealing with reactions with electrophiles. 1. Oxidation As has been mentioned above, electrochemical or chemical oxidation of `proton sponges' results first in the formation of radical cations and then of dications. Thus treatment of the `double sponge' 28 with an excess of iodine at 710 8C gave black needles of the salt 104 (with the I73 anion), which could be studied by the X-ray method.121 It is noteworthy that increase in temperature to 25 8C or heating of the salt 104 results in its isomerisation, presumably via the immonium cation 105, into the dication 106.121 The `proton sponge' undergoes similar conver- sions under the action of certain complexes of Rh3+, Ru3+, and Ru2+, which result in the formation of the 1,1,3-trimethyl-2,3- dihydroperimidinium cation 42 (R1 ±R3=Me).129 Me2N Me2N NMe2 + + etc. 104b NMe2 Me2N NMe2 Me2N + + 104a NMe2 Me2N NMe2 NMe2 Me2N +. 28 7e +e 7e +e 28+. 6-H+C6Cl5O7 6 + 6-H+C6Cl5O7_HOC6Cl5 . Table 18. Ionisation potentials, IP1, and electrochemical oxidation poten- tials (E1=2 ox ) of 1,8-diaminonaphthalenes and the long-wave absorption maximum (UV spectra) of their molecular complexes with 1,3,5-trinitro- benzene.Compound IP1 /eV E1=2 ox /V lmax /nm (in MeCN) (inCHCl3) 2 EI 2 PS 118 1 7.47 7.10 7 530 2 7.40 7 7 560 3 7.43 7 7 575 4 7.38 7 7 540 5 7.40 7 7 580 6 7.38 7.05 0.36, 1.02 119 500 23 7 7 0.11, 0.72 120 7 16 A F PozharskiiOxidation of compound 6 with thallium triacetate or lead tetraacetate in dichloromethane at low temperature 102 or, which is better, with iodine in boiling acetonitrile 130 gives the 1,10- binaphthyl `sponge' 90 in moderate yields. The latter is also formed as a by-product (yield 10%) in the nitration of compound 6.130 Apparently, in all these cases it is the intermediate radical cation 6+.that undergoes dimerisation. 2. Electrophilic substitution reactions `Proton sponges' manifest unusually high activity in reactions of electrophilic substitution.To avoid resinification and to ensure desired regioselectivity, the syntheses are usually carried out at low temperatures (< 0 8C). In some cases (the Friedel ± Crafts acylation), the substrate should be passivated by being converted into a salt. It should be noted that such catalysts as AlCl3 or BF3 do not give n-complexes with `proton sponges'.As a rule, the first substituent enters only at positions 4 and 5 (for exceptions see Section V. 2. b). The second and subsequent substituents can occupy both the para- and ortho-positions. The specific orienta- tion is determined by the nature of the substituent already present and, consequently, by the pKa of the substrate and the degree of symmetry of the IHB in the corresponding cation.There is good reason to believe that some reactions of naphthalene `proton sponges' with electrophiles occur through a radical cation mech- anism, where the radical cation generated first undergoes further nucleophilic attack by the anion present in the reaction mixture (see Section V. 2. b). a. Nitration Nitration of the `proton sponges' 6 and 31 in acetic acid gives directly the 2,4,5,7-tetranitro-derivative of the type 107 irrespec- tive of the amount of nitric acid.46 This was explained by the fact that due to the decrease in basicity of each subsequent substitution product and the increase in the concentration of the correspond- ing free base the rate of each successive nitration increases.Mononitration, which gives the 4-nitro-derivatives 84 and 85 in 50%± 70% yields, can be carried out only in concentrated sulfuric acid with one equivalent of HNO3 46, 55 (as mentioned above, the binaphthyl `sponge' 90 is also formed as a by-product). Obviously, in strongly acidic media, compounds 84 and 85 are present exclusively in the form of cations, and the reaction does not proceed further.However, an attempt at subsequent nitration of compound 84 gave only the tetranitro-derivative 107.In the case of the ethyl analogue 85, dinitration could be accomplished with one equivalent of HNO3 to give the 2,4- and 4,5-dinitro-deriva- tives 108 and 109 in yields 32% and 28%, respectively.46 The nitration of compound 6 can be controlled to a greater extent when nitrogen dioxide is used as a nitrating agent.130 Nitration of 4-bromo-1,8-bis(dimethylamino)naphthalene 51 with one equivalent of HNO3 in concentrated H2SO4 gave 4-bromo-5-nitro- and 4-bromo-2-nitro-derivatives (yields 27% and 15%, respectively).27 b.Halogenation On treatment of the `proton sponge' 6 with one equivalent of bromine in acetic acid or CCl4, a crimson-coloured complex of an unknown structure precipitates, and gradually resinifies.Treat- ment of the complex with concentrated H2SO4 converts it into 4-bromo-1,8-bis(dimethylamino)naphthalene 51 in good yield. Subsequent bromination could be achieved only with NBS.27 First, with 1 equiv. of NBS, a mixture of 2,4- and 2,5-dibromo- derivatives 110 and 111 is formed in a 54 : 46 ratio. Further bromination of this mixture (1 equiv.of NBS) gives 2,4,7- tribromo-1,8-bis(dimethylamino)naphthalene 112 as the only product. An attempt to convert it into a tetrabromo-derivative was unsuccessful. Thus unlike nitration, acylation, and formyla- tion (see below), peri-dibromination in naphthalene `proton sponges' does not take place, presumably due to larger steric hindrances exerted by bromine atoms. It is noteworthy that bromination of the `proton sponge' 6 with NBS (1 equiv.) in chloroform occurs in quite a different way.93 In this case, 2-bromo- 71 and 2,7-dibromo-derivatives 73 are formed in 36% and 52% yields, respectively.ortho-Substitu- tion occurs nearly quantitatively in the chlorination of compound 6 with N-chlorobenzotriazole (CBT) to give 2-chloro- 70 and 2,7- dichloro-derivatives 72.28 Probably, these reactions proceed with the participation of the `sponge' base (rather than the cation) in which the p-electron density at positions 2 and 7 is greater than at positions 4 and 5.At the same time, one should not rule out the radical mechanism of substitution. 28 4I27MeCN 710 8C 104 + Me2N+ NMe2 Me2N N Me C H H2 N Me Me2N NMe2 H + + 105 106 Me2N 2I¡3 2I¡3 NR2 R2N 6, 31 HNO3 H2SO4,715 8C NR2 R2N NO2 84, 85 R=Me (6, 84), Et (31, 85). 85 HNO3/H2SO4 NO2 Et2N NEt2 NO2 O2N NO2 Et2N NEt2 + 108 109 HNO3/AcOH 84 O2N O2N NO2 Me2N NMe2 NO2 107 6 Complex 111 112 Br2 AcOH or CCl4 H2SO4 710 8C Me2N NMe2 Br NBS THF,790 to720 8C 51 NBS THF,760 to7208C Me2N NMe2 Br Br Me2N NMe2 Br Br + 110 Me2N NMe2 Br Br Br Naphthalene `proton sponges' 17Recently, in our laboratory we have established that treatment of compound 6 with sodium nitrite in hydrochloric acid gives 4-chloro-1,8-bis(dimethylamino)naphthalene 50 in a nearly quan- titative yield. If hydrobromic acid is used instead of hydrochloric acid, the yield of the 4-bromo-derivative 51 is much lower (*20%).130 The formation of these somewhat unusual products is a convincing proof of the radical-cation mechanism of halogen- ation.Obviously, at the first stage the `proton sponge' is oxidised with nitrous acid to the radical cation, which is subjected to nucleophilic attack by the halide anion at position 4, where the highest positive charge is accumulated. c. Formylation and acylation Formylation of the `proton sponges' 6 131 and 31 132 under conditions of the Vilsmeier reaction in a deficiency of POCl3 (0.5 equiv.) gives the corresponding 4-formyl-derivative 80 and 113 in moderate yields.When an equimolar amount of POCl3 is used, compound 6 gives the 4,5-dialdehyde 81 and small amounts of the 2,5-dialdehyde 96 along with the monoaldehyde 80.131 1,8-Bis(dimethylamino)naphthalene 6 does not undergo the Vils- meier ± Haack acylation or acylation with carboxylic acids in polyphosphoric acid (as takes place in the series of perimidones, 2,3-dihydroperimidines, and perimidines).133 At the same time, under conditions of the Friedel ± Crafts reaction in the presence of AlCl3 or AlBr3, acyl chlorides acylate the `sponge' base even at 790 8C.110 The reaction is not very smooth, therefore it is more convenient to perform it with the salt 6-H+.The yields of the ketones 82 and 101 are virtually quantitative. The reaction of the base 6 with trifluoroacetic anhydride does not require a catalyst and proceeds at730 8C. This results in the formation of the 4-trifluoroacetyl derivative 83 in moderate yield. Treatment of the base 6 with an excess of benzoyl chloride gave the 4,5-benzoyl derivative 102 (yield 7%).Under the same conditions, the reaction with excess acetyl chloride gives phenale- none 89 (yield 27%), presumably as a result of intramolecular aldol condensation of the intermediate, the 4,5-diacetyl derivative. The progenitor of this phenalenone series (114) was synthesised (yield 15%) by intramolecular acylation of ethyl acrylate 115 upon heating in polyphosphoric acid (PPA).110 d.Hydroxymethylation `Proton sponges' undergo hydroxymethylation with paraformal- dehyde in polyphosphoric acid at 45 8C to give the alcohols 116a,b.132, 134 At higher temperatures, compound 6 is converted into naphthopyran 88 in good yield, apparently as a result of dehydration of the intermediate 4,5-bishydroxymethyl derivative 117. The alcohols 116 were also obtained by reduction of the aldehydes 80 and 113 with LiAlH4.132 e.Miscellaneous reactions The `proton sponge' 6 reacts with alkanesulfonyl chlorides to give the 4-alkylsulfinyl derivatives 118 and 119.135 It was established that the active species, which directly interacts with compound 6, is the corresponding chlorosulfine generated in situ. Compounds 6 CBT or NBS (1 equiv.) Me2N NMe2 Hal + 70, 71 Hal=Cl (70, 72), Br (71, 73).CHCl3 Me2N NMe2 Hal Hal 72, 73 6 NaNO2 HHal 6-H+. Hal7 Me2N NMe2 Hal 50, 51 R=Me (80), Et (113). 6 R2N NR2 CHO 80, 113 Me2N NMe2 CHO OHC + Me2N NMe2 OHC CHO +80 81 96 HCONMe2 POCl3 (1 equiv.) HCONMe2 POCl3 (0.5 equiv.) R2N NR2 6, 31 6-H+ RCOCl7AlCl3 CH2Cl2, 20 8C Me2N NMe2 COR 82, 101 Me2N NMe2 COPh PhOC 102 Me2N NMe2 COCF3 83 6 MeCOCl AlCl37CH2Cl2 NMe2 Me2N C C Me O Me O NMe2 Me2N O Me 89 80 (EtO)2PCH2CO2Et O NaNH27MePh NMe2 Me2N CH 115 PPA D NMe2 Me2N O 114 CH EtO2C 6, 31 (CH2O)n PPA, 73 8C (CH2O)n PPA, 45 8C R=Me (a), Et (b). 7H2O Me2N NMe2 O 88 Me2N NMe2 CH2OH HOH2C 117 R2N NR2 CH2OH 116a,b 18 A F Pozharskii119 can be also obtained by the reaction of the `proton sponge' 6 with alkanesulfinyl chlorides.Compound 6 behaves as a strong CH-nucleophile with respect to 4,6-dinitro-derivatives of benzofuroxan and benzofurazan to give the corresponding adducts 120.136 The `proton sponge' easily adds to perfluorocycloalkanes with exocyclic double bonds. This reaction involves both peri-positions and the phenalene derivatives are formed 121,122.137 3. Reactions involving dialkylamino groups The following four types of these reactions are known: (1) quater- nisation, (2) dealkylation, (3) substitution of the dialkylamino groups, and (4) oxidative conversions.The latter have already been described in Section V. 1. Here, only the first three types will be considered. Practically all these reactions have been performed with compound 6. Only one example of quaternisation of the `proton sponge' is known.Contrary to the original report,1 it was established that its treatment with methyl fluorosulfate gives the non-crystalline salt 68 (as the SO3F7 anion), which is converted into the crystalline tetrafluoroborate 67 under the action of NaBF4.37 An attempt to synthesise the borate complex 123 by treating the cation 6-H+ with pyridine ± borane was unsuccessful.138 At the same time, complexes of this type are easily formed with tetramethyl-o- phenylenediamine and 2,20-bipyridyl.This circumstance can be regarded as additional proof of the extremely low nucleophilicity of the `proton sponge'. Heating of `proton sponge' salts containing mild nucleophiles, such as thiocyanate,26 phenylmercaptide, and phenylselenide 139 leads to demethylation and the formation of 1-dimethylamino-8- methylaminonaphthalene 5.This reaction proceeds especially smoothly with selenophenol; the yield of the amine 5 is close to quantitative. It is of note that if the `sponge' 6 is heated with thiophenol and selenophenol, the formation of the cation 6-H+ proceeds at a very low rate (>1 h). Demethylation of `proton sponge' bases occurs only if the ring contains strong electron-acceptor groups and if the reaction is carried out in a superbasic medium.30 Thus, treatment of the `nitro sponge' 84 with a solution of KOH in DMSO (40 8C, 24 h) gave the trimethyl-substituted compound 58 (yield 40%).The two other reaction products were naphthol 57 (yield 7%) and, which is especially unexpected, lactam 124 (yield 11%).The formation of naphthol 57 seems to be a result of nucleophilic substitution of the dimethylamino group in the more activated position 1. Some complex reactions also entail hydrolytic substitution of the dimethyamino group by the car- bonyl group (see Section V. 5). The mechanism of formation of the lactam 124 is not known with certainty. Presumably, this reaction begins with the generation of an equilibrium amount of the carbanion 125 in which intramolecular nucleophilic substitu- tion of the 8-NMe2 group takes place.The 1-methyl-6-nitro-1,2- dihydrobenzo[c,d]indole 126 formed undergoes further autoox- idation, which is also characteristic of other compounds of this kind. R=H, Me, PhCH2. Me2N NMe2 S CH(Cl)R O 118 Me2N NMe2 S CH2R O 119 6+R C(Cl) S O 3 6+2RCH2SO2Cl MeCN, 20 8C 76 .HCl, 76 .RCH2SO3H R=H, Me, CF3, F, Cl. 119 2 6+RCH2SCl O MeCN, 20 8C 76 . HCl X=N, N O. 6+ X O N NO2 O2N Me2N+ NMe2 H O X H NO2 O2N 7 120 N 6 F3C CF3 MeCN, 20 8C Me2N NMe2 CF3 CF3 F F 121 Me2N NMe2 F 122 F F F MeCN, 20 8C 6-H++ 7 BH3 + NMe2 B Me2N H H + 123 N Nu7=SCN7, PhS7, PhSe7. 6+NuH 6-H+ Nu7 D NHMe Me2N +Nu7Me 5 84 KOH7DMSO 40 8C NO2 Me2N NHMe NO2 Me2N OH NO2 N Me O + + 58 57 124 84 NO2 Me2N N Me 7 125 CH2 KOH NO2 N Me O2 OH7 124 126 Naphthalene `proton sponges' 19It is known that the 8-NMe2 group can be substituted by hydrogen in the catalytic hydrogenation of the `nitro sponge' 84 (see Section V. 4). 4. Reactions of functional groups The greatest number of `proton sponge' derivatives (azomethines, hydrazones, oxime, nitrile, etc.) were synthesised from the 4-alde- hyde 80.129 The latter was also converted by the Wittig reaction into 4-vinyl-1,8-bis(dimethylamino)naphthalene 74 109 and com- pound 115.131 A number of other 4-vinyl derivatives (75, 127, 128) were synthesised by condensation of the aldehyde 91 with the corresponding CH-acids.The conditions for polymerisation of the vinyl `sponge' were investigated.109 It was found that heating of compound 127 with boron trifluoride etherate results in the reduction of the exocyclic C=C bond and the concomitant formation of the naphthylmethyl derivative of diethyl malonate 129 instead of the expected intramolecular cyclisation into the corresponding phenalenone.131 It was assumed that the ethyl groups of the etherate play the role of a hydride donor, and the reaction can be realised due to polarisation of the C=C bond, which is enhanced by the donor effect of the dimethylamino groups. Upon boiling in water, the dialdehyde 81 undergoes the intramolecular Cannizzaro reaction to give the naphtho[1,8-c,d]- pyranone derivative 130 in 36% yield.131 The unusual feature of this reaction is that it does not require any alkali.Apparently, the alkalinity of the medium sufficient for the formation of the anion 131 is created by the `proton sponge' itself. It should be noted that unlike the naphthalene-1,8-dicarbaldehyde, which exists under ordinary conditions only in the form of a cyclic hydrate, the dialdehyde 81 is stable. This may be ascribed to the stabilising effect of the dimethylamino groups, which, when entering into conjugation, reduce the partial positive charge on the carbonyl carbon atoms and thus hamper the hydration.However, the formation of the hydrate 132 which is needed for the Cannizzaro reaction, does take place upon boiling of the dialdehyde 81 in water. Catalytic hydrogenation of the `sponge' 84 gave the amine 76,45 which is extremely unstable and oxidisable in air. 4,5-Dia- mino-1,8-bis(dimethylamino)naphthalene was obtained by hydrogenation of the dinitro-derivative 40.24 Subsequent hydro- genation of compound 76 is accompanied by elimination of the 8-NMe2 group and the formation of 1-dimethylamino-4-amino- naphthalene 133.The amine 76 gives azomethine with p-nitro- benzaldehyde and the N-acetyl derivative 77; the latter was methylated to give quite stable 4-methylamino- 78 and 4-dime- thylamino-derivatives 79 of the `proton sponge'.45 The latter can be regarded as a kind of a `sesqui-sponge'. 5. Transformations of naphthylmethyl carbocations derived from the `proton sponge' It has recently been established in our laboratory that treatment of the alcohol 116a with concentrated hydrochloric or phosphoric acid gave unexpectedly the spiro compound 93 containing two fragments of the `proton sponge' in a nearly quantitative yield.134, 140 There are reasons to assume that the key intermediate in this synthesis is the naphthylmethyl carbocation 134a (structure A) in which strong resonance stabilisation involving both dimethyla- mino groups provides the fixation of the diene system containing the exo- and endo-cyclic bonds CH2=C and C(4a)=C(5) (struc- tures B and C).In the subsequent [4p+2p]-cycloaddition, one molecule of the carbocation behaves as a diene, whereas the second one plays the role of a dienophile; this reaction occurs in a `head-to-head' mechanism as shown in the scheme. An analo- gous spiro compound (136) is obtained from the alcohol 116b; however, this reaction is much slower and the yield is lower (*22%).132 This may evidently be due to insufficient stabilisation of the carbocation 134b due to the more pronounced non- coplanarity of the ring and the diethylamino groups.The cycloaddition reaction observed when the alcohol 116a was heated in benzene in the presence of solid acidic adsorbents (Al2O3, SiO2, or TiO2) occurs in a different direction.132, 141 In this 80 Me2N NMe2 H Y X 74, 75, 115, 127, 128 127 74: X=Y=H; 75: X=Y=CN; 115: X=CO2Et, Y=H; 127: X=Y=CO2Et; 128: X=CO2Et, Y=CN.Me2N NMe2 CH2CH(CO2Et)2 129 BF3 . Et2O, 110 8C 81 7OH7 H2O D O NMe2 Me2N H OH HO H 7H+ O NMe2 Me2N H O7 HO H O NMe2 Me2N O 132 131 130 NH2 NMe2 Me2N 84 H2 Pd/C H2 Pd/C NH2 NMe2 76 133 116a,b H+ R2N NR2 CH2 + R2N CH2 + NR2 CH2 + A B C 134a,b R2N NR2 R2N O 93, 136 R=Me (116a, 93), Et (116b, 136).NR2 R2N 135a, b H2O/OH7 7Me2NH H R2N R2N R2N + + R2N NR2 R2N R2N + 7H+ NR2 20 A F Pozharskiicase, the main reaction product is the isomeric spiro compound 92 of the `head-to-tail' type (yield 23%). This reaction also gives dinaphthylmethane 91 (10%), the aldehyde 80 (19%), and the 4-dimethylaminomethyl derivative of the `proton sponge' 137 (22%) (all the yields are given for Al2O3; for other supports they vary somewhat).Most probably, all these compounds are the products of transformation of the carbocation 134a. Thus the formation of the aldehyde 80 can be explained by the well-established ability of carbenium ions to oxidise alcohols to aldehydes through elimination of the hydride ion.The formation of the dinaphthyl- methane derivative 91 seems to be due to the ipso-substitution of the CH2OH group in the original alcohol 116a by the carbocation 134a. An attack by the carbocation at the second possible direction of the free peri-position 5 in the molecule of compound 116a should lead to the formation of the alcohol 138, which further generates the carbocation 139.Subsequent intramolecular ipso-attack by the carbenium centre on the other residue of the `proton sponge' at position 4 gives ultimately the spiro product 92 via the immonium salt 140. The dimethylamine liberated in the hydrolysis of the immonium group reacts with the carbocation 134a to give compound 137. The different direction of cycloaddition reactions in protic media and in the presence of Lewis catalysts is explained in the following way.In a strongly acidic protic medium favouring the formation of the non-symmetrical spiro compounds 93 and 136, the original alcohol appears to be fully protonated due to the high basicity of the `proton sponge'. It is therefore hardly likely that it will easily interact with the carbocation 134a.As a result, the [4p+2p]-cycloaddition remains the only possible reaction of the carbocation, especially when its concentration is high enough. At the same time, on the surface of an adsorbent the carbocation 134a is formed in a low concentration. Being surrounded by an excess of the original alcohol, it will primarily react with the latter to give the symmetrical spiro compound 92, or with another strong nucleophile.Thus for naphthyl carbocations derived from `proton sponges' there exist two different cycloaddition reactions, which follow different mechanisms and give spiro compounds of two different types. The most surprising feature in the formation of the spiro compounds 93 and 136 is that contrary to electrostatic laws, the positively charged carbon atoms from two methylene groups combine.Quantum mechanical calculations 140, 142, 143 suggest that this process is favoured by the symmetry of their boundary orbitals. This process may not be synchronous and occurs via the formation of a biradical intermediate. The secondary alcohols 141 ë 143 behave differently in the presence of acids. Whereas the two former do not change even on prolonged heating in concentrated HCl or CF3CO2H, their phenyl analogue forms the spiro compound 144 in 87% yield.140 At first, this compound was erroneously identified as 93; however, sub- sequent X-ray analyses confirmed the validity of the symmetrical structure.144 At present, it still remains obscure whether com- pound 144 is formed by a mechanism of two-step electrophilic substitution, as is the case with the spiro adduct 92, or as a result of a [4p+2p]-cycloaddition of two molecules of the cation 147 and its abnormal direction is due to steric factors.The behaviour of the alcohols 141 ± 143 with respect to Al2O3 is also different. When boiled in benzene in the presence of Al2O3, compound 141 underwent dehydration, presumably, by a mech- anism of E1-elimination via the carbocation 145.This reaction gives the 4-vinyl derivative 74 in good yield. The alcohols 142 and 143 do not change under these conditions.109 The inertness of the alcohol 142 in both protic media and in the presence of Al2O3 might be due to stabilisation of the cation 146 by the electron- acceptor group CF3, so that it is not formed at a steady-state concentration that is sufficient for this reaction.Apparently, the specific reactivity of naphthalene `proton sponges', which is ultimately due to the potent +M-effect of two dimethylamino groups, is manifested in cycloaddition reactions to a far greater degree than in any other conversion. Indeed, although these reactions have been later discovered in alcohols derived from 1,8-dimethoxy-, 1-dimethylamino-8-methoxy-,145 and even 1-dimethylaminonaphthalene,92 they proceeded with considerable difficulty and gave spiro compounds in low yields and of only one specific type. 116a Al2O3 C6H6 Me2N NMe2 O NMe2 CH2 Me2N NMe2 Me2N NMe2 + + 92 91 + + CHO Me2N NMe2 80 CH2NMe2 Me2N NMe2 137 Me2N NMe2 NMe2 Me2N+ H2O 92+Me2NH. 140 Me2N NMe2 Me2N NMe2 CH2 CH2OH 138 Me2N NMe2 NMe2 CH2 CH2 + Me2N 139 116a+134a Al2O3 7OH7 R=Me (141, 145); CF3 (142, 146), Ph (143, 147).Me2N NMe2 CH(OH)R 1417143 H+ H2O Me2N NMe2 CH R + 1457147 (R =Ph) Me2N NMe2 NMe2 O H H Ph Ph 144 Naphthalene `proton sponges' 21VI. Applications of the `proton sponges' in organic syntheses There are numerous indications of the use of `proton sponge' in organic syntheses, mostly as a strong but low-nucleophilic base.Its application is useful in those cases where it is necessary to ionise the acidic X7H bond or to bind an acid liberated in the course of the reaction without any effect on other base-sensitive groups. A typical example is the cyclisation of the imidazoles 148 in 2-dimethylsila-3H-imidazo[2,1-b]thiazoles 149, which occurs in the presence of the `proton sponge' 6 and in more than 80% yield.Substitution of compound 6 by ordinary bases (OH7, MeO7, etc.) leads to the cleavage of the S7Si bond.146 In reactions of chiral compounds requiring the use of a base, the `proton sponge' hardly causes any racemisation and favours the retention of high optical purity.147 ± 149 An example is the conversion of optically active alcohols into ethers under the action of triethyloxonium tetrafluoroborate:147 Quite unexpected was the use of the `proton sponge' as a debrominating agent in the conversion of vic-dibromides into alkenes.150 Thus, heating of compound 6 with dibromoace- naphthene in dimethoxyethane gives acenaphthylene in a nearly quantitative yield.Dibromides of coumarin, isocoumarin, chal- cone, etc., enter into this reaction.However, the fate of the `proton sponge' in this conversion could not be followed. Other applications of the `proton sponge' in organic syntheses are documented in several publications.151 ± 154 VII. Conclusion The discovery of the high basicity of 1,8-bis(dialkylamino)- naphthalenes has given a strong impetus to the search for other, still stronger neutral organic bases.Developments in this field have led to the appearance of an interesting branch of organic chemistry, namely, the chemistry of `proton sponges'. Although naphthalene `proton sponges' are no longer record holders among neutral bases, their potential in the design of novel compounds possessing unusual physicochemical properties has by no means been exhausted. 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出版商:RSC    

年代:1998

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Abstract. The results of simulations of the reactions and reactivity of fluoroalkenes by semiempirical and non-empirical methods of quantum chemistry are summarised. In addition to the isolated molecule approximation, methods based on the calculations of potential energy surfaces of reactions and transition states are used. The shortcomings of such calculations are analysed. The nature of chemical bonds in fluoroalkenes and the applicability of quantum chemistry methods for the calculation of their physico chemical and energy characteristics (with ionisation potentials and dipole moments as examples) are discussed. The bibliography includes 106 references.I. Introduction Investigation into the reaction mechanisms of fluoroalkenes is not only of theoretical significance but it is of great practical impor- tance as well.For instance, polymerisation and copolymerisation of these compounds lead to the formation of heat-resistant fluoroplastics, the properties of which depend, in particular, on the sequence of monomers in the copolymer. Fluoroorganic compounds (including those obtained in reactions of fluoroal- kenes) are widely used as lubricants, dyes, medicines, etc.The kinetics and mechanisms of the reactions depend on the condi- tions (temperature, pressure, solvent), structures and physico- chemical properties of reagents, and structures of transition states. The methods of quantum chemistry can be applied to the estimation of the influence of structural and electronic factors on the course of a reaction . As a rule, quantum chemistry calculations of potential energy surfaces for reactions and transition states give useful informa- tion, which allows a qualitative description of the reaction mechanism.However, these calculations are time-consuming and can be performed only for relatively simple systems. Further- more, due to the approximations implied in the computational methods, the results of the calculations do not always agree with the experimental data.General principles for the construction of potential energy surfaces for reactions using quantum chemistry methods were described in detail elsewhere.1±3 For reactions of fluoroalkenes with the simplest radicals, these calculations were performed for the first time in the early 1980's (see below). Therefore, calculations within the isolated molecule approxima- tion are still a current topic in the theory of reactivity of alkenes since they allow one to determine the most probable site of the bond to be cleaved, or to trace changes in reactivity along certain series of molecules, in terms of different reactivity indices.Simulation of reaction mechanisms and reactivities is closely connected with the problem of the description of chemical bond- ing and with physicochemical parameters of molecules. However, in quantum-chemical calculations of such characteristics as ion- isation potentials, electron affinity, dipole moments, etc., prob- lems as to their accuracy arise.These questions will be also discussed below. Three chapters of our earlier monograph 4 were devoted to the quantum chemistry of fluorine-containing molecules (including fluoroalkenes).However, a number of new studies using modern computational methods have appeared, and these studies should also be summarised. Many results of these studies are based on those of earlier investigations discussed in the monograph 4, and thus we have to cite this reference often. II. Isolated molecule approximation The problem of the strength of the double bond in ethylene and fluoroethylenes is still far from being completely solved.Accord- ing to the experimental data on bond lengths, frequencies of valence vibrations, and calculated force constants for the C=C bond, the energy of this bond should increase along the series 4 H2C=CH2<FHC=CH2<cis-FHC=CHF<trans-FHC=CHF& &F2C=CH2<F2C=CHF<F2C=CF2.(1) The strength of the p-component of the C=C bond changes presumably in the same order (though, this conclusion was derived in earlier studies based on the primitive HuÈ ckel method). According to the results of non-empirical calculations,5 some deviations for non-symmetrical fluoroethylenes were observed. In the case of symmetrical molecules, the energy of the double bond increases along the series: H2C=CH2<FHC=CHF<F2C=CF2.For non-symmetrical fluoroalkenes energy of the C=C bond changes as follows: A V Fokin A N Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, 117813 Moscow, Russian Federation. Fax (7-095) 135 50 85. Tel. (7-095) 135 64 89 MA Landau N N Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul.Kosygina 4, 117877 Moscow, Russian Federation. Fax (7-095) 938-21 56. Tel. (7-095) 936 17 22 Received 6 March 1997 Uspekhi Khimii 67 (1) 28 ± 38 (1998); translated by Ya V Zubavichus UDC 539.192; 541; 547.413 Simulation of reactions of fluoroalkenes by quantum chemistry methods A V Fokin,MA Landau Contents I.Introduction 25 II. Isolated molecule approximation 25 III. Potential energy surfaces of reactions and structure of transition states 29 IV. Quantum-chemical calculations of the physicochemical parameters of fluoroalkenes 30 V. Conclusion 32 Russian Chemical Reviews 67 (1) 25 ± 34 (1998) #1998 Russian Academy of Sciences and Turpion LtdH2C=CH2&FHC=CH2>H2C=CF2; FHC=CHF>H2C=CF2. However, these conclusions 5 (as well as the results of other non-empirical and semiempirical calculations that use the opti- mised geometry) should be used with certain precautions (see below).The maximum force required for the cleavage of the double bond in fluoroethylenes was taken as a criterion of its strength.5 Non-empirical calculations within the Hartree ± Fock approxi- mation with the 5-31G basis set (modified program HONDO) were used.The thermochemical estimation of the energy of the p-bond is based on the heats of different reactions that occur with the cleavage of this bond (chlorination, bromination, hydrobromina- tion, polymerisation, cyclisation, etc.). In all cases, the heat of the corresponding reaction of tetrafluoroethylene exceeds that of ethylene by 10 ± 16 kcal mol71.4, 6 However, further calculations of the energy of theC=Cbond were performed assuming equality of the energies of the C7F bonds in CF4 and C2F4, and of the C7H bonds in CH4 and C2H4, which is in our opinion a very rough approximation.In one of our previous studies we attempted to explain the differences observed for the heats of reactions of ethylene and tetrafluoroethylene.4 Quite rough estimates using the HuÈ ckel method lead to agreement between the changes in the p-bond energies for a series of fluoroethylenes and the known data on bond lengths and frequencies of stretching vibrations [series (1)].However, this result was also obtained within rather rough approximations. An attempt to explain the thermochemical differences in energies of the double bonds in C2H4 and C2F4 was undertaken6 based on non-empirical calculations in the 6-31G* basis set with the use of the unrestricted Hartree ± Fock (UHF) wavefunctions and Mùller ± Plesset second order perturbation theory (MP2).The energy required for the cleavage of the p-bond in ethylene and tetrafluoroethylene as a result of rotation of the molecule to a biradical transition state in the process of cis ± trans-isomerisation was calculated.Thermochemical estimates of the bond energy were confirmed, and weakening of this bond in C2F4 compared to C2H4 was explained by a `planarisation' of two CF2 groups (since the carbon atoms in these groups in the biradical transition state are `pyramidised' to a great extent).An approach to estimating the energy of the p-bond as the difference between the energies of a triplet twist-biradical and a planar alkene 6 does not take into account changes in the energy of C=C, C7F, and C7H s-bonds in the rotation. Therefore, we suppose that the data on bond lengths and frequencies of stretch- ing vibrations of the double bond in alkenes [series (1)] describe more adequately the order of changes in the energy of theC=Cp- bond in fluoroalkenes (since, in general, substitution hardly affects the length of the s-bond).The rates of different reactions of fluoroalkenes that occur with cleavage of the C=C p-bond increase along the series (1). This refers to the reactions with nucleophilic reagents 4, 7 and to some radical reactions.Tetrafluoroethylene reacts with alcohols even at room temperature, whereas an analogous reaction for ethylene is impossible,8 ± 10 which is explained only by kinetic rather than thermodynamic factors. For instance, at 25 8C the equilibrium for the reaction of ethylene is almost totally shifted towards etherification.11 The reactions with nucleophilic reagents should proceed more easily the lower the energy of the lowest unoccupied molecular orbital (LUMO), i.e.the higher the affinity of the molecule for the electron. According to results of CNDO and INDO calcula- tions,12 the LUMO of fluoroethylenes is an antibonding p-orbital of ethylene perturbed by fluorine atoms. The values of E n for ethylene and tetrafluoroethylene are 5.36 eV and 3.72 eV (CNDO) and 5.91 eV and 4.20 eV (INDO), respectively.These values clearly explain why tetrafluoroethylene reacts much more easily with nucleophiles than ethylene.{ Of course, the values given are relative, but they agree qualitatively with the results of calculations by different modifi- cations of non-empirical methods that use basis sets with only s- and p-orbitals. However, an introduction of d-orbitals into the basis leads to opposite results, which are in agreement with rather scarce and approximate experimental data (see below).Adecrease in the calculated values of E n along the series F2C=CF2> F2C=CFCF3>F2C=C(CF3)2 is also in agreement with the experimentally observed increase in the reactivity of the above- mentioned molecules in reactions with nucleophiles.13 Populations on carbon atoms and overlapping populations of the C=C p-bond on lower occupied p-orbitals formed by con- jugation of lone pairs on the fluorine atom with the p-bonding orbital of ethylene are negligibly small compared to the popula- tion of the highest occupied p-orbital formed primarily by the p-orbital of ethylene (see Ref. 4 in which the results of the ab initio calculations are given).In reactions with alcohols, thiols, and amines, tetrafluoro- ethylene gives only addition products.7, 13 Together with addition products (2), perfluoropropylene also gives products of vinylic (3) and allylic (4) substitution. In the case of perfluoroisobutylene, only reactions (2) and (3) occurred.6, 13, 14 Vinylic substitutions F2C=CFCl+NaSC2H5 C2H5SFC=CFCl+NaF and F2C=CFCl+PhLi PhFC=CFCl+LiF could be accomplished in the case of chlorotrifluoroethylene 4 and perfluoro-tert-butylethylene.15 In the latter, only the fluorine atom trans to the (CF3)3 group undergoes substitution in reactions with carbonylmetallates. However, in the case of (CF3)3CCF=CFX (X=Cl or Br), in the reaction with carbon- ylmetallates, the substitution products are not formed.15 Other examples of analogous reactions with fluoroalkenes are given elsewhere.4, 14 ± 17 Products of vinylic substitution, together with addition prod- ucts, are also formed in reactions of 1,2-dibromo-1,2-difluoro- ethylene, tribromofluoroethylene, and bromotrifluoroethylene with alkoxides.18, 19 In the case of F2C=CFBr, the main site of attack by a nucleophile is the carbon atom of the CF2 group (the yield of the addition products and the products of substitution of a fluorine atom in the CF2 group by OR is 80%± 94%).Oligomers or polymers and trifluoroethylene are also formed as by-products (less than 0.5%). The above-mentioned features of reactions of perfluoropro- pylene and trifluorochloroethylene were interpreted in terms of the results of quantum-chemical calculations by the CNDO method.20 ± 22 Allylic substitution in the case of perfluoropropy- lene occurs because, according to the results of the calculation, the carbon atom of the trifluoromethyl group carries the maximum positive charge.However, due to great steric hindrances, reaction (2) appears to be preferable (attack at the CF2 group, which also carries a relatively large positive charge). In the case of perfluoro- isobutylene, the reaction (4) is impossible due to steric hindrance.The same holds for the reaction of perfluoropropylene with bulky thiol molecules, which follow only pathways (2) and (3).13 ROCF2 CFHCF3 (2) ROCF CFCF3+HF (3) CF2 CFCF2OR+HF (4) F2C CFCF3+ROH { It should be noted that many features of reactions with fluoroethylenes and of changes in their reactivity have already been rationalised in the first quantum-chemical studies using the primitive HuÈ ckel method (for more details, see Ref. 4). 26 A V Fokin, MA LandauVinylic substitution (3) can be explained by a substantial increase in the polarity of the C± F bond of the CF2 group on going from tetrafluoroethylene to perfluoroisobutylene and chlor- otrifluoroethylene.20 Despite the fact that the negative charge on the fluorine atom is almost constant for all the four molecules considered, the positive charge on the carbon atom of the CF2 group in perfluoropropylene, perfluoroisobutylene, and trifluor- ochloroethylene is substantially higher than in tetrafluoroethylene (Table 1).In the case of phosphorus fluorides for which substitution of the fluorine atoms is typical, the P7F bond is even more polar according to calculations by the CNDO method.4, 22, 24 Vinylic substitution in fluoroethylenes for which experimental data are lacking, has been analysed based on the bond polarity criterion.22 According to the polarity criterion for the C7F bond, the probability of vinylic substitution is low in monofluoroethylene and both (cis and trans) 1,2-difluoroethylenes: the difference between the charges on the carbon and fluorine atoms Dq (calculated by the CNDO method) is substantially smaller than in tetrafluoroethylene. However, these reactions are highly prob- able for 1,1-difluoroethylene and trifluoroethylene: the difference in charges on the carbon and fluorine atoms for these molecules is more than 0.55 (as for perfluoropropylene, perfluoroisobutylene, and chlorotrifluoroethylene), which is larger than the correspond- ing value for tetrafluoroethylene (0.48, Table 1).Similar results were obtained in ab initio calculations, which are summarised in Table 1, viz. atomic charges and Dq values calculated from the data on populations of the corresponding atomic orbitals.23 Values of Dq for 1,1-di- and tri-fluoroethylenes are also substantially larger than for other members of the series.Similar results were obtained by the INDO method.22 Charge distributions in non-symmetrical fluoroethylenes indi- cate 4, 12 that the addition of hydrogen halides to these molecules should obey the Markownikoff rule, which is in agreement with the experimental data.7, 13 The results of the corresponding semi- empirical (CNDO, INDO) and non-empirical calculations 4 also support this conclusion (see also charges of the carbon atoms given in Ref. 5). In reactions with alcohols and thiols, the RO7(RS7) group also adds to the carbon atom, which according to calculations 12 has a relatively high positive charge. A similar conclusion was derived 4, 12, 20 from calculations of 1,1-dichloro- 2,2-difluoroethylene, chlorotrifluoroethylene, perfluo- ropropylene, and perfluoroisobutylene which is confirmed by the experimental data.The orientation of borane addition to mono- fluoroethylene according to calculations by theMNDOmethod,25 is also in accordance with the experimental data (see also charges of fluoroalkenes calculated by the CNDO method26).Generally, in vinylic substitution in non-symmetrical halo- ethylenes, nucleophilic attack can occur at both carbon atoms of the double bond (see, for example, Ref. 15). The probability of such attack should increase with a decrease in the charge differ- ence between the two carbon atoms. If a molecule contains a CFX group (X=Cl, Br), substitution of both F and X is possible.15 This is observed, for example, in the reaction of b-chloro-a,b- difluorostyrene with phenyllithium and KFe(CO)2Cp.15 The difference in charges on the carbon atoms of the double bond is small (0.171, calculated by the AM1 method).27 Despite the fact that the charge of the carbon atom on the PhFC group is greater than that of the CFCl group, a nucleophile attacks only the =CFCl fragment (presumably, due to steric hindrances because of the presence of the benzene ring in PhFC=).A competitive substitution of both fluorine and chlorine atoms occurs. Unfortunately, the charges on the F and Cl atoms were not given in Ref. 27, and thus it is impossible to estimate the probabilities of the two reactions in terms of polarity of the C7X bond.Presumably, the substitution of the F atom is more probable. This assumption is in agreement with the results of calculations for other molecules (chlorotrifluoroethylene, in par- ticular 20) and with the above-mentioned data on reactions of tribromofluoroethylene.19 In addition, we believe that KFe(- CO)2Cp or Re(CO)5, in general, cannot be considered as classical nucleophiles.In the course of the reaction, intermediate com- plexes can be formed through dative bonds between the Fe (Re) atom and the p-orbital of alkene as well as by donor ± acceptor bonds between CO and the vinyl group. A detailed study of electrochemically activated reactions of nucleophilic substitution of polyfluorinated vinyl halides was performed,27, 28 and quantum-chemical calculations for the nine alkenes considered, four fluorinated butadienes, and fluorinated acetylenes were accomplished.27 The semiempirical AM1 method with full geometry optimisation was used. Energies of frontier p-orbitals of the molecules studied and charges on the vinylic carbon atoms were determined.Other parameters (in particular, charges on halogen atoms, interatomic distances, bond angles) were not given in Ref. 27.However, certain problems in the interpretation of the results of quantum-chemical calculations that use optimised geometry may arise (see below). The electron acts as the simplest nucleophile. In the case of the formation of rather stable radical-anions upon interaction of a haloalkene with an electron [for instance, (RFC=CFX) .7], the nucleophilic sub- stitution is a `multi-step' process.27 If no radical-anions are formed, the nucleophilic substitution is a `one-step' process.Changes in frontier p-orbitals of ethylene upon substitution of halogens for hydrogen were thoroughly analysed.27 Energetic, spatial, and kinetic factors were considered. The energy of the p-orbitals of the double C=C bond perturbed by halogen atoms depends both on conjugation of a lone p-electron pair of the halogen with the electrons of the p-bond Table 1.Charges on the carbon (qC) and the fluorine (qF) atoms in fluoroalkenes calculated by the CNDO22 (I) and ab initio 23 (II) methods and the difference between the charges of C and F (Dq). Molecule qC qF Dq I II I II I II FHC=CH2 +0.209 +0.263 70.186 70.361 0.395 0.624 cis-FHC=CHF +0.143 +0.101 70.166 70.335 0.309 0.436 trans-FHC=CHF +0.141 70.054 70.169 70.335 0.310 0.281 F2C=CF2 +0.317 +0.534 70.159 70.274 0.476 0.808 F2C=CClF +0.398 ± 70.159 (70.165) ± 0.557 (0.563) ± F2C=CFCF3 +0.418 ± 70.163 (70.158) ± 0.581 (0.576) ± F2C=C(CF3)2 +0.520 ± 70.164 ± 0.684 ± F2C=CH2 +0.448 +0.654 70.192 70.313 0.640 0.967 F2C=CFH +0.379 +0.600 70.173(70.174) 70.294 (70.291) 0.552 (0.553) 0.894 (0.891) Note.The fluorine atoms for which values of qF are given are underlined. Values of qF are given for the fluorine atom in the cis- (no parentheses) and trans- (in parentheses) positions to the fluorine atom of the CFX group. In calculations on chlorotrifluoroethylene, vacant 3d-orbitals of the Cl atom were included into the basis set.Simulation of reactions of fluoroalkenes by quantum chemistry methods 27and on the inductive influence of the halogen atoms. Both these factors should favour easier reduction of alkenes in the series C=CBr<C=CCl<C=CF.27 The spatial factor is interrelated with the orbital size of the halogen atom and acts in the opposite direction to the energetic factor: C=CF<C=CCl<C=CBr. The kinetic factor depends on the rate of fragmentation of the radical-anion that is formed in the first stage of the process.The influence of the kinetic factor changes in the same order as that of the spatial factor (for details, see Ref. 27). The vinylic carbon atom of the CF2(CFCl, CFH) group has the greatest (in absolute value) charge in tert-C4F9CF=CF2, tert- C4F9CF=CFH, and tert-C4F9CF=CFCl,27 though in the two latter cases, the difference in charges on the carbon atoms of the double bond is small, and the charge on the carbon atom of the CFCl group is close to zero.In the case of trifluorostyrene, a nucleophilic attack at the CF2 group for which qC=+0.112 is preferable; for the PhFC group, qC=+0.046. Due to such a small difference in charges, an attack at both carbon atoms of the double bond is to some extent probable.In contrast, the carbon atom of the PhFC group in PhFC=CHF and PhFC=CFCl carries the positive charge whilst another carbon atom of the double bond carries a small negative charge.27 It should be noted meanwhile that according to our results29 obtained by spectral methods (19F NMR, IR, UV), a mixture of cyclic dimers is formed as a result of g-irradiation of planar trifluorostyrene (which is rather typical of fluoroalkenes).On the other hand, g-irradiation of non-planar trans-difluorostilbene adsorbed on aluminosilicate leads to a trimer 30 formed by elimi- nation of hydrogen atoms of the benzene rings of the monomer, the double bond being retained (for details, see Refs 4, 29, and 30).Preliminary experiments carried out under identical conditions demonstrated that the trimerisation with retention of the double bond is specific for trans-difluorostilbene and is not due to differ- ences in experimental conditions for the two monomers. A linear correlation was established between the calculated energies E n of LUMOs for compounds studied and experimental electrochemical potentials.27 Electrochemical reduction becomes easier with a decrease in E n,27 but no parameters and statistical criteria for the equation derived were given.Seventeen perchlorofluoroalkenes were calculated using the MNDO method in combination with the perturbation theory.31 Changes in energy in the formation of a bond between an alkene (s) and a nucleophile (t) in the interaction of these alkenes with CH3OLi in ether (DE) were calculated: DE a qsqt Rste a 2XOcsctstU2 EH ¢§ EL , (5) where qs and qt are charges on s and t atoms in isolated molecules, e=4.34 is the dielectric constant, Rst=1.7 A is the distance between the s and t atoms in the transition state, cs and ct are the coefficients of AOs of these atoms in frontier MOs, and EH and EL are the energies of the frontier orbitals of the nucleophile and alkene, respectively.The first term in equation (5) accounts for the contribution of electrostatic interactions, the second corresponds to perturbation of the covalent bonds. For all of the compounds studied, the second terms in equation (5) for both carbon atoms of the double bond are approximately equal, and thus a nucleophile attacks the atom with the maximum positive charge.However, there are several exceptions to this rule. For example, molecules with an allylic chlorine atom are more reactive than the corresponding perfluoro analogues, and a nucleophile attacks the carbon atom of the double bond most remote from the chlorine atom even if this atom carries a lower positive charge than the another carbon atom of the double bond.This was rational- ised by the fact that the allylic chlorine atom stabilises the transition state and decreases the activation energy of the reaction. According to the calculated DE values, it was established that the reactivity of the compounds studied with respect to hard nucleophiles (to CH3OLi, in particular) decreases along the series CF2=CFRF>CF2=CF2>RFCF=CFCl> RFCF=CClR0F> RFCF=CFR0F> RFCF=CCl2 (where RF is a perfluoroalkyl radical), which is in accordance with the known experimental data.The results obtained allowed the prediction of the site of nucleophilic attack in an alkene molecules. For instance, CH3OLi should attack the carbon atom of the CF2 group in CF2=CFCF3 (DE=723.01 kcal mol71 as compared to76.78 kcal mol71 for the carbon atom of the CFCF3 group involved in the formation of the double bond);31 this coincides with the experimental data.13 However, in RFCF=CCl2, the values of DE are similar for both carbon atoms, and the direction of the reaction is determined by the coefficient of the AO in the frontier MO: a nucleophile attacks the carbon atom with higher cs (in this case, it is the carbon atom of the CCl2 group).It was pointed out 31 that the direction of the reactions studied is determined not only by the rate of the bimolecular stage leading to the carbanion but also by the relative stability of the inter- mediate products. The enthalpies of formation of the possible intermediate products were calculated.For instance, it was found that in the reaction of CF3CF=CFCFCl2 with OH7, the anion (CF37CF(OH)7CF7CFCl2)7 is more stable than [CF37CF7CF(OH)7CFCl2]7. Calculations of intermediate products 31 lead to better agreement with the experimental data than calculations by perturbation theory. The acceptor strength of perchlorofluoroalkyl radicals (esti- mated from charges of the carbon atoms of the double bond linked to them) increases along the series CF3<CF2Cl< CFCl2<CCl3 (for details, see Ref. 31). The structures of the cations formed by protonation of fluoroethylenes were analysed 5 using non-empirical calculations with atomic Hartree ¡¾ Fock functions in the 5-31G basis set). In the case of monofluoroethylenes, the following symmetrical carbocation should be formed:5 Protonation of trans-1,2-difluoroethylene and tetrafluoro- ethylene also results in the formation of symmetrical p-complexes with retention of the covalent C7F bonds 1,1-difluoroethylene and trifluoroethylene should form s-com- plexes on protonation 5 and the configuration A is more favourable than B.p-Complexes are much more stable than s-complexes.5 Other examples of similar studies are given elsewhere.4 A problem arises in the choice of the molecular geometry.In non- empirical and semiempirical calculations, the energy minimisation procedure is often applied, and further calculations of quantum- chemical characteristics are performed for the structure, which corresponds to the global energy minimum. However, noticeable deviations between the calculated and experimental geometrical parameters for the gas phase are often observed.For example, for fluoroethylenes, the difference between the experimental and calculated bond lengths does not exceed 0.03 A, which can hardly affect substantially the calculated physicochemical parameters. Only for calculations by the MNDO method do the calculated bond lengths deviate strongly from the experimental ones.How- C C H H H H F + . C C F H F H H + and , C C F F F F H + C C F F H H H + C C F F H F H + C C F H F F H + A B , and , 28 A V Fokin, MA Landauever, the difference between the calculated and experimental bond angles in some cases reaches 4 8, which inevitably introduces errors in the values of some physicochemical parameters and quantum- chemical indices (dipole moments, ionisation potentials, etc.).4 For instance, changes in ionisation potentials for the series of fluoroethylenes obtained by the LCAO SCF ab initio calculations in the DZ basis not only disagree with the experimental data, but do not reflect any regularity in contrast to the results of semi- empirical calculations (EHT, CNDO, INDO), which use the experimental molecular geometry (see Refs 4 and 12).{ The same problem was discussed in another study.32 It was shown that in the case of full geometry optimisation by the MNDO and ab initio methods, not only the calculated bond angles, but also the C=C and C7F bond lengths differ consid- erably from the experimental values.Recently,33 enthalpies of formation, moments of inertia, vibration frequencies, and geometrical parameters for more than 100 fluorinated hydrocarbons and radicals containing one or two carbon atoms were calculated using the non-empirical BAC-MP4 method (fourth order Mùller ± Plesset perturbation theory cor- rected for additivity of bonds).It was found that the bond angles coincide with the experimental data within 1 8 on average. How- ever, as can be seen from the tables given in Ref. 33, this is not quite true for fluoroethylenes, especially for non-symmetrical ones. In some cases, as well as in the studies mentioned above, deviations from the experimental data reach 4 8, for example, Angle Value of angle /deg. Angle Value of angle /deg. calcula- experi- calcula- experi- tion 33 ment 4 tion 33 ment 4 CCF 122.4 120.98 CCF1 125.4 125.43 CCH1 125.7 127.70 CCF3 120.6 118.81 CCH3 119.8 121.41 CCH 123.4 127.20 CCH2 121.5 123.93 The problems discussed in Ref. 33 lie somewhat beyond the objectives of the present review (though the authors point to the need for kinetic studies). In the above-mentioned study, neither data on the energy of molecular orbitals, nor populations of atomic orbitals and atomic charges were given, and the mecha- nisms of the reaction with participation of the compounds studied were not considered.III. Potential energy surfaces of reactions and structure of transition states First studies devoted to calculations of potential energy surfaces of reactions with the participation of fluoroethylenes appeared in 1980.34, 35 The unrestricted Hartree ± Fock method with split valence basis set 4-31G was used.In particular, a mechanism of reaction of monofluoroethylene with atomic hydrogen was dis- cussed.34 Potential energy surfaces of the following reactions were studied by non-empirical MO LCAO methods: H2CCH2F . H2C=CHF +H . , H2C=CHF +H . H2CCH2F . (6) as was of a similar reaction of ethylene.35 The main contribution to the potential barrier (E) of reaction (6) equal to 5.6 kcal mol71 arises from deformation of theH2CCHF segment in the transition state (4.5 kcal mol71); the rest (1.1 kcal mol71) is due to inter- actions of the FHC=CH2 fragment with H.The potential barrier of reaction (6) exceeds by 3.3 kcal mol71 that of the reaction H2CCH2+H . H2CCH3 . . The lifetime of H2CCH2F . is relatively great (1079 s).If the process has adiabatic character, the energy E converts to the translation energy of the reaction products 35 (see also Ref. 4). The studies 34, 35 were based on general principles of analysis of potential energy surfaces for radical reactions developed by Nagase, Morokuma, and Kern.34, 36, 37 Reactions of atomic hydrogen with 1,1-difluoroethylene and monofluoroacetylene were studied in detail.38 An ab initio method within the unrestricted Hartree ± Fock theory (modified version of Gaussian 70) was used.It was demonstrated that in both cases the preferential site for a nucleophilic attack is the carbon atom not bonded with the fluorine atom. The potential barrier to the reaction of 1,1-difluoroethylene (4.1 kcal mol71) 38 is higher than that of ethylene (2.2 kcal mol71).34 Potential energy surfaces, structures of transition states for reactions of fluoroethylenes with alkyl radicals and heteroradicals as well as the regioselectivity of alkyl radicals in reactions with non-symmetrical fluoroethylenes were discussed in a number of papers 39 ± 44 (see also Ref. 4). The extended HuÈ ckel method (EHT) was used.39, 40 The conclusions derived in Refs 39 and 40 are not always correct from our viewpoint.For instance, attempts to analyse the mechanisms of the reactions studied of ethylene and fluoroethylenes 40 with CH3 . and CF3 . using different correlations between the experimental activation energy and the calculated quantum-chemical indices do not meet any criteria of mathemat- ical statistics.} A more thorough analysis of the mechanisms of radical reactions was performed using the concept of `phility' of free radicals in reactions of addition to alkenes.41 ± 44 This concept is based on a hypothesis 45 of a parallelismbetween the direction of the charge transfer and localisation of the bonds formed and cleaved and on an assumption 46 that the potential barrier to a reaction depends on the charge transfer stage and that the strongest charge transfer in a transition state corresponds to the most preferable reaction pathway.A hypothesis 47 stating that the interaction of centres with the maximum electron density on frontier orbitals of the reagents (the HOMO of the alkene and the orbital with the unpaired electron of the radical for the stage of charge transfer from alkene to radical; and theLUMOof the alkene and the same orbital of the radical for the stage of charge transfer fromthe radical to the alkene) is the most preferable, is also implied.Depending on the substrate, a radical can act as a nucleophile and an electrophile, and can exhibit ambiphilic properties (in the case that the charge transferred from the orbital of the radical occupied by one electron to the lowest unoccupied p-orbital of the alkene is close to the charge transferred in the opposite direction from the highest occupied p-orbital of the alkene to the radical orbital).In general, certain radicals and an alkene can act as nucleophiles and electrophiles (donors and acceptors) simulta- neously donating and accepting electrons. The `phility' is deter- mined by the predominating process.The `phility' concept,41 first, allows one to determine the most reactive centre of a non-symmetrical alkene molecule; second, to predict changes in the reactivity of a given radical with different alkenes or a given alkene with different radicals; and third, to estimate the preferred direction of charge transfer in the system (from the alkene to the radical or, conversely, from the radical to the alkene). To determine the reaction rate constants, the stabilisation energy method,1 the concept of a `quasi-p-electron system' of the C C H2 H3 H1 F F3 H F2 F1 C C { Unfortunately, there are no published data on the orbital populations and all atomic charges of fluoroalkenes calculated ab initio with a more complete basis set.} Plots of dependences of (E7E0)exp on the theoretical value (E7E0)theor, the sum of contributions of charge transfer, and contributions of exchange repulsion were discussed 40 (E and E0 are activation energies of fluoro- ethylene and ethylene, respectively). However, only four or five exper- imental points (equal to the number of molecules studied) were used in data processing, and strong deviations from the theoretical line were observed.No statistical criteria, except for the correlation coefficient, for the dependences obtained were given 40 (for details, see Ref. 4). Simulation of reactions of fluoroalkenes by quantum chemistry methods 29transition state (i.e. the electron system that comprises p-systems of the initial molecules),48 and a one-electron method of pertur- bation theory within the frontier orbitals approximation have been used.42, 43 Reactions of ethylene and fluoroethylene with six alkyl and halogenoalkyl radicals (CH3 ., CH2F . , CHF2 . , CF3 . , C2F5 . , and CCl3 . ) and four heteroradicals [PH2 . , P(CH3)2 . , SH . , S(CH3)2 . ] were discussed.42, 43 An analogous approach was also used for the qualitative prediction of the direction of radical substitution reactions in an aromatic ring 49 and for a investigation of the reactions of fluoroethylene copolymerisation.50 The results obtained 41 ± 44, 49, 50 reflect adequately the exper- imental data on reaction mechanisms and reactivities of the molecules studied.Generalisation of these results has led to the development of a model of intermolecular ambivalence in organic reactions.51 After publication of the monograph,4 several studies devoted to quantum-chemical calculations of potential energy surfaces for reactions of fluoroalkenes appeared.Hydrogen migration in the monofluoroethylene molecule was studied 52 ab initio on a Har- tree ± Fock atomic orbital basis using perturbation theory.It was shown that a local minimum on the energy surface does not correspond to the singlet state of the rearrangement product. H2CCHF HCCH2F . The problems of accounting for the correlation effects that arise when determining the geometry of transition states are the same as in calculations of the ground state structures of molecules. The authors of a study 53 of addition reactions of H ., OH . , NH2 . , and CH3 . radicals to mono- and 1,1-di-fluoroethylenes using non- empirical calculations (up to MP4/6-31G**) came to a conclusion on the necessity of accounting for the electron correlation when determining the structures of transition states for reactions of fluoroalkenes with electrophilic radicals. The importance of accounting for the electron correlations in quantum-chemical calculations of reactions of fluoroethylenes with radicals was also discussed in other studies.52, 54, 55 In all these studies, different modifications of the ab initio method were used [Hartree ± Fock atomic orbitals MP2±MP4/DZ+P; 6-31G**; 6-311G**; 6- 311+G(2d,p); 6-311+G(3df,2p), etc.].The relative reactivity of the above-mentioned radicals in reactions with monofluoroethylene and difluoroethylene obtained from calculations 53 is in agreement with the experimen- tal data.However, the regioselectivity ofOH . andNH2 . radicals is very `sensitive' to the level of theoretical calculations, and an increase in this level up to MP4/6-31G** may lead to a contra- diction with the experimental data (an attack at the carbon atom of the FHC and F2C groups according to calculations rather than of the H2C group according to experiment).53 Non-empirical calculations of potential energy surfaces of addition reactions ofH .,CH3 . ,CH2OH . ,CH2CN . , and tert-C4H9 . radicals to alkenes H2C=CHX (X=H, CH3, NH2, OH, F, SiH3, Cl, CN, CHO, NO2) were performed.54, 55 A correlation between the potential barrier and the reaction enthalpy was established (R2=0.973).However, besides the correlation coefficient, no other statistical criteria were used for the dependence obtained, which does not allow one to judge its correctness. Problems of the statistical significance of correlation equations were analysed in detail elsewhere.56 ± 59 The methyl radical does not manifest nucleophilic properties in reactions with the alkenes studied (X=F, H, OH, CH3, SiH3, and NH2) and acts as an electron acceptor (i.e.the charge is transferred from the alkene to CH3 . ).54 In the case where X=Cl, according to the results of calculations, charge transfer does not occur at all; for X=CHO, NO2, and CN, methyl acts as an electron donor (i.e. exhibits nucleophilic properties: the charge is transferred from CH3 .to the alkene). Electron-acceptor groups increase the reactivity of alkenes due to an increase in the exothermicity of the reaction rather than the polar character of the transition state.54 The stability of fluorine-containing cations including H2C=CHFCHá3 and H2C=CHCHF+ was studied by the ab initio SCF method in the 6-31G basis set (Gaussian 82 programme).60 An analysis of singlet and triplet transition structures in the process of trifluoroethylene formation in the photolysis of 2,2,2-trifluoro- diazoethane was performed 61 using the ab initio method at the QCISD(T)-FC/6-311(2s,2p)/MP2-FC/6-31G** theory level.The results of non-empirical calculations for C3H2F+, C3HFá2 , and C3Fá3 ions are given elsewhere 62 (SCF, Hartree ± Fock, 6-311G). Potential energy surfaces for molecules of X2F4 type (X=C, Si, Ge, Sn, and Pb) were studied in detail using ab initio methods (RHF and UHF, DZ and DZd basis sets).63 In particular, it was established that the planar structure of C2F4 corresponds to the energy minimum.Similar energy minima were found for ground states of Si2F4, Ge2F4, Sn2F4, and for a singlet excited state of Pb2F4.It was shown that a biradical triplet particle F2X . ±X . F2 has higher energy than the singlet particle in the ground state. IV. Quantum-chemical calculations of the physicochemical parameters of fluoroalkenes Different aspects of the quantum chemistry of fluoroalkenes, such as the nature of the chemical bonds therein, relative stability of geometrical isomers, and calculations of physicochemical param- eters, have been discussed in detail previously.4 Problems arising from the interpretation of the results of calculations and from calculations of certain parameters (ionisation potentials, dipole moments, different energy characteristics, magnetic shielding constants of 19F and 13C nuclei, and so on) have been also analysed. Most of these parameters affect the character and quantitative characteristics of reactions of fluoroalkenes and thus they are closely connected with the objectives of the current review.The results of quantum-chemical calculations of physico- chemical parameters of fluoroalkenes obtained in the 1990's have been analysed in another review.64 Here, we will discuss in brief only the problems that concern the reactivity and mechanisms of reactions of compounds of this class.Difficulties arising from the calculations of geometrical parameters of molecules have already been mentioned. Non- empirical calculations of neutral molecules and dications of type C2X2Y2á 2 were performed 65 in the 3-21G*(d) basis set using the Gaussian 88 programme. In particular, it was shown that dication C2F2á 4 has a planar configuration in the ground state in contrast to the twist-structure of C2H2á 2 .Serious problems also arise in the calculation of the ionisation potentials and dipole moments of molecules. 1. First ionisation potentials; electron affinity The first ionisation potential I, which corresponds to detachment of an electron from the HOMO, is the most interesting for chemists: the value of I to some extent correlates with the reactivity of a molecule in reactions with electrophilic reagents.Linear correlations between the logarithms of rate constants of certain reactions and I values are often found for narrow subsets of closely related compounds. In order to estimate I, the difference between the energies of the ion formed and the initial molecule DE0 has to be calculated.According to Koopmans theorem,66 if wavefunctions are not changed after detachment of an electron from certain MO, the corresponding ionisation potential is equal to the energy of this orbital Ei. This implies that the difference between the correlation energies of the ion and molecule DEc is equal to the reorganisation energy DEr taken with the opposite sign,67 since I+Ei=DEc+DEr , where DEr is the difference between the energy of the HOMO calculated in the optimum modification of the Hartree ± Fock method and the ionisation potential.68 30 A V Fokin, MA LandauUsually, ionisation potentials I calculated according to Koop- mans theorem using different semiempirical and non-empirical methods of the self-consistent field (SCF) approach exceed the experimental values.At the same time, the use of DE0 may lead to errors due to the difference in the correlation energies of the ion formed and the initial molecule. This often results in lower values of the ionisation potentials I compared to the experimental ones. The results obtained according to Koopmans theorem may be corrected (for example, by changing the calculated values of I by 8%), however, such corrections give satisfactory results only for ionisation from the two lowest MOs.As an alternative, averaged values of ionisation potentials obtained by the two above-mentioned methods can be used for a comparison of calculated and experimental results.23 But this artificial approach does not lead to good results in all cases.4 According to the Frank ± Condon principle, the internuclear distance should be kept constant as an electron is detached.But there is no reason to state that the ion formed in this case will be in its ground state. It is more probable that the interatomic distances of the ground state of the initial molecule correspond to an excited vibrational state of the ion. In this case, the so-called vertical ionisation potential Iv, which can be determined, for instance, by the electron impact method, includes energy from the excitation of the molecule in addition to the adiabatic ionisation potential (Ia).Discrepancies between the values of Iv determined in different studies are sometimes equal to 0.15 ± 0.40 eV.4, 64 At the same time, the accuracy of the experimental determination of the adiabatic ionisation potentials is very high (0.01 ± 0.02 eV).4 Adiabatic ionisation potentials, which correspond to the difference in the energy of ground states of the ion and molecule, can be measured with high precision using spectral and photo- ionisation methods.Obviously, calculation of the adiabatic ionisation potentials requires that the difference in energies of the ion and molecule, calculated for optimised geometries corresponding to the energy minima of both particles, should be determined.Ionisation potentials calculated according to Koopmans theorem should correspond to the experimentally determined vertical ionisation potential. However, the change in, for example, energies of HOMOs calculated by the HuÈ ckel method as well as by EHT, CNDO, INDO methods 12, 69 ± 74 for a series of haloethylenes corresponds to a change in the first adiabatic potentials and at the same time does not reflect changes in the vertical potentials.Therefore, in many studies the first ionisation potentials Ia of haloethylenes calculated using the corresponding parametrisation are taken to be equal to the energy of the HOMO.Surprisingly, the best results were obtained using the HuÈ ckel method. Presumably, it is a relatively simple procedure to adjust the parameters required for the calculations using experimental data for a small subset of molecules that accounts for this fact. Of course, it is possible to try to adjust the corresponding parameters for calculations of ionisation potentials of haloethy- lenes within semiempirical SCF quantum-chemical methods.However, such a procedure and subsequent calculations are much more time-consuming than the corresponding calculations by the HuÈ ckel method. Thus, merely from a pragmatic viewpoint, the results 71 ± 73 are still timely, and a similar approach can be successfully applied for other classes of compounds. Attempts at calculations of ionisation potentials of fluoro- ethylenes using various semiempirical and non-empirical versions of the SCF method did not, in general, lead to positive results.The most complete study of ionisation potentials of molecules of the Table 2. Calculated and experimental values of the first adiabatic ionisation potentials of haloethylenes (eV). Molecule Calculation 79 Calcula- Experiment 67, 68, 80 Experiment 67, 81 Experiment 26, 82 ± 85 (see a) tion 72, 73 FHC=CH2 7 10.40 10.37 10.360.015 10.370.02 F2C=CH2 7 10.32 10.31 10.290.01 10.310.02 10.300.02 cis-FHC=CHF 7 10.30 7 10.230.02 10.250.02 trans-FHC=CHF 7 10.30 7 10.210.02 10.190.02 F2C=CHF 7 10.21 7 10.140.02 10.140.02 F2C=CF2 7 10.12 10.11 10.14 10.120.02 ClHC=CH2 10.06 10.00 10.000.02 9.9950.01 10.000.02 cis-ClHC=CHCl 7 9.64 9.650.01 9.630.01 9.660.02 trans-ClHC=CHCl 9.65 9.64 9.640.02 9.650.01 9.640.02 Cl2C=CH2 9.89 9.89 9.83 9.790.02 7 Cl2C=CHCl 9.49 9.50 9.480.03 9.470.01 7 Cl2C=CCl2 7 9.32 9.340.03 9.320.02 7 BrHC=CH2 9.93 9.89 7 9.800.02 9.820.02 cis-BrHC=CHBr 7 9.45 7 9.450.02 9.450.02 trans-BrHC=CHBr 9.46 9.45 7 9.470.02 9.460.01 Br2C=CHBr 9.22 9.30 7 9.270.02 7 cis-FHC=CHCl 9.92 9.91 7 7 9.860.01; 9.870.01 trans-FHC=CHCl 9.89 9.91 7 7 9.870.02 cis-FHC=CClF 9.93 9.89 7 7 9.860.02 trans-FHC=CClF 9.90 9.89 7 7 9.830.02 F2C=CHCl 7 9.84 9.84b 7 7 F2C=CCl2 7 9.70 9.65 7 9.690.01 F2C=CClF 7 9.81 9.84 7 9.82 9.76 FClC=CH2 7 9.98 9.97b 7 7 F2C=CBrF 7 9.71 9.67b 7 7 a In Ref. 79, the results of calculations on fluoroalkenes by the HuÈ ckel method with different complicating corrections were also given.b Experimental data were obtained after publication of the corresponding results of calculations.72, 73 Simulation of reactions of fluoroalkenes by quantum chemistry methods 31class considered was performed by the ab initio MO LCAO method in the DZ basis set,23 these values were calculated both according to Koopmans theorem and as a difference in total energies of the ion formed and the initial neutral molecule.However, even the direction of change of the calculated ionisation potentials did not correspond to the change in experimental adiabatic and vertical ionisation potentials (see Refs 4, 23, 64 and results of Refs 32, 75 ± 78). Possible reasons for this fact have already been mentioned.Table 2 shows adiabatic ionisation potentials of haloethylenes calculated by the HuÈ ckel method. Differences between the calcu- lated and experimental values do not exceed a few hundredths of an electron-volt. This suggests that the calculated ionisation potentials of 28 haloethylenes (Table 3) for which there are no experimental data are also reliable.After publication of the studies 72 ± 74, adiabatic ionisation potentials of some mixed halo- ethylenes were measured; for them, good agreement between the calculated and experimental data was also observed (Table 2). Table 2 shows that experimental ionisation potentials of cis- and trans-1,2-haloethylenes are virtually identical, and this fact justifies the application of the computational methods that do not take into consideration geometrical differences between the isomers. It should be noted that according to mass spectrometric data,84, 86 the first ionisation potential of fluoroethylenes can be interpreted as the energy of a p-electron of the C=C bond (taken with the opposite sign).This conclusion can also be derived from the calculations on these molecules by the CNDO, INDO, MNDO, and ab initio methods (see Refs 4 and 64): the orbital formed by a p-bonding orbital of ethylene perturbed by fluorine atoms has the highest energy of all the p-orbitals.The great scatter of experimental data on electron affinity (A) does not allow one to derive unambiguous conclusions on changes in A for fluoroethylenes. For example, in a reference book,83 the value A>72.15 eV is indicated for ethylene, while for fluoro- ethylenes the data are absent. In more recent studies,54, 87 the values of A for C2H4 and CH2=CHF are 71.74 eV and 71.91 eV, respectively, i.e.ethylene manifests a higher electron affinity than monofluoroethylene. However, a non-empirical calculation 54 (MP2, 6-31G**/HF/6-31G* basis) gives other results: 71.86 eV and 71.62 eV.The same result was obtained in Ref. 88: calculations by the CNDO and Xa methods as well as non-empirical calculations using the Hartree ± Fock functions in 6-31G and 6-31G* basis sets lead to a more stable lowest vacant orbital in fluoroethylenes compared to that in ethylene. Only the inclusion of d-functions into the 6-31G* basis set gives agreement with the above experimental results.Probable experimental errors have also to be taken into account. However, due to the lack of experimental data, this question is open to discussion. 2. Dipole moments of molecules The polarity of a molecule can substantially affect its reactivity and reaction mechanisms. Problems arising in quantum-chemical calculations of dipole moments have been considered in a number Table 3.Calculated 73 first adiabatic ionisation potentials (Ia/ eV) for haloethylenes (experimental data for the given molecules are absent). Molecule Ia Molecule Ia Br2C=CH2 9.81 ClBrC=CHBr 9.34 Br2C=CBr2 9.09 FBrC=CHBr 9.42 FBrC=CH2 9.88 Cl2C=CClF 9.45 ClBrC=CH2 9.84 Cl2C=CClBr 9.26 FHC=CHBr 9.80 Br2C=CBrF 9.26 ClHC=CHBr 9.54 Br2C=CClBr 9.15 ClBrC=CHF 9.74 F2C=CBr2 9.61 FBrC=CHCl 9.52 Cl2C=CBr2 9.21 FClC=CHBr 9.50 FClC=CClF 9.57 F2C=CHBr 9.73 FBrC=CBrF 9.39 Cl2C=CHF 9.79 ClBrC=CBrCl 9.20 Cl2C=CHBr 9.39 F2C=CClBr 9.64 Br2C=CHF 9.70 Cl2C=CBrF 9.36 Br2C=CHCl 9.41 Br2C=CClF 9.36 FBrC=CHF 9.79 FClC=CBrF 9.48 FClC=CHCl 9.61 FClC=CClBr 9.40 ClBrC=CHCl 9.45 FBrC=CClBr 9.30 Table 4.Calculated and experimental values of dipole moments of fluoroalkenes.Molecule Method Dipole moment /D of calculation a calculation experiment FHC=CH2 PPP89 0.77 1.427 98 MNDO90 1.70 CNDO91 1.51 CNDO12 1.585 CNDO92 1.587 INDO12 1.649 INDO92 1.686 INDO93 1.483 HAM/394 2.64 ab initio [STO-3G] 95 0.71 ab initio [STO-4G] 95 0.79 ab initio [STO-3G] 96 0.90 ab initio [4-31G]96 2.10 MM97 1.43 F2C=CH2 CNDO99 0.88 1.37 101 ± 103 CNDO12 1.389 INDO93 1.420 INDO12 1.493 MNDO90 2.03 HAM/394 2.69 ab initio b (see 100) STO-3G.7.3 1.71 (1.83) 7.3.1 1.22 9.5 2.31 9.5 2.25 MM97 1.39 cis-FHC=CHF CNDO99 2.74 2.42 102, 103 CNDO12 2.806 INDO12 2.900 INDO93 2.523 ab initio b (see 100) STO-3G.7.3 3.10 (3.36) 7.3.1 2.62 9.5 3.47 9.5 3.55 ab initio b (RHF)104 4.31G; 6.31G 3.50 (3.60) 6.311G 3.53 6.31G* 2.81 6.311G* 2.91 MM97 2.42 F2C=CHF CNDO12 1.459 1.30 ± 1.32 98 INDO12 1.534 MNDO90 1.82 HAM/394 2.83 MM97 1.32 F2C=CClF CNDO21 0.33 0.38 98 aMM is the molecular mechanics method.b Results of calculations with different basis sets for experimental (gas phase),100 standard 104, and optimised (in parentheses) molecular geometry; fixed standard parameters are as follows:104 bond lengths C7C, C7F, and C7H are equal to 1.34, 1.33, and 1.08 A, respectively, all angles are equal to 120 8. 32 A V Fokin, MA Landauof studies.4, 56 The calculated and experimental values of dipole moments of fluoroethylenes are summarised in Table 4. In the study,94 which appeared at the same time as the monograph 4, the HAM/3 method was used for the calculation of dipole moments of 97 molecules including fluoro-, 1,1-difluoro-, and trifluoro-ethyl- enes.However, as indicated by the authors of Ref. 94, this method overestimates the results compared to experiment. In most cases, the CNDO method gives the dipole moments that are in better agreement with the experimental data than those obtained by the INDO, MNDO, HAM/3, and ab initio methods (see Table 4). This holds for calculations of dipole moments of other fluorine-containing molecules, such as phosphorus fluo- rides 4, 24, 105 and nitrogen fluorides.4, 106 However, it should be noted that calculations by molecular mechanics are more reliable than quantum-chemical calculations.V. Conclusion Quantum-chemical calculations on fluoroalkenes even within the isolated molecule approximation allow one to explain or predict many subtle features of their reactions.A more complete descrip- tion can be achieved by determination of the structure and stability of possible intermediates 5 (carbanions, radicals, etc.) and by calculation of potential energy surfaces of the reactions. In calculations of physicochemical parameters of fluoroal- kenes, semiempirical and non-empirical methods that make use of experimental molecular geometry lead to better results despite the rapid development of techniques of non-empirical calculations and determination of molecular geometry by a procedure of energy minimisation.Presumably, this is due to the fact that the geometrical parameters of fluoroalkenes (especially of non-sym- metrical ones) first of all, bond angles calculated in such a way deviate strongly from the experimental values.Probably, an extension of the basis sets will finally allow one to overcome this problem. Let us indicate some other problems that are still subjects of discussion, primarily the strength of the p-bonds in ethylene and fluoroethylenes. Our opinion on this point was outlined in the second section of this review. Another unsolved problem was not considered here but it was extensively discussed in the monograph:4 the relative stability of geometrical non-halogenated isomers (in particular, 1,2-alkyl- substituted ones) and 1,2-dihaloethylenes. It was established experimentally that in the first case, the trans-isomer is more stable than the cis-isomer, whilst in the second case the situation is opposite.However, discussions on the probable reasons for the phenomenon still continue.We hope that all the problems mentioned will be successfully resolved in the near future. The authors are most grateful to A S Kabankin and L A Piruzyan for a long-lasting and fruitful collaboration, and to I P Beletskaya for valuable advice in the discussion of the paper. References 1. 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J A Sell, D M Mintz, A Kuppermann Chem. Phys. Lett. 58 601 (1978) 82. F I Vilesov Usp. Fiz. Nauk 81 669 (1963) g 83. L V Gurvich, G V Karachentsev, V N Kondrat'ev, Yu A Lebedev, V AMedvedev, V K Potapov, Yu S Khodeev Energii Razryva Khimicheskikh Svyazei. Potentsialy Ionizatsii i Srodstvo k Elektronu (Energies of Rupture of Chemical Bonds. Ionisation Potentials and Electron Affinities) (Moscow: Nauka, 1974) 84. J Momigny Nature (London) 191 1089 (1961) 85. J Momigny Nature (London) 199 1179 (1963) 86. J I Majer, in Advances in Fluorine Chemistry Vol. 2 (EdsMStacey, J C Tatlow, A G Sharpe) (London: Butterworths, 1961) p. 55 87. S G Lias, J E Bartmess, J F Liebman, J L Holmes, R D Levin, W G Mallard J. Phys. Chem. Reference Data, Suppl. 17 1 (1988) 88. N S Chiu, P D Burrow,K D Jordan Chem. Phys. Lett. 68 121 (1979) 89. I Ficher-Hjalmars, S Meza Acta Chem. Scand. 26 2991 (1972) 90. M J S Dewar, H S Rzepa J. Am. Chem. Soc. 100 59 (1978) 91. J A Pople, M S Gordon J. Am. Chem. Soc. 89 4253 (1967) 92. C Leibovici J. Mol. Struct. 6 158 (1970) 93. R Iha, A N Sigh J. Phys. 53B 117 (1979) 94. P R Livotto, Y Takahata Acad. Brasil. Ciencias 61 135 (1989) 95. W Hehre, J A Pople J. Am. Chem. Soc. 92 2191 (1970) 96. W L Jorgenson, M E Cournoyer J. Am. Chem. Soc. 100 5278 (1978) 97. A Y Meyer J. Comput. Chem. 1 111 (1980) 98. O A Osipov, V I Minkin, A D Garnovskii Spravochnik po Dipol'nym Momentam (Handbook on Dipole Moments) (Moscow: Vysshaya Shkola, 1971) 99. C C Nascimento, I M Brinn Z. Naturforsch., A Phys. Sci. 33 366 (1978) 100. C W Boch, P George, G J Mains, M Trachtman J. Chem. Soc., Perkin Trans. 2 814 (1979) 101. A L McClellan Tables of Experimental Dipole Moments (San Francisco: Freeman, 1963) 102. V W Laurie J. Chem. Phys. 34 291 (1961) 103. V W Laurie, D T Pence J. Chem. Phys. 38 2693 (1963) 104. J S Binkley, J A Pople J. Chem. Phys. 45 197 (1977) 105. MA Landau, A S Kabankin, A V Fokin Zh. Fiz. Khim. 47 2916 (1973) f 106. AV Fokin,AS Kabankin,MALandau, VI Yakutin Zh. Fiz. Khim. 51 1073 (1977) f a�J. Struct. Chem. (Engl. Transl.) b�Russ. Chem. Bull. (Engl. Transl.) c�Theor. Exp. Chem. (Engl. Transl.) d�Russ. J. Org. Chem. (Engl. Transl.) e�Pharm. Chem. J. (Engl. Transl.) f�Russ. J. Phys. Chem. (Engl. Transl.) g�Physics-Uspekhi (Engl. Transl.) 34 A V Fo

ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. The data on the synthesis, chemical transformations and practical use of aliphatic nitro alcohols published over the last 25 years are described systematically and analysed. The bibliography includes 316 references. I. Introduction Aliphatic nitro-derivatives attract considerable attention from researchers, because they are readily accessible and convenient starting compounds for the synthesis of organic derivatives of various classes.The diversity of chemical transformations of aliphatic nitro-compounds provided the basis for the paper `Aliphatic nitro-compounds as perfect intermediates' published in 1979.1 Nitro alcohols of the aliphatic series fully conform to this definition. Furthermore, since nitro alcohols contain the hydroxy group, their reactions can involve simultaneously several reaction sites; hence, the range of chemical transformations of these derivatives is even broader.The results of numerous studies devoted to the chemistry of aliphatic nitro alcohols and published before 1972 have been surveyed quite comprehensively in reviews and monographs.2 ±4 However, no reviews describing systematically the data on the synthesis, transformations and practical use of aliphatic nitro alcohols obtained during the last 25 years have been published so far.The purpose of this review is to fill this gap. We only briefly outline the preparation of nitro alcohols by nitroaldol condensa- tion (Henry reaction), because its conditions have been described quite comprehensively in previous publications.3, 4 The studies containing some novelty either in the procedure of condensation of nitro-compounds with carbonyl derivatives or in the use of the target compounds are discussed in greater detail.New data about other known methods for the synthesis of nitro alcohols and new information on their chemical transformations are presented. We also consider most of the studies that mention potential practical applications of these compounds, because this aspect has received little attention in earlier reviews.II. Methods for the synthesis of aliphatic nitro alcohols 1. Nitroaldol condensation (the Henry reaction) The Henry reaction consisting of condensation of aliphatic nitro- compounds with aldehydes or ketones is the most general method used widely for the synthesis of aliphatic nitro alcohols.The mechanism of this reaction has not yet been completely eluci- dated. It is generally accepted that nitroalkanes are condensed with carbonyl compounds according to the following scheme: a. Reactions of nitromethane and its derivatives with aldehydes The reaction of nitromethane 1 with formaldehyde 2 catalysed by Ca(OH)2 makes it possible to prepare both 2-nitropropane-1,3- diol 3 5 and 2-hydroxymethyl-2-nitropropane-1,3-diol 4.6 The diol 3 has also been prepared from the compounds 1 and 2 at 0 ± 10 8C in the presence of sodium carbonate.7 The synthesis of 3 from 1 and an aqueous solution of form- aldehyde or paraformaldehyde is carried out most often in the presence of alkali, NaOH8 ±12 or KOH,13 as catalysts.Sodium and potassium hydroxides are also used as catalysts to synthesise the triol 4 from MeNO2 and the aldehyde 2.14 The condensation of nitromethane 1 with paraformaldehyde catalysed by sodium methoxide in methanol gives the sodium salt 5.15, 16 The condensation of MeNO2 with paraform in the presence of Bu4N+Br7 and KF under an inert atmosphere at 18 ± 22 8C for C R1 R2 NO2 H +B7 7HB CNO2 R1 R2 R3R4C O 7 HB C R1 NO2 R2 C R3 R4 OH+B7.C R1 NO2 R2 C R3 R4 O7 R=H(3); CH2OH (4). MeNO2+CH2O 1 2 Ca(OH)2, H2O HOCH2CCH2OH R NO2 3, 4 MeNO2+(CH2O)x MeONa, MeOH (HOCH2)2C NOO7Na+. 076 8C 1 5 M-G A Shvekhgeimer A N Kosygin Moscow State Textile Academy, ul. Malaya Kaluzhskaya 1, 117918 Moscow, Russian Federation. Fax (7-095) 952 14 40. Tel. (7-095) 955 35 96 Received 8 September 1996 Uspekhi Khimii 67 (1) 39 ± 74 (1998); translated by Z P Bobkova UDC 547.43 Aliphatic nitro alcohols.Synthesis, chemical transformations and applications M-G A Shvekhgeimer Contents I. Introduction 35 II. Methods for the synthesis of aliphatic nitro alcohols 35 III. Chemical transformations of nitro alcohols 49 IV. Practical use of aliphatic nitro alcohols 64 Russian Chemical Reviews 67 (1) 35 ± 68 (1998) #1998 Russian Academy of Sciences and Turpion Ltd72 h affords a mixture of the diol 3 (yield5%± 10%) and the triol 4 (yield 80%± 85%).16 Treatment of the nitronate 5 with trifluoroacetic acid at low temperatures gives rise to the aci-form 6, which is rapidly converted into the diol 3 and 2-nitroprop-2-en-1-ol.16 The diol 3 obtained from MeNO2 and paraformaldehyde in the presence of KF and Bu4N+Br7 exists as a nitronate 7 stabilised by intramolecular interactions.17 The condensation of bromonitromethane with formaldehyde 2 in the presence of NaOH in methanol gives the diol 8.18 Treatment of nitromethane 1 with bromine in an alkaline medium followed by the reaction with formaldehyde in the presence of NaOH19 or the condensation of MeNO2 with form- aldehyde followed by treatment with bromine 20 afford the same compound 9, the yield of which is up to 60%.Nitro alcohols 10 containing a furyl substituent have been obtained by the condensation of MeNO2 with aldehydes 11 in the presence of sodium hydroxide.21, 22 R R0 n Yield (%) Ref. H Me 2 49 21 Me H 2 78 22 H H 1 66 ± 73 21, 22 H Me 1 59 ± 75 21, 22 The reaction of chiral complex 12 with nitromethane has been carried out in the presence of NaOH or KF and gave a mixture of diastereoisomeric nitro alcohols 13a (1S,2S) and 13b (1S,2R) in a high yield.It was found that the ratio of the resulting isomers varies over a wide range depending on the procedure used and the temperature (see Table 1).23 When nitromethane was treated with butyllithium (THF, HMPT, 790 to 760 8C), and the product was made to react with aldehyde 12 (THF, 770 to 760 8C, 1.5 h; 745 to 740 8C, 3 h), the isomer 13a was obtained as the only product.23 Sodium methoxide has been used as the catalyst in the reactions of nitromethane 1 with both aliphatic 24 and aro- matic 25, 26 aldehydes. Potassium fluoride is an efficient catalyst of the reaction of MeNO2 with aldehydes.27 Study of the condensation of nitromethane with aldehydes carried out using IR and NMR spectroscopy has shown that the process occurs efficiently in DMF at 20 8C in the presence of a complex of nickel acetate with 2,20-dipyridyl (Dipy) and gives nitro alcohols 15 in 81%± 87% yields.28 The reaction of MeNO2 with acetaldehyde catalysed by Rh(PMe3)3Cl and carried out at 23 8C for 0.5 h affords 1-nitro- propan-2-ol in 90% yield.29 Nitromethane readily reacts with aromatic aldehydes in liquid ammonia giving rise to nitro alcohols 16 in good yields.30 (HOCH2)2C N O OH 3+HOCH2C CH2 .NO2 6 (HOCH2)2C N O O 5 7 Na+ CF3COOH, MeCN 730 to725 8C MeNO2+(CH2O)x 1 KF, Bu4N+Br7, PriOH, argon 20 8C, 3 days 7 H O CH2 C N O O H O CH2 7 BrCH2NO2+CH2O NaOH, H2O, MeOH pH 9710, 45 8C (HOCH2)2CNO2 .Br 8 (76.2%) MeNO2+Br2 1 HOCH2C(NO2)Br2 , 9 1+CH2O 2 9 . 1. NaOH, H2O 2. Br2 1. NaOH, H2O, 0720 8C 2. CH2O, NaOH, 0720 8C O R CH(CH2)nCHO+MeNO2 R0 NaOH, EtOH 20 8C, 3 h 11 O R CH(CH2)nCHCH2NO2 R0 OH 10 2 Me C O H Cr(CO)3 12 Me C H CH2NO2 OH Cr(CO)3 13a,b 1 MeNO2 B R=C13H27, 3-MeOC6H4, 4-PhCH2OC6H4 (43%), Ph (59%), 4-MeC6H4 (50%), 4-ClC6H4 (63%).RCHO +1 MeOH, MeONa RCHCH2NO2 OH R=C6H13, C7H15, CH2, , Me2C CH(CH2)2C(Me) CH(CH2)2. MeNO2+RCHO KF, PriOH 23 8C, 6 h RCHCH2NO2 14 OH RCHO +MeNO2 Ni(OAc)2, Dipy, DMF 208C RCHCH2NO2 OH 1 15 R= ,PhCH2CH2, 4-NO2C6H4, 4-NCC6H4. N Table 1. Reaction conditions and the ratio of the products 13a and 13b. Base Temperature Yield Ratio /8C (%) 13a : 13b NaOH 20 100 64 : 36 NaOH 720 95 92 : 8 NaOH 740 90 97 : 3 KF/PriOH 20 80 40 : 60 KF/PriOH 0 90 88 : 12 36 M-G A ShvekhgeimerTriethylamine has been used as the catalyst in the reaction of MeNO2 with 4-chloropentanal 17; this resulted in the formation of nitro alcohol 18 in 42.5% yield.31 Difluoronitromethane condenses with trifluoroacetaldehyde on heating in the presence of sodium carbonate.The reaction gives 1,1,1,3,3-pentafluoro-3-nitropropan-2-ol in 38.5% yield.32 The condensation of difluoronitromethane with aromatic aldehydes has been carried out using KOH as the catalyst.33 It has been reported 34 that chlorofluoronitromethane reacts with aliphatic aldehydes without a catalyst. b.Reactions of nitromethane homologues and their derivatives with aldehydes It should be noted that even when primary nitro-compounds are used, the synthesis usually affords products resulting from con- densation with one formaldehyde molecule. 2-Nitrobutan-1-ol has been obtained in an almost quantitative yield by the reaction of 1-nitropropane with formaldehyde 2 in the presence of Ca(OH)2 at 30 8C.35 In a number of publications,36 ± 39 synthesis of nitro alcohols 19 from nitroalkane 20 and formaldehyde catalysed by an alkali (NaOH or KOH) or by Amberlites IRA-410 has been described (Table 2).Condensation of nitrated butadiene or isoprene oligomers with formaldehyde in the presence of alkaline catalysts gave the corresponding nitro alcohols.40 The process of condensation of nitroalkanes with formalde- hyde has been optimised by experiment design.41, 42 It was found that the reaction of paraformaldehyde with nitroethane in the presence of KOH occurs over a period of 2 ± 3 min.41 The highest yields of nitro alcohols in the reaction between an aqueous solution of the aldehyde 2 and a mixture of C1 ±C4 nitroalkanes are attained when the process is carried out at an RNO2 :CH2O ratio of 2.5 : 1 and at a temperature of 50 ± 60 8C for 35 ± 40 min.42 The condensation of 2-nitropropane with paraformaldehyde in the presence ofKFaffords 2-methyl-2-nitropropan-1-ol in 86% yield.43 2-Nitropropan-1-ol has been obtained in a yield of 62% ± 74% by the reaction of nitroethane with the aldehyde 2 in the presence of triethylamine.44 Tributylphosphine has been used as the catalyst in the syn- thesis of 2-methyl-2-nitropropan-1-ol from 2-nitropropane and paraformaldehyde.45 As a rule, high yields of nitro alcohols are achieved when formaldehyde is condensed with functionally substituted nitro- alkanes 21 in the presence of NaOH (Table 3).46 ± 48 It can be seen from Table 3 that in the case of fluorine- containing nitroalkanes, approximately the same yields of nitro alcohols are obtained when the reaction is carried out with heating.On the other hand, homologues of ethyl nitroacetate readily react with formaldehyde at room temperature. Treatment of diethyl fluoronitromalonate with potassium hydroxide gives rise to salt 22, which reacts with the aldehyde 2 to yield compound 23; the latter reacts with one more molecule of formaldehyde being thus converted into 2-fluoro-2-nitropropane- 1,3-diol in a yield of more than 70%.49 Ar=2-H2NC6H4 (53%), 3,4-(MeO)2C6H3 (98%), 4-Br-3,4-(MeO)2C6H2 (80%), 4-MeC6H4 (92%), 3,4-(CH2O2)C6H3 (83%), 4-BrC6H4 (93%), 3-BrC6H4 (91%), 3-ClC6H4 (75%), 4-MeOC6H4 (82%), 1-naphthyl (53%). 1 ArCHO MeNO2+ NH3(liq.) 16 ArCHCH2NO2 OH MeCH(CH2)2CHCH2NO2 . Cl OH 1+MeCH(CH2)2CHO Cl 18 17 Et3N, EtOH 5 ± 7 8C, 48 h; 20 8C, 24 h F2C(NO2)H+ArCHO ArCHC(NO2)F2 OH KOH, DMF 150 8C, 5 ± 15 h Ar=2-HOC6H4 (47%), 2-ClC6H4 (61%), 2-O2NC6H4 (31.5%), 3-O2NC6H4 (56%), 4-MeC6H4 (60%), 2,4-(MeO)2C6H3 (53.5%).R=H (83%), Me (55%), Et (73%), Pr (86%). F(Cl)C(NO2)H+RCHO RCHC(NO2)(Cl)F refluxing, 1 h OH RR0CHNO2+CH2O 20 2 H2O 20730 8C RR0C(NO2)CH2OH . 19 Me2CHNO2+(CH2O)x Bu3P, PriOH 35740 8C, 45 min Me2CCH2OH . NO2 NaOH, H2O RR0C(NO2)CH2OH .CH2O+RR0CHNO2 2 21 HOCH2CNO2 F COOEt 23 KOH H2O HOCH2CNO2 F COO7 7CO2 C F COOEt COOEt O2N KOH EtOH K+C NO¡2 F COOEt 22 CH2O, H2O 20 8C, 3 h Table 2. Reaction conditions and yields of products in the condensation of the nitro-derivatives 20 with formaldehyde. R R0 Catalyst Time Yield Ref. /h (%) But Me KOH 7 56.5 36 MeC(NO2)2CH2 H NaOH 48 7 37 Et H NaOH 5 see a 38 C12H257C18H37 H Amberlites 7 30 39 IRA-410 a The reaction also yields EtC(NO2)(CH2OH)2.Table 3. Reaction conditions and yields of products for the condensation of the nitro-derivative 21 with formaldehyde. R R0 Tempera- Time Yield Ref. ture /8C, /h (%) Et F 76 ± 80 2 ± 3 80 46 Pr F 76 ± 80 2 ± 3 87 46 Bu F 76 ± 80 2 ± 3 86 46 C5H11 F 76 ± 80 2 ± 3 79 46 C6H13 F 76 ± 80 2 ± 3 83 46 C7H15 F 76 ± 80 2 ± 3 85 46 C8H17 F 76 ± 80 2 ± 3 81 46 F2CCl H 710 7 7 47 COOEt Me 20 2 60 a 48 COOEt Et 20 2 67 a 48 COOEt Pr 20 2 67 a 48 COOEt Pri 20 2 79 a 48 COOEt Bu 20 2 82 a 48 a Found for the acetylated product.Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 37Reactions of aldehydes with nitro ketones carried out in the presence of strong bases are substantially complicated by side reactions of the initial compounds and reaction products.To avoid these problems and to obtain the products of nitroaldol condensation in higher yields, a convenient procedure was devel- oped48 according to which the condensation of nitro ketones with formaldehyde was carried out in the presence of triphenylphos- phine. The intermediate alcohols were isolated as the correspond- ing acetates after treatment with acetic anhydride.Thus the reaction of nitro ketones 24 with formaldehyde in the presence of triphenylphosphine gave nitro alcohols 25, which were treated in situ with acetic anhydride in the presence of pyridine to give the corresponding acetates 26 (yields 73%± 91%).48 1,4-Difluoro-1,4-dinitrobutane reacts with two molecules of the aldehyde 2 in the presence of potassium carbonate giving 2,5- difluoro-2,5-dinitrohexane-1,6-diol in 75% yield.50 Some researchers have synthesised nitro alcohols by reactions of formaldehyde with the salts of nitro-compounds prepared previously.The sodium salt 27 prepared from ethyl nitroacetate and the aldehyde 2 under the action of sodium ethoxide has been made to react with a second molecule of 2; this resulted in the synthesis of compound 28.51 The reaction of bis-salts 29 with the aldehyde 2 at 1 ± 3 8Cgave bis-salts 30, which were converted into the corresponding diols 31.Thus treatment of the salt 30b withHONH2 . HCl in water at pH 4 gives the diol 31b in 54.5% yield; treatment of the salt 30a with 18% hydrochloric acid or with 75% acetic acid affords 31a in a yield of 9.2% or 6.6%, respectively.52 Under the same conditions, the bis-salt (7O2N=CHCH2..CH2CH=NO2 7) 2Na+ reacts with two molecules of CH2O and is thus converted into the salt of bis-hydroxymethyl derivative [7O2N=C(CH2OH)CH2CH2C(CH2OH)=NO2 7] 2Na+ in 93.6% yield.52 Whereas the anions of non-conjugated mononitro-derivatives react with formaldehyde only at pH>7, the dianions of con- jugated dinitroalkanes 32 are able to react with the aldehyde 2 giving rise to diols 33 both at pH > 7 (yields 70%± 90%) and at pH=4 (yields 40%± 70%).53 The reaction of conjugated nitroalkenes 34 with the aldehyde 2 in the presence of Et3N in acetonitrile starts with the formation of allylic anions 35, which are attacked by the aldehyde at the carbon atom carrying the nitro group to give nitro alcohols 36 (yields 60%± 94%).54 Nitroalkenes 37 react successively with lithium phenylthiolate and the aldehyde 2 to afford salts of aci-forms 38, which are converted, on treatment with acetic acid, into mixtures of erythro- and threo-isomers (in a ratio ranging from 86 : 14 to 93 : 7) of nitro alcohols 39 (yields 57%± 91%).55 The reaction of the nitroalkenes 37 with PhSH and CH2O in the presence of bases yields mixtures of the erythro- and threo- isomers of 39 (1 : 1) together with nitro diols RCH(SPh) ± C(CH2OH)NO2.55 Sodium methoxide has been used as the catalyst in the reaction of 1-(nitromethyl)cyclopentene with paraformaldehyde.56 HOCH2CNO2 F 7 HOCH2CCH2OH.F NO2 CH2O, KOH H2O R, R0=Alk.Ph3P, PriOH 20 8C, 24 h RCCHR0 +CH2O O NO2 RC O CCH2OH NO2 R0 24 25 Ac2O, C5H5N RC O CCH2OAc NO2 R0 26 O2NCH(CH2)2CHNO2+2CH2O F F K2CO3, H2O 60 8C, 2 h HOCH2C(CH2)2CCH2OH . F F NO2 NO2 2 O2NCH2COOEt +CH2O NaOEt, EtOH 27 EtO C N O 2, H2O Na C CH2OH + 7 (HOCH2)2CCOOEt. NO2 28 O O 30a,b NO¡2 C C CH2OH R R 2Na+ H+ 7O2N C C CH2OH 29a,b NO¡2 ) 2Na++2CH2O (7O2N CHC CCH H2O 1 ± 3 8C, 24 h R R R=H (a), Me (b).CCHCH2OH NO2 NO2 31a,b R R HOCH2CHC n=0, R=H; n=1: R=H, R0 =H, Me. CR0)n HOCH2C (R0C NO2 R 33 CCH2OH NO2 R CR0)n C NO¡2 +2CH2O R 1 ± 5 8C, 18 h 7O2N C (CR0 32 R CNO2 R3 Et3N 7Et3NH+ 35 R1R2C 2 7 CH CNO2 R3 R1=H, Bu, PhCH2; R2=H; R3=Me, Et, C7H15, (CH2)2COOMe. 34 R1R2CHCH 7 CHCCH2O NO2 R3 Et3NH+ 7Et3N 36 R1R2C CHCCH2OH R3 NO2 R1R2C R=Me, Et, Pr, Pri, Bu, C5H11, Ph, PhCH2CH2.R OH NO2 SPh 39 HOCH2 SPh H R N O O 38 Li+ AcOH 778 8C, 1 h 7 1. PhSLi, THF, 20 8C, 1 h 2. 2, H2O, 20 8C, 3 h CHNO2 37 RCH 38 M-G A Shvekhgeimer1-Halo-1-nitroalkanes 40 smoothly react with aldehydes 41 in the presence of potassium carbonate at 20 8C, and this leads to halo-substituted nitro alcohols 42 (yields 37%± 75%).57 Several nitro-compounds 43 have been condensed with ali- phatic or arylalkyl aldehydes 44 in the presence of sodium hydroxide in methanol or ethanol; this resulted in the formation of threo-isomers of nitro alcohols 45, as a rule, in good yields.59, 60 However, the reactions of butyraldehyde with nitrobutane (43, R1=Pr) and nitrooctane (R1=C7H15) gave the corresponding nitro alcohols in yields of 26% and 30% (Table 4).60 When the same nitro-compounds 43 were made to react with aldehydes in the presence of KF, the erythro-isomers of 45 were obtained.59, 60 In some cases, water was added to the reaction mixture.60 Potassium hydroxide has been used as the catalyst in the reactions of myristaldehyde 46 with nitroethane 61 and of unsatu- rated nitro-compounds 47 with aldehydes 48;62 these reactions gave nitro alcohols 49 and 50 in 64% and 30%± 65% yields, respectively.A method for the synthesis of nitro alcohols 51 by condensa- tion of aromatic aldehydes with nitroalkanes has been patented.63 According to this method, aldehydes are treated first with sodium hydrogensulfite and then with nitroalkanes in the presence of aqueous sodium hydroxide at 20 8C.63 In the synthesis of 1,1,1-trichloro-3-nitrobutan-2-ol (yield 30%), the initial chloral was also converted first into the hydro- gensulfite derivative, and the latter was made to react with nitro- ethane in the presence of aqueous sodium hydroxide at 20 8C for 12 h.64 In the same study,64 condensation of primary or secondary nitroalkanes with aliphatic or aromatic aldehydes at 20 8C in the presence of NaHCO3,K2CO3 or NaOH was studied.The yields of the target nitro alcohols 52 were found to depend crucially on the nature of the initial aldehyde (Table 5). Synthesis of 2-nitro-1-phenylpropan-1-ol from benzaldehyde and nitroethane under mild conditions in the presence of bases in DMSO or DMF has been reported.65 Condensation of the complex 12 with nitroethane in the presence of NaOH yields a mixture of diastereoisomers 53a,b; on treatment with hydrogen peroxide, this mixture is converted into a mixture of diastereoisomeric nitro alcohols 54a (1R*,2R*) and 54b (1R*,2S*) in a ratio of 36 : 64.23 CH2NO2+(CH2O)x MeONa CHCH2OH.NO2 RC O H 41 +R0CHNO2 X 40 K2CO3, H2O 20 8C, 12 h R=Alk, CCl3; R0=H, Cl, Br, Me, Et; X=Cl, Br. RCH CR0 X NO2 42 OH NaOH, MeOH or EtOH 44 R1CH2NO2 +R2R3CHC O H 43 45 R2R3CHCHCH(R1)NO2 .OH 46 C13H27C +EtNO2 O H C13H27CHCH(Me)NO2 , 49 KOH, MeOH Et2O, 20 8C OH R1, R2=H, Me; R3=Me, Et, Pr, Pri. 47 R1CH CCH2NO2+R3C 48 O H R2 C(R2)CHCH(R3)OH 50 R1CH NO2 KOH EtOH, H2O 4-RC6H4C O H +NaHSO3 H2O R=But, Bus; R0=Me, Et. 4-RC6H4CH(OH)CH(R0)NO2 51 4-RC6H4C OH SO3Na H NaOH, R0CH2NO2 20 8C, 12 ± 16 h R1CHO+R2R3CHNO2 H2O 20 8C R1CH(OH)C(NO2)R2R3. 52 (a) NaOH, EtOH, H2O, 740 8C, 0.5 h; (b) NH4Cl, H2O.a, b CHCH(Me)NO2 Me OH Cr(CO)3 53a,b H2O2 C O H Me Cr(CO)3 12 +EtNO2 CHCH(Me)NO2 Me OH 54a,b Table 4. Reaction conditions and yields of the compounds 45. R2 R3 R1 Tempera- Time Yield Ref. ture /8C /h (%) Me H Cl(CH2)3 5 ± 10 7 65 a 58 Ph H H 30 ± 35 45 67 a 59 Ph H Me 30 ± 35 45 69 a 59 Ph H Et 30 ± 35 45 65 a 59 H H Ph 30 ± 35 45 63 a 59 Me H Ph 30 ± 35 45 58 a 59 Ph H Ph 30 ± 35 45 60 a 59 Me Me Ph 30 ± 35 45 58 a 59 H Ph Ph 20 144 85 a 59 Me H Et 38 65 80 b 60 Et H Et 38 65 63 b 60 Bu H Me 38 65 52 b 60 Bu H Et 38 65 58 b 60 Et H C7H15 38 65 30 b 60 C9H19 H Me 38 65 61 b 60 Bu H C7H15 38 65 70 b 60 Me Me Et 38 65 54 b 60 Et H Pri 38 65 26 b 60 a (R*,R*) threo-Isomers are formed; if the reaction is carried out in the presence of KF, the nitro alcohols 45 are mostly formed as (R*,S*) erythro-isomers; b water was added to the reaction mixture.Table 5. Reaction conditions and yields of the nitro alcohols 52. R1 R2 R3 Catalyst Time Yield /h (%) Me Me H K2CO3 5 54 Pri Me H K2CO3 5 40 CF3 Me H NaOH 5 8.2 4-O2NC6H4 Et H NaHCO3 5 16.3 4-O2NC6H4 Me Me K2CO3 5 7 Br3C Me H K2CO3 0.5 10 Aliphatic nitro alcohols.Synthesis, chemical transformations and applications 39The reaction of 12 with the nitronate prepared previously from nitroethane resulted in the formation of a mixture of diastereoisomers 54a,b in a ratio of 77 : 23.23 Nitro-compounds 55 containing a CF3 group condense with aldehydes 56 in the presence of potassium fluoride in isopropyl alcohol or without a solvent to give nitro alcohols 57.66 R R0 Solvent Yield (%) H Pr PriOH 71 H Pr 7 73 H C6H13 PriOH 65 H C6H13 7 71 H Ph(CH2)2 7 83 H Me(Ph)CH 7 46 H Ph PriOH 26 H Ph 7 9.5 Me Pr PriOH 54 Me Pr 7 35 Me C6H13 PriOH 54 Me PhCH2CH2 7 40 During the last 10 ± 15 years, neutral alumina (Brockmann activity I) has been used as a catalyst in the Henry reaction.In the presence of this catalyst, nitro-compounds 58 condense with various aliphatic aldehydes 59 at ambient temperature.67 The reaction of nitro-compound 60 with propanal carried out in the presence of Al2O3 at 20 8C affords condensation product 61 as a mixture of erythro- and threo-isomers in a 54 : 46 ratio and in 22% overall yield.68 Nitro alcohols 62 have been prepared by the reactions of primary or secondary nitro-compounds 63 with aldehydes in the presence of neutral Al2O3 without solvents.66, 69 R1 R2 R3 Yield Ref.(%) Me Et H 80 69 Et Me H 71 69 Et (CH2)2COOMe H 84 69 Pri MeCH(OH) H 69 69 Me H 75 69 R1 R2 R3 Yield Ref. (%) C6H13 H 86 69 C6H13 H 86 69 Ph(CH2)2 Me H 71 69 Ph(CH2)2 Me Me 69 69 Ph(CH2)2 see a H 82 69 See b Me H 78 69 Pr CF3 H 73 66 Ph CF3 H 9.5 66 CF3 H 3 66 Pr CF3 Me 33 66 a R2=Me(CH2)3OCH(Me)OCH2; b R1=Me2C=CH(CH2)2CH(- Me)CH2.The product 64 resulting from interaction of acrolein with two molecules of nitroethane has been synthesised by two procedures: by the reaction of the aldehyde with a fivefold excess of EtNO2 (yield 39%) or by the addition of one molecule of EtNO2 to the double bond of the aldehyde followed by condensation of the resulting adduct 65 with a second molecule of the nitro-derivative (yield 89%).70 It has been reported 71 that the following nitro alcohols are formed as mixtures of erythro- and threo-isomers upon reaction of the corresponding aldehydes with nitro-compounds in the pres- ence of Al2O3 (Brockmann activity I) at 20 8C for 23 h: It can be seen from published results 66, 69 that the condensa- tion of nitro-compounds with aromatic aldehydes in the presence of KF or Al2O3 gives the corresponding nitro alcohols in low yields.When the reactions of nitroalkanes 66 with aromatic aldehydes 67 are conducted 30 in liquid ammonia, the correspond- ing nitro alcohols 68 are formed in 53% ±98% yields. CF3CHNO2+R0C F3CCCH(R0)OH NO2 55 56 57 R R O H KF, argon R1, R2, R3 : Me, Et, H; Me, , H; Pri, Et, H; C6H13, Et, H; R1CHC(NO2)R2R3 OH R1CHO+R2R3CHNO2 Al2O3, CH2Cl2 20 8C, 23 h 58 59 O O H2C Me C6H13, , H; CH2 CH(CH2)2, Me, Me; Ph(CH2)2, C6H13, MeOCO(CH2)2, H; Ph(CH2)2, , H.O O H2C Me Me, H; CH2 CH(CH2)2, Me, H; C6H13, , H; H2C O O O O H2C Me 60 EtCHO+MeCHCH2 O O 61 CCH2 NO2 EtCH OH Me Al2O3 20 8C, 3 days O O C NO2 R1C 63 +R2R3CHNO2 R1CHC(NO2)R2R3 OH 62 O H 20 8C, 24 h Al2O3 CH2O O O O Me H2C O O MeCH(CH2)2CHCHNO2 NO2 OH Me 64 MeCH(CH2)2CHO NO2 65 5 equiv.EtNO2, Al2O3 CH2Cl2, 20 8C, 17 h EtNO2, Al2O3 CH2Cl2, 20 8C, 24 h 1 equiv. EtNO2, Al2O3 CH2Cl2, 20 8C, 3 h CHC H2C O H OH Me OH MeCHCHNO2 , where R=Me, Et, (CH2)4COOMe, OH R Me OH CH(CH2)2CHCHNO2, Ph(CH2)2CHCHNO2, OH Me EtCH EtCHCH2NO2, C6H13CHCH2NO2, EtCHCHNO2, OH OH NO2 OH NO2 EtCHCH(CH2)6OH, EtCHCHCH2OH, C6H11CHCHCH2OH.OH NO2 40 M-G A ShvekhgeimerNitro alcohols have been obtained in good yields by the condensation of nitromethane, nitroethane or 1-nitropropane with the corresponding aliphatic, aromatic or heterocyclic alde- hydes in the presence of KF supported on Al2O3 (Scheme 1).72 Scheme 1 R R0 Time /h Yield (%) Et H 5 71 Pri Me 5 75 Et Me 5 75 Pri Et 5 78 Me Et 5 77 Bu Me 5 62 See a Me 5 79 See a H 5 50 Ph Et 6 55 Me 15(N2) 77 a R=Me2C=CH(CH2)2CH(Me)CH2.Sodium methoxide 73 and sodium ethoxide 74 have been used as catalysts in the nitroaldol condensation of primary nitroalkanes with functionally substituted aldehydes 69 and 70. The reaction of sulfide 71 with acetaldehyde carried out in the presence of potassium tert-butoxide (0 8C, 20 min) gives nitro alcohol 72 (yield 95%), whereas the reaction with isobutyralde- hyde carried out under the same conditions affords nitroalkene 73 (yield 83%).75 Esters of chloronitroacetic acid 74 react with aliphatic alde- hydes in the presence of sodium acetate.76 The reaction of nitro-compounds 76 with aldehydes in the presence of butyllithium led to g-nitro alcohols 75 in 75% ±80% yields instead of the normal products of nitroaldol condensa- tion.77 This unusual reaction pathway was explained 77 by the fact that at low temperatures nitro-compounds 76 are doubly depro- tonated at the a- and b-positions with respect to the nitro group to give dilithio-derivatives 77.The formation of g-nitro alcohols in the condensation of nitro-compounds with aldehydes in the presence of BuLi is not a specific feature peculiar only to 2-arylnitroethanes.It was found that g-nitro alcohols are also formed in the reactions of some secondary nitro-compounds with aldehydes in the presence of BuLi or ButLi. For example, 3-nitro-1-phenylbutan-1-ol was synthesised in 50% yield by the reaction of 2-nitropropane with benzaldehyde in the presence of BuLi and ButLi.77 The condensation of 1,1,1-trifluoro-2-nitropropane with ali- phatic and aromatic aldehydes in the presence of BuLi and ButLi occurs via unusual doubly deprotonated products 78 and yields g-nitro alcohols 79.67 Primary nitro-compounds 80, doubly deprotonated upon treatment with BuLi, react with aldehydes to give products of nitroaldol condensation, namely, b-nitro alcohols 81.66 The reactions of primary nitro-compounds 82 with aldehydes in the presence of butyllithium and TiCl3(OPri) at nitroalkane : al- dehyde ratios of 1 : 1 or 2 : 1 have been studied in detail.This process was found to afford b-nitro alcohols 83 as mixtures of erythro- and threo-isomers.78 R=H, Me; R0=H, Me; Ar=3-ClC6H4, 4-ClC6H4, 3-BrC6H4, 4-BrC6H4, 4-MeOC6H4, 4-MeC6H4, 2-O2NC6H4, 2-H2NC6H4, 3,4-(MeO)2C6H3, 6-Br-3,4-(MeO)2C6H2, ArCHO+RR0CH2NO2 NH3(liq.) ArCHCRR0 OH 67 66 68 NO2 , .H2C O O RCHO +R0CH2NO2 OH KF/Al2O3 RCHCH(NO2)R0 O 69 1. MeONa, 20 8C, 18 h 2. H3O+ R(R0O)PCH2CHO+EtNO2 O R(R0O)PCH2CHCHNO2 O OH Me R, R0 (yield, %): MeO, Me (66); EtO, Et (60); PriO, Pri (50); Bu, Pri (46); MeCH CHCH2, Pri (20); MeCH CHCH2, Pr (20).R=Me (52%), Et (51.5%), Pr (54%), Pri (28%), Ph (65%). EtOCCHO+RCH2NO2 O 70 RCHCHCOOEt NO2 OH 1. EtONa, EtOH, PhH, 20 8C, 12 h 2. H3O+ PhSCHCHMe NO2 OH 72 0 8C, 20 min MeCHO 0 8C, 14 h PhSCH2NO2 71 ButOK, ButOH, THF Me2CHCHO PhS O2N C H Pri 73 C R, R0 (yield, %): Me, Me (65); Me, Et (92); Et, Me (98); Et, Et (45); PhCH2, H (71). O2NCHCOOR +R0CHO 74 Cl 20 8C, 172 h R0CHC(NO2)COOR OH Cl AcONa .3H2O, EtOH, H2O 76 Ar, R: 4-ClC6H4, Et; Ph, Ph; Ph, 4-ClC6H4. 778 8C, 0.5 h BuLi, C6H14, THF ArCH2CH2NO2+RCHO RCHCH(Ar)CH2NO2 75 OH ArCH2CH2NO2 BuLi 790 to778 8C 77 Ar H N H O7 O7 2Li+. 7 7 Ar H H + N O O7 7 Ar H + N O7 O7 H F3CCHMe 1. BuLi, THF, argon,770 8C 2. ButLi, THF, HMPT,770 8C NO2 R=C6H13 (12%), PhCH2CH2 (9%), Ph (21%) . 78 LiO N OLi C F3C CH2 1.RCHO,780 8C 2. H2O F3CCHCH2CHR NO2 OH 79 X, R (yield, %): F, But (90); H, But (40); F, Ph (67); H, Ph (32); H, 4-O2NC6H4 (52); F, 3,4-(CH2O2)C6H3 (40). F2CCH2NO2 X 80 BuLi, DMF,THF, argon 790 8C F2(X)C Li C 7 O N OLi 2. H2O,775 8C, 1 h 1. RCHO, THF, argon F2CCHCH(R)OH NO2 X 81 Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 41The ratio of the erythro- and threo-isomers formed varies over a wide range.For a nitroalkane : aldehyde ratio of 1 : 1, it varies from 3.9 : 1 (in the case of R0 = 4-O2NC6H4, R = Bu) to 11.2 : 1 (in the case of R0 = MeOOCC6H4, R = Et); for a nitroalka- ne : aldehyde ratio of 2 : 1, it ranges from 2 : 1 (R0=COOMe, R=Et) to 8 : 1 (R0 =PhCH=CH, R=Et).78 The reaction of nitroethane with benzaldehyde in the presence of butyllithium has been studied systematically using two approaches.79 According to one of them, EtNO2 was treated with BuLi in THF and HMPT at various temperatures and various HMPT/BuLi ratios, and after that, the aldehyde was added at a temperature between 778 and 735 8C, and the reaction mixture was quenched with acetic acid (procedure A).The second approach included treatment of the aldehyde with butyllithium in THF and HMPT at 790 8C and subsequent addition of the nitro-compound at various temperatures and various HMPT/BuLi ratios followed by acidification of the reaction mixture (procedure B).It was found that in both cases, the reaction resulted in the formation of a mixture of b-nitro alcohol 84 and g-nitro alcohol 85.When the reaction was carried out by procedure A at an HMPT/BuLi ratio of 2.6 and at a temperature of790 to725 8C, the yields of the compound 84 varied in the range 68.8% ± 11.3%. In the absence of HMPT, the yield of 84 was 47.3%, and that of the compound 85 was 3% (at 790 8C). At higher temperatures, the yield of 85 increased, whereas the overall yield decreased. The reaction carried out by procedure B withoutHMPAafforded only the nitro alcohol 84 (yield 56.8%).At HMPT/BuLi ratios greater than 2, the nitro alcohol 85 was formed as the major product (yields 50.8% ± 72.6%, the yields of 84 formed simultaneously being 3.8% ± 10.6%. Triethylamine has been used as the catalyst in the condensa- tion of nitro-compounds with both aliphatic 30 and heteroaro- matic 80 aldehydes.Mixtures of erythro- and threo-isomers of nitro alcohols 86 are formed when the nitro compound 60 or its derivative 87 is made to react with acetaldehyde or propionaldehyde in the presence of tetrabutylammonium fluoride trihydrate (Table 6).69 In a study on the development of the synthesis of b-amino alcohols according to the scheme it was found 81 ± 83 that the nitroaldol condensation of nitroalkanes with molecular weights much greater than that of nitromethane in the presence of conventional catalysts gives b-nitro alcohols in low yields.In order to increase the yields of the target compounds, the researchers studied the nitroaldol condensation of aldehydes with silyl ethers 88, which were obtained from nitroalkanes and chlorotrimethylsilane or tert-butyl(chloro)dimethylsilane under the action of Pri 2NLi (LDA), in the presence of tetrabutylammo- nium fluoride. This resulted in the formation of nitro alcohol derivatives 89 in good or high yields.Silyl ethers of nitro alcohols 90 have been prepared in 30%± 40% yields by treatment of 2-nitropropane derivative 91 with aldehydes.81 The reaction of the silyl ether 87 with aldehyde 92 gives rise to four isomeric nitro alcohols (overall yield 59%, isomer ratio R=Et: R0=4-O2NC6H4, Ph, 4-MeOC6H4, 2-CF3C6H4, 4-MeOCOC6H4, R=Bu: R0=4-O2NC6H4 .(a) BuLi, THF,778 8C, 15 min; (b) R0CHO, TiCl3(OPri), CH2Cl2,778 8C, 1 h; 20 8C, 3.5 h. RCH2NO2 82 R0CHCH(R)NO2 OH 83 a, b CH, Bu, C5H11, But, COOMe; , PhCH MeCH2NO2+PhC PhCHCH(Me)NO2+PhCHCH2CH2NO2 . 84 85 OH OH O H Me2CHNO2+MeCH(CH2)2CHO Cl Et3N, EtOH 577 8C, 2 days; 20 8C, 24 h MeCH(CH2)2CHC(NO2)Me2 , Cl (42.5%) OH O C O H O +O2NCH2CHO Me Et3N 20 8C, 12 h O NO2 OH CHCHCHO O Me Me 86 O O NO2 CCH2 RCH OH R0=ButMe2Si. Bu4N+F7. 3H2O RCHO O MeCCH2 N O O R0O 87 MeCHCH2 NO2 O O 60 RCHO +R0CH2NO2 RCHCH(R0)NO2 OH RCHCH(R0)NH2, OH R2CH2NO2 1. LDA, THF,778 8C 2. Me3SiCl or ButMe2SiCl R1=SiMe3, SiMe2But: R2=C5H11, R3=C6H13 (71 ± 80%); R2=C5H11, R3=But (57%); R2=C5H11, R3=Ph (75783%); R2=C5H11, R3=4-MeOC6H4 (70%); R2=C5H11, R3=4-O2NC6H11 (91%); R2=Et, R3=Pr (82%, erythro : threo=20 : 1); R2=Et, R3=Pri; R2=Pri, R3=Pr; R2=Me, R3=C5H11 (erythro : threo=20 : 1); R2=Et, R3=C5H11 (erythro : threo=20 : 1); R2=C7H15, R3=Pr (erythro : threo=20 : 1); R2=Me, R3=C10H21; R2=Me, R3=C9H19; R2=Et, R3=Ph (erythro : threo=3.5 : 1).+ R2CH O7 OR1 N R3CH(R2)NO2 OH 88 89 R3CHO Bu4N+F7 R=C6H13, But, Ph, 4-MeOC6H4, 4-O2NC6H4. Me2CHNO2 1. LDA, THF,778 8C 2. ButMe2SiCl 91 + Me2C O7 OSiMe2But N 90 Me2CCHOSiMe2But NO2 R RCHO, Bu4N+F7 Table 6. Reaction conditions, yields and isomeric ratios of the nitro alcohols 86. R Nitro- Tempera- Time Yield Ratio compound ture /8C /h (%) erythro : threo Me 87 778 ± 20 15.5 20 56 : 44 Me 87 0 2.3 89 55 : 45 Et 87 0 0.75 74 54 : 46 Me 60 0 ± 6 23 52 62 : 38 42 M-G A Shvekhgeimer58 : 28 :12 : 2).The isomer 93 was isolated from this mixture in a pure state in a yield of 34%.69 The condensation of 1-nitrohex-3-ene 94 with aldehydes catalysed by Amberlyst A-21 occurs successfully at 20 8C; this gives nitro alcohols 95a,b, their yields being 74% and 71%, respectively.84 The reactions of nitroalkanes RCH2CH(R0)NO2 with 4-ethoxybenzaldehyde carried out in the presence of 1,5-diazabi- cyclo[4.3.0]non-5-ene (DBN) give rise to the corresponding nitro alcohols 4-EtOC6H4CH(OH)C(R0)(NO2)CH2R (R, R0 : H, H; H, Me; Me, H).85 Nitro alcohols 96 resulting from the condensation of alde- hydes 97 with methyl 4-nitrobutyrate in the presence of 1,8- diazabicyclo[5.4.0]undec-7-ene (DBU) have been treated in situ with acetic anhydride in the presence of sulfuric acid; the corre- sponding acetates 98 were prepared in this way in 75% yield.86 The reaction of nitroalkenes MeCH(R)CH=CHNO2 (R = H, Me) with propionaldehyde in the presence of DBN is accom- panied by isomerisation of the double bond in the nitroalkene and gives unsaturated b-nitro alcohols EtCH(OH)CH(NO2) ± CH=C(Me)R (R=H, Me).54 To synthesise nitro alcohols 99, a mixture of nitroethane and aldehydes was treated with a mixed acetal of dimethylketene in the presence of [Rh(C5Me5)Cl]2(m-Cl)2 and then with tetrabutylam- monium fluoride.The reaction conditions and the product yields are listed in Table 7.87 c.Reactions of nitroalkanes with ketones Reactions of nitro-compounds with ketones have been much less studied than the nitroaldol condensation involving aldehydes. This is due to some features peculiar to the reactions with ketones. These reactions are often accompanied by spontaneous dehydra- tion of the adducts yielding nitroalkenes, which enter into the Michael reaction with the initial nitro-derivatives. Only few studies dealing with the reactions of nitro-com- pounds with ketones have been published over the last 25 years.The reaction of phenylnitromethane with acetone in the presence of sodium methoxide at 20 8C for 50 h gave the corresponding nitro alcohol, 2-methyl-1-nitro-1-phenylpropan- 2-ol, in a yield of only 10%.59 When nitromethane 1 was made to react with ethyl methyl ketone in the presence of sodium methoxide, the product of nitroaldol condensation, 2-methyl-1-nitrobutan-2-ol 100, was obtained.When this reaction was carried out in the presence of piperidine, 2-methyl-1-nitrobut-2-ene 101 was isolated from the reaction mixture. In the presence of sodium methoxide, 101 reacted with 1,3,5-trioxane being thus converted into nitro alcohol 102.56 The method for the synthesis of b-nitro alcohols from alde- hydes and nitroalkanes developed by Colvin et al.81, 82 and described above proved to be inapplicable to nitroaldol conden- sation between nitroalkanes and ketones.Therefore, a modified procedure for the synthesis of silyl ethers of b-nitro alcohols 103 from 1-nitrohexane and ketones RCOR0 was proposed.81 When trifluoroacetophenone was treated with nitroethane in the presence of an aqueous solution of potassium carbonate for 5 h, nitro alcohol 104 was formed.6 g-Nitro alcohol 105 was synthesised in 77% yield by con- densation of 1-nitropropane with benzophenone in the presence of butyllithium in THF and HMPT at low temperatures.79 O O Me Me C O H 92 +87 Bu4N+F7. 3H2O 0 8C, 3.5 h 93 Me NO2 OH CH Me Me O O CH2 C O O CH(CH2)2 (b). EtCH CHCH2CHCHR NO2 OH 95a,b 94 EtCH CH(CH2)2NO2+RCHO R=Me (a), CH2 R=Me, (CH2)2COOMe. 98 RCH CH(CH2)2COOMe NO2 OAc RCHO +O2N(CH2)3COOMe 97 MeCN DBU Ac2O H2SO4 96 RCHCH(CH2)2COOMe OH NO2 Me EtNO2+RCHO Me2C 2. Bu4N+F7 1. [Rh(C5Me5)Cl]2(m-Cl)2, C(OEt)OSiMe3 RCHCHNO2 . 99 OH (CH2O)3, MeONa MeNO2 NH MeONa EtCCH2NO2 100 MeCH CCH2NO2 Me 101 OH Me MeCEt O CCH(NO2)CH2OH.Me 102 MeCH 7 C5H11C C O7 R R0 NO2 a or b CCHC5H11 OSiMe3 R R0 NO2 103 C5H11CH2NO2 2 equiv. BuLi, HMPT, THF RCR0 O [C5H11C NO2]27 R=R0 =Ph, R,R0 =(CH2)5; (a) Me3SiCl; (b) 1. AcOH; 2. Me3SiCl7(Me3Si)2NH. PhCCF3+MeCH2NO2 K2CO3, H2O 20 8C, 5 h C CF3 OH Ph CHNO2 . Me 104 O EtCH2NO2+Ph2CO Ph2CCH(Me)CH2NO2 . OH 105 BuLi, THF, HMPT 790 to770 8C, 0.5 h Table 7.Reaction conditions and yields of the nitro alcohols 99. R Solvent Number of Yield (%) equivalents of EtNO2 Ph CH2Cl2 5 7 Ph THF 10 16 Ph THF 43 82 a PhCH=CH THF 43 48 PhCH2CH2 THF 43 54 a The ratio syn : anti=39 : 61. Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 43The reactions of 2-arylnitroethanes 76 with ketones in the presence of butyllithium also afford g-nitro alcohols 106 rather than the b-derivatives.77 d.Reactions of 1,1-dinitro- and 1,1,1-trinitro-derivatives with aldehydes In a study dealing with the kinetics and mechanism of the condensation of acetaldehyde with 1,1-dinitroethane or with methyl 2-nitropropionate in DMSO,88 it has been found that the addition of the anions derived from nitro-compounds to the carbonyl group of the aldehyde is the rate-determining step of this reaction.The researchers determined the equilibrium and rate constants of the process, which characterise the basicity and nucleophilicity of the nitronate anions. It was found that there exists a normal correlation between the basicity and nucleophi- licity of the two anions in DMSO: the more basic anion reacts faster than the less basic one.In the presence of alkali, 1,1- dinitroethane reacts with an aqueous solution of formaldehyde to give 2,2-dinitropropan-1-ol in 80% yield.89 Condensation of the potassium salt of 1,1-dinitroethane, formed upon treatment of 1-chloro-1-nitroethane with an aque- ous solution of potassium carbonate for 1 h at 25 ± 30 8C, with excess formaldehyde resulted in the formation of 2,2-dinitropro- pan-1-ol in 20%± 25% yield.90 2-Fluoro-2,2-dinitroethanol 107 was prepared by the reaction of chlorofluorodinitromethane with potassium iodide and form- aldehyde at 70 8C.91 2,2-Dinitro-3-vinyloxypropan-1-ol is formed in 19.2% yield when 2,2,2-trinitroethyl vinyl ether is treated successively with alkaline hydrogen peroxide and with formaldehyde in the pres- ence of sodium hydroxide, and the reaction mixture is then acidified by HCl to pH 2.92 The ether 109 containing an HC(NO2)2 group reacts readily and rapidly with formaldehyde in the presence of sodium hydrox- ide and is thus converted into hydroxymethyl derivative 110.93 Compound 111 containing two HC(NO2)2 groups reacts with two molecules of formaldehyde in the presence of sodium hydro- gencarbonate to give tetrol 112; under the action of a base, it eliminates a glyoxal molecule giving rise to salt 113.The latter reacts with the aldehyde 2 to give 2,2-dinitropropane-1,3-diol 114 in an overall yield of 74%.94 The compound 111 can also react with formaldehyde without a catalyst yielding the tetrol 112, which eliminates a dinitro- methane molecule and CH2O even at 20 ± 30 8C being thus converted into aldehyde 115, which cyclises in situ into tetrahy- drofuran derivative 116.94 When the dipotassium salt of 1,1,2,2-tetranitroethane 117 is made to react with formaldehyde, this gives tetranitrodiol 118, which is cleaved on treatment withKOHto give 2,2-dinitroethane salt. This salt reacts with the aldehyde 2 yielding diol 114.95 The sodium 95 or potassium 96 salts of dinitromethane react with glyoxal to give salts of 1,1,4,4-tetranitrobutane-2,3-diol 119.The salt 119 (Y = Na) is converted into the tetrol 112 by the reaction with formaldehyde.95 Treatment of the salts 119 (Y=Na or K) with 10% sulfuric acid 95 or hydrogen chloride 96 results in the formation of the diol 111 in a yield of 71.8% or 57.2%, respectively.Potassium salts of fluoro-containing dinitroalkanes 120 or tetranitroalkanes 121 react readily with formaldehyde to give the corresponding nitro alcohols 122 or tetranitro-diols 123.97 R, Ar (yield, %): Me, Ph (70); Ph, 4-ClC6H4 (67); (CH2)5 , Ph (69). OH ArCH2CH2NO2+R2CO 106 76 R2CCH(Ar)CH2NO2 BuLi 790 to778 8C, 0.5 h NO¡2 MeC NO2 MeC(NO2)2CH2OH.NO¡2 7Cl7 NO¡2 MeC Cl CH2O H+ NO¡2 MeC OH NO¡2 +MeCHO, MeCHNO2 Cl K2CO3, H2O 25730 8C, 1 h H2O 7HCl NO¡2 MeC Cl F(Cl)C(NO2)2 FC(NO2)2CH2OH. 107 KI, CH2O, EtOH, H2O 70 8C (a) H2O2, NaOH, H2O, MeOH,74 8C; (b) CH2O, H2O; (c) HCl, MeOH, 40 8C. a, b, c (O2N)3CCH2OCH CH2 CHOCH2C(NO2)2CH2OH 108 CH2 F(NO2)2CH2OCH2CH(NO2)2+CH2O NaOH, H2O 18725 8C, 20 min 109 2 FC(NO2)2CH2OCH2C(NO2)2CH2OH. 110 OH OH HOCH2C(NO2)2CHCHCH(NO2)2CH2OH 112 7OHCCHO OH HC(NO2)2CHCHCH(NO2)2+2CH2O 111 2 NaHCO3, H2O 075 8C, 4 h OH 2 7 HOCH2C(NO2)2 HOCH2C(NO2)2CH2OH. H2CO 114 113 30 8C, 15 min; 20 8C, 14 h 111+2CH2O 112 7CH2C(NO2)2,7CH2O HOCH2C(NO2)2CHC 115 116 OH O H HO HO NO2 NO2 O (O2N)2C 2K+ +2CH2O H2O, Et2O 117 C(NO2)2 27 118 HOCH2C(NO2)2C(NO2)2CH2OH KOH +7 KC(NO2)2CH2OH H2CO H2O 7OH7 HOCH2C(NO2)2CH2O7 114 . 7 2(O2N)2CH Y++ C C O H O H YOH, H2O H2SO4, H2O, Et2O, 2 8C or HCl (gas), 075 8C Y=Na, K 112 111 2CH2O, H2O, Et2O Y=Na 119 C(NO2)2 C(NO2)2 CHOH CHOH 27 2Y+ 44 M-G A ShvekhgeimerSynthesis of 2,2,2-trinitroethanol by the condensation of trinitromethane with formaldehyde in the presence 98 or in the absence 99 of CuSO4 has been reported. 2. Synthesis of nitro alcohols by introduction of a nitro group into hydroxyl-containing compounds This route for the preparation of nitro alcohols has not found wide application, because the two existing methods for the introduction of a nitro group into compounds containing a hydroxy group suffer from serious drawbacks restricting their use.One of these methods, namely, treatment of halo-substituted alcohols with some metal nitrites, normally permits preparation only of nitro alcohols containing a primary nitro group.In addition, there are some limitations as regards the use of the initial halohydrins and solvents. The second method, namely, nitration of alcohols, has also found limited use, because only in some cases does it permit the target compounds to be obtained in reasonable yields.The main reaction pathway in this case is the oxidation of alcohols yielding carboxylic acids and the corresponding nitroalkanes. During the last 25 years, the former method has hardly been used to synthesise aliphatic nitro alcohols. It was not until 1987 that the synthesis of 12-nitrododecan-1-ol (yield 79.5%) by the Meyer reaction of 12-iodododecan-1-ol was reported.100 Over the period of time considered, only in rare cases has nitration of alcohols been used to prepare nitro alcohols; for example, nitro alcohols 124 have been obtained in this way by treating alcohols 125 with dinitrogen tetroxide.101 The reaction of Ph2C(OH)Et withN2O3 orN2O4 at 20 ± 25 8C in chloroform affords nitro alcohol Ph2C(OH)CH(Me)NO2 in a yield of 47% or 62%.When the same alcohol is treated withN2O5 in chloroform or with HNO3 (20 8C, 1 h; 80 8C, 4 h), nitration involves not only the side chain but also the aromatic ring and the hydroxy group and results in the formation of O2NC6H4C(Ph)(O- NO2)CH(Me)NO2 (yields 35% and 70%, respectively).102 An original method for the synthesis of nitro alcohol 126 reported in a patent 103 consists of the transformation of butyne- 1,4-diol into 1,4-dihydroxybutan-2-one by the Kucherov reaction followed by replacement of the b-hydroxyl with respect to the carbonyl group by a fluorodinitromethyl group by treatment with fluorodinitromethane. 3. Synthesis of nitro alcohols by introduction of a hydroxy group into compounds containing a nitro group Syntheses of nitro alcohols by this route can be clearly divided into two groups: (a) formation of a hydroxy group in nitro-compounds from other oxygen-containing groups incorporated in the initial molecule and (b) the formation of nitro alcohols from nitro- compounds containing no oxygen-containing groups. a.The formation of a hydroxy group from other oxygen-containing groups present in nitro-compounds The reaction of unsaturated nitro ketones 127 (A = C=CH) or saturated nitro ketones (A = CHCH2) with lithium aluminium hydride at 20 8C and at a LiAlH4 : 127 ratio of 1 : 4 involves only transformation of the carbonyl group into a hydroxy group and affords nitro alcohols 128 (yields 64%± 95%).104 When nitro ketones 129 containing an epoxy group are reduced with lithium aluminium hydride, the structure of the reaction product depends on the LiAlH4 : 129 ratio.When this ratio is 1 : 4, only the carbonyl group is reduced, and nitro alcohols 130 are formed (yields 67% and 74%); at a ratio of 1 : 2, cleavage of the epoxide ring occurs, in addition to the reduction of the carbonyl group, and the process gives nitro-diols 131.104 Unsaturated nitro alcohols 132 have been synthesised in 62%± 68% yields by the reduction of nitro ketones 133 with lithium aluminium hydride at 715 8C and at an LiAlH4 : 132 ratio of 1 : 4.105 Ketone 134 is converted into nitro alcohol 135 in 71% yield on treatment with sodium tetrahydridoborate.32 R=C6F13 (36%); C10F21 (62%); n=4 (55%), 6 (75%).RCH2C(NO2)¡2 K++CH2O 120 RCH2(NO2)2CH2OH 122 2 H2O 20 8C, 2 h HOCH2CCH2(CF2)nCH2CCH2OH NO2 NO2 123 NO2 NO2 + 7 7+ 2 121 H2O 20 8C, 2 h NO2 NO2 K CCH2(CF2)nCH2C K+2CH2O NO2 NO2 I(CH2)12OH+AgNO2 O2N(CH2)12OH.Et2O, in the dark 20 8C, 18 h R=H: R0=Me, Et, Bu; R =R0 =Me. 125 124 (53% ± 62%) N2O4, ClCH2CH2Cl 710 8C, 1 h; 22 8C, 12 h Ph2CH OH CRR0 NO2 Ph2CHCHRR0 OH HOCH2CH2CCH2OH O F(NO2)2CH, N2 HOCH2C CCH2OH+H2O HgSO4, H2SO4 50 8C, 1 h FC(NO2)2CH2CH2CCH2OH. 126 O Me2AC(CH2)2CRR0 127 Me2ACH(CH2)2CRR0 OH 128 O NO2 LiAlH4, Et2O 20 8C, 1 h NO2 R=H, Me; R0=H, Me, Et, Pr; A =C CH, CHCH2.O CHC(CH2)2C(R)Me Me2C O 129 NO2 R=H, Me. LiAlH4, Et2O 20 8C, 1 h 131 (62%) Me2CHCH2CH(CH2)2C(R)Me OH OH NO2 O CHCH(CH2)2C(R)Me Me2C OH 130 (61%) NO2 R=H, Me, Cl, Br. 4-RC6H4CHCH 132 OH O2N HC 715 8C, 1.5 h LiAlH4, Et2O 133 4-RC6H4CCH O O2N HC Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 45The outcome of the reduction of esters 136 with sodium tetrahydridoborate depends on the nature of the radical R; if R=PhCH2, the reaction gives rise to the nitro alcohol 107 (yield 70%), whereas with R=Me2CH, hemiacetal 137 is formed.106 When cyclic a-nitro ketones 138 are treated with sodium tetrahydridoborate, the reduction is accompanied by ring opening and yields nitro alcohols 139.107 3-Nitropropanal was converted into 3-nitropropan-1-ol in 89% yield by treatment with the complex BH3 .SMe2 in Et2O at 20 8C for 4 h under a nitrogen atmosphere.108 Nitro alcohol 140 was synthesised in 77.5% yield by the reduction of 5-nitropentan-2-one with diborane in THF.31 The reaction of tetrahydrofuran derivatives 141 with borane in THF results in ring opening and formation of diols 142.109, 110 Nitro ketone 143 was converted into nitro alcohol 144 in 72% yield on treatment with SnCl4 and Et3SiH at 0 8C.66 The reduction of the compounds RCOCH(R0)(CH2)2NO2 (R, R0 = Me, H;111 Ph, H; MeO, 2-MeOC6H4 112) in the presence of baker's yeast occurs stereoselectively to give (S)-nitro alcohols as the major products; the highest stereoselectivity (99%) is observed in the case of R=MeO, R0 =2-MeOC6H4.111, 112 When epoxide 145 (R = Me) is heated in water at 50 8C, the corresponding diol MeCH(OH)CH(OH)CH2NO2 is formed in 56% yield.113 Heating of 145 in aliphatic alcohols leads to ethers 146 (yields 55%± 68%).113 In the presence of triethylamine at 18 ± 20 8C, the epoxide 145 (R=Me) isomerises into the unsaturated nitro alcohol MeCH(OH)CH=CHNO2 (yield 62%).113 2,3-Epoxy-1-nitropropane reacts with hydrazines RC6H4NHNH2 (R=2-NO2, 4-NO2) at 20 8C in aqueous ethanol affording 2- or 4-O2NC6H4NHNHCH(CH2OH) ±CH2NO2 in 86% yield.114 Epoxides 147 with a more complicated structure have been converted into mixtures of unsaturated nitro alcohols 148 and 149 in four ways: by treatment with triethylamine in aqueous acetoni- trile (procedure A); in the presence of alumina in diethyl ether and in dichloromethane (procedure B); at 25 8C over silica gel (proce- dure C); over aluminium isopropoxide (procedure D) (Scheme 2).115 The reaction conditions and the yields of the alcohols 148 and 149 are listed in Table 8.Scheme 2 On treatment with hydrohalic acids, 3-fluoro-3-nitrooxetane 150 is converted into nitro alcohols 151 in high yields (96.5% and 93.1%), whereas on treatment with phosphorus trifluoride, it is converted into polymer 152 (yield 82.6%).49 O2NCF2CCF3 134 O O2NCF2CHCF3 .Et2O. 135 OH 5720 8C, 15 min NaBH4, Et2O FC(NO2)2CH2OH 107 FC(NO2)2CHOH OCHMe2 137 a R=PhCH2 b R=Me2CH F2C(NO2)COOR 136 (a) NaBH4, THF, H2O, 10712 8C, 0.5 h; (b) NaBH4, (MeOCH2)2, 20725 8C, 0.5 h.O NO2 R 138 (CH2)n R=H, n=0710 (50%785%); R=But, n=1 (75%). HO(CH2)3CH(CH2)nCH2NO2 139 R NaBH4, MeCN H2O, 0 8C, 4 h O2N(CH2)3CMe O2N(CH2)3CHMe. O 140 BH3 .THF 20 8C, 172 h OH R1=NO2, R2=H, R3=OH (35 8C, 72 h, 98%);109 R1=F, R2=H, R3=OH (25 8C, 24 h, 100%);109 R1=NO2, R2, R3=O (35 8C, 72 h, 98%).110 O R2 R3 R1(NO2)2C HOCCH2CHCH2OH R2 R3 142 141 BH3 .THF, N2 C(NO2)2R1 C(CH2)2CMe NO2 F3C Me 143 O C(CH2)2CHMe. NO2 F3C Me 144 OH SnCl4, Et3SiH CH2Cl2, 0 8C, 2 h R=H; R0 =Me, Et, Pri; R=Me, R0 =Me, Pri. 145 O CHCH2NO2+R0OH RCH refluxing RCHCH(OR0)CH2NO2 OH 146 CCHR0 OH RCH 149 CH2NO2 148 RCH2CCHR0 + OH CHNO2 O CHR0 RCH2C CH2NO2 147 HX, H2O 20 8C, 0.571 h X=Cl, Br. PF3, CH2Cl2 O F NO2 150 XCH2CCH2OH F NO2 151 HO CH2CCH2O H n 152 F NO2 Table 8.Reaction conditions and yields of the nitro alcohols 148 and 149. R R0 Time/ h Yield of (148+149) (%) Yield of 148 (%) E/Z Ratio in 148 A B C D A B C D A B C D A B C D H Me 2 2 2 7 97 92 92 7 16 15 0 7 7 7 78/22 7 H Et 2 4 4 7 82 96 96 7 15 11 0 7 7 7 78/22 7 H C5H11 2 4 5 7 89 82 84 7 33 18 0 7 7 7 80/20 7 H Pri 2 3 3 7 90 87 83 7 17 15 0 7 7 7 84/26 7 Me Me 7 3 2 7 7 85 85 7 7 9 0 7 7 7 65/35 7 Et Et 7 4 48 1 7 86 0 80 7 7 7 traces 7 7 7 70/30 Bu Bu 7 5 48 1 7 80 0 83 7 2 7 traces 7 83/17 7 71/29 46 M-G A ShvekhgeimerKetone 153 has been treated with ketene diethyl acetal at 730 8C; this gave compound 154 (yield 88%). Acid 155 was synthesised by reaction of the ketone 153 with malonic acid in the presence of pyridine.This acid was converted in situ into ester 156 (yield 48%) by reaction with ethanol in the presence of sulfuric acid.116 When the trinitromethane derivative Hg[C(NO2)3]2 is refluxed with an aqueous solution of formaldehyde, the HgC(NO2)3 group is substituted resulting in the formation of 2,2,2-trinitroethanol in 95% yield.117 On heating with formalin at 60 ± 70 8C for 20 h, compounds of the formula (O2N)3CCH(R)CH2HgC(NO2)3 (R=H, Me) eliminate an alkene and are thus converted into 2,2-dinitropro- pane-1,3-diol 114 in 91% yield.117 b.Formation of a hydroxy group in nitro compounds containing no oxygen-containing groups The rates of hydration of CH2=CHNO2, MeC(NO2)=CH2, MeCH=CHNO2 and MeC(NO2)=CHMe have been studied by spectrophotometry at 40 8C and pH 6 in an AcOH±Me2CO buffer solution.The effective pseudo-first-order rate constants for these reactions were found to be (78.82.2)1073, (6.50.1)1073, (57.81.0)1073 and (5.70.15)1073 min71, respectively. The rate of formation of the corresponding nitro alcohol depends on the charge on the b-carbon atom in the p-system and also on the steric effect of the b-substituent.118 Having studied the kinetics of hydration of nitroethylene in buffer solutions by spectrophotometry, Fakhrutdinov et al.119 proposed the following scheme for this process: It was found that the rate of hydration increases with an increase in the pH, and at a constant pH it increases with an increase in the concentration of the base in the solution.At low pH, the reaction rate is limited by deprotonation of CH2(áOH2) ¡CHNO2, whereas at high pH, it is limited by the interaction of the nitroalkene with water.It was found that at pH = 0.96, 2.01, 2.98, 6.00 and 7.15, the activation energies of hydration are 10.9, 14.1, 15.2, 13.5 and 11.1 kcal mol71, respectively, and the corresponding activation entropies are 743.0, 730.1, 729.2, 726.4 and735.4 e.u., respectively.119 The kinetics and mechanism of hydration of 1-nitroprop-1- ene have been studied.120 The researchers proposed the following scheme for this process: At low pH and a low buffer capacity, the reaction rate is limited by proton abstraction, and when these parameters are increased, the interaction of the nitroalkene with water becomes the rate-determining step of the process.120 g-Nitro alcohols 157 have been synthesised by radical addition of aliphatic alcohols to nitroalkenes 158 in the presence of tert- butyl peroxide at 150 8C.121 Treatment of 3-fluoro-3,3-dinitropropylammonium sulfate with sodium nitrite in an acid medium yields 3-fluoro-3,3-dini- tropropan-1-ol in 56% yield.122 When tetranitroalkane 159 reacts with potassium iodide in methanol, in parallel with the replacement of one of the nitro groups by potassium, replacement of one more nitro group by a hydroxy group occurs, and the reaction affords the nitro alcohol salt 160.123 Unexpectedly, the reaction of an excess of nitroalkanes 161 with methyl bromoacetate in the presence of sodium methoxide in dimethylacetamide resulted in the formation of compounds 162.124 Evidently, the compounds 162 result from the condensation of nitroalkanes RR0CHNO2 with methyl glyoxylate generated according to the following scheme:124 4.Synthesis of nitro alcohols by simultaneous introduction of nitro and hydroxy groups Reactions of alkenes with nitrogen oxides are used most often for the synthesis of nitro alcohols with simultaneous introduction of NO2 and OH groups to vicinal positions.O2NCF2CCH2COEt 156 CF3 OH O O2NCF2CCF3 O 153 O2NCF2CCH C(OEt)2 154 CF3 OH refluxing for 25 h O2NCF2CCH2COH 155 CF3 OH O EtOH, H2SO4 CH2(COOH)2 C5H5N 730 8C, 2 h 20 8C, 0.5 h CH2 C(OEt)2 + 7 CH2(OH2)CHNO2+B 7 CH2(OH)CHNO2+BH+, HOCH2CH2NO2+B. 7 CH2(OH)CHNO2+BH+ CH2 CHNO2+H2O 7 + CH2(OH2)CHNO2 , MeCH CHNO2+H2O k1 k71 + 7 MeCHCHNO2 , OH2 + 7 MeCHCHNO2+B OH2 k2 k72 7 MeCHCHNO2+BH+, OH k3 k73 7 MeCHCHNO2+BH+ OH MeCHCH2NO2+B.OH R0 =Me: R=C5H11 (28%), Pr (11%), Bu (20%); R0 =Et: R=Pr (6%). RCHCH(R0)CH2NO2 OH 157 RCH2OH+O2NCH CHR0 158 (ButO)2 150 8C, 12715 h FC(NO2)2CH2CH2NH2 .H2SO4 NaNO2, H2O (pH 1.271.5) 60 8C, 1.5 h FC(NO2)2CH2CH2OH. 7 + KC(NO2)2CH2CCH2COH. 160 Me Me Me Me (O2N)3CCH2CCH2CNO2 159 Me Me Me Me KI, MeOH 20725 8C, 2 days R=Me: R0 =Me (48%), Et (52%); R=H: R0=Et (11%), Ph (68%). 161 RR0CHNO2+BrCH2COOMe RR0CCHCOOMe NO2 OH 162 MeONa 20 8C, 12 h + + 7 7 7 RR0C N O ONa BrCH2COOMe + RR0C N O OCH2COOMe RR0C NOH+HCCOOMe.O Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 47The nitro alcohol C12H25CH(OH)CH2NO2 (yield 10%) and its nitrate C12H25CH(ONO2)CH2NO2 (yield 86%) have been prepared by the reaction of C12H25CH=CH2 with N2O4 in CCl4 in the presence of sulfuric acid at 0 ± 5 8C for 0.5 h.125 The nitro alcohol Ph2C(OH)CH(Me)NO2 has been synthes- ised by the reactions of the alkene Ph2C=CHMe with N2O4 (CHCl3, 20 ± 25 8C, 6 h), N2O3 (CHCl3, 20 8C, 17 h) or HNO3 (d 1.51, CHCl3, 100 8C, 4 h), the yields of the product being 56%, 46% or 50.8%, respectively.103 When the same alkene was treated with N2O5 (CHCl3, 18 8C, 12 h), the product O2NC6H4 ± (Ph)C(ONO2)CH(Me)NO2 containing a nitro group instead of a hydrogen atom in one of the benzene rings was obtained in 32.2% yield.102 The reactions of various alkenes with NO2 in hexane at 20 8C have been studied in detail.Nitro alcohols and their derivatives 163 ± 170 were synthesised in this way.126 The formation of the compounds 165, 166 and 168 was explained 126 in terms of the following scheme: Diethyl vinylphosphonate 171 readily reacts with N2O4 in dichloromethane to give nitro alcohol 172 (yield 34%).127 2-Hydroxy-3-nitropropionic acid was synthesised in 60% yield by the reaction of acrylic acid with N2O4 in dichloroethane at 18 ± 20 8C over a period of 10 h.128 Nitro alcohol 173 is formed in 23% ± 25% yield in the reaction of nitrogen oxide with 4-phenylbut-1-ene in various solvents (dichloroethane, tetrachloromethane or benzene) at 20 8C.129 Simultaneous introduction of NO2 and OH groups into alkenes 174 was accomplished by treating them with alkyl nitrites with heating in acetic acid under an inert atmosphere; the yields of nitro alcohols 175 were 20%± 33%.130 The synthesis of compound 176 (yield 55%) from alkene 177 was carried out using acetyl nitrate formed in situ from Ac2O and HNO3.131 The reaction of isoprene with acetyl nitrate at 20 8C affords a mixture of nitroalkenyl acetates 178 and 179 in a ratio of 7 : 3.132 Treatment of unsaturated ketones 180 with nitronium tetra- fluoroborate at 730 8C in a solution in SO2 followed by treat- ment with water gives compounds 181 in low yields.133 An original method has been proposed 134 for the synthesis of dichloro-substituted nitro alcohols 182.It is based on the reaction of trichloronitromethane with aldehydes in the presence of SnCl2. Me2CC(ONO)Me2+ NO2 163 (17%) 20 8C, 5 min NO2 . +Me2CC(ONO2)Me2+O2NCH2C(Me)C(NO2)Me2 , 164 (3%) 165 (3%) NO2 OH 20 8C, 5 min NO2 .CHEt Me2C CMe2 Me2C 167 (36%) O2NCH2C(Me)CHEt+Me2CCH(OH)Et , 166 (8%) OH NO2 NO2 CHBu CH2 170 (48%) O2NCH2CHBu . 20 8C, 5 min NO2 . CEt2 CH2 20 8C, 5 min NO2 . 169 (47%) OH 168 (4%) O2NCH2C(Et)CHMe +O2NCH2CEt2 , OH NO2 OH C C CH2R NO2 C C CH2 NO2 R NO2 . . C C CH NO2 R NO2 C C CH NO2 R NO2 NO2 . . C C CH NO2 R NO2 ONO C C CH NO2 R. NO2 OH (EtO)2PCHCH2NO2 . 172 N2O4, CH2Cl2 0720 8C, 3 h (EtO)2PCH CH2 171 O O OH Ph(CH2)2CH CH2 Ph(CH2)2CHCH2NO2 . OH 173 NO 20 8C R1R2C CHR3 174 R1, R2, R3: Ph, Ph, H; Me, Ph, H; Me, COOMe, H; Et, Et, Me. AlkONO, AcOH, N2 115 8C, 30750 min R1R2C 175 CHR3 OH NO2 1. Ac2O, HNO3, 10 8C, 1 h 2. H2O, 20 8C, 12 h 177 O2N NO2 N N N CH2C CHMe Me 176 O2N NO2 N N N CH2CCH(Me)NO2 . OH Me CH2 C CH CH2 Me AcONO2 20 8C, 1 h O2NCH2C CHCH2OAc+O2NCH2CCH CH2 . 178 179 Me Me OAc R=Me (4%), Pr (17%), Pri (34%). 180 + 1. NO2BF¡4 , SO2, CH2Cl2730 8C, N2 2. H2O RC(CH2)2C CH2 Me O RC(CH2)2CCH2NO2 181 OH Me Cl3CNO2 SnCl2, Et2O 0 8C ClCNO2 SnCl3 Cl RCHO 4 h 48 M-G A ShvekhgeimerThe reaction of ethyl 4-methylpent-2-enoate 183 with nitryl chloride or nitrosyl chloride yields nitrite 184, which is converted into compound 185 on refluxing with water, the overall yield of the latter being 34%.74 III.Chemical transformations of nitro alcohols 1. Reactions involving the nitro group The reduction of 2-nitrobutan-1-ol in the presence of Raney nickel at 35 ± 40 8C under a hydrogen pressure of 10 atm results in the formation of 2-aminobutan-1-ol in 62%± 74% yield.44, 135 The same catalyst is effective in the reduction of nitro alcohols 186 to amino alcohols 187 (Table 9).66 Raney nickel has also been used to reduce the compounds PhCH(OH)(CH2)2NO2 and Ph2CHCH(OH)CH2NO2 to amino alcohols PhCH(OH)(CH2)2NH2 (yield 49%) and Ph2CH± CH(OH)CH2NH2 (yield 18%).79 The reduction of the erythro-isomers of silyl ethers of nitro alcohols in the presence of Raney nickel occurs stereoselectively; treatment of the reaction mixture with Bu4N+F7 gives the erythro-isomers of amino alcohols 188.82 2-Hydroxymethyl-2-nitropropane-1,3-diol 4 is converted into the amine H2NC(CH2OH)3 (yield 89%) by hydrogenation in the presence of the Pd/C catalyst at 59 ± 63 8C for 4.5 h.136 When 4-MeOC6H4CH2C(OH)(Et)NO2 is reduced with ammonium formate in the presence of Pd/C in THF and MeOH, the amino alcohol 4-MeOC6H4CH2C(OH)(Et)NH2 is formed in 73% yield.137 The amino-diols HOCH2C(R)(NH2)CH(OH)C13H27 (R = H, Me) have been synthesised in high yields (83% and 86%) by the reduction of the corresponding nitro-diols with ammonium for- mate in the presence of Pd/C in MeOH at 20 8C for 16 h under an atmosphere of nitrogen.61 An active catalyst of reduction, Ni2B, which is formed in situ from NiCl2 . 6H2O and NaBH4, has been used to convert the nitro alcohol O2NCMe2(CH2)2CH(OH)Me into the corresponding amino alcohol (yield 76%). The reaction occurs in MeOH at 20 8C over a period of 5 min.138 Reduction of nitro alcohols to amino alcohols with hydrogen has also been carried out in the presence of a number of rhodium- containing catalysts. For example, 2-nitrobutan-1-ol has been converted into 2-aminobutan-1-ol by hydrogenation in the pres- ence of di(rhodiumnorbornadienechloride) and Ph2PCH2CH(PPh2)Me at 60 8C under a hydrogen pressure of 67 atm for 20 h.139 The reduction of 2-nitrobutan-1-ol to 2-aminobutan-1-ol catalysed by rhodium-, iridium- and palladium-containing sys- tems has been studied in detail (Table 10).140 Nitro glycols 189 have been hydrogenated at 80 ± 100 atm in the presence of Ru/C catalyst or Raney Ni ± Ru for 2 ± 5 h; this gave amino glycols 190 in 78%± 96% yields.141 Hydrogenation of the nitro alcohols 4-RC6H4CH(OH). .CH2NO2 (R = H, Me, Cl) in the presence of PdO2 in 80% AcOH at 1 atm gives rise to the corresponding amino alcohols RC6H4CH(OH)CH2NH2 in 59%, 50% and 63% yields, respec- tively.26 The nitro groups in nitro alcohols and their derivatives 191 and 192 have been reduced to amino groups using LiAlH4.23, 25, 81, 83, 142 R=H (67%), Me (76%), Et (82%), Pr (83%), Bu (92%), C5H11 (85%), Ph (52%), CCl3 (77%), 4-MeC6H4 (57%), 4-ClC6H4 (59%), 2-furyl (53%), 2-thienyl (61%). O2NC CHR OSnCl3 Cl Cl H3O+ O2NC CHR OH 182 Cl Cl Me2CHCHCHCOOEt NO2 ONO Me2CHCHCHCOOEt .NO2 OH 185 184 D, 5.5 h H2O CHCOOEt 183 Me2CHCH NO2Cl, PhH, 0 8C, 5 h; 20 8C, 3 days or NOCl, PhH, 0 8C, 3 h; 20 8C, 5 days H2, Raney Ni, EtOH 24 h F3CC(R)CHR0. 187 OH NH2 F3CC(R)CHR0 186 OH NO2 R, R0: Pr, Et; Pri, Et; C5H11, Me; C5H11, Et; Pr, C7H15; C10H21, Me; C9H19, Me; Ph, Et. RCHCHR0 OSiMe2But NO2 188 RCHCHR0 OH NH2 1. H2, Raney Ni 2. Bu4N+F7 EtCHCH2OH EtCHCH2OH. NH2 H2, Cat., EtOH 65 atm NO2 R=H, Me, Et, CH2OH.HOCH2CCH2OH 189 R NO2 H2, Ru/C or (Raney Ni) ±Ru 275 h HOCH2CCH2OH 190 R NH2 Table 9. Reaction conditions and yields of the amino alcohols 187. R R0 Pressure /atm Temperature /8C Yield (%) H Pr 1 20 52 H C6H13 25 50 76 H Ph(CH2)2 1 20 79 H PhCHMe 25 50 82 H Pr 30 50 59 Me Pr 30 50 5 Me C6H13 25 50 22 Me Ph(CH2)2 25 50 53 Table 10. Hydrogenation of 2-nitrobutan-1-ol.140 Catalyst Phosphine Catalyst: Time/h Tempera- Yield phosphine ture/ 8 (%) Rh2Cl2(NBD)2 a see b 1 : 1 20 60 78 Rh2Cl2(NBD)2 PPh3 1 : 1 16 50 36 Rh2Cl2(NBD)2 DIOPc 1 : 1 65 80 57 Ir2Cl2(COD)2 d PPFA e 1 : 2 16 50 67 Ir2Cl2(COD)2 BPPFA f 1 : 2 16 50 56 Ir2Cl2(COD)2 DIOP 1 : 2 16 50 54 Ir2Cl2(COD)2 DIPHOS g 1 : 2 16 50 47 PdCl2(COD)2 DIPHOS 1 : 2 18 75 24 PdCl2(PhCN)2 BPPFOH h 1 : 1 18 75 41 a NBD�norbornadiene; b Ph2PCH2CH(Me)PPh2; c DIOP�(S,S)-2,3- O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane; d COD � cyclooctadiene; e PPFA � (R)-a-[(S)-2-(diphenylphosphino)- ferrocenyl]ethyldimethylamine; f BPPFA � (R)-a-[(S)-2,10-bis(diphenyl- phosphino)ferrocenyl]ethyldimethylamine; g DIPHOS � bis(1,2-di- phenylphosphino)ethane; g BPPFOH � (R)-a-[(S)-2,10-bis(diphenyl- phosphino)ferrocenyl]ethane.Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 49When unsaturated nitro ketones 193 are reduced with a twofold excess of lithium aluminium hydride, the nitro groups are converted into amino groups, whereas the carbonyl groups are reduced to hydroxy groups; thus, the reaction yields amino alcohols 194.104 Nitro-diols 195 have been converted into amino-diols 196 by treatment with zinc and hydrochloric acid in ethanol.143 Electroreduction of 2-methyl-2-nitropropane-1,3-diol giving 2-amino-2-methypropane-1,3-diol in 99% yield has been patented.144 Nitro alcohols or nitro-diols 197 have been converted into the corresponding amino-derivatives 198 by electroreduction at bimetallic electrodes consisting of copper deposited on zinc or cadmium.145 Electroreduction of the triol 4 in the presence of sulfuric acid at 75 ± 80 8C and at a current density of 0.1 A cm72 for 12 ± 13 h affords amino-derivative H2NC(CH2OH)3 in 40%± 50% yield.6 The hydrochloride (HOCH2)3CNH2 .HCl has been synthes- ised by electroreduction of the triol 4 at a perforated palladium cathode at 75 ± 80 8C in the presence of 6.5% HCl.146 Electroreduct3-hydroxy-2-nitrobutyrate giving rise to the DL-threonine ester 199 (substance yield 72%± 77%; current yield 7%± 10%) can be described by the following scheme:147, 148 Treatment of nitro alcohol acetates 200 with tributylstannane in the presence of azobis(isobutyronitrile) in boiling benzene results in the replacement of the nitro group by a hydrogen atom.48 R R0 Yield (%) PrCO Et 74 PriCO Et 75 C5H11CO Me 83 C6H13CO Me 84 EtCO C5H11 83 C6H13CO (CH2)2COMe 85 C6H13CO (CH2)2COOMe 80 COOMe (CH2)2CH(OAc)Me 88 COOMe (CH2)2COOEt 83 The reduction of the nitro group in an acetate of a more complicated structure occurs under the same conditions.48 The researchers cited 48 proposed yet another method for the introduction of a hydrogen atom instead of the nitro group in nitro alcohol acetates 201.The method involves treatment of 201 with 1,8-diazabicyclo[5.4.0]undec-7-ene in boiling benzene. 2. Reactions involving hydrogen atoms attached to a carbon atom carrying nitro groups Nitro alcohols containing one or two hydrogen atoms at the carbon atom carrying the nitro group are able to enter into nitroaldol condensation.The reaction of the salt [7O2N=CHCH(OH)CH(OH)CH=NO72 ] 2Na+ with formalde- hyde at 0 ± 5 8C and pH=10 in Et2O gives the tetrol HOCH2C(- NO2)2CH(OH)CH(OH)C(NO2)2CH2OH.95 The reactions of salts derived from 2-nitropropane-1,3-diol 3 or from 2-nitropropan-1-ol with an aqueous solution of form- aldehyde have been studied.149 Reversible reactions give 2- hydroxymethyl-2-nitropropane-1,3-diol 4 or 2-methyl-2-nitro- propane-1,3-diol, respectively.R1CHCH(R3)NO2 191 OR2 LiAlH4, Et2O, N2 R1CHCH(R3)NH2 OR2 R1=Ph, R2=H, R3=H (55%);23 R1=Ph, R2=H, R3=Me;23 R1=PhCH2 , R2=H, R3=H (28%);25 MeO R1= , O R2= , R3=H (80%);142 O R1CHC(NO2)R3R4 192 OR2 1. LiAlH4, Et2O, 36 8C, 5 h 2. Na2SO4, H2O R1CHC(NH2)R3R4 (50%785%) OH R2=SiMe3 or SiMe2But; R3=H, Me; R4=C5H11, Me.81, 83 R1=C6H13, But, Ph, 4-MeOC6H4, 4-O2NC6H4; R=H (35%), Me (37%), Et (38%).CHCH(CH2)2CHR OH 194 Me2C CHC(CH2)2CHR O 193 NO2 LiAlH4, Et2O 20 8C, 1 h Me2C NH2 R=C7H15 (25%), C9H19 (33%), C11H23 (38%), C13H27 (40%). 196 RCHCHCH2OH NH2 OH 195 RCH(OH)CHCH2OH NO2 Zn, HCl, H2O, EtOH R1, R2: H, Me; Me, CH2OH; H, Et; Et, CH2OH. R1R2C(NO2)CH2OH 197 R1R2C(NH2)CH2OH 198 CHCOOMe NO2 MeCH OH 2H+, 2e7 CHCOOMe NO MeCH OH 2H+, 2e7 CHCOOMe N HOH MeCH OH 2H+, 2e7 CHCOOMe NH2 199 MeCH OH 4H+, 4e7 NOH CCOOMe MeCH OH RR0C(NO2)CH2OAc 200 Bu3SnH, AIBN, PhH 80 8C, 2 h RR0CHCH2OAc EtOOC(CH2)2C(NO2)(COOMe)CH2OAc Bu3SnH, AIBN, PhH 80 8C, 2 h EtOOC(CH2)2CH(COOMe)CH2OAc.(83%) R=Me (66%), Et (71%), Pr (88%), Pri (83%), Bu (78%). RCCOOEt CH2OAc NO2 201 DBU, PhH 80 8C, 5 h RCHCOOEt CH2OAc HC(CH2OH)2NO¡2 +CH2(OH)2 K1 O2NC(CH2OH)3+OH¡ 4 MeC(CH2OH)2NO¡2 +CH2(OH)2 K2 HOCH2C(NO2)CH2OH+OH7. Me 50 M-G A ShvekhgeimerThe equilibrium constants K1 and K2 are (4.30.4)61073 and (3.60.2)61072, respectively.The characteristics of these reac- tions are retained over a broad range of concentrations: [CH2(OH)2]=0.058 ± 3.06 M and [OH7]=1.81072 ± 0.36 M.In the presence of Dowex-1 anion-exchange resin, the con- densation of nitro alcohols with formaldehyde or acetaldehyde at 20 8C is completed over a period of 1 ± 2 h.150 2-Nitroethanol 202 reacts with aliphatic aldehydes in the presence of sodium methoxide to give salts 203 (yields 67%± 99%), which are converted into nitro-diols 204 (yields 91%± 98%) on treatment with hydrochloric acid.143 Alumina (Brockmann activity I) has been used as the catalyst in the condensation of nitro alcohols 205 with aldehydes.69 The reaction of the nitro alcohol 202 with benzaldehyde in the presence of triethylamine at 0 8C is completed over a period of 23 h resulting in the formation of DL-threo-2-nitro-3-phenylpro- pane-1,3-diol (yield 98.7%).151 The reaction of furfural with 1-nitropropan-2-ol in the pres- ence of triethylamine gave the corresponding nitro-diol 206 in a yield of only 35%.152 The condensation of the nitro alcohol 202 with hexadec-2-enal in the presence of K2CO3 or Et3N at 20 8C affords nitro-diol 207 and a minor quantity of the product of its cyclisation 208.153 When this reaction is carried out in methanol in the presence of K2CO3 at 20 8C, the tetrahydropyran derivative 208 (yield 50%) is formed together with compound 209 (yield 12%) (see Scheme 3).153 On treatment with butyllithium, compounds 210 were con- verted into the corresponding lithio-derivatives. Condensation of the latter with carbonyl compounds R1R2C=O followed by hydrolysis of the condensation products gave nitro-diols 211 in 52%± 88% yields.154 A single diastereoisomer of nitro-tetrol 212 was obtained in a similar way from compound 213 and benzaldehyde (yield 73%).154 Aldehydes have been made to condense with 4-nitrobutan-2- ol in the presence of the Amberlyst A-21 ion-exchange resin at 0 8C for 15 h.This resulted in the formation of nitro-diols 214 (yields 45%± 70%).155 Dinitro-diols 215 have been prepared in two stages, namely, by the reaction of salts 216 with nitroalkenes 217 and subsequent treatment of the resulting reaction mixtures with formaldehyde in the presence of phosphorous acid at pH 6 ± 5.5.156 The reaction of 1-nitropropane with formaldehyde in the presence of Ca(OH)2 in water at 30 8C for 1 h followed by treatment of the reaction mixture with bromine in CCl4 at 20 8C gives rise to 2-bromo-2-nitrobutan-1-ol in 90% yield.35 R1=R2=Me, R3=H (63%); R1=Bu, R2=H, R3=Me (79%); R1=Me, R2=H, R3=H (91%).R1 CH(OH)CH2NO2 R2 R3CHO R1 C(OH)CHCHOH R2 NO2 R3 O2NCH2CH2OH+RCHO 202 MeONa, MeOH 6724 h R=C7H15, C9H19, C11H23, C13H27. RCHCH(NO2)CH2OH. OH 204 HCl, H2O 7 C CH2OH 203 RCH OH + NO2Na R, R0: Me, MeCH(OH)CH2; Et, (CH2)6OH; Et, CH2OH.RCHO +R0CH2NO2 205 Al2O3 20 8C, 24 h RCH(OH)CH(NO2)R0 CHCH(NO2)CHMe . OH OH 206 +MeCHCH2NO2 OH Et3N O O C H O O OCH2CHR3 NO2 210 C6H14 BuLi 2. H3O+ 1. R1R2C O R1R2C CCH2OH R3 NO2 211 OH O OCH2CLi NO2 R3 R1, R2, R3: H, Pri, H; H, Br, H; H, , H; trans-PhCH CH, H, H; H, C13H27C C, H; H, Ph, H; H, 4-MeOC6H4, H; Me, Ph, H; H, (S)-MeCH2CH(Me), H; H, Pri, Me; H, 4-CNC6H4, H; H, , Me; H, Ph, Me; R1,R2=(CH2)5, R3=H, Me.(a) BuLi, THF, HMPTA790 to740 8C, 3 h; (b) PhCHO,790 to760 8C, 1.5 h; (c) AcONa, H2O. HOCH2CHCHCH(NO2)CHPh OH OH OH 212 O O CH Me Me O O 213 CH2NO2 a, b, c RCHCH(NO2)CH2CHMe OH OH 214 RCHO +O2NCH2CH2CHMe OH R=Me, Pri, C5H11, C10H21, , Ph(CH2)2, (Z)-Me(CH2)7CH CH(CH2)7, Me(CH2)4CH(NO2)(CH2)2. NO2 R R=Me: R0 =Me (45%), Et (42%); R=R0 =Et (31%). 216 217 HOCH2C NO¡2 Na++R0C 2.CH2O, H2O, H3PO3 CH2 1. MeOH, H2O, 072 8C, 1 h; HOCH2CCH2CCH2OH NO2 R 215 NO2 R0 Scheme 3 O2N(CH2)2OH+ K2CO3(Et3N) Ar, 20 8C K2CO3, MeOH 20 8C, 19 h Me(CH2)12CH CHCHO Me(CH2)12CHCH2CHCHCH2OH+208 209 (12%) (50%) Me(CH2)12CH CHCHCHCH2OH+C13H27 OH NO2 207 OH O NO2 208 202 OMe OH NO2 Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 51When nitro-diol 218 or 219 is treated with an aqueous solution of sodium hydroxide and then with a mixture of fluorine and nitrogen, fluoro-derivative 220 or 221 is formed in a yield of 71% or 68%, respectively.50 The potassium salt of 2,2-dinitroethanol is converted into 2-chloro-2,2-dinitroethanol on treatment with chlorine in water at 0 ¡À 5 8C.157 When salts of nitro alcohols react with chlorine or bromine, halogenation products are formed: When the products resulting from reactions of MeNO2 with aldehydes (CH2O or Cl3CCHO) were treated with bromine, 2,2- dibromo-2-nitroethanol 158 or 3-bromo-1,1,1-trichloro-3-nitro- propan-1-ol 35 were synthesised.In a number of patents,5, 7, 9 ¡À 12, 20 it has been proposed to synthesise 2-bromo-2-nitropropane-1,3-diol 8, which is a known bactericide agent `Biocide', according to the following scheme (the reaction conditions and product yields are listed in Table 11): Similarly, Cl3CCH(OH)C(NO2)Br was prepared from MeNO2 and Cl3CCHO;5, 7 MeCH(OH)C(NO2)BrCH(OH)Me was synthesised from MeNO2 and MeCHO;10 and MeCH(OH)C- Me(NO2)BrCH(OH)Me was obtained from EtNO2 and MeCHO.11 The reactions of salts of nitro alcohols 222 with aryldiazonium cations 223 in water at low temperatures yield arylhydrazones 224.159 R R0 Yield (%) H 4-Me 51 Me H 60 Me 4-Me 45 Me 4-Cl 23 Ph 4-Cl 22 H 4-Br 31 Me 4-Br 20 H 4-NO2 20 H 4-NH=C(NH2)NHSO2 25 H 3-NO2 17 Me 4-NO2 35 H 4-H2NSO 22 H 4-MeCONHSO2 20 H 4-H2NCONHSO2 21 H 56 2-Nitropropane-1,3-diol 3 is nitrated with tetranitromethane in an alkaline medium giving 2,2-dinitropropane-1,3-diol 114.8 However, an attempt to carry out alkaline nitration of the salt 30b was unsuccessful; the reaction resulted in the formation of diene 225 (yield 21.4%).52 Dinitro alcohol acetates 226 [the compound 226 with n = 3 has been obtained by catalytic oxidative nitration of the nitro alcohol O2NCH(CH2)3OH] have been introduced into the Michael reaction with acrolein.This afforded compounds 227, which were then converted into dinitro-diols 228.160 CHCH2NO2 OH 218 O2NCH2CH OH 1. NaOH, H2O 2. F2/N2, 5 8C, 1 h MeCHCH(NO2)CHMe OH OH MeCHC(NO2)CHMe . HO 219 221 1. NaOH, H2O 2. F2/N2, 1 h F OH O2NCHCH CHCHNO2 , OH OH 220 F F OH 7 K+[C(NO2)2CH2COH] Br(NO2)2CCH2COH (Ref. 123), (91%) Me Me Br2 CCl4 Me Me 7 C(Me)CNO2] 2Na+ CH2OH 7 [O2NCC(Me) CH2OH Br2, Et2O 0 8C, O2NC C(Me) HOCH2 Br C(Me) (44%) CBr (Ref. 52), NO2 CH2OH X=Cl (73.8%), Br (75%). (O2N)2CCHCHC(X)(NO2)2 (Ref. 96) OH X OH 7 7 [(O2N)2CCHCHC(NO2)2] 2Na+ OH 5710 8C X2, Et2O HOCH2CCH2OH . Br NO2 8 MeNO2+CH2O (HOCH2)2C NO¡¦2 K+ Br2 B R0C6H4NHN CCHR NO2 OH 224 H2O 8 8C, 16 h RCHCHNO2 OH 222 7 Na+ +R0C6H4N�¢2 223 S N 4-Et NHSO2 N HOCH2CH(NO2)CH2OH 3 114.NaOH, C(NO2)4, H2O 15 8C, 1 h HOCH2C C NO2 Me 225 C Me CCH2OH. NO2 NaNO2, AgNO3 H2O, pH 778, 0 8C, 1 h 2Na+ C NO2 CH2OH 7 O2N C Me 30b C Me C HOCH2 Table 11. Reaction conditions and yields of the compound 8. Conditions of nitroaldol Conditions Yield Ref. condensation of bromination (%) Base Sol- ¡ä /8C t/ ¡ä /8C t /h vent min Ca(OH)2 H2O 0 30 0 1 74 5 Na2CO3 0 ¡À 10 10 1 80 7 NaOH H2O 5 ¡À 10 5 ¡À 10 86 9 NaOH MeOH 20 0.5 83 10 NaOH H2O, 20 60 5 ¡À 20 98 11 DEGa MeONa MeOH 5 5 86 b 20 NaOH H2O see c 12 aDEG D diethyleneglycol; bCHCl3, c paraform, CCl4, BrCl. 52 M-G A ShvekhgeimerThe nitro alcohols O2NCH2CH(R0)OH add to a,b-unsatu- rated ketones in liquid ammonia to give adducts 229 (yields 45%± 96%).161 Adduct 230 was synthesised by the reaction of the nitro alcohol 202 with 2-(4-methylphenyl)-1-nitroethene under similar conditions (yield 23%).161 The reaction of salts of nitro alcohols 231 with nitroalkenes 232 in aqueous methanol gives adducts 233 in quantitative yields.On treatment with an aqueous solution of hydroxylamine hydro- chloride, the adducts 233 are converted into dinitro alcohols 234 in 37%± 55% yields.162 Chloro-substituted dinitro alcohols 235 have been synthesised by the reaction of the salts 231 with 2-nitroprop-1-ene in aqueous methanol followed by treatment of the resulting adducts 236 with chlorine.163 3.Reactions involving the hydroxy group When dinitro alcohols 237 react with compound 238 in the presence of toluene-p-sulfonic acid, intermolecular elimination of water occurs to give acetals 239 (yields 73%± 100%) as mixtures of cis- and trans-isomers (in a ratio of 15 ± 35 : 85 ± 65).109 Ethers derived from nitro alcohols are often synthesised by reactions of nitro alcohols with halo-derivatives in the presence of bases.The reaction of 3-nitropropan-1-ol with ButMe2SiCl carried out in DMF in the presence of imidazoline at 20 8C for 15 h gave the ether O2N(CH2)3OSiMe2But in 91% yield.108 Ethers 240, 241 and 242 have been prepared in high yields by the reactions of triol 243 with various halo-derivatives at 20 ± 25 8C in the presence of KOH, NaH or Et3N.164 R X Time /h Yield (%) Me I 2 89 Bu Br 0.5 78 The reaction of 2,2-difluoro-2-nitroethanol 244 with com- pounds RX in the presence of 5% aqueous sodium hydroxide at 25 ± 30 8C affords ethers 245.165 Condensation of the nitro alcohol 107 with allyl bromide or propargyl bromide catalysed by sodium hydroxide has been carried out in the presence of formaldehyde; this led to ethers 246 in 66% or 55% yield, respectively.93 The role of formaldehyde was to prevent the nitro alcohol 107 from decomposing in the alkaline medium.93 1,3-Dichloropropan-2-ol reacts with two molecules of the nitro alcohol 244 in the presence of 20% aqueous sodium hydroxide to give the diether (NO2CF2CH2OCH2)2CHOH in a yield of 78.6%.166 n=1, 3.(O2N)2C(CH2)nCH2OH 228 CH2CH2CH2OH (O2N)2C(CH2)nCH2OAc 227 CH2CH2CHO (O2N)2CH(CH2)nCH2OAc+CH2 226 CHCHO O2NCH2CHOH R1 NH3 (liq.), 10748 h R2CH CHCOR3 R3=Me, Ph, 4-MeC6H4, 4-MeOC6H4; R1=Me, R2=R3=Ph. R1=H, R2=Ph, 4-BrC6H4, 4-O2NC6H4, O ; R2CHCH2COR3 CH(NO2)CH(R1)OH 229 202 4-MeC6H4CH CHNO2 4-MeC6H4CHCH(NO2)CH2OH CH2NO2 230 (23%) NH3 (liq.), 48 h .[RC(NO2)CH2OH]7Na+ +R0C 231 232 CH2 NO2 MeOH, H2O 072 8C, 1 h R, R0 =Me, Et. 233 [HOCH2C(NO2)(R)CH2C(R0)NO2]7Na+ H2O, pH 4 HONH2 . HCl, HOCH2C(NO2)CH2CH(NO2) R 234 R0 Me R=Me, Et. Cl2 (pH*171.5) 0710 8C HOCH2C(NO2)CH2C 236 R 7 + NO2Na HOCH2CCH2CCl NO2 R 235 NO2 Me 231+CH2 CNO2 Me MeOH, H2O 072 8C, 1 h R=F, Me, NO2; R0=F, NO2. O C(NO2)2R0 RC(NO2)2CH2OH+HO 237 238 TsOH, ClCH2CH2Cl 1057110 8C, 2 h RC(NO2)2CH2O O C(NO2)2R0 239 O2NC[(CH2)3OH]3+3RX KOH, DMSO 20725 8C 243 O2NC[(CH2)3OR]3 240 243+R0Cl NaH, DMSO 25 8C O2NC[(CH2)3OR0]3 241 R0=Bu (78%); 4-ClC6H4 (63%); 243+ButMe2SiCl Et3N, DMF, DMAP 25 8C, 24 h O2NC[(CH2)3OSiMe2But]3 242 DMAP= .N NMe2 R, X (yield, %): Me, MeOSO2 (78); CH2CHCH2, Cl (75.5); F2C(NO2)CH2OH+RX NaOH, H2O 25730 8C 244 F2C(NO2)2CH2OR 245 CCH2, Br (27). HC R (time /h): CH2CH CCH2 (30). CH2 (45); HC FC(NO2)2CH2OR 246 FC(NO2)2CH2OH+RBr 107 CH2O NaOH, H2O, 20725 8C FC(NO2)2CH2OH OH7 FC(NO2)2CH2O7 FC¡(NO2)2 +CH2O. Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 53Ethers 247 containing ester groups have been prepared by the reaction of the nitro alcohol 244 with alkyl haloacetates 248 in boiling acetone in the presence of potassium carbonate.167 The reaction of the nitro alcohols 237 with bis(chloromethyl) ether involves the replacement of only one chlorine atom and gives products RC(NO2)2CH2OCH2Cl (R=Me, F, Cl, NO2).168 2-Fluoro-2-nitropropane-1,3-diol resulting from the reaction of diethyl 2-fluoro-2-nitromalonate 249 and the aldehyde 2 in the presence of potassium hydroxide has been introduced in situ in the reaction with chlorotrimethylsilane in boiling acetonitrile; this gave bis-silyl ether 250.49 The reactions of dichlorosilanes 251 with nitro alcohols 252 giving rise to compounds 253 (yields 10%± 69%) have been studied in detail.169 Polysiloxanes 255 have been synthesised by the condensation of dichlorosilanes 251 with nitro-diols.169 Compounds 256 containing one, two or three residues of nitro alcohols 257 are formed in the condensation of 257 with chloro- silanes 258 in the presence of pyridine.170 n R1 R2 R3 X Yield (%) 3 Me H Me Cl 44 2 Me H Me Cl 66 2 Me Me H Cl 40 3 Me CCl3 H H 14 2 Me CCl3 H H 15 1 H H Me Cl 7 1 H CCl3 H H 30 3 Me Et Et Br 12 The reaction of the nitro alcohol 107 with epibromohydrin has been carried out at 0 ± 2 8C in the presence of sodium hydroxide for 48 h; the ether resulting from the replacement of bromine in epibromohydrin was obtained in 32% yield.171 The reaction of the nitro alcohol 107 with epichlorohydrin has been described in two publications.In a patent,172 ether FC(NO2)2CH2O was reported as the reaction prod- uct (yield 31%). However, in another study 171 the same process gave a different compound containing two nitro-alcohol residues, namely, FC(NO2)2CH2OCH2CH(OH)CH2OCH2 ± C(NO2)2F. The nitro alcohol 244 reacts with ethylene oxide or propylene oxide in the presence of aqueous sodium hydroxide at 0 8C for 16 h to give ethers RCH(OH)CH2OCH2C(NO2)F2 (R = H, Me) (yields 32% and 67%); in the case of ethylene oxide, the com- pound F2C(NO2)CH2O(CH2)2OCH2CH2OH is also formed (yield 4%).173 The reaction of the nitro alcohol 202 with epoxides 259 at 50 8C in the presence of boron trifluoride etherate or sulfuric acid results in the formation of ethers 260 (yields 51% and 46.4%).174 The compounds CH:C(R)C(OH) CH(R0) ±OCH2CH2NO2 have been obtained by the reaction of the nro alcohol 202 with epoxides HC:C(R)7 ) (R and R0 are H or lower alkyls) in the presence of 4-MeC6H4SO3H or the KU-2 ion- exchange resin.175 Nitro alcohols 261 react with trifluoromethanesulfonic acid to give cations 262, which add in situ to nitro alcohols 263 yielding compounds 264 (Table 12).176 The ether Me3SiCH2OCH2C(NO2)2F has been prepared in 45% yield by the reaction of the nitro alcohol 107 with Me3Si- CH2OSO2CF3 in dichloromethane in the presence of K2CO3 at 20 8C for 16 h.177 Refluxing of the nitro alcohols 237 with divinyl ether in the presence of HgO and trifluoroacetic acid leads to transetherifica- tion yielding vinyl ethers 265.92 X=Cl, Br; R=Me (74%), Et (67%), Bu (65%). 244 +XCH2COOR 248 O2NCF2CH2OCH2COR 247 K2CO3, Me2CO D, 475 h O Me3SiOCH2CCH2OSiMe3 . NO2 F 250 C F COOEt COOEt O2N 249 KOH, CH2O, EtOH, H2O 20 8C, 3 h HOCH2CCOOEt NO2 F HOCH2CCH2OH NO2 F Me3SiCl, MeCN D, 5.5 h; 20 8C, 12 h KOH, H2O CH2O, R1=Me, Et, MeO, EtO; R2=Me, Et, Ph, MeO, EtO; R3=H, Me. R1R2SiCl2+HOCHCH2NO2 R3 251 252 R1R2Si(OCHCH2NO2)2 R3 253 n=3, 4; R1=Et, Ph, MeO; R2=Me, Et, MeO; R3=Me, Et. 251+HOCH2CCH2OH NO2 R3 254 40755 8C 6 ± 16 h HO SiOCH2CCH2O R1 R2 255 NO2 R3 H n R1n SiCl4-n+(4-n)HOCHCNO2 258 257 R2 R3 X C5H5N Et2O R1n Si(OCHCNO2)4-n 256 R2 R3 X FC(NO2)2CH2OCH2CHOCH2 CH2CHOCH2 R=Et, CCl3. RCH(OH)CH2OCH2CH2NO2 260 +O2NCH2CH2OH 259 202 R O BF3 . Et2O or H2SO4 50 8C, 3 h COCH(R0 RC(NO2)2(CH2)2O(CH2)nC(NO2)2R0. 264 RC(NO2)2(CH2)á2 ] 262 R0C(NO2)2(CH2)nOH (263) 7H2O + RC(NO2)2(CH2)2OH 261 [RC(NO2)2(CH2)2OH2 CF3SO3H, N2 ClCH2CH2Cl R=Me (64.2%), F (64%), NO2 (25.9%).RC(NO2)2CH2OH+CH2 CH2 237 CHOCH RC(NO2)2CH2OCH CH2 265 CH2Cl2, D, 22.5 h HgO, F3CCOOH Table 12. Reaction condition and yields of the compounds 264. R R0 n Temperature /8C Time /h Yield (%) Me F 2 60 20 9 Me Me 2 60 20 80 F F 2 60 96 15 NO2 NO2 2 65 ± 70 72 16 F F 1 30 ± 35 168 20 NO2 Me 1 70 ± 75 72 46 NO2 F 1 60 ± 70 72 48 54 M-G A ShvekhgeimerThe addition of nitro alcohols to alkenes or to buta-1,3-diene and the addition of alcohols to nitroalkenes have been used to synthesise ethers of nitro alcohols.For example, the reaction of 3-nitropropan-1-ol with 2-methylpropene in CH2Cl2 in the pres- ence of sulfuric acid (20 8C, 12 h) gave the ether ButO(CH2)3NO2 (yield 95%).108 Adduct 266 was obtained by reaction of 107 with 2-methyl- pent-1-ene at 20 8C in the presence of HgSO4 or Hg2SO4 (yields 74% and 58%, respectively).178 The reaction of the nitro alcohol 107 with buta-1,3-diene affords a mixture of 1,2-addition product 267 and 1,4-addition product 268 in a ratio of 77 : 23, their total yield being 53%.178 Two products 269 and 270 resulting from radical C- and O-addition (yields 28% and 70%, respectively) have been syn- thesised by the reaction of hexan-1-ol with 1-nitroprop-1-ene in the presence of tert-butyl peroxide.179 1,1,1-Trinitroethanol adds to the C=N bond in perfluoro- guanidine at ambient temperature to give compound 271.180 Acyclic and cyclic acetals containing one or two nitro-alcohol residues are promising compounds for practical purposes.Acetals 272 have been synthesised by the reactions of the nitro alcohols 237 with 1,3,5-trioxane or with paraldehyde in the presence of sulfuric acid 181, 182 or sodium hydroxide. The reaction conditions and the yields of products are presented in Table 13.89 Mixed acetals 273 182, 183 and 274 184 have been prepared by condensation of three components.The acetal 273 (R=F) was synthesised by the reaction of the nitro alcohol FC(NO2)2CH2OH with the ether MeC(NO2)2- CH2OCH2Cl in the presence of TiCl4.183 Polycondensation of formaldehyde with the diol HOCH2C(- NO2)2CH2CH2C(NO2)2CH2OH in the presence of sulfuric acid affords poly(nitroformal).185 Nitro alcohols add to acyclic 178, 186 or cyclic 108, 142, 178 vinyl ethers 275 in the presence of catalysts to give acetals 276 (see Table 14).The addition of nitro alcohols 277 to 2,3-dihydropyran in THF and HMPT in the presence of butyllithium at a temperature between790 8C and740 8C leads to the compounds 209.154 The nitro alcohol 107 adds to ethoxyacetylene in CH2Cl2 at 20 8C in the presence of HgSO4 187 or Hg(OAc)2 178 or in the presence of HgO in CF3COOH92 giving rise to 2 : 1 adduct 278 (yields 100%, 95%, or 70%, respectively).When this reaction is carried out in the presence of Hg(OAc)2 in CH2Cl2 and hexane at 20 8C, the 1 : 1 adduct 279 is produced in 73% yield in addition to the adduct 278 (yield 27%).178 CPr Me FC(NO2)2CH2OH +CH2 107 FC(NO2)2CH2OCH2CHPr. 266 Me FC(NO2)2CH2OH+CH2 107 CH CH CH2 HgSO4, CCl4, sealed tube 55 8C, 16 h CHCH2OCH2C(NO2)2F. 268 267 Me CHCHOCH2C(NO2)2F+MeCH CH2 Me(CH2)4CH2OH+MeCH (ButO)2 CHNO2 C5H11CHCH2NO2+C6H13OCHCH2NO2 . 269 270 OH Me (O2N)3CCH2OH+(F2N)2C NF H2NCONH2, MeCN 23 8C, 72 h 271 (O2N)3CCH2OC(NF2)2NHF. RC(NO2)2CH2OH+(R0CHO)3 [RC(NO2)2CH2O]2CHR0. 237 272 (NO2)3CCH2CH2OH+FC(NO2)2CH2CH2OH+F5SN CCl2 C[OCH2CH2C(NO2)3]OCH2C(NO2)2F. 274 F5SN R=NO2 ,182 F;183 237+(CH2O)x+MeC(NO2)2CH2OH 80% H2SO4 RC(NO2)2CH2OCH2OCH2C(NO2)2Me 273 R1OH+R2OCH R2OCH(OR1)CH2R3. CHR3 20 8C 275 276 R=H, Me.RCH(NO2)CH2OH+ O 277 BuLi, THF, HMPT 790 to740 8C, 3 h O OCH2CH(NO2)R 209 Table 13. Reaction condition and yields of the compounds 272. R R0 Catalyst Yield (%) Ref. F H H2SO4 7 181 Me H NaOH 78 89 Me H H2SO4 78 182 Me Me NaOH 35 89 NO2 H H2SO4 7 182 Table 14.Conditions for the formation of the acetals 276. R1 R2 R3 Catalyst Solvent T /8C Time /h Yield (%) Ref. O2N(CH2)3 7(CH2)37 TsOH Et2O 20 5 95 108 7(CH2)37 TsOH Et2O 20 12 90.5 142 FC(NO2)2CH2 Et H Hg(OAc)2 CH2Cl2 20 16 73 178 FC(NO2)2CH2 7(CH2)37 Hg(OAc)2 CH2Cl2 42 17 100 178 FC(NO2)2CH2 FC(NO2)2CH2 H Hg(OAc)2 CH2Cl2 20 24 61 178 (O2N)3CH2CH2 FC(NO2)2CH2 H BF3 .Et2O 7 0 12 7 186 MeC(NO2)2CH2 FC(NO2)2CH2 H Mol. sieve 5A 7 0 12 7 186 FC(NO2)2CH2 FC(NO2)2CH2 H Mol. sieve 5A 7 0 12 7 186 O CHCH2NO2 Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 55Two compounds, 1 : 1 adduct 280 and 2 : 1 adduct 281, were obtained by refluxing the nitro alcohol 107 and divinyl ether in CH2Cl2 in the presence of HgSO4 or Hg2SO4.It was found that the ratio of 280 to 281 depends appreciably on the 107 : ether ratio. For example, when this ratio is 1 : 1, reaction in the presence of HgSO4 gives 280 in 12% yield and 281 in 58% yield; at 2 : 1 ratio, the yield of 280 is 70%, while that of 281 is 20%. When this reaction is carried out in the presence of Hg2SO4 and 107 : ether= 1 : 2 (refluxing for 26 h), the yields of 280 and 281 are 40% and 34%, respectively.178 The orthoformate HC[OCH2C(NO2)3]3 or orthocarbonate C[OCH2C(NO2)3]4 have been synthesised from 2,2,2-trinitroetha- nol and CHCl3 or CCl4 in the presence of FeCl3.188 Cyclocondensation of nitro-diols of various structures with aldehydes or ketones in the presence of acid catalysts has found wide application as a method for the preparation of nitro- derivatives of 1,3-dioxane. 2-Nitropropane-1,3-diol 3 reacts with acetone or with aro- matic aldehydes in boiling benzene in the presence of toluene-p- sulfonic acid to yield 5-nitro-1,3-dioxane derivatives 282.189 2-Bromo-2-nitropropane-1,3-diol 8 has been involved in the condensation with acetone or aliphatic aldehydes in the presence of various catalysts; this resulted in the formation of 5-bromo-5- nitro-1,3-dioxane derivatives 283.The reaction conditions and the yields of reaction products are listed in Table 15.190 ± 192 Acetals 284, which are promising as monomers, have been synthesised from diol 285 and carbonyl compounds 286 in the presence of toluene-p-sulfonic acid.193 The stereochemistry of the reactions of halo-2-nitropropane- 1,3-diols with acetaldehyde in the presence of toluene-p-sulfonic acid has been studied.194 It was found that these reactions yield mixtures of diastereoisomers 287 and 288.the ratio 287 : 288 =16 : 83 (X=Cl), 5.5 : 94 (X=Br). 2-Alkyl-5-hydroxymethyl-5-nitro-1,3-dioxanes 289 were pre- pared in*90% yields by cyclocondensation of 2-hydroxymethyl- 2-nitropropane-1,3-diol 4 with aliphatic aldehydes in the presence of toluene-p-sulfonic acid and CuSO4 at 20 8C.194 A series of 5-nitro-1,3-dioxane derivatives 290 have been synthesised by cyclocondensation of nitro-diols 291 with esters of aldehydo acids 292 in the presence of zinc chloride and orthophosphoric acid in boiling benzene.195 R1 R2 R3 Yield (%) H Me 2-OCH2COOEt 90 H Et 2-OCH2COOEt 96 H Br 2-OCH2COOEt 95 H Me 3-OCH2COOEt 92 H Br 3-OCH2COOEt 84 H Me 4-OCH2COOEt 92 H Et 4-OCH2COOEt 90 H 3-O2NC6H4 4-OCH2COOEt 50 H Br 4-OCH2COOEt 75 2-MeO Me 4-OCH2COOEt 74 2-MeO Cl 4-OCH2COOEt 88 2-MeO Br 4-OCH2COOEt 87 2-EtO Me 4-OCH2COOEt 92 2-EtO Et 4-OCH2COOEt 91 2-EtO Br 4-OCH2COOEt 78 C EtO OCH2C(NO2)2F OCH2C(NO2)2F Me 278 279 +H2C C(OEt)OCH2C(NO2)2F . 107+EtOC CH FC(NO2)2CH2OH+CH2 107 CHOCH CH2 CH2Cl2 36 8C, 16 h 280 CHOCHMe OCH2C(NO2)2F CH2 +MeCHOCHMe 281 OCH2C(NO2)2F OCH2C(NO2)2F R, R0: Me, Me; H, 4-Cl6H4; H, Ph; H, 4-BrC6H4.HOCH2CHCH2OH +RR0C O NO2 3 O O H O2N R R0 282 TsOH, PhH 80 8C, 3 h HOCH2CCH2OH +RR0C O 8 Br NO2 O O Br O2N R R0 283 R, R0 (yield, %): H, (47); H, Ph (65); H, 4-O2NC6H4 (70); O 285 286 TsOH, PhH 80 8C, 476 h HOCH2CCH2OH +RR0C O CH NO2 CH2 O O CH O2N R R0 CH2 284 Me, Me (71); (CH2)5 (62).H, 3,4-(CH2O2)C6H3 (56); H, PhCH=CH (67); +MeCHO TsOH, PhH 80 8C C X CH2OH CH2OH O2N X O O Me + O2N O O Me 287 288 NO2 X 289 O O HOCH2 O2N Alk 4 HOCH2CCH2OH +AlkCHO CH2OH NO2 20 8C, 7 days TsOH, CuSO4 R1 290 O O R2 O2N R3 +HOCH2CCH2OH NO2 R2 291 H O C 292 80 8C, 3 h ZnCl2, H3PO4, PhH R3 R1 Table 15. Reaction conditions and yields of the compounds 283.R R0 Catalyst Solvent � t Yield Ref. /8C /h (%) H Me p-TsOH PhH 80 0.5 7 190 H Et p-TsOH PhH 80 0.5 7 190 H Pr p-TsOH PhH 80 0.5 7 190 H Pri p-TsOH PhH 80 0.5 7 190 H H H2SO4 DCEa 83 7 7 191 Me Me BF3 . Et2O 7 447 7 40 192 H Me BF3 . Et2O 7 447 7 85 192 H Et BF3 . Et2O 7 447 7 7 192 H Pr BF3 . Et2O 7 447 7 7 192 H Pri BF3 . Et2O 7 447 7 7 192 a DCE �dichloroethane. 56 M-G A ShvekhgeimerThe rate of the reaction of 5-substituted furfural derivatives 293 with 2-ethyl-2-nitropropane-1,3-diol in the presence of the KU-2 ion-exchange resin (H-form) has been shown to decrease in the series NO2>I>Br>H>Me. It was assumed that attack by the diol on the protonated formyl group is the rate-determining step of the reaction.196 Cyclocondensation of mono- and disubstituted 2-nitropro- pane-1,3-diol derivatives 295 with aldehydes or ketones in the presence of TsOH gave di-, tri- or tetra-substituted derivatives of 5-nitro-1,3-dioxane 296.197 The compounds 296 were obtained in low yields for R1 = H, R2 = R3 = R4 = R5 = Me (27%); R1 = H, R2 = R3 = Me, R4= R5 = H (30%) and R1 = R4 = R5 = H, R2 = R3 = Pri (32%).197 1,3-Dioxane derivatives 297 were synthesised from the nitro- diol 206 using acetone dimethyl ketal or benzaldehyde dimethyl acetal as the second reactant.152, 198 The reaction of 206 with benzaldehyde dimethyl acetal affords three diastereoisomers 297a,b,c in 55.5% overall yield and in a ratio of 4 : 1 : 1.152 Refluxing of the nitro-diol HOCH2CH(NO2)CH(OH)C13H27 with acetone dimethyl ketal in acetone in the presence of toluene- p-sulfonic acid results in the formation of 2,2-dimethyl-5-nitro-4- tridecyl-1,3-dioxane in 61.5% yield.61 Yet another method for the synthesis of acyclic and cyclic acetals of aromatic aldehydes and nitro alcohols consists of the reaction of dichloromethyl arenes with alcohols or 1,3-diols.This procedure was used, for example, to prepare acetals 298 or 299 from dichloromethylarenes 300 and polynitro alcohols 301 or 2,2- dinitropropane-1,3-diol 114.199 The reactivity of nitro alcohols decreases in the order NO2> Cl>HCCH2>Me, i.e.in parallel with their acidity.199 By refluxing nitro-diol 302 with arylboronic acids 303 in acetone, 2-bora-1,3-dioxane derivatives 304 have been synthes- ised.200 Cyclic compound 305 containing two oxygen atoms and a phosphorus atom in the six-membered ring has been obtained by cyclocondensation of the triol 4 with phosphorous acid.201 2-Bromo-2-nitropropane-1,3-diol 8 reacts with dichloro- derivatives 306 at 50 8C to give 5-bromo-5-nitro-2-R-2-oxo- 1,3,2-dioxaphosphorinanes 307.202 Cyclocondensation of the diol 8 with 2,2,2-trichloro-1,3- dimethyl-1,3,2l5-diazaphosphetidin-4-one 308 gives rise to com- pound 309.203 5-Chloromethyl-5-nitro-1,3,2-dioxathian-2-one 310 was syn- thesised by the reaction of the triol 4 with thionyl chloride in the presence of pyridine.204 Esters derived from nitro alcohols are usually synthesised either by reactions of nitro alcohols with acid chlorides or anhydrides or by reactions of nitro alcohols with carboxylic acids in the presence of anhydrides of other acids.The preparation O R C O H +HOCH2CCH2OH Et NO2 293 KU-2, PhH R=H (66%), Me (70%), Br (84%), I (79%), NO2 (82%). O R O O Et NO2 294 296 O O R4 R5 R1 O2N R2 R3 HOCHC(NO2)CHOH+R4CR5 295 TsOH PhH R3 O R1 R1=H, Me, CH2OH; R2=H, Me, Pr, Pri; R3=H, Me, Pr, Pri; R4=H, Me; R5=H, Me, Et, Pri, Bu, Ph, 2-HOC6H4, 4-O2NC6H4, PhCH CH; R4, R5=(CH2)5.R2 R=H, R0 =Ph; R=R0 =Me. 297 O O O Me R0 R NO2 O CHCHCHMe +RR0C(OMe)2 OH OH NO2 206 TsOH, DMF 60 8C, 1 h O O H Ph 297a 297b 297c O2N Ph H O O H Ph O O Me Me Me Y Y Y O2N O2N Y= . O R=Cl, Me, NO2; Ar Ph, 3-O2NC6H4, 4-O2NC6H4 ; ArCHCl2+RC(NO2)2CH2OH 300 301 507100 8C AlCl3 or FeCl3 ArCH[OCH2C(NO2)2R]2 298 300+HOCH2CCH2OH 114 NO2 NO2 Ar=Ph, 3-O2NC6H4 (AlCl3,), 4-O2NC6H4 (FeCl3).AlCl3 or FeCl3 507100 8C Ar O O NO2 NO2 299 Ar=Ph (90.5%), 4-ClC6H4 (67%). HOCH2CCH2OH +ArB(OH)2 Me2CO 50 8C, 2 h 302 303 304 ArB COOEt NO2 O O NO2 COOEt HOCH2CCH2OH +H3PO3 4 NO2 CH2OH 305 O POH O O2N HOCH2 R=Me (60%), Cl (28%), PhO (56%), 4-MeC6H4O (49%), 3-MeC6H4O (42%), 4-O2NC6H4O (60%), OH (39%). 307 O O Br ON2 O R P 50 8C HOCH2CCH2OH +RPCl2 8 306 NO2 Br O , N2 O O 8+O N PCl3 N Me Me CH2Cl2 O N P N Me Me Cl O O Br NO2 308 309 HOCH2CCH2OH +3SOCl2 4 NO2 CH2OH 30 8C, 1 h C5H5N 310 O S O O2N ClCH2 O Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 57of nitro alcohol acetates has been described in numerous publica- tions.There is no need to consider these studies in detail, because the acetates have been synthesised by conventional procedures, which have been described fairly comprehensively in mono- graphs.3, 4 Syntheses of halocarbonates 311 and 312 (yield 91%) by the reactions of nitro alcohols 313 with carbonyl dichloride or difluoride or by the reaction of 2,2-dinitropropane-1,3-diol 114 with carbonyl difluoride have been reported in patents 205, 206 and papers.207, 208 Carbonates 314 205 or 315 (yield 64%) 208 were prepared by the reaction of nitro alcohol 316 with the chloroformate 311 (R=F, X=Cl) or of 2,2,2-trinitroethanol with carbonyl difluoride. 2,2-Dinitropropane-1,3-diol 114 reacts with two molecules of chloroformate 317 giving rise to diester 318 (yield 83%).209 The exothermic reaction of the nitro alcohols 237 or 202 with chloride 319 in the presence of iron(III) chloride has led to the synthesis of O-alkyl S-ethyl thiocarbonates 320 210 or 321.The three-component reaction involving the nitro alcohol 107, thiophosgene and Cl3CSH, taken in a ratio of 3 : 1 : 1, carried out in the presence of a base under conditions of phase transfer catalysis affords compound 322, which is converted into com- pound 323 (yield 81%) upon treatment with 1,1,1-trinitroethanol and chlorine.211 When 2,2-dinitropropanol is made to react with bis(1,2,4- triazol-1-yl) thioketone in acetone in the presence of pyridine at 20 8C, the ester [MeC(NO2)2CH2O]2C=S is formed in 83% yield over a period of 2 days.212 Similarly, the reaction of (O2N)3CCH2OH or the alcohol 107 with bis(1,2,4-triazol-1-yl) sulfoxide yields the corresponding esters.212 The reaction of 2-nitropropane-1,3-diol with trimethylacetyl chloride in dichloromethane carried out for 3 h at 40 8C and then for 12 h at 25 8C affords the corresponding bis-ester in 95% yield.15 Esters 324, which are of interest as monomers, have been synthesised in 60%± 70% yields by the reactions of dinitro alcohols 325 with chlorides of a,b-unsaturated acids.213 ± 215 It should be noted that the yield of 324 (R1= F, R2= R3 = Me) was as low as 5%.214 The reaction of the nitro alcohol 325 (R1=Me, R2=H) with acryloyl chloride carried out for 3 h at 20 8C in the presence of TiCl4 affords the corresponding ester 324 (R1=Me, R2=R3=H) in 93% yield.213 The reaction erfluoroacryloyl chloride with the nitro alcohol 244 in the presence of triethylamine at 740 to 735 8C follows two competing pathways: esterification of the nitro alcohol giving the ester F2C=CFCOOCH2C(NO2)F2 (yield 7.9%) and the addition of the nitro alcohol to the double bond of the chloride leading to the ether F2C(NO2)CH2OCF2 ± CH(F)COCl (yield 7%).216 Esters derived from nitro alcohols and unsaturated dicarbox- ylic acids have been described in the literature.217, 218 Tris-esters 326 have been prepared by the reactions of the nitro-triol 4 with benzoyl, 4-chlorobenzoyl and toluene-p-sulfonyl chlorides in the presence of pyridine.164 Ar Y � /8C Time /h Yield (%) Ph CO 70 15 51 4-ClC6H4 CO 20 30 93 4-MeC6H4 SO2 0 12 47 R, X: F, Cl; F, F; Me, F; NO2, F; FCOOCH2, F; RC(NO2)2CH2OH+XCX 313 RC(NO2)2CH2OCX 311 O C5H5N or NaF O HOCH2CCH2OH +FCF 114 312 NaF FCOCH2CCH2OCF .NO2 NO2 O O NO2 NO2 O 316 311 FC(NO2)2CH2OCCl +O2NCF2CH2OH O C5H5N 314 FC(NO2)2CH2OCOCH2C(NO2)F2 , O C5H5N (O2N)3CCH2OCOCH2C(NO2)3 . 315 O 2(O2N)3CCH2OH +FCF O 2(NO2)3CCH2OCCl+HOCH2C(NO2)2CH2OH O 317 114 (NO2)2C[CH2OCOCH2C(NO2)3]2 . 318 O CH2Cl2, MeCN RC(NO2)2CH2OCSEt 320 O RC(NO2)2CH2OH 237 ClCSEt (319), FeCl3 O R=F, Me, NO2 ; NO2CH2CH2OH 202 NO2CH2CH2OCSEt . 321 O 319, FeCl3 CH2Cl2, MeCN FC(NO2)2CH2OH+ClCCl +Cl3CSH 107 S B 75 to 5 8C [FC(NO2)2CH2O]3COCH2C(NO2)3 . 323 [FC(NO2)2CH2O]3CSSCCl3 322 (NO2)3CCH2OH, Cl2 60770 8C, 75 h R=F, Me, NO2. +2RC(NO2)2CH2OH C5H5N, Me2CO 20 8C, 48 h [RC(NO2)2CH2O]2C S C S N N N N N N 2ButCCl+CHNO2 O CH2Cl2 40 8C, 3 h; 25 8C, 12 h CH2OH CH2OH CHNO2 O O CH2OCBut CH2OCBut R1=H, F, Me; R2=H, Me; R3=H, Me.R1C(NO2)2CHOH +CH2 325 R2 CCOCl R3 65780 8C, 578 h CH2 CCOOCHC(NO2)2R1 R3 324 R2 O2NC(CH2OH)3+ArYCl C5H5N O2NC(CH2OYAr)3 4 326 58 M-G A ShvekhgeimerCompound 327 has been obtained in 40% yield by treatment of the bis-ester, resulting from the reaction of 2-butyl-2-nitro- propane-1,3-diol with methanesulfonyl chloride, with chlorophe- nol in the presence of sodium hydride.219 Synthesis of hydroxy ketone 328 has been accomplished using trifluoroacetic anhydride.103 The reaction of the dinitro-diol 114 or the nitro-diol 4 with trifluromethanesulfonic anhydride 329 at 20 8C in the presence of pyridine results in the formation of mixtures of mono-triflates 330 (yields 46.8% or 44.4%) and bis-triflates 331 (yields 14% or 16.2%).49 The researchers cited 49 also proposed conditions under which the reaction of 2-fluoro-2-nitropropane-1,3-diol with 329 gives either mono-triflate 332 (yield 78.9%) or bis-triflate 333 (yield 75.5%).The nitro-diol HOCH2C(NO2)(Br)(CH2OH) has been esteri- fied with formic acid in the presence of acetic anhydride and pyridine at 40 8C to give the corresponding bis-ester (HCOOCH2)2C(NO2)Br.220 Esters 334 (yields 47% ± 100%) have been synthesised from polynitro alcohols 335 and dicarboxylic acids in the presence of the anhydride 329 and aluminium chloride.221 Under the same conditions, the following esters were obtained: (NO2)2(R)CCH2O2CCH(Et)CO2CH(R)(NO2)2 (R = NO2, 63%; R = F, 100%) and RCOOCH2C(NO2)2F (R = Ph, 92%; R=PhCH=CH, 100%).221 The urethane FC(NO2)2CH2CH2OCONHCH2CH2C(NO2)2F is formed in 72% yield when the isocyanate FC(NO2)2CH2CH2NCO and the alcohol FC(NO2)2CH2CH2OH are refluxed for 6 h in chloroform in the presence of iron acetylacetonate.122 Study of the kinetics of the reactions of 2-R-2-nitropropane- 1,3-diols with the bis-isocyanateCH2(C6H4NCO)2 has shown that the nitro group adjacent to the hydroxy group decreases the reaction rate owing to its inductive effect and also owing to the formation of intra- and intermolecular OH.. .O2N hydrogen bonds.222 It has been found that the reaction between 2-methyl-2-nitro- propane-1,3-diol and o-phenylene diisocyanate yielding polyur- ethane follows second-order kinetics, its rate constant being 0.685 mol71 h71 at 20 8C or 1.127 mol71 h71 at 30 8C.223 Compounds 336 have been synthesised from isothiocyanates and nitro alcohols.224 Substitution of the hydroxy group in nitro alcohols has been studied fairly comprehensively.The replacement of all three hydroxy groups in the triol 243 by fluorine, chlorine, bromine or iodine atoms has been described.164 The hydroxy group in nitro alcohols 337 is substituted by an amine residue on treatment with primary or secondary amines in water; this gives compounds 338 in 60%± 89% yields.225 ± 227 If the reaction of formaldehyde with trinitromethane is carried out in water in the presence of urea at 70 8C for 2 h, the resulting (O2N)3CCH2OH reacts in situ with urea, and [(O2N)3CCH2NH]2CO is formed as the reaction product.98 Three-component condensation of the nitro-triol 4, form- aldehyde and primary amines in the presence of sodium hydro- gencarbonate gives rise to tetrahydro-1,3-oxazine derivatives 339.228 HOCH2CCH2OH +MeSO2Cl NO2 Bu MeSO2OCH2CCH2OSO2Me Bu NO2 4-ClC6H4OH, NaH, DMF 90 8C, 17 h 4-ClC6H4OCH2CCH2OSO2Me. 327 NO2 Bu FC(NO2)2(CH)2COCH2OCOCF3 . 328 (82%) FC(NO2)2(CH)2COCH2OH (F3CCO)2O R=NO2 (114) ; R=CH2OH (4). CF3SO2OCH2CCH2OH+CF3SO2OCH2CCH2OSO2CF3 331 330 R NO2 R NO2 HOCH2CCH2OH +(CF3SO2)2O NO2 R C5H5N, Et2O 20 8C, 172 h 329 4, 114 (b) 2 equiv.(CF3SO2)2O, C5H5N, CHCl3, 5 8C, 3 h. (a) 1 equiv. (CF3SO2)2O, C5H5N, Et2O, 20 8C, 16 h; HOCH2CCH2OH NO2 F 332 HOCH2CCH2OSO2CF3 NO2 F [CF3SO2OCH2]2C(NO2)F 333 a b n=1, 3, 4, 5, 6, 8, 10; R=F, NO2. RC(NO2)2CH2OH+HOC(CH2)nCOH 329, AlCl3 20 8C, 1.5 h 335 O O RC(NO2)2CH2OC(CH2)nCOCH2C(NO2)2R 334 O O R1ZCNS+HOCH(R2)CR3R4NO2 70 8C, 6 h R1ZNHC(S)OCH(R2)CR3R4NO2 336 Z=SO2, R1=Ph, R2=H, R3=H, Br, R4=H, Me (23% ± 31%); Z=CO, R1=Ph, 4-ClC6H4, 4-O2NC6H4, 2-furyl, R2=H, Me, R3=H, Br, R4=H, Me, Et (18% ± 75%). +7 +7 (a) Bu4NF, THF, MeCN, 90 8C, 12 h (75%); (b) Bu4NF, 4-MeC6H4SO2F, THF, 4A mol.sieve (86%); (c) PBr3, C5H5N, PhH, 50770 8C, 2 h (69%); (d) SOBr2, C5H5N, CH2Cl2, refluxing for 2 h (64%); (e) NaBr, (HOCH2CH2)2O, 1507170 8C, 3 h (55%). a or b c, d or e NaI, Me2CO 25 8C, 12 h SOCl2, C5H5N, CHCl3 50 8C, 2 h O2NC[(CH2)3F]3 O2NC[(CH2)3Br]3 O2NC[(CH2)3Cl]3 (80%) O2NC[(CH2)3I]3 (89%) O2NC[(CH2)3OH]3 243 R1R2C(NO2)CH2OH+R3R4NH H2O 20 8C, 20 h 337 R1R2C(NO2)CH2NR3R4 338 R1=F, Me; R2=H, Me, NO2; R3=Me, Et, Pri, Bu, Me2CH(CH2)2; R4=H, Me.Aliphatic nitro alcohols.Synthesis, chemical transformations and applications 59The compounds 339 (R = Me, Et, Pr, Pri, Bu and But) have also been obtained by another method, namely, the reaction of the nitro-triol 4 with the corresponding 1,3,5-trialkylhexahydro- 1,3,5-triazines.229 The pyrazolidine derivative 340 has been synthesised by the reaction of hydrochloride 341 with 2,2-dinitropropane-1,3-diol 114 (yield 86%) or with the potassium salt of 2,2-dinitroethanol (yield 23.5%).230 Cyclocondensation of the nitro-diols 302 or 114 with form- aldehyde and primary amines leads to hexahydropyrimidine derivatives 342. The reaction conditions and the yields of reaction products are listed in Table 16.200, 231, 232 When the nitro alcohol 202 is treated successively with phosphorus tribromide and sodium thiophenoxide, the corre- sponding thioether is formed in an overall yield of 73%.233 The hydroxy group in nitro alcohols 343 is replaced by aryl groups when 343 are treated with arenes 344 in the presence of sulfuric acid; this yields diphenylmethane derivatives 345.63, 234 ± 236 R1 R2 R3 R4 Ref.But Me H EtO 63 EtCHMe Et H EtO 63 Bui Me OCH2O 234 EtCHMe Et OCH2O 234 EtO H H EtS 234 Cl H H EtS 235 HC:CCH2O Me H H, F, Me, Pri 236 Et H MeO, PrO, HC:CCH2O On treatment of acetates 346 with sodium tetrahydridoborate in DMSO or ethanol at 20 ± 25 8C for 1.5 h, the acetoxy group is substituted by a hydrogen atom giving nitro-compounds 347 (yields 41%± 90%).27, 237 The formation of 347 from the acetates 346 involves the intermediate formation of nitroalkenes 348.237 4.Reactions involving several sites Oxidation of nitro alcohols to nitro-carbonyl derivatives can involve simultaneously two reaction sites, namely, the hydroxy group and the hydrogen atom attached to the carbon atom carrying the hydroxy group. Nitro alcohols 349 have been converted into nitro ketones 350 by treatment with chromium(VI) oxide supported on montmor- lonite at715 8C accompanied by ultrasonic treatment.72 Pyridinium chlorochromate has proved to be a good reagent for the transformation of nitro alcohols into nitro ketones.Treat- ment of nitro alcohols 349 with this reagent in the presence of molecular sieves at 20 8C gave nitro ketones 350 (yields 61%± 87%).83, 238 R=Me (32%); Pri (22%); But (77%). O2NC(CH2OH)3+CH2O+RNH2 NaHCO3, H2O 60 8C, 3 h 4 HOH2C O2N R O N 339 341 EtOCCH2NHNH2 .HCl O pH 4, 55760 8C, 15 min H2O, AcONa HOCH2C(NO2)2CH2OH HN NCH2COOEt , NO2 O2N 340 +7 341+KC(NO2)2CH2OH 340 . 60 8C, 15 min EtOH, H2O N N R R O2N R0 342 (HOCH2)2C(NO2)COOEt 302 (HOCH2)2C(NO2)2 114 CH2O, RNH2 O2NCH2CH2OH+PBr3 0 8C O2NCH2CH2Br PhSNa, THF 20 8C 202 O2NCH2CH2SPh . 4-R1C6H4CHCHR2 NO2 R3 R4 CHR2 + NO2 4-R1C6H4CH OH H2SO4 R3 R4 345 343 344 R1=Me, Et, Bu, Ph, MeCH(Et)(CH2)3; R2=H, Me; R3=H, Me, Et, Pr; R2, R3=(CH2)5.237 R1R2CCHR3 346 R1R2CHCHR3 347 NaBH4, DMSO 20725 8C OAc NO2 NO2 R1=C6H13, C7H15, CH2, , Me2C CH(CH2)2C(Me) CH(CH2)2; R2=R3=H;27 346 348 NaBH4 R1R2CHCHR3 347 R1R2CCHR3 NO2 OAc CR3 NO2 R1R2C NO2 B 7AcOH R, R0 (yield, %): Me, Me (90); Et, H (76); Pri, Me (93); Pri, Et (90). 715 8C, 373.5 h RCCH(NO2)R0 O 350 RCHCH(NO2)R0 349 OH Table 16. Reaction conditions and yields of the compounds 342. R R0 Solvent � Time Yield Ref. /8C /h (%) But COOEt EtOH 78 2 25 200 Me NO2 EtOH 5 ± 20 19 231 But NO2 MeOH 20 1 29 231 Mea NO2 see b 20 24 35 232 CH2COOMea NO2 see b 20 18 22 232 CH2COOEt a NO2 see b 20 18 15 232 a Hydrochlorides of the amines were used; b in the presence of AcONa, pH 4. 60 M-G A ShvekhgeimerIt has been found 239 that both the rate of the oxidation of EtCH(OH)CH(NO2)Et with pyridinium chlorochromate on silica gel in CH2Cl2 and the yield of the nitro ketone EtCOCH(NO2)Et formed in this reaction markedly increase upon ultrasonic treat- ment: without ultrasound the yield of the product is 60% over a period of 1.5 ± 2.5 h at 25 8C, whereas the reaction carried out with sonication is completed over a period of 20 min at 718 8C (yield 71%). On treatment with K2Cr2O7 or K2CrO4 in the presence of Bu4N+HSO74 , H2SO4 and FeSO4 at 710 8C, nitro alcohols 351 are readily oxidised to give nitro ketones 352 in high yields (70% ± 93%),67 and nitro-diols 353 are easily oxidised to nitro diketones 354.155 3-Chloro-6-nitrohexan-2-one has been synthesised in 75% yield by the oxidation of 3-chloro-6-nitrohexan-2-ol with a solution of Na2Cr2O7 in H2SO4 at 10 ± 20 8C for 10 h.58 Heating of 1-nitropropan-2-ol with hydrochloric acid (18% ± 36%) at 95 ± 107 8C for 5 ± 30 h affords lactic acid in 14% ±16% yield.240 Attempts to dehydrate compounds 356 with phthalic anhy- dride, P2O5 or MeCOCl were unsuccessful: the reactions were accompanied by rupture of the C± P bond.Dehydration of these compounds was accomplished by heating them with thionyl chloride in the presence of pyridine. This gave the corresponding unsaturated compounds 357.127 Primary and secondary nitro alcohols 358 are readily dehy- drated on treatment with cyclohexylcarbodiimide, being con- verted into mixtures of diastereoisomeric nitroalkenes 359.241 R R0 Time Yield Isomeric /h (%) ratio Z/E H Me 3.5 35 Me Me 17 82 70 : 30 H C6H13 2.5 60 Bu H 10 90 83 : 17 see a H 14 75 But H 17 94 MeCH=CH H 60 70 Bu Me 17 99 55 : 45 2-Furyl H 24 66 aMe2C=CH(CH2)2CH(Me)CH2 .The methanesulfonyl chloride ± triethylamine system has also been used for the transformation of nitro alcohols 358 into nitro- alkenes 359 (yields 30%±80%).70, 242 The nitroalkene Z-MeCH(NO2)CH2CH2CH=C(NO2)Me is formed in 80% yield when the nitro alcohol MeCH(NO2)CH2 ± CH2CH(OH)CH(NO2)Me is refluxed with MeSO2Cl and Et3N in dichloromethane under nitrogen.75 The E-isomers of nitroalkenes 359 were synthesised by dehy- dration of the nitro alcohols 358 over alumina (Brockmann activity I) at 40 8C.71 R R0 Time /h Yield (%) Me Me 8 75 Me Et 7 77 Me (CH2)4COOMe 7 76 Me CH2CH(OH)Me 7 73 EtCH=CH(CH2)2 Me 7 85 Ph(CH2)2 Me 9 68 Et H 48 60 C6H13 H 50 61 Et (CH2)6OH 7 68 cyclo-C6H11 C5H11 24 72 The dehydration occurs selectively: when the molecule con- tains both primary and secondary hydroxy groups, the secondary group is eliminated preferentially.71 It is of interest that the dinitro-diol resulting from acidification of the solution of the salt 30 with hydrochloric acid to pH 1 is dehydrated under the reaction conditions to give triene 360 in 26.7% yield.52 CHCH2;83 R=Me, Pri, R=Me,CH2 CH(CH2)2; R0=EtCH RCCH(NO2)R0 O 350 3A mol.sieves; 20 8C, 36 h PCC, CH2Cl2 RCHCH(NO2)R0 OH 349 PCC is pyridinium chlorochromate. Ph(CH2)2, O O Me H2C ; R0=Me, Et, (CH2)2COOMe, ;238 O O Me H2C CH2Cl2,710 8C, 2 h R1CC(NO2)R2R3 O 352 R1CHC(NO2)R2R3 OH 351 R2=Me, Et, H2C O O O O Me H2C , , (CH2)2COOMe; R3=H, Me.R1=Me, Pri, C6H13, Ph(CH2)2, CH2 CH(CH2)2 ; R=Me (53%), Pri (45%), C5H11 (47%), C10H21 (58%), (58%), RCHCH(NO2)CH2CHMe OH OH 353 CH2Cl2,710 8C, 2 h RCCH(NO2)CH2CMe O 354 O Me(CH2)4CH(NO2)(CH2)2 (65%). PhCH2CH2 (70%), Z-Me(CH2)7CH CH(CH2)7 (58%), R, R0 (yield, %): Me, Me (58); Me, Et (69); Et, Pri (59); Pri, Et (53).(RO)2P CCH2NO2 OH R0 356 C R0 CHNO2 357 SOCl2, C5H5N, CHCl3 60765 8C, 15 min O (RO)2P O 358 RCH CHR0 OH NO2 25735 8C Cu2Cl2, argon C6H11N C NC6H11 C C R NO2 R0 H 359 R=Me, Et, R0=Me, Et;242 R=Me, R0=PhS.75 358 RCH CHR0 OH NO2 0 8C, 15 min MeSO2Cl, Et3N, CH2Cl2, N2 359 C C R R0 NO2 H 40 8C 358 RCH CHR0 OH NO2 Al2O3, CH2Cl2 359 C C H R0 NO2 R HOCH2 CH2OH Me Me NO¡2 C C C C 7O2N 30 2Na+ HCl, H2O C H2C 360 C NO2 Me Me C C NO2 CH2 .Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 61When a mixture of 2-nitrobutan-1-ol and isoprene is kept at 150 8C, the nitro alcohol is dehydrated, and the resulting 2-nitro- but-2-ene reacts in situ with isoprene to give a mixture of [4+2]- adducts 361 and 362.243 On treatment with potassium hydroxide in Et2O for 2 h at 715 to 720 8C, the nitro alcohol O2NCH2CH(OH)CH2Cl is converted into the aldehyde O2NCH2CHCH2CHO (yield 21%).244 The reactions of 1,1,1-trinitroethanol or trinitromethane with formaldehyde in the presence of CuSO4 followed by treatment of the reaction products with ammonia at 100 ±105 8Cfor 8 ± 9 h lead to the same product, 2,2-dinitroethylamine.98, 245 Under conditions of the Mitsunobu reaction (treatment with triphenylphosphine and diethyl azodicarboxylate under an inert atmosphere),246 g-nitro alcohols 363 are converted into nitro- derivatives of cyclopropane 364.247 R R0 Yield (%) Ratio trans : cis C5H11 H 82 10 : 1 C5H11 Me 87 10 : 1 4-PhC6H4 H 75 trans 4-MeOC6H4 H 92 7 : 1 PhCH2OCH2CH=CH H 64 trans Under the same conditions, PhCH2OCH2CH(OH) ± CH(CH2OCH2Ph)CH2NO2 is converted into 1,3-bis(benzyl- oxy)-2-nitrocyclopropane in 98% yield (the ratio of the isomers was not determined).247 When hydrochlorides 365 are introduced into the Mitsunobu reaction, they are converted into hydrochlorides of 1-alkyl-3- hydroxymethyl-3-nitroazetidines 366.229 When acetates of nitro alcohols 367 and isonitrile 368 are heated in the presence of DBU, 367 are converted into nitro- alkenes 369, while the isonitrile is deprotonated to give carbanion 370.The nitroalkenes and the carbanion react to give pyrrole derivatives 371.248 Pyrrole derivatives 372 have been obtained in high yields (70% ± 86%) by cyclocondensation of the acetates 367 with alkyl cyanoacetates 373 in the presence of DBU at 20 ± 25 8C.107 Pyrazole derivatives 374 are formed in the reaction of 2,2- dinitroethanol with diazo compounds 375 in benzene or ether.249 2-(1-Hydroxy-2-nitroethyl)furan 376 reacts with bromine and sodium methoxide to give 2,5-dihydrofuran derivative 377 in 77.5% yield.142 2,4,4-Trimethyl-2-nitromethyltetrahydrofuran (yield 30%) is formed when the nitro alcohol BrCH2CMe2CH2C(OH)(Me). .CH2NO2 is kept over Al2O3 in benzene at 25 8C.250 Treatment of (S)-1-nitropentan-1-ol with 30% H2O2 and K2CO3 in methanol (0 8C, 24 h) followed by the addition of hydrochloric acid results in the formation of (S)-4,5-dihydro-5- methylfuran-2(3H)-one (yield 7%).111 Under the conditions of the Mitsunobu reaction, nitro alco- hols 378 cyclise to give eitr isoxazoline N-oxides or dihydro-1,2- oxazine N-oxides 379 depending on the mutual arrangement of the nitro and hydroxy groups.When R=PhSO2, the yield of the corresponding isoxazoline is relatively low; in this case, 1-nitro-1- phenylsulfonylcyclopropane is formed as the main reaction prod- uct. For the cyclisation products to be formed in high yields, it is necessary that an electron-withdrawing group or a double bond, which stabilise the aci-form 380 of the initial compounds, be present in the a-position with respect to the nitro group.251 R R0 A Yield (%) Ph H CH2 98 Ph H CH2CH2 93 EtOCO Me CH2CH2 94 Me H COCMe2 81 NO2 H CH2 98 PhSO2 H CH2 15 2-Hydroxy-6-methyl-5-nitrotetrahydropyran has been syn- thesised in 68% yield by cyclocondensation of acrolein with 1-nitropropan-2-ol in the presence of diethylamine and formic acid at 60 ± 62 8C for 20 h.252 EtCHCH2OH+CH2 NO2 C CH CH2 Me MeOH 150 8C, 14 h 361 362 Me + Et NO2 Me Et NO2 RCHCH2CHR0 OH NO2 363 20 8C, 14 h Ph3P, EtOOCN NCOOEt, PhH R R0 NO2 364 R=Me, Et, Pr, Pri, Bu, But.CH2OH CH2OH + RNH2CH2C(NO2) Cl7 365 EtOOCN Ph3P, NCOOEt CH2OH NO2 Cl7 366 + RHN DBU, THF refluxing for 16 h 368 367 R1CHCHR2+CNCH2COOCH2Ph NO2 OAc R, R0 (yield, %): Me, Me (72); Et, Me (74); Me, Et (76); Et, Et (74); N R1 R2 H 371 COOCH2Ph 369 R2 C R1CH NO2 7 CNCHCOOCH2Ph 370 (CH2)2COOMe, Me (60); Me, (CH2)2COOMe (53).R1=Et, (CH2)2COOMe; R2=Me, Et, (CH2)2COOMe; R3=Et, But. N R1 R2 H COOR3 372 367+CNCH2COOR3 373 DBU, THF 20725 8C, 10718 h R=OEt (PhH, 50 8C, 6 h, 46%); Me (PhH, 50 8C, 4 h, 64%); Ph (Et2O, 36 8C, 14 h, 63%).N N NO2 H 374 RCO HOCH2C(NO2)2H+N2CHCOR 375 PhH or Et2O 2. MeONa, 730 8C 1. Br2, MeOH,735 8C O CHCH2NO2 OH 376 O CHCH2NO2 MeO OH OMe 377 RCH A NO2 378 C N OH O 380 CHR0 OH R A CHR0 OH O N A R0 R O 379 62 M-G A ShvekhgeimerThe reaction of 4-nitrobutan-2-ol with sodium glyoxylate in the presence of Na2CO3 resulted in the isolation of compound 381 as two isomers (a and b), their yields being 32% and 10%, respectively.253 Tetrahydro-1,3-oxazine derivatives 382 have been prepared by refluxing compounds 383 with aldehydes 384 in benzene in the presence of sodium hydrogencarbonate.254 Hydrazones 385 react with carbon disulfide in the presence of potassium hydroxide to give 1,3,4-oxadiazine-2-thiones 386.159 On heating with sodium hydride, cyclic ketones 387 or 388 containing nitro and 3-hydroxyalkyl groups in the a-position with respect to the oxo group isomerise with ring expansion giving rise to compounds 389 or 390.255 n Time /min Yield (%) 4 45 91 10 30 87 5.Cleavage of nitro alcohols Under certain conditions, b-nitro alcohols (or b-nitro-diols) are cleaved to give the initial carbonyl derivatives and nitro-com- pounds (or their salts).In the presence of alumina at ambient temperature, nitro alcohols 391 are cleaved to nitromethane and ketones 392.250, 256 AcONa, BaO or KNaHPO4 can be used instead of Al2O3.256 Benzyltrimethylammonium hydroxide cleaves the nitro alco- hol Me2C(OH)CH(NO2)(CH2)2NO2 on refluxing in methanol for 6 h to give acetone and 1,3-dinitropropane (yield 50%).257 This process can be carried out in ethanol, dioxane, THF or DMF in the presence of Na2CO3, Na3BO3, MgO or BaCO3.257 Treatment of the alcohol 107 with sodium dichromate and sulfuric acid results in its cleavage giving formaldehyde and fluorodinitromethane (yield 65%).258 When the dinitro-diol 114 157 or 2,2-dinitropentane-1,3- diol 259 is treated with potassium hydroxide in methanol, the salt K+C7(NO2)2CH2OH and CH2O or MeCHO are formed.The reaction of compound 114 with potassium hydroxide and sodium carbonate yields salt 393, which reacts with chlorine to give 2-chloro-2,2-dinitroethanol (overall yield 48%).157 Treat- ment of the dinitro-diol 114 with chlorine fluorosulfonate affords dichlorodinitromethane (yield 15%).260 The decomposition of nitro alcohols 394 in an aqueous buffer solution with m 1.0 D and pH 6.5 ± 8.4 has been studied at 25.3 8C by spectrophotometry.The rate constants for the reaction were determined. It was postulated that deprotonation occurs rapidly, while the rupture of the C±C bond is the rate-determining step of the process.261 6. Intramolecular hydrogen bonds in nitro alcohols and nitro-diols Studies discussing the problems concerning intramolecular hydro- gen bonding in nitro alcohols and published before 1977 have been surveyed in a review.262 The configurations of the erythro- and threo-isomers of the nitro alcohols RCH(OH)CH(NO2)R0 (R, R0 =Me, Me; Pri, Me; CF3, Me; CCl3, Me; CBr3, Me; 4-O2NC6H4, Et; 4-O2NC6H4, Pri) have been established by IR and 1H, 13C and 15N NMR spectro- scopy.It was found that the erythro-isomers occur mostly in the gauche conformation A, and the threo-isomers exist predomi- nantly in the gauche conformation B. These conformers predom- inate because they are stabilised by intramolecular hydrogen bonds between OH and NO2 groups.64 O2NCH2CH2CHMe + O Me R1 R3 R2 R4 381a,b NaHCO3 25 8C OH 7+ CCONa O O H 381a: R1=R3=H, R2=NO2, R4=OH; 381b: R1=NO2, R3=OH, R2=R4=H. R, R0 (yield, %): PhCH2, Me (80); cyclo-C6H11, Me (60); PhCH2, Ph (60). NO2 RNHCH2C(Me)CH2OH+R0CHO 383 384 NaHCO3, PhH 80 8C, 5 h N O R0 R O2N Me 382 CCH(OH)R0 +CS2 NO2 60770 8C, 0.5 h; 20 8C, 6 h KOH, DMF 4-RC6H4NHN 385 R=H, R0 =Me (71%); R=Me, R0 =H (88%), Me (78%).N O N NO2 R0 S 4-RC6H4 386 NaH, MeOCH2CH2OMe 85 8C 389 (81%) NO2 387 (CH2)2CHMe OH O O Me O NO2 NaH, MeOCH2CH2OMe 85 8C, 1 h (CH2)n O NO2 (CH2)3OH 388 O NO2 O (CH2)n 390 R=Br (20%),250 NO2 (72%).256 RCH2CCH2CCH2NO2 391 OH Me Me Me Al2O3, PhH 7MeNO2 RCH2CCH2CMe 392 Me Me O Cl2C(NO2)2 .ClSO2F 725 to720 8C, 1.5 h; 0 8C, 2.5 h 114 HOCH2CCH2OH NO2 NO2 114 KOH, MeOH 0 8C ClC(NO2)2CH2OH, 393 +7 KC(NO2)2CH2OH Cl2, H2O Ar, 104 k (litre mol71 s71) : Ph, 3.02 (pH 4.6 ± 8.4); 4-MeC6H4, 3.37; 4-MeOC6H4, 5.09; 4-ClC6H4, 4.22; 4-O2NC6H4, 4.54; 3-ClC6H4, 3.79; 3-MeC6H4, 3.07.O2NCMe2CHO7 k2 k72 Ar O2NCMe2CHOH Ar 394 k1[B] k71[BH+] 7 O2NCMe2+ArCHO Aliphatic nitro alcohols. Synthesis, chemical transformations and applications 63The conformation equilibria for the alcohols RCH(OH)CH2NO2 [R = H (202), Me, Ph, CCl3] and for 2- nitropropane-1,3-diol 3 have been studied.In the case of the alcohols, equilibrium conformations A1, B1 and C1 were consid- ered.263 For the diol 3, equilibrium conformations A2, B2 and C2 were discussed. It was found that the nitro alcohol 202 (R = H) exists predominantly in conformations A1 and C1. In other cases (R = Me, Ph, CCl3), conformation C1 predominates.The dihedral angle of 100 ± 125 8 is in agreement with the synclinal arrangement of the OH and NO2 groups and with the formation of a hydrogen bond between them. In the case of the nitro-diol 3, conformer A2 with the synclinal arrangement of the CH2OH and OH groups predominates in the equilibrium. Thus, the interaction between the two hydroxy groups is stronger than that between the OH and NO2 groups.263 IV.Practical use of aliphatic nitro alcohols Possible applications of aliphatic nitro alcohols have been dis- cussed in a large number of publications. Aliphatic monoatomic and polyatomic nitro alcohols and their derivatives both in a pure state and in various compositions are used most widely as bio- logically active compounds and as components of rocket fuels or explosives.Other applications of these compounds have also been proposed. 2-Bromo-2-nitropropane-1,3-diol 8 is used most often as a biologically active compound known as `Biocide'. This nitro-diol as well as nitro alcohols of the general formula R1R2C(OH)CR3(Br)NO2 (R1 = H, alkyl, phenylalkyl; R2 = H; R1R2 = cycloalkyl; R3 = H, Me, Et, Br) are active against Staphylococcus aureus and Pseudomonas aeruginosa.264 ± 266 The nitro-diol 8 has been used as an antimicrobial additive 267 and as a component of agents controlling the amount of slime in wood pulp during the manufacture of paper,265, 268 antimicrobial com- positions controlling Chlorella,269 antiulcer compositions 270 and compositions controlling the development of Cosmarium.271 It has been proposed to use the bis-formate of nitro-diol 8 and 5-bromo-5-nitro-1,3-dioxane and its 2-substituted derivatives, resulting from condensation of 8 with aldehydes, as bactericides and fungicides;190, 191, 220 the mixed ester 4-ClC6H4OCH2 ± C(NO2)(Br)CH2OSO2Me has been used as a component of an agent that fully controls the growth of Erysiphe gramin.219 The nitro-diol 8 has been patented as a deodorant for the toilets of buses and campers,272 as a deodorant for refuse bins 273 and as a component of compositions removing NH3, H2S and MeSH from aircraft toilet waste waters.274 The use of the nitro-diol 8 and 2-chloro-2-nitropropane-1,3- diol as inhibitors of light- and heat-induced paint changes has been patented.275 The polyester obtained by polycondensation of 8 with tereph- thaloyl dichloride has been claimed as an agent for cotton processing in order to make cotton fabrics resistant against Escherichia coli JFO 3134 and Staphylococcus aureus JFO 12732 and more comfortable for wearing.276 The nitro alcohols R1C(NO2)(X)CH(OH)R2 (R1, R2 = H, halogen, substituted alkyl; X = halogen) have been patented as components of microbicide agents, in particular, of those active against Alternaria tenuis,47 Pseudomonas aeruginosa,277 Plasmo- diophora brassicale 278 and Baccillus subtilis.279 The nitro alcohols Cl3CCH(OH)CH(Br)NO2,7 2-nitro-1-(2- and 4-pyridyl)propan-1-ols,280 EtC(NO2)(Br)CH2OH,35 2-nitro- 1-(2- and 4-quinolyl)propan-1-ols and 2-nitro-1-(2 and 4-quino- lyl)propane-1,3-diols 281 have been used as bactericides and fungi- cides. 2-Hydroxymethyl-2-nitropropane-1,3-diol 4 has been recom- mended as an agent for treating water infected with Streptococ- cus.282 FC(NO2)2CH2OH 107 suppresses the growth of Escherichia coli or Staphylococcus aureus, while 2-chloro-2,2-dinitroethanol exhibits antispasmodic activity.283 2-Methyl-4-nitropent-3-en-2-ol has been reported to improve blood circulation.284 A broad spectrum of biological activities is exhibited by the nitro alcohols R,R0CH(OH)C(NO2)(X) (R,R0,X=CCl3, Me, Cl; H, Pr, Cl; Me, Cl, Cl; Pri, H, Br; Bu, H, Br; C5H11, H, Br; C6H13, H, Br; Me, Me, Br; Et, Me, Br; CCl3, Me, Br; H, Et, Br; Me, Et, Br; Me, Br, Br; Et, Br, Br; Pri, Br, Br.57 It has been proposed to use formate, methyl carbonate and some other esters of nitro alcohols RC(NO2)2CH2OH (R = Cl, Br, Me) as bactericides and fungicides.157, 285 The compounds R1ZNHC(S)OCH(R2)CR3R4NO2 (R1, Z, R2, R3, R4 = Ph, SO2, H, H, H; Ph, SO2, H, Br, Me; Ph, CO, H, H, H; 4-O2NC6H4, CO, H, Br, Me; 4-O2NC6H4, CO, Me, H, Et; 4- ClC6H4, CO, H, H, H; Ph, CO, H, H, Et; 4-O2NC6H4, CO, Me, H, Me; 2-furyl, CO, H, H, H) have been patented as fungicides and bactericides for deodorants, creams and cleaning compositions,224 while the esters (2-thienyl)COOCHRCHR0NO2 (R=H, Me, Pr, Pri;R0=H, Me, Et) have been used as fungicides and bactericides for deodorants, soaps and creams.286 The nitro alcohols RR0C(NO2)(CH2)nOH (R, R0 =H, Cl, Br, Br; n = 1 ± 3) and their acetates have been recommended as components of compositions improving the properties of paper, dyes, cosmetics and adhesives.287 The addition of the nitro-diol 8 to a mixture used for the development of photographic materials inhibits contamination of waste waters.288 The nitro alcohols RC(NO2)BrCH(R0)OH (R = CH2OH, lower alkyls; R0 =H, lower alkyls) increase the stability of dyes in photographic materials with improved photosensitive layers.289 These nitro alcohols combined with isothiazol-3-one derivatives permit high-quality development of photographic materials even at low pH without decreasing the sensitivity.290 The nitro alcohols RC(NO2)(Br)CH(OH)R0 (R, R0 =H, Me, Et) are added to paper pulp in order to improve the properties of photographic paper.291 A film coated by a composition of polyamide with 2-hydr- oxymethyl-2-nitropropane-1,3-diol 4 becomes more resistant to abrasive wear.292 H OH R NO2 H R0 H OH R R0 H NO2 A B NO2 H H OH R H NO2 H H R H OH NO2 H H H HO R A1 B1 C1 NO2 H HOCH2 H HO H NO2 H HOCH2 OH H H NO2 H HOCH2 H H OH A2 B2 C2 S COOCHRCHR0NO2 R=H, Me, Pr, Pri; R0=H, Me, Et. 64 M-G A ShvekhgeimerThe use of the nitro alcohols R1R2C(NO2)CH(OH)R3 [R1, R2= H, C1 ± 5 alkyl, CH2OH; R3 = H, alkyl (C1 ± 5)] as compo- nents of toning agents protecting colloidal photographic materials from X-rays has been patented.293 The nitro-diol 4 has been used as a stabiliser of developers for colour photography.294 The compound 4 and 2-methyl-2-nitro- propane-1,3-diol have been recommended as agents increasing light resistance of dyes.295 2-Methyl-2-nitropropan-1-ol can be used for the preparation of polymerisation inhibitors for esters of unsaturated carboxylic acids.296 The nitro alcohols XX0C(NO2)CH(R)OH (X, X0=Hal; R= H, alkyl) are used to prevent fur formation.297 2-Methyl-3-nitropropan-2-ol has been used as a plasticiser for cellulose triacetate.298 The same nitro alcohol blended with the product of condensation of urea and formaldehyde imparts permanent-press and wrinkle-resistant characteristics to cotton textiles.299 The nitro-diols O2NC(R)(CH2OH)2 (R = Me, Et, CH2OH) react with melamine at 70 8C and pH 9.5 ± 10 to give adhesive resins that join surfaces under elevated pressures at 250 ± 300 8C.300 2-Methyl-2-nitropropan-1-ol has been used to improve adhe- sion of rubber to a steel cord.301 The product of reaction between 2-ethyl-2-nitropropane-1,3- diol and tetraethylenepentamine, which solidifies over a period of 15 min at room temperature, has been recommended as a coating for steel sheets.302 The nitro alcohols RC(NO2)(X)CH(OH)R0(R=H, CH2OH, lower alkyls; R0= H, lower alkyls; X = Hal) have been patented as agents protecting metal working fluids from bacteria and fungi.303, 304 2-Nitropropan-1-ol increases the covering power, the time of hardening and the strength of cements.305 3-Nitrobutan-2-ol has been used as a selective solvent for dearomatisation of directly distilled petrols.306 The nitrate MeCH(ONO2)CH(NO2)Me increases the cetane number of diesel fuels.307 Halo-substituted nitro alcohols of the general formula R1[CH(OH)]nC(NO2)(X)CR2R3OH [R1, R2, R3=H, alkyl (C1 ± 7); X = Cl, Br, I; n = 0, 1], in particular, the nitro-diol 8 have been claimed as antimicrobial agents for hydrocarbon fuels.308 Polynitro alcohols and their derivatives are important com- ponents of rocket fuels and explosives.The compound (O2N)2C(F)CH2OCOOCH2CF3 has been used as a plasticiser for plastic explosives.205 The formate, methyl carbonate and some other esters of MeC(NO2)2CH2OH have been used as plasticisers for rocket fuels;157 the compound [FC(NO2)2CH2]2NCOOCH2C(NO2)2F has been patented as a plasticiser for explosive compositions.309 Among the derivatives of polynitro alcohols proposed as plasticisers for high-energy compositions, the following com- pounds should be noted: [EtOCOCH2C(NO2)2F]2Me,187 (RO)2CHMe, MeCH(OR)OCH2C(NO2)2R0 [R= CH2C(NO2)2F, R0=F, Me, NO2]186 [RC(NO2)2CH2O]2CH2 (R=Me,182, 310 F,311 NO2 182), [RC(NO2)2CH2O] ± CH2[OCH2C(NO2)2R0] (R, R0=NO2, Me;182 Me, Et 310), RC(NO2)2CH2OC[=CHCOOCH2C(NO2)2R]COOCH2(NO2)2- R0 (R=R0=H, F, Me, NO2).218 The use of the orthoester [FC(NO2)2CH2O]2CH±[OCH(- Me)C(NO2)2F] as a co-plasticiser in high-energy compositions has been patented;312 the orthocarbonate [FC(NO2)2CH2O]3- COCH2C(NO2)3 has been used as a plasticiser and as an energetic explosive.211 2-Fluoro-2,2-dinitroethanol 107 can be used as a plasticiser and a binding agent for rocket fuels and explosive compositions;91 the ether (O2N)3CCH2OC(NF2)3 can be used as a plasticiser and an oxidant for rocket fuels.180 The following compounds have been patented as energetic binding agents for high-energy compositions: the vinyl ethers RC(NO2)2CH2OCH=CH2 (R=F, NO2) 313 and FC(NO2)2 ± CH2C(NF2)2CH2OCH=CH2,213 nitroformals synthesised by polycondensation of the tetranitro-diol [HOCH2C(NO2)2CH2]2 with formaldehyde 185 or by polymerisation of the epoxide 314 and esters formed by 2,2-di- nitropropane-1,3-diol 114 with oxalic, malonic or fumaric acids.315 The polyester resulting from the reaction of the dichloride [ClCOCH2CH2]2C(NO2)2 with the diol [HOCH2C(NO2)2CH2]2O has been patented as a component of energy-producing composi- tions.316 The nitro-derivatives [RC(NO2)2CH2O]2C=S (R = F, Me, NO2) have been used as intermediate compounds in the synthesis of energetic explosives.212 References 1.D Seebach, E W Colvin, F Lehr, T Weller Chimia 33 1 (1979) 2. 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ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. Data on hydrogenation of mechanical alloys and intermetallic compounds formed in the course of mechanical alloying and mechanical milling are generalised. It is shown that mechanochemical methods make it possible to solve a number of problems arising in the hydrogenation of metals and alloys and to synthesise novel hydrides and nanocomposite materials. The mechanism of hydrogenation of mechanical alloys is discussed.The bibliography includes 98 references. I. Introduction The diversity of practical tasks associated with the accumulation of hydrogen and the use of metal hydrides and intermetallic compounds (IMC) in various technologies stimulate the develop- ment of research directed to the search for novel metal ± hydrogen systems with definite structural, kinetic, and thermodynamical characteristics.This type of research consists of the use of alloys and solid solutions based on the known hydride-forming metals, IMC and composite materials obtained from hydride-forming components containing binders that are inert with respect to hydrogen.1 LaNi5, TiFe, and magnesium and its alloys are considered to be the most promising from the viewpoint of the accumulation of hydrogen.2, 3 These materials are traditionally prepared by alloying and by the methods of powder metallurgy.One substantial problem is the activation of these materials which are usually covered by a layer of oxides and hydroxides, thus preventing the chemisorption of hydrogen at the surface and resulting in long induction periods and low reaction rates in the initial hydrogenation.To achieve the highest reaction rate it is necessary to perform several cycles of hydrogenation and dehydrogenation (cycling), which result in the dispersion of the alloy and the development of its microstructure. The microstructure plays a significant role in further cycling and affects considerably the reaction rate and the amount of absorbed hydrogen.In addition, traditional methods of preparation of alloys are mainly restricted by thermodynamically stable compounds and solid solutions. Mechanochemical methods (mechanical alloying, mechanical milling, etc.) consist in mechanical treatment of metal powders in high-energy planetary ball mills. This treatment results mainly in plastic deformation of the material under conditions that fix its metastable state.These methods can be applied to the synthesis of metastable phases: amorphous phases,4 supersaturated solid solutions, non-stoichiometric intermetallic compounds,5 quasi- crystals,6, 7 and composites with different microstructure and composition including those with non-interacting components.8 These phases often exhibit unusual physicochemical properties and enhanced reactivity.8, 9 Mechanical alloying, as shown in the present review, can be a promising method for the preparation of materials with high reactivity with respect to hydrogen.II. Magnesium systems 1. Problems arising in the hydrogenation of magnesium and its alloys Magnesium and its alloys are promising materials for the accu- mulation of hydrogen.The stoichiometric hydrogen content in magnesium hydride is 7.6 mass% (Table 1), which exceeds its content in other known hydrides and also in gas cylinders.10, 11 However, the relatively high thermal stability of magnesium hydride (equilibrium hydrogen pressure of 0.1 MPa is achieved at *560 K), insufficiently high rates of hydrogenation of magne- sium and dehydrogenation of MgH2, long-lasting activation, and incomplete conversion of magnesium into the hydride (which decreases the hydrogen content to 5 mass %) have stimulated the appearance of research projects aimed at the possibility of improving the above-mentioned characteristics.It has been established to date that the formation of magne- sium hydride is connected with the processes of formation and growth of MgH2 nuclei.For the majority of metal ± hydrogen systems, the first stage of the interaction of hydrogen with a metal is characterised by the formation of a solid solution of hydrogen in a metal, in which nuclei of the hydride phase are crystallised.12 The formation of solid solutions of hydrogen is not typical of magnesium, or more precisely, their concentrations are low.13 ± 15 The hydride nuclei appear at the surface of magnesium particles; when the nuclei overlap, the reaction rate is controlled by diffusion processes.`Diffusion retardation' then occurs, which prevents the complete conversion of magnesium into the hydride. In the first cycle of hydrogenation, the extent of conversion does I G Konstanchuk, V V Boldyrev Institute of Solid State Chemistry and Processing of Mineral Raw Materials, Siberian Branch of the Russian Academy of Sciences, ul.Kutateladze 18, 630128 Novosibirsk, Russian Federation. Fax (7-383) 232 28 47. Tel. (7-383) 232 96 00 (I G Konstanchuk), (7-383) 232 15 50 (V V Boldyrev) E Yu Ivanov TOSOH SMD, 3515 Grove City Road, Grove City, OH 43123, USA. Fax (1-614) 875 00 31. Tel. (1-614) 875 79 12 Received 10 March 1997 Uspekhi Khimii 67 (1) 75 ± 86 (1998); translated byMG Ezernitskaya UDC 541.44+541.124 Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen I G Konstanchuk, E Yu Ivanov, V V Boldyrev Contents I.Introduction 69 II. Magnesium systems 69 III. Hydrogenation of FeTi and LaNi5 undergoing mechanical milling 76 IV. The direct formation of hydrides during mechanical milling 77 V.Conclusion 77 Russian Chemical Reviews 67 (1) 69 ± 79 (1998) #1998 Russian Academy of Sciences and Turpion Ltdnot usually exceed 0.9, in subsequent cycles it does not exceed 0.6 ± 0.7.16 ± 18 Kinetic characteristics of hydrogenation processes in the first cycle differ substantially from those in the later cycles.The surface layer of magnesium oxide, which is always present on the original metal particles before the first hydrogenation, inhibits the for- mation of magnesium hydride nuclei, because no dissociative adsorption of hydrogen on this layer occurs.19 Therefore, the rate of hydrogenation of magnesium in the first cycle is deter- mined by the processes of destruction of the oxide film and formation of a metallic surface with hydrogen chemisorption ability.19 ± 21 A model of the first cycle of hydrogenation of magnesium taking into account the dynamics of the destruction of the oxide film and kinetics of the nucleus growth has been proposed.22 During activation through cycling, exfoliation of the oxide film occurs. The initial reaction rate is determined by the dis- sociative chemisorption of hydrogen on a pure metal surface, and the nucleus growth occurs due to the surface diffusion within the layer of the chemisorbed hydrogen.23 An increase in the number of nuclei with an increase of DP (the difference between the pressure in the reactor and the hydrogen equilibrium pressure for magne- sium hydride at the temperature of the experiment), which is a driving force of the reaction, or the introduction of catalysts accelerates the initial stage of hydrogenation, leads to the faster formation of the continuous layer of the hydride, and to a change over to diffusion conditions at lower conversion rates.24 It was shown by experiments on the movement of the Kirkendal mark that hydrogen atoms or ions are the diffusing species.25 The rate of decomposition of magnesium hydride depends on the dynamics of the formation and growth of nuclei of magnesium metal.23 The desorption of hydrogen from the surface of the solid reaction product, metallic magnesium, is the limiting stage of the reaction; for this reason, the size of the Mg/MgH2 interface plays a significant role.Thus, the rate of magnesium hydrogenation in the first stage and the rate of decomposition of magnesium hydride are deter- mined by the processes of adsorption (or desorption) of hydrogen on the metal surface (in the first hydrogenation, by the formation of the metal surface), by the rates of formation and growth of nuclei of the reaction product, and after the creation of the continuous layer of MgH2 in the hydrogenation, by the rate of diffusion of hydrogen through the hydride.Hence, changes in the kinetics of hydrogenation and dehydrogenation can be achieved by controlling these stages. In the majority of studies dealing with the improvement of kinetic and thermodynamic characteristics of magnesium in the interaction with hydrogen, its alloys with other metals have been used. However, it should be noted that in the case of magnesium, the choice of these metals is rather limited for the enthalpy of mixing of magnesium with many elements of the Periodic Table is positive.To date, only two magnesium intermetallic compounds are known that absorb hydrogen reversibly, viz., Mg2Ni and Mg51Zn20, and the corresponding hydrides, MgNiH4 (Ref. 26) and Mg51Zn20Hy (y = 90 ± 95).27 The hydride Mg2NiH4 is regarded as a promising one for the accumulation of hydrogen, because it interacts with hydrogen at a high rate at temperatures of about 473 ± 573K and PH2 '0.5 ± 1.0 MPa,28 although its hydro- gen capacity is less than that of magnesium (see Table 1).At the first stage of hydrogenation of Mg2Ni under the conditions ensuring fast heat removal and at rather high hydrogen pressures, chemisorption of hydrogen on the surface of IMC with an effective activation energy of 14.00.8 kJ mol71 is the limiting stage.29 However, due to high rates of hydrogenation and dehy- drogenation, the processes of heat and mass transfer play a significant role and may become limiting, especially if this IMC is used on a technological scale.The creation of composite materials containing inert metallic fillers is the way to solve the problems of heat and mass transfer,30 but this further decreases the hydrogen capacity of an accumulator.The interaction of IMC of magnesium and of some other elements, such as rare earth metals, Cu, Al, Ca, with hydrogen leads to the decomposition of the intermetallic compound to form a mixture of two hydrides or a hydride and a metal (or another IMC).This process was termed hydrogenolysis,31, 32 in general it can be represented by the equation AxBy+ m 2 H2 xAHm7n+yBHn , where A, B are elements constituting the IMC; n=0 if the second element does not give a hydride. Hydrogenolysis can be reversible, as in the case of Mg2Cu,33 and irreversible, as for most IMC of magnesium with rare earth metals and Al.11, 32, 34 ± 40 The reversibility is determined by the relationship between the values of the heat of formation of the binary hydride and the initial IMC.The mechanism of hydro- genolysis has not been studied in detail. No complete separation of hydrides or a hydride and a metal (IMC) resulted from hydro- genolysis occurs. The material obtained is composed of particles with a developed interface between its constituents 35, 36 which determines the reactivity of these particles in further hydrogena- tion ± dehydrogenation cycles. Of the systems that undergo hydrogenolysis, the main atten- tion has been paid to IMC of magnesium with rare earth metals.Hydrogenolysis results in a non-stoichiometric hydride LnHx, which plays the role of a `hydrogen pump', which facilitates the delivery of hydrogen to the magnesium surface and thus accel- erates hydrogenation.35 ± 37, 41 However, no significant enhance- ment of the kinetics of dehydrogenation of these reaction mixtures occurs in comparison with pure MgH2.34 ± 36, 40 In order to accelerate dehydrogenation, it is necessary to introduce elements that facilitate the processes of hydrogen desorption and the formation of the metallic phase nuclei, for example, d-met- als.34, 37, 42 ± 44 These additives form a separate phase as a result of cycling. After hydrogenolysis the material is a virtually multi- phase system, its features being typical of those for multi-phase alloys.Table 1. Characteristics of hydrogen storage systems.10, 11 Method Hydrogen content Hydride density Accumulated energy Hydrogenation of storage /g cm± 3 rate (mass %) /1022 at cm± 3 /kJ g71 /kJ cm73 H2, gas 1.2a 0.9 7 142.7 2.13 7 20 MPa, 300 K H2, liquid 412 a 4.2 7 142.7 9.96 7 LaNi5H6 1.4 5.5 6.2 2.0 13.04 High TiFeH2 1.9 6.3 5.5 2.7 14.93 " MgH2 7.6 6.6 1.5 10.8 15.64 Low MgNiH4 3.6 5.8 2.7 5.1 13.75 High Gasoline 7 7 7 48 38 7 a With account of the mass of the storage system. 70 I G Konstanchuk, E Yu Ivanov, V V BoldyrevTwo- and multi-phase systems are alloys of magnesium with other elements, the composition of which does not correspond to the stoichiometric composition of intermetallic compounds. These can be alloys with Ni, Cu, rare earth, and some other metals. Hydrogenation of alloys of magnesium with rare earth metals yields a mixture of binary hydrides, as in the case of hydro- genolysis of IMC.The compositions formed differ, first of all, in specific features of the microstructure and in the size of the magnesium/lanthanide hydride interface. In the decomposition of a non-stoichiometric alloy, the interface is, as a rule, smaller than that in hydrogenolysis of IMC, which results in a decrease in the rates of the interaction of these systems with hydrogen.The alloys Mg± Ni and Mg± Cu are two-phase systems containing magnesium and IMC Mg2Ni or Mg2Cu. Hydrogena- tion of these alloys can be carried out in two versions: under conditions where both phases are hydrogenated and under con- ditions where only magnesium is hydrogenated and the interme- tallic compound serves only as a catalyst.However, in both cases dissociative hydrogen adsorption takes place on clusters of the d- element resulting from segregation occurring in the course of hydrogenation ± dehydrogenation.{ Studies of the surface of alloys by ferromagnetic resonance,45 X-ray photoelectron and Auger spectroscopy 46, 47 gave reason to adopt a model of the structure of the surface of the alloy and the scheme of the initial stage of hydrogenation, which accounts for the catalytic activity of transition metals.47 ± 49 The essence of the model is as follows: 1.Activation of the material in hydrogenation ± dehydro- genation cycles leads to dispersion of the sample, that is, to the formation of a new clean surface. 2. In the interaction with traces of oxygen and water, clusters of a d-element are formed on this surface (the so-called oxidative segregation). 3. Dissociative chemisorption of hydrogen takes place on the clusters of the d-element. 4. The hydrogen atoms formed diffuse into the bulk of the material along the metal/oxide interface . The course of the reaction, as was mentioned above, depends largely on the size of the magnesium/catalyst interface and accordingly, on the size of magnesium particles in the alloy.The size of magnesium crystallites determines the degree of its conversion into the hydride. The process of nucleation is accel- erated under the action of catalytic additives, which leads to faster formation of a continuous layer of the hydride and to a change to the diffusion control of the reaction.24 In the case of larger magnesium particles, these processes slow down and practically completely stop the reaction long before complete conversion.The portion of magnesium that does not enter the reaction decreases and the hydrogen content increases with a decrease in the size of particles in the alloy. Hence, the main problems in the creation of two- and multi- component materials for accumulation of hydrogen are, first, the choice of a catalyst and determination of its optimum concen- tration at which the maximum hydrogen content is achieved over a given time and second, the creation of the microstructure of the sample ensuring the maximum magnesium/catalyst interface.In addition, the problem of activation exists for all the alloys. The rate of the first hydrogenation is determined by the rate of the rupture of the oxide film, by the rates of the formation and growth of magnesium hydride nuclei.The effect of additives reduces most commonly to a decrease in the induction period and to an increase in the rate of the first hydrogenation as compared to analogous characteristics for pure magnesium,17, 24, 50, 51 however, the dura- tion of the first cycle remains too high. 2. The interaction of mechanical alloys of magnesium with hydrogen A new approach to the creation of two- and multi-component systems interacting with hydrogen has been proposed.52 ± 55 It is based on the use of mechanical alloying. In the initial stage of processing of a mixture of metal powders (in *5 min) in centrifugal planetary mills, composites are formed with a consid- erable interface similar to those resulting from hydrogenolysis of IMC, but the hydrogenolysis stage is excluded.These composites were termed mechanical alloys (MA). A typical (rather porous) structure of a particle of a mechanical alloy with uniformly distributed particles of a metal catalyst within the magnesium matrix is shown in Fig. 1.55 Wide angle X-ray scattering (WAXS) of these materials did not reveal the formation of either interme- tallic compounds or solid solutions.The processes of hydrogenation and dehydrogenation have been studied for the systems Mg± Ce;56 Mg± Fe, Mg± Co, Mg± Ti, Mg± V, Mg± Nb, Mg± Cr, Mg± C, Mg± Si;52 ± 54, 57 ± 59 Mg± Ni;29, 52, 53, 60 ± 63 Mg± TiO2;64 ± 66 Mg±V2O5, Mg± Cr2O3;66 Mg±MnO2; Mg± Fe2O3, Mg± NiO;67 Mg±MmM05 (MM05 is IMC of the type LaNi5, Mm is a mischmetall, M0 is Ni with admixtures of Al, Co, and Mn);68, 69 Mg± FeTi,70 etc.Thus, mechanical alloying allows the considerable extension of the range of systems under study including mixtures that could not be alloyed by traditional methods, such as Mg± Fe, Mg± Ti, Mg± Nb, and Mg± Cr with positive enthalpy of mixing of the components, or mixtures of magnesium with oxides.It is also very difficult to prepare from the latter composites with homogeneous distribution of the components by traditional alloying methods. The additives to magnesium used in mechanical alloys differ in their nature. Thus the above-mentioned additives can be divided into six groups. 1. Ni forms with magnesium the intermetallic compound Mg2Ni, which can absorb and liberate hydrogen. 2. Ce, Nb, and Ti form hydrides CeH3, NbH2, and TiH2, which can serve as `hydrogen pumps' due to the change in stoichiometry. 3. MmM0 and FeTi are intermetallic compounds, which absorb hydrogen reversibly under milder conditions compared to magnesium. 4. Fe, Co, and Cr are metal catalysts, which do not form hydrides themselves under the conditions studied.{ Segregation processes occur due to the presence of traces of oxygen and water in the hydrogen. Figure 1. A particle of mechanical alloy Mg± Ni at a fracture; the image in the Ni Ka radiation. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 715. Si and C form with magnesium compounds with predom- inantly covalent bonds. 6. Metal oxides. All the types of catalytic additives accelerate remarkably the first hydrogenation as compared to that of pure magnesium 52, 67 (Fig. 2). The character of the action of additives (except for Si and C) seems to be the same and consists in the acceleration of the stage of chemisorption of hydrogen on the surface of a catalyst.It is of note that mechanical alloys Mg± Ni are hydrogenated during the first cycle faster than the corresponding ordinary alloys Mg± Ni (Fig. 3).61 The rate of the first hydrogenation depends on the nature and concentration of the catalyst, but in almost all the systems examined, the reaction begins at the maximum rate (see Figs 2, 3). This means that the surface of the catalyst in mechan- ical alloys, unlike that in traditional alloys, is accessible to hydro- gen.X-ray photoelectron spectroscopic studies 29, 71 of the surface of MA have shown that it is covered with a layer of organic impurities and oxygen-containing magnesium compounds. The thickness of this layer is small, and it does not seem to affect the hydrogenation process. The profiles of concentration of the elements for magnesium MA containing 50 mass% Ni are shown in Fig. 4. The surface layer (3 ± 5 nm) contained neither metallic magnesium, which appeared at a depth of 40 nm, nor nickel, which appeared at a depth of *4 ± 10 nm in the oxidised state and at a depth of 40 nm in the metallic state. The presence of oxygen throughout the entire layer provides evidence for the destruction of the surface MgO film and its dispersion in the matrix.Carbon-containing impurities also penetrate into the bulk upon mechanical alloying. The chemical state of the elements in the near-surface layer of MA differs significantly from that in the original metals. The contour of the nickel 2p3/2 line at a depth of 40 nm corresponds to metallic nickel and to the nickel implanted into an inert matrix as isolated atoms.In addition, charge transfer from magnesium to non-bonding levels of the nickel atoms has been observed. In this case, a portion of magnesium is in the state to which the formal oxidation degree of +1 can be ascribed. The chemical state of oxygen also differs from that in the original metals and MgO, which might be due to defectiveness of the surface layer of MA.29, 71 The nickel atoms resulting from the chemical interaction between Mg and Ni in the course of mechanical alloying bear an excessive electron density and are active catalysts of hydrogen dissociation.The nickel atoms are, probably, present not only in the surface layer, but also in the bulk of the sample forming a 0 50 100 150 t /min 1.0 2.0 [H] (mass %) b 1 2 3 10 20 t /h 2.0 4.0 [H] (mass %) a 1 2 3 5 4 7 6 8 Figure 2.Kinetics of the first hydrogenation of magnesium and its mechanical alloys. (a): T=625K and PH2=1.5 MPa; (1) Mg+5 mass%of Co; (2) Mg+ 5 mass% of Nb; (3) Mg + 5 mass% of Fe; (4) Mg + 5 mass% of C; (5) Mg+8 mass%of Ce; (6) Mg+5 mass%of Ti; (7) Mg+5 mass% of Si; (8) metallic magnesium, particle size is 20 mm; (b): T = 615 K, PH2 = 1.5 MPa; (1) Mg + 8 mass% of Fe; (2) Mg + 10 mass%of Fe2O3; (3) Mg+10 mass%of NiO. 0 10 20 t /min 0.2 0.4 0.6 1 2 3 4 5 a Figure 3.Kinetics of the first hydrogenation of mechanical alloys of magnesium with nickel. [Ni] (mass %): (1) 60, (2) 53, (3) 35, (4) 1; (1 ± 4) at PH2 = 1.8 MPa, T = 583 ± 593 K; (5) ordinary alloy of Mg with 25 mass% of Ni at PH2 = 2.1 MPa, T=673 K.[Mg, Ni, O, C] (at.%) 15 10 5 0.2 0.1 Ni/Mg 0 100 200 h /nm 5 3 4 2 1 Figure 4. The dependence of the chemical composition on the etching depth (h) for a mechanical alloy of Mg with 50 mass%of Ni. (1) Mg, (2) Ni, (3) O, (4) C, (5) Ni/Mg. 72 I G Konstanchuk, E Yu Ivanov, V V Boldyrevsupersaturated solid solution around the nickel particles upon dispersion of nickel within the matrix.Other MA of magnesium with metals and IMC, even those having a positive enthalpy of mixing, seem to have a similar struc- ture. This can be indirectly proved by the fact that the initial hydro- genation rate of mechanical alloys Mg± Fe and Mg± Ni depends linearly on the square of the concentration of the catalyst inMA.59 Hence, disordering of the surface layer, the rupture of the oxide film, and dispersion of the catalyst take place in mechanical alloys.Disordering of the surface layer makes the catalyst accessible to hydrogen, which is the main reason for the rapid first hydrogenation of MA as compared to that of alloys and mixtures of the same composition.29, 71 It should be noted that this surface structure is quite stable over a long period: the initial rate of hydrogenation of MA that have been stored in air for 9 ± 12 months did not differ from that of freshly prepared samples.29 The high reactivity (with respect to hydrogen) of mechanical alloys of magnesium with transition metal oxides may also be accounted for by the presence of metallic particles and partially reduced metal oxides at the surface formed in the course of mechanical processing.64 ± 67 The efficiency of the action of metal oxides as catalysts depends on the ease of their reduction.66 For the majority of systems examined (except for Mg± Ce and, under special experimental conditions that will be considered below, Mg± Fe and Mg± Co), magnesium hydride (the second phase, catalyst) is the product of the first hydrogenation.The degree of conversion of magnesium into the hydride, as in two- phase systems, in these MA depends on their microstructures.A model of a microstructure reflecting qualitatively the initial stage of hydrogenation is presented in Fig. 5: MgH2 is formed around the catalyst particles. It is difficult for hydrogen to penetrate through the layer of hydride; that is why the reaction slows down with an increase in the thickness of the layer.The thickness of the hydride layer, which depends on the distance between particles of the catalyst, decreases with an increase in the concentration of the catalyst, and the degree of conversion also increases. However, the hydrogen content (mass %) in the alloy decreases due to the large mass of catalyst. It has been found experimentally 59, 63 that the optimum catalyst content was in the range of 2 mass%± 25 mass %.In subsequent cycles, MA retain high rates of hydrogenation and dehydrogenation (Figs 6, 7) and rather high hydrogen capacity.52 In the case of the Mg± Ni mechanical alloy, the intermetallic compound Mg2Ni is formed after 3 ± 4 cycles of hydrogenation ± dehydrogenation, mainly according to the equation 61 2MgH2+Ni =Mg2Ni +2H2 .Further, kinetic characteristics of the system do not differ from those of the ordinary two-phase system Mg± Ni or the interme- tallic compound Mg2Ni.29, 63 Kinetic characteristics of the mechanical alloy Mg± Ce in the cycling are also comparable with those of hydrogenation and dehydrogenation obtained in the hydrogenolysis of Mg± CeHx mixtures (Fig. 8).56 This shows that the size of the interface inMA has the same order of magnitude as the interface in the mixtures obtained in the hydrogenolysis. As for MA composed of thermodynamically immiscible components, one could expect rapid destruction of a sample in repeating hydrogenation ± dehydrogenation cycles due to exfolia- tion of the system. However, these systems appeared to be rather stable in cycling.A mechanical alloy Mg± Fe retains its hydrogen capacity for *70 cycles, and the rate of hydrogenation even Mg Ni Ni Ni Ni a Solid solution of Ni in Mg b MgH2 Mg± 5 mass%of Ni Mg± 55 mass%of Ni Figure 5. A model of the microstructure of mechanical alloy Mg± Ni (a) and the illustration of the effect of the Ni concentration on the degree of conversion (b). 0 20 40 t /min 2 4 [H] (mass %) 1 23 4 5 6 7 8 Figure 6. Kinetics of hydrogenation for the fifth cycle, PH2 = 1.5 MPa, T=625 K. (1) Mg, particle size is 20 mm, mechanical alloy; (2)Mg+5 mass%of Nb, (3) Mg+5 mass%of Fe, Co or Ni, (4) Mg+5 mass%of Ti, (7) Mg+ 5 mass% of Si, (8) Mg + 50 mass% of C; (5) intermetallic compound Mg2Ni; (6) Mg, particle size is equal to the size of particles of the mechanical alloy (P=0.7 MPa, T=583 K). 2 4 0 20 40 t /min [H] (mass %) 1 2 3 4 5 6 Figure 7. Kinetics of dehydrogenation, P=0.1 MPa. Mechanical alloy: (1) Mg + 5 mass% of Ni, T = 615 K; (2) Mg + 5 mass% of Fe, T = 625 K; (3) Mg + 5 mass% of Nb, T = 625 K; (4) Mg+5 mass% of Ti, T = 625 K; (6) Mg + 5 mass% of Si, T= 621 K; (5) Mg, particle size is 20 mm, T=625 K. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 73gradually increases;59 a mechanical alloy Mg± TiO2 retains its kinetic characteristics at least up to the 39th cycle 64 (there are no data on longer cycling). It was shown 59, 72 that complex competing processes includ- ing sintering and dispersion of the sample occur in a Mg± Fe system in the hydrogenation ± dehydrogenation.Hydrogen under- goes dissociation on iron and it further diffuses into the bulk of the sample along the interface and grain boundaries to form an Mg/Fe adsorption layer at the interface in which rather high mobility of both hydrogen and metals is observed. Some experi- ments on the annealing of a contact pair Mg/Fe using a micro- probe analysis have shown that the principal possibility is for iron to diffuse along the surface of magnesium in a hydrogen atmos- phere.59, 72 This favours an increase in the Mg/Fe contact area and in the hydrogenation rate.When a definite hydrogen concentra- tion in the adsorption layer at the interface Mg/Fe is achieved, a ternary hydride of the composition Mg2FeH6 crystallises at a noticeable rate, which was observed at T5615 K.At hydrogen pressures exceeding the equilibrium pressure over MgH2, catalytic hydrogenation of magnesium occurs in parallel, often at a higher rate; the synthesis of Mg2FeH6 can also occur according to the equation 2MgH2+Fe+H2 Mg2FeH6 . However, since the equilibrium hydrogen pressure forMg2FeH6 is lower than that for magnesium hydride 57 (Fig. 9), the synthesis of the latter was successfully accomplished directly from the ele- ments. This reaction has been described in detail.59, 72 Analogous processes seem to occur in the hydrogenation of a mechanical alloy Mg± Co. This system does not belong to immiscible ones; however, for this system only one intermetallic compound MgCo2 is known, which does not absorb hydrogen under ordinary hydrogenation conditions. In the hydrogenation of mechanical alloys in this system, two hydrides, viz., Mg2CoHx (x=4 ± 5) and Mg3CoH5 , were detected 58, 59 which, like Mg2FeH6 , can be synthesised from the elements (without the intermediate stage of formation of magnesium hydride).Decom- position of ternary magnesium ± cobalt hydrides gave a previously unknown IMC with composition close to Mg2Co and having a cubic structure.This compound interacts with hydrogen even at room temperature to form solid solutions and at elevated temper- atures to form ternary hydrides (Table 2). It should be noted that ternary magnesium ± iron and magne- sium ± cobalt hydrides could not be detected for a long time due to the lack of hydride-forming IMC in the systems Mg± Fe and Mg± Co.Yvon et al.73 considered the hydride Mg2NiH4 as a complex compound composed of the Mg2+ cation and the [NiH4]47 anion. Assuming that the 18 electron rule, which determines the content of a ligand (H in this case), is valid for these complexes, they have predicted the possibility that hydrides Mg2FeH6 and Mg2CoH5 exist. Research by these authors in 1984 ± 1985 aimed at synthesising these hydrides was success- ful.74, 75 At the same period, the hydrides were found in the hydrogenation of mechanical alloys.76 The lack of a method for the preparation of a material with a large Mg/Fe and Mg/Co interface caused the authors 74, 75 to use a mixture of pressed metal powders.The reaction had to be conducted for weeks under drastic conditions, nevertheless, complete conversion of these mixtures into the hydrides was not achieved.Neither the second hydride Mg3CoH5 existing in the Mg± Co system nor the inter- metallic compound Mg2Co could be isolated and identified. The magnesium ± iron and magnesium ± cobalt ternary hydrides were examined as catalysts for the hydrogenation of unsaturated hydrocarbons. These catalysts manifested catalytic activity comparable with that of catalysts based on platinum- group metals, and high selectivity in processes of partial reduction 0 30 t /min [H] (%) 50 4 3 2 1 Figure 8.Kinetics of hydrogenation at T=603K and PH2=1.9 MPa. (1) mechanical alloy Ce + 25Mg; (2) CeMg12; mixture: (3) Mg + 10 mass%of LaNi5; (4) Ce+12Mg. T /K 1073 (1/T) /K71 1.4 1.5 1.6 600 650 700 1 0 71 1 2 0.2 1.0 2.0 3.0 P /MPa ln (P /MPa) Figure 9.Temperature dependence of equilibrium pressures of hydrogen over the hydride phases MgH2 (1) and Mg2FeH6 (2). Table 2. Crystallographic data for novel ternary hydrides and intermetallic compounds. Compound Unit cell Cell parameters /nm Ref. Mg2FeH6 Cubic, a=0.6443 74 Fm3m a=0.64420.0002 59 Mg2CoHx , Tetragonal a=0.44770.0001, 59 (low-tempera- c=0.66120.0001, ture modifica- a=0.4480(2) 75 tion) a c=0.6619(3) Mg2CoHx , Cubic a=0.644 59 (high-tempera at 480K ture modifica- tion) Mg3CoH5 Orthorhombic a=0.4675+ 0.0002, 59 b=0.8073+0.0003, c=1.00910.0003, Z=4 `Mg2Co' Cubic a=1.143+ 0.001 58, 59 a The temperature of a reversible phase transition of Mg2CoHx from the low-temperature modification to the high-temperature one is 470 K, DH for the phase transition is*2 kJ mol± 1.58, 59 74 I G Konstanchuk, E Yu Ivanov, V V Boldyrevof acetylene derivatives and compounds with several double bonds (Table 3).59, 77 Hydrogenation and dehydrogenation of MA containing, for example, two or more IMC can be more complicated.Depending on the composition of MA, cycling can result in the interaction between the components, their decomposition, and the formation of other phases, which can be both hydride-forming and hydride- non-forming.Thus a study of hydrogenation of a MA containing 66.6 mass%of La2Mg17 and 33.3 mass%of LaNi5 has shown 78 that several hydrogenation ± dehydrogenation cycles result in their decomposition and the formation of Mg, Mg2Ni, and LaHx (x=2.00 ± 2.99).Then, depending on the temperature and hydro- gen pressure used in hydrogenation, hydrogen is absorbed either by magnesium or by magnesium and Mg2Ni, and LaHx serves as a `hydrogen pump'. After 20 cycles, the sample was a conglomerate of all the phases with a granule size of *0.2 mm. Although these MA have lower hydrogen capacity as compared to that of pure La5Mg17, the rates of their hydrogenation and dehydrogenation are higher in the first cycles and depend, other conditions being the same, on the duration and intensity of mechanical processing.Thus MA obtained under the most drastic conditions (at an acceleration of 130 m s72 for 25 min) absorbs 3.5 mass% of hydrogen (90% of full capacity) in less than 1 min at 523K and liberates the same amount in 6 min, whereas under the same conditions it takes 2.5 h for pure La5Mg17 to absorb 4.9 mass% of hydrogen (also 90% of its full capacity) and 3 h to desorb it.78 This was explained 78 by the small size ofMAparticles (*10 mm), their multi-phase composition and catalytic activity of one or two components.It was thus concluded that a solution of the problem of improvement of the kinetic characteristics of hydrogenation and dehydrogenation lies in the creation of analogous composite materials. 3. Hydrogenation of Mg2Ni obtained by mechanical alloying and mechanical milling The intermetallic compound Mg2Ni interacts with hydrogen at a high rate at 473 ± 573K and a hydrogen pressures of 0.5 ± 1.0 MPa; however, a problem of activation exists. From 10 to 20 cycles of hydrogenation ± dehydrogenation are necessary for activation 79 in the course of which dispersion of the material and the formation of a metallic surface occur.These difficulties can be avoided if Mg2Ni is subjected to mechanical milling or is synthes- ised by mechanical alloying using the same procedure as for the synthesis of MA but increasing the duration of the treatment.It was shown 80, 81 that mechanical alloying of a mixture of Mg and Ni powders gives nanocrystalline Mg2Ni with the size of crystal- lites of about 20 ± 30 nm. This material interacts rapidly with hydrogen, and reproducible rates of hydrogenation are achieved even by the second cycle (Fig. 10). If a small amount of palladium (less than 1 mass%) is added to the reaction mixture during mechanical alloying, the thus modified Mg2Ni absorbs hydrogen at room temperature (Fig. 11).80 ± 83 An analogous result was obtained without addition of palladium.84 However, in this case hydrogenation of Mg2Ni was performed immediately after its mechanical milling without exposure to air (Fig. 12). Mechanical milling implies practically the same procedure as mechanical alloying except that the initial material is not a mixture of metal powders but an intermetallic compound obtained by traditional alloying.As is shown by Fig. 12, the reaction rate and the amount of absorbed hydrogen increase as the duration of mechanical Table 3. Catalytic characteristics of ternary hydrides and traditional catalysts.59 Catalyst Degree of Selectivity (%) conversion (%) Compound to be hydrogenated is acetylene Mg2CoHx 44 100 (with respect to C2H4) Pd/Al2O3 21 90 Compound to be hydrogenated is butadiene Mg2FeH6 51 ± 55 99.2 ± 99.7 Pt ± S/Al2O3 23 100 Pd ± S/Al2O3 36 72 10 30 50 t /min 1.0 2.0 3.0 1 2 x 0 Figure 10.Kinetics of hydrogenation of nanocrystalline Mg2Ni prepared by mechanical alloying at T=573 K, PH2=1.16 MPa; (1) the first cycle; (2) the second and subsequent cycles.The x values in the formula Mg2NiHx are shown in the y-axis. 10 30 50 t /min 2.0 1.5 1.0 0.5 0 x Figure 11. Kinetics of hydrogen sorption by nanocrystalline Mg2Ni modified with palladium during mechanical alloying (room temperature, PH2=1.2 MPa, without activation). The x values in the formula Mg2NiHx are shown in the y-axis. 0.2 0.1 0 H/M 0 0.5 1073 t /s 4 3 2 1 Figure 12.Kinetics of hydrogen sorption for Mg2Ni (285 K, PH2= 2 MPa) after its mechanical milling in an argon atmosphere for 30 (1), 5 (2), 1 min (3), and without milling (4). The ratio of hydrogen atoms to metal atoms in Mg2NiHx is shown in the y-axis. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 75milling is increased, which can be due to gradual dispersion of the sample and the formation of pure metal surface during this milling.However, at room temperature hydrogenation does not go to completion; the hydrogen content corresponds to the formula Mg2NiHx (x&2). This might be connected with the fact that as the temperature decreases, following the formation of a hydride layer on the surface of particles, the diffusion of hydrogen through this layer, the rate of which can be small at room temperature, becomes the limiting stage.The `diffusion retardation' occurs similarly to that observed in the hydrogena- tion of magnesium. Moreover, at T<515K Mg2NiH4 exists as a low-temperature modification, the structure of which is the monoclinic distortion of a cubic high-temperature modification.85 The latter normally results from hydrogenation at temperatures above the temperature of phase transition.The explanation of this fact calls for additional studies. III. Hydrogenation of FeTi and LaNi5 undergoing mechanical milling As compared to magnesium systems, LaNi5 and FeTi are low- temperature accumulators of hydrogen. They absorb hydrogen at room temperature.However, commercial polycrystalline samples covered with oxygen-containing compounds require, as a rule, preliminary activation. For instance, the procedure for activation of FeTi consists of heating the sample in a vacuum to 673 ± 723K and subsequent annealing in hydrogen at a pressure of*0.7 MPa followed by cooling to room temperature and increase in the hydrogen pressure to 3.5 ± 6.5 MPa.86 This procedure is repeated several times so that hydrogenation is reproducible. Activation of LaNi5 proceeds much more easily than in the case of FeTi, but this is still required and consists of a prolonged exposure of the sample at high hydrogen pressure (about 5.0 MPa) at room 87 or elevated (*623 K) temperatures in vac- uum.88 Mechanical milling of these intermetallic compounds in ball mills allows omission or simplification of the activation proce- dure.The kinetic curves of hydrogen sorption for mechanically milled FeTi are presented in Fig. 13.84 After mechanical milling, contact of the sample with air was excluded. It is seen that in this case hydrogenation starts at the maximum rate, and the degree of conversion depends on the duration of the treatment.If the sample was exposed to air after mechanical milling and prior to hydro- genation, activation was necessary, but it occurred much more easily than for ordinary alloys: it is sufficient to heat the sample at 673K in vacuum for 30 min.89 If FeTi is modified with a small amount of palladium, which is added during mechanical milling, neither annealing of the sample nor preliminary exposure of the sample to hydrogen are required.This sample immediately absorbs hydrogen at room temper- ature.81, 83, 90 Isotherms of `pressure ± composition' at room temperature without preliminary activation of samples were obtained 91 for amorphous and nanocrystalline FeTi formed upon mechanical alloying. It was shown that the shape of the isotherms is very `sensitive' to the microstructure of the samples and changes significantly after their annealing.The relaxation of structural defects and mechanical strains upon annealing leads to a decrease in the solubility of hydrogen in both cases and to a change in the length and slope of the plateau in the isotherm of nanocrystalline FeTi. The shape of the adsorption isotherm of the initial nano- crystalline powder indicates the significant contribution of an amorphous component.This fact together with the electron microscopy and WAXS data allowed the authors 91 to draw the following conclusion: the microstructure of particles of nano- crystalline FeTi formed upon mechanical alloying is nanocrystal- lites with a highly disordered intercrystallite region; the portion of this structure in the bulk material is 20%± 30%.It should be emphasised that these data give direct evidence that the degree of disorder and mechanical strains affect the character of the hydro- gen absorption in the sample. During preliminary activation, which is required for hydrogenation of intermetallic compounds not doped with palladium the relaxation processes have, as a rule, already occurred, which prevents the unambiguous interpretation of the results.Analogous kinetic data have been obtained for LaNi5.81, 83 The kinetic curves for hydrogenation of mechanically milled samples of LaNi5�one of which was hydrogenated immediately after mechanical milling, another was kept in air for a long period, and the third was prepared with the addition of palladium � are presented in Fig. 14. Hence, mechanical milling of intermetallic compounds Mg2Ni, FeTi, and LaNi5 yields a metallic surface, on which dissociative adsorption of hydrogen occurs. However, upon con- tact of these samples with air the surface is rapidly oxidised and the kinetic characteristics of hydrogenation are impaired. Modifica- tion with palladium (one of the best catalysts for the dissociative adsorption of hydrogen) during mechanical milling of these intermetallic compounds allows hydrogenation to be carried out at room temperature without preliminary activation. 0 0.5 1073 t /s 0 0.5 H/M 1 2 3 4 Figure 13. Kinetics of hydrogen sorption for FeTi (283 K, PH2=2 MPa) after its mechanical processing in an argon atmosphere for 24 (1), 3 h (2), 30 min (3), and without processing (4).The ratio of hydrogen atoms to metal atoms in FeTiHx is shown in the y-axis. 5 4 3 2 1 0 20 40 60 80 t /min 1 2 3 x Figure 14. Kinetics of hydrogenation of LaNi5 milled mechanically at T=313K and PH2=1.5 MPa; (1) the sample kept in air for several months; (2) the sample immediately after mechanical activation; (3) the sample modified with palladium. The x values in the formula LaNi3Hx are shown in the y-axis. 76 I G Konstanchuk, E Yu Ivanov, V V BoldyrevIV. The direct formation of hydrides during mechanical milling The formation of metal hydrides is possible during mechanical milling.92 ± 96 To this end, mechanical milling is run in a hydrogen atmosphere at elevated pressure. In this way, binary hydrides of the compositions TiH1.9, ZrH1.66 and MgH2 have been prepared and characterised by WAXS analysis, differential thermal analy- sis, and electron microscopy.92 ± 95 It is remarkable that magne- sium usually absorbs hydrogen in noticeable amounts only at temperatures above 573 K, whereas hydrogenation concurrent with mechanical milling yields magnesium hydride without heat- ing.To date it remains unclear which factor is determining in the formation of hydrides during mechanical milling in a hydrogen atmosphere. It might be crushing, plastic deformation, the for- mation of a fresh metal surface, local increase in temperature upon collision of grinding bodies, etc. Most likely, all these processes contribute to the formation of hydrides. Mechanochemical processes are known to be non-equilibrium ones and often occur via metastable states.This is the case in the hydrogenation of ZrNi during its mechanical milling in a hydro- gen atmosphere.96 This process includes the stages differing from those in the traditional hydrogenation, namely, the consecutive formation of the hydride phases ZrNiH and/or ZrNiH3 followed by their amorphisation, and then the decomposition of the amorphous phase to ZrH2 and metastable hydride a-Zr1 ± dNiHx.As a result, composite particles composed of amorphous (a-Zr1 ± dNiHx) and crystalline (a-ZrNiH3 and/or ZrH2) hydride phases are formed according to the hydrogen pressure used, which varies from 0.1 to 1.0 MPa. Particles resulting from mechanical processing in a hydrogen atmosphere appeared to be much smaller in size (0.5 ± 1.0 mmat a hydrogen pressure of 1.0 MPa) than those obtained under similar conditions in an argon atmosphere (50 mm) due to the higher fragility of the hydride phases as compared to that of ZrNi.Taking this into account, it was concluded 96 that mechanical processing in a hydrogen atmosphere can be a promising method for the creation of hydrogen-accumulating materials with a very large interface between the amorphous and crystalline phases and for the creation of hard magnets with anisotropic nanosized grains, which can be obtained by consecutive hydrogenation, amorphisation, and dehydrogenation with simultaneous crystal- lisation.When mechanical milling of Mg2Ni was carried out in a hydrogen atmosphere at a pressure of 1.0 MPa in a `Fritsch P7' ball planetary mill at a rotation rate of 400 rev min71, the sorption of hydrogen was also detected.97 According to the WAXS data, the samples obtained corresponded to the a-phase, a solid solution of hydrogen in the intermetallic compound Mg2NiH0.3.The latter is usually formed in the initial stages of hydrogenation and at higher hydrogen concentrations, the for- mation of Mg2NiH4 begins.98 However, upon hydrogenation during mechanical milling, the amount of hydrogen absorbed exceeded significantly that corresponding to the formula Mg2NiH0.3.For instance, after processing for 80 h this amount achieved 1.6 mass% (Mg2NiH1.8) without noticeable changes in X-ray patterns. In this case, a correlation between the value of specific surface of the sample and the hydrogen content in the sample was observed (Fig. 15). The authors 97 associate this fact not only with the disintegration of the sample, but also with the formation of a nanocrystalline composite material during mechanical milling composed of crystalline particles with this phase disordered at the grain interface where hydrogen can occupy more sites that in the crystalline phase.The thickness of this disordered phase might be only several nanometers and is not observed in X-ray patterns. In our opinion, the data given in Ref. 97 do not rule out the principal possibility for the formation of the hydride Mg2NiH4 during mechanical milling since the investigations were performed in a low-energy `Fritsch P7' apparatus.Under more drastic conditions (more powerful mills, higher hydrogen pressure), hydrogenation can occur to reach the stage of formation of the ternary hydride, which, probably, would be a metastable phase. In any case, the formation of a solid supersaturated solution of hydrogen in Mg2Ni at the grain interface 97 indicates that the processes of traditional hydrogenation and hydrogenation during mechanical milling differ in the initial stages.V. Conclusion The above review of published data on the interaction between hydrogen and mechanical alloys and intermetallic compounds undergoing mechanical milling allows the following conclusions to be drawn. Mechanical alloying is a promising method for the prepara- tion of materials with high reactivity with respect to hydrogen.As compared to traditional methods for the preparation of alloys and intermetallic compounds, mechanical alloying is sub- stantially less time- and energy-consuming. There is no need for high temperatures. Depending on the apparatus design, the process of mechanical alloying takes from several minutes to several hours to yield homogeneous materials or composites with a sufficiently uniform phase distribution, while in the case raditional alloying, as a rule, special prolonged homogenising annealing is necessary.This is especially important for obtaining alloys of metals differing considerably in their specific densities. Mechanical alloys formed in the early stages of mechanical alloying are metastable systems with a large interface where the `atomic' contact of metals occurs and their chemical interaction is possible.This interaction consists of both electron transfer from one element to another thus increasing the catalytic activity of the metal-catalyst and the reduction of oxides used as the second phase in magnesium systems thus creating centres of hydrogen adsorption. Ahighly disordered specific structure of theMAsurface with a catalyst dispersed in the form of atoms in the near-surface layer is responsible for the high hydrogenation rate in the first cycle.This structure is stable to oxygen and the operation with the sample in air does not require any special conditions. The possibility to vary the microstructure during mechanical alloying allows the reaction rate and hydrogen capacity of MA to be varied.The method of mechanical alloying provides a possibility to create composites from non-interacting components, which favours the synthesis of novel hydrides and IMC in the reaction of these MA with hydrogen. [H] (mass %) 1 2 3 4 s /m2 g71 0.5 1.0 1.5 0 1 2 3 4 5 Figure 15. The correlation between the specific surface of the samples (s) prepared by hydrogenation of Mg2Ni during mechanical milling and the hydrogen content in the samples.t /min: (1) 5, (2) 15, (3) 60, (4) 300, (5) 7800. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 77IV. The direct formation of hydrides during mechanical milling The formation of metal hydrides is possible during mechanical milling.92 ± 96 To this end, mechanical milling is run in a hydrogen atmosphere at elevated pressure.In this way, binary hydrides of the compositions TiH1.9, ZrH1.66 and MgH2 have been prepared and characterised by WAXS analysis, differential thermal analy- sis, and electron microscopy.92 ± 95 It is remarkable that magne- sium usually absorbs hydrogen in noticeable amounts only at temperatures above 573 K, whereas hydrogenation concurrent with mechanical milling yields magnesium hydride without heat- ing.To date it remains unclear which factor is determining in the formation of hydrides during mechanical milling in a hydrogen atmosphere. It might be crushing, plastic deformation, the for- mation of a fresh metal surface, local increase in temperature upon collision of grinding bodies, etc.Most likely, all these processes contribute to the formation of hydrides. Mechanochemical processes are known to be non-equilibrium ones and often occur via metastable states. This is the case in the hydrogenation of ZrNi during its mechanical milling in a hydro- gen atmosphere.96 This process includes the stages differing from those in the traditional hydrogenation, namely, the consecutive formation of the hydride phases ZrNiH and/or ZrNiH3 followed by their amorphisation, and then the decomposition of the amorphous phase to ZrH2 and metastable hydride a-Zr1 ± dNiHx.As a result, composite particles composed of amorphous (a-Zr1 ± dNiHx) and crystalline (a-ZrNiH3 and/or ZrH2) hydride phases are formed according to the hydrogen pressure used, which varies from 0.1 to 1.0 MPa.Particles resulting from mechanical processing in a hydrogen atmosphere appeared to be much smaller in size (0.5 ± 1.0 mmat a hydrogen pressure of 1.0 MPa) than those obtained under similar conditions in an argon atmosphere (50 mm) due to the higher fragility of the hydride phases as compared to that of ZrNi. Taking this into account, it was concluded 96 that mechanical processing in a hydrogen atmosphere can be a promising method for the creation of hydrogen-accumulating materials with a very large interface between the amorphous and crystalline phases and for the creation of hard magnets with anisotropic nanosized grains, which can be obtained by consecutive hydrogenation, amorphisation, and dehydrogenation with simultaneous crystal- lisation. When mechanical milling of Mg2Ni was carried out in a hydrogen atmosphere at a pressure of 1.0 MPa in a `Fritsch P7' ball planetary mill at a rotation rate of 400 rev min71, the sorption of hydrogen was also detected.97 According to the WAXS data, the samples obtained corresponded to the a-phase, a solid solution of hydrogen in the intermetallic compound Mg2NiH0.3.The latter is usually formed in the initial stages of hydrogenation and at higher hydrogen concentrations, the for- mation of Mg2NiH4 begins.98 However, upon hydrogenation during mechanical milling, the amount of hydrogen absorbed exceeded significantly that corresponding to the formula Mg2NiH0.3. For instance, after processing for 80 h this amount achieved 1.6 mass% (Mg2NiH1.8) without noticeable changes in X-ray patterns.In this case, a correlation between the value of specific surface of the sample and the hydrogen content in the sample was observed (Fig. 15). The authors 97 associate this fact not only with the disintegration of the sample, but also with the formation of a nanocrystalline composite material during mechanical milling composed of crystalline particles with this phase disordered at the grain interface where hydrogen can occupy more sites that in the crystalline phase.The thickness of this disordered phase might be only several nanometers and is not observed in X-ray patterns. In our opinion, the data given in Ref. 97 do not rule out the principal possibility for the formation of the hydride Mg2NiH4 during mechanical milling since the investigations were performed in a low-energy `Fritsch P7' apparatus.Under more drastic conditions (more powerful mills, higher hydrogen pressure), hydrogenation can occur to reach the stage of formation of the ternary hydride, which, probably, would be a metastable phase. In any case, the formation of a solid supersaturated solution of hydrogen in Mg2Ni at the grain interface 97 indicates that the processes of traditional hydrogenation and hydrogenation during mechanical milling differ in the initial stages.V. Conclusion The above review of published data on the interaction between hydrogen and mechanical alloys and intermetallic compounds undergoing mechanical milling allows the following conclusions to be drawn. Mechanical alloying is a promising method for the prepara- tion of materials with high reactivity with respect to hydrogen.As compared to traditional methods for the preparation of alloys and intermetallic compounds, mechanical alloying is sub- stantially less time- and energy-consuming. There is no need for high temperatures. Depending on the apparatus design, the process of mechanical alloying takes from several minutes to several hours to yield homogeneous materials or composites with a sufficiently uniform phase distribution, while in the case of traditional alloying, as a rule, special prolonged homogenising annealing is necessary.This is especially important for obtaining alloys of metals differing considerably in their specific densities. Mechanical alloys formed in the early stages of mechanical alloying are metastable systems with a large interface where the `atomic' contact of metals occurs and their chemical interaction is possible.This interaction consists of both electron transfer from one element to another thus increasing the catalytic activity of the metal-catalyst and the reduction of oxides used as the second phase in magnesium systems thus creating centres of hydrogen adsorption.Ahighly disordered specific structure of theMAsurface with a catalyst dispersed in the form of atoms in the near-surface layer is responsible for the high hydrogenation rate in the first cycle. This structure is stable to oxygen and the operation with the sample in air does not require any special conditions.The possibility to vary the microstructure during mechanical alloying allows the reaction rate and hydrogen capacity of MA to be varied. The method of mechanical alloying provides a possibility to create composites from non-interacting components, which favours the synthesis of novel hydrides and IMC in the reaction of these MA with hydrogen. [H] (mass %) 1 2 3 4 s /m2 g71 0.5 1.0 1.5 0 1 2 3 4 5 Figure 15.The correlation between the specific surface of the samples (s) prepared by hydrogenation of Mg2Ni during mechanical milling and the hydrogen content in the samples. t /min: (1) 5, (2) 15, (3) 60, (4) 300, (5) 7800. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 77IV. The direct formation of hydrides during mechanical milling The formation of metal hydrides is possible during mechanical milling.92 ± 96 To this end, mechanical milling is run in a hydrogen atmosphere at elevated pressure. In this way, binary hydrides of the compositions TiH1.9, ZrH1.66 and MgH2 have been prepared and characterised by WAXS analysis, differential thermal analy- sis, and electron microscopy.92 ± 95 It is remarkable that magne- sium usually absorbs hydrogen in noticeable amounts only at temperatures above 573 K, whereas hydrogenation concurrent with mechanical milling yields magnesium hydride without heat- ing.To date it remains unclear which factor is determining in the formation of hydrides during mechanical milling in a hydrogen atmosphere. It might be crushing, plastic deformation, the for- mation of a fresh metal surface, local increase in temperature upon collision of grinding bodies, etc. Most likely, all these processes contribute to the formation of hydrides.Mechanochemical processes are known to be non-equilibrium ones and often occur via metastable states. This is the case in the hydrogenation of ZrNi during its mechanical milling in a hydro- gen atmosphere.96 This process includes the stages differing from those in the traditional hydrogenation, namely, the consecutive formation of the hydride phases ZrNiH and/or ZrNiH3 followed by their amorphisation, and then the decomposition of the amorphous phase to ZrH2 and metastable hydride a-Zr1 ± dNiHx.As a result, composite particles composed of amorphous (a-Zr1 ± dNiHx) and crystalline (a-ZrNiH3 and/or ZrH2) hydride phases are formed according to the hydrogen pressure used, which varies from 0.1 to 1.0 MPa.Particles resulting from mechanical processing in a hydrogen atmosphere appeared to be much smaller in size (0.5 ± 1.0 mmat a hydrogen pressure of 1.0 MPa) than those obtained under similar conditions in an argon atmosphere (50 mm) due to the higher fragility of the hydride phases as compared to that of ZrNi.Taking this into account, it was concluded 96 that mechanical processing in a hydrogen atmosphere can be a promising method for the creation of hydrogen-accumulating materials with a very large interface between the amorphous and crystalline phases and for the creation of hard magnets with anisotropic nanosized grains, which can be obtained by consecutive hydrogenation, amorphisation, and dehydrogenation with simultaneous crystal- lisation.When mechanical milling of Mg2Ni was carried out in a hydrogen atmosphere at a pressure of 1.0 MPa in a `Fritsch P7' ball planetary mill at a rotation rate of 400 rev min71, the sorption of hydrogen was also detected.97 According to the WAXS data, the samples obtained corresponded to the a-phase, a solid solution of hydrogen in the intermetallic compound Mg2NiH0.3.The latter is usually formed in the initial stages of hydrogenation and at higher hydrogen concentrations, the for- mation of Mg2NiH4 begins.98 However, upon hydrogenation during mechanical milling, the amount of hydrogen absorbed exceeded significantly that corresponding to the formula Mg2NiH0.3.For instance, after processing for 80 h this amount achieved 1.6 mass% (Mg2NiH1.8) without noticeable changes in X-ray patterns. In this case, a correlation between the value of specific surface of the sample and the hydrogen content in the sample was observed (Fig. 15). The authors 97 associate this fact not only with the disintegration of the sample, but also with the formation of a nanocrystalline composite material during mechanical milling composed of crystalline particles with this phase disordered at the grain interface where hydrogen can occupy more sites that in the crystalline phase. The thickness of this disordered phase might be only several nanometers and is not observed in X-ray patterns.In our opinion, the data given in Ref. 97 do not rule out the principal possibility for the formation of the hydride Mg2NiH4 during mechanical milling since the investigations were performed in a low-energy `Fritsch P7' apparatus. Under more drastic conditions (more powerful mills, higher hydrogen pressure), hydrogenation can occur to reach the stage of formation of the ternary hydride, which, probably, would be a metastable phase. In any case, the formation of a solid supersaturated solution of hydrogen in Mg2Ni at the grain interface 97 indicates that the processes of traditional hydrogenation and hydrogenation during mechanical milling differ in the initial stages. V. Conclusion The above review of published data on the interaction between hydrogen and mechanical alloys and intermetallic compounds undergoing mechanical milling allows the following conclusions to be drawn. Mechanical alloying is a promising method for the prepara- tion of materials with high reactivity with respect to hydrogen. As compared to traditional methods for the preparation of alloys and intermetallic compounds, mechanical alloying is sub- stantially less time- and energy-consuming. There is no need for high temperatures. Depending on the apparatus design, the process of mechanical alloying takes from several minutes to several hours to yield homogeneous materials or composites with a sufficiently uniform phase distribution, while in the case of traditional alloying, as a rule, special prolonged homogenising annealing is necessary. This is especially important for obtaining alloys of metals differing considerably in their specific densities. Mechanical alloys formed in the early stages of mechanical alloying are metastable systems with a large interface where the `atomic' contact of metals occurs and their chemical interaction is possible. This interaction consists of both electron transfer from one element to another thus increasing the catalytic activity of the metal-catalyst and the reduction of oxides used as the second phase in magnesium systems thus creating centres of hydrogen adsorption. Ahighly disordered specific structure of theMAsurface with a catalyst dispersed in the form of atoms in the near-surface layer is responsible for the high hydrogenation rate in the first cycle. This structure is stable to oxygen and the operation with the sample in air does not require any special conditions. The possibility to vary the microstructure during mechanical alloying allows the reaction rate and hydrogen capacity of MA to be varied. The method of mechanical alloying provides a possibility to create composites from non-interacting components, which favours the synthesis of novel hydrides and IMC in the reaction of these MA with hydrogen. [H] (mass %) 1 2 3 4 s /m2 g71 0.5 1.0 1.5 0 1 2 3 4 5 Figure 15. The correlation between the specific surface of the samples (s) prepared by hydrogenation of Mg2Ni during mechanical milling and the hydrogen content in the samples. t /min: (1) 5, (2) 15, (3) 60, (4) 300, (5) 7800. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 77

ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC