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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