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Stereochemistry of the Diels–Alder reaction: steric effects of the dienophile onendo-selectivity |
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Journal of the Chemical Society, Perkin Transactions 2,
Volume 1,
Issue 1,
1974,
Page 17-22
John M. Mellor,
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摘要:
1974 17Stereochemistry of the Diels-Alder Reaction : Steric Effects of theDienophile on ends-SelectivityBy John M. Mellor * and Colin F. Webb, Department of Chemistry, The University, Southampton SO9 5NHStructures are assigned to adducts of cyclopentadiene with +unsaturated nitriles, ketones, and esters. Productratios are determined for series of dienophiles which differ in their position of alkyl substitution relative to thedouble bond. In each series increased size of the alkyl group enhances the endo-selectivity of that group. Thiseffect is attributed to non-bonding repulsive interactions in the transition state.THE synchronous or non-synchronous nature of theDiels-Alder reaction has been discussed extensively.14The extreme views of a synchronous reaction requiringthe two new bonds to be formed to an equal extentin the transition state,5 and of a non-synchronous re-action involving a &radical intermediate have givenway to the suggestion of a one step mechanism in whichin the transition state the two new bonds are formedto an unequal extent.'+ More recently discussion hascentred on explanations of the regiospecificity lo, l1 andendo-selectivity of the reaction.12*13The first explanation of high erndo-selectivity was givenby the Alder rule of maximum accumulation of un-saturation.14 Subsequently high endo-selectivity hasbeen explained by secondary orbital interactions 15*16stabilising the endo-transition state, more favourable1 J.G. Martin and R. K. Hill, Chern. Rev., 1961, 61, 537.J. Sauer, Angew.Chem., 1967, 79, 76.3 S. Seltzer, Adv. Alicyclic Chem., 1968, 2, 1.W. C. Herndon, Chem. Rev., 1972, 72, 157.M. Charton, J . Org. Chem., 1966, 81, 3745. * R. P. Lutz and J. D. Roberts, J . Amer. Chem. SOC., 1961,W . von E. Doering, M. Franck-Neumann, D. Hasselmann,M. T. H. Liu and C. Schmidt, Tetrahedron, 1971, 27, 5289. * M. J. S. Dewar and R. S. Pyron, J . Amer. Chem. Sot., 1970,10 J. Feuer, W. C. Herndon, and L. H. Hall, Tetrahedron, 1968,11 0. Eisenstein, J. M. Lefour, and N. T. Anh, Chew. Comm.,88, 2198.and R. L. Kaye, J . Amer. Chem. SOC., 1972, 94, 3833.92, 3098.24, 2676.1971, 969.geometry for primary overlap of orbitals in the endo-transition state,l' and attractive van der Waals dis-persion forces favouring an enzdo-product l3 or repulsiveforces favouring an eado-product.12 The relative im-portance of different factors in controlling endo-selec-tivity has been examined using both substituted dienesand substituted dienophiles but rarely have conditionsbeen chosen to isolate the different factors.In a series of paw-substituted cinnamic acids electron-withdrawing substituents augment l8 the aryl endo-selectivity.In addition to a series of substitutedcyclobutenes 1 9 9 2o remote substituents, which couldnot alter the steric requirements of an attacking diene,markedly influence reactivity. Such effects must beentirely due to non-steric factors. The addition ofl2 K. N. Houk, Tetrahedron Letters, 1970, 2621; K. N. Honkand L. J. Luskus, J .Amer. Chem. SOC., 1971, 93, 4606.l3 T. Kobuke, T. Fueno, and J. Furukawa, J . Amer. Chern.SOC., 1970, 92, 6548; Y . Kobuke, T. Sugimoto, J. Furukawa,and T. Fueno, ibid., 1972, 94, 3633.I6 R. Hoffmann and R. B. Woodward, J . Amer. Chern. SOC.,16 R. B. Woodward and R. Hoffmann, Angew. Chem. Internat.1' W. C . Herndon and L. H. Hall, Tetrahedron Letters, 1967,18 C . S. Rondestvedt and C. D. Ver Nooy, J . A m y . Chem. SOC.,19 M. N. Paddon-Row and R. N. Warrener, Tetrahedron Letters,20 M. N. Paddon-Row, Tetrahedron Letters, 1972, 1409.K. Alder and G. Stein, Angew. Chem., 1937, 50, 510.1965, 87, 4388.Edn., 1969, 8, 781.3096.1955, 77, 4878.1972, 140618 J.C.S. Perkin I1methyl methacrylate to cyclopentadiene has beenrecognised for sometime as an example of the failureof the endo-rule21,22 and this has been attributed tosteric factors. More recently two explanations forthe ertdo-selectivity in this and related additions havebeen given.Comparison of the endo-selectivity inadditions to 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-di-enone (1) and related polysubstituted cyclopentadieneshas led to the view that a repulsive non-bonding inter-action between a substituent entering on the exo-faceand a hydrogen of the methylene of a cyclopentadieneleads to a preference for efido-attack.l2 An alternativeexplanation of attractive non-bonding forces, which0a ; R = H a ; R1=R2=Hb; R = Me b; R1= H, R2 = Mec ; R = Et c; R1=Me, R2= Hd ; R = P r i d; R1=R2=Nlee; R=But e ; R1=Et, R2=Hf ; R1 = Et, R2 = Meg ; R1 = Pri, R2 = Meh; R1 = But, R2 = Me&R2(7) d O R ra ; R1 = R2 = Hb; R1= H, R2 = Mec ; R1 = Me, R2 = Hd; Rf = R2 = Mee; R1 = Et, R2 = Hf ; R1 = Et, R2 = Meg ; R1 = Pr*, R2 = Meare important only in the endo transition state, has beengiven.l3 To clarify the importance of factors influencingendo-selectivity a study with cyclic dienes avoids con-formational uncertainties inherent in the use of acyclicdienes.In this and the accompanyingwe analyse the effect of alkyl substitution of both J’ dieneand dienophile upon erzdo-selectivity and regioselec-tivity. Here we describe the effect of varying thestructure of the dienophile upon the endo-selectivitywith cyclopentadiene. In the accompanying papers 23, 24we describe the importance of diene structure uponeado-selectivity and regioselectivity.Dienophiles (2a-e) with a-substitution to the double21 J.A. Berson, A. Remanick, and W. A. Mueller, J . Anzer.22 J. A. Berson, 2. Hamlet, and W. A. Mueller, J . Amer.23 B. C. C. Cantello, J. M. Mellor, and C. F. Webb, followng24 J. M. Mellor and C. F. Webb, J.C.S. Perkin 11, 1974, 26.25 J. C. Davis and T. V. Van Auken, J . Amer. Chem. SOL,Chem. Soc., 1960, 82, 5501.Chem. SOL, 1962, 84, 297.paper.1965, 8’4, 3900.bond, dienophiles (3a-d) with p-substitution, anddienophiles (4a-1) with y-substitution were preparedby standard methods or by suitable modificationsdescribed in the Experimental section. Adducts withcyclopentadiene were prepared at a variety of tem-peratures (see Experimental section) and pairs ofadducts were separated by chromatography oversilica gel where necessary.Structures were assignedto adducts by consideration of earlier studies, theirn.m.r, spectra (discussed fully in the Experimentalsection), and in one case from g.1.c. analysis.Adducts of cyclopentadiene with acrylonitrile and2-met hylacrylonitrile-( 4 1a; R1 = Me, R2 = CNb; R1= Et, R2 = CNc ; R1 = Pri, R2 = CNd ; R1 = But, R2 = CNe ; R1 = Me, R2 = Hf ; R1 = Et, R2 = Hg; R1 = Pri, R2 = Hh; R1 = But, R2 = Hi ; R1= R2=Mej ; R1 = Et, R2 = Mek; R1 = Pri, R2 = Me1; R1 = But, R2 = Mehave been previously prepared(5) CNa; R = Hb; R=Med; R = P r *e; R = ButC ; R = E t(10) R 2a ; R1 = Me, R2 = CN g; R1 = Pri, R2 = Hh; R1 = But, R2 = H b; R1 = Et, R2 = CNc; R1 = Pri, R2 = CN i; R1= R2=Med; R1 = But, R2 = CN j ; R1 = Et, R2 = Mek; R1 = Pri, R2 = Me e; R1 = Me, R2 = H1; R1 = But, RZ = Me f ; R1 = Et, R2= Hand structures as~igned.~~*~G Similarly we assign struc-tures to the adducts of 2-ethylacrylonitrile (2c) and2-isopropylacrylonitrile (2d) from their n.m.r.spectra.Adducts of 2-t-butylacrylonitrile were not isolated, butmass spectroscopy-g.1.c. established their formation inlow yield. Respective structures were assigned byconsideration of g.1.c. retention times of these and theother adducts (5a-d) and (6a-d).Structures of adducts of cyclopentadiene with acro-lein,27 methacrolein,28 and methyl vinyl ketone 27,29are well established.Structures were assigned toadducts (7d-g) and (8d-g) by considering the chemicalshifts of the signals associated with the protons of thesubstituents.26 J. C. Muller, J. P. Fleury, and U. Scheidegger, Org. Magnetic27 P. Laszlo and P. v. R. Schleyer, J . Amer. Chem. Soc., 1963,28 S. Beckmann and R. Bamberger, Annalen, 1953, 580, 198.29 J. G. Dinwiddie and S. P. McManus, J . Org. Chem., 1966,30,Resonance, 1970, 2, 71.85, 2709.7661974 19Structures of adducts of esters of acrylic acid and and to dienophiles (4a-1) at a single temperature.methacrylic acid and their stability under the con- Results were obtained by reaction in dilute solution inditions of their formation were assumed on the basis of benzene, p-xylene, and ether; g.1.c.analyses were ofearlier ~ t u d i e ~ . ~ ~ * ~ ~ ~ ~ ~ Structures were assigned to crude reaction mixtures. In all cases the observedadducts of esters of 2-cyanoacrylic acid by considering exo : endo ratios were independent of reaction time.chemical shifts of the signals associated with the Increase of size of the alkyl group in the series ofTABLE 1Product composition for Diels-Alder reactions of cyclopentadiene with substituted acrylonitrilesSubstituentHHMeMeMeMeE tEtPriPriButButSolvent (t/"C)Benzene (80)p-Xylene (138)Benzene (80)p-Xylene (138)Diethyl ether (18)Diethyl ether (0)Benzene (80)p-Xylene (138)Benzene (80)p-Xylene (1 38)Benzene (80)fi-Xylene (138)Column used forg.1.c.separationAdducts (%) Yield (%) t/"C(5a) 54.9 (6a) 45-1 D(120)(5a) 53.8 (6a) 46.2(6b) 15.9 (6b) 84.1 F(110)(5b) 17.1 (6b) 82.9 86(5b) 12.3 (6b) 87-7(5b) 11.8 (6b) 88.2( 5 ~ ) 12.3 ( 6 ~ ) 87-7 F(120)( 5 ~ ) 13-5 ( 6 ~ ) 86.5 52(6d) 92 F(125)No reaction observed(5e) 2.4 (6e) 97.6 5 F( 130)(5d) ( 5 4 9 (6d) 91 30TABLE 2Product coniposition for Diels-Alder reactions of cyclopentadiene with unsaturated ketones and unsaturated aldehydesSubstituentsAlkyl AcylH CHOMe CHOMe CHOH COMeH COMeMe COMeMe COMeH COEtH COEtMe COEtMe COEtMe COPriMe COPriSolvent (t/"C)Benzene (20)Benzene (20)Benzene (80)Benzene (20)Benzene (80)Benzene (20)Benzene (80)Benzene (20)Benzene ( S O )Benzene (20)Benzene (80)Benzene (20)Benzene (80)Adducts (%)(7a) 76.4 (8a) 23.6(7b) 15-4 (8b) 81.6(7b) 20.4 (8b) 79.6( 7 ~ ) 84.4 ( 8 ~ ) 15.6(7c) 83 (8c) 17(7d) 35.9 (8d) 64.1(7d) 41.8 (8d) 58.2(7e) 86 (8e) 14(7e) 84-4 (8e) 15-6(7f) 37-3 (8f) 62.7(7f) 43.1 (8f) 56-9(7g) 40.6 (8g) 59.4(7g) 46.2 (8g) 53-8Column usedfor g.1.c.D (75)D(75)D ( WD ( WD ( WD(88)D(88)Yield (%) separation (t/"C)908073776250TABLE 3Product coniposition for Diels-Alder reactions of cyclopentadiene with 2-substituted acrylate estersAlkyl groupof esterMeEtPriButMeEtPriButMeEtPriButSolvent (t/"C)Benzene (40)Benzene (40)Benzene (40)Benzene (40)Benzene (20)Benzene (20)Benzene (20)Benzene (20)Benzene (20)Benzene (20)Benzene (20)Benzene (20)protons of the alkoxy-groups.In the endo-esterprotons were shielded relative to the exo-ester.Results of product studies are given in Tables 1-3.In most cases the kinetic nature of products was estab-lished; in other cases [using dienophile (2e) and acryl-ate esters] it is assumed. Addition to dienophiles(2a-e) and (3a-g) was examined at two temperaturesColumn usedfor g.1.c.Adducts (%) Yield (%) separation (t/"C)(9a) 74.5 (loa) 25.5 52 4 8 3 )(9b) 75.2 (10b) 24.8 50 4 8 0 )(9c) 78.5 ( 1 0 ~ ) 21.5 50 A(80)(9d) 83-6 (10d) 16-4 40 4 9 0 )(9e) 74.7 (1Oe) 25.3(9f) 75.2 (10f) 24.8(9g) 76.6 (log) 23.4(9h) 78.2 (10h) 21.8(9i) 31-2 (1Oi) 68.8(9j) 31.8 ( l O j ) 68.2(9k) 32-8 (10k) 67.2(91) 34-0 (101) 66.02-alkylacrylonitriles enhances the endo-selectivity ofthat group. As previously noted13 there is littleselectivity with acrylonitrile (2a) but considerableselectivity with methacrylonitrile (2b).Now we notefurther enhancement of this selectivity with 2-ethyl-acrylonitrile (Zc) and 2-isopropylacrylonitrile (2d) and a3O R. Frazer, Canad. J. Chern., 1962, 40, 7820 J.C.S. Perkin IImarked increase with 2-t-butylacrylonitrile (2e). Yieldsand rates decrease with increased size of dienophile.Increase of size of an alkyl group in the series ofalkyl vinyl ketones and alkyl isopropenyl ketonesenhances the endo-selectivity of that group ; the effectis less marked than with the 2-alkylacrylonitriles butyields are again reduced with increased size of dieno-phile.In the three series of esters examined increasedsize of the alkoxycarbonyl group leads to slight en-hanced eado-selectivity. This enhancement is morepronounced with esters of 2-cyanoacrylic acid than withesters of acrylic or methacrylic acid.The Alder rule l4 or the Woodward-Hoffmann l5view of secondary orbital overlap fail to account forour results. This failure could be explained by thedominance of other factors or by the conformationalrequirements of secondary orbital overlap which areabsent in the above examples. With acrylonitrileand other ap-unsaturated nitriles the centrosymmetricnature of the nitrile group leads to unfavourable geo-metry for secondary orbital overlap and thereforeother factors must account for the observed products.This may partly, but we believe, not entirely, accountfor the low endo-selectivity of the nitrile group.Ana-lysis of the role of secondary orbital overlap with theap-unsaturated ketones, aldehydes, or esters is com-plicat ed by conf ormational uncertainties. Althoughmethyl isopropenyl ketone (3d) exists mainly in thes-tvans-conformation 3l reaction from the s-cis-con-formation of this and related compounds is possible asthe barrier to rotation is low. Without either a know-ledge of the relative reactivities, or endo-selectivitiesof the s-cis- or s-trans-conformers the role of secondaryorbital overlap is in these cases less clear. Howeverwe note that in the three series of esters examined similartrends are observed.If conformational differenceswere responsible for these trends it would be surprisingif all three series with different conformational require-ments gave the same results. We conclude that thediffering product ratios obtained within each seriesof dienophiles are explained neither by secondary orbitaloverlap nor by differences in the conformational equi-libria of the dienophile.Houk and Luskus l2 have compared the reactions ofcyclopentadiene and 2,5-dimethyl-3,4-diphenylcyclo-penta-2,4-dienone (1) with methyl acrylate, methylmethacrylate, and methyl crotonate. Adduct ratios(eado-C0,Me : exo-C0,Me) with dienone (1) were methylacrylate 1743, methyl methacrylate 15.7, and methylcrotonate 12.2 ; with cyclopentadiene ratios weremethyl acrylate 2-73, methyl methacrylate 0.43, andmethyl crotonate 1.05.It was concluded that withdienone (1) secondary orbital overlap stabilised theenzdo-transition state but with cyclopentadiene thesteric interference between methyl substituents and themethylene hydrogens [absent in (l)] led to greateramounts of exo-adducts. Such an explanation assumesthat in dienone (1) the effect of the substituent methyland phenyl groups and the effect of dipolar interactionsbetween dienone and dienophile, are small. Laterstudies with l-methylcyclopentadiene and 2-methyl-cyclopentadiene show the first assumption to be question-able. ex0 : endo-Ratios with these dienes are differentfrom those of cyclopentadiene. With polysubstitution,and with phenyl groups which will adopt out-of-planeconformations larger interactions would be expected.Although the importance of dipolar interactions arerecognisedl, in the case of addends with highly polargroups, solvent studies suggest that such interactionscannot be neglected even in the absence of polar groups.24Therefore, although a sharp distinction can be madebetween dienone (1) and cyclopentadiene it is not clearwhether this distinction arises from interaction betweenmethyl substituent of dienophile and methylene hydro-gens of cyclopentadiene, as suggested,lZ or from inter-actions between methyl substituents of dienophileand substituents (mainly the phenyl groups) of dienone(1).Both possibilities might account for the resultsand the role of dipolar interactions between the car-bony1 groups is uncertain.In considering our own studies it is clear that increasedsteric bulk of substituents on the dienophile destabilisesboth the endo- and exo-transition states.Alkyl sub-stituents, both at the a- and @-positions, retard bothexo- and endo-attack. However the transition statewith the bulky substituent entering into the exo-positionis the more affected. Although the effect of sub-stitution a t a y-position upon rate is not established,results in Table 3 show that the exo-transition state isdestabilised with respect to the endo-transition state.The retardation of both exo- and endo-modes of additionimplies repulsive non-bonding interactions betweenalkyl substituent and the diene in both transition states.The observation of the decreasing importance of suchinteractions as the site of substitution is further re-moved from the double bond is consistent with thisview. At the a-position replacement of hydrogenby methyl markedly influences the endo-selec-tivity 21922*25,26532 but replacement through the seriesethyl, isopropyl, and t-butyl further influences theendo-selectivity (Table 1). In particular substitutionby a t-butyl group leads to almost entirely endo-t-butylproduct but both additions are very slow.The view that endo-selectivity is modified by non-bonding repulsive interactions,12 which are greater inthe exo-transition state, accords with our observations.The alternative view of non-bonding attractive inter-actions l3 requires, in order to account for reduction inreactivity by alkyl substitution, and of changes in endoselectivity by substitution a t ct-, p- and y-positions,the operation of both attractive and repulsive inter-actions.We prefer the former view and considerthat alkyl substitution in dienophiles introduces re-pulsive interactions in the transition states of both31 I?. H. Cottee, B. J. Straughan, C . J. Timmons, W. F. Forbes,33 A. I. Konovalov, G. I. Kamasheva, and M. P. Loskutov,and R. Shilton, J . Chena. SOC. (B), 1967, 1146.Doklady Akad. Nauk. S.S.S.B., 1972, 204, 1031974 21exo- and wdo-addition but this interaction is greaterin the exo-transition state.*EXPERIMENTAL1.r. spectra were measured for chloroform solutions with aUnicam SP 200 spectrophotometer.N.m.r. spectra weremeasured for deuteriochloroform solutions with a VarianHA 100 spectrometer. U.V. spectra were measured forsolutions in ethanol with a Unicam SP 800 spectrophoto-meter. Mass spectra were measured with an A.E.I.MS 12 spectrometer. Analytical g.1.c. was carried outwith a Perkin-Elmer F-1 1 chromatograph using the follow-ing columns: A, 2 m 20% TCEP on Chromosorb W;B, 2 m 15% Ucon on Chromosorb W; C, 2 m 25% y-nitro-y-methylpimelonitrile on Chromosorb W; D, 2 m 17%isopropenyl ketone, methyl acrylate, and methyl metha-crylate. Full details of the n.m.r. spectra of the adductsare presented later. Unless otherwise stated all adductswere shown to be stable under the reaction conditionsof their formation: in no case was epimerisation observed,Synthesis of Dienophiles.-2-Ethylacrylonitrile (Zc), b.p.112-113°, was prepared in 60% yield by heating Z-cyano-butyric acid 33 (7.25 g), dimethylammonium hydrochloride(5.25 g), and 37% aqueous formaldehyde solution (6.1 ml)under reflux for 3 h and isolating the product by etherextraction.Similarly 2-isopropylacrylonitrile (2d), b.p.125-127" was prepared in 41% yield from 2-cyano-3-methylbutyric acid.33 2-t-Butylacrylonitrile (2e), b.p. 73-75" at 80 mmHg was prepared in 38% yield by dehydrationof 2-cyano-3,3-dimethylbutan-2-01 with phosphorus oxy-TABLE 4N.m.r. data for adducts of cyclopentadienez Values1 -H 4-H6.83 7.037.20 7.047-08 7-086-98 7-086-82 6.986-98 7.066.93 7 -086.86 7.077-32 7.206.78 7.126-80 7.137.22 7.227-22 7-227-23 7-247.20 7-207.03 7-137.09 7.097.05 7.257-02 7.236.97 7.243,z-J37.90-8.35-8.40-8.458.087.737-807.828.1 58-228-218.037.998.107.778.138.157-627.627.573,,-H 5- and 6-H 7,- and 7,-H8.80-8.35-8.40-8.458.508-95-8.404 - 4 58-658.428.418.678.688.669-298.804 - 4 09.279.279.283-70, 3.853.733.743.743.80, 3.973.78, 4.023.79, 4.053.78, 4.043-90. 4.193-80, 4.223.93, 4.043.92, 4.063.993.76, 3.943.904.093-79, 3.943.79, 3-953.77, 3.953-83Ref.25.TCEP on Chromosorb W; E, 2 m 15% PPG on Chromo-sorb W; F, 2 m 20% Ucon on Chromosorb W; and G,2 m 20% XE6O on Chromosorb G.Preparative g.1.c.was carried out with a Pye-Unicam 105 chromatographusing columns of 3/8 in internal diameter. cyclopenta-diene was obtained by cracking dicyclopentadiene at 170'and was stored in a dry ice-bath. The following dieno-philes were commercially obtained and distilled beforeuse : acrylonitrile, methacrylonitrile, acrolein, metha-crolein, methyl vinyl ketone, ethyl vinyl ketone, methyl* A Referee has commented that ' inductive effects must playa considerable part' since in the light of recent results (E. T.McBee, M. J. Keogh, R. P. Levek, and E. P. Wesseler, J . Org.Chem., 1973, 38, 632) they might be expected to reproduce theobserved trends. Although the experimentally determineddifferences in free energy GAG$ between the transition statesleading t o epimeric adducts are always small ( < I kcal mol-l)we observe that endo-selectivity is influenced by the nature ofthe alkyl substituent at branching points M, (3, and y to thedouble bond of the dienophile.It is probable that inductiveeffects of the type suggested by McBee et al. will contribute tothe observed selectivity of dienophiles branched at the cc-position.There is no evidence t o suggest that such effects ' play a con-siderable part' in those dienophiles branched a t the p- or y-position. Our results establish that increasingly bulky dieno-philes suffer a major reduction in rate of both sndo- and exo-modes of addition, an effect which we attribute to steric repulsionand which is greater in the exo-transition state.Ref.26.chloride8.60, 8-80-8.35-8.4043-458.45, 8.578-22, 8.358-32, 8.388-73, 8.828.41, 8.528.59, 8-668.59, 8.668.42, 8-618-42, 8.618-46, 8-668-54, 8.668.67, 8.678.67, 8.768.68, 8-788.65, 8-788.67, 8-822,,-Substituent6.158.487-36, 8.87, 8.91-8.20, 8.918-757.047.028-668.678.670-377.817.51, 8.957-827.45, 8.946.95, 8.942,,,-Substituent '7-828.808-30, 8.97, 8.990.667.907-59, 8.997.937-67, 9-017-16, 9.01, 9.059.037-64-7.638.968.968-88-8.40, 8.98in pyridine. Ethyl isopropenyl ketone (3f),b.p. 120-121°, was prepared in 44% yield by heatingdiethyl ketone (38.7 g) , dimethylammonium hydrochloride(40.5 g), and 37% aqueous formaldehyde solution (40 ml)at 80' for 18 h and isolating the product by steam distillationand ether extraction of the distillate.Similarly isopropylisopropenyl ketone (3 g ) , b.p. 126-127", was preparedin 44% yield from ethyl isopropyl ketone by reaction a t85" for 72 h. The attempted preparation of the ketone(3h) by this route failed. Ethyl 2-cyanoacrylate (4b) wasprepared according to the method of Ardus34 and wasused directly. Similarly esters (4a), (4c), and (4d) wereprepared. Esters (4f-h) and (4j-1) were prepared byreaction of the appropriate acid chloride and alcohol inpyridine, were isolated by extraction with ether, and weredistilled before use.Reaction of Dienophiles with Cyc1opentadiene.-Adductswere prepared as shown in Tables 1-3. For examplemethacrylonitrile (2b) (3.58 g) and cyclopentadiene (3.58 g)were heated 35 under reflux in p-xylene (70 ml) for 16 h.Pairs of adducts were separated by chromatography on33 J. Kovas and C. S. Marvel, J . Polymer Sci., 1967, 563.84 A. E. Ardus, U.S.P. 2,467,926 (Chem. Abs., 1949, 43, 6222).35 W. Kraus and P. Schutte, Tetrahedron, 1968, 24, 153722 J.C.S. Perkin I1silica gel and individual adducts were characterised (i.r.,n.m.r., and mass spectra.to adducts of dienophiles (2a+)~ (3a-g)~ and (4a and b)by a consideration of their spectra (see Table 4). Note-worthy features which facilitated assignments were therelative chemical shifts of 3,,-H and 3,,-H and the relativechemical shifts of protons of an endo- and of an exo-sub-We gratefully acknowledge an S.R.C. studentship (to[3/651 Received, 28th March, 19731N."JIz.Y. Spectra of Addzccts.-Structures were assigned stituent. The data accord with related s t ~ d i e s . ~ ~ ~ ~ ~c. F. w.)
ISSN:1472-779X
DOI:10.1039/P29740000017
出版商:RSC
年代:1974
数据来源: RSC
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