1974 2671Enamines. Part XXXVIII.l Configurations of I -Aryl-I -dialkylamino-propenesBy Riccardo Stradi,' Donato Pocar, and Cesare Cassio, lstituto di Chimica Organica, Facolth di Farmacia,The configurations of several 1 -aryl-I -dialkylaminopropenes have been assigned from l H n.rn.r. data. Theolefinic protons of the euro;-isomers resonate a t higher field than those of the corresponding Z-isomers.Universith di Milano, Viale Abruzzi 42, 201 31 Milano, ItalyA 1H n.m.r. study of several 3-N-methylanilino- and di-alkylaminopent-2-enes showed that in all cases the E- or Z-configuration could be assigned from the chemical shiftsof the olefinic protons and magnitudes of the allyliccoupling constants. All the compounds studied werefound to be mixtures of the two configurational isomers,with the thermodynamically more stable one as the majorcomponent.In all cases but that of 3-N-methylanilino-pent-2-ene the greatest stability is shown by the E-isomer. This exception can be explained in terms ofsteric and electronic effects.The foregoing results have been shown to apply ingeneral to a large number of enamines of aliphatic ke-tones, both symmetrical and unsymmetrical and derivedfrom both dialkylamines and N-alkylaniline~.~tsuggested theconfiguration of some enamines derived from propio-phenone and its $-substituted derivatives and secondaryamines, from examination of molecular models and U.V.spectra. The enamines investigated were all isomericallypure. Also, Duhamel et al. have established, by nuclearOverhauser effect studies the configuration of l-dimethyl-amino-l-phenylpropene.6We now report a lH n.m.r.investigation of the con-figurations of a homogeneous series of enamines derivedfrom aryl ethyl ketones. In most cases, mixtures con-taining both E- and the Z-isomers were studied; thusdata for both isomers could be obtained simultaneously.The enamines listed in Table 1 were prepared. Con-figurational assignments were based exclusively on lHchemical shifts, coupling constants for each pair of isomerst In ref. 4 the straightforward application of the Pascualrule to some N-methylaniline derivatives led us incorrectly toassign the lower field signal to the E-isomer. However this rule,which takes into account only steric features, cannot be applied toenamines, in which electronic factors are also important.Part XXXVII, D.Pocar and R. Stradi. Ann. Chim. (ItaZy),1971, 61, 181.Recently, Sollemberger and Martinbeing apparently identical. The signals of the olefinicprotons and, to a minor extent, of the methyl groups,occurred at higher field for the major form in each pair.Molecular models showed the complete lack of sterichindrance of the amino-group in the E-enamines, conjug-ation of the nitrogen lone pair with the x-system beingeasily achieved. In the Z-enamine the methyl grouphinders the amine residue from assuming the more favour-able conformation. This is confirmed by the resultsobtained with enamines of aliphatic ketones containingvery bulky amine residues, where this effect is particu-larly marked.2 Thus one could expect the isomer show-ing the high-field methyl and olefinic proton signals tohave the E-configuration. In other words, for all theenamines studied, the E-isomer is the major component.The chemical shifts of the olefinic protons of the E-isomers are very similar, whereas those of the Z-isomersshow a wider range of values.This is in agreement withthe above conclusion: the chemical shift of the olefinicproton will be more affected by changes in the aromaticnucleus when the latter lies on the same side of the doublebond. Accordingly, in the Z-series, the highest 6 valuesare found for compounds (Ih and g) in which the effect ofthe strongly electronegative heteroatom on the olefinicproton is greatest.To confirm this effect a series of heterocyclic l-amino-l-arylethylenes (11) (Table 2) was synthesized.In thesecompounds the difference between the chemical shifts ofthe two olefinic protons was in the range 0-27-0.38p.p.m. when the vinyl group was bonded to the 2-positionof the heterocycle, whereas no values greater than 0.16R. Stradi and D. Pocar, Chimica e Industria, 1971, 53, 266.3 G. Bianchetti, unpublished work.D. Pocar, R. Stradi, and B. Gioia, Gazzetta, 1968, 98, 969.6 D. Y . Sollemberger and R. B. Martin, J . Amer. Chem. SOC.,6 L. Duhamel, P. Duhamel, S. Combrisson. and P. Siret,1970, 92, 4261.Tetrahedron Letters, 1972, 36032672 J.C.S. Perkin Ip.p.m. were found for the 3- and 4-pyridyl derivatives.Similar behaviour has been found by Coletta et al.' forseveral pyridylethylenes.The aromatic solvent-induced shift effect in the case ofEXPERIMENTALPreparation ofEnamines.-The ketone (0.05 mmol) wasdissolved in anhydrous hexane or benzene (for the lesssoluble ketones) (ca.100 ml). The secondary amine (0-30TABLE 1ArPhPhC6H,*OMe( -4)2-Pyrid yl2-Pyrid yl4-Pyrid yl4-Pyrid y l3-Pyrid yl3-Pyrid yl2-Thien yl2-ThienylC6H,*Oamp;Ie( -4)C,H,*NO,( -4)C6H,*NO, (-4)2 - F u ~ l2-FuVlR2NMorpholinoEt,NMorpholinoEt2NMorpholinoEt2NMorpholinoEt,NMorpholinoEt,NMorpholinoEt2NMorpholinoE t2NMorpholinoEt2NAr2-Pyrid yl2-Pyridyl3-Pyridyl3-Pyridyl4-P yrid yl4-Pyrid yl4-Pyrid yl2-Thien yl2-Thien yl2-FuvlEIZlOO/O901 10lOO/O9218lOO/O80/2087/137013078/229515961485/1578/2292/890/1070130p 1 4CH,1.551-521-561.571.581.551.611-561.581-571.581.541.741.711.731-70gC6D6 gcc14CH, H1-63 4-611-64 4.511-67 4-571-70 4.441.43 4-801-40 4-681.78 4.801-82 4-701-49 4-751-54 4.641-52 4.781-54 4-641.74 4.751-76 4.721.77 4.741-80 4.70R2NMorpholinoEt2NPiperidinoEt,NMorpholinoPiperidinoEt,NEtiNMorpholinoEt2NTABLE 2the enamines (I) is very strong (see Table l), but notreadily explicable; however the magnitudes of the in-duced shifts are quite different for the two sets of isomers.The foregoing results, obtained on mixtures of E-and Z-isomers, axe in accord with those described byDuhamel in relation to isomerically pure l-dimethyl-amino-l-phenylpropene. Moreover, in this homogeneousseries, the differences in chemical shifts of the olefinicprotons of the E- and Z-isomers are great enough to allowassignment of configuration even when only one isomeris present.* Birkhofer et aZ.8 obtained o-aminophenols by reactions ofacylfurans and with secondary amines and interpreted the reac-tion in terms of an intermediate enamine. However, we foundthe fury1 enamines (10 and p) and (IIh) to be stable and easilydistilled.*C-,D,H4.624.624.604-6 14.554-624-804-824.584.624.644.614-684-754-704.76z7 h , euro;3.range IT)8CC14CH,1.761-731-821-871-851.861.821-851-791.801.781.801.80gCDCl,H A4.754.504.27 or 4-234-19 9r 4.164.454.404.334-504.634.44gC6D6CH,1.751-781.681.781-821.651.671-701-671-671.701.701.76amp;CDCl,HB4.404-234.23 or 4-274.16 or 4.194.294.284-204.124.204.13p 1 4H5-074.955-305-616.005.1 15-354-075.145.225.455.465.758 ~ 6 ~ 6 (at 1 amp; '0.8Torr) H95-1035.17 93-102135-1 455.13 112-120(m.p.71)5.13 145-1565.75 80-906.27 75-854.9'3 87-965.26 75-854.89 88-965.06 75-855.28 70-805-56 67-755.57 67--755.90 58-65B. range ("C)(at 1 f 0.5 Torr)80-10082-9086-9585-9490-1028G-10085-9359-7092-10063-70mmol) was then added, followed dropwise with stirring,under nitrogen, by a solution of titanium tetrachloride (0.028mmol) in anhydrous hexane (25 ml). After 24 h the mix-ture was filtered or centrifuged and the filtrate was evapor-ated. The crude enamine was distilled in a microapparatusat 1-2 Ton.* Only the enamine (Ie) was isolated directly,as a crystalline solid, m.p. 71 "C (from hexane). G.1.c. con-firmed the purity of all the products, the only impurity beingca. 1-2 of the starting ketone.The lH n.m.r. spectra were recorded with a Varian A-60spectrometer operating at 60 MHz with Me,Si as internalstandard.4/176 Received, 28th January, 197417 F. Coletta, A. Gambaro, and L. Pasimeni, Gazzefta, 1973,8 L. Birkhofer and G. Daum, Chew. B e y . , 1962, 95, 183.103, 266
展开▼
