N.M.R. SPECTRA OF SOME Α–β UNSATURATED ESTERS

Abstract

The results of analysis of methyl citraconate and mesaconate, methyl cis-and transcrotonate, and methyl cis-and trans-2-petitenoate are reported. A previously reported analysis of the crotonates by first-order perturbation theory gave significantly dilferent values for the 1,3 coupling constants. The discrepancy in results, which is due to the i~nportance of secondorder effects, emphasizes the need for caution in applying the first-orcler neth hod. From values of 1,3 coupling constants reported here and elsewhere it is concluded that there is no simple relation between the magnitude of 1,3 coupling constants and geometrical configuration of the interacting nuclei.Much attention has been devoted recently to the N.M.R. spectra of substituted propenes and butenes (1-7). Two aspects of the spectra have received special attention. The first is the magnitude of the 1,3 coupling constants." Whipple et al. have reported 1,3 coupling to be consistently greater in the trans 2-substituted propenes (J1,3 than in the cis isomers (2) (J1,3 trans). In tiglic and angelic acids and their nlethyl esters the opposite was found (1). Subsequently Jackinan and Wiley (4) provided evidence which gave a different assignment to the protons of a-methacrolein from that of Whipple et al. and thus provided another case where J I ,~ trnns was greater than J I ,~ cis. T h e second aspect was the basis of Jackman's studies. He presented compellillg evidence for the use of chemical shifts to assign configuration to a-P unsaturated esters. Among the compounds which he has studied were the methyl esters of mesaconic, citraconic, and cisand trans-crotonic acids. We have examined these same four esters in addition to the methyl esters of cis-and trans-2-pentenoic acid. The more exact method of analysis employed in our case has provided some results of particular interest in this field. Methyl Mesaconate and Methyl Citraco7zateT h e olefinic and C-methyl protons in these esters comprise an AX3 system. Analysis of such a system can be made by first-order perturbation theory. T h e complete results are listed in Table I. All coupling constants in this paper are reported with a n estimated accuracy of f 0.05 cycle/sec as the standard deviation of each lneasurement was e 0 . 0 5 cycle/sec. The coupling constants in Table I

Proton Magnetic Resonance and Infrared Spectra of Some Complexes of N,n-Dimethylacetamide

“…At high resolution, a slight broadening of the high field N-methyl signal can be detected which can be attributed to a small trans spin-spin coupling between the N-methyl and C-methyl groups. Similar couplings have been noted in ethylenic systems (15).…”

Thermal rearrangement of α,β- to β,γ-unsaturated esters. Evidence for a 1,5-hydrogen transfer mechanism

“…the corresponding values of 6.9 and 7.3 Hz found in cis-and trans-crotonate respectively (16). If however Y is CH, (8) or CO,CH, (1 and 7) then the coupling constants are 9.6, 9.2, and 9.6 Hz respectively.…”

“…The corresponding shift in methyl cis-and trans-crotonates is 0.26 AT (16). This difference is undoubtedly due to the closer net distance between the methine hydrogen --Equilibrium could not be reached satisfactorily and the carbonyl group in 1 and 7, than the net at 297 "C but the equilibrium ratio approached distance between the hydrogens on the methyl the value of 15:55:30 for 10,11, and 12.…”

Studies on the mechanism and stereochemistry of phosphonate anion condensations

“…Our structural assignments were based on the following considerations: a) the g-Me of the trans- Fantazier (75) in their study of ethyl a,g-dimethyl cinnamates. Similar results were also reported by other workers (70,71,76,77). Examination of models also show that the ethoxyl protons are located, at least part of the time, directly over the phenyl ring in the cis-isomer and are in points are illustrated in Figure 3.…”