Dérivés allyliques de la colonne IV-A, de type R3ECH2CH=CH—CH3 et R3ECH(CH3)CH=CH2. Approche des effets électroniques par résonance magnétique nucléaire 13C et 1H

Abstract

A comparative 1H and 13C nmr study of allylic derivatives of the IV-A group elements, of the type R3ECH2CH=CH—CH31E, Z and R3ECH(CH3)CH=CH2, 2, was performed (R = —C6H5, —CH3, E = C, Si, Ge, Sn, Pb). The present paper dealing with the allylic moiety concerns both the chemical shifts, δ1H and δ13C, and the homonuclear J(H—H) and heteronuclear couplings 1J(13C—H), nJ(13C—E), n = 1 to 4, nJ(E—C—H), n = 2 and 5. The spectra are consistent with a σ-allylic structure. In order to obtain an inductive order for the groups G(E) = —E(C6H5)3 a coherent group electronegativity scale was established. The intrinsic group electronegativities, without appreciable change, obey the following sequence:[Formula: see text]Both the changes along the series, in spectroscopic parameters, δ1H, δ13C, and the unusually large heteronuclear couplings, 2J(E—C—H), 5J(E—C—H), 4J(E—13C) (E = Sn, Pb), are interpreted in terms of an electronic mesomeric donating effect for the substituent σ, R3ECH2— and R3ECH(CH3)—, by σ–π hyperconjugation. Factors intrinsic to the main-group element control the chemical shifts δ1Hα and δ13Cα. The s-character of the bond Cα—Hα appears to control the 1J(13Cα—Hα) couplings; no correlation with a σ → π hyperconjugation could be established.