Manifestation of stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one-bond (1)J(C-H) NMR coupling constants. Relative acceptor ability of the carbonyl (C=O), thiocarbonyl (C=S), and methylidene (C=CH2) groups toward C-H don

摘要

Theoretical examination [B3LYP/6-31G(d,p), PP/IGLO-III//B3LYP/6-31G(d,p), and NBO methods] of six-membered cyclohexane 1 and carbonyl-, thiocarbonyl-, or methylidene-containing derivatives 2-27 afforded precise structural (in particular, C-H bond distances) and spectroscopic (specifically, one-bond (1)J(C-H) NMR coupling constants) data that show the consequences of stereoelectronic hyperconjugative effects in these systems. Major observations include the following. (1) sigma(C)-H-ax --> pi*(C=Y) and pi(C=Y) --> sigma*(C-Hax) (Y = O, S, or CH2) hyperconjugation leads to a shortening (strengthening) of the equatorial C-H bonds adjacent to the T group. This effect is reflected in smaller (1)J(C)-(Hax) coupling constants relative to (1)J(C)-H-eq. (2) Comparison of the structural and spectroscopic consequences of sigma(C-Hax) --> pi*(C=Y) hyperconjugation in cyclohexanone 2, thiocyclohexanone 3, and methylenecyclohexane 4 suggests a relative order of acceptor orbital ability C=S > C=O > C=CH2, which is in line with available pK(a) data. (3) Analysis of the structural and spectroscopic data gathered for heterocyclic derivatives 5-12 reveals some additivity of sigma(C-Hax) --> pi*(C=Y) --> sigma*(C-Hax), n(x) --> sigma*(C-Hax), n(betaO) --> sigma*(C- Heq), n(x) --> sigma*(C-Hax), n(betaO) --> sigma*(C-Heq), and sigma(S-C) --> sigma*(C-Heq) stereoelectronic effects that is, nevertheless, attenuated by saturation effects. (4) Modulation of the C=Y acceptor character of the exocyclic pi group by conjugation with alpha-heteroatoms O, N, and S in lactones, lactams, and methylidenic analogues 13-24 results in decreased sigma(C-Hax) --> pi*(C=Y) and pi(C=Y) --> sigma*(C-Hax) hyperconjugation. (5) Additivity of sigma(C-Hax) --> pi*(C=Y) and pi*(C=Y) --> sigma*(C-Hax) hyperconjugative effects is also apparent in 1,3-dicarbonyl derivative 25 (C=Y equal to C=O), 1,3-dithiocarbonyl derivative 26 (C=Y equal to C=S), and 1,3-dimethylidenic analogue 27 (C=Y equal to C=CH2).