Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Abstract

. Can. J. Chem. 69,496 (1991). The 'H nuclear magnetic resonance spectra of benzyl silane and benzyl trichlorosilane, obtained in CS2 and benzene-d6 solutions, are analyzed. The long-range coupling constants between the methylene and para ring protons are used to derive apparent twofold barriers about the Csp2-Csp3 bonds of 7.4 2 1.6 and 8.1 + 1.1 kJ/mol for the silane and the trichlorosilane, respectively. These are higher than that for ethylbenzene and are attributed mainly to the stabilization of the perpendicular conformer, that with the C-Si bond in a plane perpendicular to the phenyl plane, by u-IT conjugation (hyperconjugation) of the C-Si bond and the IT electron system. Molecular orbital computations confirm the predominantly twofold nature of the internal barrier in benzyl silane and also for benzyl germane and stannane. The calculated barriers for the silane derivatives are rather higher than the experimental values. The computed barriers have magnitudes that appear to change with X in much the same order as do the hyperconjugative interactions deduced in other ways for CH2X(CH3)3 groups (X = Sn, Ge, Si, C). The angles CCX in benzyl-X (X = CH3, SiH3, SiC13, GeH3, SnH3) are all computed to decrease smoothly as sin2$, where $ is the angle by which the C-X bond twists out of the phenyl plane.Key words: conformations, benzyl silane and trichlorosilane; NMR, benzyl silane and trichlorosilane; MO calculations, benzyl silane and trichlorosilane. Des calculs d'orbitales molCculaires confirment la nature principalement binaire de la barrikre interne dans le benzyl silane ainsi que dans les benzyl germane et stannane. Les barrikres calculCs pour les dCrivCs silanes sont sensiblement plus hautes que les valeurs expCrimentales. La variation des amplitudes des barrikres calculCes pour les dCrivCs silanes avec X semble suivre l'ordre des interactions d'hyperconjugaison dCduites sur d'autres bases pour les groupements CH2X(CH3), (X = Sn, Ge, Si, C). Les calculs indiquent que les angles CCX, des benzyl-X (X = CH3, SiH3, SiC13, GeH3 et SnH,), doivent diminuer doucement comme le sin2$, oh IJJ est l'angle que fait la liaison C-X avec le plan du noyau phCnyle.

The Rotational Barriers of Groups Containing Silicon in Substituted Benzenes. A Theoretical Approach to the Silicon Substituent Effect

“…) H) and its stabilization is attributed 3 to the hyperconjugative effect. The rotational barriers of the molecules with X ) H and Cl are higher than those of the corresponding molecules in which carbon replaces silicon, and this feature has been attributed 3 to the greater hyperconjugative ability of the CsSi bond with respect to the CsC bond.…”

“…) H) and its stabilization is attributed 3 to the hyperconjugative effect. The rotational barriers of the molecules with X ) H and Cl are higher than those of the corresponding molecules in which carbon replaces silicon, and this feature has been attributed 3 to the greater hyperconjugative ability of the CsSi bond with respect to the CsC bond. A different effect is observed when X ) CH 3 , because the barrier becomes lower than in the corresponding carbon derivative: 2,3 this behavior has been interpreted 2,3 as being due to a consistent decrease in steric effects in the rotational maximum of type B in the molecule containing silicon owing to the CsSi being longer than the CsC bond distance.…”

“…8,12 The conformational minimum of phenylsilane is staggered, 7,13 A in Scheme 1, as in toluene, and the small rotational barrier, amounting to a few cal/mol, probably originates from steric repulsion effects. 7 The barrier for internal rotation in PhCH 2 SiX 3 molecules, with X ) H, Cl, or CH 3 , amounts to a few kcal/mol, the conformational minimum is of type A (R 1 ) SiX 3 , R 2 ) R 3 ) H) and its stabilization is attributed 3 to the hyperconjugative effect. The rotational barriers of the molecules with X ) H and Cl are higher than those of the corresponding molecules in which carbon replaces silicon, and this feature has been attributed 3 to the greater hyperconjugative ability of the CsSi bond with respect to the CsC bond.…”

“…The internal rotation process of SiR 3 substituents, where R spans from the small H atom to bulkier groups, has been studied intensively. [1][2][3][4] The understanding of the fundamentals of the structures of organosilicon compounds is important for their wide potential use in the industry of silicone polymers and silicon based ceramic materials 5 and for new classes of biologically active compounds. 6 Analysis of the origin of the barrier for internal rotation around the SisC bond can be employed to investigate the interactions between groups containing silicon and an unsaturated system and, in view of the possible d-orbital participation in bond formation in the former derivatives, enables comparisons to be made with the corresponding carbon compounds.…”

“…Reduced steric effects in silicon compounds with respect to the analogous carbon derivatives have been reported 14 for the conformational behavior of cyclohexyl derivatives as well. The X group in the CH 2 SiX 3 rotor affects the hyperconjugative character of the CsSi bond 3 , becoming smaller when X ) Cl with respect to X ) H and CH 3 .…”