Kinetic and stereochemical factors in the hydralumination of alkenes and the configurational stability of the resulting carbonaluminum bond

Abstract

A series of cyclic olefins was subjected to hydralumination by diisobutyl-aluminum hydride, in order to determine the stereochemistry, the regiochemistry and the influence of structure on reactivity. The reactivity gradation observed was: acenaphthylene > 1,1-dimethyl-3-trimethylsilylindene > 1-methylacenaphthylene > 1,1-dimethylindene > 1-phenylacenaphthylene ⪢ 1,1-dimethyl-3-phenylindene. By working in donor solvents it was demonstrated that cis-hydralumination is the kinetically controlled mode of addition. The reactivity for 3-substituted-1,1-dimethylindenes reflected polar effects, whereas that for 1-substituted acenaphthylenes responded to steric effects. The regiochemistry of hydralumination was generally determined by steric factors, except for the case of the silylindene. The isomerization of the hydralumination adduct could occur either by carbonaluminum bond inversion or by dehydralumination. The former process took place readily in the absence of Lewis bases, but the latter process required elevated temperatures and protracted times. The stable cis-hydralumination adducts could be protodealuminated with retention of configuration at the CAl bond, although acenaphthenyl systems gave significant amounts of 1,3-dihydroacenaphthylenes. The nature of these unusual hydrolysis products was elucidated by deuterium labeling, 1H NMR and chemical degradation studies. A complete series of C 2- and C 3-deuterated-1,1-dimethylindenes was synthesized unambiguously in order to permit these foregoing studies. The configurational stability of these organoaluminum compounds toward heat, neutral Lewis bases and carbanions was evaluated. These observations, together with the temperature-dependent 1H NMR spectrum of 1-acenaphthenyl(diisobutyl)aluminium, lend some insight into the pathways of carbonaluminum bond inversion.