The Influence of Heteroatom-substituted Methyl Groups and of Vinyl Groups (CR2=CR—) Attached to C-2 of 1,3-Dioxolanes on the Ease and Direction of Hydrogenolysis of the 1,3-Dioxolanes by AlH2Cl, AlH3, or LiAlH4

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Abstract
Heteroatom substituents on the methyl group of 2-methyl-1,3-dioxolane retard the rate of hydrogenolysis by AlH2Cl of ether solutions of the 2-substituted 1,3-dioxolanes. The effectiveness of the heteroatoms in decreasing the ease of hydrogenolysis is H < S < O < Br < NR2. This retardation is thought to be due to the destabilization of the transition state leading to the intermediate oxocarbonium ion, caused by the electronegativity of the heteroatom and/or coordination of some of the AlH2Cl with the heteroatom.AlH2Cl or AlH3 in diethyl ether at room temperature reduces 2-vinyl- or 2-[alkyl (or aryl) substituted vinyl]-1,3-dioxolanes to only the β,γ-unsaturated alkyl β-hydroxyethyl ether, the product expected from hydride ion addition to C-2 of the 1,3-dioxolane. The ease of hydrogenolysis increases with increasing alkyl (or aryl) substitution on the 2-vinyl group.LiAlH4 in diethyl ether at room temperature, or in di-n-propyl ether or 1,2-dimethoxyethane (DME) at room temperature or reflux temperature, reduces 2-vinyl-l,3-dioxolane only to α-propenyl β-hydroxyethyl ether. This product is the result of hydride addition to the β-carbon of the vinyl group accompanied by ring opening and double bond migration. Alkyl substituents attached to the vinyl group of 2-vinyl-1,3-dioxolane markedly retard the rate of hydrogenolysis by LiAlH4 in the dialkyl ethers at room temperature, but at the reflux temperature (90°) of di-n-propyl ether the 2-alkylated vinyl-1,3-dioxolanes are hydrogenolyzed, but only to the β,γ-unsaturated alkyl β-hydroxyethyl ether. In refluxing DME (80°) the 2-alkylated vinyl-1,3-dioxolanes give primarily, if not exclusively, the α,β-unsaturated alkyl β-hydroxyethyl ether. A rationale is suggested to account for these results.

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