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

Abstract

Heteroatom substituents on the methyl group of 2-methyl-1,3-dioxolane retard the rate of hydrogenolysis by AIHzCl of ether solutions of the z-substituted 1,3-dioxolanes. The effectiveness of the heteroatoms in decreasing the ease of hydrogenolysis is H < S < 0 < Br < NR,. This retardation is thought to be due to the destabilization of the transition state leading to the intermediate oxocarboniurn ion, caused by the electronegativity of the heteroatom and/or coordination of some of the AIHzCl with the heteroatom.AIHzCl or AlH, in diethyl ether at room temperature reduces 2-vinyl-or 2-[alkyl (or aryl) substituted vinyl]-1,3-dioxolanes to only the P,y-unsaturated alkyl P-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.LiAlH, in diethyl ether at room temperature, or in di-n-propyl ether or 1,2-dimethoxyethane @ME) at room temperature or reflux temperature, reduces 2-vinyl-1,3-dioxolane only to a-propenyl P-hydroxyethyl ether. This product is the result of hydride addition to the P-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 LiAIH4 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 P,y-unsaturated alkyl P-hydroxyethyl ether. In refluxing DME (80") the 2-alkylated vinyl-l,3-dioxolanes give primarily, if not exclusively, the a,P-unsaturated alkyl P-hydroxyethyl ether. A rationale is suggested to account for these results.LiAlH4 in diethyl ether reduces 2-styryl-1,3-dioxolane only to f3-phenylethyl-1,3-dioxolane, by hydride addition to the olehic carbon P to the phenyl group.Les substituants heteroatomiques situes sur le groupe rnethyle du methyl-2 dioxolane-1,3 ralentissent la vitesse d'hydrogbnolyse par AIHzCl des solutions etherbes des dioxolanes-1,3 substitues en 2. L'efficacite des heteroatomes dans la diminution de la facilite d'hydrogenolyse est H < S < 0 < Br < NR,.On pense que ce ralentissement est du a la destabilisation de l'etat de transition qui conduit a I'ion oxocarbonium intermediaire a cause de l'electronegativite de l'heteroatorne et/ou de la coordination d'une certaine quantite de AIHzCl avec l'het6roatorne.AIHzCl ou AlH3 dans 1'6ther diethylique reduit quantitativement a la temperature ordinaire les vinyl-2 ou les vinyl-2 [substituks par un-groupe,alkyle'(ou aryle)] dioxolanes-1,3 en alkyl p,y-insature 0-hydroxyethyl Bther qui est le produit attendu lors de I'addition d'un ion hydrure sur le C-2 d'un dioxolane-1,3. La facilite de I'hydrogknolyse croit avec I'augmentation de la substitution par un groupe alkyle (ou aryle) sur le groupe vinyle en position 2.LiAIH4 dans 1'6ther diethylique la temperature ordinaire, ou dans Ether di-n-propylique ou dans le dimkthoxy-1,2 ethane (DME) L...