Hydrogenolysis by LiAlH 4 –AlCl 3 of an ether solution of norcamphor isobutylene ketal

Abstract

An approximately 1:1 mixture of the two isomers (3 and 4) of norcamphor isobutylene ketal was hydrogenolyzed by AlH 2 Cl in ether at room temperature. The slow reaction (62% complete in 20 h; ∼96% complete in 168 h) produced only 1-(2-endo-norbornyloxy)-2-methyl-2-propanol (7), 2-(2-exo-norbornyloxy)-2-methyl-1-propanol (6), 2-exo-norborneol (10), and unchanged 3 and 4 in the proportion 64.5:26.5:4.5:4.5. Recovery of materials was at least 95%.Compound 10 is believed to arise from slow hydrogenolysis of the product 6. Since the unreacted ketal was still an approximately 1:1 mixture of the isomers 3 and 4, and because it is known that in the hydrogenolysis of acetals and ketals, the reduction by hydride donation is much faster than is the isomerization of the ketals, it is inferred that the two isomers (3 and 4) of norcamphor isobutylene ketal hydrogenolyze at approximately the same rate.Arguments are presented to support the view that isomer 4 reacts with two molecules of AlH 2 Cl, one as the Lewis acid and the other as the hydride donor, to provide product 7 and possibly some of product 6, while isomer 3 reacts with one molecule of AlH 2 Cl, which acts as the Lewis acid and the hydride donor, to provide most if not all of product 6 and some of product 7.