STEREOSELECTIVITY IN NONCONCERTED REDUCTIVE REARRANGEMENT OF SOME BICYCLIC SPIRO OXIDES

The reductive rearrangement of the series of bicyclic spiro oxides 1 and 4-8 has been examined using mixtures of lithium aluminum deuteride (or hydride) and aluminum trichloride. Three types of monodeuterated products are found: primary-deuterated tertiary alcohol in the case of 4 and 6, tertiary-deuterated primary alcohol in the case of 1, and α-deuterated primary alcohol in the case of 5-8. The stereochemistry of the products has been determined in each case and the mechanisms of the reductions are discussed. Evidence is presented in the cases of 1,6, and 8 that the reductive rearrangement occurs exclusively or predominately in a nonconcerted fashion by way of a zwitterionic intermediate. It is suggested that the tendency of a terminal spiro oxide to be reduced in a nonconcerted fashion, in addition to being dependent upon the nature of the reducing species in solution, is governed by the stability of the cationic portion of the intermediate. Further, it is suggested that the tendency of such an intermediate to react by hydride migration (internal nucleophilic attack) rather than deuteride (or hydride) transfer from the coordinated alane (external nucleophilic attack) is governed by the extent of charge delocalization in the cationic portion of the intermediate. Finally, the ratio of exo/endo attack by the internal nucleophile, hydride, in the intermediates formed during the reduction of 6 and 8 is shown to be unusually low for a carbonium ion type of process, 3.3-7.1 and ~5.2, respectively. It is suggested that these ratios are consistent with the idea of steric hindrance to endo attack on the norbornyl ion or with the operation of a torsional effect. © 1969, American Chemical Society. All rights reserved.