Additions to bicyclic olefins. III. Stereochemistry of the epoxidation of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins. Steric effects in the 7,7-dimethylnorbornyl system

A systematic study of addition reactions of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins has been undertaken in order to define more precisely the role of steric effects in controlling the stereo- chemistry of the additions and the nature of the intermediates in such addition reactions. The addition of borane to norbornene proceeds almost exclusively exo (99.573, whereas the corresponding addition to 7,7-dimethylnor- bornene proceeds preferentially endu (78%). Similarly, hydroboration of 2methylenenorbornane gives pref- erentially exo (85%), whereas 2-methylene-7,7-dimethylnorbornane gives preferentially (85%) endo product. Similar results were realized with 1-methylnorbornene, 2-methylnorbornene, bornene, and 2,7,7-trimethylnorbor- nene. Consequently, hydroboration of norbornene, 1- and 2-methylnorbornene, and 2-methylenenorbornane goes predominantly exo, evidently reflecting the greater steric availability of the exu position in this bicyclic system. However, the presence of 7,7-dimethyl substituents causes the addition to proceed preferentially from the endu direction. Consequently, in hydroboration the 7,7-dimethyl substituents alter the normal direction of addition to olefins of the norbornane structure, irrespective of whether the double bond is endocyclic, directly under the 7,7- substituents, or exocyclic, located to the side of the substituents. major argument for the u-bridged norbornyl cation A is the almost exclusive exo substitution realized in the solvolysis of 7,7-dimethylnorbornyl derivatives. 334