Carbocations conformation

The mechanism in Scheme 19 is somewhat speculative, and relies heavily on the assumption that axial attack of solvent is more energetically favorable than equatorial attack of solvent on carbocations similar in structure to 64. However, the stereochemistry of acid-catalyzed hydrolysis of two diastereomeric hexa-hydrophenanthrene 9,10-epoxides (65 and 67) provides support for this proposal. These two epoxides contain transfused cyclohexane moieties that restrict the geometry of each epoxide to a single conformation. Reaction of 65 with H+ yields a single carbocation conformation 66, in which the hydroxyl group is forced to occupy an axial position (Scheme 20). 

Whereas acid-catalyzed hydrolyses of 65a and b yield almost exclusively trans diol products, the acid-catalyzed hydrolysis of 67a yields 85% of cis diol and 15% of trans diol.69 The reaction of 67 with H+ also yields a single carbocation conformation 68, in which the hydroxyl group is forced to occupy an equatorial position (Scheme 21). The major cis diol product can be rationalized by energetically favored axial attack of water on carbocation 68. In unpublished work, we have also determined that the acid-catalyzed hydrolysis of 67b, which is much more reactive than 67a and yields a stabilized carbocation 68b, also yields cis and trans diols in a ratio of 85 15. Axial attack of water on 68 to yield cis diol is therefore favored over equatorial attack to yield trans diol, but not to the same extent that axial attack of water on carbocation 66 is favored. 

Fig. 6. Reaction profiles of carbocation conformers, Ci, C2. a) Rapid equilibration b) conformational control...

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