Stereoselectivity Studies of the SP2 Carbon on 4-tert-Butylcyclohexylidene Analogues

Abstract

A study of possible mass effect on the steric course of nucleophilic attack on three analogues of 4-tert-butylcyclohexylidene has been undertaken. 1HNMR analyses have been used to determine the ratios of the trans isomers over the cis ones resulting from axial and equatorial attack of the nucleophile. The preference for axial attack is largest for the heavier brominated indandione substituent and least for the non-halogenated one. Computer simulation studies of our molecular structures were also carried out. Using PCMODEL executed on a Sun Workstation and run on Windows®, the cis and trans structures were optimized by energy minimization procedures in the program. For the minimized structures, it was found that the cis structures have higher energy values than the corresponding trans isomers. The latter is favored by further separation of the tert-butyl groups and the indandione substituent. A pyramidalization experiment was also performed using this program. This was done by displacing the 4-tert-butylcyclohexylidene portion vertically upwards and downwards by 0.1A and then calculating the energies of the structures without minimization. It was found from this study, that the energies of the molecules distorted upwards to 0.4A have lower energies compared to similar molecular structures distorted downwards.