CYCLIZATIONS OF 3-CHLOROCARBANIONS TO CYCLOPROPANES - STRAIN-FREE TRANSITION-STATES FOR FORMING HIGHLY STRAINED RINGS

Ab initio calculations at the MP2/6-31+G(d,p)//MP2/6-31+G(d) level hav e been used to investigate the cyclizations of a series of stabilized 3-chlorocarbanions (ClCH(2)CH(2)CHZ(-) where 2; = C(O)H, CCH, or CN) t o cyclopropane derivatives. In each case, the cyclization barrier is s maller than the Sh 2 barrier of an analogous acyclic system, despite t he fact that the cyclization is over 25 kcal/mol less exothermic. The surprisingly small enthalpic barrier to the cyclizations is the result of the nucleophile being held in close proximity to the electrophilic site in the cyclization substrate. This destabilizes the ground state of the cyclization process and leads to 6-9 kcal/mol of barrier lower ing, enough to overcome the angular strain of the transition state. Al though cyclizations to three-membered rings have well-known entropic a dvantages, it appears that the proximity effect may be the dominant ba rrier-lowering factor in many eases. Other examples are given, and the results are compared to the available condensed-phase data.