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 have been used to investigate the cyclizations of a series of stabilized 3-chlorocarbanions (ClCH 2 CH 2 CHZ - where Z = C(O)H, CCH, or CN) to cyclopropane derivatives. In each case, the cyclization barrier is smaller than the S N 2 barrier of an analogous acyclic system, despite the 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 lowering, enough to overcome the angular strain of the transition state. Although cyclizations to three-membered rings have well-known entropic advantages, it appears that the proximity effect may be the dominant barrier-lowering factor in many cases. Other examples are given, and the results are compared to the available condensed-phase data.