FORMATION OF BETA-LACTONES THROUGH LEWIS ACID-PROMOTED [2- A THEORETICAL-STUDY(2]CYCLOADDITION REACTION )

The formation of beta-lactone through Lewis acid-promoted [2 + 2] cycl oaddition is studied using semiempirical (AM1/RHF and AM1/CI) and nb i nitio (HF/6-31G and MP2/6-31G*) calculations. After a preliminary sem iempirical study of the BF3-catalyzed parent reaction through two dist inct reaction paths, ab initio and/or semiempirical studies on solvent and Lewis acid (BH3 and BF3) effects concentrate on the mechanism inv olving the prior formation of the C-C bond. At the HF/6-31G level of theory the introduction of BS induces a reduction of the activation en ergy from 40.8 to 11.9 kcal/mol, and calculations performed with AM1/C OSMO showed that the introduction of a solvent results in the formatio n of an earlier transition state. The case of BH3 is somehow more comp licated since the studied system induces, both at the semiempirical an d nb initio levels, a hydride transfer leading to a very stable produc t.