ATOMIC MOTIONS AND PROTONATION STEREOCHEMISTRY IN NUCLEOPHILIC ADDITIONS TO BICYCLOBUTANES

Several nucleophilic reactions on bicyclobutanes activated at the brid gehead carbon by electron withdrawing groups (SO(2)Ph, CO(2)Me, COPh, and CN) were performed in MeOH. In all cases, the less stable 1,3-disu bstituted cyclobutanes isomer was preferentially obtained (compared to the equilibrium ratio). The results for the two charge localizing gro ups CN and SO(2)Ph oppose the existing knowledge regarding the protona tion stereochemistry of such carbanions. Ab initio calculations (6-31G ) have shown that as the nucleophile approaches the bicyclobutane, th e bridgehead activating group moves inward toward an axial position. W ith a charge localizing group (CN and S(H)-SO2) the carbanion remains pyramidal, whereas with C(H)=O as an activating group, the carbanion i s nearly planar. It is suggested therefore that under conditions where the carbanion undergoes rapid protonation, it is trapped in its initi al pyramidal geometry. Whereas, in cases where the lifetime of the car banion is long enough to allow appreciable equilibration, protonation map result in a different product distribution. This hypothesis was te sted by slowing down the protonation rates. As a result, the more stab le isomer was indeed preferentially obtained.