Atomic Motions and Protonation Stereochemistry in Nucleophilic Additions to Bicyclobutanes<sup>1</sup> Shmaryahu Hoz Carmela Azran Ariel Sella 10.1021/ja960370g.s001 https://acs.figshare.com/articles/dataset/Atomic_Motions_and_Protonation_Stereochemistry_in_Nucleophilic_Additions_to_Bicyclobutanes_sup_1_sup_/3660030 Several nucleophilic reactions on bicyclobutanes activated at the bridgehead carbon by electron withdrawing groups (SO<sub>2</sub>Ph, CO<sub>2</sub>Me, COPh, and CN) were performed in MeOH. In all cases, the <i>less</i> stable 1,3-disubstituted cyclobutanes isomer was preferentially obtained (compared to the equilibrium ratio). The results for the two charge localizing groups CN and SO<sub>2</sub>Ph oppose the existing knowledge regarding the protonation stereochemistry of such carbanions. <i>Ab initio </i>calculations (6-31G<sup>*</sup>) have shown that as the nucleophile approaches the bicyclobutane, the bridgehead activating group moves inward toward an axial position. With a charge localizing group (CN and S(H)SO<sub>2</sub>) the carbanion remains pyramidal, whereas with C(H)O as an activating group, the carbanion is nearly planar. It is suggested therefore that under conditions where the carbanion undergoes rapid protonation, it is trapped in its initial pyramidal geometry. Whereas, in cases where the lifetime of the carbanion is long enough to allow appreciable equilibration, protonation may result in a different product distribution. This hypothesis was tested by slowing down the protonation rates. As a result, the <i>more</i> stable isomer was indeed preferentially obtained. 1996-06-12 00:00:00 isomer bridgehead 2 Ph CO bicyclobutane charge localizing group carbanion charge localizing groups CN protonation