Atomic Motions and Protonation Stereochemistry in
Nucleophilic Additions to Bicyclobutanes<sup>1</sup>
Shmaryahu Hoz
Carmela Azran
Ariel Sella
10.1021/ja960370g.s002
https://acs.figshare.com/articles/journal_contribution/Atomic_Motions_and_Protonation_Stereochemistry_in_Nucleophilic_Additions_to_Bicyclobutanes_sup_1_sup_/3660036
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