Atomic Motions and Protonation Stereochemistry in Nucleophilic Additions to Bicyclobutanes

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• 作者：Shmaryahu Hoz ; Carmela Azran ; and Ariel Sella
• 刊名：Journal of the American Chemical Society
• 出版年：1996
• 出版时间：June 12, 1996
• 年：1996
• 卷：118
• 期：23
• 页码：5456 - 5461
• 全文大小：181K
• 年卷期：v.118,no.23(June 12, 1996)
• ISSN：1520-5126

文摘

Several nucleophilic reactions on bicyclobutanes activated at thebridgehead carbon by electron withdrawinggroups (SO₂Ph, CO₂Me, COPh, and CN)were performed in MeOH. In all cases, the less stable1,3-disubstitutedcyclobutanes isomer was preferentially obtained (compared to theequilibrium ratio). The results for the two chargelocalizing groups CN and SO₂Ph oppose the existingknowledge regarding the protonation stereochemistry ofsuchcarbanions. Ab initio calculations (6-31G^*)have shown that as the nucleophile approaches the bicyclobutane,thebridgehead activating group moves inward toward an axial position.With a charge localizing group (CN and S(H)SO₂) the carbanion remains pyramidal, whereas with C(H)=Oas an activating group, the carbanion is nearly planar.It is suggested therefore that under conditions where thecarbanion undergoes rapid protonation, it is trapped initsinitial pyramidal geometry. Whereas, in cases where the lifetimeof the carbanion is long enough to allow appreciableequilibration, protonation may result in a different productdistribution. This hypothesis was tested by slowingdown the protonation rates. As a result, the morestable isomer was indeed preferentially obtained.