Carbene Fragmentation in the Bicyclo[3.1.0]hexyl System:  Disconnecting the Trishomocyclopropenyl Cation

Abstract

[structure: see text] Fragmentation of cis-3-bicyclo[3.1.0]hexyloxychlorocarbene (4) affords cis- and trans-3-bicyclo[3.1.0]hexyl chlorides, cis- and trans-2-bicyclo[3.1.0]hexyl chlorides, and 2-bicylo[3.1.0]hexene. The promiscuity of product formation, taken together with kinetics and labeling studies, suggests that the fragmentation of 4 proceeds via a 3-bicyclo[3.1.0]hexyl cation-chloride ion pair but largely bypasses a trishomocyclopropenyl cation intermediate.

Adventures in Reactive Intermediate Chemistry: A Perspective and Retrospective

“…92 Instead, the bicyclo[3.1.0]hexyl cation chloride ion pair formed from carbene 34 in DCE yields an array of products, including the cis-and trans-chloride return products, cis and trans hydride-shifted chlorides, and even some alkene formed by HCl elimination. 93 Strikingly, carbon "scrambling", a hallmark of solvolysis proceeding through cation 35, is greatly suppressed in chloride formation from 34. The low activation energy for carbene fragmentation (measured as 2.4 kcal/mol, 93 compared to ΔH ⧧ = 24.1 kcal/mol for acetolysis of the analogous tosylate 92 ) obviates the need for assistance by the neighboring cyclopropyl C−C bond.…”

“…93 Strikingly, carbon "scrambling", a hallmark of solvolysis proceeding through cation 35, is greatly suppressed in chloride formation from 34. The low activation energy for carbene fragmentation (measured as 2.4 kcal/mol, 93 compared to ΔH ⧧ = 24.1 kcal/mol for acetolysis of the analogous tosylate 92 ) obviates the need for assistance by the neighboring cyclopropyl C−C bond.…”

“…Instead, the bicyclo[3.1.0]­hexyl cation chloride ion pair formed from carbene 34 in DCE yields an array of products, including the cis - and trans -chloride return products, cis and trans hydride-shifted chlorides, and even some alkene formed by HCl elimination . Strikingly, carbon “scrambling”, a hallmark of solvolysis proceeding through cation 35 , is greatly suppressed in chloride formation from 34 .…”