Carbene Rearrangements - Rearrangements - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part II. Introduction

Chapter 18. Rearrangements

18.D. Carbene Rearrangements86

Carbenes can rearrange to alkenes in many cases.87 A 1,2-hydrogen shift leads to an alkene, and this is often competitive with insertion reactions.88 Benzylchlorocarbene (47) rearranges via a 1,2-hydrogen shift to give the alkene.89Similarly, carbene 48 rearranges to alkene 49, and replacement of H on the α-carbon with D showed a deuterium isotope effect of ~5.90 Vinylidene carbene (H2C=C:) rearranges to acetylene.91 Rearrangement of alkylidene carbene (50) has been calculated to give the highly unstable cyclopentyne (51), which cannot be isolated, but can give a [2 + 2] cycloaddition product (Reaction 15-63) when generated in the presence of a simple alkene.92 The spiro carbenes undergo rearrangement reactions.93

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