Bamford–Stevens reaction

The Bamford–Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens (1900–2000). The usage of aprotic solvents gives predominantly Z-alkenes, while protic solvent gives a mixture of E- and Z-alkenes. As an alkene-generating transformation, the Bamford–Stevens reaction has broad utility in synthetic methodology and complex molecule synthesis.

The treatment of tosylhydrazones with alkyl lithium reagents is called the Shapiro reaction.

The first step of the Bamford–Stevens reaction is the formation of the diazo compound 3.

In protic solvents, the diazo compound 3 decomposes to the carbenium ion 5.

In aprotic solvents, the diazo compound 3 decomposes to the carbene 7.

The Bamford–Stevens reaction has not proved useful for the stereoselective generation of alkenes via thermal decomposition of metallated tosylhydrazones due to the indiscriminate 1,2-rearrangement of the carbene center, which gives a mixture of products. By replacing an alkyl group with a trimethylsilyl (TMS) group on N-aziridinylimines, migration of a specific hydrogen atom can be enhanced. With the silicon atom beta to H, a σ_C-Si → σ^*_C-Hstereoelectronic effect weakens the C-H bond, resulting in its exclusive migration and leading to the nearly exclusive formation of allylsilanes instead of equal amounts of allylsilanes and isomeric homoallylsilanes, analogous to the mixture of products seen in the dialkyl case, or other insertion products (i.e. cyclopropanes). See beta-silicon effect.

N-tosyldydrazones can be used in a variety of synthetic procedures. Their use with arynes has been used to prepare 3-substituted indazoles via two proposed pathways. The first step is the deprotonation of the hydrazone of diazo compounds using CsF. At this point, the conjugate base could either decompose to give the diazo compound and undergo a [3+2] dipolar cycloaddition with the aryne to give the product, or a [3+2] annulation with aryne which would also give the final product. While strong bases, such as LiOtBu and Cs₂CO₃ are often used in this chemistry, CsF was used to facilitate the in situ generation of arynes from o-(trimethylsilyl)aryl triflates. CsF was also thought to be sufficiently basic to deprotonate the N-tosylhydrazone.

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