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.

Reaction mechanism

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.

Directed Bamford-Stevens reaction

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 σC-Si → σ*C-H stereoelectronic 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.

Synthesis of 3-substituted indazoles from arynes and N-tosylhydrazones

N-tosylhydrazones 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.

N-tosylhydrazones as reagents for cross-coupling reactions

Barluenga and coworkers developed the first example of using N-tosylhydrazones as nucleophilic partners in cross-coupling reactions. Typically, nucleophilic reagents in coupling reactions tend to be of the organometallic variety, namely organomagnesium, -zinc, -tin, -silicon, and –boron. Combined with electrophilic aryl halides, N-tosylhydrazones can be used to prepare polysubstituted olefins under Pd-catalyzed conditions without the use of often expensive, and synthetically demanding organometallic reagents.

The scope of the reaction is wide; N-tosylhydrazones derived from aldehydes and ketones are well tolerated, which leads to both di- and trisubstituted olefins. Moreover, and variety of aryl halides are well tolerated as coupling partners including those bearing both electron-withdrawing and electron-donating groups, as well as π-rich and π-deficient aromatic heterocyclic compounds. Stereochemistry is an important element to consider when preparing polysubstituted olefins. Using hydrazones derived from linear aldehydes resulted in exclusively trans olefins, while the stereochemical outcomes of trisubstituted olefins were dependent on the size of the substituents.