M. Carmen Perez-Aguilar and Carlos Valdes
Angew. Chem. Int. Ed.
Due to the expense, complexity and, often, toxicity of transition metal catalyst systems, transition metal-free coupling carbon-carbon bond forming reactions are a highly attractive target in synthetic methodology. The difficulty in achieving these processes comes in requiring strong activation of at least one of the coupling partners. In the case of carbon addition to carbonyls, this activation is usually achieved by generating an anionic nucleophile. But addition of weaker nucleophiles, such as boronic acids, can be achieved by activation of the carbonyl species by condensation with tosylhydrazine.
Previous studies have shown that aryl and alkyl boronic acids can arylate and alkylate tosylhydrazones effectively. However, additions of alkenes to hydrazones give mixtures of products varying in both the position and geometry of the double bond. This problem has been solved in a recent report, which demontrates that the presence of two equivalents of CsF in the reaction gives high product selectivity.
When arylvinylbronic acids are used, the product of gamma-protodeboronation is observed as the only product of reaction. This process can be employed with a wide range of arylvinylboronic acids and tosylhydrozones.
In contrast, the addition of alkylvinylboronic acids is marginally less effective and affords selectively the product of alpha-protodeboronation.
This difference in selectivity is postulated to be due to the stabilisation of electron density at the benzylic position of arylvinylboronic acids, thus promoting the gamma-protodeboronation pathway in those cases. Similarly, the presence of electron-withdrawing groups in the hydrazone coupling partner are shown to bias selectivity in favour of the alpha-protodeboronation pathway.