Fe-catalyzed cross-coupling of 1-substituted cyclopropyl tosylates & Rh(III) complexes in carbene transfer reactions: development of an azo metathesis

Abstract

Firstly, a novel Fe-catalyzed cross-coupling of 1-alkynylcyclopropyl tosylates and alkyl Grignard reagents was developed, which represents the first Fe-catalyzed cross-coupling of tert-alkyl electrophiles. The protocol allows regioselective access to the direct substitution products, in contrast to related Fe-catalyzed processes involving propargylic substrates which furnish exclusively the allene products. In addition, 1-vinyl¬cyclopropyl tosylates afforded the desired cross-coupling products when reacted with MeMgCl. If β-hydrogens were available on the nucleophile, mixtures of the cross-coupling product along with the reduced and the dimerized starting material were obtained. 1-Aryl- and 1-alkylcyclopropyl tosylates do not undergo this transformation which suggests that coordination of the substrate to the iron catalyst is necessary to allow for successful cross-coupling. Secondly, a Rh(III)-catalyzed metathesis of diazo compounds and azoarenes affording α-imino esters was developed. Key to the success and scalability of the reaction was irradiation of the reaction mixture with blue LEDs which facilitate the trans-to-cis isomerization of the azoarene and sets the scene for the metathesis to occur. This trans¬formation represents a new type of reaction of donor/acceptor carbenes and shows a broad scope with high yields and an appreciable compatibility with polar and apolar substituents giving rise to α-imino esters which could not easily be synthesized by traditional means.