Unveiling the role of boroxines in metal-free carbon–carbon homologations using diazo compounds and boronic acids† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sc02264f, additional data is available from the University of Cambridge Data Repository Website: https://doi.org/10.17863/CAM.10823.

Carbon–carbon bond forming reactions have been at the centre of organic synthesis for decades allowing efficient and quick assembly of complex molecular structures. Among many others, organoboron compounds are oen the reagents of choice as they promote C–C bond formation reactions with high chemoand stereoselectivity without the need for expensive, sometimes unstable and toxic transition metal catalysts. A large number of highly efficient methods have been described where a boron atom plays initially a role of the Lewis acid followed by a s-bondmigration thus allowing ametal-free coupling reaction between an electrophile (organoborane, organoboronic ester, dihaloborane) and a nucleophile (organolithium intermediates, sulfur ylides, diazo compounds). Coupling between boronic acids and diazo compounds has attracted considerable interest in the synthetic community as boronic acids are readily available, usually stable and more atom efficient reagents when compared to their ester analogues. Additionally, the homologation using TMSdiazomethane (TMSCHN2) and organoboron compounds is useful to install a trimethylsilyl and boron functionality in a metal-free fashion. These doubly functionalised carbon substrates are of interest, as they may be used to generate sequential or orthogonal functionalization at the same carbon atom. To date, the homologation with TMSCHN2 has only been performed in combination with boronic acids to access the TMS-free homologation products. While several proposed mechanisms have been reported, the role of a boronic