Alkyl formates as transfer hydroalkylation reagents -Hydrobenzylation of imines

International audienceHydrofunctionalization of unsaturated bonds, such as carbonyl, imines, alkenes or alkynes, provide a rapid and atom-economic access to functionalized molecules. Among those processes, hydroalkylation, which formally adds a R-H alkane across a C=C, C=O or C=N bond, has been intensively studied. However, in these reactions a sacrificial hydride or hydrogen donor is required, usually a hydrosilane or a hydroborane. Therefore, hydroalkylation by transfer was explored to increase the atom economy. In 2011, Oshima's group proposed a transfer from allenyl or homoallyl alcohols to aldehydes or imines under Cu-catalysis. Then, in 2013, Tang's group designed efficient and versatile hydroalkyl-donors based on Hantzsch esters or benzothiazoles. Yet, these two reactions did not improved the atom economy, although recyclable but under harsh conditions. Following previous work on transfer hydrosilylation, formate appeared as a new potential transfer reagent. We assumed that alkyl formates can perform transfer hydroalkylation reactions through transition-metal catalysis. This process would present a high atom economy and CO$_2$, as the only byproduct, can be recycle in formic acid, which can also be biosourced. As a proof of concept, we developed a novel catalytic reaction of transfer hydroalkylation from alkyl formates using benzyl formates and imines as model reagents