SYNTHESIS AND CATALYTIC APPLICATIONS OF PHOSPHINITE COMPLEXES OF NICKEL

Abstract

A series of imino- and aminophosphinite pincer complexes of nickel(II) (POCN)NiR (where R = Alk, OtBu, SPh, OAc, OTf, etc.) were synthesized and characterized. The prepared compounds were subjected to comparative studies of their catalytic activity in deoxygenative hydroboration of carboxamides. Using 5 mol% of nickel(II) pre-catalysts, primary and secondary amides were converted to the corresponding N-borylamine products within 24 hours at 60-80°C, presenting rare examples of catalytic systems for mild deoxygenative reduction of amides to secondary and primary amines. On the other hand, in contrast with the previously reported systems, the reduction of tertiary amides resulted in no or very low conversion of the substrates. The highest catalytic activities were observed for complexes substituted with alkyl and tert-butoxy ligands. Such reactivity was found consistent with the rates of the pre-catalysts activation with HBPin to generate the POCN Ni(II) hydride derivatives, which were suggested as active hydroboration catalysts. Mechanistic studies by means of stoichiometric and control experiments support formation of the Ni(II) hydride species and for primary amides additionally suggest the presence of an amide pre-activation step via dehydrogenative coupling with HBPin to produce N-borylamides. Based on mechanistic studies, deoxygenative reduction of both primary and secondary carboxamides was proposed to proceed via intermediate formation of the imine derivatives, such as RC(H)=N(Bpin) and RC(H)=NR, respectively, which further undergo addition of HBPin to produce the corresponding N-borylamine products