Construction of Bulky Ligand Libraries by Ru^(II)-Catalyzed P^(III)-Assisted ortho-C–H Secondary Alkylation

Abstract

Modification of commercially available biaryl monophosphine ligands via ruthenium (II) -catalyzed P (III) -directed-catalyzed ortho C−H secondary alkylation is described. The use of highly ring-strained norbornene as a secondary alkylating reagent is the key to this transformation. A series of highly bulky ligands with a norbornyl group were obtained in excellent yields. The modified ligands with secondary alkyl group outperformed common substituted phosphines in the Suzuki−Miyaura cross-coupling reaction at a ppm mole level of Pd catalyst.

Sterically Controlled Late‐Stage Functionalization of Bulky Phosphines

“…Previously reported arylphosphine C−H borylation reactions, which are catalyzed by rhodium, ruthenium or iridium, or mediated by boron bromide reagents, all rely on the directing effect of the phosphine substituent to achieve high regioselectivity (Scheme 1). [6c,7] Directed arylation, [8] alkylation, [9] alkenylation [9b,10] and silylation [11] provide efficient access to additional substituents, but the use of a directing group strategy still limits functionalization to the C−H bond that lies closest to a catalyst engaged in dative bonding with the phosphorus lone pair, [8,9b,e,f,10–12] or the C−H bond para to this position [9a,c,d] . Here we report an undirected C−H borylation reaction of bulky phosphines, where the most sterically accessible position(s) of the scaffold undergo functionalization, so that a distinct regioisomer is obtained compared with phosphorus directed C−H borylation reactions.…”