Functionalization of Alkanes by Isolated Transition Metal Boryl Complexes

We report the photochemical reaction of transition metal boryl complexes of the type Cp*M(CO)nB(OR)2 (M = Fe, Ru, W) with alkanes to form alkylboronate esters in yields as high as 85% for reaction of Cp*W(CO)3[Bcat(Me)2] {[Bcat(Me)2] = B-1, 2-O2C6H2-3, 5-Me2} with pentane. Synthesis of a series of Cp and Cp* catecholboryl complexes showed that sterically blocking or eliminating sp2 positions on the metal boryl complex was important for alkane functionalization to occur. The metal boryl complexes reacted exclusively at the terminal C−H position of alkanes. Functionalization of 2-methylbutane occurred preferentially at the least sterically hindered terminal position with a selectivity of 10:1. This selectivity data, in addition to kinetic isotope effects measured for reaction of metal boryls with a mixture of pentane and pentane-d12, argues against radical chemistry. Several experiments were conducted to probe for CO dissociation. An experiment employing added 13CO, one conducted under 2 atm pressure of CO, and one conducted in the presence of PMe3 indicated that the mechanism involves CO dissociation to form a 16-electron intermediate that reacts faster with alkane solvent than it recoordinates CO. The effect of boryl electronics on the functionalization of alkanes was studied by examining the reactions of ruthenium dialkylboryl, dithioboryl, and dialkoxyboryl complexes with pentane. The dialkoxyboryl complexes gave the highest yields of functionalized product. A comparison between reactions of the different boryl complexes in arene and alkane solvents showed that the electronic properties of the boryl group had a greater effect on the reaction of the unsaturated intermediate with alkane than they did on the generation of the intermediate.