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)(n)B(OR)(2) (M = Fe, Ru, W) with alkanes to form alkylbor onate 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-Me-2) with pentanel Synthesis of a series of Cp and Cp* catecholboryl complexes showed that sterically blockin g or eliminating sp(2) positions on the metal boryl complex was important f or alkane functionalization to occur. The metal boryl complexes reacted exc lusively at the terminal C-H position of alkanes. Functionalization of 2-me thylbutane occurred preferentially at the least sterically hindered termina l position with a selectivity of 10:1. This selectivity data, in addition t o kinetic isotope effects measured for reaction of metal boryls with a mixt ure of pentane and pentane-d(12), argues against radical chemistry. Several experiments were conducted to probe for CO dissociation. An experiment emp loying added (CO)-C-13, one conducted under 2 atm pressure of CO, and one c onducted in the presence of PMe3 indicated that the mechanism involves CO d issociation to form a 16-electron intermediate that reacts faster with alka ne solvent than it recoordinates CO. The effect of boryl electronics on the functionalization of alkanes was studied by examining the reactions of rut henium dialkylboryl, dithioboryl, and dialkoxyboryl complexes with pentane. The dialkoxyboryl complexes gave the highest yields of functionalized prod uct. A comparison between reactions of the different boryl complexes in are ne and alliant 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.