The Chemoselective, Enantiospecific Cross-Coupling of Secondary Boronic Esters and the Stability of Mesoporous Silica Supports for Pd Catalysis
Glasspoole, Ben William
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The Suzuki-Miyaura Cross-Coupling of aryl halides and aryl boronic esters has become one of the most important and oft used C-C bond forming reactions in industry and academia alike. Recently, substantial effort has been invested in expanding this reaction to include alkyl boronic esters as coupling partners, though until recently, success has been limited to primary alkyl boronic esters. Secondary alkyl boronic esters, with the inherent possibility of being chiral, have proven to be more difficult to couple. As a means of expanding our program on the enantio- and regioselective hydroboration of styrene derivatives, we sought to develop conditions that could couple benzylic (secondary) boronic esters. Not only was the coupling to aryl iodides achieved in moderate to good yield with a commercially available (and relatively cheap) catalyst system and phosphine, but the coupling reaction proceeds with almost complete retention of the stereochemistry installed during the hydroboration reaction. Interestingly, these conditions leave primary (linear) alkyl boronic esters completely untouched. Further examination of the chemoselectivity of the reaction revealed that, despite being unable to cross-couple strictly aliphatic secondary boronic esters, our silver-mediated protocol was able to effectively cross-couple chiral allylic boronic esters in high yield and good regioselectivity. The asymmetric syntheses of novel secondary boronic esters have also been developed to overcome the substrate limitations of the hydroboration reaction. Together with our effective cross coupling strategy, these novel chiral boronic esters have led to the synthesis of exciting new classes of molecules, most notably, the asymmetric triarylmethanes. Finally, the stability of mesoporous silica supports used in Pd catalysis was assessed. Though silica supports effectively reduce Pd-contamination in reaction mixtures to sub-ppm levels, their long-term reusability is hindered by material degradation caused by harsh reaction conditions. It was found that aqueous base, required for the Suzuki-Miyaura reaction, is responsible for silica degradation and the collapse of mesostructure. Interestingly, it was determined that the reaction itself had a protective effect on the material, with the boric acid side-product mitigating the deleterious effect of the base.