Introduction to Alkylsilicates

The now well-known cross-coupling of sp-sp² and sp²-sp² carbon centers has proved to be highly useful in synthetic organic chemistry. Lacking in the various C-C cross-coupling arsenal is the ability to couple sp³-sp² and sp³-sp³ carbon centers. The Molander group at the University of Pennsylvania was the first to employ a photoredox/Ni dual catalysis approach to the resolution of this omission (Scheme 1). A general method for the coupling of sp³-sp² carbon centers is particulary important for drug discovery as it has been reported that compounds with lower sp³ fractions have higher rates of attrition in drug development.¹ Their first attempts were to use a metal photoredox catalyst and an alkyltrifluoroborate as the radical provider. The second catalyst was a Ni⁰ complex, which reacts with the radical generating a Ni^I intermediate. This then undergoes an oxidative addition of the aryl or vinyl halide to form a Ni^III intermediate, followed by a reductive elimination to Ni^I and the cross-coupled product. This work was followed by both Fensterbank and Molander utilizing alkylbis(catecholato)silicates as the alkyl radical source. A summary of that chemistry is the topic of this review. The photoredox-catalyzed, nickel-based cross-coupling of alkylsilicates and alkyltrifluoroborates has been reviewed.²

In the interest of space and repetition the two commonly used silicates are generalized in structures 1 for the triethylammonium silicate moiety and 2 for the diisopropylammonium silicate moiety . Thus the abbreviations Si and Si* are used for the triethylammonium and diisopropylammonium silicate, respectively, throughout this review.