Nickel-Catalyzed Cross-Coupling Reactions: Stereospecific Arylations, Formation of 2-PyridylZinc Reagents & Directed Hydroarylation of Alkynes
- Author(s): Hanna, Luke Edward
- Advisor(s): Jarvo, Elizabeth R
- et al.
Cross-coupling technology has become an indispensable tool for the rapid and efficient synthesis of complex molecules. Over the past few decades a foundational understanding of organometallic chemistry has been laid using palladium and other precious metals. Recent research on first row base metal catalysts such as nickel, cobalt and iron has uncovered new and complementary modes of reactivity compared to their more well-studied precious metal counterparts. While nickel sits one row above palladium on the periodic table, ongoing research has illustrated that nickel possesses a unique reactivity profile. Thus, while nickel is commonly thought of as a cheaper alternative to palladium, research in the field of nickel catalysis has demonstrated far more potential than this. The unique propensity of nickel to undergo single electron chemistry as well as its ability to break strong carbon oxygen bonds make research into nickel reactivity an immensely beneficial endeavor to the fields of inorganic, organometallic and synthetic organic chemistry.
Chapter 1 describes the development of a stereospecific Suzuki coupling of benzylic carbamates and pivalates with aryl- and heteroarylboronic esters. The reaction proceeds with selective inversion or retention at the electrophilic carbon, depending on the identity of the ligand used. Tricyclohexylphosphine ligand provides products with retention of configuration at the electrophilic carbon, while an N-heterocyclic carbene ligand SIMes provides products with inversion.
Chapter 2 discusses the development of a regio- and stereoselective nickel-catalyzed hydroarylation of alkynes using propargylic carbamates as directing groups. The reaction proceeds under mild reaction conditions using arylboronic acids in the absence of base. A range of heterocycles and functional groups are tolerated under the reaction conditions. Additionally, the method is applied to the synthesis of tamoxifen.
Chapter 3 details a nickel-catalyzed cross-electrophile coupling reaction of benzylic esters and aryl halides. Both inter- and intramolecular variants proceed under mild reaction conditions. A range of heterocycles and functional groups are tolerated under the reaction conditions. Additionally, the first example of a stereospecific cross-electrophile coupling of a secondary benzylic ester is described.
Chapter 4 presents secondary benzylzinc reagents generated from 2-pyridylcarbinols using a nickel catalyst and diethylzinc. Substrates are activated in situ using a chlorophosphate reagent. Quenching the organozinc reagents allows for facile deoxygenation of 2-pyridylcarbinols in a one-pot reaction with straightforward incorporation of a deuterium label from deuteromethanol. An intramolecular conjugate addition of a secondary benzylzinc reagent with an α,β-unsaturated ester is also demonstrated.