Disilenes, silylenes and silyl anions: fundamentals and catalysis

Abstract

Disilenes, silylenes and silyl anions are examples of compounds with silicon in oxidation state +II. They are fundamentally interesting due to their analogy with common, reactive organic compounds and intermediates, such as alkenes and carbanions. Equally, low-valent main-group compounds, including disilenes and silylenes, are often able to mimic the reactivity of expensive and rare transition metals. Chapter 1 provides a background and introduction to the history and chemistry of low-oxidation state silicon in relation to this thesis.

Chapter 2 studies the equilibrium between disilenes and silylsilylenes. The equilibrium between disilenes (R2Si=SiR2) and their silylsilylene (R3Si-SiR) isomers has been previously inferred but not directly observed, except in the case of the parent system (H2Si=SiH2). Chapter 2 reports a new method to prepare base-coordinated disilenes with hydride substituents. By varying the steric bulk of the coordinating base and other substituents, I have been able to control the rearrangement of disilene adducts to their silylsilylene tautomers. Remarkably, 1,2-migration of a trimethylsilyl group is preferred over hydride migration. A DFT investigation of the reaction mechanism provides a rationale for the observed reactivity Chapter 3 focuses on synthesising phospha-amidinato silylenes. Recent studies have shown that phosphorus substituents may reduce the singlet-triplet gap of silylenes relative to more common nitrogen substituents, and therefore enhance their reactivity. Four phospha-amidinato silanes have been prepared from three novel phospha-amidinate ligands. Six silylenes were generated by reduction or reductive dehydrochlorination. A four coordinate phospha-amidinato silylene was isolated and fully characterised. Chapter 4 investigates the use of silicon based initiators for the hydroboration of carbonyls and alkynes. A silyl anion was found to be the most effective initiator for both aldehydes and ketones. The mechanism was studied through stoichiometric reactions and found to proceed through coordination of the silyl anion to pinacolborane (HBpin), generating a reactive borohydride. Furthermore, the silyl anion was shown to decompose HBpin to BH3 under certain conditions which can catalyse the hydroboration of alkynes.