Rhodium catalysed borylation reactions via direct and indirect C-H activation
This thesis describes both direct and indirect C-H activation borylation processes, catalysed by several Rh-based catalyst precursors. Chapter One presents an overview of the processes investigated, namely diboration, dehydrogenative borylation, and direct C-H activation of hydrocarbon substrates, which give borylated species that are of interest to synthetic chemists. The uses of such borylated species are also highlighted. Chapter Two highlights the synthetic procedure for the synthesis of the catalyst precursor [Rh(acac)(C0E)(_2)], which can be used to prepare bis-phosphine catalyst precursors of the form [Rh(acacXP(_2))]- Although a procedure appears in the literature, it is not well cited. Also, this new procedure replaces [Tl(acac)] with [Na(acac)] and hence avoids the use of thallium salts. Chapter Three investigates the reaction of two vinyl(boronate) esters (VBEs) with Bzcat], and a wide array of catalyst precursors, which yield, among other species, the tris(boronate) esters ArCH(_2)C(Bcat)(_3) and ArCH(Bcat)CH(Bcat)(_3); the former results from the addition of both borons of the B(_2) unit to the same carbon atom, and are of interest due to their wide synthetic versatility. Chapter Four investigates the dehydrogenative borylation of alkenes using both HBpin and B2pin2, and several catalyst precursors. Most significantly, this route allows the synthesis of 1,1-disubstituted vinyl(boronate) esters that cannot be made by alkyne hydroboration. Chapter Five investigates the direct C-H activation of benzylic and aromatic hydrogens using the catalyst precursor [Rh(Cl)(N(_2))(P(^i)Pr(_3))(_2)]. This allows the functionalisation of hydrocarbon substrates, which are ubiquitous. Chapter Six investigates the stability of B-chlorocatecholborane to phosphines with the view to a boron analogue of the Heck reaction. In such a reaction, phosphmes would likely be employed on the catalyst. An understanding of the stability of the boron reagent under typical reaction conditions is needed, therefore, in order to prevent degradation of B-chlorocatecholborane, a process that is known for catecholborane.