Potassium Organotrifluoroborates: Chemistry Beyond Cross-Coupling

Over the years, organoboron species have been vastly utilized in synthetic organic chemistry. Traditional methods to synthesize these compounds, such as metal-halogen exchange, C-H activation and Miyaura borylation, often require the use of bisboronates as borylating partners [e.g., bis(pinacolato) diboron (B2Pin2), pinacolborane (HBPin) or neopentylglycolborane]. When the boronic acid is the target, the use of these reagents requires extra deprotection step, affording wasteful diol byproducts. Recently, the palladium-catalyzed synthesis of arylboronic acids employing the atom economical tetrahydroxydiboron (BBA) reagent has been reported. The high cost associated with palladium, combined with several limitations of both palladium and copper-catalyzed processes, prompted us to develop an alternative method. Thus, the nickel-catalyzed borylation of aryl and heteroaryl halides and pseudo-halides using tetrahydroxydiboron (BBA) has been formulated. The reaction proved to be widely functional group tolerant and applicable to a number of heterocyclic systems. Because of their tetracoordinate nature, potassium organotrifluoroborates do not undergo undesirable side reactions with commonly employed organic reagents, and therefore the organic substructure of simple organotrifluoroborates can be functionalized to build molecular complexity while leaving the carbon-boron bond intact. This valuable bond can then be further converted into a variety of groups in a later synthetic step. Furthermore, in contrast to the corresponding aryl and heteroarylboronic acids, organotrifluoroborates are air and moisture stable and can be stored on the bench for months without appreciable decomposition. The major thrust of this thesis research has been the development of mild and metal-free methods for the hydrolysis, oxidation, chlorination and nitrosation of potassium organotrifluoroborates. All developed conditions were efficient for a variety of trifluoroborates containing diverse functional groups and especially heteroaryl units. Moreover, to explore the reactions of the unique nitrosoarenes synthesized, these species were used in a 1,3-dipolar cycloaddition with (trifluoromethyl)diazomethane and alkenes to afford trifluoromethylated isoxazolidines.