Potassium Organotrifluoroborates: Reliable Cross-Coupling Partners and Stepping Stones to Functionalized Azaborine Cores

The synthesis of enantioenriched, air-stable secondary alkylmetallic reagents has gained much attention over the past few years; however, competition between product-forming reductive elimination and β-hydride elimination has hindered their efficient cross-coupling. The paradigm of employing a hemilabile ligand to favor reductive elimination has been extended to include an array of amides and benzylic ethers. The synthesis of protected secondary benzylic alcohols through a cross-coupling reaction was explored. Potassium 1-(benzyloxy)alkyltrifluoroborates, synthesized through a diboration of aldehydes, serve as nucleophilic partners in a stereospecific Suzuki-Miyaura reaction, where the benzyl group serves as a stabilizing group to prevent β-hydride elimination. ^ An improved synthesis of potassium β-trifluoroboratoamides utilizing tetrahydroxydiboron and tetrakis(dimethylamino)diboron as greener, more atom-economical dibora sources was explored. Enantioenriched potassium β-trifluoroboratoamides were cross-coupled with aryl chlorides to afford β-arylated amines. In this coupling, the carbonyl of the amide serves as a stabilizing group to prevent β-hydride elimination. ^ The replacement of a C=C bond with an isoelectronic B-N unit within an aromatic system results in azaborines, a class of compounds with stability and aromaticity similar to the all-carbon parent. Based on the potential of azaborines in the fields of medicinal chemistry and materials science, methods for the synthesis and functionalization of azaborines could have an impact by increasing structural diversity and creating new chemical space. A convergent, modular synthesis of 2,1-borazaronaphthalenes was developed from 2-aminostyrenes and potassium organotrifluoroborates, providing a transition metal-free route to elaborated azaborines. ^ Functionalization of 2,1-borazaronaphthalenes was envisioned to build molecular complexity and demonstrate the stability of azaborines to various transformations. The first general method for the Suzuki-Miyaura cross-coupling of brominated azaborine with potassium (hetero)aryltrifluoroborates was developed to afford 3-(hetero)arylated-2,1-borazaronaphthalenes in high yield. ^ Whereas 3-bromo-2-aryl-2,1-borazaronaphthalenes undergo cross-coupling with an aryltrifluoroborate, 1-alkyl-2-aryl-3-bromo-2,1-borazaronaphthalenes undergo a self-arylation, in which the azaborine serves as both the electrophilic and nucleophilic species in the cross-coupling. A Kumada coupling with Grignard reagents provides access to the 1,2,3-trisubstituted 2,1-borazaronaphthalenes. ^ A nickel-catalyzed reductive coupling was explored with brominated 2,1-borazaronaphthalenes and alkyl iodides. This method demonstrates the inherent stability of azaborines to transition-metal catalysis and provides the first examples of C3 alkylated 2,1-borazaronaphthalenes. ^ ^Please refer to dissertation for diagrams.