Electrophilic C–H Borylation and Related Reactions of B–H Boron Cations

Abstract

Catalytic procedures are described for aminedirected borylation of aliphatic and aromatic tertiary amine boranes. Sequential double borylation is observed in cases where two or more C–H bonds are available that allow 5-center or 6-center intramolecular borylation. The HNTf2 catalyzed borylation of benzylamine boranes provides a practical means for the synthesis of ortho-substituted arylboronic acid derivatives, suitable for Suzuki-Miyaura cross-coupling applications.

Fundamental and Applied Properties of Borocations

“…Catalytic (in activators such as HNTf 2 ) variants using borenium equivalents were concurrently developed ( Fig. 10) [7,49]. The key step in the catalytic cycle involves hydride transfer from (amine)BH 3 to 15 to regenerate 13 (or its functional equivalent).…”

“…Thus Vedejs and co-workers demonstrated that intramolecular aliphatic C-H borylation using highly electrophilic [(R 3 N)BH 2 ] + borenium cations, or their equivalents, was also possible. Activation of amine borane 19 with [Ph 3 C][B (C 6 F 5 ) 4 ] led to the evolution of H 2 and formation of the spiro borenium cation 20 (which was subsequently characterised by X-ray diffraction studies) [49,57]. A C-H insertion mechanism was proposed proceeding via a four-membered transition state.…”

“…Vedejs and co-workers extended intramolecular aliphatic borylation to other [(NR 3 )BH 2 ] + systems and in internal competition studies determined that the formation of five-membered boracycles is preferred over six membered, whilst the dehydroboration of aromatic and aliphatic C-H bonds was kinetically comparable [49]. Finally, they demonstrated that the replacement of [Ph 3 C][B(C 6 F 5 ) 4 ] with HNTf 2 was possible and that catalytic (in electrophilic activator) aliphatic C-H borylation was also feasible.…”

Mono- and Polynuclear Boron Dications

“…A stepwise hydride abstraction of the borane complexes is identified. 56 For example, the addition of 0.5 equivalent of trityl salt to the NHC-BH 3 complex (19) [13] 2+ .…”

Synthesis and reactivity of N,N′-1,4-diazabutadiene derived borocations

“…6 Borocations, which feature a cationic boron centre and thus act as boron electrophiles, are traditionally classified by their coordination number: two-coordinate boriniums, three-coordinate boreniums, and four-coordinate boroniums. Due to the fact that boriniums are often highly unstable, thereby requiring extensive steric or electronic stabilisation, 7 and boroniums often possess low Lewis acidity as a result of the saturated coordination sphere at boron, boreniums represent an attractive target of investigation, occupying a middle ground between stability and reactivity (Lewis acidity).…”