Pyro-Borates, Spiro-Borates, and Boroxinates of BINOL—Assembly, Structures, and Reactivity

Abstract

VANOL and VAPOL ligands are known to react with three equivalents of B(OPh) to form a catalytic species that contains a boroxinate core with three boron atoms, and these have proven to be effective catalysts for a number of reactions. However, it was not known whether the closely related BINOL ligand will likewise form a boroxinate species. It had simply been observed that mixtures of BINOL and B(OPh) were very poor catalysts compared to the same mixtures with VANOL or VAPOL. Borate esters of BINOL have been investigated as chiral catalysts, and these include meso-borates, spiro-borates, and diborabicyclo-borate esters. Borate esters are often in equilibrium, and their structures can be determined by stoichiometry and/or thermodynamics, especially in the presence of a base. The present study examines the structures of borate esters of BINOL that are produced with different stoichiometric combinations of BINOL with B(OPh) in the presence and absence of a base. Depending on conditions, pyro-borates, spiro-borates, and boroxinate species can be generated and their effectiveness in a catalytic asymmetric aziridination was evaluated. The finding is that BINOL borate species are not necessarily inferior catalysts to those of VANOL and VAPOL but that, under the conditions, BINOL forms two different catalytic species (a boroxinate and a spiro-borate) that give opposite asymmetric inductions. However, many BINOL derivatives with substitutents in the 3- and 3'-positions gave only the boroxinate species and the 3,3'-PhBINOL ligand gave a boroxinate catalyst that gives excellent inductions in the aziridination reaction. BINOL derivatives with larger groups in the 3,3'-position will not form either spiro-borates or boroxinate species and thus are not effective catalysts at all.

2,2′-Biphenol/B(OH)₃ Catalyst System for Nazarov Cyclization

“…This result indicated that a powerful Lewis or Brønsted acid was generated in situ upon mixing the weak 2,2′-biphenol (pK a1 7.6) and B(OH) 3 (pK a 9.2), 44) promoting undesired side reactions. Similar to past studies [45][46][47][48] in which a cyclic boroxinate Brønsted acid was isolated from a mixture of 1 equiv. of the biphenol derivative and 3 equiv.…”

Boron‐Based Catalysts for C−C Bond‐Formation Reactions

“…Upon further examining effective catalysts, they found that B(OPh) 3 was more effective than that of BH 3 ⋅ THF and other boron sources . Later, studies on the reaction mechanism, scope, and synthetic applications of asymmetric aziridine reactions were comprehensively conducted …”

Chiral Diol-Based Organocatalysts in Enantioselective Reactions

“…In 1993, Wulff’s group introduced VANOL and VAPOL as chiral ligands for a metal-catalyzed asymmetric Diels–Alder reaction [ 59 ]. During subsequent decades, many modified VANOL and VAPOL derivatives have been shown to be crucial in asymmetric reactions the aziridination of imines [ 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 ], the 2-aza-Cope rearrangement [ 69 ], the Ugi reaction [ 70 ], and other reactions [ 71 ]. Since this review is focused on diol organocatalysts, those reactions involving transition metals with VANOL or VAPOL ligands will not be covered here.…”

“…The mixture could be utilized as a precatalyst in the synthesis of aziridines 92 from the combination of imines 90 and diazoacetates 91 ( Scheme 36 ). A comprehensive study on asymmetric aziridination reaction with BINOL boroxinates, including the reaction scope, mechanism, and synthetic applications, was also reported by the Wulff group recently [ 71 ]. However, the BINOL complexes gave lower yields and stereoselectivities than VANOL and VAPOL derivatives.…”