1246888-31-2Relevant academic research and scientific papers
Pyro-Borates, Spiro-Borates, and Boroxinates of BINOL - Assembly, Structures, and Reactivity
Hu, Gang,Gupta, Anil K.,Huang, Li,Zhao, Wenjun,Yin, Xiaopeng,Osminski, Wynter E. G.,Huang, Rui H.,Wulff, William D.,Izzo, Joseph A.,Vetticatt, Mathew J.
supporting information, p. 10267 - 10285 (2017/08/09)
VANOL and VAPOL ligands are known to react with three equivalents of B(OPh)3 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)3 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)3 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′-Ph2BINOL 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.
The iso-VAPOL ligand: Synthesis, solid-state structure and its evaluation as a BOROX catalyst
Gupta, Anil K.,Zhang, Xin,Staples, Richard J.,Wulff, William D.
, p. 4406 - 4415 (2015/02/02)
The new vaulted biaryl ligand iso-VAPOL is an isomer of VAPOL but has the chiral pocket of VANOL. The synthesis of iso-VAPOL involves a cycloaddition/electrocyclization cascade (CAEC) similar to one that is used for VAPOL except that the starting material
BOROX catalysis: Self-assembled AMINO-BOROX and IMINO-BOROX chiral Bronsted acids in a five component catalyst assembly/ catalytic asymmetric aziridination
Gupta, Anil K.,Mukherjee, Munmun,Hu, Gang,Wulff, William D.
supporting information, p. 7932 - 7944,13 (2020/10/15)
A five-component catalyst assembly/aziridination reaction is described starting from an aldehyde, an amine, ethyl diazoacetate, B(OPh)3, and a molecule of a vaulted biaryl ligand (VAPOL or VANOL). A remarkable level of chemoselectivity was observed since, while 10 different products could have resulted from various reactions between the five components, an aziridine was formed in 85% yield and 98% ee and only two other products could be detected in 3% yield. Studies reveal that the first in a sequence of three reactions is an exceedingly rapid amine-induced assembly of an AMINOBOROX chiral Bronsted acid species from VAPOL and B(OPh)3, which is followed by imine formation from the amine and aldehyde and the concomitant formation of an IMINO-BOROX chiral Bronsted acid and finally the reaction of the imine with ethyl diazoacetate mediated by the IMINO-BOROX catalyst to give aziridine-2-carboxylic esters with very high diastereo- and enantioselectivity.
How the binding of substrates to a chiral polyborate counterion governs diastereoselection in an aziridination reaction: H-bonds in equipoise
Vetticatt, Mathew J.,Desai, Aman A.,Wulff, William D.
supporting information; experimental part, p. 13104 - 13107 (2010/12/19)
The stereochemistry-determining step of the self-assembled chiral Bronsted acid-catalyzed aziridination reactions of MEDAM imines and three representative diazo nucleophiles has been studied using ONIOM(B3LYP/6-31G:AM1) calculations. The origin of cis selectivity in the reactions of ethyldiazoacetate and trans selectivity in reactions of N-phenyldiazoacetamide can be understood on the basis of the difference in specific noncovalent interactions in the stereochemistry-determining transition state. A H-bonding interaction between the amidic hydrogen and an oxygen atom of the chiral counterion has been identified as the key interaction responsible for this reversal in diastereoselectivity. This hypothesis was validated when a 3° diazoamide lacking this interaction showed pronounced cis selectivity both experimentally and calculationally. Similar trends in diastereoselection were observed in analogous reactions catalyzed by triflic acid. The broad implications of these findings and their relevance to chiral Bronsted acid catalysis are discussed.
