Modulating Electrostatic Interactions in Ion Pair Intermediates To Alter Site Selectivity in the C−O Deoxygenation of Sugars

Abstract

Controlling which products one can access from the predefined biomass‐derived sugars is challenging. Changing from CH2Cl2 to the greener alternative toluene alters which C−O bonds in a sugar are cleaved by the tris(pentafluorophenyl)borane/HSiR3 catalyst system. This increases the diversity of high‐value products that can be obtained through one‐step, high‐yielding, catalytic transformations of the mono‐, di‐, and oligosaccharides. Computational methods helped identify this non‐intuitive outcome in low dielectric solvents to non‐isotropic electrostatic enhancements in the key ion pair intermediates, which influence the reaction coordinate in the reactivity‐/selectivity‐determining step. Molecular‐level models for these effects have far‐reaching consequences in stereoselective ion pair catalysis.

Achieving Site-Selective C–O Bond Reduction for High-Value Cellulosic Valorization

“…For example, in chiral counterion catalysis, a well-structured contact ion pair is required for efficient asymmetric induction, and consequently, lower polarity solvents typically increase enantioselectivity. − While searching for greener, nonhalogenated solvents for site-selective deoxygenations, we noticed, unexpectedly, that toluene was more reactive than dichloromethane; unexpected because the generation of an ion pair should be less favored in low polarity solvents. For example, the reduction of 25 in CH 2 Cl 2 gave 1-deoxyglucose ( 26 ), a pyranose species uniquely able to withstand subsequent reduction . In toluene, however, this intractable bond was broken to provide 1-deoxysorbitol ( 27 ), a species not previously seen with BCF/HSiR 3 catalysis (all in CH 2 Cl 2 ).…”

“…For example, the reduction of 25 in CH 2 Cl 2 gave 1-deoxyglucose (26), a pyranose species uniquely able to withstand subsequent reduction. 63 In toluene, however, this intractable bond was broken to provide 1-deoxysorbitol (27), a species not previously seen with BCF/HSiR 3 catalysis (all in CH 2 Cl 2 ). Divergent reactivity was seen across a set of monosaccharides.…”

Defunctionalisation catalysed by boron Lewis acids

“…30 More recently, the Gagné group found the polarity of solvent to exert a profound inuence on reactivity and regioselectivity of the deoxygenation of sugars using the B(C 6 F 5 ) 3 /hydrosilane catalytic system (not shown). 31 Mechanistic investigations indicated low-dielectric solvents can shorten inter-ion bond lengths of the key ion-pair intermediates due to electrostatic compressive forces. By tuning the electronic properties of uoroaryl borane catalysts and utilising different reductants, chemo-and siteselective modications of various complex natural products to yield divergent products were achieved by Gagné and coworkers (Scheme 11).…”