Computational Study of B(C₆F₅)₃-Catalyzed Selective Deoxygenation of 1,2-Diols: Cyclic and Noncyclic Pathways

Abstract

The selective deoxygenation of polyols has emerged as an attractive approach to transform biomass-derived polyols into valuable building blocks. Herein, we present a theoretical study on the boron-catalyzed selective deoxygenation of terminal 1,2-diols. The computational results explain the different product distributions obtained with different silanes and unveil the critical role of the cyclic siloxane intermediate. Compared to noncyclic pathways, the cyclic pathway facilitates the initial deoxygenation process because the cyclic structure minimizes the steric repulsions between the reagents. It avoids overreduction because the generated bulky disiloxane moiety hinders the second deoxygenation.

Sequence‐Controlled Catalytic One‐Pot Synthesis of Siloxane Oligomers

“…[46] The Piers-Rubinsztajn reaction also proceeds via asimilar S N 2-Si transition statet of orm ad isilyl oxonium ion intermediate (Scheme 14). [47] The S N 2S im echanism explains well that the undesired Piers-Rubinsztajn reactions with monohydrosilanes are kinetically unfavorable, on account of steric hindrance of the isopropoxysilanes. Although cleavage of the CÀ Ob ond in the disilyl oxonium ion through nucleophilic attack of the borohydrideH ÀB(C 6 F 5 ) 3 À affords the desired siloxane (Scheme 14, path A), cleavageo ft he SiÀOb ond leads to the reverser eaction or alkoxysilane/hydrosilane metathesis, which eventually resultsi nu ndesired side reactions (path B).…”

“…Oestreich and Rendler experimentally demonstrated that the B(C 6 F 5 ) 3 ‐catalyzed hydrosilylation of ketones proceeded via an S N 2 Si transition state triggered by Si−H bond activation by B(C 6 F 5 ) 3 ; a mechanism that is supported by DFT calculations . The Piers–Rubinsztajn reaction also proceeds via a similar S N 2‐Si transition state to form a disilyl oxonium ion intermediate (Scheme ) . The S N 2 Si mechanism explains well that the undesired Piers–Rubinsztajn reactions with monohydrosilanes are kinetically unfavorable, on account of steric hindrance of the isopropoxysilanes.…”

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

“…Communications bond reduction (see the Supporting Information) and lower than that of similarly calculated C À O bond reductions involving BCF/HSiR 3 . [33] The data suggest that a chair flip to the 1 C 4 conformer is required to minimize steric clashes between neighboring -OSi groups ( Figure S2). An analysis of the coupling constants ( 3 J HH ) showed that Et 3 Si-1-deoxyglucose and A adopt the 4 C 1 and 1 C 4 conformers, respectively ( Figures S3 and S4), consistent with the computed conformer stabilities.…”

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

“…Zuschriften bond reduction (see the Supporting Information) and lower than that of similarly calculated C À O bond reductions involving BCF/HSiR 3 . [33] The data suggest that a chair flip to the 1 C 4 conformer is required to minimize steric clashes between neighboring -OSi groups ( Figure S2). An analysis of the coupling constants ( 3 J HH ) showed that Et 3 Si-1-deoxyglucose and A adopt the 4 C 1 and 1 C 4 conformers, respectively ( Figures S3 and S4), consistent with the computed conformer stabilities.…”