Harnessing the reactivity of poly(methylhydrosiloxane) for the reduction and cyclization of biomass to high-value products

Abstract

Poly(methylhydrosiloxane) (PMHS) has been examined for its ability to reduce and subsequently cyclize carbohydrate substrates using catalytic tris(pentafluorophenyl)borane (BCF).

Selectivity and Mechanism of Iridium-Catalyzed Cyclohexyl Methyl Ether Cleavage

“…Yamamoto reported that the electron-deficient triaryl borane tris­(pentafluorophenyl)­borane (BCF) catalyzes the deoxygenation of alcohols with silanes via initial dehydrosilylation to the silyl ether and showed that this system also cleaves dialkylethers. , (Figure ). Gagné has applied this catalyst to both the partial and complete deoxygenation of carbohydrate substrates. − Work by Brookhart showed that a cationic pincer-supported iridium complex ( 2 ) operates by a similar mechanism; however, attempts by Gagné to apply this system in complex polyether substrates provided products resulting from unselective C–O cleavage. , In recent studies, we have shown that a simple cationic bis­(phosphine)­iridium dihydride complex ( 1 ) is capable of effecting the selective cleavage of alkyl ethers with silane . Crucially, the resulting silyl ethers are stable with respect to further reduction, unlike in the Brookhart system. …”

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

“…Tris(pentafluorophenyl)borane (BCF) has emerged as a highly effective catalyst for the site‐selective deoxygenation of biomass‐derived carbohydrates . The choice of the sugar's protecting group and reductant (hydrosilane or hydroborane) can tailor which C−O bonds are cleaved to yield high‐value chiral products .…”

“…As part of a broader effort to improve the “greenness” of site‐selective sugar deoxygenations, we discovered that Et 3 Si‐1‐deoxyglucose ( Et 3 Si‐2 , generated in situ from Et 3 Si‐MeO‐glucose) provided an excellent yield of 1‐deoxysorbitol ( 1 ) when the reaction was carried out in toluene (Scheme b). This new product from BCF/HSiR 3 catalysis was unexpected, as previous attempts to selectively reduce 1‐deoxyglucose by BCF/HSiR 3 in CH 2 Cl 2 failed because of the formation of an unreactive (but spectroscopically characterizable) ion pair [Si‐1‐deoxyglucose][H‐BCF] ( A , Scheme b) that sequestered the catalytic quantities of silylium ions .…”

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

“…Tris(pentafluorophenyl)borane (BCF) has emerged as a highly effective catalyst for the site-selective deoxygenation of biomass-derived carbohydrates. [1][2][3][4][5][6][7][8][9] The choice of the sugars protecting group and reductant (hydrosilane or hydroborane) can tailor which C À O bonds are cleaved to yield high-value chiral products. [7] Since the pool of biorenewables is fixed, new approaches that alter the site selectivity diversifies the high-value compounds that can be accessed from it.…”

“…[28,29] Low-polarity solvents enhance the electrostatic interaction between the cation and anion, and in cases where a tighter ion pair is beneficial to stereochemical transfer, for example, from a chiral anion to a cation, these solvents enhance enantioselectivity. [30,31] As part of a broader effort to improve the "greenness" of site-selective sugar deoxygenations, [6] we discovered that Et 3 Si-1-deoxyglucose (Et 3 Si-2, generated in situ from Et 3 Si-MeO-glucose) [5] provided an excellent yield of 1-deoxysorbitol (1) when the reaction was carried out in toluene (Scheme 1 b). This new product from BCF/HSiR 3 catalysis was unexpected, as previous attempts to selectively reduce 1deoxyglucose by BCF/HSiR 3 in CH 2 Cl 2 failed because of the formation of an unreactive (but spectroscopically characterizable) ion pair [Si-1-deoxyglucose][H-BCF] (A, Scheme 1 b) that sequestered the catalytic quantities of silylium ions.…”