Origin of Catalytic Selectivity from Sn(OTf)₂ in Methanol Solution for the Conversion of Glucose to α-Hydroxyesters

Abstract

Tin triflate (Sn(OTf) 2 ) shows good catalytic performance toward the conversion of glucose into α-hydroxyesters (AHEs). Here, we report the catalytic mechanisms for the conversion of β-D-glucopyranose into AHEs in methanol solution with Sn(OTf) 2 at the PBE0/6-311++G(d,p), def2-TZVP theoretical level, combining the ESI-MS verification. From the alcoholysis of Sn(OTf) 2 in methanol solution, the catalytic active species involves both [Sn(CH 3 O)(CH 3 OH)] + and [Sn(OTf)(CH 3 OH) 2 ] + Lewis acids, together with [CH 3 OH 2 ] + Bro̷ nsted acid. There are six vital kinds of reaction stages, i.e., the ring-opening of β-D-glucopyranose, retro-aldol fragmentation, aldol-condensation, aldose−ketose/ketose−aldose tautomerization, Cannizzaro reaction, and the etherification with CH 3 OH. The Bro̷ nsted acid ([CH 3 OH 2 ] + ) is in charge of both ringopening of β-D-glucopyranose and etherification with CH 3 OH, whereas the Lewis acidic species ([Sn(OTf)(CH 3 OH) 2 ] + , [Sn(CH 3 O)(CH 3 OH)] + ) are responsible of the remainder. From chain-glucose, both the C2−C3 retro-aldol fragmentation and the aldose−ketose tautomerization are the selectivity-controlling steps for generating C4-AHEs and C3-AHE, respectively.[Sn(OTf)(CH 3 OH) 2 ] + play an important role in the C2−C3 retro-aldol fragmentation, followed by generating C4-AHEs, which originates from the −CH 3 OH ligand with as a H-donor, making the chain-glucose initially being protonated. Alternatively, [Sn(CH 3 O)(CH 3 OH)] + plays a critical role in the aldose−ketose tautomerization, followed by yielding C3-AHE, which stems from the −OCH 3 -ligand with a H-acceptor, making the chain-glucose initially being deprotonated.