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

Tin triflate (Sn(OTf)(2)) shows good catalytic performance toward the conversion of glucose into alpha-hydroxyesters (AHEs). Here, we report the catalytic mechanisms for the conversion of beta-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(CH3O)(CH3OH)](+) and [Sn(OTf)(CH3OH)(2)](+) Lewis acids, together with [CH3OH2](+) Bronsted acid. There are six vital kinds of reaction stages, i.e., the ring-opening of beta-d-glucopyranose, retro-aldol fragmentation, aldol-condensation, aldose-ketose/ketose-aldose tautomerization, Cannizzaro reaction, and the etherification with CH3OH. The Bronsted acid ([CH3OH2](+)) is in charge of both ring-opening of beta-d-glucopyranose and etherification with CH3OH, whereas the Lewis acidic species ([Sn(OTf)(CH3OH)(2)](+), [Sn(CH3O)(CH3OH)](+)) 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)(CH3OH)(2)](+) play an important role in the C2-C3 retro-aldol fragmentation, followed by generating C4-AHEs, which originates from the -CH3OH ligand with as a H-donor, making the chain-glucose initially being protonated. Alternatively, [Sn(CH3O)(CH3OH)](+) plays a critical role in the aldose-ketose tautomerization, followed by yielding C3-AHE, which stems from the -OCH3-ligand with a H-acceptor, making the chain-glucose initially being deprotonated.