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

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(CH3O)(CH3OH)]+ and [Sn(OTf)(CH3OH)2]+ Lewis acids, together with [CH3OH2]+ Bro̷nsted 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 CH3OH. The Bro̷nsted acid ([CH3OH2]+) is in charge of both ring-opening of β-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.