Journal
ACS CATALYSIS
Volume 6, Issue 7, Pages 4379-4387Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.6b00618
Keywords
organocatalysis; thiourea; mechanism; hydrogen bonding interactions; acid-base catalysis; computational analysis; tetrahydropyranylation
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Funding
- Hungarian Scientific Research Fund (OTKA) [K-112028]
- Janos Bolyai Scholarship from Hungarian Academy of Sciences
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Combined computational experimental studies were carried out to parallel two mechanistic models for tetrahydropyranylation of alcohols catalyzed by Schreiner's thiourea. The results challenge the common mechanistic view that the catalytic effect is related to stabilizing double hydrogen bonding interactions between the thiourea and the alcohol, which promote the attack on 3,4-dihydro-2H-pyran (DHP) (hydrogen bonding (HB) mechanism). In the alternative mechanism that we propose, thiourea acts as a Bronsted acid, protonating DHP to form an oxacarbenium ion, which reacts with the alcohol (Bronsted acid (BA) mechanism). Computations point to clear preference of transition states associated with the BA mechanism and, accordingly, predict similar catalytic activity for N-methylated thiourea and thiouracil. These predictions are confirmed experimentally. Reactions with deuterated alcohols yield both syn and anti products, providing further support for the Bronsted acid mechanism.
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