Correlations Between Nucleophilicities and Selectivities in the Substitutions of Tetrahydropyran Acetals

Abstract

Selectivities that deviate from SN1 stereoelectronic models in the nucleophilic substitutions of tetrahydropyran acetals were investigated. When weak nucleophiles were employed, stereoselectivities conformed to known SN1 stereoelectronic models. In contrast, stereoselectivities in the substitutions of acetals with strong nucleophiles depended on reaction conditions. Erosions in selectivities were observed when strong nucleophiles were employed in the absence of coordinating counterions. These erosions in selectivities are attributed to rates of nucleophilic additions to oxocarbenium ion intermediates that approach the diffusion limit. When triflate counterions were present, however, SN2-like pathways became accessible with strong nucleophiles. In most cases examined, the major stereoisomers formed from reactions that proceeded through SN2-like pathways were opposite to the major stereoisomers formed from the analogous reactions that proceeded through SN1 pathways.

Blue Light Photocatalytic Glycosylation without Electrophilic Additives

“…There is a steadily increasing body of kinetic evidence 28–37 that a variety of stereoselective glycosylation reactions proceed by associative reaction pathways rather than by dissociatively free glycosyl oxocarbenium ions that have only been observed in super acidic media. 22,23,38,39 In such classical glycosylation reactions, the associative pathways are more prevalent with more reactive acceptor alcohols, 40–43 consistent with the observed pattern in Table 2. The hexafluorophosphate counterion is not considered to intervene directly in the reaction mechanism except for providing charge stabilization consistent with studies on the influence of a variety of counterions on the stereoselectivity of glycosylation reactions.…”

The influence of acceptor nucleophilicity on the glycosylation reaction mechanism

“…12,13 Next a series of carbohydrate acceptors is used to place the reactivity of these alcohols in the context of the reactivity of the ethanol model acceptors (see Fig. 2B and C).…”

Cation Clock Reactions for the Determination of Relative Reaction Kinetics in Glycosylation Reactions: Applications to Gluco- and Mannopyranosyl Sulfoxide and Trichloroacetimidate Type Donors

“…132 This in turn is consistent with methallyltrimethylsilane being a much weaker nucleophile than isopropanol and requiring the potent electrophile at the dissociative end of the mechanistic spectrum. 133 …”