Diastereofacial Selectivity ofO-Protected α-Hydroxy Aldehydes: Temperature and Solvent Effect

Abstract

Temperature dependence measurements allow the evaluation of stereoselectivity in terms of differential enthalpy and entropy of activation. An analysis of diastereoselectivity in the addition reaction of nBuLi to O-protected α-hydroxy aldehydes revealed the great importance of the entropic contri- [a]

Solvent effects on stereoselectivity: more than just an environment

“…We observed this phenomenon when n-BuLi was added to O-(TBS)-mandelic aldehyde in n-hexane. 22 Our interpretation of the solvation-dependent nature of T inv fits well, even in those cases of non-linear Eyring plots with multiple T inv values, as well as in some other unusual temperature behaviours, like that observed in the diastereoselective allylation of the O-(TBS)-N-triisopropylsilylimine of lactal (Fig. 9).…”

“…(ii) for each aldehyde, T inv depends on the chain length of the solvent. 22 Measurements of the diastereomeric ratio of n-BuLi addition to 2-phenylpropanal in a homologous series of linear hydrocarbons showed some striking regularities in their corresponding Eyring plots (Fig. 7).…”

Diastereoselective Additions of Allylmagnesium Reagents to α-Substituted Ketones When Stereochemical Models Cannot Be Used

“…Solvents or counterions, which should affect aggregation and the precise composition of the reagent, , led to only small changes in diastereoselectivity (entries 1–4). In contrast to what might be anticipated, the diastereoselectivity increased somewhat upon warming (entries 1, 5, 6), which indicates that entropic differences in activation energies could be important in controlling stereoselectivity. − Addition of allylmagnesium chloride to ketone 5b in hot THF gave an unidentifiable mixture of products that did not include allylated product anti - 10b , cyclooctanone, or 2-bromocyclooctanone 5b (entry 7). The stereoselectivity also increased slightly with decreasing concentration (entries 8, 9, and 1) but did not increase upon further dilution (cf.…”

Chemical reactivity and solution structure: on the way to a paradigm shift?

“…Basing on extensive experimental studies, [140][141][142][143][144][145][146][147][148][149][150][151][152][153] which have been reviewed, 140 Cainelli et al analysed the inuence of variety of solvents (aliphatic and aromatic hydrocarbons, halohydrocarbons, ethers) and their mixtures on temperature dependences of the ratio of stereoisomers formed in many types of stereoselective reactions (nucleophilic additions, cycloadditions, photochemical and enzymatic reactions). Eyring plots of enantiomeric (or diastereomeric) ratios were found to be nonlinear in many cases, two or more linear segments being separated by "inversion temperature (T inv )" (Fig.…”