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Hydrolysis of r-Alkyl-r-(methylthio)methylene Meldrum’s Acids. A Kinetic and Computational Investigation of Steric Effects

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dc.contributor.advisor Claude F. Bernasconi
dc.contributor.author Claude F. Bernasconi, Shoshana D. Brown, Mahammad Ali, Zvi Rappoport, Hiroshi Yamataka, and Hatim Salim.
dc.date.accessioned 2018-03-01T07:36:41Z
dc.date.accessioned 2022-05-22T08:29:00Z
dc.date.available 2018-03-01T07:36:41Z
dc.date.available 2022-05-22T08:29:00Z
dc.date.issued 2006-06-23
dc.identifier.uri http://localhost:8080/xmlui/handle/123456789/8001
dc.description.abstract The rates of hydrolysis of R-R-R-(methylthio)methylene Meldrum’s acids (8-R with R ) H, Me, Et, s-Bu, and t-Bu) were determined in basic and acidic solution in 50% DMSO-50% water (v/v) at 20 °C. In basic solution (KOH), nucleophilic attack to form a tetrahedral intermediate (TOH - ) is rate limiting for all substrates (k1 OH). In acidic solution (HCl) and at intermediate pH values (acetate buffers), water attack (k1 H2O) is rate limiting for 8-Me, 8-Et, and 8-s-Bu; the same is presumably the case for 8-t-Bu, but rates were too slow for accurate measurements at low pH. For 8-H, water attack is rate limiting at intermediate pH but at pH < 4.5 MeS- departure from the tetrahedral intermediate becomes rate limiting. Our interpretation of these results is based on a reaction scheme that involves three pathways for the conversion of TOH - to products, two of which being unique to hydrolysis reactions and taking advantage of the acidic nature of the OH group in TOH - . This scheme provides an explanation why even at high [KOH] TOH - does not accumulate to detectable levels even though the equilibrium for OH- addition to 8-R is expected to favor TOH - , and why at low pH water attack is rate limiting for R ) Me, Et, s-Bu, and t-Bu but leaving group departure becomes rate limiting with the sterically small R ) H. The trend in the k1 OH and k1 H2O indicates increasing steric crowding at the transition state with increasing size of R, but this effect is partially offset by a sterically induced twisting of the CdC double bond in 8-R which leads to its elongation and makes the substrate less stable and hence more reactive. Our computational results suggest that this effect becomes particularly pronounced for R ) t-Bu and explains why k1 OH for 8-t-Bu is somewhat higher than for the less crowded 8-s-Bu. en_US
dc.description.sponsorship This research was supported by Grant Nos. CHE-0098553 and CHE-0446622 from the National Science Foundation (C.F.B.) and a grant from the U.S.-Israel Binational Science Foundation (Z.R.). en_US
dc.language.iso en en_US
dc.publisher American Chemical Society en_US
dc.relation.ispartofseries J. Org. Chem. 2006, 71, 4795-4802;
dc.relation.ispartofseries ;doi: 10.1021/jo060254o
dc.title Hydrolysis of r-Alkyl-r-(methylthio)methylene Meldrum’s Acids. A Kinetic and Computational Investigation of Steric Effects en_US
dc.type Article en_US

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