Hydrolysis of α-Alkyl-α-(methylthio) Methylene Meldrum's Acides. A Kinetic and Computational Investigation of Steric Effects.

Claude Bernasconi 1Shoshana D. Brown 1Mahammad Ali 1Zvi Rappoport 2Hiroshi Yamataka 3Hatim Salim 2

1. University of California, Department of Chemistry, California, Santa Cruz 95064, United States
2. The Hebrew University of Jerusalem (HUJI), Givat Ram, Jerusalem 91904, Israel
3. Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan


Steric effects are known to play an important role in SNV reactions that follow the two-step mechanism ( eq 1). However, most studies aimed at quantifying the steric effects have been hampered by the difficulty of distinguishing between steric and


electronic effects on the rate of these reactions. The present investigation of the hydrolysis of 1 with R = H, Me, Et, s-Bu and t-Bu was designed to minimize this problem since the R groups


can be expected to have very similar electronic effects but should lead to substantial variation in the steric crowding at the transition state of the k1-step. Even though this expectation was met up to a point, our study indicates that the situation is more complex than anticipated because steric crowding in the reactant plays an important role as well and, for R = t-Bu, leads to a steric acceleration of the reaction. Furthermore, changing the size of the R-group leads to a change in rate limiting step under acidic conditions, i.e. for R = H conversion of the intermediate to products is rate limiting while for the other R groups nucleophilic attack is rate limiting.

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Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 2, by Claude Bernasconi
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-26 07:31
Revised:   2009-06-07 00:44
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