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

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

Abstract

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

image003_1.gif

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

Rysunek1.bmp

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