Predominant role of basicity of leaving group in alpha-effect for nucleophilic ester cleavage |
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| Authors: | Nomura Yasuo Kubozono Takayasu Hidaka Makoto Horibe Mineko Mizushima Naoki Yamamoto Nobuyuki Takahashi Toshio Komiyama Makoto |
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| Institution: | Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan. |
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| Abstract: | It has been found that alpha-effects in nucleophilic reactions, unexpectedly large nucleophilicity due to adjacent unpaired electrons, are strongly dependent on the structure of substrate. The nucleophilic cleavages of 4-nitrobenzoate esters and 4-methylbenzoate esters by HOO- have been systematically investigated in detail. When the leaving groups of substrates are sufficiently good (aryl, 2,2,2-trifluoroethyl, and 2,2-dichloroethyl esters), alpha-effect is evident. However, this effect drastically decreases as the leaving group gets poorer, and is only marginal for the cleavages of 2-fluoroethyl and methyl esters. In the nucleophilic cleavages by salicylaldoxime and acetohydroxamic acid, alpha-effect is also notable only for the esters having good leaving groups. These enormous dependences of alpha-effects on the substrate-structure have been interpreted in terms of the difference in the position of transition-state in the reaction coordinate. |
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| Keywords: | α-Effect Nucleophilic ester cleavage 4-Nitrobenzoate ester 4-Methylbenzoate ester Hydroperoxide ion Hydroxyl ion Rate constant Substrate-structure Transition-state Reaction coordinate |
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