π Nucleophilicity: The Effect of Charge Delocalization on the Efficiency of Internal Displacements in ElcB Reactions

Shmaryahu Hoz, Zeev Gross, D. Cohen

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Abstract

The effect of negative charge (de)localization by the activating group on the efficiency of the second step of an ElcB reaction was investigated. Rate constants for the reactions of carbanions 1(LG)- and 2(LG)- with LG = CN, MeO were determined in 25% sulfolane-75 % water at 25 °C, The results show that the relatively localized dicyano-stabilized carbanion 2(CN)- is much more reactive than the delocalized nitro-stabilized carbanion 1(CN)-. The proximity of the β-cyano group does not greatly affect the pKa of the nitromethyl moiety in l(HCN) whereas a decrease of ca. 6 pKa units was observed for the malononitrile moiety in 2(HCN). The pKas obtained are 9.6 and 5.6, respectively. This increased acidity was not observed for the methyl analogue 3 of 2(HCN) (pKa = 10.4), providing additional support for the assumption that cyano-stabilized carbanions are more localized than the nitro ones. A short ab initio calculation at the 4-31G level was performed on CH2NO2 and CH2CN- in order to determine whether (de)localization occurs at the HOMO level or is an overall effect. The results indicate that unlike the case of the enolate ion (de)localization in the two carbanions occurs at both the HOMO and inner levels as well.

Original languageEnglish
Pages (from-to)832-836
Number of pages5
JournalJournal of Organic Chemistry
Volume50
Issue number6
DOIs
StatePublished - Mar 1985

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