Volume 9, Issue 10 p. 2209-2218
Full Paper

Structure–Nucleophilicity Relationships for Enamines

Bernhard Kempf Dipl.-Chem.

Bernhard Kempf Dipl.-Chem.

Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F) 81377 München, Germany, Fax: (+49) 89-2180-77717

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Nathalie Hampel

Nathalie Hampel

Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F) 81377 München, Germany, Fax: (+49) 89-2180-77717

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Armin R. Ofial Dr.

Armin R. Ofial Dr.

Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F) 81377 München, Germany, Fax: (+49) 89-2180-77717

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Herbert Mayr Prof. Dr.

Herbert Mayr Prof. Dr.

Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F) 81377 München, Germany, Fax: (+49) 89-2180-77717

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First published: 16 May 2003
Citations: 170

Graphical Abstract

Enamines are commonly characterized as “strong nucleophiles”. Kinetic investigations of the reactions of 28 enamines and related compounds with benzhydrylium ions show that their reactivities span over ten orders of magnitude corresponding to relative reactions times of 1 min versus 20 000 years. Their reactivities are compared with those of other carbon nucleophiles.

Abstract

The kinetics of the reactions of benzhydryl cations with 22 enamines, three pyrroles, and three indoles were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these electron-rich π-systems were derived from equation log k (20 °C)=s(E+N) and compared with the nucleophilicities of other π-systems (silyl enol ethers, silyl ketene acetals) and carbanions. It is shown that the nucleophilic reactivities of enamines cover more than ten orders of magnitude, comparable to enol ethers on the low reactivity end and to carbanions on the high reactivity end. Since the products of Nattack are thermodynamically less stable than the reactants, the observed rate constants refer to the formation of the carbon carbon bonds. In some cases, equilibrium constants for the formation of iminium ions were measured, which allow one to determine the intrinsic rate constants of these reactions.