Macrocyclic Polyethers as Enolate Activators in Homogeneous and Heterogeneous Systems

Abstract

The ability of macrocyclic polyethers to activate enolates has been studied in the alkylation of deoxybenzoin (1) with butyl derivatives nBuY (Y = Br, I, OMes) catalyzed by crown ether PHDB18C6 (7) or cryptand [2.2.2, C₁₀] (8) under phase-transfer catalysis (PTC) and homogeneous (chlorobenzene) conditions. The enolate reactivity is mainly determined by the ligand (cryptand>crown ether) and solvent (increasing with the polarity, in the order: toluene<chloroben-zene<1,2dichlorobenzene). Regioselectivity of the reaction is also remarkably affected by ligand and alkylating agent.     

Benzophenone Schiff bases of glycine derivatives: Versatile starting materials for the synthesis of amino acids and their derivatives

This review focuses on the introduction and early development, in solution, of phase-transfer catalyzed (PTC) reactions to afford racemic or enantioenriched natural and unnatural amino acids. To form monosubstituted amino acids alkylation reactions are performed on the benzophenone Schiff base of glycine. For α,α-disubstituted amino acids the activated intermediate is an aldimine derivative of the monosubstituted amino acid. Enantioenriched products are produced by organocatalysis using derivatives of Cinchona alkaloids as the phase-transfer catalyst. Selectivity for monoalkylatation and lack of product racemization depend on the acidities of the glycine imines, and dialkylated products are formed from aldimine esters of monoalkyl amino acids. The racemic and catalytic enantioselective reactions of a cationic glycine equivalent with organoboranes, organometallics and malonate anion are discussed as are other reactions of these versatile Schiff bases derivatives.     

Some problems in PTC theory: solvation,crystalline lattice strength,and topology

It was shown thatS _N2-type reactions both in solid-liquid and liquid-liquid systems can proceed at the interface through the formation of cyclic adsorption complexes. The conditions for the formation of the latter were formulated, and the kinetic and thermodynamic parameters of reaction with solid ionophoric salts were estimated. A linear relation between the energy of the crystalline lattice of the salt and its melting point was estimated, the latter being shown to be useful as a simplified estimation of the reactivity of solid salts in reactions where the crystalline lattice is destroyed. In solid-liquid systems the benzene effect of solvents was discussed. In highly basic nitrogen-containing solvents (pyridine,etc.), a substitution reaction proceeds in the absence of a PT catalyst. The role of hydration in liquid-liquid systems was discussed.Reviews published in this issue were prepared on the basis of lectures presented at the conference Phase Transfer Catalysis. New Ideas and Methods (Moscow, March 21–25, 1994).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2085–2093, November, 1995.