Metal ambident character

Addition reactions of 2-chloro-3-pyridylcarbonyl isothiocyanate with primary amines yield acylated thioureas 1. Because of the ambident character of the thiocarbamoyl group, in the following cyclization step the 2-chloro substituent of the pyridine may react either with the sulfur or the nitrogen center, depending on the reaction conditions. In the example below, heating under reflux in toluene yields 2-aminopyrido[3,2-e][l,3]thiazin-4-ones, whereas the anion prepared by metalation with lithium hydride in dimethylformamide at room temperature leads to 2-thioxo-l,2-dihydropyrido[2,3-c/]pyrimidin-4(3//)-ones 2.6... 

Alkoxypropargylation. The ambident character of the propargylic anion, which may be in equihbrium with the aUenic form, is responsible for its limited use in synthesis. In general the structure and reactivity of the ambident anion depend on the nature of the substrate, the counter cation, and the solvent. There is also an erythro-threo stereoselectivity problem when alkylated propargylic anions react with aldehydes or unsymmetrical ketones. In contrast, the zinc and titanium reagents derived from the title compound possess the allenic structure and, upon reaction with aldehydes, lead almost exclusively to the )8-acetylenic alcohol (eq 1), presumably by a chelate transition state (Sei process). The reaction also leads preferentially to the erythro diastereomer. The stereoselectivity is highest with titanium as the metal and THE as the solvent. ... 

The selectivity of reaction of alkenes with carbenes (produced by a-eliminations using metal alkoxides), in the presence or absence of crown compounds, has been used as a probe for carbenoid character, since in the former case carbenes are assumed to be formed free of any interactions with metal ions. The ambident carbanion of ethyl acetoacetate, as its sodium salt in THF solution, not only shows enhanced reactivity in the presence of complexing agents for the sodium ion, but also an increase in 0-alkylation with respect to C-alkylation. ... 

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