Chemistry of Stable α‐Halogenoorganolithium Compounds and the Mechanism of Carbenoid Reactions

Abstract

Some years ago we found that a-halogenoorganolithium compounds (carbenoids) previously postulated as transient intermediates in organolithium-initiated a-eliminations can be obtained in a stable form. The present paper is a review of the methods for their preparation and of their reactivity. They possess both nucleophilic and electrophilic properties, which can be utilized singly or combined for the synthesis of substances of very different types. Their thermolability, which is considerably reduced (with only one known exception) by the solvent tetrahydrofuran, is due to electrophilic secondary reactions, in which carbenes evidently do not occur as intermediates. A mechanism is proposed which fits the experimental data for various carbenoid reactions.

Stereospecific Synthesis of Alkenes by Eliminative Cross‐Coupling of Enantioenriched sp3‐Hybridized Carbenoids

“…Theprocess is trivially regiospecific, and stereochemical information encoded within the two carbenoid subunits is translated via as equence of three stereospecific processes [electrophilic substitution (1 + 2!3), 1,2-metalate rearrangement (3!4), and b-elimination (4! [8] Hodgson and coworkers deliberately exploited this phenomenon to make 2-buten-1,4-diol derivatives by homocoupling of 1-lithiooxiranes and demonstrated that the E/Z ratio of the olefin product was dependent on the enantiopurity of the carbenoid, as anticipated. Herein, the realization of this eliminative cross-coupling approach for the synthesis of aseries of trisubstituted alkenes belonging to the styrene class is reported.…”

Halomethyllithium Carbenoids: Versatile Reagents for the Homologation of Electrophilic Carbon Units

“…In this sense, Köbrich considered it the most unstable of the three monohalocarbenoids (except LiCH 2 F), because of decomposition at temperatures above À1108C. [5] This behaviour was also confirmed by Villieras in the 1980s. [12] However, studies published in the late 1980s [9] and in the 1990s [13] point out its stability at temperatures up to À788C, and thus it is now well recognized that working at this nonprohibitive temperature does not compromise the efficiency of the reactions in which such species are involved.…”

“…[4] In this scenario, monohalomethyllithium reagents (i.e. LiCH 2 X, X ¼ Cl, Br, I) have received particular attention, and since their introduction by Köbrich as nucleophilic agents in the 1960s (Scheme 2), [5] have become a versatile tool to perform homologation reactions as an alternative to diazomethane-based procedures [6] or sulfur ylides (e.g. Corey-Chaykovski-type chemistry).…”

A practical guide for using lithium halocarbenoids in homologation reactions

“…A significant advancement in the field originated from the work of Gert Köbrich and coworkers in the 1960s [9]. These milestones still represent the key concepts in carbenoid chemistry and put the bases for the rational design and understanding of reactions involving these versatile synthetic tools.…”