Nucleophilic carbenes catalytic activity

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Lewis bases (B ) that can occupy the open coordination site inhibit catalytic activity. The electrophilic nature of the metal carbene is seen in its subsequent reactions with nucleophiles (S ), which occur with the transfer of the carbene entity from the metal to a nucleophile without ever having generated or transformed an actual free carbene. 

In the thiazolium cation the proton in the 2-position is acidic and its removal gives rise to the ylide/carbene 227. This nucleophilic carbene 227 can add, e.g., to an aldehyde to produce the cationic primary addition product 228. The latter, again via C-deprotonation, affords the enamine-like structure 229. Nucleophilic addition of 229 to either an aldehyde or a Michael-acceptor affords compound(s) 230. The catalytic cycle is completed by deprotonation and elimination of the carbene 227. Strictly speaking, the thiazolium salts (and the 1,2,4-triazolium salts discussed below) are thus not the actual catalysts but pre-catalysts that provide the catalytically active nucleophilic carbenes under the reaction conditions used. This mechanism of action of thiamine was first formulated by Breslow [234] and applies to the benzoin and Stetter-reactions catalyzed by thiazolium salts [235-237] and to those... 

Recently, palladium complexes of carbene ligands have been recognized as highly reactive catalysts for palladium-promoted reactions, in particular for the Heck reaction [42-44]. The polymer-supported palladium carbene complexes 18 and 19 were prepared by the nucleophilic substitution of the bromomethy-lated Wang resin with 17 under basic reaction conditions (Scheme 7) [45]. The catalytic activity of 18 and 19 was examined for the Heck reaction of aryl bromides with acrylates or styrene to exhibit high TONs up to 5,000. The polymer-supported palladium-carbene complexes are air-stable and recyclable. 

In a general (and generalizing) view on this issue it can be stated that suitably selected ionic liquids are very likely to form catalytically active ionic catalyst solutions with a given transition metal catalyst if the latter is neither extremely electrophilic (acidic) nor extremely nucleophilic (basic). While extremely electrophilic catalyst complexes are likely to coordinate strongly even with those anions of the ionic liquid solvent which are generally regarded as weakly coordinating, extremely nucleophilic catalytic centers are likely to react with the ionic liquid s cation. Carbene complex formation by oxidative addition as well as dealkylation of the cation are possible deactivation pathways of the catalyst in such a case. 

A -Alkylated thiazolium and benzothiazolium salts also experience base-promoted deprotonation at the 2-position to form ylides. Such compounds, often referred to as TV-heterocyclic carbene (NHC), are nucleophilic catalysts in benzoin condensation. In 1943, Ugai and co-workers reported that thiazolium salts catalyze self-condensation of benzaldehyde to generate benzoin via an umpoulong process. Breslow at Columbia University in 1958 proposed thiazolium ylide as the actual catalyst for this transformation. In this mechanism, the catalytically active species was represented as a thiazolium zwitterion, the resonance structure of an NHC, and the reaction was postulated to ensue via the enaminol or the Breslow intermediate. ... 

The mechanism operating in A-heterocyclic carbene-mediated reactions was proposed initially by Breslow in 1958 for the thiazolium salt-catalyzed benzoin condensation (Scheme 6.1). This proposal involves the formation of a carbene as the catalytically active species by deprotonation of the thiazolium cation, which subsequently adds to one molecule of the aldehyde, generating a nucleophilic intermediate known as the Breslow intermediate. Next, this... 

TEBA is an interesting case. It is normally used in generations and subsequent reactions of carbanions and carbenes its catalytic efficiency is mainly due to the salting-out effect of the aqueous phase, since the latter is normally 50% NaOH. On the other hand, it shows very low catalytic activity in nucleophilic displacements... 

Nyce, G.W, Lamboy, J.A., Connor, E.F., Waymouth, R.M. and Hedrick, J.L., Expanding the catalytic activity of nucleophilic N-heterocyclic carbenes for transesterification reactions. 

The 1,2,4-triazolium salt 30 serves as precatalyst, it is transformed by deprotonation to the catalytically active substrate 31, a heterocyclic (nucleophilic) carbene (cf. p. 221). 

More importantly, after full optimization, they allowed for decreasing the amount of hydrosilane required for the reduction and were compatible with lower reaction temperature. " Mechanistic studies using in situ H NMR led the authors to propose decoordination of one NHC prior to entering the catalytic cycle and subsequent activation of the hydrosilane by the nucleophilic carbene released in solution. 

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