Carbene reactions metal-bound intermediates

When a reaction appears to involve a species that reacts as expected for a carbene but must still be at least partially bound to other atoms, the term carbenoid is used. Some carbenelike processes involve transition metal ions. In many of these reactions, the divalent carbene is bound to the metal. Some compounds of this type are stable, whereas others exist only as transient intermediates. In most cases, the reaction involves the metal-bound carbene, rather than a free carbene. 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

Thus, the photolysis of Fischer carbenes opens up, under extremely mild (neutral) conditions, a synthetic access to electron-rich alkoxy- or amino ketenes which are difficult or impossible to synthesize through other means. These ketene intermediates are generated as metal-bound species in low stationary concentrations (less side reactions ), but can be further reacted in a variety of synthetically useful ways. In recent years, several synthetic methods exploiting this chemistry have been developed to a remarkable level of maturity. This article wants to briefly highlight this chemistry by discussing a few selected applications. 

Irradiation of Fischer carbene complexes generates, by insertion of carbon monoxide, a metal-bound ketene intermediate. Photolytic reactions of carbene complexes are synthetically attractive, in that the reaction conditions are mild and the reactions of ketene intermediates with a variety of reagents is of significant scope. A low concentration of metal-bound ketene is probably obtained and in the absence of a nucleophile, the starting material can usually be recovered even after prolonged irradiation. The ketene intermediates are readily trapped with nucleophiles for example, dipeptides are formed in excellent yield and with very high diastereoselectivity upon irradiation of optically active carbenes in the presence of natural or urmatural a-amino acids (Scheme 28). Dipeptides and PEG-supported amino acids and dipeptides can also be used as nucleophiles. 

The catalytic cycle begins with a metal carbene complex (96), which may be added directly to the reaction mixture or is afforded rapidly upon displacement of a suitable ligand on the metal center by the alkene. Subsequent addition to this carbene complex by another alkene (97) forms a metallacyclobutane intermediate, which can readily dissociate to a metaUacarbene complex and an alkene. In some catalysts, the metal bound carbene species has a high rotation barrier, which allows interaction of an empty pz orbital of the carbene complex with the incoming alkene (Scheme 22). 

This model is modified by Pino [300,301], Corradini [302], Kissin [303], Keii [304], Terano [305], Cecchin [306] to other titanium complexes. Bimetallic models between the titanium compound and the cocatalyst were discussed by Sinn and Patat [137], Pino [301], and Zakharov [307]. Others suggest that the growing polymer chain is bound to the transition metal through a double bond (carbene complex) and that the insertion reaction occurs through formation of a metal-cyclobutane intermediate [308,309]. 

Use of Rh2(OAc)4 suggested that there was no inherent selectivity attributable to the coordinated carbene or to rhodium(ll). However, modification of dirhodium(ll) ligands to imidazolidinones provided exceptional diastereocontrol, obtained by influencing the conformational energies of the intermediate metal carbene [19, 23], as well as high enantiocontrol. Representative examples of products from these highly selective intramolecular C-H insertion reactions with cyclic systems is given in Scheme 15.6. Additional examples of effective insertions in systems from which diastereomeric products can result are illustrated in processes of the synthesis of 2-deoxyxylolactone (Scheme 15.7) [64, 65]. Here the conformation of the reactant metal carbene that is responsible for product formation is 32 rather than 33. Other examples in non-heteroatom-bound systems (for example, as in Eq. 15) confirm this preference. 

Reaction of diazo compounds with a variety of transition metal compounds leads to evolution of nitrogen and formation of products of the same general type as those formed by thermal and photochemical decomposition of diazoalkanes. These transition metal-catalyzed reactions in general appear to involve carbenoid intermediates in which the carbene becomes bound to the metal.83 The metals which have been used most frequently in synthesis are copper and rhodium. 

In this dissociative pathway (which is assumed to be the major one today) first a phosphine is displaced from the metal center to form an active 14-electron-intermediate 42. After alkene coordination cis to the alkylidene fragment the 16-electron-olefine-complex 43 undergoes [2 + 2]-cycloaddition to give a metallacylobutane 44. Compound 44 breaks down in a symmetric fashion to form carbene complex 45. The ethylene is replaced in the conversion to complex 46. In the next steps (they are not further discribed above), another intramolecular [2 + 2]-cycloaddition joins up the eight-membered ring 11 regenerating the catalyst 42. Each step of the reaction is thermodynamically controlled making the whole RCM reversible. With additional excess of phosphine added to the reaction mixture an associative mechanism is achieved, in which both phosphines remain bound. 

The [2+2] reactions of the zirconium-imido compounds with alkynes and alkenes occurs by a mechanism similar to that for the [2+2] reactions of carbenes with alkynes and alkenes. The alkene or alkyne first binds to an open coordination site at the metal, and this coordination is followed by conversion of the alkyne or alkene complex to the metallacyhc product (e.g. Equation 13.76). Thus, the [2+2] reaction requires a 16-electron intermediate to bind the olefin or alkyne, even though the metallacyHc product and the imido complex have the same overall electron count. In support of the coordination of alkyne or alkene, albeit weakly, to the d° metal center, the rate of the reaction of alkynes with the 18-electron zirconocene-imido compound containing bound pyridine-N-oxide was inhibited by added pyridine-N-oxide (Equation 13.76). ... 

These reactions give carbenes (Chapter 11) or carbene-like intermediates. The reaction of Eq. 4.10 is particularly important because it is one of the rare ways in which the tightly bound CO can be removed to generate an open site at the metal. In this way a ligand L, which would normally not be sufficiently strongly binding to replace the CO, can now do so. 

These transition metal-catalyzed reactions in general appear to involve carbenoid intermediates in which the carbene becomes bound to the metal7 ... 

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