Acyclic carbene-metal complexes

Reports of catalysis with acyclic carbene-metal complexes first appeared in 2005 [19,22a,38] and dealt with Pd- and Ni-catalyzed coupling reactions for which effective NHC-based catalysts were already known [39]. Gradually, the focus has shifted toward identifying reactions that benefit from the distinct steric and electronic properties of acyclic carbenes. In this section, key examples of catalysts that exhibit imusual stabilities, activities, or selectivities resulting from the presence of acyclic carbene ligands are highlighted. Recent review articles provide more detailed compilations of catalytic applications of this ligand class [9]. 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

A useful metalation approach that has little precedent in cyclic carbene chemistry [18] is the use of 2-chloroamidinium or chloroiminium ions as precursors for acyclic carbene ligands. Fiirstner and coworkers prepared cationic Pd complexes of acyclic diamino-, aminooxy-, aminoarjd, and aminothiocarbenes by oxidative addition of chloroiminium precursors to Pd(PPhs)4 (route d. Scheme 16.1), an approach that was also effective for ADC-Ni complexes [19]. This route permits complexation of sterically nonhindered acyclic carbenes that would not be stable in the free state. Chloroamidinium precursors can be meta-lated without a change in metal oxidation state via lithium-halogen exchange followed by transmetalation (route e). This strategy has been successfully employed with Pd", Rh, and Ir [20]. 

In addition to their thermodynamic propensity to dimerize [16], free acyclic car-benes containing alkyl groups are prone to decomposition via intramolecular C—H insertion reactions that lead to net elimination of an alkene (Scheme 16.3a) [35]. Loss of two alkene equivalents has been observed to occur from an Alder-type ADC bound to W or Mo tetracarbonyl fragments, resulting in conversion to an amidine ligand (Scheme 16.3b) [36]. This process appears to be limited to zerovalent metal complexes and may be facihtated by the unusual -(C,N) binding mode of the carbene. 

Figure 5.1 Metal complexes comprising the classical imidazol-2-ylidene ligand (A) and representative non-classical carbene ligands (B-N), including normal carbenes (B-E), abnormal carbenes (F-I), remote carbenes (E, G, I), cyclic alkyl(amino)carbenes (J), acyclic carbenes (K, L, M) and amino(ylide)-carbenes (N). Substituted nitrogen centres may be replaced by oxygen or sulfur. The M=C bond representation— while strongly over-emphasizing the differences in the nature of the metal-carbon bond in these non-classical carbene complexes— was used to accentuate normal and abnormal bonding.

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

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