Metal-alkyne complexes chromium

Other late transition metals used in special cases of hydrosilation include cobalt, iron, ruthenium (vide infra for reactions with alkynes), osmium, chromium, molybdenum, tungsten and copper. Metallocenes (see Metallocene Complexes) of early transition metals and lanthanides have also been found to catalyze the hydrosilation of a number of unsaturated compounds including alkenes and esters (vide infra). 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Several reactions of metal-carbene complexes with alkynes leading to five-membered ring compounds have been described. The action of acetylenes on the chromium phenyl(pyrrolidino)carbene complex 472 results in mixtures of indanones 473 and indenes 474297 Terminal alkynes (pent-l-yne or hex-l-yne) react with the molybdenum carbene complex 475 to afford, after oxidative work-up, indanones 476 in contrast, trimethylsilylacetylene gave only the naphthoquinone 477. ... 

Reaction of [S(CCPh)2] with [Co2(CO)g] afforded 2 jhe crystallographically characterised adduct [(PhCC)S Co2(CO)6(Ti2-CCPh) ] which, on treatment with further [Co2(CO)g] afforded [S Co2(CO)6(Tl2-CCPh) 2l. The reaction of pentacarbonyl allyloxy((4-methylphenyl)ethynyl)carbene) chromium and tungsten complexes with dicobalt octacarbonyl afforded 63 tpe alkyne complexes (62) (M = Cr, W), which failed to undergo intramolecular Pauson-Khand reactions instead, at room temperature, a 1,2-elimination occurs, regenerating the metal hexacarbonyl with predominant formation of dinuclear cobalt complexes (63) - (65). 

Transition-metal carbene complexes have proven especially useful in the construction of six-membered rings, primarily in the synthesis of naphthalenes and higher aromatic ring systems. The major method employed is the Dbtz benzannulation reaction (depicted in Scheme 17.12), where an a,p-unsaturated or aryl carbene complex of chromium (78) couples with an alkyne to afford a phenol derivative (76). This reaction has been demonstrated for a vast array of molecular architectures and functionalities and has proven to be a very reliable and efificient process. Perhaps the major limitation on the synthetic utility is the construction of the carbene complex for systems that require structural complexity in the carbene complex. In addition to the Dotz reaction itself, several efforts to develop alternative procedures have been inspired by a detailed understanding of the mechanism of the Dotz reaction. 

Metalla-Diels-Alder Reaction via Metal Exchange A significant metal effect on the reactivity of a,(3-unsaturated Fischer carbene complexes is found in the reaction with alkenes and alkynes. Thus, the reaction of electron-poor alkynes with chromium alkenyl(alkoxy)car-benes is usually slow [26], whereas rhodium-catalyzed reactions of chromium alkenyl(methoxy)carbenes 75 with electron-poor alkynes 76 readily afford polysubstituted cyclopentenones 78, 79, and 80 (Scheme 5.17) [27]. Although the reactions proceed with high regioselectivity, the regioselectivity depends on the substituent (R ) of... 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

A transmetalation of the styrylcarbene chromium complex 62 in the presence of stoichiometric amounts of [Ni(cod)2] to give the nickel carbene intermediate 63 was applied to the synthesis of Cr(CO)3-coordinated cycloheptatriene 64 upon reaction with terminal alkynes [57] (Scheme 37). The formation of pen-tacarbonyl(acetonitrile)chromium is expected to facilitate the metal exchange. 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

The single best piece of evidence for the intermediacy of vinylketene complexes is however the isolation and characterization by Wulff22 of amine-stabilized T74-vinylketenechromium(0) complexes (42) from the thermolysis of the chromium carbene complexes 43, containing a tethered alkyne functionality. This was the first time that a d6-rf-vinylketene complex of any group 6 metal had been isolated. 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

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