Metallocarbene complex

Both are stable metallocarbene complexes, but they have very different reactivity profiles. The molybdenum catalyst is highly reactive and is effective widi sterically demanding olefins. Its drawbacks are diat it is not highly tolerant of diverse functional groups and has high sensitivity to air, moisture, and solvent impurities. The ruthenium system, on die odier hand, is catalytically active in die presence of water or air, and it exhibits a remarkable functional group tolerance. It is not a reactive as the molybdenum catalyst, particularly toward sterically bulky substrates. However, it is readily available and is die reagent of choice for all but die most difficult substrates. 

The new metallocarbene complex l,3-bis(2,6-diisopropylphcnyl)-4,5-dimethyl-3//-imidazolidenylpalladium(O) has been reported to catalyse the dimerization of butadiene in the presence of propan-2-ol to afford octa-l,3,7-triene, rather than the usual telomer-ization products, typical for other palladium catalysts. The new catalyst is characterized by an unprecedented efficiency (TON > 80000 and TOF > 5000 h 1)82... 

As pointed out in this woric, the mild conditions of the Cr reaction resulted in no observed epimeriza-tion, and offered an excellent alternative to the Schlosser modification of the Witdg reaction. Both the C1CI2 chemistry and the TiCU/Zn method discussed in Section 3.1.12.1.2 have the advantage of preparing the metallocarbene complex in situ. 

Early studies identified hydroxylamine hydrochloride as an essential component for the success of the reaction. When added to an unbuffered aqueous solution, this additive results in dramatically enhanced catalytic activity, presumably through the binding of the oxygen atom to the remaining vacant coordination site of the bimetallic metallocarbene complex (species... 

Alkene metathesis occurs by way of an intermediate metallacycle 87, followed by ring opening to give either the starting materials or one of the new alkenes and a new metallocarbene complex (2.111). Further metallocycle formation using another alkene and ring-opening provides the other product alkene and recovered catalyst to continue the cycle. 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

Carbonyl insertions into metallocarbenes have previously been observed for several different metals, including iron48 (see Section VI,C) and manganese.49 Indeed, carbonyl insertions into chrominum and tungsten diphenyl-carbenes have been shown to be viable processes.50 Most importantly, Wulff has isolated51 an 774-vinylketenecobalt (I) complex from the reaction between a cobalt carbene and an acetylene, a transformation that necessitates such a carbonyl insertion (see Section V,B). 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

A similar reaction of ylide 200 can also be carried out under thermal conditions or in the presence of catalytic amounts of Cu(acac)2 [143]. The carbenoid reactions of iodonium ylides can also be effectively catalyzed by rhodium(II) complexes [144, 145]. The product composition in the rhodium(II) catalyzed reactions of iodonium ylides was found to be identical to that of the corresponding diazo compounds, which indicates that the mechanism of both processes is similar and involves metallocarbenes as key intermediates as it has been unequivocally established for the diazo decomposition [144]. 

The interaction of metallocarbene (or alkylidene) complexes with unsaturated compounds via the formation of a metallacyclobutane intermediate plays an important role in several stoichiometric and catalytic reactions, e.g., metathesis, cycloaddition and polymerization. 

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