Diphenylcarbene complexes

The reaction " of the diphenylcarbene complex (21) with organonitriles, RCN, leads to the insertion product (22), as well as the nitrile complexes [(C0)5W(NCR)] and d5-[(CO)4W(NCR)2ketene]. In 1,1,2-trichloroethane, the reaction obeys the two-term rate law (1). Carbonyl ligand substitution occurs via... 

Carbonylation of the rhodium diphenylcarbene complexes 26 and 28 resulted in the formation of diphenylketene in good to excellent yields (reactions 8.51 and 8.52). It was assumed that the diphenylketene is formed by carbon-carbon coupling in the coordination sphere of rhodium [83]. 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

Fig. 10.6. Dimeric (Ar = 2,6-dimethylphenyl) (a) and monomeric (Ar = 2,4,6-trimethylphenyl) (b) copper complexes with diphenylcarbene. Reproduced from J. Am. Chem. Soc., 126, 10085 (2004), by permission of the American Chemical Society.

In comparison with other catalysts, (diphenylcarbene)tungstenpenta-carbonyl is surprisingly sluggish (26). This observation is significant as it relates to diverse views regarding the carbene-to metallocyclobutane interconversion. Whereas Casey emphasizes a need to accommodate the incoming olefin within the coordination sphere of the metal prior to rearrangement to a metallocycle [Eq. (5)], Katz (28) has described the process essentially as a dipolar attack of a polarized carbene-metal (R2C+—M-) on the olefin [Eq. (6)]. The latter does not specify a need for w-complexation of the olefin as a precondition to metathesis. 

Important evidence in support of the chain-reaction theory was the synthesis of carbene complexes, such as diphenylcarbenepentacarbonyl-tungsten(O) (4), and their reaction with olefins. The results in Eq. (12.16) demonstrate69 the scission of the carbon-carbon double bond of the alkene and the combination of the ethyl-idene fragment with the diphenylcarbene group of 4 ... 

C. P. Casey, and T. J. Burkhardt, Reactions of (Diphenylcarbene)pentacarbonyl-tung-sten(0) with Alkenes. Role of Metal-Carbene Complexes in Cyclopropanation and Olefin Metathesis Reactions, J. Am. Chem. Soc. 96, 7808-7809 (1974). 

This study did not lead to an unambiguous definition of the photochemistry of Cp(CO)2Mn=CPh2, but the authors suggested that their results were consistent with initial formation of a high-spin Mn(II) complex and a diphenylcarbene radical anion [Eq. (146)]. Such reaction is reasonable... 

The reactivity of the corresponding phenylcarbene complex contrasts with the behavior of the above diphenylcarbene. Thus, the in situ reaction of olefins with the nonisolated pentacarbonylbenzylidenetungsten, obtained by protonation of this ammonium salt derivative, exclusively gives cyclopropanes without any organic metathesis products or inorganic alkylidene derivative (see Tabled) ... 

The difference between the reaction condition for the two carbenes (40°C vs. — 70°C) suggests that the reaction of diphenylcarbene proceeds via the dissociation of a molecule, and that the n-complexation of reactant and product is an essential step in the mechanism. 

The formation of diphenylcarbene by laser flash photolysis of diphenyldiazomethane was studied in SC-CO2 and other supercritical media. [18] A laser-flash induced ring-closure reaction of a bipyridyl complex (Scheme 2) revealed solvation properties. [19] Laser-flash impact to metal carbonyl complexes activated hydrogen and simple alkanes like CH4, C2H4, C2H6, and further inorganic reactions in supercritical fluids have been reviewed. [20]... 

The fundamental interconversion of a ketene iridium complex with a carbene-CO iridium complex has been investigated by experimental [84] and theoretical [85] methods. A controlled, reversible conversion of a diphenylketene ligand to diphenylcarbene and CO ligands on a single iridium center was discovered and rationalized (reaction 8.53). 

Casey CP, Burkhardt TJ. Reactions of (diphenylcarbene)pen-tacarbonyltungsten(O) with alkenes. Role of metal-carbene complexes in cyclopropanation and olefin metathesis reactions. J. Am. Chem. Soc. 1974 96 7808-7809. 

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