Carbene-carbon monoxide coupling

The carbene-carbon monoxide coupling was suggested as a potential elementary C—C coupling step in catalytic carbon monoxide reduction systems based on the observation of the reaction product (r -C5(CH3)5)2ZrH 2(p-OCH=CHO) [31]. The zirconoxycarbene complex 3 was found to react with carbon monoxide at room temperature to afford a zirconium-coordinated ketene complex 4 in 30% isolated yield (reaction 8.18). The X-ray structures of both 3 and 4 were determined [32]. 

Diphenylketene was isolated in up to 70% yield from (CO)5Cr=CPli2 under 1 bar carbon monoxide at 50 °C. An intramolecular carbene-carbon monoxide coupling was assumed in the reaction based on the observation that no incorporation of CO... 

The iron carbene complex (ri -CsHs) Fe(CO)(CH2) generated by the electron impact from (Ti -C5H5)Fe(C0)2CH20CH3 in the gas phase was fotmd to react with cyclohexene or n-donor bases such as NH3, CH3CN, or CD3CDO by displacement of ketene from the iron center (reaction 8.25). Methylene-carbon monoxide coupling was assumed to occur in the coordination sphere of iron during the reaction [51]. 

Despite the fragmentary results known to date, the dediazotation of diazoalkanes coupled with the carbene-carbon monoxide trapping reaction seems to be a promising synthetic path for the preparation of various carboxylic acid derivatives through the ketene intermediate product. Especially the highly effective and selective cobalt-catalyzed examples of the reactions are worth to explore in more detail. 

Alvarez-Toledano has investigated149 the reactivity of tertiary amines toward vinylketene complexes of type 221 and found that a number of unusual transformations take place. The formation of 237 and 238 is not directly linked to the presence of the amine, but instead relies on a thermo-lytic coupling of two carbene complexes formed from the vinylketene ligand, with additional loss of carbon monoxide. It is thought that 239 is formed by the deprotonation of the labile C-4 proton, followed by subsequent coordination of a capping Fe(CO)3 group. 

Radicals and carbenes ch37 ch38 metal to carbon are key steps Carbon monoxide inserts into metal-carbon bonds Palladium is the most important metal C-C, C-0, and C—N bonds can be made with Pd catalysis Cross-coupling of two ligands is common Allyl cation complexes are useful electrophiles ... 

According to theoretical calculations, there is a low-energy pathway for the direct interaction of diazomethane with carbon monoxide [4j. However, experimental results so far show only evidences for two-step pathways. These involve first the dediazotation of the diazoalkanes in thermal, photochemical, or transition metal mediated reactions [5] resulting in free carbenes (Equation 8.2) or transition metal coordinated carbenes (Equation 8.3), which under proper reaction conditions couple in the second step with carbon monoxide (Equation 8.4 or 8.5) to form ketene products. 

To show a metal-mediated coupling of a carbene with carbon monoxide, the isolation of the ketene complex Co2(CO)7(CH2=C=0) in 32% yield in the reaction of Co2(CO)8 with the carbene precursor CH2Br2 in the presence of Znat 20 °C for 2 days in TH F was taken into account. This cobalt ketene complex was found to catalyze the methoxycarbonylation or aminocarbonylation of a mixture of CH2Br2 and Zn under 5 bar carbon monoxide pressure at —40-20 °C in THF solution, yielding methyl acetate (385% based on C02(CO)7(CH2=C=O)) and N,N-diethylacetamide (184% based on Co2(CO)7(CH2=C=0)) using methanol and diethylamine as the additional reagents in the reaction, respectively [75]. 

Experiments at atmospheric pressure of carbon monoxide have shown that complex 24 converts rapidly first into 23 and diethyl malonate and in a much slower reaction, complex 23 converts further into octacarbonyl dicobalt and an other mole of diethyl malonate. In both reactions, the formation of highly reactive ethoxycarbo-nylketene was assumed by coupling of the ethoxycarbonyl carbene ligand with carbon monoxide, whose intermediate is scavenged by ethanol [76]. Reactions of complex 24 with in the presence of excess ethanol result in diethyl malonate with natural isotopic distribution of[77]. The combination of these steps led to an effective one-pot procedure for the preparation of various malonic acid derivatives by catalytic carbonylation of ethyl diazoacetate in the presence of a few mol% octacarbonyl dicobalt catalyst precursor [76]. Two new example procedures illustrate the usefulness of the method. 

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