Aldehydes, coupling decarbonylation

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

Morimoto, Kakiuchi, and co-workers were the first to show that aldehydes are a useful source of CO in the catalytic PKR [68]. Based on 13C-labeling experiments, it was proposed that after decarbonylation of the aldehyde, an active metal catalyst is formed. This was proven by the absence of free carbon monoxide. As a consequence CO, which is directly generated by previous aldehyde decarbonylation, is incorporated in situ into the carbonylative coupling. The best results were obtained using C5F5CHO and cinnamaldehyde as CO source in combination with [RhCl(cod)]2/dppp as the catalyst system. In the presence of an excess of aldehyde the corresponding products were isolated in the range of 52-97%. 

Similarly, the perfluoroacyl radical CF3C (—O)203 was obtained by photolysis of a cyclopropane solution containing dw-butyl peroxide and trifluoroacetaldehyde below -80 °C after abstraction of the aldehydic hydrogen atom, the trifluoroacetyl radical is formed. It decarbonylates above -80 °C and exhibits a coupling of 11.54 G with three equivalent 19F nuclei. 

Acetylenic ketones of type (1) are decarbonylated to give the coupled product (2) when heated for many hours with the rhodium complex in benzene or xylene solution. In contrast with decarbonylation of aldehydes, an equivalent of the rhodium complex is required.12... 

The rhodium(I) catalyst (COl Rhlacac) was found to promote the decarbonylative coupling of aromatic aldehydes with norbomene to generate new C-C bonds with high stereoselectivity (Scheme 22.5) [10]. The reaction took place through an oxidative addition of the aldehyde C-H... 

SCHEME 22.5 Decarbonylative coupling of aromatic aldehydes and norbomenes catalyzed by rhodium. 

Yang, L., Guo, X., Li, C.-J. (2010). The first decarbonylative coupling of aldehydes and norbwnenes catalyzed by rhodium. Advanced Synthesis and Catalysis, 352, 2899-2904. 

Whilst acetylenic alcohols can be employed directly in Cadiot-Chodkiewicz reactions [9], protection of the alcohol (usefully as the Thp ether) is necessary for Castro coupling [14]. A variation based upon these two processes involves coupling of terminal alkynes with 3-bromopropynol (10) in the presence of pyridine [15]. For primary alcohol products, oxidation to the aldehyde with nickel peroxide followed by base-catalyzed decarbonylation generates the new terminal acetylene e.g. Fig. 1.10. 

Decarbonylation of the acylmetal intermediates has been extended to a variety of carbon-carbon forming reactions. For example, Li et al. reported decarbonylative addition reactions of aromatic aldehydes to alkynes (Scheme 7.17) [24] and acrylates [25]. An oxidative decarbonylative coupling reaction of aromatic aldehydes with 2-arylpyridines was promoted by a rhodium catalyst (Scheme 7.18) [26]. Carboxylic acid derivatives were also employed for analogous carbon-carbon bond-forming reactions through decarbonylation [27]. 

Other sources for cross-coupling reactions are aldehydes and carboxylic acids after decarbonylation and decarboxylation, respectively, which can be reacted with aryl halides to form biaryls. The following Experimental Procedure illustrates the potential of this quite atom-economic reaction. In this case, a copper co-catalyst promotes both the decarboxylation and the cross-coupling. 

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