Palladium complexes acylation

Unsymmetrical oc-diketones RCOCOR have been prepared by treatment of an acyl halide RCOCl with an acyltin reagent R COSnBus, with a palladium complex catalyst. 

Sparteine 1 was also used in a palladium complex-catalyzed enantios-elective benzoylation of alcohols using monoxide and the organobismuth(V) compound (Scheme 37). The carbonylative acylation of alcohols using carbon monoxide (CO) is known to be an alternative tool for the prepar-... 

The carbonylation was explained by the following mechanism. Formation of dimeric 7r-allylic complex 20 from two moles of butadiene and the halide-free palladium species is followed by carbon monoxide insertion at the allylic position to give an acyl palladium complex which then collapses to give 3,8-nonadienoate by the attack of alcohol with regeneration of the zero-valent palladium phosphine complex. When halide ion is coordinated to palladium, the formation of the above dimeric 7r-allylic complex 20 is not possible, and only monomeric 7r-allylic complex 74 is formed. Carbon monoxide insertion then gives 3-pentenoate (72). 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

An acyl-palladium complex might undergo a series of follow up reactions. Subsequent transmetalation and reductive elimination lead to the formation of a carbonyl compound. This process is also coined carbonylative coupling, referring to the cross-coupling reaction, which would take place in the absence of carbon monoxide under similar conditions (for more details see Chapter 2.4.). 

If the reaction mixture also contains a nucleophile, then the acyl-palladium complex might undergo displacement of the metal, which usually leads to the formation of a carboxylic acid derivative. The side product in this process is a palladium(II) complex that undergoes reductive elimination to regenerate the catalytically active palladium(O) complex. 

The fate of the acyl palladium complex depends on the circumstances. In the presence of a suitable nucleophile (alcohol, amine) it is converted into the corresponding carboxylic acid derivative. The side product, a palladium hydride is converted to the active form of the catalyst in a reductive elimination step, resulting in the formation of an equimolar amount of acid, which is quenched by an added base (in most cases the excess of the nucleophile). 

Acylation of thiochromanones with boron trifluoride and acetic anhydride readily affords (92), which may be elaborated into (93) (equation 36) (79CJC3292), while conversion of the keto function to an oxime (94) permits the formation of the palladium complex (95) as a first step towards functionalization of the aromatic ring (equation 37) (78BCJ3407), or ring expansion by Beckmann rearrangement (equation 38) (76JCS(P1)2343). 

The oxidative addition is quite general with alkyl, allyl, benzyl, vinyl, and aryl halides as well as with acyl halides to afford the palladium (II) complex VII. The frans-bis( triphenylphosphine )alkylpalladium halides can also be carbonylated in an insertion reaction to give the corresponding acyl complexes, the stereochemistry of which (17, 18) proceeds with retention of configuration at the carbon bonded to palladium. The acyl complex also can be formed from the addition of the corresponding acid halide to tetrakis (triphenylphosphine) palladium (0). 

Initially the Pd(0) complex oxidatively adds to enol triflate 6 to form a vinyl-Pd(II) species. Carbon monoxide then inserts into the new Pd—C o-bond to yield a palladium(ll)-acyl complex which captures methanol. The methanolysis step is formally a reductive elimination reaction in which the Pd(0) catalyst is regenerated to propagate the catalytic cycle (Scheme 6.8).7... 

Cavell and co-workers later demonstrated the feasibility of the CO insertion in a (NHC)methylpalladium complex [22], The resulting acyl-palladium complex is prone to decomposition to yield acylimidazolium salts and Pd(0) (Scheme 4), which might explain the deactivation of the catalyst during the copolymerization reaction. 

The reactions may also be carried out under an atmosphere of carbon monoxide, CO (Scheme 10.22), when the usual catalytic cycle occurs. CO inserts easily into the palladium complex Ar-Pd -X. The aryl ligand migrates on to the carbonyl group to form a metal-acyl species, X-Pd - C(0)Ar. A transmetallation-reductive elimination sequence follows, forming the ketone and regenerating the Pd catalyst. 

Recently it has been shown by Chikashita et al. that hydride transfer is possible from the benzimida-zoline (21) to an acyl chloride, giving the corresponding aldehyde and benzimidazolium salt (22 equation 1) The reaction is most effective in the presence of 1 mol equiv. of acetic acid. Although it has only been used for a few aldehydes, it is successful with aromatic and aliphatic examples (e.g. p-nit-robenzaldehyde, 82% g.c. yield cyclohexanecarbaldehyde, 80% g.c. yield) and may have substantial potential. Another new method is the treatment of aroyl chlorides with dialkylzinc reagents in the presence of catalytic palladium complexes. However, the applicability would seem to be limited by the rather low yields. 

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