Diketones decarbonylation

Casey et al. have studied the decarbonylation reactions of [cis-(OC)4M(MeCO)(PhCO)], in which M is Mn or Re (16,17). These complexes lose a carbonyl ligand to form five-coordinate intermediates of the type [(OC)3M(MeCO)(PhCO)]. Reversible methyl migration proceeds much more rapidly than does phenyl migration. In the course of these studies, a phosphine substituted rhena-/3-diketonate complex, [fac-(OC)3(Et3P)Re(MeCO) (PhCO)], was prepared. 

Product studies have been carried out on the irradiations of several highly strained cw-a-diketones. Although there have been no mechanistic studies, the observed products can be rationalized by a mechanistic scheme involving initial carbon-carbon bond cleavage and decarbonylation. 

Frequently, the decomposition of fluorine compounds results in a product with fewer carbon atoms through loss of carbon monoxide, carbon dioxide, or even an organic molecule Perfluonnated ot-diketones undergo decarbonylation with cesium fluoride in diglyme [90] (equation 57)... 

In a straightforward refinement of their concept Rubin et al. have turned to cyclophyne 14 [32] in which the vinylic hydrogens of 13 are replaced by 1,2-dioxocyclobuteno groups. This cyclic diketone moiety has been used previously [18, 33, 34] as a synthetic equivalent for alkynyl groups which can be generated from the dione by thermally or photochemically induced CO expulsion. Successive decarbonylation of 14 should ultimately lead to cyclophyne 15 with the composition Q0H6. 

If decarbonylation of the acyl radical is slow, aldehydes and diketones are formed. 

They found that at room temperature using benzene as the solvent only 3% consisted of the diketone 25 and mainly the ring-contracted products were obtained 33% of keto aldehyde 24 and 28% of ketone 26. From an industrial point of view the desired compound is the keto aldehyde 24, which is an interesting intermediate for the synthesis of other cyclopentanone derivatives with floral and fruity smells. The acid catalyzed reaction mechanism leading to the synthesis of keto aldehyde 24 had been discussed earlier (25). Therefore, it is of interest whether the product distribution changes in the presence of a heterogeneous catalyst system and also whether the decarbonylation of the compound 24 to compound 26 can be suppressed. 

Mechanistic studies strongly suggest the intermediacy of a samarium acyl anion. For example, the phe-nylacetyl radical (PhCHiCO ) is known to rapidly decarbonylate k = 5.2 X 10 s" ), providing a benzyl radical which dimerizes to bibenzyl. However, addition of phenylacetyl chloride to a solution of Sml2 in THF leads to a 75% yield of the expected diketone, and neither toluene nor bibenzyl is detected. Apparently, intermolecular reduction of this acyl radical to the corresponding anion proceeds at a rate which is greater than 5.2 x 10 s . ... 

An interesting ring-contraction biogenetic pathway from puupuhenone (45) to haterumadienone (46) might proceed via benzylic acid rearrangement of the diketone form of 45, followed by oxidative decarbonylation. Haterumadienone (46) is the first ring-contracted... 

Kaneda, K., Azuma, H., Wayaku, M., Teranishi, S. Decarbonylation of a- and 3-diketones catalyzed by rhodium compounds. Chem. Lett. 1974, 215-216. 

The rhodium complex [RhCl(PPh3)3] readily brings about stoichiometric decarbonylation of aldehydes, acyl halides and diketones. A typical aldehyde decarbonylation is illustrated by equation (69). a,3-Unsaturated aldehydes are decarbonylated stereospecifically (equation 70), while with chiral aldehydes the stereochemistry is largely retained (equation 71). ° ... 

By heating above 200 C catalytic decarbonylation is possible using [RhCl(PPh3)3], and is particularly suitable for aromatic aldehydes since aliphatic aldehydes tend to dehalogenate under these conditions to form alkenes (equation 72). Cationic rhodium complexes, for example [Rh(Ph2P(CH2)2PPh2)2], are much more active catalysts and hence reactions can be carried out at below 100 Because of the milder conditions aliphatic aldehydes can be decarbonylated to the alkane using this catalyst system. Rhodium catalysts can also be used to decarbonylate a- and -diketones and keto esters (equations 73 and 74). ... 

Acylstannanes will couple with acyl halides to give mixtures of mono- and diketones, but decarbonylation can be prevented by an atmosphere of CO, when the diketone is the predominant product.48... 

An efficient route for the synthesis of [2.2]cyclophanes has also been described which involves the photochemical double decarbonylation of the diketones (160), (161) and (162). The reactions are carried out in argon-degassed benzene solution and give high yields of products efficiently. In the case of the meta systems (160) both mono (163) and double decarbonylation products (164) are formed, but with (161) and (162) only the bisdecarbonyla-tion is observed affording (165) and (166) respectively. 

Acyltins. Acyltins, which are sensitive toward oxygen giving stannyl carboxylate, can be coupled with acid chlorides to yield a-diketones.f A CO atmosphere suppresses decarbonylation which is the major side process. 

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