Chiral aldehydes, decarbonylation

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). ° ... 

In 1983, James and co-workers disclosed the first example of enantioselec-tive intramolecular hydroacylation reaction of racemic a,a-disubstituted aliphatic aldehyde 1 when they investigated aldehyde decarbonylation reaction using a chiral Rh complex as the catalyst (Scheme 8.2). The enantioen-riched product 2 (69% ee) could be obtained via kinetic resolution, but only a low conversion was achieved. One possible reason for the low reactivity was the presence of a quaternary stereogenic center at the a-position of the carbonyl group in the substrate. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The chiral aldehyde 88 was decarbonylated to give 89 with overall retention of the stereochemistry [45]. The unsaturated aldehyde in the polyfunctionalized molecule 90 was decarbonylated smoothly to afford alkene 91 [46]. The decarbonylation of aldehydes catalysed by a supported Pd or Rh complex is carried out at high... 

It has been proposed that the decarbonylation of aldehydes by the Wilkinson catalyst [RhCl(PPh3)3] involves a radical pair disproportionation or recombination reaction. A radical pair intermediate in solution is equivalent to a cage reaction (Scheme 6). Table 15 shows the results obtained from the decarbonylation of a series of chiral cyclopropyl aldehydes ... 

Use of the chiral carbon pool for cyclopentenone preparation is also known. The fungal metabolite terrein [88] was selectively monoacetylated and then reduced with chromous chloride to enone [89]. Acetylation and olefin cleavage with ruthenium tetroxide aiwi sodium periodate led to aldehyde [90], which was readily decarbonylated to [65] (51). An alternative route (52) began with the less common S,S-tartaric acid [91], converted in four steps to diiodide [92]. Dialkylation of methyl methylthiomethyl sulfoxide with [92] gave the cyclopentane derivative [93]. Treatment of [93]... 

Decarbonylations. Wilkinson s catalyst has been known for some time to decarbonylate aldehydes, even heavily functionalized ones, to the corresponding hydrocarbons. Some examples are shown in eqs 30-33, illustrating high stereochemical retention in the decarbonylation of chiral, cyclopropyl, and unsaturated aldehydes. Acid chlorides are also decarbonylated by RhCl(PPh3)3-... 

Chiral aldehydes can be decarbonylated under full retention of the configuration, but occasionally partial racemization may take place [10]. In general, decarbonylation with stoichiometric rhodium-arylphosphine complexes can be achieved at ambient temperature, but usually the catalytic version requires more severe conditions. Goldman et al. [11] discovered that trialkylphosphines form significantly more active catalysts, as exemplified with the binuclear complex [Rh(PMe3)(CO)Cl]2. The complex operates even at room temperature. A similar effect was also noted with iridium-phosphine catalysts [12]. 

Hoveyda et al. reported the enantioselective synthesis of (+)-africanol through asymmetric olefin metathesis followed by one-carbon contraction via decarbonylation (Scheme 8.16) [65]. When symmetrical norbornene 84 was treated with a chiral molybdenum catalyst, desymmetrization took place to afford the chiral bicycle 85 in an enantioselective fashion. The resulting vinyl terminus was subsequently transformed into a methyl group via hydroboration, oxidation to aldehydes, and decarbonylation. Further manipulations of the functional groups gave (+)-africanol. 

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