Rhodium catalyzed decarbonylation

Increasing use is being made of pyran syntheses based upon [4 + 2] cycloadditions of carbonyl compounds. The appropriate unsaturated aldehyde with ethyl vinyl ether yields 53 with peracids this affords an epoxide that undergoes ring contraction to the aldehyde 54 (Scheme 23) and rhodium catalyzed decarbonylation affords the required 3-alkylfuran with the optical center intact.116 Acetoxybutadiene derivatives add active carbonyl compounds giving pyrans that contract under the influence of acids to give... 

However, the decarbonylation reaction can be suppressed by the use of specially tailored chelating groups. Intermolecular processes involving dienes and salicylaldehydes are now known, and are thought to proceed via a double chelation mechanism, akin to the Jun-type system. Rhodium-catalyzed reactions lead to hydroacylated products, under relatively mild conditions (Equation (134)).117... 

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... 

Fristrup P, Kreis M, Pahnelund A, Norrby PO, Madsen R (2008) The mechanism for the rhodium-catalyzed decarbonylation of aldehydes a combined experimental and theoretical study. J Am Chem Soc 130 5206-5215... 

Oxidative addition of aldehydes is expected from the mechanism of their decarbonylation reactions, catalyzed by rhodium and palladium catalysts 9-10>. Harvie and Kemmitt reported the formation of the following diacyl complex by the reactions of the aldehydes with Pt(PPh3)4 u). 

In the foregoing, the formation of organic molecules on transition metal complexes is explained by stepwise processes of oxidative addition, insertion, and reductive elimination. One typical example, which can be clearly explained in this way, are the carbonylation and decarbonylation reactions catalyzed by rhodium complexes 10-137). Tsuji and Ohno found that RhCl(PPh3)3 decarbonylates aldehydes and acyl halides under mild conditions stoichiometrically. Also this complex and RhCl(CO) (PPh3)2 are active for the catalytic decarbonylation at high temperature. 

Rhodium-Catalyzed Regioselective C-H Functionalization via Decarbonylation of Acid Chlorides and C-H Bond Activation Under Phosphine-Free Conditions... 

In addition to Pd-catalyzed decarbonylation, rhodium complexes catalyze the decarbonylation efficiently. In this section, decarbonylation of acyl halides and aldehydes using palladium catalysts is surveyed. ... 

A related approach involves the direct decarbonylation of stable ketones. DauguUs and Brookhart demonstrated that the rhodium-catalyzed decarbonylation of diaryl ketones was feasible [11]. Efficient extrusion of CO from alkyl aryl ketones to form alkylarenes was easily achieved by rhodium(I) catalysis directed by apyridyl ortho to the RCO group (Scheme 22.6) [12]. (CO)2Rh(acac) was found to be the optimal catalyst and the methodology had a broad substrate scope. This method offers an alternative way to synthesize alkyl benzenes through an ARCIS reaction, complementary to the known Friedel-Crafts alkylation reaction of arenes. 

Shuai, Q., Yang, L., Guo, X., Basle, O., Li, C.-J. (2010). Rhodium-catalyzed oxidative C-H arylation of 2-arylpyridine derivatives via decarbonylation of aromatic aldehydes. Journal of the American Chemical Society, 132, 12212-12213. 

Maetani, S., Fukuyama, X, Ryu, I. (2013). Rhodium-catalyzed decarbonylative C-H arylation of 2-aryloxybenzoic acids leading to dibenzofuran derivatives. Organic Letters, 15, 2754-2151. 

Scheme 8.2 Rhodium-catalyzed decarbonylation of an a,(i-unsaturated aldehyde.

Scheme 8.4 Rhodium-catalyzed decarbonylation of heptanal with rhodium complexes bearing chelating diphosphine ligands.

Scheme 8.6 Side products in the rhodium-catalyzed decarbonylation of pent-4-enal in hs presence of ethylene.

Scheme 8.8 Rhodium-catalyzed decarbonylation of 3-methyl-3-phenylbutanal.

Scheme 8.13 Rhodium-catalyzed decarbonylation of an aldehyde derived from a Diels-Alder reaction.

Rhodium catalyzed reactions of ethyl isocyanoacetate 353 with 3-fluoroacetylacetone 352 provides a new facile method for the catalytic synthesis of substituted pyrroles. The key step of the reaction is the activation of the C-H bond of isonitrile 353 induced by the a-heteroatom effect. 3-Fluoropyrrole 44 was obtained in 40 % by this method [115]. The mechanism of the transformation includes rhodium promoted decarbonylation of formamide 354 followed by cyclocondensation of intermediate 355 to form the corresponding pyrrole 44. 

The rhodium-catalyzed successive C-C/C-O bond cleavage reaction of a cyclobutanone 77 containing a phenoxymethyl side chain was affected by the employed bidentate diphosphine ligand (Scheme 3.44) [53]. In the presence of [Rh(nbd)(dppe)]PF 5 (nbd, norborna-2,5-diene dppe, l,2-bis(diphenylphos-phino)ethane) (5 mol%) and diphenylacetylene (20 mol%), cyclobutanone 77 was transformed into the alkenoic ester 78 in 88% yield via C-C bond cleavage, P-oxygen elimination, and reductive elimination. In contrast, the [Rh(nbd)(dppp)]PFg-catalyzed (dppp, l,3-bis(diphenylphosphino)propane) reaction afforded cyclopentanone 79 in 81% yield through a rhodacyclohexanone species that was formed by 6-endo cyclization. The reaction of the cyclobutanone 77 catalyzed by [Rh(nbd)(dppb)]PFg (dppb, l,4-bis(diphenylphosphino)butane) led to exclusive formation of cyclopropane 80 via decarbonylation. 

A 2-phenylpyridine system was used in the rhodium-catalyzed decarbonyla-tion reaction of ketones (Eq. (6.7)) [16]. Unlike the reaction shown in Eq. (6.6), P hydrogen is not available for intermediate 33, and this alters the course of the reaction to decarbonylation. 

Decarbonylation of acyl cyanides has also been known since Blum reported a rhodium-catalyzed reaction in 1967 (Eq. (6.16)) [35]. Later, this reaction was significantly improved by using a palladium-based catalyst [36]. 

The first example that we discuss is the rhodium-catalyzed decarbonylation of aldehydes. This reaction was investigated owing to its potential in the conversion of biomass into chemicals or fuel as this contributes to the reduction of the oxygen content, which is one of the major challenges in biomass utilization [46]. 

Scheme 8.3 Proposed catalytic cycle for the rhodium-catalyzed decarbonylation of aldehydes.

In 2013, Ryu and coworkers reported a rhodium-catalyzed arylation of 2-aryloxybenzoic acids through decarbonylative intramolecular C-H functionalization (Scheme 6.14) [24]. The existence of AC2O andKl could significantly enhance the reactivity. Based on the control experiments, a mixed anhydride will be produced through the condensation reaction of carboxylic acid and AC2O, which oxidize the rhodium(I) catalyst to rhodium(III) species. 

The major drawback in the development of efficient catalytic PK protocols is the use of carbon monoxide. Many groups probably refuse to use this reaction in their synthetic plans in order to avoid the manipulation of such a highly toxic gas. Carbonylation reactions without the use of carbon monoxide would make them more desirable and would lead to further advances in those areas. Once the use of rhodium complexes was introduced in catalytic PKR, two independent groups realized these species were known for effecting decarbonylation reactions in aldehydes, which is a way to synthesize metal carbonyls. Thus, aldehydes could be used as a source of CO for the PKR. This elegant approach begins with decarbonylation of an aldehyde and transfer of the CO to the enyne catalyzed by rhodium, ruthenium or iridium complexes under argon atmosphere (Scheme 36). 

Considerable information about the course of aldehyde decarbonylations has been gleaned from the decarbonylations of alk-4-enals. Pent-4-enals form cyclopentanones in high yield in decarbonylations catalyzed by [RhCl(PPh3)3], The major product from the decarbonylation of hex-4-enal is 2-methylcyclopentanone. As shown in Scheme 5, the cyclization reaction requires a vacant site on rhodium. The other products result from decarbonylation of the unsaturated acyl before cyclization can take place. In these cases, there is competition between addition of deuterium to C-1 of the alkenyl ligand or its addition to the alkene bond and the formation of an unstable metallocycle. ... 

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