Rhodium-Catalyzed Decarbonylation of Aldehydes

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]. 

B3LYP/LACVP level of theory. Oxygen red, phosphorus pink, palladium dark gray, carbon light gray, and hydrogen hite. 

A KIE of 1.77 for the reaction with benzaldehyde was measured by comparing the rates of the reaction of benzaldehyde-Dj with the reaction rate for the undeuterated substrate in a competition experiment. In this manner, the full KIE of the selectivity-determining step can be measured even in cases where this step is not the overall rate-determining step [7]. A reason for choosing this particular approach was that it was suspected that the overall turnover-Umiting step was in fact simply the dissociation of the bound carbon monoxide formed during the 

A theoretical estimation of the KIE was carried out using complex 10 as resting state and one of the three possible transition states (oxidative addition 11, migratory insertion 13, or reductive elimination 14). When the migratory insertion was used as the rate-determining step, the calculated KIE value of 1.80 was found to be in excellent agreement with the experimental value of 1.77. 

Another important piece of evidence for the rate-limiting nature of the migratory insertion step is the fact that the Hammett p-value for the reaction of phenyl acetaldehyde is similar to the one obtained for benzaldehyde. Had the oxidative addition been rate determining, then a lower slope in the Hammett plot would be expected as the homo-benzylic position at which the reaction occurs is not in conjugation with the Jt-system. 

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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... 

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

In contrast to a number of studies on the homogeneous hydrogenation of carbon-carbon multiple bonds [25], there had been few papers about hydrogenation of simple ketones before Schrock and Osborn [26] reported in 1970 a catalytic activity of cationic rhodium complexes with relatively basic phosphines as ligands. In fact, the Wilkinson s rhodium(I) complex usually lacks activity towards hydrogenation of carbonyl groups, and rather catalyzes decarbonylation of aldehydes. The catalytic cycle of the hydrogenation of ketones proposed by Schrock and Osborn is depicted in Scheme 3. 

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

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

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... 

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 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. ... 

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 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. ... 

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. 

The migratory deinsertion process is important in the context of the transition metal-catalyzed or -mediated extrusion of the carbonyl group from aldehydes, ketones, and acid derivatives [70]. For example, treatment of aldehydes with a rhodium complex induced decarbonylation through oxidative addition of the... 

The cychzation of pent-4-enals and related compounds to cyclopentanones is catalyzed by [RhCl(PPh3)3] in a reaction that involves oxidative addition of the -CHO group to the rhodium(I) catalyst. This reaction has much in common with the decarbonylation that most aldehydes undergo in the presence of this catalyst (see Decarbonylation Catalysis). 

Chlorotris(triphenyIphosphine)rhodium, [(C Hs)3P]3RhCI. Mol. wt. 913.09 This organometallic complex is obtained as purple-red crystals by interaction of ethanolic solutions of RhCla-SHsO and a sixfold molar excess of triphenyl-phosphine. It catalyzes exceedingly rapid hydrogenation of double and triple bonds. Aliphatic aldehydes are decarbonylated to the corresponding paraffins according to the equation ... 

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. 

Koneko et al. (1999) developed a synthesis for 6-[ F]FDOPA (O Fig. 42.29b). The 4-[ F] fluorocatechol was prepared by nucleophilic aromatic substitution of the nitro group on 6-nitroveratryl aldehyde followed by decarbonylation with tris(triphenylphosphine) rhodium(I) chloride and hydrolysis (9.2% yield). 6-[ F]FDOPA was then prepared from the 4-[ F] fluorocatechol by a P-tyrosinase catalyzed reaction in the presence of ammonium, pyruvate, and ascorbate in an ethanolic Tris-HCl buffer within 5 min in about 60% radiochemical yield without any isomers. 

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

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