Formyl complexes decarbonylation

Two formyl transfer" reactions of isolated formyl complexes are shown in Eqs. (20) (37, 42) and (21) (37, 38, 42, 47, 66). Control experiments indicate that these reactions do not involve metal hydride intermediates (formed via decarbonylation). Straightforward intermolecular H transfer (rather than formyl ligand transfer) is believed to be taking place. 

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

It is well known that formyl complexes easily undergo the reverse reaction — decarbonylation (Equation (7)). 

For similar reasons, CO rarely inserts into metal hydrides. The M-H bond energy is usually high enough that insertion to form a formyl derivative is tirermodynamically uphill. Therefore, decarbonylation of formyl ligands is more typical (Equation 9.29), and formyl complexes of the transition metals are usually prepared by indirect means. ... 

Reactions between [Cp Ir(PMe3)(Me)(OTf)] and aldehydes (RCHO) proceed with high selectivity to give the hydrocarbyl carbonyl salts [Cp Ir(PMe3)(R)(CO)]OTf (137, R = Me, Et, Pr, Ph, 1-ethylpropyl,/>-Tol, Mes, (Z)-l-phenyl-l-propen-2-yl, vinyl, Bu, 1-adamantyl). The tandem C-H bond activation/decarbonylation reaction afforded the first isolated tertiary alkyl complexes of Ir. X-ray diffraction studies were carried out on Mes, Bu, and 1-adamantyl derivatives. Hydride reduction of the /)-Tol complex provided an example of a rare transition metal formyl complex, [Cp lr(PMe3)(p-Tol)(CHO)]. ... 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

Not unexpectedly, cationic complexes like [Re(CO)4 (dppe)]+ react readily with NaBH4 to give the formyl [Re C(0)H (C0)3(dppe)]+ that decarbonylates in solution to [ReH(CO)3(dppe)]. 

Chelation-assisted additions of formyl C-H bonds to olefins and dienes have been reported by Jun et al. [120]. In the case of the reaction of 8-quinolinecar-boxaldehyde, they proposed that the formation of the stable 5-membered met-allacyclic complex [121] suppressed the undesired decarbonylation reaction (Eq. 53) [120]. The intermolecular hydro acylation of 1-alkene with 2-(diphenyl-phosphino)benzaldehyde by rhodium(I) catalyst has been conducted on the basis of this working hypothesis [122]. 

The mechanism (Scheme 48) ° is expected to proceed through the acylpalladium species much as in the Rosenmund reduction. Indeed, the acyl complex 56 from oxidative addition of vinyl chloride with Pd(CO)(Ph3P)3 was isolated (Scheme 49). " The reaction of acid chlorides with the same catalyst provides aldehydes. However, aliphatic acid chlorides do not reduce effectively. The phosphine ligands present in the Heck acylpalladium intermediate are thought to be the cause, allowing decarbonylation and elimination to occur. Interestingly, the formylation will not occur with the Rosenmund catalyst. 

A mechanism was suggested on the basis of deuteration labeling studies and by the isolation of some intermediates (Scheme 8.17) [4]. In the first stage, the catalyst 1 is loaded with the substrate aldehyde. Under the effect of benzoic acid, the hydrido-Rh-acyl complex 2 is formed. After decarbonylation of the acyl unit, the corresponding Rh-alkyl complex 4 is obtained, which is immediately converted into the jc-olefin complex 5. Exchange of the olefin by the formyl acceptor olefin (here NBD) and subsequent hydroformylation of the latter result in the net transfer of the formyl group from one aldehyde to the other. 

Exchange of formyl hydrogens for tritium is observed to occur in both aryl aldehydes (with concurrent ortho labeling in appropriate cases) and aliphatic aldehydes using [(cod)Ir(PCy3)(py)]PF6. Partial reduction of aldehydes to alcohols may occur in some cases. Labeling by other iridium phosphine catalysts has not been reported but is likely to occur. This type of catalytic activity, which likely involves reversible oxidative addition of the iridium center into the formyl C—H bond, is different in outcome from that of organorhodium complexes, whose insertion into formyl C—H bonds proceeds instead to decarbonylation. 

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