Decarbonylation ligands

Given in Eq. (32) is a general mechanistic scheme for the decarbonylation ligand substitution is included for comparison ... 

Little work has been done with analogous compounds of rhenium. Treatment of p-ClC6H4Re(CO)5 with PPhj at 50°-60°C yields cis-p-ClC5H4Re(CO)4PPh3 by an unknown pathway. The same product results from p-ClC5H4CORe(CO)5 and PPhj the decarbonylation followed by ligand substitution has been proposed. The reaction of OT-CIC5H4CO-Re(CO)j with PPhj proceeds similarly 4). 

For example, treatment of MgO-supported [HIr4(CO)n] in flowing He at 573 K caused essentially complete removal of the CO ligands, as shown by IR and EXAFS spectra, with the Ir4 tetrahedra remaining essentially intact, as shown by EXAFS spectra [12]. IR spectra indicated the formation of carbonate and formate on the basic MgO, which evidently was not an inert platform [19]. When the decarbonylation took place in the presence of H2, the iridium aggregated into larger clusters more readily than when the de-... 

These comparisons prompted the Rosch group [32,33] to conclude that some Ugands remained on the supported clusters after decarbonylation this conclusion may be quite general—supported metal clusters are highly reactive, and typical oxide and zeoUte supports are not unreactive. Thus, a representation of supported clusters such as tetrairidium on 7-AI2O3 as 4/7-AI2O3 is a simplification that fails to account for the ligands on the cluster. 

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]. 

Scheme 9.13 Selective decarbonylation of a cyclobutanone to form the corresponding ketone with PR ligands or the aldehyde with NHC ligands...

Afonso, Gois, and co-workers followed this report with an unexpected decarbonylation of diazo-acetamides (Scheme 9.14) using 43-NHC (Fig. 9.8) [58]. The reaction generated three different products, with low selectivity for the decarbonylated product. The authors tested other substrates with different R groups and bulk at the amine position but found no correlation to the amount of decarbonylation product formed. However, 43-IPr was more selective than 43-SIPr for the decarbonylation product. The authors attributed the decarbonylation to the axial coordination of the NHC ligand to the dirhodium (11) complexes. 

Mononuclear acyl Co carbonyl complexes ROC(0)Co(CO)4 result from reaction of Co2(CO)8 with RO-.77 These also form via the carbonylation of the alkyl precursor. The ROC(0)Co(CO)4 species undergo a range of reactions, including CO ligand substitution (by phosphines, for example), decarbonylation to the alkyl species, isomerization, and reactions of the coordinated acyl group involving either nucleophilic attack at the C or electrophilic attack at the O atom. 

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. 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

The facility of these decarbonylation reactions is obviously related to the donor capacity of the ligand groups. The halogens follow the variation that may be anticipated for this series. The reactions of Os COlnL with the halogen acids HX (X = Cl, Br, I) involve sequential evolution of carbon monoxide, but their facility increases with the donor capacity of the halogen, Cl < Br < I (157,162). 

The greater lability of complex 146.C (compared to 145.c), as evinced by the much shorter reaction time, is typical of those that bear a carbomethoxy or acetyl substituent at the central carbon of an i73-allylic ligand. The temperature required for complete decarbonylation of complexes of type 146 and 148 increases with the size of the R-substituent, which suggests a mechanism involving hydride transfer.111 This would also explain the observed activating effect of the centrally located carbomethoxy group in 146.C, which would clearly labilize the methyl proton shown explicitly in 146. 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

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