Decarbonylation, phosphine

Replacement of CO in MeCOMn(CO)5 with PPh3 seems to have little effect on the rate of the decarbonylation. As shown in Table IV, MeCO-Mn(CO)4PPh3 (an isomeric mixture) reacts only slightly faster than MeCOMn(CO)5 after provision is made for the difference in temperature 169). However, a recent kinetic study on the decarbonylation of CpMo-(CO)2L(COMe) (L = a tertiary phosphine) has shown that both inductive and steric properties of L are important 19a). Sterically demanding and weakly a-bonding phosphines increase the reaction rate. 

Recently a kinetic study has been reported of the decarbonylation of various CpMo(CO)2L(COMe) (L = a tertiary phosphine) to CpMo(CO)2-LMe (19a). 

The reactions of MeMn(CO)5 with various phosphines and phosphites (L) to give/ac-MeCOMn(CO)3L2 and/or wicr(L srrflMi)-MeCOMn(CO)3L2 have been monitored by NMR spectroscopy 166). Sterically demanding L s promote formation of the meridional over the facial isomer. Interestingly, the meridional acetyls decarbonylate more easily than their facial counterparts, perhaps owing to a weaker Mn—CO bond cis to COMe in the former. 

Chloroacylation of terminal aryl, alkyl or alkenyl alkynes [Le. the addition of RC(=0)-C1 across the CC triple bond] with aromatic acyl chlorides was catalysed by [IrCl(cod)(lPr)] (5 mol%) in good conversions (70-94%) in toluene (90°C, 20 h). Z-addition products were observed only, hitemal alkynes were umeactive. Surprisingly, a phosphine/[lr(p-Cl)(l,5-cod)]2 system under the same conditions provides decarbonylation products (Scheme 2.34) [117]. 

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]. 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

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. 

Decarbonylation of the acyl is likely to be metal-assisted (Ru11) giving rise to a Ru carbonyl, which is subsequently decarbony-lated by nucleophilic attack by nBu3P. This phosphine can displace coordinated carbonyl, as exemplified by reaction 3 ... 

This reaction occurs thermally in toluene at 30°C with an equilibrium constant (K) equal to 1.5 (13). Both bis(phosphine) and (carbonyl)phosphine Soret bands are present in the active catalyst solutions (see Decarbonylation Procedure), together with the unassigned, and likely critical, band at 420 nm. This could be due to some species giving rise to, or resulting from, a RuII + RCO reaction this is equivalent, of course, to a (Ru H-COR) acyl or a RuIII(C0)R (carbonyl)alkyl species, and the final elimination reaction after loss of CO could be written as ... 

In an attempt to resolve this question of stereochemistry and also to determine whether or not the decarbonylation of an acid chloride containing a f3 hydrogen takes place stereospecifically, erythro- (XI) and fhreo-2,3-diphenylbutanoyl chlorides (XII), obtained by the reaction of the known acids (13, 14) with oxalyl chloride, were synthesized. The reaction of these acid chlorides (see Reaction 8) with chlorotris( triphenyl-phosphine) rhodium gave the corresponding acyl complexes of type lib [R = C6H5CH(CH3)CH(C6H5)]. Decarbonylation of the erythro- cy complex in benzene at 30 °C gave a 90% yield of frans-a-methylstilbene while decarbonylation of the threo-acyl complex under similar reaction... 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Decarbonylation of Aldehydo Sugar Derivatives with Chlorotris(methyldiphenyl-phosphine)rhodium(I), D. J. Ward, W. A. Szarek, and J. K. N. Jones, Chem. Ind. (London), (1976) 162-163. 

Decarbonylation of 2-methoxyethanol provides the CO ligands for formation of the five-coordinate bulky phosphine complexes [RuHCl(CO)L2] (L = PCy,2472 PBu 2R (R = Me, Et)1712]. These compounds readily take up CO to give [RuHCl(CO)2L2] (L = PCy3,1813 PBu 2R1712) with trans L and... 

It is convenient to note here that a common reaction of alcohols with transition metal halides in the presence of phosphine is the decarbonylation and sometimes dehydrogenation to give metal carbonyls. These reactions proceed via alkoxide intermediates which then undergo 8-H transfer ... 

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