Hydroalkoxycarbonylation

The mechanism of the pyridine-modified cobalt-catalyzed hydromethoxycarbonyla-tion of 1,3-butadiene was studied by high-pressure IRand NMR spectroscopy. It was found that pyridine accelerates the conversion of ti -C4H7Co(CO)3 to methyl-3-pentenoate and the methanolysis of the intermediate CH3CH=CHCH2C(=0)Co (CO)4 1144). 

Thiourea-based ligands were evaluated for the palladium-catalyzed bis(methox-ycarbonylation) of terminal olefins. The best results were obtained by using tetra-substituted thioureas such as 40 and 41 (Equation 7.16) [145]. 

Palladium(II) complexes with l,l -bis(diphenylphosphano)ferrocene, l,l -bis(di-phenylphosphano)octamethylferrocene, l,l -bis(diphenyphosphano)ruthenocene, and l,l -bis(diphenylphosphano)osmocene were used to catalyze the methoxycar-bonylation of styrene. The regioselective (up to 85%) formation of 3-phenylpropano-ate was observed. The highest turnover frequency (334h at 100°C and 42 bar CO pressure) was obtained with the l,l -bis(diphenyphosphano)ruthenocene precursor in the presence of p-toluene sulfonic acid cocatalyst [146]. 

The mechanism for palladium-catalyzed alkoxycarbonylation ofcinnamyl chloride was investigated. An associative mechanism was observed at low pressure, while an insertion mechanism was observed at high pressure or when an excess of ligand (PPh3, P Hx3, or hemilabile P-N ligands 42, respectively) was used [147]. 

Highly active, selective, and recyclable palladium catalyst systems for the hydroesterification of styrene (at room temperature and 6 bar CO pressure) and vinyl acetate (at 40-60 °C and 6-10 bar CO pressure) were found by using l,2-bis(di-tert-butylphosphinomethyl)benzene as ligand and polymeric sulfonic adds of limited SO3H loadings as promoter [148]. 

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Carboxylic acids and their derivatives like esters, amides, anhydrides, and acyl halides are formally synthesized from olefins, carbon monoxide, and compounds represented by Nu-H such as H2O, ROH, RNH2, RCOOH (Equations (4) and (5)). Alkynes also react under similar conditions to afford the corresponding unsaturated carboxylic acid derivatives. These reactions have been named hydrocarboxylation, hydroalkoxycarbonylation, and hydroaminocarbonylation. 

Two possible mechanisms are suggested for the hydroalkoxycarbonylation of alkenes. One is similar to that of hydroformylation in which a catalytic cycle starts with a hydridometal complex (Scheme 14, path Here, olefin... 

Asymmetric hydroalkoxycarbonylation of a-methylstyrene is also examined, leading to 3-phenyl-butanoic acid, the //< r///< /-product. The highest ee is at best 60% for this substrate.When an allylic alcohol is treated with a chiral Pd complex under GO, cyclohydrocarbonylation takes place to give the corresponding 7-butyrolactone in an asymmetric manner (Equation (6)). Similarly, chiral -lactones and other lactams can also be synthesized. 

Among the great number of hydrocarbonylation reactions occurring according to Scheme 3, only the hydroalkoxycarbonylation reaction (X = OR) has been investigated up to now. 

The best optical yields obtained in the hydroformylation are comparable with the highest yield obtained in hydroalkoxycarbonylation using Pd catalysts. In Table 15 the results obtained in hydroformylation with rhodium or platinum catalysts are compared with those obtained in hydroalkoxycarbonylation using identical substrates and identical optically active ligand 9). 

The same regularities observed in hydroformylation seem to exist also in hydroalkoxycarbonylation, the prevailing chiralities in the rhodium-catalyzed hydroformylation being opposite to those in the palladium-catalyzed hydroalkoxycarbonylation. These regularities show that the model used to interpret the experimental hydroformylation data might also be applied to other hydrocarbonylation reactions. 

Hydroesterification is one of the Reppe reactions in which carbon monoxide, an alcohol, and an alkene react to esters (Scheme 21). This reaction is also called alkoxycarbonylation, hydroalkoxycarbonylation, and hydrocarbalkoxylation. 

As an example of the trend toward using homogeneous catalysts to assemble complex organic intermediates as building blocks via sequential insertions, Sisak, Ungvary, and Marko developed an efficient one-pot hydroesterification-Michael addition [22]. Thus, hydroalkoxycarbonylation of acrylonitrile with alcohols proceeds in the presence of catalytic amounts of Co2(CO)g and pyridine bases to yield 2,4-dicyano-2-methylbutanoic acid esters. 

In the second step, the concentration of pyridine as ligand must be low because it has an inhibitory effect on the hydroalkoxycarbonylation. In situ isomerization to the 4-pentenoic acid ester is a prerequisite for the subsequent carbonylation which provides dimethyl adipate. To ensure internal double-bond rearrangement, the temperature of the reaction is increased to 160-200 °C to give dimethyl adipate with 80 % selectivity. After hydrolysis of the ester, adipic acid is obtained with an overall selectivity of about 70% [1]. So far, this process has been performed on pilot-plant scale. 

In 2004, a comprehensive experimental and theoretical study on the hydroformylation and hydromethoxycarbonylation of A -vinyl acetamide was reported by Klaus et al. [58]. The goal of this study is to get insight into the observed regioselectivity in favor of the branched iso) product (Fig. 5). Both hydroformylation and hydroalkoxycarbonylation have the same reaction mechanism, and they differ only in the step of oxidative addition with hydrogen (H2) leading to aldehydes or alcohol (ROH) yielding amino acid esters. 

Scheme 13 Free energy profiles for hydroformylation (solid line) and hydroalkoxycarbonylation (dotted line) for the branched products...

Toste has demonstrated the hydroalkoxycarbonylation of P-allenyl carboxylic acid derivatives catalyzed by a mixture of a chiral, enantiomerically pure bis(gold) complex and a chiral, enantiomerically enriched silver phosphonate salt [107]. As an example, treatment of 6-methyl-4,5-heptadienoic acid with a catalytic mixture of [(S)-binapj (AuC1)2 and Ag-(J )-67 formed the yluctone 83 in 88% yield with 82% ee (Eq. (12.43)) [107]. 

He has reported a gold(I)-catalyzed protocol for the intermolecularhydroalkoxylation and hydroalkoxycarbonylation of alkenes with phenols and carboxylic adds, respec-... 

Pd-catalyzed carbonylation of unsaturated substrates incorporating a nucleophilic function in a suitable position may result in ring formation with CO incorporation into the ring (cyclocar-bonylation). In the absence of external nucleophiles, the cyclocarbonylation process usually results in intramolecular hydroalkoxycarbonylation of the unsaturated bond, both exo and endo cychzation modes being possible, as shown in Scheme 10 for a generic alkynol. 

Much attention has been focused on the palladium-catalyzed Reppe-type carbonylation (Scheme 6.10) [88-92]. A strong influence of the imidazolium-based IL structure on the branched-to-linear selectivity was observed in the hydroalkoxycarbonylation of styrene with alcohols (ethanol, n- and i-propyl alcohol, benzyl alcohol, n-octyl alcohol, etc.). While reactions performed in BMIPFe IL result mainly in the formation of the linear esters, a completely different picture is observed in BMI-BF4 IL the carbonylation of styrene with w-octyl alcohol gives 100% linear and 83% branched selectivities in BMP PFfi and BMI BF4, respectively [89]. The branched selectivity is also favored in ILs derived from the methylimidazolium cation with 3-benzyl or 1-acetonyl groups [91]. 

The effect of various alcohols on the regioselectivity of the palladium-catalyzed hydroalkoxycarbonylation of styrene in [bmim][BF4] and [bmim][PF6] ionic liquids was investigated. The highest regioselectivities toward branched ester were observed with preformed PdCl2(PPh3)2 catalyst. The addition of diphosphanes favored the formation of the linear isomer [150]. 

Using the hydroformylation reaction conditions, the hydroalkoxycarbonylation of enamides to amino acid esters has also been studied recently (see Equation (2))." Using these conditions, it has been possible to obtain a-amino acid derivatives via an aminocarbonylation process with catalytic amounts of Co2(CO)8." " " ... 

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