Styrenes hydroesterification

Similar 0—4 generations silica-supported Pd-PAMAM dendrimers with various spacer lengths were used by Alper et al. as recyclable catalysts for the hydroesterification reaction of alkenes (55) and the oxidation of terminal alkenes to methyl ketones (56). The hydroesterification experiments (Scheme 16) showed that (PPh3)2Pd-PPh2-PAMAM-Si02 complexes were highly active catalysts for styrene derivatives and linear long-chain alkenes (numbers of turnovers up to 1200). 

In the hydroesterification of styrene (equation 123) the isomer distribution was found to be very dependent on the phosphorus ligand used. 

When the cluster [Pd4(C0)4(02CMe)4]-2MeC02H was dissolved in styrene and treated with methanol under an atmosphere of carbon monoxide, methyl cinnamate was formed.516 This reaction was also believed to occur by an alkoxycarbonyl route. The reaction became catalytic when [Pd(OAc)2] (106) was used in presence of NaOAc and a stoichiometric amount of copper(II) as reoxidant for the palladium(O) formed. Stille and co-workers have investigated this reaction, sometimes called carboalkoxylation, in detail. A basic difference between this reaction and the hydroesterification described above is that the oxidative nature of carboalkoxylation permits double functionalization of a double bond. Thus (E)- and (Z)-2-butene react readily with CO and methanol in the presence of a catalytic amount of PdCl2 and a stoichiometric amount of CuCl2 to give methyl 3-methoxy-2-methylbutanoate (equation 124).517,518... 

With [Pd(DBA)2] (105) in the presence of (+)-neomenthyldiphenylphosphine, the hydroesterification of styrene with CO and methanol gave a best optical yield of 52% for the methyl 2-phenylpropionate formed.578 The reactions were carried out in presence of trifluoroacetic acid. Other acids were of far lesser value. It was believed that the complex [PdH(02CCF3)PPh2 (neomenthyl)] was formed. 

Much higher enantiomeric excesses have been achieved in the hydroesterification of a-methyl-styrene.90-91 In this case, the asymmetric center is generated by hydrogen addition at the tertiary site. Tlie reaction is sensitive to the alcohol, the highest enantiomeric excesses being obtained with isopropanol or r-butanol (equation 60). 

Other Asymmetric Reactions. Asymmetric synthesis using the new ligand 1 is still limited. When 1 is used for Pd-clay catalyzed hydroesterification of styrene with carbon monoxide and, ... 

Palladium-catalyzed hydroesterification of styrene gives predominantly the branched ester, in which a new chiral center has b n created at the 2-position (equation 34). Carrying out this reaction at low pressure (1-2 bar) in the presence of the bulky chiral phosphine neomenthyldiphenylphosphine and tri-fluoroacetic acid leads to significant asymmetric induction (50% ee) this work seems to represent the only significant advance towards stereocontrol of catalytic hydroesterification reactions. 

Asymmetric induction in the palladium-catalyzed hydroesterification of z-methylstyrene, styrene and other alkenes in the presence of ligands with rigid 5//-dibenzophospholyl units linked to chiral cyclobutane, dioxolane or cyclohexane skeletons (CBDBP, DIPHOL, CHDBP) is compared with the effect of the analogous diphenylphosphane ligands18 23. 

Asymmetric hydroesterification of styrenes is suggested as an alternative route to optically active 3-arylbutanoic acids, a series of nonsteroidal anti-inflammatory drugs18. Thus, with palladium(0)bis(dibenzylideneacetone) in the presence of NMDPP and trifluoroacetic acid 6-methoxy-2-vinylnaphthalene gives the pharmaceutically interesting methyl (— )-2-(6-methoxy-2-naphthvl)propanoate (naproxen) with 42% ee20. 

PdCl2(PhCN)2 associated with phosphines such as (+)-neomenthyldiphenylphosphine (NMDPP) and /)-toluenesul-fonic acid (TsOH) dissolved in [G4CiIm]BF4 catalyzes the hydroesterification of styrene derivatives to produce 2-arylpropionic esters in very good yields and regioselectivities (Scheme 18). The simple separation of the products and the use of lower CO pressures are the major advantages of the use of ILs in the hydroesterification of styrene derivatives as compared with the reactions performed under homogeneous conditions. ... 

The cationic paUadium(II) complex [Pd(MeCN)2(PPh3)3](BF4)2 is an active catalyst for the hydroesterification of styrene using CO and methanol under very mild conditions. The catalytic system, Pd(Oac)-PPh3-p-toluenesulfonic (p-TsOH) could also produce the branched ester regioselectively in excellent yield at ambient temperature. The catalytic asymmetric hydroesterification of styrene by the use of chiral phosphines, such as (5)-l-[(l )-r,2-bis(diphenylphosphino)ferrocenyl]ethyldimethylamine, and (5)-(i )-BPPFA as ligands (Eq. 19). ... 

Table 5.3 Recycling of IL catalyst solution used in the hydroesterification of styrene" [29].

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]. 

The electronic effect of diphosphanes on the regioselectivity of the palladium-catalyzed hydroesterification of styrene was studied. It was found that in the presence of electron-poor phosphanes, branched esters are produced and high activities of the catalytic systems were observed [154]. 

Seayad A, Jayasree S, Damodaran K, Toniolo L, Chaudhari RV. On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex. J. Org. Chem. 2000 601 100-107. 

Oi S, Nomura M, Aiko T, Inoue Y. Regioselective hydroesterification of styrene catalyzed by cationic palladium(II) complexes under mild conditions. J. Mol. Catal. A Chem. 1997 115 289-295. 

Wang L, Kwok WH, Chan ASC, Tu T, Hou X, Dai L (2003) Asymmetric hydroesterification of styrene using catalysts with planar-chiral ferrocene oxazoline ligands. Tetrahedron Asym 14 2291-2295... 

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