Styrene, palladium-catalyzed

Palladium-catalyzed hydrosilylation of styrene derivatives usually proceeds with high regioselectivity to produce benzylic silanes, 1-aryl-1-silyle thanes, because of the... 

However, in the palladium-catalyzed addition of HSiCl3 to -substituted styrenes, the size of the substituent on the MOP ligand is crucial. While (R)-MeO-MOP/[Pd(//3-C3H5)Cl]2 yielded (R)-phenylethanol with poor enantioselectivity (14% ee),110 replacement of the methoxy group with hydrogen (H-MOP, (36d)) affords the same product with 93% ee (Equation (ll)) 111... 

A summary of the ligand effects of the palladium-catalyzed enantioselective hydrosilylation of styrene is given in Table 9. 

Table 9 Palladium-catalyzed asymmetric hydrosilylation of styrene.

Vinyl- or arylboronic acids also react with allenes, affording 1,3-dienes or styrene derivatives, respectively.89 This palladium-catalyzed addition proceeds with good regioselectivity and high stereoselectivity in favor of the formation of (ft)-trisubstituted isomer. 

Palladium-catalyzed addition of the selenium-silicon bond of PhSe-SiMe3 to arylacetylenes proceeds in a regio- and stereoselective manner to afford (Z)-a-(phenylseleno)-/ -(trimethylsilyl)styrenes (Equation (123)).250 Aliphatic alkynes fails to undergo the addition reaction. Analogous addition of the Se-Ge bond to alkynes occurs under similar conditions. 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

The palladium-catalyzed asymmetric hydrosilylation of styrenes has been applied to the catalytic asymmetric synthesis of l-aryl-l,2-diols from arylacetylenes (Scheme 6).46 Thus, ( )-l-aryl-2-(trichlorosilyl)ethenes, which are readily generated by platinum-catalyzed hydrosilylation of arylacetylenes, were treated with trichlorosilane and the palladium catalyst coordinated with MOP ligand 12f to give 1 -aryl-1,2-bis(silyl)ethanes, oxidation of which produced the enantiomerically enriched (95-98% ee) 1,2-diols. 

Table 2 Palladium-catalyzed asymmetric hydrosilylation of substituted styrenes 25 with trichlorosilane Substrate 25...

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

Using quantum chemical molecular modelling tools we have examined the reaction mechanism of palladium catalyzed hydrosilylation of styrene by the precatalyst system, 1, developed by Togni and co-workers. One fundamental question that we have focused on is whether the reaction proceeds by the classical Chalk-Harrod mechanism or by an alternative mechanism such as the modified-Chalk-Harrod mechanism. In this section, the general features of the catalytic cycle are examined. 

In this and subsequent sections, we investigate the reaction mechanism of the palladium catalyzed hydrosilylation of styrene via ah initio molecular dynamics and combined quantum mechanics and molecular mechanics (QM/MM) techniques. Both methodologies constitute powerful approaches for the study of the catalytic activity and selectivity of transition metal... 

A detailed study of the mechanism of the enantioselective palladium catalyzed hydrosilylation of styrene with trichlorosilane was carried out with combined QM/MM ab initio molecular dynamics simulations. A number of fundamental mechanistic questions have been addressed, including the main features of the catalytic cycle, as well as the specific nature of the regioselectivity and enatioselectivity. 

In the second series of experiments, O. Nuyken et al. investigated the Heck reaction of iodobenzene with styrene as a model for the numerous palladium-catalyzed coupling reactions (Scheme 6.6) [54]. In the literature only a few Heck coupling reactions of hydrophobic substrates in aqueous solution are known. The most detailed study has been performed by Jeffery et al. on the model reaction of iodobenzene with methylacrylate in the presence of tetrabutylammonia salts. Product yield was at least 95% after 2 h reaction time at 50 °C in pure aqueous solution [55], however more... 

Much less attention has been focused on carbonylation reactions in ionic liquids. The biphasic palladium-catalyzed alkoxy carbonylation of styrene. Scheme 2, in [bmim][BF4]—cyclohexane has been reported. ... 

Heck in one of his first papers already demonstrated the feasibility of applying the palladium-catalyzed crosscoupling of aryl and alkenyl halides with alkenes repetitively on appropriate oligofunctional substrates. For example, twofold coupling of 1,4-diiodobenzene with styrene furnished 1,4-distyrylbenzene in 67% yield (Scheme 1). Since then, a large number of ortho-, meta-, and / r< -dihaloarenes and -heteroarenes have been subjected to twofold Heck reactions with various alkenes (Schemes 2-4). 

The palladium-catalyzed coupling of aryl iodides with vinylstannanes (Stille coupling) leads to the formation of styrenes. With resin-bound vinylstannanes, this reaction can be conducted in such a way that simultaneous detachment from the support of the newly formed styrenes occurs. This has been realized intramolecularly in the preparation of macrocyclic lactones (Entry 4, Table 3.43). The required resin-bound vinylstannanes were prepared either by hydrostannylation of alkynes with a resin-... 

A number of methods are now available for the palladium-catalyzed arylation of alkenes. Mizoroki et al. showed that, in the presence of PdCl2, iodobenzene reacted with ethylene to give styrene.623 These reactions require a stoichiometric amount of base to neutralize the hydriodic acid formed (equation 172). 

Aryl chlorides Aryl chlorides will substitute alkenes only under very special conditions, and then catalyst turnover numbers are generally not very high. Palladium on charcoal in the presence of triethylphos-phine catalyzes the reaction of chlorobenzene with styrene,58 but the catalyst becomes inactive after one use.59 Examples employing an activated aryl chloride and highly reactive alkenes, such as acrylonitrile, with a palladium acetate-triphenylphosphine catalyst in DMF solution at ISO C with sodium acetate as base react to the extent of only 51% or less.60 Similar results have been reported for the combination of chlorobenzene with styrene in DMF-water at 130 C, using sodium acetate as the base and palladium acetate-diphos as a catalyst.61 Most recently, a method for reacting chlorobenzene with activated alkenes has been claimed where, in addition to the usual palladium dibenzilideneacetone-tri-o-tolylphosphine catalyst, nickel bromide and sodium iodide are added. It is proposed that an equilibrium concentration of iodobenzene is formed from the chlorobenzene-sodium iodide-nickel bromide catalyst and the iodoben-zene then reacts in the palladium-catalyzed alkene substitution. Moderate to good yields were reported from reactions carried out in DMF solution at 140 C 62... 

Mukhopadhyay, S., Rothenberg, G., (oshi, A., Baidossi, M. and Sasson, Y. (2002) Heterogeneous palladium-catalyzed Heck reaction of aryl chlorides and styrene in water under mild conditions. Adv. Synth. Gated., 344, 348. 

Asymmetric hydrocarboxylation of styrenes.1 Use of (S)- or (R)-l as a chiral ligand in the palladium-catalyzed hydrocarboxylation of p-isobutylstyrene (2) results in (S)- or (R)-2 (ibuprofen) in 83-84% ee. Similar enantioselectivity obtains in hydrocarboxylation of a 2-vinylnaphthalene to form naproxen. 

It was shown earlier that palladium-catalyzed hydrovinylation of styrene using phosphino ester-type ligands leads to isomerization of the external al-kene (kinetic product) to the internal alkene (thermodynamic product) at higher substrate conversion. In this regard, the idea was to suppress this isomerization by running the reaction at lower conversion in a CFMR system in order to minimize the catalyst-substrate contact time. 

Scheme 5 Schematic synthetic pathways and structure of dendritic P,0 ligands applied in the palladium-catalyzed hydrovinylation of styrene...

Polymers containing chromophores such as acridine and azobenzene have also been prepared by palladium-catalyzed amination according to two approaches [222-224]. The first approach involved the polymerization of monomers containing the chromophore. For example, 4-aminoazobenzene was condensed with 1,3-dibromobenzene (Eq. (39)) or 4,4 -dibromobiphenyl ether to form polymeric materials with Mw values of 9.0 x 103 and 19 x 103, respectively. Alternatively, polymers prepared by polymerization of 4-bromostyrene or copolymerization of styrene and 4-bromostyrene were coupled with N-phenyl-4-amino azobenzene. Substitution of the aryl bromides by the amino azobenzene unit was essentially quantitative when using P(tBu)3 as the ligand. 

Aryl tellurium trichlorides arylated styrene and other olefins in palladium-catalyzed reactions. The arylated olefins were obtained in low yields. Elemental tellurium precipitated during the reactions7. 

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