Phosphine-catalyzed System

Carboxylic nitrile liquid and solid elastomers are used to prepare elastomer-modified epoxy liquid and solid resins when it is desirable to have the modifier in the epoxy portion of the system. This is effected through alkyl-hydroxy esterification reactions. This is covered in the literature for uncatalyzed liquid resins (9), for tert-amine catalyzed systems (10), for tert-phosphine catalyzed systems (11), for low molecular weight solid resins advanced from the liquid state (12) and for liquid and solid resins where an additional rubber vulcanization step is carried out in addition to the alkylhydroxy ester adducting step (13). Such adduct preparations offer formulation stability with a wide range of hardener types amines, anhydrides, catalytic, Lewis acids/bases. 

An analogous nickel catalyzed coupling was also reported recently, where 3-, and 4-cyanopyridine were coupled with different ethynylzinc derivatives in the presence of a nickel-phosphine catalyst system (7.43.),61 Although this reaction is not a Sonogashira coupling, it constitutes an efficient alternative approach to ethynylpyridines. It is also interesting to... 

In 2001, Oi et al. [54] reported on the ruthenium(II) phosphine catalyzed re-gioselective arylation of 2-arylpyridines using aryl halides (Eq. 29). C-C bond formation occurs predominantly at the position ortho to the pyridyl group. The same catalyst system is also effective for the arylation of aromatic imines (Eq. 30) [55]. Although the reaction mechanism has not been elucidated, it was proposed that a tetravalent arylruthenium complex,for example,Ru(Ph)(Br)(Cl)2(I) ,reacts electrophilically with the arylimines. Therefore, C-H bond cleavage is believed to proceed via an electrophilic substitution pathway. 

In order to address the issue of phosphine-derived by-products in nickel-catalyzed systems, a number of researchers have investigated the use of alternative ligands to promote cross-coupling reactions. In particular, Herrmann... 

Trost and co-workers found that the Pd(OAc)2 tris(2,6-dimethoxyphenyl)phosphine (TDMPP) system catalyzes the homodimerization of terminal alkynes 1 to produce conjugated enynes 2 (Scheme This method is very simple and efBcient reactions are... 

Even though aminophosphine- and phosphite-based pincer complexes, as well as other systems such as SCS-based pincer-type Heck catalysts, were shown to serve as stable and clean sources of palladium nanoparticles in Heck reactions [24a, 25], and also xylene-derived phosphine-based systems were found to decompose under Heck reaction conditions in the presence of organic bases and hence palladium nanoparticles generally are considered to be the catalytically active form of palladium pincer Heck catalysts [27], catalytic cycles with the involvement of Pd intermediates could not have been excluded to be operative in pincer-catalyzed versions of the Heck reaction. In contrast, experimental observations, such as halide exchange reactions... 

Various a-formyl esters can be generated from the hydroformylation of a,P-unsaturated esters and diesters. Ethyl 2-formylpropanoate 33 can be obtained from the hydroformylation of ethyl acrylate 32. The reaction was catalyzed by the Rh2Cl2(CO)2/phosphine/NEt3 system at 20-40 °C in the presence of H2/CO (1/1, 20 atm), and yielded the product with excellent regioselectivity (98-100%). Particularly effective ligands for this reaction include l,4-bis(diphenylphosphino)butane (DPPB) 34, phosphole (o-TDPP) 35, and phosphanorbomadiene (DMTPPN) 36. 

The important discovery by Wilkinson [1] that rhodium afforded active and selective hydroformylation catalysts under mild conditions in the presence of triphenylphosphine as a hgand triggered a lot of research on hydroformylation, especially on hgand effects and mechanistic aspects. It is commonly accepted that the mechanism for the cobalt catalyzed hydroformylation as postulated by Heck and Breslow [2] can be apphed to phosphine modified rhodium carbonyl as well. Kinetic studies of the rhodium triphenylphosphine catalyst have shown that the addition of the aUcene to the hydride rhodium complex and/or the hydride migration step is probably rate-limiting [3] (Chapter 4). In most phosphine modified systems an inverse reaction rate dependency on phosphine ligand concentration or carbon monoxide pressure is observed [4]. 

In 2001, Mori and coworkers reported a nickel-NHC-catalyzed system for the reductive coupling of aldehydes and 1,3-dienes that had distinct properties from a nickel-phosphine based system. Both systems generated the homoallylic silyl ether however, use of phosphines resulted in selectivity, while using an NHC reversed selectivity and resulted in the Z isomer (Figure 13.42) [94]. 

A digital functional approach has been employed to investigate important steps in the Heck reaction catalyzed by a bis(carbene)Pd complex and one in which the Pd is coordinated by a bidentate carbene-phosphine ligand. The crucial steps of olefin insertion into the palladium-aryl bond and / -hydride elimination were investigated. For the bis(carbene)Pd catalyst, a mechanism was proposed, which proceeds via halide abstraction, to give a cationic species, prior to olefin coordination and insertion. The total insertion/elimination process was found to be exothermic (—8.9 kcal moP ). For the carbene-phosphine ligated system, the vacant site for olefin coordination was provided by phosphine dissociation. The energetics for the total insertion/elimination process was very similar to that of the bis-carbene system. 

NHQRh(COD)Cl] complexes are active catalysts for various cyclizations involving alkenes and alkynes. In 2005, Evans et al. showed that [(IMes)Rh (COD)Cl] catalyzed the diastereoselective [4 + 2 + 2] carbocyclization of 1,6-enynes in the presence of 1,3-butadiene and silver tiiflate (Equation (8.10)). Classical phosphine/rhodium systems generally displayed lower activities. For example, the use of the electron-rich tiicyclohexylphosphine gave only 30% yield, demonstrating that strong a-donation is crucial for the catalytic turnover in this carbocyclization. 

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