Phosphine, in palladium-catalyzed

In a related study, Srivastava and Collibee employed polymer-supported triphenyl-phosphine in palladium-catalyzed cyanations [142]. Commercially available resin-bound triphenylphosphine was admixed with palladium(II) acetate in N,N-dimethyl-formamide in order to generate the heterogeneous catalytic system. The mixture was stirred for 2 h under nitrogen atmosphere in a sealed microwave reaction vessel, to achieve complete formation of the active palladium-phosphine complex. The septum was then removed and equimolar amounts of zinc(II) cyanide and the requisite aryl halide were added. After purging with nitrogen and resealing, the vessel was transferred to the microwave reactor and irradiated at 140 °C for 30-50 min... 

Manger CC, Mignani G. Synthetic applications of Buchwald s phosphines in palladium-catalyzed aromatic-bond-forming reactions. Aldrichimica Acta 2006 39 17-24. 

The use of palladium(II) 7i-allyl complexes in organic chemistry has a rich history. These complexes were the first examples of a C-M bond to be used as an electrophile [1-3]. At the dawn of the era of asymmetric catalysis, the use of chiral phosphines in palladium-catalyzed allylic alkylation reactions provided key early successes in asymmetric C-C bond formation that were an important validation of the usefulness of the field [4]. No researchers were more important to these innovations than Prof. B.M. Trost and Prof. J. Tsuji [5-10]. While most of the early discoveries in this field provided access to tertiary (3°) stereocenters formed on a prochiral electrophile [Eq. (1)] (Scheme 1), our interest focused on making quaternary (4°) stereocenters on prochiral enolates [Eq. (2)]. Recently, we have described decarboxylative asymmetric allylic alkylation reactions involving prochiral enolates that provide access to enantioenriched ot-quatemary carbonyl compounds [11-13]. We found that a range of substrates (e.g., allyl enol carbonates,... 

As mentioned in Sect. 3.1.1, secondary phosphine-boranes also react efficiently with aryl iodides in palladium-catalyzed substitution reactions (Pd(PPh3)4) [73]. In all cases the boranato functional group remains unchanged. 

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

Fig. 6 Schematic representation of the Meijer-Reek noncovalently immobilized phosphine ligands applied in palladium-catalyzed allylic aminations...

Scheme 11 Van Leeuwen s core-functionalized ferrocenyl-phosphine carbosilane den-drimers used as ligands in palladium-catalyzed allylic alkylation reactions...

Palladium and platinum form numerous alkyl complexes of the types M(R)(X)L2 and MR2L2, usually stabilized by phosphine ligands. The palladium compounds are prone to reductive elimination this is important in palladium catalyzed cross-coupling reactions (Section 21-14) ... 

Ligand size determines coordination numbers as well as reactivity. Thus phosphine complexes of Pd°, frequently used as precursors in palladium catalyzed reactions, may be 4-, 3-, or 2-coordinate, as in Pd(PMe3)4, Pd(PPr )3, and Pd(PPhBu 2)2, respectively. For nickel phosphine and phosphite complexes, the dissociation constant Kd for the equilibrium... 

Some of the most significant improvements in catalyst development were the discoveries of alternative palladium ligands made by Farina in the early 1990s. In particular, tri(2-ftuyl)phosphine (15) and triphenylarsine proved to be far superior to triphenylphosphine when used as ligands in palladium-catalyzed Kosugi-Stille reactions. Perhaps... 

Tom and coworkers published three new types of phosphines derived from diarylsul-fones, tritylimidazole or 2-bromobenzophenone (Figure 13). Their activity in palladium-catalyzed amination of aryl bromides is not general, but they do catalyze the coupling of certain substrates. For example, the coupling of morpholine with l-bromo-4-nitrobenzene (97% with 47), 4-bromo-f-butylbenzene (81% with 48) and 4-bromoanisole (96% with 49) was achieved in high yields160. 

The tetratertiary phosphine (110) has been successfully used in palladium-catalyzed allylic substitution reactions,226 while dinuclear palladium complexes of (105) catalyze the electrochemical reduction of C02 to CO in acidic DMF solutions.236... 

Application of Sterically Demanding Phosphine Ligands in Palladium-Catalyzed Cross-Coupling leading to C(sp )—E Bond Formation (E = NH, OH, and F)... 

Phosphates and phosphinates are also recommended as ligands. (R)- or (S)-Binaphthol phosphates 3.54 are used in palladium-catalyzed asymmetric hydro-carboxylation of olefins [923] or in rhodium-catalyzed cycloadditions of diazo compounds to olefins, albeit with modest selectivities in the latter case [924], Seebach and coworkers [925] tested phosphinates and phosphites prepared from diol 2.50 (R = R = Me, Ar = Ph) as ligands for rhodium and palladium in various enantioselective metal-catalyzed reactions [925], Rhodium-catalyzed hydrosilyla-tions of arylmethyl- or ethylketones by Ph2SiH2 were the only interesting reactions with these ligands. 

Transition metal catalyzed processes are useful tools for the synthesis of functionalized thiophenes. Thus for instance, a phosphine-free, palladium catalyzed coupling protocol for the synthesis of 2-arylbenzo[d]thiophenes from various 3-substituted benzo[6]thiophenes and aryl bromides or iodides has been reported <04T3221>. Likewise, 2,2 -bithiophenes have been 5,5 -diarylated directly with aryl bromides in the presence of Pd(OAc)2, bulky phosphine ligands and CS2CO3 <04T6757>. A series of electron-deficient and relatively electron-rich benzo[6]thienyl bromides have been shown to participate in palladium catalyzed amination reactions, as exemplified by the interesting conversion of 63 to the tetracyclic system 64 upon reaction with 2-aminopyridine 65 <04EJO3679>. 

Activated methylene compounds such as dimethyl malonate have found substantial utility in palladium catalyzed allylic substitution reactions. Accordingly, the Krapcho decarboxylation is often used in conjunction with these reactions. As an example, the first total synthesis of enantiomerically pure (-)-wine lactone has utilized the sequence of reactions.27 First, the allylic substitution reaction of 2-cyclohexen-l-yl acetate (49) with alkali sodium dimethylmalonate yielded 51 with high enantioselectivity, as a result of the use of chiral phosphine ligand 50. The malonate was then subjected to Krapcho decarbomethoxylation using NaCl, H2O, and DMSO at 160 °C to yield 52. This reaction has been used similarly following the allylic substitution reaction with other malonate derivatives.28-30... 

Path (b) is the direct path, simply taking place from a four-coordinate, 16-electron complex. Many intermediates presumed in palladium-catalyzed reactions are considered to follow this path. On the other hand, in path (c), reductive elimination is induced by association with external ligands such as olefins and phosphines. This path has been observed mainly for nickel complexes. 

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