Rh- and Pd-complexes

A review8 with more than 186 references discusses the synthesis of Rh and Pd complexes with optically active P,N-bidentate ligands and their applications in homogeneous asymmetric catalysis. The effect of the nature of the P,N-bidentate compounds on the structure of the metal complexes and on enantioselectivity in catalysis was examined. Allylic substitution, cross-coup-ling, hydroboration and hydrosilylation catalyzed by Rh or Pd complexes with optically active P,N-bidentate ligands are considered. 

In fact, a variety of hansihon metals have been used as efficient catalysts in a range of cycloaddihon reachons. Among these, the Co, Ni, Ru, Rh and Pd complexes have been the major players, whilst the Ir complexes have played only a minor role. Nonetheless, several Ir-catalyzed cycloadditions have been recently reported, which cannot be realized by other metal complexes. This chapter summarizes Ir complex-catalyzed cycloadditions, which include several types of cycloi-somerizahon and cycUzahon. 

As the bulk of the substituents on silicon increases, the shorter become the chains in the coupled product, to the limit of disilane. This limit occurs for both primary and secondary silanes. One difference between the metallocene (Group 4) promoted dehydrocoupling reactions and that of metals such as Rh or Pt, is the difficulty in the condensation of dialkylsilanes from the former grouping. Dialkylsilanes condense in the presence of metallocene catalysts only if a hydrogen acceptor (such as an olefin) is present. An interesting exception to the condensation of a secondary silane center comes from the reactions of hydrogen terminated disilanes such as 1,2-dimethyldisilane which forms polymers from both metallocene promoted reactions as well as with Rh and Pd complexes. 

Materials Based on Rh and Pd Complexes with jt-Bonding Ligands (Phosphines and Dienes)... 

Other electrophilic systems investigated include Pd"-acetate mediated oxidation of CH4 to CF3C02CH3 in trifluoroacetic acid by Sen, including a catalytic version of this reaction using H202 as oxidant.149 Direct carboxylation of alkanes including CH4 can be catalyzed by Rh and Pd complexes 150... 

In chemistry developed particularly by Halcon (37) ethylidene diacetate production is catalyzed by both Rh and Pd complexes in the presence of iodide promoters. In addition, an organic promoter (tertiary phosphine or N-base (pyridine or amine)) is required for catal3d ic activity under rather higher pressures than those used for methyl acetate carbonylation, namely, 140 bar CO/H2 (1 2) and 150° C Cr complexes are also found to have an important promotional effect. Se-lectivities of up to 50% ethylidene diacetate have been claimed. However, none of this chemistry has progressed toward commercialization beyond the pilot plant stage, possibly because of the requirement for extensive acetic acid recycle in the carbonylation reactor (see also the preceding section ). 

The mode of rearrangement of 1,8-bishomocubanes (1) catalysed by Rh and Pd complexes depends, not only on the metal and the ligands attached, but also on the level of substitution at the C-4 and C-5 positions of the carbocyclic species. Both snoutanes and the exo- and e Steric effects are significant and since the endo- som r (8) of the diene is formed preferentially in reactions of 9,10-disubstituted compounds, attack by the metal from the direction syn to the 9,10-substituted position must occur in most cases. 

Mikami s groups have already demonstrated that chirally flexible 2,2 -bis(diphenylphosphanyl)biphenyl (biphep) can act as a chiral ligand for Ru, Rh, and Pd complexes through axial chirality control by chiral diamines [25]. Recently, they reported that the racemic Pt complexes, even with chirally flexible biphep ligands, can be resolved as the atropos complex at 50 °C but converted at higher temperatures into enantiopure [26]. The complex (76)-(S) was obtained with (R)-2,2 -bis (trifluoromethanesulfonylamino) -1,1 -binaphthyl(dabn)Tf, and (76) -(R) was obtained with (R)-dabn by the addition of aqueous HCl. In carbon-carbon... 

Rh + and Pd " complexes are easily reduced on bonding to macroligands, especially in alcoholic solution, for a variety reasons. For example, if Pd cations are complexed to a Nylon-66 fiber, immobilization on the polymer powder is accompanied by partially formed Pd even in the step that involves macrocomplex formation. One of the causes is probably rapid oxidation of Pd to Pd " involving CuJ used as a stabihzer in polymer fiber fabrication. Thus redox processes in such systems can be either stimulated or inhibited using particularly a purposeful selection of reagents, solvents and reaction conditions. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

Methanoic acid and methanoates were amongst the most effective donors used for reduction of alkenes in aqueous solution, using Rh, Ru, Pt, and Pd complexes of (115), the most active catalyst being [RuCl2(115)2].344... 

Similar studies with the bromo- and iodo-analogues showed the reactivity order I > Cl > Unlike corresponding Rh and Pt" complexes, K2[Pd-... 

Akita and coworkers established a dehydrative condensation procedure, starting from hydroxo metal precursors containing the hydrotris(3,5-diisopropylpyrazolyl) borato ligand, and they were able to obtain dinuclear (/r-peroxo)Pd complexes, as indicated in Scheme 4. With the same procedure peroxo and hydroperoxo species (for Pd and Rh) and alkyl peroxides (for Mn, Co, Ni and Pd) complexes may be obtained. 

Alkenes and Alkynes. A series of metal carbonyl cations, such as [Au(CO)n]+,287,288 [c-Pd( i-CO)2]2+,289 [Rh(CO)4]+,290 and [(Pt(CO)3)2]2+,291 was found to induce the formation of carboxylic acids from alkenes and CO in the presence of H2SO4 under mild conditions. A novel water-soluble Pd catalyst292 and Pd complexes of calixarene-based phosphine ligands293 showed high activity in the regioselective carboxylation of vinyl arenes to yield 2-arylpropionic acids or... 

These were prepared by tethering Rh and Pt complexes to silica-supported metal catalysts (metal = Pd, Ni, Ru, Au). The catalysts are very active in the hydrogenation of benzene derivatives to the corresponding substituted cyclohexanes under mild conditions. The activities are higher than those of the separate homogeneous complexes, complexes just tethered to silica, or the silica-supported heterogeneous catalysts. When the sol-gel-entrapped [Rh2Co2(CO)12] complex was heat-treated at 100°C, immobilized metallic nanoparticles were formed.425 The catalyst thus prepared efficiently catalyzed substituted benzene derivatives. 

In the case of Ir the analogous [Ir(ju-RNYNR)(CO)2]2 was prepared from IrCl(CO)3 and Li RNC(H)NR. The dimeric Rh and Ir complexes probably have structures similar to [Pd( /3-allyl)(RN3R)]2 (vide infra).60,61 The carbonyl groups can be substituted by PPh3 and also by dienes such as cod and nbd giving complexes, e.g. of the composition [Rh RN3R nbd]2.58... 

Rh and Pd-catalysed Reactions of Diazo Compounds via Electrophilic Carbene Complexes... 

Various monocomponent catalysts based on nickel compounds, such as Ni(All)2, Ni(All)Cl, Ni(All)OCOCF3 and Ni(Cod)2, have been used for the polymerisation of allene crystalline polymers of rather high molecular weight were yielded, but they were characterised by lower structural regularity as a result of diads made up of 1,2- and 2,1-linked monomeric units [438,441, 442]. Complexes of other transition metals, especially m-Rh(CO)2ClPPh3, Co(A11)3, Co2(CO)8 (in hydrocarbon solutions) and Pd complexes (in acetic acid solutions) are also active in allene polymerisation [241]. 

A ligand of a metal complex is one of the most appropriate templates for a molecular-imprinted metal-complex catalyst. Several ligands have been reported as candidates because of their analogy to transition states or reaction intermediates for target reactions [51-64], Several metal complexes with single-site Co, Cu, Zn, Ti, Ru, Rh, and Pd species have been used as active metal sites coordinated with template ligands (Table 22.1). Acrylate polymers [54, 55, 60, 63, 64] or polystyrene-divinylbenzene (DVB) polymers [51, 53, 56] are common polymer supports for molecularly imprinted catalysts. 

Pd, Pt, Rh, and Ru complexes were used as catalysts for the hydrogenation of alkenes with molecular hydrogen. In many cases, higher activity and enhanced selectivity for the desired reaction were accompanied hy successful re-use of the ionic liquid and the catalyst. Examples are reported for cyclohexadiene [10],... 

In the first reported direct A -carbonylation of nitroaromatics to isocyanates, simple Pd- or Rh-based systems were used to catalyze the reaction of aromatic mononitro compounds with carbon monoxide [11, 12]. Later, it became possible to work without the drastic reaction conditions that had been required initially, by using Lewis acid co-catalysts [13], Various catalysts and catalyst mixtures, normally based on Ru, Rh, or Pd complexes with co-catalysts, were described in numerous patents and publications [1, 3, 14—16], The careful choice of the composition of the triad consisting of metal salt, co-catalysts and ligand (preferably aromatic amines) led to efficient catalyst systems [14a-e] for the direct reductive carbonylation process. A quite active Pd-phenanthroline-H system with noncoordinating carboxylic acids such as 2,4,6-trimethlybenzoic acid as proton source is worth mentioning [14 d]. 

The use of transition metal species can lower appreciably the decomposition temperature of ot-diazo-carbonyl compounds they can also alter the reactivity of the carbene intermediate (resulting from the initial nitrogen elimination see Section 3.9.2.1) by complex formation. Hence, the Wolff rearrangement may occur with difficulty or, usually, not at all. Thus, some copper species (excepting, for example, Cul), or Rh and Pd catalysts are inappropriate. Freshly prepared silver(I) oxide has been used most frequently, but silver salts (especially silver benzoate) are sometimes preferred.Silver-based catalysts are usually employed in combination with an alkaline reagent e.g. sodium carbonate or a tertiary amine). Even under silver catalysis competing reactions may be observed, and sometimes the products of Wolff rearrangement may not be obtained (see Section 3.9.2.3). 

Triphenylphosphine complexes of Ru, Rh, and Pd continue to play a crucial role in homogeneous catalysis for a wide range of transformations. Palladium complexes with PPh3 are extremely useful in organic synthesis.112,113... 

---