Reduction metal complexes catalyzed

In conclusion, phase transfer catalysis is a method of considerable potential for metal complex catalyzed reduction, oxidation and carbonylation reactions. 

The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154-159]. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [160]. Stable, uncomplexed imidazoline-2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [162-166]), are also convenient starting materials for the preparation of carbene complexes [167,168]. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169-171]. Examples of such preparations are given in Table 2.8. 

In different models for cytochrome P-450, Zn,152 Zn amalgam,153 and sodium ascorbate154 are used as the reductant. In contrast with all the abovementioned metal complex-catalyzed oxidations, the Mn(TPP)Cl-ascorbate system oxidizes alkanes with predominant formation of ketones.154 Certain complexes, however,... 

The reactions of nitric oxide involve either oxidation or reduction (or both simultaneously in disproportionation and decomposition). Except for oxidation, these reactions of nitric oxide require catalysts for them to proceed at significant rates. An important stimulus to studying these catalyzed reactions lies in the environmental hazards posed by oxides of nitrogen, of which NO is the parent member, as discussed at the beginning of this section. Particular attention in the area of metal complex catalyzed reactions has focused in the last five years on the reduction of nitric oxide by carbon monoxide, (113). 

Organic synthesis via transition metal complex-catalyzed electrochemical and photochemical reduction of CO2 has been developed [2,122b, 145-147]. Among transition metal complexes, ruthenium bipyridine complexes show high catalytic activity a typical reaction is shown in Eq. 11.79. [Ru(bpy)2(CO)2] and [Ru(bpy)2(CO)Cl] efficiently catalyze the electrochemical reduction of CO2 to CO and HC02. The nature of the products is dependent upon the pH of the solution. A catalytic cycle involving [Ru(bpy)2(CO)]°, ]Ru(bpy)2(C0)(C02 )] and [Ru(bpy)2(C0)C02H] was proposed (Eq. 11.79) [1461]. 

The idea that transition-metal complexes can act as PT agents was presented in some early studies of metal-complex-catalyzed reactions under PTC conditions. Lately, several approaches to the design of bifunctional catalysts have been made. Organometallic complexes capable of effecting phase transfer reactions were prepared for the first time in 1982 [192, 193]. Tertiary phosphines containing polyether substituents react with Pd(PhCN)2Cl2 to afford complexes capable of catalyzing the reduction of bromobenzene with NaH suspended in toluene. 

In order to discover new modifiers in the hydrogenation of both C=0 and C=C bonds we have tested fS7-Qr,a-diphenil-2-pyrrolidinmethanol (DPPM) (Scheme 1), which was used as a ligand in the transition metal complex catalyzed enantioselective reduction of prochiral ketons, like acetophenone and pinacolone [8, 9, 10]. 

A number of metal complexes catalyze the hydrosilylation of various carbonyl compounds by triethylsilane. Stereoselectivity is observed in the hydrosilylation of ketones as in the reactions of 4-t-butylcyclohexanone and triethylsilane catalyzed by ruthenium, chromium, and rhodium metal complexes (eq 4). Triethylsilane and Chlorotris(triphenylphosphine)rho-dium(I) catalyst effect the regioselective 1,4-hydrosilylation of Q ,/3-unsaturated ketones and aldehydes. Reduction of mesityl oxide in this manner results in a 95% yield of product that consists of 1,4- and 1,2-hydrosilylation isomers in a 99 1 ratio (eq 5). This is an exact complement to the use of phenylsilane, where the ratio of respective isomers is reversed to 1 99. ... 

First, for reasons of clarity, the currently-accepted mechanism of transition-metal complex catalyzed-hydrosilylation reactions will be described briefly. Furthermore, consideration of selective, if not asymmetric, reduction of certain carbonyl compounds by way of rhodium(I)-catalyzed hydrosilylation (Section 4) is included in this review because the catalytic process and stereochemical course of this reaction correlate closely with those of their asymmetric reduction under similar conditions that will be described in the succeeding section. 

The incorporated film can catalyze Oj reduction Metal complexes of me o-... 

The utility of Na[BH4] as a reducing agent is enhanced by the presence of catalysts. For example, CgHs-C C-CeHs is not reduced by Na[BH4], but in the presence of Rh(CO)Cl3 or HRh[P(C6H5)3]3 in C6H6/C2H5OH (1 1 25 C), a 5 1 mixture of trans- to c/s-stllbene is formed with the former, and pure frans-stilbene is formed with the latter [13]. Similarly, porphyrin metal complexes catalyze the reduction of ketones with Na[BH4] in tetrahydrofuran [14]. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

After the initial two reports of Rh- and Co-catalyzed reductive aldol couplings, further studies did not appear in the literature until the late 1990s. Beyond 1998, several stereoselective and enantioselective reductive aldol reactions were developed, which are catalyzed by a remarkably diverse range of metal complexes, including those based upon Pd, Cu, Ir, and In. In this chapter, transition metal-catalyzed aldol, Michael, and Mannich reactions that proceed via transition metal hydride-promoted conjugate reduction are reviewed. 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

A tremendous amount of progress has been made over the past decade in the understanding of the catalyzed reductive coupling of unactivated alkenes and alkynes. Both early and late transition metal complexes accomplish the reaction with good yields and with low catalyst loadings. Enynes and dienes can... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

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