Transition metal complexes catalyzed

Monodentate dipolarophiles such as acrolein, methacrolein, and a-bromoacrolein could be successfully utilized in the l ,J -DBFOX/Ph-transition metal complex-catalyzed asymmetric nitrone cycloadditions [76]. The reactions of N-benzylideneani-line N-oxide with acrolein in the presence of the nickel(II) aqua complex R,R-DBF0X/Ph-Ni(C104)2 3H20 (10mol%) and MS 4 A produced a mixture of two regioisomers (5-formyl/4-formyl regioisomers ca 3 1). However, enantio-... 

A variety of transition metal complexes catalyze hydrosilylation of alkynes. Catalysis of hydrosilylation by rhodium gives T -alkenylsilanes from 1-alkynes.74... 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

Kondo, T., Uenoyama, S.-y., Fujita, K.-i., and Mitsudo, T.-a., First transition-metal complex catalyzed addition of organic disulfides to alkenes enables the rapid synthesis of vicinal dithioethers, /. Am. Chem. Soc., 121, 482, 1999. 

Transition metal complex-catalyzed carbon-nitrogen bond formations have been developed as fundamentally important reactions. This chapter highlights the allylic amination and its asymmetric version as well as all other possible aminations such as crosscoupling reactions, oxidative addition-/3-elimination, and hydroamination, except for nitrene reactions. This chapter has been organized according to the different types of reactions and references to literature from 1993 to 2004 have been used. 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

Transition Metal-Complex Catalyzed Hydrogenation of Carbonyl Compounds... 

Thus far, we have discussed the transition metal complex-catalyzed hydrogenation of C=C, C=0, and C N bonds. In this section, another type of transition metal complex-mediated reaction, namely, the hydroformylation of olefins, is presented. 

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. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

Ethers, sulfides, amines, carbonyl compounds, and imines are among the frequently encountered Lewis bases in the ylide formation from such metal carbene complex. The metal carbene in the ylide formation can be divided into stable Fisher carbene complex and unstable reactive metal carbene intermediates. The reaction of the former is thus stoichiometric and the latter is usually a transition metal complex-catalyzed reaction of a-diazocarbonyl compounds. The decomposition of a-diazocarbonyl compounds with catalytic transition metal complex has been the most widely used approach to generate reactive metal carbenes. For compressive reviews, see Refs 1,1a. 

Transition metal complex catalyzed reactions exhibit different outcomes depending on whether they are conducted in water or not. Chao-Jim Li highlighted the greening of chemical reactions, focusing on the use of water as an alternative solvent for reactions involving metals, R = CH2Ph (Li, 1996) (see fig. 6.8). 

The transition metal-complex-catalyzed carbonylation of benzyl halides to yield phenylacetic acids is an extensively studied reaction [Eq. (7)]. 

Certain transition metal complexes catalyze the decomposition of diazo compounds, where the metal-bound carbene intermediates behave differently from the free species generated by their photolysis or thermolysis. 

Noncarbonyl transition metal complexes catalyze dimerization and aromatic cyclo-trimerization of ethynylcyclopropane. The product composition depends on the catalyst and the reaction conditions. Thus, Co(acac)2 in the presence of phosphines and AIEt2Cl afforded either the dimer 1,3-dicyclopropyl-1 -butyn-3-ene or a mixture of 1,2,4- and 1,3,5-tris(cyclopropyl)benzenes, whereas Pd(0 Ac)2 gave the same dimer in the presence of PPh3 but only a tris(cyclopropyl)fulvene in the absence of phosphines (equation 176)245. 

The transition metal complex-catalyzed formation of 1,3-dioxepanes from vinyl ethers has also been described. For example, reaction of allyl vinyl ether 157 with a nonhydridic ruthenium complex at higher temperatures and without any solvent produced 1,3-dioxepane 159 whereas, the use of a hydridic ruthenium complex resulted in the formation of vinyl ether 158 by double-bond isomerization (Scheme 43). It was suggested that cyclic acetal formation proceeds via a 7i-allyl-hydrido transient complex, which undergoes nucleophilic attack of the OH group at the coordinated Jt-allyl <2004SL1203>. 

Other transition metal complexes catalyze interesting cocyclization of alkynes and alkenes by a completely different mechanism. Efficient 671-clcctrocyclization of the... 

Fig. 4. Chalk-Harrod mechanism for the transition metal complex-catalyzed hydrosilation of olefin.

Several comprehensive reviews on the preparation, structure, properties, photochemistry, and redistribution of polysilanes prepared by the Wurtz coupling method have been published.1516 31 33 There are also several excellent reviews on the transition-metal-complex-catalyzed dehydrocoupling of hydrosilanes to polysilanes.34 36 The present section reviews the more recent work on the dehydropolymerization of hydrosilanes to polysilanes, catalyzed by transition-metal complexes under homogeneous conditions. Extensions of the methodology to the dehydropolymerization of hydrogermanes, hydrostannanes, and hydrophosphanes to the corresponding polymers are also reviewed. The literature is covered up to early 1997. 

Two factors are important in determining the relative values of kd and fce-the oxidation potential of the catalyst and its Lewis acidity. In general, the ease with which transition metal complexes catalyze the decomposition of hydroperoxides is related to their redox potentials (see Table V). Hydroperoxides are strong oxidants but weak reducing agents. Hence, reaction (312) is the slower,... 

A transition metal complex catalyzes the trimerization of ethylene to give 1-hexene. Draw a catalytic cycle like the one shown in Fig. 6.3 for this reaction. 

There has been a mutual fertilization between transition-metal complex-catalyzed interactions of silanes and olefins and attempted syntheses of silicon-transition-metal compounds. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

Transition metal complexes catalyze desulfurization of thiiranes. Complexes of Rh <1986TL3573>, Re <1997TL7701, 1999CC1003>, and Mo <2003JA3871> have been successfully employed in catalytic quantities. The presence of a stoichiometric sulfur acceptor such as carbon monoxide or an alkene, phosphine, or arsine is required. In Re-catalyzed desulfurization, the formation of an intermediate Re =S species via initial coordination of the thiirene ligand has been suggested (Schemes 12 and 13) <1997TL7701>. Kinetics and mechanism of sulfur... 

As chiral ligands for transition metal complex-catalyzed asymmetric reactions, a variety of novel chiral ferrocenylchalcogen compounds, which possess a planar chirality due to the 1,2-unsymmetrically disubstituted ferrocene structure, have been prepared from chiral ferrocenes (Scheme 1). Thus, chiral diferrocenyl dichalcogenides bearing an optically active dimethylaminoethyl or p-tolyl-sulfoxide moiety 1-10 were prepared by lithiation of the corresponding chiral ferrocenes, highly diastereoselectively, in moderate to high chemical yields. 

Several Ru-based transition metal complexes catalyze the hetero Diels-Alder reaction between aldehydes, in particular benzaldehyde and Danishefsky s diene. Using the [Ru(Cp)(CHIRAPHOS)] (18) complex, a modest e.e. value of 25% is obtained (Entry 1, Scheme 10.25) [48]. This reaction is also catalyzed by irradiating the chiral complex (3) in the presence of the diene and the hetero-dienophile. The product is obtained with a good chiral induction (Entry 2, Scheme 10.25) [49, 50],... 

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]. 

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