Catalyst zerovalent

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Pringle found that zerovalent metal complexes were active catalysts (Scheme 5-3) for the addition of PH3 to formaldehyde (see also Scheme 5-2, Eq. 1). 

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

Several nickel(II) complexes (e.g., (173)-(176)) have successfully been used to catalyze ATRP, especially when coupled with bromo-initiators, although activities are usually lower than with copper, ruthenium or iron systems.416-419 The alkylphosphine complex (175) is thermally more stable than (174) and has been used to polymerize a variety of acrylate monomers between 60 °C and 120 °C.418 Complex (176) is an unusual example of a well-defined zerovalent ATRP catalyst it displays similar activities to the Ni11 complexes, although molecular weight distributions (1.2-1.4) are higher.419 Pd(PPh3)4 has also been investigated and was reported to be less controlled than (176).420... 

A. Cyclo-Oligomerization with Zerovalent PR3/P(0R)3-Stabilized Nickel Complexes as the Catalyst... 

Scheme 7. Interplay of the C8- and C12-production channels for the cyclo-oligomerization of 1,3-butadiene with zerovalent bare nickel complexes as the catalyst. Free energies (AG, AG in kcalmol-1) are given relative to the favorable v -syn,T[l Cl) -cis isomer of 2b. 

Intermolecular bis-silylation of unactivated alkenes has been achieved initially with a zerovalent platinum catalyst such as Pt(PPh3)4 (Equations (30) and (31)).101 1,2-Difluorotetramethyldisilane undergoes addition to ethylene and norbornene in the presence of Pt(PPh3)4 catalyst at 150 °G to give the corresponding adducts in 95% and 26% yields, respectively. For the addition of 1,2-diphenyltetramethyldisilane to ethylene, Pt(PMe3)4 (33% yield) was found to be more active than Pt(PPh3)4 (4% yield). 

More recently, Dupont and coworkers studied the impact of the steric effect in the hydrogenation of monoalkylbenzenes by zerovalent nanoparticles (Ir, Rh, Ru) in the ionic liquid BMI PF6. The results, when compared with those obtained with the classical supported heterogeneous catalysts, showed a relationship between the reaction constants and the steric factors [106]. 

Ruthenium complexes are active hydrogenation catalysts for the reduction of dienes to monoenes. Both zerovalent and divalent ruthenium complexes containing various (alkene, diene and phosphine) ligands have been employed as catalysts that have met with different degrees of success. 

Palladium(O) forms a complex with quinone that is now electron rich and can be protonated to give hydroquinone and palladium(II). The latter can start a new cycle via a carbomethoxy species after reaction with methanol and CO (c.f. reaction (6), Figure 12.4). Thus we have formally switched from a hydride initiator to a carbomethoxy initiator species. Addition of quinone to a nonactive or moderately active palladium system is a diagnostic tool that tells us whether zerovalent palladium is involved as an inactive state. Likewise, one might add dihydrogen to a system to see whether palladium(II) salts need to be converted to a hydride to reactivate our dormant catalyst. 

The use of highly dehydroxylated AI2O3 and zerovalent M(CO)6 (M = Cr, Mo, W) led to highly dispersed low-valent, surface-subcarbonyl catalysts, in which the oxidation of M by the hydroxyl groups of alumina was prevented [19, 20] ... 

However, the bromide ion in [BMIM]Br or [TBA]Br has been found to become beneficial when used to stabilize the noble metal catalysts that are prone to form zerovalent nanoparticles, which could agglomerate further under reaction conditions, resulting in the loss of the catalysts 84). 

In this contribution we describe facile, high-yield syntheses of the series of zerovalent iron isocyanide complexes Fe(CO)5 (CNC6H3Me2-l,3)n ( n = 1-5). The starting material is iron pentacarbonyl, and cobalt(II) chloride is used as a catalyst to achieve the stepwise replacement of carbonyl groups by 2-isocyano-l,3-dimethylbenzene.4,9... 

HCN adds more readily to alkynes than to alkenes.179 The addition of HCN to acetylene catalyzed by Cu+ ions was once a major industrial process to manufacture acrylonitrile carried out in the presence of copper(I) chloride, NH4CI, and HC1180 (see Section 6.2.4). Zerovalent Ni and Pd complexes are effective catalysts... 

A similar approach has been used to synthesize isochroman-3-ones (80JA4193). o-Bromomethylbenzyl alcohol oxidatively adds to the zerovalent complex and carbonylation and reductive elimination of the catalyst follow. The catalyst is kept active by the presence of base which absorbs the hydrogen bromide formed in the reaction. 

Two reports have appeared on the catalyzed reaction of C02 with epoxides to form alkylene carbonates. One of the processes uses phosphine complexes of zerovalent nickel as the catalyst (157), and appears closely related to the more recent isolation of (PCy3)2Ni(C02) (115). Ethylene oxide reacts in benzene under 500 psi pressure of C02 in a stainless steel autoclave at 100°C to form ethylene carbonate with 95% selectivity, (77), using as the catalysts NiL2, L = PCy3 or PPh3. 

The copper-catalysed, Ullman-type coupling of aryl, heteroaryl and alkenyl halides may be achieved at ambient temperature using copper(I) thiophene-2-carboxylate as catalyst.60 A new semiconducting poly(anthraquinone-l,5-diyl) with nitro groups at the 4- and 8-positions has been prepared by Ullman-type coupling using metallic copper or a zerovalent nickel complex as catalyst.61... 

The Cr A and several other zeolites containing transition metal ions, which may exist in two or more valence states, were also found to be oxidation catalysts. One such system of note is the copper containing Type Y zeolite, the redox chemistry of which was studied in several recent investigations (2, 3.4, 5). These studies established the range of conditions at which copper exists in divalent, monovalent, or zerovalent state and in particular determined the reduction conditions in hydrogen and carbon monoxide atmospheres for a complete conversion of Cu Y to Cu Y but no further to Cu°. The Cu ions in type Y zeolite were reported to be specific adsorption centers for carbon monoxide ( 6), ethylene ( 7), and to catalyze the oxidation of CO (8). In the present work the Cu ions were also found to be specific adsorption centers for oxygen. 

---