Copper complex hydrides

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

Hydridic copper complexes have been discussed in the literature for a very long time. Recently they have been shown to have a variety of interesting chemical and structural properties. Unfortunately, a good workable synthesis leading to stable isolated compounds does not currently exist in the literature. 

A solution of Li[GaH4] in diethyl ether is prepared by the standard methods,J1 and is stored in a needle-valve O-ring flask. The concentration of Li[GaH4]-ether solution is determined by hydrolysis of a known volume of the solution followed by gallium determination by ethylenediamine-tetraace-tic acid, disodium salt using copper-PAN [l-(2-pyridylazo)-2-naphthol copper complex] as the indicator.4 It is assumed that all the gallium is present as the tetrahydrogallate. Only an approximate concentration need be determined since an excess of sodium hydride or potassium hydride is used. 

There are many examples of borohydride compounds of these metals, e.g., Cu, Ag, Zn and Cd-BH as neutral and anionic complexes in which the mode of bonding of BH is dependent on the coordination number of the metaP. Higher borane anions also combine with Cu and Ag, yielding both neutral and anionic complexes. Although no borohydrides of Au are isolated, treatment of Au-halide complexes with, e.g., NaBH, is a standard method for the preparation of Au-cluster compounds Copper(I) hydride, first reported in 1844, has the ZnS structure [d(Cn-H) = 0.173 nm (1.73 A) d(Cu-Cu) = 0.289 nm (2.89 A)] and decomposes to the elements when heated. At >100°C the decomposition is explosive. 

Methylation Diazomethane-Boron fluoride etherale. Dimethylcopperlithium. Dimethyl-sulfonium methylide. Iodine Methyl iodide. Methyl-(tri-n-butylphosphine) copper complex. Silver perchlorate. Simmons-Smith reagent. Sodium hydride. 

The complex is obtained from tri-n-butylphosphine and copper(I) hydride, prepared by reduction of copper(I) bromide with diisobutylaluminum hydride at —50°. The complex is a useful reducing agent it reduces iodobenzene to benzene (80% yield) and benzoyl chloride to benzaldehyde (50% yield). In addition the complex reduces primary, secondary, and tertiary alkyl-, vinyl-, and arylcopper(l) compounds to the corresponding hydrocarbons in high yields, under mild conditions and with no rearrangements.1... 

The intramolecular cyclopropanation of several unsaturated diazo carbonyl compounds83 is most efficiently catalyzed by the Aratani complex (A)-4. Thus, 1-diazo-5-hexen-2-one is converted into (15",5i )-2-oxobicyclo[3.1.0]hexane with 77% ee, An interesting aspect of this study is the activation of the catalyst by bis(2-methylpropyl)aluminum hydride, which reduces the copper(II) to give a copper complex. Other unsaturated diazoketones with the / -complex 4 gave inferior results and with a-diazo /i-oxo esters, which require higher temperatures for carbenoid formation, the enantiomeric excesses were close to zero. 

Various high conductivity materials have been alloyed with metal hydrides to form enhanced heat transport composite materials. Eaton et al. [31] experimented with various alloyed metal additives including copper, aluminum, lead, and lead-tin. The samples were alloyed at elevated temperature (200-600 °C) and cycled. In many samples, cycling resulted in the separation and fracture of the alloy and thus a reduction in composite thermal conductivity. Sintered aluminum structures of 20% solid fraction have been integrated with LaNis hydride materials with success, resulting in effective thermal conductivities of 10-33 W/mK [32-34]. Temperatures required for this process and added mass and volume may exclude application to some complex hydrides. 

Copper(I) hydride complexes. Stabilization of CuH with various phosphines enables the use of such complexes to reduce carbonyl groups (in the presence of a double bond). ... 

Lipshutz and coworkers have developed copper hydride complexes with diphosphine ligands that catalyze the asymmetric hydrosilylation of aryl ketones at low temperatures (-50 to -78 °C) [68]. Nolan and coworkers discovered that copper complexes with NHC ligands are very efficient catalysts for the hydrosilylation of ketones, including hindered ketones such as di-cyclohexyl ketone and di-tert-butyl ketone [69]. 

The incorporation of pyridine or triazole improves the adhesion between poly(imide)s and copper [71]. Poly(3,3, 4,4 -benzophenone tetracarboxylic dian-hydride-3,5-diamino-1,2,4-triazole) (BTDA-DATA) contains the triazole moiety as repeating units. Poly(4,4 -oxydiphthalic anhydride-1,3-aminophen-oxybenzene-8-azaadenine) (ODPA-APB-8-AA) bears the triazole moieties at the end [72]. BTDA-DATA starts to decompose at 350 °C. However, ODPA-APB-8-AA starts to decompose at 400 °C. The polymers have been tested as adhesives for copper surfaces. The adhesion is increased by the formation of copper complexes. 

Displacement of the mesyloxy group is formally a Sn2 process. The hydride reaction with the bromo compounds probably involves electron transfer, capture of bromine, and back-donation of hydrogen (deuterium) to the substrates within the ligand sphere of the copper complexes. The reason for the dichotomy must be hinged on the acceptor characteristics of bromine vis-d-vis the harder carbon. 

Mechanistic studies have been carried out for neutral and cationic Cu systems [12,13b]. The proposed mechanism for [Cu(Cl)(NHC)j complexes involves the formation of [Cu(0 Bu)(NHC)] by reaction of the chloride complex with the base (Scheme 8.3). [Cu(H)(NHC)j would be formed in situ by o-bond metathesis between the terf-butoxide copper complex and the hydrosilane. The hydride copper complex is highly unstable (observable by NMR) however, it is the active species. Hence, by addition of the hydride species to the carbonyl, a second o-bond metathesis with the silane affords the expected silyl ether and regenerates the active catalyst. In the case of cationic derivatives, dissociation of one NHC occurs as the first step, which is displaced by the fert-butoxide moiety, and is the direct precursor of the active species. The hydrosilane is activated by the nucleophilic NHC, leading to the formation of the silyl ether. The activation of the silane appears to be the decisive step for this transformation. 

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