Hydride derivatives

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

The nomenclature of boron hydride derivatives has been somewhat confusing and many inconsistencies exist in the Hterature. The stmctures of some reported boron hydride clusters are so compHcated that only a stmctural drawing or graph, often accompanied by explanatory text, is used to describe them. Traditional nomenclature systems often can be used to describe compounds unambiguously, but the resulting descriptions may be so long and unwieldy that they are of Htde use. The lUPAC (7) and the Chemical Abstract Service (8) have made recommendations, and nomenclature methods have now been developed that can adequately handle nearly all clusters compounds however, these methods have yet to be widely adopted. Eor the most part, nomenclature used in the original Hterature is retained herein. 

An asymmetric synthesis has used the reduction of imonium salts to optically active tertiary amines with lithium aluminum alkoxy hydrides derived from optically active alcohols (538,539). 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Synthesis and reactions of aluminium hydride derivatives. S. Cucinella, A. Mazzei and W. Marconi, Inorg. Chim. Acta, Rev., 1970, 4, 51-71 (183). 

The reaction of the coordinatively unsaturated ruthenium amidinates with [Cp RuCl]4 tetramer or [CpRufMeCNlsJPFg provides access to novel amidinate-bridged dinuclear ruthenium complexes (Scheme 146), which in turn can be transformed into cationic complexes or hydride derivatives. In these complexes, a bridging amidinate ligand perpendicular to the metal-metal axis effectively stabilizes the highly reactive cationic diruthenium species. 

Alkyl or hydride derivatives are formed by addition of a variety of Grignard reagents or sodium isopropoxide to the halide precursors. Cationic complexes may be obtained by the interaction with AgSbFg in the presence of an appropriate... 

O-Bond metathesis of the Ln-alkyl with the phosphine gives a Ln-phosphido complex. (This initiation step was observed to be faster when the hydride derivative [Cp 2bnH]2 was used.) Since the reactions were zero-order in substrate, the next... 

IV. Terminal Hydride Derivatives of the s- and p-Block Metals Supported by Poly(pyrazolyl)borato Ligation... 

By analogy with the alkyl derivatives described earlier, poly(pyrazo-lyl)hydroborato ligation may also be utilized to support monomeric terminal hydride derivatives [TpRR ]MHn. However, at present, the chemistry of such complexes is effectively restricted to that of four-coordinate derivatives of the type [TpBut]MH. 

The beryllium hydride derivative [TpBut]BeH is obtained by metathesis of either [TpBut]BeCl or [TpBut]BeBr with LiAlH4 in Et20 at room temperature [Eq. (27)] (56). 

The zinc hydride complex [TpBut]ZnH, the first metal hydride derivative of the type [Tp1 ]MH to be prepared, is readily synthesized by metathesis of ZnH2 with Tl[TpBut] [Eq. (28)] 80,138). 

Zinc fluoride complexes of i-para-toXyX and 3-t-butyl of hydrotris(3-R-5-methylpyrazol-l-yl) borato zinc complexes have been synthesized via complex formation in the presence of potassium fluoride. Facile substitution of the fluoride yields the zinc pyridyl, diethyl ether, or hydride derivatives.241... 

Peters, F. M. Aluminium Hydride Derivatives from the Reactions of Lithium Aluminium Hydride with Trimethylamine and Dimethyl-amine. Canad. J. Chem. 42, 1755 (1964). 

The participation of hydride species in this process is corroborated by the ability of complexes [( 75-C5Me5)RhIII(bpy)Cl]+ (M = Rh or Ir)28 and [Rhni(bpy)(PPh2Et)2(Cl)2]+32 in films to perform ECH of organics. Indeed, such complexes are readily transformed to hydride derivatives upon electroreduction in protic electrolytes (see Section 9.10.3.1.1). However, these complexes give more stable hydride species and are much less active than [Rh(bpy)2(Cl)2]+ complexes. 

The selective and almost quantitative electrocatalytic formation of CO is obtained when [Ru(bpy)2(CO)2]2+ is used as electrocatalyst in aqueous acetonitrile (20 80) medium. In the absence of added water, formation of the [Ru(bpy)(CO)2]ra polymer is in competition with that of the hydride derivative [Ru(bpy)(CO)H]+, the latter being the active catalyst as demonstrated in a separate experiment using an authentic sample of this hydrido-complex.92 In those conditions HCOO" and H2 are also formed besides CO. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

ICOSAHEDRAL CARBORANES AND INTERMEDIATES LEADING TO THE PREPARATION OF CARBAMETALLIC BORON HYDRIDE DERIVATIVES... 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

The hydrogenation of unfunctionalized alkenes is readily performed by Group III and lanthanide cyclopentadienyl hydride derivatives, one key feature being the high TOFs of these systems (up to 120000 IT1 for hydrogenations catalyzed by Lu, Tables 6.8 and 6.9) [119, 120]. The reaction rate depends heavily on the metal and the ligands. It is inversely proportional to the metal radius (Lu>Sm>Nd>La), and it is faster for the Cp M derivatives than for the ansa di-... 

Recently, Kang and Kim developed new chiral ferrocenyl tin hydride derivatives 72 and 73 (Scheme 19) [60]. The authors screened the new chiral reagent in the reduction of a-bromoesters. Using one equivalent of 73 good ee s were obtained for ester 71. One drawback for this reagent, however, is the lengthy synthetic route for its preparation. 

It was shown that direct attack of 7 on THF was unlikely, and the reduction was attributed to dissociation of 7 to a tricoordinate aluminum hydride derivative, as in eq. [8]. 

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