Optical metal hydrides

Although various electrochromic devices have been demonstrated, their performance still needs to be drastically improved. This will require a major research and development effort. On the other hand, the fiber optic metal hydride hydrogen sensor already shows that metal hydride applications may provide a clear advantage over competing systems. 

Reactions of propynyl alcohols and their derivatives with metal hydrides, such as lithium aluminum hydride, constitute an important regio- and stereoselective approach to chiral allenes of high enantiomeric purity63-69. Formally, a hydride is introduced by net 1,3-substitution, however, when leaving groups such as amines, sulfonates and tetrahydropyranyloxy are involved, it has been established that the reaction proceeds by successive trans-1,2-addition and preferred anti-1,2-elimination reactions. The conformational mobility of the intermediate results in both syn- and ami- 1,2-elimination, which leads to competition between overall syn- and anti-1,3-substitution and hence lower optical yields and/or a reversal of the stereochemistry. 

When activated by metallic catalysts, hydrogen may be transferred from the metallic center to unsaturated organic molecules. The nature and reactivity of transition metal hydrides depend on the central metals as well as on the electronic and steric properties of the ligands. Metal hydrides with optically active ligands are chiral and thus, are capable of asymmetric hydrogenation. 

The milder metal hydride reagents are also used in stereoselective reductions Inclusion complexes of amine-borane reagent with cyclodextrins reduce ketones to optically active alcohols, sometimes in modest enantiomeric excess [59] (equation 48). Diisobutylaluminum hydride modified by zmc broniidc-iV./V.A V -tetra-methylethylenediamine (TMEDA) reduces a,a-difluoro-(3-hydroxy ketones to give predominantly erythro-2,2-difluoro-l,3-diols [60] (equation 49). The threo isomers arc formed on reduction with aluminum isopropoxide... 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

The most popular methods of preparing optically active l-octyn-3-ol involve asymmetric reduction of l-octyn-3-one with optlcally-active alcohol complexes of lithium aluminum hydride or aluminum hydride. These methods give optical purities and chemical yields similar to the method reported above. A disadvantage of these metal-hydride methods is that some require exotic chiral alcohols that are not readily available in both enantiomeric forms. Other methods include optical resolution of the racemic propargyl alcohol (100 ee) (and Note 11) and microbial asymmetric hydrolysis of the propargyl acetates (-15% ee for l-heptyn-3-ol)... 

Crystallographic studies of transition metal hydride complexes Stereochemistry of six-coordination Five-coordinate structures Stereochemistry of five-coordinate Co complexes Absolute stereochemistry of chelate complexes Stereochemistry of optically-active transition metal complexes Electron density distributions in inorganic compounds... 

After a metal hydride complex was prepared from LAH and quinine (1 1), irradiation of a mixture of the resulting solution containing the above chiral hydride agent and the enamide (133) led to the formation of two optically active lactams 158 [6%, [a]D —63° (c = 0.48, CHC13)] and 155 [ 13%, [a]D — 102° (c = 0.44, CHC13)] with 37% optical purity. Reduction of the lactam 155 with LAH furnished (—)-xylopinine (20) in 48% chemical yield. 

A two-step hydrolysis of crude 2 with trifluoroacetic acid and lithium hydroxide gives the optically pure a-hydrazino acids 3 (ee >98 %). Reduction with hydrogen on platinum(II) oxide reduces 3 to the a-amino acids 4 in high yields. Alternatively, 2 can also be reduced with metal hydrides (LiAIH4/Et20) to, V-inethylcphedrinc and /i-hydrazino alcohols 5. After transformation into the Mosher ester [with (- )-(S)-methoxy-4-(tri fluoromcthyljphenylacctyl chloride], 5 (R = CH3) shows a diastereomeric ratio d.r. [(2,S,2, S )/(2,S, 2 7f)] >95 5. 

Optically active aliphatic propargylic alcohols are converted to corticoids (90% ee) via biomimetic polyene cyclization, and to 5-octyl-2(5ii)-furanone. The ee s of propargylic alcohols obtained by this method are comparable with those of the enantioselective reduction of alkynyl ketones with metal hydrides, catalytic enantioselective alkylation of alkynyl aldehydes with dialkyIzincs using a chiral catalyst ((S)-Diphenyl(l-methylpyrrolidin-2-yl)methanol) (DPMPM), and the enantioselective alkynylation of aldehydes with alkynylzinc reagents using A(A-dialkylnorephedrines. °... 

Most often, asymmetry is created on conversion of a prochiral trigonal carbon of carbonyl, enol, imine, enamine, and olefin groups to a tetrahedral center. One of the easiest methods for the preparation of optically active alcohols is the reduction of prochiral ketones. This transformation is achieved using chiral reductants in which chiral organic moieties are ligated to boron or to a metal hydride (Table 4.9). 

Optically active 3-keto sulfoxides are readily available from esters and methyl p-tolyl sulfoxide. The ketone function can be reduced with metal hydrides (DIBAL and ZnCl2/DIBAL) giving, with very high diastereoselectivity, opposite configurations in the resulting 3-hydroxy sulfoxides (Scheme 58). ... 

Metal hydrides offer considerable interest as hydrogen storage systems and are ideal candidates for INS spectroscopy especially if high point molecular symmetry is limiting the effectiveness of optical techniques, as in the case of the hexahydrides [22]. 

The binary alkali metal hydrides, MH (M = Li, Na, K, Rb, Cs) crystallise with the sodium chloride structure and are ionic [68]. Their INS spectra. Fig. 6.23, show peaks related to the density of transverse and longitudinal optic states due to the antiphase motions of the hydride ion and the M" cations in the lattice unit cell [69] ( 4.3). The first overtones were also seen. The acoustic bands, at lower energy transfer, were weaker than the optical bands since the acoustic bands arise from in-phase motions of the hydride ion and the cations the hydrogen mean square displacement is smaller in the acoustic than in the optical modes. 

The use of (i7-C5Hs)2TiCl2 as catalyst provides a more powerful reduction method similar to the use of LiAlH4 (230). In this case, reduction of optically active functional silanes occurs with a high degree of retention of configuration (eq. [65]). The intervention of a transition-metal hydride catalyst is probably involved in these reductions. 

Asymmetric reductions with chiral complex metal hydrides and tricoordinate hydride reagents are rare. Iminium salts25 26 and imines27 have been reduced by chiral complex aluminum hydrides. Optically active 2-substituted Ar-methylpiperidine was obtained by reduction of the corresponding 3,4,5,6-tetrahydropyridinium perchlorate with (—)-menthol lithium aluminum hydride. The optical purity for the -propyl derivative was 12% in favor of the S configuration. Similar reductions of imines prepared from acetophenone and propiophenonc with (-)-mcn-thol-lithium aluminum hydride and ( + )-borneol-lithium aluminum hydride reagents resulted in low (<10%) optical yields in those examples where optical yields could be calculated. 

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