Hydride and equivalents

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

The carbon-carbon double bond of 2,3-diphenylthiirene 1,1-dioxide is reduced by aluminum amalgam in wet ether to cis-2,3-diphenylthiirane 1,1-dioxide (16%). 

The triphenyllead anion reacts with thiirane to give 2-mercaptoethyltriphenyllead. 

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N,N-Dimethy1aniline from Nakarai Chemicals was dried over calcium hydride and freshly distilled. Three molar equivalents of N,N-dimethylaniline are used to achieve complete conversion of the n-butyllithium, because In the present particular case free n-butyllithium, if present, causes the isomerization of the (Z)-alkene to the (E)-isomer. 

A 1.5 to 2 M solution of methylsulfinyl carbanion in dimethyl sulfoxide is prepared under nitrogen as above from sodium hydride and dry dimethyl sulfoxide. An equal volume of dry tetrahydrofuran is added and the solution is cooled in an ice bath during the addition, with stirring, of the ester (0.5 equivalent for each 1 equivalent of carbanion neat if liquid, or dissolved in dry tetrahydrofuran if solid) over a period of several minutes. The ice bath is removed and stirring is continued for 30 minutes. The reaction mixture is then poured into three times its volume of water, acidified with aqueous hydrochloric acid to a pH of 3-4 (pH paper), and thoroughly extracted with chloroform. The combined extracts are washed three times with water, dried over anhydrous sodium sulfate, and evaporated to yield the jS-ketosulfoxide as a white or pale yellow crystalline solid. The crude product is triturated with cold ether or isopropyl ether and filtered to give the product in a good state of purity. 

We now tum our attention to the C21-C28 fragment 158. Our retrosynthetic analysis of 158 (see Scheme 42) identifies an expedient synthetic pathway that features the union of two chiral pool derived building blocks (161+162) through an Evans asymmetric aldol reaction. Aldehyde 162, the projected electrophile for the aldol reaction, can be crafted in enantiomerically pure form from commercially available 1,3,4,6-di-O-benzylidene-D-mannitol (183) (see Scheme 45). As anticipated, the two free hydroxyls in the latter substance are methylated smoothly upon exposure to several equivalents each of sodium hydride and methyl iodide. Tetraol 184 can then be revealed after hydrogenolysis of both benzylidene acetals. With four free hydroxyl groups, compound 184 could conceivably present differentiation problems nevertheless, it is possible to selectively protect the two primary hydroxyl groups in 184 in... 

Analogous to this reaction, thiocarbamates have been prepared using ImCSIm, which is generated in situ from two equivalents of imidazole, CS2, sodium hydride, and one equivalent of a thiazolium salt 2163 ... 

A 20 g sample, prepared and stored in a dry box for several months, developed a thin crust of oxidation/hydrolysis products. When the crust was disturbed, a violent explosion occurred, later estimated as equivalent to 230 g TNT. A weaker explosion was observed with potassium tetrahydroaluminate. The effect was attributed to superoxidation of traces of metallic potassium, and subsequent interaction of the hexahydroaluminate and superoxide after frictional initiation. Precautions advised include use of freshly prepared material, minimal storage in a dry diluent under an inert atmosphere and destruction of solid residues. Potassium hydrides and caesium hexahydroaluminate may behave similarly, as caesium also superoxidises in air. 

The nitrosyls RuH(NO)(PR3)3 are 5-coordinate with trigonal bipyramidal structures and linear Ru-N-O geometries the hydride and nitrosyl ligands occupy the apical positions (for RuH(NO)(PPh3)3, z/(Ru-H) 1970 cm-1, i/(N—O) 1640 cm-1 H NMR, 8 = +6.6 ppm for the hydride resonance). The high-field NMR line is a quartet showing coupling with three equivalent phosphines, which would not be possible in a square pyramidal... 

Rapid rotation of the end groups and/or bridging hydrides is required to account for apparent magnetic equivalencies. The molecule does, however, have built in pseudo-cylindrical symmetry, i.e., one set of metal TT-type orbitals binds the bridging hydrides and the other set forms the TT-component of the metal-metal double bond. 

Conditions Method A Epoxidation using Sharpless reagent method B addition of 0.05-0.1 equivalent of calcium hydride and 0.1-0.15 equivalent of silica gel to the Sharpless reagent, ee = Enantiomeric excess. 

From the equation showing the mechanism it is evident that 1 mol of lithium aluminum hydride can reduce as many as four molecules of a carbonyl compound, aldehyde or ketone. The stoichiometric equivalent of lithium aluminum hydride is therefore one fourth of its molecule, i.e. 9.5 g/mol, as much as 2 g or 22.4 liters of hydrogen. Decomposition of 1 mol of lithium aluminum hydride with water generates four molecules of hydrogen, four hydrogens from the hydride and four from water. 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

To a stirred suspension of 0.25 g LAH in 10 mL anhydrous THF, stirred, under nitrogen and at room temperature, there was added a solution of 0.33 g 4-acetoxyindol-3-yl-N,N-diisopropylglyoxylamide in 10 mL anhydrous THF. This was added dropwise at a rate that maintained the reaction at reflux. When the addition was complete, the reflux was maintained for an additional 15 min and then the reaction was cooled to 40 °C. The excess hydride and the product complex were destroyed by the addition of 0.5 mL followed by 1.5 mL H20. The solids were removed by filtration, the filter cake washed with THF, the filtrate and washings pooled, and the solvents removed under vacuum. The residue was distilled at the KugelRohr and the distillate dissolved in 1 mL MeOH. One equivalent of dilute HCI was added, and the volatiles were removed under vacuum. The solid residue was recrystallized from MeOH/Et20 to give 0.13 g (47%) of 4-hydroxy-N,N-diisopropyltryptamine hydrochloride (4-HO-DIPT) with a mp 263 °C with decomposition. Anal C,H,N. 

This method has essentially been applied only to iron hydrides. A correlation between Mossbauer and H NMR chemical shifts has been noted.112 The method has also been applied in cluster hydrides, and the non-equivalence of the metals in [Fe4H(CO)13] was shown in this way, for example. 

Lithium borohydride is intermediate in activity as a reducing agent between lithium aluminium hydride and sodium borohydride. In addition to the reduction of aldehydes and ketones it will readily reduce esters to alcohols. It can be prepared in situ by the addition of an equivalent quantity of lithium chloride to a 1m solution of sodium borohydride in diglyme. Lithium borohydride should be handled with as much caution as lithium aluminum hydride. It may react rapidly and violently with water contact with skin and clothing should be avoided. 

Sinex et al. [44] have described a method for the determination of methyltin compounds based on reaction with sodium borohydride to form tin hydrides and then purge and trap analysis followed by gas chromatography with mass spectrometric detection. Down to 3-5pg absolute (as tin) of methyltin compounds (equivalent to the sub xg/kg range) can be determined by this procedure. 

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