Superhydride

Li[Ph4P2N4S2R].274 Alkali-metal derivatives of the dianion [Ph4P2N4S2]2 are prepared by treatment of 111 (E=S, R=Ph) with alkali-metal superhydrides M[Et3BH] (M=Li, Na].275 The methylene bridge between the two sulfur atoms in 116 is introduced by the addition of diiodomethane to... 

Other reagents used for the preparation of lactones from acid anhydrides are lithium borohydride [1019], lithium triethylborohydride (Superhydride ) [1019] and lithium tris sec-butyl)borohydride (L-Selectride ) [1019]. Of the three complex borohydrides the last one is most stereoselective in the reduction of 3-methylphthalic anhydride, 3-methoxyphthalic anhydride, and 1-methoxynaphthalene-2,3-dicarboxylic anhydride. It reduces the less sterically hindered carbonyl group with 85-90% stereoselectivity and is 83-91% yield [1019]. 

Unless otherwise stated, all operations are carried out in flame-dried glassware under an inert gas atmosphere using standard Schlenk techniques.7 Solvents were dried over sodium (light petroleum, bp 40-60°C) or sodium benzophenone (diethyl ether, THF) or calcium hydride (acetonitrile). Li[BHEt3] in THF (Superhydride ) and sodium bis(trimethylsilyl)amide were used as purchased (Aldrich). 

Superhydride has also been used for the hydrogenolysis of alkyl toluene-p-sulphonates, and an illustrative example is given in Expt 5.4 for the preparation of cyclooctane from cyclooctyl toluene-p-sulphonate. A sodium borohydride hydrogenolysis of alkyl halides and of alkyl methane- and toluene-p-sulphonates under PTC conditions provides an interesting variant of this reaction.9... 

Primary and secondary alkyl bromides, iodides, and sulfonates can be reduced to the corresponding alkanes with LiBHEt3 (superhydride) or with lithium aluminum hydride (LiAlH4, other names lithium tetrahydridoaluminate or lithium alanate). If such a reaction occurs at a stereocenter, the reaction proceeds with substantial or often even complete stereoselectivity via backside attack by the hydride transfer reagent. The reduction of alkyl chlorides to alkanes is much easier with superhydride than with LiAlH4. The same is true for sterically hindered halides and sulfonates ... 

Scheme 2 Synthesis of benzyl-substituted titanocenes through the hydridolithiation reaction of aryl-substituted fulvenes using superhydride...

Only a few special tertiary amides can be reduced to alcohols, e.g., with superhydride. 

What is interesting to note here is that the catalyst that facilitates the conversion of the borohydride to borate can be an acid, metal, or metal boride. So, as this reaction proceeds, it is producing the very catalyst that degrades the starting reactant. It is possible to control the boride formation with the use of borohydride esters such as lithium triethoxyborohydride, commonly referred to as superhydride. 

Another, quite general route is to prepare a sulfide in situ by reduction of sulfur using superhydride (equation 39). 

Li[Et3BH] (Superhydride) (lithium triethylborohydride) Tetrahydrofuran -78 to RT ester —> alcohol ketone —> alcohol aldehyde —> alcohol alkyl halide —> alkane epoxide —> alcohol... 

Another modified metal hydride, lithium triethylborohydride, the so-called superhydride , has been introduced as a powerful reducing agent especially suitable for trisubstituted, tetrasubstituted and bicy-clic epoxides (Table 3). With trisubstituted epoxides the regiochemistry is completely controlled to give only tertiary alcohols. No skeletal rearrangement is observed for benzonorbomadiene oxide. 

Furthermore, superhydride is used as an alternative to LAH in the reduction of carbohydrate epoxides (equation 18), In this example, the C—O bond away from the hydroxy group is selectively cleaved, but a tosyloxy group is also reduced in epoxides having a hydroxy group protected with tosyl. A kinetic study of reductions with superhydride has been reported. ... 

Figure 24 The structure of the sodium derivative of the superhydride anion, Na[HBEt3].

Reduction of primary as well as secondary tosylates with LiEtjBH (Superhydride) provides a convenient method for the deoxygenation of alcohols to the corresponding... 

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