Displacement using hydride

Allylic acetates are reduced in good yield to alkenes by a new reductive displacement using hydride reagents and catalytic activation with Pd(Ph3P)4 (Scheme 1). In a similar study allylic or benzylic alcohols have also been shown... 

Tin tetrachloride has been used to prepare the stericaHy hindered triisopropylchlorosilane [13154-24-0] (119). Organobromosdanes are obtained under similar conditions through reaction with cupric and mercuric bromide. These reactions are most suitable for stepwise displacement of hydrogen to form mixed hydridochlorosilanes or in systems sensitive to halogen (120). Hydrides have also been displaced using organic bromides. Heating triethylsilane and... 

Nucleophilic displacement by hydride ion is an excellent method for the reduction of halides. In etheral solution a wide range of halides are reduced by LAH at room temperature66-79. In some cases sodium borohydride has been used as a milder reagent with sensitive substrates80,81. This reagent has also been used under phase-transfer conditions82,83. 

Hydride Displacement. The first preparation of H3BNH3 was effected by hydride displacement using BH4 and an NH4" salt 19). The appropriate equation is BH4 + NH4+ H3BNH3 + H2. Further replacement of H to produce cations can be effected by carrying out the process at a higher temperature H) ... 

The reduction of secondary sulfonates with lithium aluminum hydride or sodium borohydride is usually a poor reaction for deoxygenating secondary alcohols [220,222], In most cases, the hydride attack will occur at sulfur and result in cleavage of the S-0 bond to afford the starting secondary alcohol as the main product. An exception from this rule is observed when tetrabutylammonium borohydride is used for reduction of secondary triflates in refluxing benzene [239]. Under these conditions clean displacement with hydride occurs to give the corresponding deoxy compounds in good yield (O Table 14). 

Addition to coordinated arenes is a reliable method for achieving overall aromatic nucleophilic substitution with formal displacement of hydride [17]. This method illustrates the use of nucleophilic addition to an arenetricarbonyl-chromium for the synthesis of aromatic compounds with unusual substitution patterns. 

It also appears possible to alkylate nitrobenzene with the displacement of hydride (or its equivalent) by the use of the anion obtained from deprotonation of methyl-sulfinylmethane (dimethyl sulfoxide [( 113)280]), viz. the methylsulfinylmethide anion (dimsyl anion [CH3SO(CH2) ]. Although the details are not known, the reaction is particularly useful because Friedel-Crafts alkylation of nitrobenzene usually fails. Indeed, the reaction of nitrobenzene with Lewis acids and alkyl (or acyl) halides is so poor that nitrobenzene has occasionally been used as a solvent for the reaction of other arenes with these substrates. It is argued that nitrobenzene is so unreactive because the nitro substituent withdraws electrons, making them unavailable for an incoming electrophile, that is, the ring is deactivated toward electrophilic substitution by the electron-withdrawing substituent. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Examples of the use of lithium aluminum hydride in stereoselective [75] (equation 9), regioselective [16] (equation 10), and product-selective [77](equation 11) displacements of fluonne are available... 

Recently, it was shown that the attack of CN on [FeCp(C6H5Cl)]+ PFortho-position. In the intermediate cyclohexadienyl complex, the CN group migrates to the ipso-carbon, whereas Cl is displaced. The monosubstituted benzonitrile complex is subjected to a second ortho-CN- attack but hydride is not removed spontaneously to give back an arene complex (Scheme XIX). Removal of the hydride is achieved by oxidation using DDQ (2,3-dichloro-... 

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

Probably more interesting is the examination of the catalytic cycle of hydrogenation studied by Rosales and co-workers43 which used [RuH(CO)(NCMe)2(PPh3)2]BF4 as catalyst (species (6)).44 The reversible displacement of the MeCN ligand trans to the hydride by cyclohexene is followed by an isomerization prior to rate-determining addition of hydrogen ((7) —> (8) —> (9)) (Scheme 4). 

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