Halogen reduction with

Both halogens of the dihalogenothiazoles can be replaced by nucleophiles. At any rate, the halogen in position 2 is always more reactive than those in positions 4 or 5, as previously discussed. Analogously,the halogen can be selectively removed only from position 2 by reduction with metals (Table V-5). 

In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as is illustrated by the preparation of 3-bromothiophene... 

Toluene from Toluidine.—It is often desirable to obtain tbe hydiocarbon from the base. The process of diazotisntion offers the only convenient method. The diazonium salt may be reduced by alcohol (Reaction 1, p. 162) or, as in the piesent instance, by sodium stannite. Less direct methods are the con-veision of the diazonium compound into (i) the hydrazine (see p. 174), (2) the acid and distillation with lime (p. 200), (3) the halogen derivative and reduction with sodium amalgam, 01, finally (4) the phenol and distillation with zinc dust. 

Ketalization of the aldehyde with neopentyl glycol (32) followed by reaction with dichloroketene gives the 2+2 cycloadduct, 33. The halogen is then removed by reduction with zinc... 

Shenlin Huang has implemented method C with 2,4-bis-OBoc-3-bromobenzyaldehyde 27 (Fig. 4.22) and 2-(trimethylsilyl)ethoxy]methyl-lithium 38 at —78 °C in THF.24 Surprisingly, lithium-halogen exchange does not happen and the intermediate benzyl alcohol undergoes reduction with sodium borohydride in the same pot to afford the desired bromophenol 39 in 68% yield. This material... 

The model is also meant to be applicable to reductions with Grignard reagents X = CH2 and M = halogenated Mg. 

Reductions with aluminum are carried out almost exclusively with aluminum amalgam. This is prepared by immersing strips of a thin aluminum foil in a 2% aqueous solution of mercuric chloride for 15-60 seconds, decanting the solution, rinsing the strips with absolute ethanol, then with ether, and cutting them with scissors into pieces of approximately 1 cm [141,142]. In aqueous and non-polar solvents aluminum amalgam reduces cumulative double bonds [143], ketones to pinacols [144], halogen compounds [145], nitro compounds [146, 147], azo compounds [148], azides [149], oximes [150] and quinones [151], and cleaves sulfones [141, 152, 153] and phenylhydrazones [154] (Procedure 30, p. 212). 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

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