Halogenation with sodium borohydride

Ill-defined, dark-colored polymers possessing aromatic structures were obtained (66-68). The same type of materials were also obtained when the reaction product of (R3P)2NiX2 (R = alkyl or aryl X = halogen) with sodium borohydride or lithium aluminum hydride was employed as the polymerization catalyst for a variety of acetylenic monomers (84-86). 

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... 

Initial halogenation of the oxime can use chlorine, hypobromite, bromine, or A-bromoacetamide. " Oxidation of the a-halonitrosoalkane can be achieved with nitric acid, nitric acid-hydrogen peroxide, " atmospheric oxygen, ozone, or a peroxyacid. Reduction of thea-halonitroalkaneis achieved with sodium borohydride or by catalytic hydrogenation, although potassium hydroxide in ethanol has been used for the conversion. 

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

As has already been mentioned, nucleophilic displacement of halogen atoms in position 2 and 4 occurs without difficulty. Reduction of 2-chloro- and 2,4-dichloro-thieno[2,3-d]pyrimidines with sodium borohydride gives the corresponding 2-chloro-3,4-dihydro derivatives (Scheme 100) (80CPB3172, 81JMC376,81JHC67). 

The adducts may be demercurated in situ, frequently with sodium borohydride or with a halogen ... 

In contrast to allyl halides substituted with one ASG, the cyclopropanation reaction proceeds relatively smoothly when a second ASG is present. Generally, the best results are obtained with sodium borohydride, sodium cyanide, potassium cyanide, and the sodium salts of alcohols or thiols as the nucleophilic species (Table 22, entries 3-26). Even spiroalkanes can be synthesized with the nucleophiles described above (Table 23). Examples illustrating this route are the conversion of a tetracyclic enamino ester with potassium cyanide to the corresponding electrophilic cyclopropane 2, and the facile one-pot synthesis of 1,1 -bis(hydroxymethyl)cyclo-propanes 3 by reduction of halogenated alkylidene malonates with lithium aluminium hydride. ... 

Preparation. Diborane is generally made by treatment of sodium borohydride with boron trifluoride etherate. During a study of the reduction of organic halogen compounds with sodium borohydride, Bell et al. noted that the stoichiometry results in generation of diborane (as monomer) ... 

Sodium borohydride (NaBH4) converts the C—Hg bond into a C — H bond. A reaction that increases the number of C—H bonds or decreases the number of C—O, C—N, or C—X bonds in a compound (where X denotes a halogen), is called a reduction reaction. Consequently, the reaction with sodium borohydride is a reduction reaction. The mechanism of the reduction reaction is not fully understood, although it is known that the intermediate is a radical. 

Similarly, reduction of both aromatic and aliphatic aldehydes and ketones where there is a chlorine (Cl) or bromine (Br) present elsewhere in the carbonyl-containing compound succeeds with sodium borohydride (NaBH,) and, occasionally, with lithium aluminum hydride (LiAIH,) at low temperatures. Catalytic reduction frequently leads to hydrogenolysis of the halogen, producing the halogen-free alcohol. 

Mercury adducts can also be reacted with sodium borohydride and converted to methoxy compounds (oxymercuration-demercuration) [321,613], or reacted with halogens in methanol to form methoxyhalogen compounds [611], Such derivatives have been utilised for locating double bonds in fatty acids by mass spectrometry. 

Preparation of the prototype starts with the radical side I liain bromination of dichloroacetophenone to give the bromoketone (23). The carbonyl group is then reduced by means of sodium borohydride displacement of halogen by means of isopropylamine... 

A phenyl ethanol amine in which the nitrogen is alkylated by a long chain alphatic group departs in activity from the prototypes. This agent, suloctidil (43) is described as a peripheral vasodilator endowed with platelet antiaggregatory activity. As with the more classical compounds, preparation proceeds through bromination of the substituted propiophen-one ( ) and displacement of halogen with octyl amine. Reduction, in this case by means of sodium borohydride affords suloctidil (43). ... 

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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