Halides reduction methods

Reductive methods form B—B bonds from B—X bonds. For B2X4 (X = Cl, Br, I) from BXj, an electric discharge is supplemented by the presence of a metal, or metal atoms, as halide scavenger. The passage of BX3 at low pressure through a rf discharge in the presence of Hg produces the diboron tetrahalides B2X4 at 300 mg h ... 

An application of the reduction method of organotin halides by simple organotin hydrides is the preparation of l, >-bis(hydridodimethylstannyl)alkanes235 ... 

We have previously seen (0-96) that dianions of carboxylic acids can be alkylated in the a position. These ions can also be acylated on treatment with a carboxylic ester1705 to give salts of p-keto acids. As in 0-96, the carboxylic acid can be of the form RCH2COOH or RR"CHCOOH. Since p-keto acids are so easily converted to ketones (2-40), this is also a method for the preparation of ketones R COCHiR and R COCHRR", where R can be primary, secondary, or tertiary alkyl, or aryl. If the ester is ethyl formate, an a-formyl carboxylate salt (R = H) is formed, which on acidification spontaneously de-carboxylates into an aldehyde.1706 This is a method, therefore, for achieving the conversion RCH2COOH — RCH2CHO, and as such is an alternative to the reduction methods discussed in 0-83. When the carboxylic acid is of the form RR CHCOOH. better yields are obtained by acylating with acyl halides rather than esters.1707... 

While removal of an AT-alkyl substituent is not always a feasible process benzyl groups can be removed by reduction with sodamide or by catalytic hydrogenolysis. If such reductive methods fail, an oxidation with chromium trioxide in acid may be successful. Other groups are not so readily displaced, but a procedure involving transalkylation with benzyl chloride followed by debenzylation can be employed to convert 1-methylimidazole into imidazole (Scheme 137). The reaction is capable of extension and operates because the quaternary salt is in equilibrium with both 1-alkylimidazoles and the alkyl halides. Under conditions in which the more volatile alkyl halide can escape from the system the 1-benzylimidazole builds up. 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

In the past decade much effort in organoindium chemistry has been devoted to study of carbonyl allylation and aUylindation of carbon-carbon multiple bonds with allylic indium reagents. Apart from the conventional transmetalation of allyl-lifhium or aUyl Grignard reagents with indium(III) halides, a method widely used for preparation of allyhndium(III) compounds is the oxidative addition of metallic indium or indium(I) iodide to aUyhc substrates [3, 6]. Transmetalation of allylstan-nane with indium(III) chloride also gives allylindium(III) [7]. Allylindium(I) was recently prepared by transmetalation of allylmercury with metallic indium in water this compound is regarded as an intermediate in the allylation of carbonyl compounds in aqueous media [8]. A new method of preparation of allylic in-dium(III) reagents - reductive transmetalation of a 7r-allylpalladium(ll) complex with indium(I) salts has been reported this enables the use of a wide variety of allylic compounds and solvents [9]. 

Froblems associated with borohydride reduction include its irreproducibil-ity, specially in the aqueous medium and the incorporation of boron in the product [158,159]. Bonneman and coworkers [160,161] suggested the use of tri-ethylborohydrides to avoid the incorporation of boron. In order to demonstrate the versatility of the borohydride reduction method, Bonneman and coworkers reduced a range of metal halides of early transition metals such as Ti, Zr, V,... 

Closely related to the alkalide reduction method is the reduction by alkah metals such as K, Li, and Na. In dry THF, diglyme or other ethers, metal halides are reduced to obtain nanoparticles of metals. NiCL has been reduced to Ni in diglyme while AICI3 has been reduced to A1 in xylene by Rieke and coworkers [203,204]. An important extension to this method is the use of the alkali naphthalides [205]. 

The first use of 5-hexenyl radical cyclization as a mechanistic tool was proposed in 1966 by Garst and Lamb for the study of alkyl halides reduction by naphthalene radical anion. Since then, Garst has extensively used this method in the study of the one-electron transfer reactions from aromatic radical anions to alkyl halides critical discussion of the use of the method may be... 

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