Triphenylphosphine acid halides

Halogenations with dihalotriphenylphosphoranes have been reviewed briefly by Fieser and Fieser.4 Dibromotriphenylphos-phorane appears to have been studied somewhat more than the dichloro compound, but both reagents effectively convert alcohols to alkyl halides, carboxylic acids and esters to acid halides, etc. The reaction of 1,2-epoxycyclohexane with dibromotriphenylphos-phorane under conditions similar to those described here gives a mixture of cis- and trans-1,2-dibromocyclohexanes. A reagent prepared from triphenylphosphine and carbon tetrachloride has been used for similar transformations.5... 

The oxidative addition is quite general with alkyl, allyl, benzyl, vinyl, and aryl halides as well as with acyl halides to afford the palladium (II) complex VII. The frans-bis( triphenylphosphine )alkylpalladium halides can also be carbonylated in an insertion reaction to give the corresponding acyl complexes, the stereochemistry of which (17, 18) proceeds with retention of configuration at the carbon bonded to palladium. The acyl complex also can be formed from the addition of the corresponding acid halide to tetrakis (triphenylphosphine) palladium (0). 

Carboxylic acid chlorides and chloroformate esters add to tetrakis(triphenylphosphine)palladium(0) to form acylpalladium derivatives (equation 42).102 On heating, the acylpalladium complexes can lose carbon monoxide (reversibly). Attempts to employ acid halides in vinylic acylations, therefore, often result in obtaining decarbonylated products (see below). However, there are some exceptions. Acylation may occur when the alkenes are highly reactive and/or in cases where the acylpalladium complexes are resistant to decarbonylation and in situations where intramolecular reactions can form five-membered rings. 

To capture electrophilic substrates such as acid halides, aldehydes, alkyl halides, isocyanates, and isothiocyanates, a variety of nucleophilic resins are commonly used. Some commercially available and representative examples are tris-(2-aminoethyl)amine (9), thiophenol (10), sulfonylhydrazide (11), triphenylphosphine (12), and methylthiourea (13) polymer resins (Fig. 6). 

ArNOi — ArNHj.1 Na+[HFe(CO) ] is a known reducing agent in a basic medium. However, the bis(triphenylphosphine)imine (PPN) salt of HFe(CO)4 1 in combination with a strong Brpnsted acid (TFA) is also a reducing agent, particularly for nitroarenes, even in the presence of aldehydes or acid halides. 

If carboxylic acid esters are readily available, they provide a convenient basis for the preparation of acid halides. Modified phosphorus halides, like 2,2,2-trichloro-l,3,2-benzodioxaphosphole can be applied successfully. This reagent forms first a 1,1-dichloroalkyl ether, which decomposes to the acid chloride (equation 16). The reaction conditions are rather drastic. For instance -butyl benzoate has to be heated for 4 h at 180 °C in order to get a 90% yield of benzoyl chloride. The only stable and isolable 1,1-dichloro ether is a,a-dichloromethyl methyl ether, which, as described above, is used as a mild reagent for the conversion of carboxylic acids to their chlorides. Similarly severe conditions are required if the chlorine or bromine adduct of triphenylphosphine is selected. This reaction may be catalyzed by BF3. It has been applied successfully to unsaturated esters and to the cleavage of lactones. 

Triphenylphosphine-carbon tetrahalides. 13, 331-332 15, 352 16, 366-368 Acid halides. The relatively mild conditions of converting acids to halides by Ph3P-CX4 can be exploited for a one-flask synthesis of A-methoxy imidoyl bromides, which give rise to nitriles on photolysis. In the presence of EtsN the PhsP-CX4 combination converts amines and CF3COOH to trifluoroacetimidoyl halides. ... 

Alkyl chlorides. Hungarian chemists have isolated and identified the salts (1) and (2) in the reaction of triphenylphosphine and carbon tetrachloride with alcohols, enolizable ketones, and acid halides. They have suggested a new mechanism in which these salts participate. 

Acyl carbonyl ferrates are involved as intermediates in two preparations of aldehydes. Alkyl bromides react with sodium tetracarbonyl ferrate(—ii) (from iron pentacarbonyl and sodium) by a process of oxidative addition to furnish the ferrate(—i) (124) this species rearranges on treatment with triphenylphosphine to a phosphonium-substituted acyl ferrate(—i) (125), which is subsequently cleaved by acetic acid to the homologous aldehyde, as outlined in Scheme 46. A related procedure employs reaction of an acid halide with sodium tetracarbonyl ferrate(—ii) to afford the acyl ferrate(—i) (126) directly this species is also cleaved by acetic acid, and affords yet another method for the reduction of acid halides to aldehydes. 

Wilkinson s catalyst and chlorocarbonylbis(triphenylphosphine)rhodium [or iridium] can catalyze the thermal decarbonylation of aromatic acid halides to aryl halides. An intermediate Rh(III) hydride is involved in the reaction (Suggs, 1978). For aliphatic acid halides, subsequent elimination of HX frequently occurs from the generated alkyl halide (Ohno and Tsuji 1968 Blum et al., 1%7, 1971 Strohmeier and Pfohler, 1976). 

The Suzuki-Miyaura synthesis is one of the most commonly used methods for the formation of carbon-to-carbon bonds [7]. As a palladium catalyst typically tetrakis(triphenylphosphine)palladium(0) has been used, giving yields of44—78%. Recently, Suzuki coupling between aryl halides and phenylboronic acid with efficient catalysis by palladacycles was reported to give yields of 83%. 

Carboxylic acids can be converted to acyl chlorides and bromides by a combination of triphenylphosphine and a halogen source. Triphenylphosphine and carbon tetrachloride convert acids to the corresponding acyl chloride.100 Similarly, carboxylic acids react with the triphenyl phosphine-bromine adduct to give acyl bromides.101 Triphenviphosphine-iV-hromosuccinimide also generates acyl bromide in situ.102 All these reactions involve acyloxyphosphonium ions and are mechanistically analogous to the alcohol-to-halide conversions that are discussed in Section 3.1.2. 

A convenient one-pot procedure for the conversion of alcohols into primary amines has been reported. The alcohol is converted into the corresponding alkyl halide by the action of bromotrichloromethane/triphenylphosphine and the product is treated successively with sodium azide, triethyl phosphite, hydrochloric acid and sodium hydroxide (equation 20)55, cf. equation 14. 

Nickel(O) triphenylphosphine can be used to couple aryl halides and alkenes to synthesize substituted olefins [149], 1,2-bis[(di-2-propylphosphino)benzene]nick-el(0) can be used to couple aryl halides [150], and l,2-bis[(diphenylphos-phino)ethane]nickel(0) can be used to prepare benzoic acid from bromobenzene in the presence of carbon dioxide [151]. 

There are other activation procedures which generate acyl halides in situ in the presence of the nucleophile. Refluxing a carboxylic acid, triphenylphosphine, bromotri-chloromethane, and an amine gives rise to the corresponding amide103. 

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