Cobalt reaction with benzyl bromide

Aryl methyl ketones have been obtained [4, 5] by a modification of the cobalt-catalysed procedure for the synthesis of aryl carboxylic acids (8.3.1). The cobalt tetracarbonyl anion is converted initially by iodomethane into the methyltetra-carbonyl cobalt complex, which reacts with the haloarene (Scheme 8.13). Carboxylic acids are generally obtained as by-products of the reaction and, in several cases, it is the carboxylic acid which predominates. Unlike the carbonylation of haloarenes to produce exclusively the carboxylic acids [6, 7], the reaction does not need photoinitiation. Replacement of the iodomethane with benzyl bromide leads to aryl benzyl ketones in low yield, e.g. 1-bromonaphthalene produces the benzyl ketone (15%), together with the 1-naphthoic acid (5%), phenylacetic acid (15%), 1,2-diphenylethane (15%), dibenzyl ketone (1%), and 56% unchanged starting material [4,5]. a-Bromomethyl ketones dimerize in the presence of cobalt octacarbonyl and... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

The reaction of o-nitrobenzaldehydes with some benzene derivatives in the presence of strong acid (H2S04, PPA) is a classical synthesis of acridinol N-oxides (373) (37BSF240) The synthesis works for benzyl alcohol, benzene, toluene and halobenzenes, but not for benzoic acid, benzonitrile, dimethylaniline, or nitrobenzene. Isoquinoline N-oxides (374) have been obtained from o-bromobenzaldoxime or the acetophenone derivative, and active methylene compounds with copper bromide and sodium hydride (77S760). The azobenzene cobalt tricarbonyl (375) reacts with hexafluorobut-2-yne to give a quinol-2-one (72CC1228), and the 3,4,5-tricyanopyridine (376) is formed when tetracyanoethylene reacts with an enaminonitrile (80S471). 

As an active initiator for a co-polymerization, acyl-cobalt complexes also work well. As demonstrated by Alper and Lee, an equimolar mixture of Go2(GO)s, benzyl bromide (BnBr), and dihydro-1,10-phenanthroline 17, possibly generating BnGOGo(GO)4 under the reaction conditions, co-polymerized PO or 1,2-butene oxide with GO, and the... 

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

Treatment of [Co(CO)4] under PT conditions with a mixture of alkyne and Mel (excess) in a CO atmosphere gives hydroxybut-2-enolides (eq. (12)) [148]. When benzyl bromide is used instead of Mel, the but-2-enolides are not formed due to the fast hydrolysis of the acylcobalt carbonyl intermediate [PhCH2COCo(CO)4] before the alkyne complex is formed. This obstacle has been overcome by carrying out the reaction in the absence of water, using solid/liquid PTC [149]. The direction of the PTC carbonylation of alkynes in the presence of Mel changes again when cobalt and ruthenium carbonyls are used simultaneously (1 1) [150]. In this case, y-ketoacids are obtained. 

Reactions at Benzylic Positions. - The catalytic oxidation, using a cobalt bromide catalyst, of 2,5-dimethylthiophen to the dialdehydeof 3-methyl-2-ethylthiophen to 2-acetyl-3-methylthiophen and l-(3-methyl-2-thienyl)-ethyl acetate, of the four isomeric methyl acetothienones to the corresponding acetylthiophencarboxylic acids, and of 2-acetoxymethylthio-phen to thiophen-2-carboxylic acid has been reported. The Wittig reaction between 2- and 3-thenyltriphenylphosphonium salts and crocetin dialdehyde has been used for the synthesis of carotenoid analogues with terminal thiophen rings. ... 

Benzylic Reactivity. 2,5-Dimethyl-3,4-dinitrothiophen has been condensed with aromatic aldehydes to give 3,4-dinitro-2,5-distyryl-thiophens. " An improved synthesis of 3-nitro-2-styryl-thiophens involves bromination of 3-methyl-2-nitrothiophen with A-bromosuccinimide, followed by a modified Wit-tig reaction. From 2,5-di(chloromethyl)thiophen, the phosphonate was prepared by the Arbusov reaction, which was used for the preparation of 2,5-distyryl-thiophens. Trichloromethyl(thienyl)carbinols have been converted into the corresponding fluoro-derivatives through the reaction with phenyl-tetrafluorophosphorane. The liquid-phase catalytic oxidation of phenyl-(2-thienyl)methane in acetic acid in the presence of cobalt(ll) acetate and sodium bromide has been investigated. ... 

Tsang SC, Zhu J, Yu KMK. A new oxidation catalyst system using fluorous cobalt(ll) species in water-supercritical carbon dioxide (C-H free environment). Catal Lett 2006 106 123-6. Martin CA, McCrann PM, Angelos GH, Jaeger DA. Reactions in microemulsion media nucleophilic displacement reaction of benzyl chloride with bromide ion. Tetrahedron Lett... 

Quite similar results have been found recently in the reaction of the cobalt(i) form of vitamin B,2 (Bus) with alkyl halides with n-butyl iodide, bromide and chloride, ethyl bromide and benzyl chloride the representative data point of vitamin B s falls several orders of magnitude above the outer sphere dissociative electron-transfer line (Walder, 1989). 

Carbonylation with iron carbonyls parallels that of cobalt carbonyls. Benzylic chlorides and bromides are carbonylated with Fe(CO)5 in the presence of base. Esters are realized when carbonylation is performed in alcohols under 1 atm of CO with catalytic amounts of iron pentacarbonyl415. Under phase transfer conditions, two predominant routes are available. With catalytic amounts of iron under a CO atmosphere and strongly basic conditions, the carboxylic acids are realized in reasonable yields415,416, whereas mild bases [Ca(OH)2l, stoichiometric amounts of iron carbonyl and the omission of CO give dibenzyl ketones417. In at least a few cases, it is possible to prepare unsymmetrical methyl benzyl ketones418, des Abbayes and coworkers have observed the formation of acyltetracarbonyl anion (52) under the reaction conditions, and have proposed the catalytic cycle in Scheme 8 for the ketone formation418. 

The use of cobalt species to catalyze cross-coupling reactions of activated or unactivated alkyl halides with alkynyl Grignard reagents in order to form C(sp)-G(sp ) bonds is also of interest. Cobalt compleres promote the crossbenzyl chlorides and bromides react with 2-trimethylsilylethynyl- and 1-octyn-l-ylmagnesium halides to provide l-aryl-2-alkynes in the presence of a catalytic amount of a cobalt complex [100,101]. 

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