Cobalt, complexes allyl

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

Most organopentacyanides are stable towards [Co(CN)jH], with the exception of allyl complexes which react to liberate propylene derivatives (105). This is one of the steps in the homogeneous hydrogenation of butadienes catalyzed by cobalt cyanide complexes (see Section VII,A). 

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. 

Cobalt hydroformylation of butadiene produced low yields (24%) of an equimolar mixture of n- and isovaleraldehyde (40). It has been established that the cobalt hydrocarbonyl adds to form a stable 7r-allyl complex (93, 94). 

A cobalt-mediated formal Alder ene reaction of an allenyne takes place to give a mixture of adducts and ( 4-cycloliexadiene)cobalt complexes (Scheme 16.77) [88], The reaction may proceed via coordination and ensuing jt-allyl complex formation. 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

The alkylidyne tricobalt nonacarbonyl complexes (2) are produced from the reaction of the cobalt tetracarbonyl anion with 1,1,1-trihaloalkanes [4], under conditions analogous to those used for the synthesis of the n-allyl complexes. Although the yields for (2) appear to be low (Table 8.3), they are better than, or comparable with, those obtained by the traditional procedures [8] and are obtained under more amenable conditions. 

General preparation of it-allyl cobalt tricarbonyl complexes (Scheme 8.1)... 

Acetylation occurs at the 2-position of allene systems (Scheme 8.14). The intermediate 7t-allyl complex breaks down via the nucleophilic displacement of the cobalt carbonyl group by the hydroxide ion to produce the hydroxyketone (7) [ 11 ]. An alternative oxygen-initiated radical decomposition of the complex cannot, however, be totally precluded. The formation of a second major product, the divinyl ketone (8), probably arises from direct interaction of the dicobalt octacarbonyl with the allene and does not require the basic conditions. 

Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentaeyanocobaltate(II) catalyst or when the a butenyl complex is reduced with the hydride complex [HCo(CN)5] . Thus 1-butene is the major product in the presence of excess CN, and major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a tr complex, and the 2-butene via an intermediate w-allyl complex. The Tr-allyl complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO)4 or from methallyl halide and NaCo(CO)4 [49). 

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

The tris-allyl complex, in each case, produced a 1.2 growth step of the butadiene molecule. With the more anionic (or less cationic) cobalt salt, the growth occured to only the dimer before it underwent anionic hydride chain transfer. With less anionic chromium the 1.2 chain growth continued on the produce polymer. 

Sulfur dioxide is capable of reacting with metal alkyl, aryl or u-allyl complexes in an insertion-type reaction to yield S-sulfinate (4), O-sulfinate (5) or 0,< -sulfinate complexes (6).13 It can also insert into the metal-metal bond in the cobalt complex (7) to give the S02-bridged complex (8).38... 

Although mechanistically different, a successful kinetic resolution of cyclic allyl ethers has recently been achieved by zirconium catalysis [2201. Other metals such as cobalt [221], ruthenium [222], and iron [2231 have been shown to catalyze allylic alkylation reactions via metal-allyl complexes. However, their catalytic systems have not been thoroughly investigated, and the corresponding asymmetric catalytic processes have not been forthcoming. Nevertheless, increasing interest in the use of alternative metals for asymmetric alkylation will undoubtedly promote further research in this area. 

It was suggested that such derivatives would have unusual stability like the corresponding 7r-allyl complexes and would be further reduced by hydrocarbonyl rather than undergoing carbonylation. Heck (63) has, however, attempted the preparation of similar derivatives from chloroacetone and phenacyl bromide with sodium cobalt tetracarbonylate, Instead of finding unusually stable complexes he reported an unusual instability. It seems likely that in fact normal alkylcobalt carbonyls are formed, e.g.,... 

Rusling et al. performed electrochemically and light mediated radical additions of alkyl iodides to cyclohexenone in conductive microemulsions catalyzed by 20 mol% of 247 in 14—81% yield [303]. Radical allylations of alkyl bromides 249 with allyl sulfides, sulfones, or phosphates catalyzed by 5 mol% of cobalt (iminate) complex 250 in the presence of zinc as reducing agent proceeded in 52-85% yield [304],... 

Jacobsen-Katsuki-type chiral (salen)cobalt(II) complex 267 (10 mol%) was used by Dunach and coworkers to catalyze electrochemical radical 5-exo cyclizations of ort/m-bromophenyl allyl ethers 293a,b to dihydrobenzofurans 295a,b (and 296a,b) [332]. Constant current or constant potential conditions... 

Under a nitrogen atmosphere, cobalt carbonyl probably experiences disproportionation by base (Scheme 5) to give Co(CO)4. One of the disproportionation by-products is the Co(II) ion which gives a blue color in aqueous NaOH due to the presence of small amounts of the Co(OH)42-ion. The subsequent reaction of the cobalt tetracarbonyl anion with 14 is probably a displacement (path a), giving the cr-allyl complex 16. However, the possibility of an electron-transfer pathway (path b), either as an alternative to, or concurrent with, the displacement pathway cannot be dismissed at this time. 

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