Allylcobalt complexes

An analogous a-allylcobalt complex is formed by treating allyl halide with KaCo(CN)5 ... 

The Co reagent 192, prepared by the reaction of Co2(CO)8 with sodium, is reactive, and the acylcobalt complex 193 is formed by the reaction of acyl halides. Insertion of butadiene at the Co-acyl bond generates the 7r-allylcobalt complex 194, from which the acylbutadiene 195 is formed by deprotonation with a base [82]. Based on this reaction, various acyldienes are prepared by Co2(CO)8-catalysed reaction of active alkyl halides, conjugated dienes and CO. The Co-catalysed reaction can be carried out smoothly under phase-transfer conditions. For example, 6-phenyl-3,5-hexadien-2-one (197) was prepared in 86% yield by the reaction of Mel, 1-phenylbutadiene (196) and CO in the presence of cetyltrimethylammonium bromide [83]. 

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

A number of r-allylmetal complexes react with nucleophiles resulting in transfer of the allyl group from the metal to the nucleophile. As previously shown the most developed are the r-allylpalladium complexes. However, tt-allyltricarbonylcoblat(I) complexes also react with carbanions to produce allylic alkylation products in good yield. Acylated r-allylcobalt complexes when reacted with a nucleophile result in an overall acylation/alkylation of 1,3-dienes, (equation 71). 

Allylic Systems. Allylic Halides. Allylic halides also undergo homolytic carbon-halogen cleavage by pentacyanocobaltate(II) to form equimolar quantities of halo- and allylcobalt complexes (21, 22, 23). It is assumed that this reaction involves generation of the allylic radical (Reaction 19), which then reacts with pentacyanocobaltate(II) (Reaction 20). 

Unlike their saturated counterparts, allylcobalt complexes are readily cleaved by hydrido complex or acids to yield mono-olefins. 

While the parent allylcobalt complex (R = R = H) was stable in an aqueous alkaline solution, the butenylcobalt complex (R = CH3, R = H) gradually evolved an approximately equimolar mixture of butenes and butadiene, suggesting a disproportionation involving the intermediate formation of hydrido complex (Reactions 3 and 4) (20, 22). Since apparently the same butenylcobalt complex is formed by adding hydrido complex to butadiene, the reduction of allylic halides is discussed with the intimately related hydrogenation of dienes later in the text. 

Since the butenylcobalt complex was too unstable, the PMR spectrum of the allylcobalt complex was studied 21, 22) to obtain information on the structure of the organocobalt intermediates in these stereoselective reductions (20). This investigation indicated that a-bonded allylic complexes are in equilibrium with 7r-allylic complexes and cyanide ion, thus providing a rationale for the stereoselectivities observed in the reduction of butadiene and butenyl chlorides. 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

Allylic cobalt tetracarbonyls are less stable than saturated alkylcobalt tetracarbonyls because they very readily evolve carbon monoxide and form 7T-aIlylcobalt tricarbonyls. Nuclear magnetic resonance studies have shown that these 7r-allylcobalt complexes possess symmetrical rather than un-symmetrical structures (6, 2S). 

Metal-mediated and -catalyzed [3 + 2 + 2]-higher-order cycloaddition reactions have also proved to be viable and mechanistically novel methods for the synthesis of seven-membered rings. The reported [3 + 2 + 2]-cycloadditions of allyliridium (Equation (30)),139 -allylcobalt (Scheme 47),140 and allylmanganese (Equation (31 ))141 complexes with alkynes involve the reaction of preformed allylmetal complexes with two separate alkynes, leading to a cycloheptadiene-metal complex. 

Comparison of the different types of cobalt catalysts shows that the in situ system [Eq.(2)] is most accessible while the Rep-, R(ind)-, and bori-ninato ligands having electron-withdrawing substitutents are the most active. The difference between the 14e" and the 12e core complexes makes itself apparent in the chemoselectivity of the reaction. Catalysts containing a 14-electron core favor pyridine formation, whereas those containing a 12-electron core (i.e., the rj -allylcobalt systems) favor the formation of benzene derivatives by cyclotrimerization of the alkynes. For example, in the reaction of propyne and propionitrile at 140°C in the presence of a 12-electron system (5), a 2 1 ratio of benzene to pyridine product is formed, whereas a catalyst containing the cpCo moiety (a 14-electron system) leads (under identical conditions) to the predominant formation of pyridine derivatives (84HCA1616). 

An allylcobalt(I) complex [Co(Tn3-C3H5) P(OMe)3 3] catalyzes the hydrogenation of aromatics at ambient temperature.160 176 Distinctive features of this catalyst are the high stereoselectivity [only cis products are formed from xylenes and naphthalene see Eq. (11.39)] and complete hydrogenation of aromatic systems177 [Eqs (11.39) and (11.40)] ... 

Allyl bromides (14) react, at room temperature, with a stoichiometric quantity of Co2(CO)8, sodium hydroxide (5 N), benzene, and benzyl-triethylammonium chloride as the phase-transfer catalyst. Fine yields (Table II) of ir-allylcobalt tricarbonyl complexes (15) were obtained by use of short reaction times (15-60 min) (28). This simple and mild method is superior to conventional routes described in the literature (29-31). 

Although butadiene reacts with Co2(CO)8 to yield the diene complexes (diene)C02(CO)o and (diene)2Co2(CO)4 (268), with alkyl- or acylcobalt tetracarbonyls it produces only the Tr-allylic species, 1-alkyl- or 1-acylmethyl-TT-allylcobalt tricarbonyls (281). These will react, in turn, with P(C3Hb)3 which displaces one CO ligand to form monotriphenyl-phosphine derivatives (281). 

In 1960 several groups of workers developed a route to the parent complex, TT-allylcobalt tricarbonyl (VII) 122, 123, 170) and also to tt-allylmanganese tetracarbonyl (VIII) 14S, 170). These syntheses were based on reactions of appropriate metal carbonyl anions with allyl halides as shown below. 

Scheme 5. Tentative reaction mechanism for the allylcobalt(I) complex-catalyzed syndiotactic 1,2-polymerization of butadiene.

Acylmetal complexes generally react with conjugated dienes to produce useful organic compounds. For example, acylcobalt carbonyls add to butadiene, producing 1 -acylmethyl-7r-allylcobalt derivatives, which then undergo elimination of the elements of HCo(CO)3 on treatment with base to give 1-acyl-diene u6). This reaction can be made catalytic with respect to the cobalt catalyst under proper reaction conditions. 

Tricari5onyl-7c-allylcobalt(I) gives 7c-fluoroalkenylCo complexes under similar conditions ... 

Co2(CO)8 and HCo(CO)4 using HP-IR. A cobalt carbonyl-butadiene complex and an allylcobalt carbonyl species were formed from the reaction of butadiene with Co2(CO)8. Reaction with HCo(CO)4 gradually afforded an alkenyl complex which reacted to form further byproducts. The reaction of cobalt carbonyls with hydrogen or deuterium, and the stoichiometric interaction of the resultant hydride with alkenes has also been studied with HP-IR. Ojima reported HP-IR studies into Co-Rh bimetallic carbonyl catalysts, and observed a CoRh(CO)7 species which was active for hydroformylation-amidocarbonylation. 

Even 4-pentenoylcobalt tetracarbonyl, which exists at 0° C under 1 atm of carbon monoxide entirely as the cyclic it complex, decomposes into 1-methyl-jT-allylcobalt tricarbonyl when heated to 35° C (6). 

The l-acylmethyl-TT-allylcobalt tricarbonyl complexes react with bases to eliminate the elements of cobalt hydrotricarbonyl and from acyidienes (15). 

Carbonyl insertion reactions may also occur, and detailed studies of the reaction of the very unstable a-allylcobalt tetracarbonyl with carbon monoxide and triphenylphosphine show the formation of butenoyl complexes via a 16-electron cobalt tricarbonyl intermediate (3a) 45, 50), viz ... 

There have been relatively few studies on either 77 -cyclopropenyl- or allylcobalt derivatives during the last decade as compared with the if - or 77 -hydrocarbyl-Co complexes. 

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