Allene complexes with cobalt

In coordination polymerization it is generally accepted that the monomer forms a 7r-complex with the transition metal prior to insertion into the growing chain. In general these complexes are insufficiently stable to be isolated although complexes of allene [69] and butadiene [70] have been reported. With allene the complex was formed prior to polymerization with soluble nickel catalysts, and cis coordinated butadiene forms part of the cobalt complex, CoCj 2H19, which is a dimerization cateilyst. 

Unsymmetrical nonconjugated allene-diynes with substituted allene groups. The cobalt catalized [2- -2- -2] cycloaddition of these allene-diynes leads to complex tricyclic compounds. The regioselectivity of... 

An allene is a very promising unsaturated partner in cobalt-mediated [2 + 2 + 2]-cycloaddition reactions. Exposure of an allenediyne to a stoichiometric amount of CpCo(CO)2 in boiling xylenes under irradiation for 5 h furnishes red-brown complexes in 42% isolated yield (Scheme 16.76) [84—87]. Treatment of a 7 3 mixture of the two diastereomers thus obtained with silica gel provides an oxygen-sensitive cobalt-free tricydic compound. 

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. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Some mechanism must exist for the facile movement of hydrogen in these complexes. Hydrogens are probably transferred from one part of the chain to another by interaction with the cobalt atoms. This idea is supported by the observation that allenes with at least one hydrogen atom (R2C=C= CHR) are pol5anerized by dicobalt octacarbonyl at room temperature... 

The palladium-catalysed oxidative acyloxylation/carbocyclization of allenynes (78) has been found to produce acyloxylated vinylallenes (82), where a new C-C bond, a new C-0 bond, and a new allene moiety are formed (Scheme 6). The reaction is believed to proceed via the initial Pd-coordination (79), followed by cyclization generating the 7 -complex (82) either via the Pd(II) (80a) or Pd(IV) (80b) complex. The reaction is completed by extrusion of the product (82) and re-oxidation of Pd(0) to Pd(II) by p-benzoquinone (p-BQ). Mechanistic evidence was gathered by isotopic labelling. An aerobic version was also realized using catalytic amounts of p-BQ together with a catalytic amount of cobalt salophen complex. ... 

The attempts to achieve enantioselective synthesis of cycloadduct compounds by using a chiral Lewis acid associated with achiral cobaltoxime resulted in moderate Another approach is to replace the dimethylglyoxime ligand set by salen ligands. Cobalt(ii) salen complexes synthesized in achiral form showed good results in Diels-Alder reactivity. In the chiral salen complex, the corresponding optically active dienylcobalt complexes 189 have been obtained by the reaction of optically active cobalt(ll) salen complexes 188 with allenic compounds (Equation (28)). 

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