Dicobalt hexacarbonyl

Stereocontrolled synthesis and recyclizations of C-acetylenylglycosides in the presence of dicobalt hexacarbonyl with formation of fused saturated 0-heterocyclic systems 98CC2665. 

Two S/P ligands derived from camphor, CamPHOS and MeCamPHOS were also developed by these authors for the diastereoselective coordination to alkyne-hexacarbonyldicobalt complexes (Scheme 10.68). These two ligands were converted in good yields into their borane-protected forms. The influence of the alkyne group (R) on their coordination to dicobalt-hexacarbonyl-alkyne complexes was evaluated. It was shown that MeCamPHOS ligand provided a... 

The cyclopropane diester (800) bearing a vicinal acetylenic moiety, when treated with Co2(CO)s, affords the formation of the dicobalt hexacarbonyl complex (801). It undergoes a smooth cycloaddition with a,N -diphenylnitrone, in the presence of Sc(OTf)3, to form the corresponding dicobalt hexacarbonyl complex of tetraydro-l,2-oxazine (802). De-complexation of adduct (802) gives 6-ethynyl-tetrahydro-l,2-oxazine (803) (Scheme 2.332) (856). 

Finally, useful stereoselectivities have been recorded for the heteroconjugate addition of organometallic reagents to 1-silyl substituted vinyl sulfones.22 The synthesis of such sulfones can be achieved starting from phenylthioacetylene.6 1 11 23 The synthesis of the dicobalt hexacarbonyl complex24 and the polymerization of phenylthioacetylene25 have been described. 

An enantioselective intramolecular Pauson-Khand reaction based on chiral auxiliary-directed 7t-face discrimination in acetylenic 0-alkyl enol ether-dicobalt hexacarbonyl complexes, which proceeds with good yields and high facial diastereoselectivity, has recently been developed by M.A. Pericas, A. Moyano, A.E. Greene and their associates. The method has been applied to an enantioselective formal synthesis of hirsutene. Moreover, the process is stereodivergent and the chiral auxiliary -rran5-2-phenylcyclohexanol- is recovered in a yield as high as 92% [18]. 

Tandem reaction generally attracts much interest because it allows us to effect multiple transformations, all in one pot. Two types are known at present. The most popular approach is a combination of several reactions with PKR. Properly functionalized 1,6- or 1,7-enynes are readily obtained from the propargyl alcohol-dicobalt hexacarbonyl complexes and a properly nucleophilic allylic moiety in the presence of a Lewis acid. The resultant enynes are subjected to the promoter-assisted PKR without purification to afford the desired PKR products (Scheme 9).82,82a-82e... 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

The formation of the butenolactone complex, XII, by the action of carbon monoxide on acetylene dicobalt hexacarbonyl complexes, XI, (89) seems to be a closely related reaction. It probably involves the following steps ... 

Dicyclopentadienyldinickeldiphenylbutadiyne-dicobalt hexacarbonyl has been prepared from diphenyldiacetylene in which one triple bond acts as a bridging group between two nickel atoms and the other between two cobalt atoms (203). Reduction of the diphenylacetylene complex (R = R = Ph) with sodium and alcohol in liquid ammonia yields dibenzyl, showing that the diphenylacetylene grouping is bonded only to the nickel atoms. The corresponding complex of acetylene (R = R = H) has also been prepared from nickeloccne and acetylene (69) ... 

Propargylation (8, 148-149). The propargylation of aromatics and /1-dicarbonyl compounds with (propargyl)dicobalt hexacarbonyl cations (I) is now used for selective alkylation of ketones and their Irimethylsilyl enol ethers and enol acetates. The reaction is regiospccific and involves attack of the more thermodynamically stable enol, In the case of ketones, yields are substantially reduced by use of a solvent. In the case of enol derivatives, CH2C12 can be used.1... 

IJ-Enynes.1 (Propargyl)dicobalt hexacarbonyl cations (8, 148-149) couple with allylsilanes to give complexes of 1,5-enynes, generally in satisfactory yields. A typical reaction is formulated in equation (I). 

The molecule of diphenylacetylene dicobalt hexacarbonyl has been shown by an x-ray investigation.57 to have the structure shown in Figure 11-19. Each cobalt atom forms six bonds, directed toward the corners... 

Pig. 11-19.—The structure of dicobalt hexacarbonyl diphenylacetylene, Coa(CO)fCi (C H )j. Large circles represent cobalt atoms, small circles carbon atoms, and circles of intermediate size oxygen atoms. 

Alkylations Using (Propargylium)Dicobalt Hexacarbonyl Salts ... 

Dendrimers with metal complex moieties in their branches require the prior incorporation of specific coordination sites into the dendrimer scaffold. Newkome et al. used such a dendrimer with twelve alkyne units for spot-on introduction of l,2-dicarba-c oso-dodecaborane groups (Fig. 4.59, above right) [127]. Moreover, on-target coordination with dicobalt-octacarbonyl to form a metallodendrimer with twelve dicobalt-hexacarbonyl units was also accomplished. These units can serve as protective groups on the one hand [128], and as catalysts on the other [129]. 

Alkylations Using (Propargyl1um)Dicobalt Hexacarbonyl Salts Preparation of 2-(l-Methyl-2-propynyl)cyclohexanone... 

Later on, Schreiber used consecutively these two reactions in the key step for the synthesis of diterpene (-i-)-epoxydictimene (73), starting from natural (R)-pulegone [116,117]. This approach was built on their preliminary studies on Lewis acid mediated intermolecular Nicholas reactions [118]. They prepared functionalized enyne 70 bearing a mixed acetal. This compound was transformed into its dicobalt-hexacarbonyl complex and, in the presence of a carefully selected Lewis acid, it formed a stabilized carbocation by release of the more accessible ethyl moiety. This cation reacted intramolecularly with the allylsilane giving the central eight membered ring of the natural product... 

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