Cobalt hexacarbonyl

The mechanism of the reactions of 3-hexyne and acetylene-cobalt hexacarbonyl in toluene solution is similar to reactions (1) and (2) of the lactone decomposition. These reactions are followed by some disproportionation of the monophosphine... 

Related structurally to the RCCo3(CO)9 complexes are the acetylenedi-cobalt hexacarbonyls which have the structure with a pseudotetrahedral... 

In a complementary contribution to this chemistry, Young assembled a [9.5.5] system by sequential metathesis reaction on a diene linked by a cobalt hexacarbonyl complexed alkyne followed by domino Nicholas-PKR which we will comment later [111]. 

Other reactions that may occur before the PK are hydrogenolysis, reductions of the alkyne complex and, when using dienes as the alkene part, a Diels-Alder reaction. When hydrogenolysis is observed in enynamides or enynethers, the resulting products usually do not cyclize, so no PK products are obtained [136]. One interesting case of previous reduction of a cobalt-hexacarbonyl complex, shown in Scheme 34, implied that in the presence of TFA, part of complex 121 was reduced to alkene 122 which reacted further with other complex molecules to give the PK product 123 [137]. 

Scheme 34 Partial reduction of the alkyne-cobalt hexacarbonyl complexes under PK conditions in the presence of TFA...

Liu has reported recently a new Co2(CO)8-mediated tandem [5+ 1]/ [2 + 2+ 1]-cycloaddition reaction that gives tricyclic 5-lactones from cis-epoxy enynes 186. This process possibly involves an initial opening of the epoxide in the cobalt hexacarbonyl complex 187 to from the complexed al-lene 189 via 188. Further coordination of the tethered olefin leads to 190 and further oxidative cyclization gives cobaltacycle 191 which inserts CO leading to the final compound 192. When performing the reaction under N2,191 suffers a reductive elimination to give cyclobutane 193 (Scheme 55) [172],... 

Upon heating, loss of a third equivalent of carbon monoxide opens a coordination site to provide 1-2 and allows cobalt to complex the alkene as in 1-3. The alkene inserts into the least hindered cobalt-carbon bond to form the cobaltacycle 1-4. Carbon monoxide inserts into the new cobalt-carbon bond to give 1-5. Next, acyl migration from cobalt to carbon forms the final carbon-carbon bond as in 1-6. Finally, a formal reductive elimination of the spectator cobalt releases the product cyclopentenone P and cobalt-hexacarbonyl. The cobalt is then ready for another catalytic cycle. In practice the cobalt is most often used stoichiometrically. The alkyne cobalt complex is a stable species that can be purified and isolated by silica gel chromatography. Conditions for catalytic turnover of the cobalt have been reported. 

It was difficult to envisage a solid-phase quantitative analysis at this level of concentration. However, for an alkyne cobalt hexacarbonyl complex such as 24 in a chlorinated solvent, use of an ultralow volume gold light-pipe can provide a detection limit as low as 300 fmol (i.e 10 nM), [108], a level fully compatible with the sensitivity range required for the assay of many medications and banned drugs. 

A series of alkyne cobalt hexacarbonyl complexes was synthesized by Jung et al. [116] and Gust et al. [117-119], These include, for example, Co2(CO)g complexes derived from aspirin 40, from diphenyl acetylene 41 and from 2-propyn-l-ol 42. 

We envisioned that an alkynyl substituent on a cyclopropane could be complexed as a cobalt hexacarbonyl derivative [8], which would enable activation of the cyclopropane toward ring opening in order to undergo subsequent cycloadditions. This is explained by the ring-opening of a cobalt complexed cyclopropanediester I under the influence of a Lewis acid to form the Nicholas carbocation 2 as illustrated in Scheme 10.3. 

This was prepared in four steps, starting with the formation of the vinyl cyclopropane, upon reaction with dimethyl-malonate and l,4-dibromobut-2-ene. Ozonolysis of the vinyl cyclopropane afforded the aldehyde, which was reacted with the Bestmann reagent to afford the alkyne substituted cyclopropane, 3. The cyclopropane was complexed with cobalt to afford the cobalt hexacarbonyl cyclopropane 4 in 85% yield. 

Kerr and coworkers have also successfully reported the use of cobalt hexacarbonyl cyclopropanes in [3-1-2] cycloadditions with nitrones via a purported Nicholas-type reaction in the presence of scandium triflate (Sc(OTf)3) to afford a variety of oxazines in high yields (Scheme 10.9) [11]. 

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