Cobalt, alkyne-substituted clusters

The area of alkyne cluster chemistry has been the subject of two previous review articles. The first is concerned largely with alkyne-cobalt chemistry (16), while the second provides a comprehensive, systematic review of alkyne-substituted homo- and heterome-tallic carbonyl clusters of the iron, cobalt, and nickel triads (17). This latter review covers the literature up to the end of 1981. The present work does not set out to be fully comprehensive, but rather reflects the authors own interests in the subject. A number of key examples are... 

In a subsequent study, they used ethylene for a dual purpose, as a substrate as well as a supercritical fluid solvent. This notoriously unreactive olefin to PKR served nicely to give 2-substituted cyclopentenones. Reaction efficiency of each alkyne substrate can be tuned by changing catalyst precursors. Not only Co2(CO)8 but also the two cobalt clusters [Co4(CO)i2] and [Co4(GO)n P(OPh)3 ] work well for some substrates (Equation (8)). The comparison with Rautenstrauch s result clearly shows the beneficial effect of this approach. 

Tetrahedrane (11) is the ruthenium analog of the much-studied tricobaltnonacarbonyl clusters Co3(CO)9CR see Cobalt Organometallic Chemistty). The substitution chemistry of (11) has been studied. A starting material is prepared from (11) by reaction with BX3 (equation 2), which gives the chloro and bromo compounds (12). In addition, (11) can also be treated directly with compounds such as diynes to yield interesting substitution products. For example, when (11) is refluxed in THF with diphenylbutadiyne, cis- and trans-alkene isomers of two alkyne insertion regioisomers are formed (equation 3). The product seems to arise from dehydrogenation of one end of the diyne to yield cis and trans enynes and an imsaturated monohydride cluster intermediate, which then reacts with the enynes to yield the allylic derivative products... 

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. ° ... 

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