Development of New Reactions Catalyzed by Low-Valent Organotransition Metal Clusters

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Various types of tetrahedral organotransition metal clusters, those consisted of three low-valent organotransition metal units and one carbon or heteroatom unit, were synthesized and studied their catalytic activity in organic transformations. In cyclization of 1-hepten-6-yne derivatives under carbon monoxide atmosphere producing cyclopentenone derivatives (so-called the Pauson-Khand reaction), Co_3(CO)_9(μ^8-CR) showed the highest catalytic activity among the clusters investigated, such as Co_3(CO)_9(μ^3-SiR), Co_3(CO)_9(μ^3-P), and Co_3(CO)_9(μ^3-S). The substituent on the carbon unit also affected to the efficiency of the reaction and the structurally simplest Co_3(CO)_9(μ^3-CH) seemed to be the best catalyst so far. The cluster, methylidynetricobalt nonacarbonyl Co_3(CO)_9(μ^3-CH), also turned out to catalyze chemoselective reaction of alkynes, such as [2+2+2]-cyclotrimerization of alkynes producing benzene derivatives, [2+2+1]-cocyclization of alkynes and carbon monoxide producing cyclopentadienones, [2+2+2+2+1]-tandem cyclization of alkynes producing hydroindenone derivatives, and hydrosilylation of alkynes. Tetrahedral clusters consisted of different kinds of organotransition metal units, such as Co_2(CO)_6[MoCp(CO)](μ^3-CPh), Co_2(CO)_6[WCp(CO)](μ^3-CPh), Co_2(CO)_6(NiCp)(μ^3-CPh), Co(CO)_3[MoCp(CO)]_2(μ^3-CPh), Co(CO)_3[WCp(CO)]_2(μ^3-CPh), and Co(CO)_3(NiCp)_2(μ^3-CPh), were also used as a catalyst in the Pauson-Khand reaction. Comparison of the catalytic activity of these clusters with the original Co_3(CO)_9(μ^3-CPh) revealed that change one Co(CO)_3 unit with MoCp(CO) increased yield of the desired compounds and accelerated the reaction.