Cobalt-catalyzed cyclotrimerization of alkynes: The answer to the puzzle of parallel reaction pathways

被引:157
作者
Agenet, Nicolas
Gandon, Vincent
Vollhardt, K. Peter C.
Malacria, Max
Aubert, Corinne
机构
[1] Univ Paris 06, CNRS, Inst Chim Mol, UMR 7611,Lab Chim Organ,FR 2769, F-75252 Paris 05, France
[2] Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA
[3] Lawrence Berkeley Lab, Div Chem Sci, Berkeley, CA 94720 USA
关键词
D O I
10.1021/ja072208r
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
To understand some experimental data at odds with the computed mechanism of the CpCo(L-2)-catalyzed [2 + 2 + 2] cyclotrimerization of ethyne, DFT computations were carried out following the fate of methyl- and hydroxycarbonyl-substituted alkynes to give the corresponding arenes. The key intermediate in all cases is a triplet cobaltacyclopentadiene obtained by oxidative coupling of the corresponding CpCo(bisalkyne) complex and subsequent spin change via a minimum energy crossing point (MECP). From that species, two different catalytic cycles lead to an arene product, depending on the nature of the alkyne and other ligands present: either alkyne ligation to furnish a cobaltacyclopentadiene(alkyne) intermediate or trapping by a sigma-donor ligand to generate a coordinatively saturated cobaltacyclopentadiene(PR3) complex. The former leads to the CpCo-complexed arene product via intramolecular cobalt-assisted [4 + 2] cycloaddition, whereas the latter may, in the case of a reactive dienophile (butynedioic acid), undergo direct intermolecular [4 + 2] cycloaddition to generate a cobaltanorbornene. The bridgehead cobalt atom is then reductively eliminated after another change in spin state from singlet to triplet. The necessary conditions for one or the other mechanistic pathway are elaborated.
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收藏
页码:8860 / 8871
页数:12
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