Chemoselective Stepwise Demetalation of Unusually Stable Fischer Biscarbene Complexes by Domino [4+2]/[2+2] Cycloaddition of 2-Isopropenyl-2-oxazoline to 1-Alkynyl Fischer Carbene Complexes of Chromium and Tungsten
journal contributionposted on 17.01.2005 by Jinzhu Chen, Zhengkun Yu, Zhaoyan Zheng, Kaichun Gu, Sizhong Wu, Fanlong Zeng, Weiqiang Tan, Xiaowei Wu, Wenjing Xiao
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Domino [4+2]/[2+2] cycloaddition of 2-isopropenyl-2-oxazoline 2 to 1-alkynyl Fischer carbene complexes (CO)5MC(OEt)C⋮CPh 1 (a, M = Cr; b, W) in a 1:2 molar ratio afforded unusually stable biscarbene complexes 3a and 3b containing a novel four-, five-, and six-membered tricyclic core in 99.6% and 45.2% yields, respectively. Chain-opening β-aminoalkenyl monocarbene complex 4b and β-amidoalkenyl monocarbene complex 5b of tungsten were also isolated from the cycloaddition upon treatment of the reaction mixture of 1b and 2 on silica gel. Partial and full oxidation of 3a,b with pyridine N-oxide underwent efficient chemoselective stepwise demetalation to afford the corresponding monocarbene complexes 6a,b and organic diester 7, respectively, under mild conditions. The X-ray crystallographic study revealed the presence of a four-, five-, and six-membered tricyclic core in compounds 3, 6, and 7, and the methyl and oxazolindinyl groups derived from oxazoline 2 are positioned syn with respect to the azabicyclo[4.2.0]octadiene bicyclic moiety, which is oriented in the opposite direction. X-ray crystal structural data are reported for the bis- and monocarbene complexes 3a, 5b, and 6b as well as for diester 7.