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

摘要

Domino [4+2]/[2+2] cycloaddition of 2-isopropenyl-2-oxazoline 2 to 1-alkynyl Fischer carbene complexes(CO)_5M=C(OEt)C ident to 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 beta-aminoal-kenyl monocarbene complex 4b and beta-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.