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

Abstract

Domino [4+2]/[2+2] cycloaddition of 2-isopropenyl-2-oxazoline 2 to 1-alkynyl fischer carbene complexes (co)(5)m=c(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 beta-aminoalkenyl 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.