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

Domino [4+2]/[2+2] cycloaddition of 2-isopropenyl-2-oxazoline <b>2</b> to 1-alkynyl Fischer carbene complexes (CO)<sub>5</sub>MC(OEt)C⋮CPh <b>1</b> (<b>a</b>, M = Cr; <b>b</b>, W) in a 1:2 molar ratio afforded unusually stable biscarbene complexes <b>3a</b> and <b>3b</b> containing a novel four-, five-, and six-membered tricyclic core in 99.6% and 45.2% yields, respectively. Chain-opening <i>β</i>-aminoalkenyl monocarbene complex <b>4b </b>and <i>β</i>-amidoalkenyl monocarbene complex <b>5b</b> of tungsten were also isolated from the cycloaddition upon treatment of the reaction mixture of <b>1b</b> and <b>2</b> on silica gel. Partial and full oxidation of <b>3a</b>,<b>b</b> with pyridine <i>N</i>-oxide underwent efficient chemoselective stepwise demetalation to afford the corresponding monocarbene complexes <b>6a</b>,<b>b</b> and organic diester <b>7</b>, respectively, under mild conditions. The X-ray crystallographic study revealed the presence of a four-, five-, and six-membered tricyclic core in compounds <b>3</b>, <b>6</b>, and<b> 7</b>, and the methyl and oxazolindinyl groups derived from oxazoline <b>2</b> are positioned <i>syn</i> 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 <b>3a</b>, <b>5b</b>, and <b>6b</b> as well as for diester <b>7</b>.