A STUDY OF THE REACTIVITY AND STRUCTURE OF CYCLIC ALPHA,BETA-UNSATURATED FISCHER-TYPE CARBENE COMPLEXES

Cycloaddition reactions with \u3b1,\u3b2-unsaturated carbene complexes of the Fischer-type bearing the carbene carbon atom and the double bond incorporated in the same ring are described. Pentacarbonyl(2H-benzopyran-2- ylidene)chromium(0) complexes (2a-c) and pentacarbonyl(4-methoxy-3,3-dimethyl-2-oxacyclopentylidene)- chromium(0) (3) show a rather low reactivity towards 1,3-dipoles and 1,3-dienes. The reactions with diazomethane are regioselective but not chemoselective; compounds 2 and 3 show two sites of attack: the \u3b1,\u3b2 carbon-carbon and the carbon-metal double bond. The crystal and molecular structures of 2a and 3 have been elucidated by single crystal X-ray analysis. Crystals of 2a are monoclinic, space group P21/c, a=7.614(3), b=14.033(3), c=12.766(3) \uc5, \u3b2=95.24\ub0, V=1358.3(7) \uc5 Z=4; crystals of 3 are triclinic, space group P1, a=6.553(1), b=9.408(1), c=10.620(1) \uc5 \u3b1=92.70(1), \u3b2=92.30(1), \u3b3=92.12(1)\ub0, V=653.0(1), \uc53, Z=2. Final agreement indices for 2a and 3 are R=0.034 and 0.033, respectively. Vibrational properties of the Cr(CO)5 moiety were interpreted by FT-IR and FT-Raman spectroscopy. Electronic spectra and \u3c0 electron distribution were interpreted by resonance Raman spectroscopy