ORGANOMETALLIC NITROSYL CHEMISTRY .41. REGIOSPECIFIC CONDENSATION OF METHYL PROPIOLATE WITH 2,3-DIMETHYL-2-BUTENE MEDIATED BY THE ELECTROPHILIC ETA-5-CYCLOPENTADIENYL DINITROSYL TETRAFLUOROBORATE SALTS OF THE GROUP-6 METALS

Chloride abstraction from Cp’M(NO)2Cl (Cp’ = Cp (η5-C5H5), Cp* (η5-C5Me5); M = Cr, Mo, W) with an equimolar amount of AgBF4 in CH2Cl2 generates reactive solutions of Cp’M(NO)2BF4. During their reactions with various nucleophiles, the Cp’M(NO)2BF4 compounds behave as though they are the formally 16-electron cations Cp’M(NO)2+. Thus, all the CpM(NO)2BF4 complexes abstract a phenyl group from the BPh4- anion to form the known CpM(NO)2Ph compounds, and CpW(NO)2BF4 catalyzes the oligomerization of 2-methylpropene. Furthermore, methyl propiolate and 2,3-dimethyl-2-butene condense re-giospecifically in the coordination spheres of all the Cp’M(NO)2+ cations to produce the cationic lactone complexes [Cp’M(NO)2C═C(H)C(Me)2C(Me)2OC(OMe)]+. All the cationic lactone complexes undergo facile O-demethylation reactions upon exposure to an acetone solution of NaI. The resulting η1- lactone-containing organometallic compounds, Cp’M(NO)2C═C(H)C(Me)2C(Me)2OC(═O), have all been fully characterized by conventional spectroscopic techniques, and a single-crystal X-ray crystallographic analysis has been performed on the CpMo member of this series of complexes. Crystal data for CpMo(NO)2C═C(H)C(Me)2C(Me)2OC(═O): a = 11.337 (6) Å, b = 12.564 (3) Å, c = 11.958 (6) Å, β = 115.12 (4)°, Z = 4, space group P21/c, number of unique reflections 2713, number of variables 245, RF = 0.021, RwF = 0.030. The compound possesses a normal three-legged piano-stool molecular geometry, the intramolecular dimensions of the lactone ligand indicating the existence of some internal derealization of π-electron density. Treatment of the neutral CpMo lactone complex with [Me3O]BF4 regenerates its cationic organometallic precursor, and treatment with iodine liberates the iodolactone from the molybdenum's coordination sphere. Interestingly, exposure of the neutral tungsten lactone complexes to air results in their being cleanly converted to the corresponding dioxo derivates Cp’W(O)2(η1-lactone). © 1990, American Chemical Society. All rights reserved.