STRUCTURE OF 2-METHYL-1-(PHENYLETHYNYL)CYCLOPENTANOL AND OF ITS CO-2(CO)(6) AND (C5H5)(2)MO-2(CO)4 CLUSTER COMPLEXES - CO-2(CO)(6) AS A CONFORMATIONAL SWITCH

Treatment of 2-methylcyclopentanone with PhC=CMgBr yields 2-methyl-1-( phenylethynyl)cyclopentanol (6'), in which the alkynyl group is axial and cis to the methyl substituent. Reaction with Co-2(CO)(8) gives the cluster complex 9, in which the (alkynyl)Co-2(CO)(6), group adopts an equatorial position and the hydroxyl and methyl groups are now axial. in contrast, (C5H5)(2)Mo-2(CO)(4) reacts with 6' to produce (phenylet hynyl)cyclopentene)Mo-2(CO)(4)-(C5H5)(2), (19). The alkyne, 6' and als o the cobalt and molybdenum complexes, 9 and 19 have all been characte rized by X-ray crystallography. The observed preference for the bulky (RC=C)Co-2(CO)(6) substituent to occupy an equatorial site in cycloalk anes may be used to rationalize the stereospecificity of the Friedel-C rafts cyclization of cobalt-stabilized propargyl cations onto suitably activated aromatic rings.