Reductive activation of arene, cyclohexadienyl and tropylium complexes of the early transition metals: Mechanisms and reactivities

The eta5-pentadienyl ligand in [Mn(eta5-C 6H7)(CO)3] can be activated by naphthalenide reduction. The course of the reaction depends on the reduction conditions---fast ("one squirt") reduction gives as the isolated product a dimer [{Mn(CO)3}2(mu2-eta4:eta 4-C6H7-C6H7)]2- held together by a new C--C bond between the hydrocarbyl ligands as established by X-ray crystallography. With slow dropwise reduction, in sharp contrast, the intermediate reduced anionic radical [Mn(eta 5-C6H7)(CO)3]-· participates in nucleophilic addition to the unreduced starting material to form [Mn(CO)3{eta4-C6H7-CO-Mn(eta 5-C6H7(CO)2}]2-. Both products can be oxidized back to the starting material. Taken together, the two reactions establish the intermediacy of the [Mn(eta5-C 6H7)(CO)3]-· radical anion in reductions of [Mn(eta5-C6H7(CO) 3].;In the second chapter substitution of unsaturated ligands for the eta 4-benzene in [Mn(eta4-C6H6(CO) 3]- has been extended from naphthalene, with which the reaction was first observed, to include cyclopentadiene. The reaction gives crystallographically characterized [Mn(eta4-C 5H6(CO)3]-.;In the third Chapter a new nucleophilic substitution reaction of an enoliminium ester by [Mn(CO)3(eta4-C6H6)] - has been established. The reaction modifies the activated benzene ligand in the anionic manganese complex, which acts as a nucleophile and displaces a triflate anion from the ester.;The fourth chapter reports electrophilic addition of [Fe(eta 5-C5H5)(CO)2{=CMe(OMe)}]+ to [Mn(eta4-C6H6)(CO) 3]-. The cross coupled product [Mn(CO)3{eta 5-C6H6-CMe(OMe)-Fe(eta5-C 5H5)(CO)2}] can itself be reduced by addition of two equivalents of naphthalenide and then reoxidized to the starting material at -78°C in high yield; oxidation of the reduced product at ambient temperature gives a new neutral compound [Mn(CO)3{eta 5-C6H6-C(=CH2)-Fe(eta5-C 5H5)(CO)2}] with one MeOH removed from the starting material, a result that implies intramolecular cyclization of the initial reduction product.;The fifth chapter explores reductive activation of tropylium complexes of the group 6 complexes [M(eta7-C7H7)(CO) 3]+ (M = Cr, W). They participate in [2+2] cycloaddition reactions similar to that reported for [Mn(eta6-C6H 6)(CO)3]+ to give [{M(CO)3} 2(mu2-eta5:eta5-C 7H7-C7H7)]2-. Control experiments provided a mechanistic account of the reaction that corrects previous accounts in the literature. An important extension of the reductively induced [2+2] cycloaddition reaction is cross cycloaddition of 6 and 7 membered rings to form intermediate [M(CO)3(mu2-eta 6:eta5-C7H7-C6H 6)Mn(CO)3] and then [Cr(CO)3(mu2-eta 5:eta4-C7H7-C6H 6)Mn(CO)3]2-, as described in the sixth chapter.