Organometallic Reaction Mechanisms: Catalytic Cyclotrimerization of Alkynes, Reduction of Carbon Monoxide and Reductive Elimination from an Alkylhydride Complex

Chapter I The cobaltacyclic compound, (n55-C5H5)Co(C(CH3)=C(CH3)c(CH3)=C(CH3)) (PPh3) (3a), formed by reaction of (n55-C5H5)Co(PPh3)2 (1a) with 2-butyne is shown to be an efficient catalyst for cyclotrimerization of alkynes to provide arenes. An investigation of the substitution reactions of PMe3 and 2-butyne with 1a and with its PMe3 containing congeners (1b and 1c) is reported. A mechanism consistent with this data and in accord with the observed dependence of the rate of formation 3a on PPh3 and 2- butyne concentration is proposed. The kinetics of the cyclotrimerization of 2-butyne catalyzed by 3a both in the presence and absence of excess PPh3 and the kinetics of substitution of PEt3 for PPh3 in 3a are reported. These results indicate a mechanism involving rate determining coordination of 2-butyne to cobalt. In contrast the reaction of C2(CO2Me)2 (dma) with the PMe3 substituted analog of 3a,(3c), is shown to proceed without prior coordination of the alkyne. Chapter II Bis (pentamethyl cyclopentadienyl)-dihydrido zirconium (IV) (ƞ5-C5Me5)2ZrH2 (2) forms unstable adducts with PF3 and CO at -80°. The carbonyl adduct (ƞ5-C5Me5)2ZrH2 (CO) yields {(ƞ5-C5Me5)2ZrH}2 (μ-OCH=CHO) and/or (ƞ5-C5Me5)2 ZrH(OCH3 upon warming depending on conditions. Carbonylation of (ƞ5-C5Me5)2 Zr(CH3)2 2 provides successively (ƞ5-C5Me5)2 Zr(CH3) (C(=O)CH3) and (ƞ5-C5Me5)2 Zr(OC(Me)=C(Me)O). Reaction of the zirconocyclopentane (ƞ5-C5Me5)2 Zr(CH2(CH2)2CH2 with CO affords (ƞ5-C5Me5)2Zr(H) (OCH=CH(CHMe2). Treatment of 2 with isobutylene gives (ƞ5-C5Me5)2Zr(H) (CH2) (13), which reacts with CO to give (ƞ5-C5Me5)2Zr(H) (OCH=CH(CHMe2))(15). The results of 13C and deuterium labeling studies indicate that (ƞ5-C5Me5)2 Zr(H) (C(=O)CH2CHMe2) is an intermediate in conversion of 13 to 15. The observed reaction patterns of alkyl and hydride derivatives of zirconium with CO are attributed to carbenoid character of the carbonyl carbon resulting from an unusual "side-on" coordination of acyl or formyl groups. Chapter III The alkyl hydride complex (ƞ5-C5Me5)2Zr(H)(CH2CHMe2) (5a), prepared by reaction of (ƞ5-C5Me5)2ZrH2 (4) with isobutylene, is a monomeric, pseudotetrahedral compound. In direct contrast to other transition metal complexes bearing cis hydride and alkyl ligands it is reasonably thermally stable. However, isobutane is evolved from Sa upon thermolysis at 75°. Reaction with H2 or ethylene also results in generation of isobutane. The isotopically labeled complexes (ƞ5-C5Me5)2Zr(D)(CH2CDMe2) (5b) and (ƞ5-C5(CD3)5)2Zr(D)(CH2CD(CH3)2)(5c) are prepared by reaction of isobutylene with (ƞ5-C5Me5)2ZrD2 and (ƞ5-C5(CD3)5)2ZrD2. respectively. The deuterium distribution in isobutanes resulting from thermolysis of 5a, b and c and from their reactions with H2, D2, C2H4 and C2D4 were determined. The rates of thermolysis of those complexes are also reported. Mechanisms consistent with these data are proposed. For the cases of thermolysis and reaction with H2, the suggested mechanisms are similar to those proposed to account for exchange processes between 4 and a deuterium atmosphere.