Investigations in cooperative catalysis: synthesis, reactivity and metal-ligand bonding

2013-07-06Research presented in this work describes investigations into the synthesis, structure, and reactivity of cooperative catalysts. Cooperative catalysis, the concept of actively involving ligands in catalytic transformations, is used extensively in enzymatic systems and has been shown to enable organometallic catalysts to realize improved enantioselectivity, chemoselectivity, or activation of otherwise unreactive bonds. Two types of cooperative catalysts are investigated in this work: non-innocent ligands and ligand-based directing groups. Initial investigations detailed in this work revolve around a catalyst of the first type: Shvo’s catalyst ({[Ph₄(η⁵-C₄CO)]₂H]}Ru₂(CO)₄(μ-H)) which is known for containing a non-innocent cyclopentadienone ligand. Shvo’s catalyst is shown to be a competent catalyst for alkylation of amines with alcohols and amines and is superior to other reported catalysts in some cases. Subsequent studies investigated the potential of Shvo’s catalyst to mediate cascade reactions, such as Pictet-Spengler type cyclizations without the need for a starting aldehyde. ❧ Later explorations were directed towards the development of a catalyst with a pendant Lewis acid, specifically boron, which could be used to direct C–H activation of substrates. Studies of the reactivity of a proposed cooperative catalyst, {[(py)₂BMe(μ-OH)]Ru(MeCN)₃}+ -OTf (py = 2-pyridyl, cym = η⁶-p-cymene), led to the discovery that ruthenium(III) chloride hydrate is a convenient catalyst for the addition of active methylene compounds to aryl alkynes. These reactions are inexpensive, operationally simple, air and water tolerant, and high yielding in cases. ❧ Further investigations towards the synthesis of a cooperative catalyst with a Lewis acid directing group focused on the synthesis of rhodium(I) and iridium(I) complexes of type [(py)₂BMe₂]ML₂ (L₂ = (tBuNC)₂, (CO)₂, (C2H₄)₂, cyclooctadiene, 1,2- bis(diphenylphosphino)ethane). These complexes were found to undergo a ring inversion of the six-membered metallocycle. Inversion recovery kinetic analysis was used to measure the rate and enthalpic barrier (ΔH‡) of this transformation across the series. Whereas the ring flip is proposed to proceed through a transition state in which a filled-filled π-interaction between the metal and the pyridines becomes available, this ring flip barrier is used to parameterize the π-acceptor ability of the ancillary ligands. The observed values were found not to correlate to either Tolman or Lever parameters, indicating that the ring flip barrier selectively reports the strength of a transient π-bond, while the other known metrics report combined π- and σ- effects. ❧ Finally, the synthesis of cooperative catalysts based on first-row metals was also explored. [(py)₂BMe₂]Ni(PPh₃)Cl and [(py)₂ BMe₂]Ni(acac) (acac = acetylacetonate) were synthesized and structurally characterized. [(py)₂BMe₂]Ni(PPh₃)Cl is observed to exhibit both inter- and intramolecular substitutional lability. Early investigations of the kinetics of this isomerization reveal that the mechanism is not straightforward