Dissertation Defense Schedule
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PROGRAM | Chemistry and Biochemistry
Synthesis and Reactivity of Acetyl Coenzyme A Synthase Active Site Analogues
By: Megan Shalaida Chair: Charles Riordan
ABSTRACT
Acetyl coenzyme A synthase (ACS) produces acetyl coenzyme A (acetyl-CoA) by combining CH3 from
a methylated corrinoid iron-sulfur protein (CH3-CoFeSP), CO from carbon monoxide dehydrogenase (COdH)
and coenzyme A (CoA). Biologically unprecedented nickel-methyl, nickel-carbonyl, and acetyl-nickel
intermediates are proposed to occur during catalysis. To probe the feasibility of Ni(II)Ni(I) or Ni(II)Ni(0)
intermediates during catalysis, the synthesis of a dinucleating thiol macrocycle with asymmetric coordination
sites was targeted. En route to this ligand, a series of mononuclear nickel thiol-based Schiff base complexes,
N,N’-bis(3-methoxycarbonylthiosalicylidene)-R-diaminonickel(II) (Ni(3-MOC-tsalR)) where R = 1,2-
diaminoethane (en), 1,3-diaminopropane (pr), 1,4-diaminobutane (but), 1,2-phenylenediamine (phen), and 4,5-
dimethyl-1,2-phenylenediamine (dimph), were synthesized. The resultant square-planar complexes were
characterized spectroscopically and crystallographically. Cyclic voltammetry studies of Ni(3-MOC-tsalR) (R =
en, pr, but) showed one reversible cathodic wave, assigned as the Ni(II)/Ni(I) reduction. Studies of Ni(3-MOCtsalR)
(R = phen, dimph) exhibited an irreversible cathodic peak assigned as a ligand reduction. Attempts to
synthesize the asymmetric dinucleating thiol macrocycle using various starting diamines and starting materials
at different temperatures and reaction times all failed to produce the desired product.
The tridentate ligand bis[2-(isopropylthio)ethyl]phenylphosphine (SiPrPSiPr, (PhP(CH2CH2SPri)2)) with
two thioether donors has been used to investigate the mechanism of methyl transfer. Reaction of SiPrPSiPr with
Ni(cod)2 (cod = 1,5-cyclooctadiene) and two equivalents of triphenylphosphine (PPh3) generated (κ2-
SiPrPSiPr)Ni(PPh3)2. The resultant tetrahedral complex was characterized spectroscopically and
crystallographically. (κ2- SiPrPSiPr)Ni(PPh3)2 was found to decompose over time in solution to (κ2-SiPrPS)2Ni.
The alkyl transfer reactivity of (κ2- SiPrPSiPr)Ni(PPh3)2 was assayed through reactions with alkylcobaloximes,
i.e., methyl, methyl-d3, ethyl, isopropyl and neopentyl. The reaction of methyl and methyl-d3 derivatives of
RCo(dmgBF2)2py (dmgBF2 = (difluoroboryl)dimethylglyoximato, py = pyridine) with (κ2- SiPrPSiPr)Ni(PPh3)2
proceeded in quantitative spectroscopic yields forming the respective alkyl species, [(κ2-SiPrPSiPr)Ni(PPh3)R]+
(R = CH3 or CD3) and [Co(dmgBF2)2PPh3]-. Reactions of (κ2- SiPrPSiPr)Ni(PPh3)2 with RCo(dmgBF2)2py, where
R = CH3CH2 or CH(CH3)2, yielded ethylene and propylene, respectively, rather than the corresponding Ni-alkyl
complex, implicating successful alkyl transfer followed by facile β-hydrogen elimination for these nickel
complexes. Kinetics data obtained for the transfer of alkyls (CH3, CH3CH2) to (κ2- SiPrPSiPr)Ni(PPh3)2 supports
an SN2 mechanism similar to that found in the enzyme.
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