Previous investigation of the kinetics and mechanism of the substitution reaction of bidentate substituted transition metal carbonyl complexes with Lewis bases (L') to yield the cis distributed bidentate substitution product, where M=Cr, Mo, or W and S-S = DTO (2,2,7,7-tetra-methyl-3, 6-dithiaoctane) or DTN (2,2,8,8-tetramethyl-3,7-dithianonane) have suggested that this reaction proceeds via a mechanism involving initial reversible dissociation of one end of the bidentate sulfur ligand followed by rapid attack of L' on the resulting five-coordinate intermediate.

No description available.

Reactions of 4-chloro-2,6-dimethoxypyrimidine (1) with enolates of acetone, pinacolone, diisopropyl ketone, and ethyl phenylacetate generated by means of potassium amide in 1 liquid ammonia were found to proceed by the S<sub>RN>1 mechanism upon photostimulation with near-UV light to give good yields of substitution products resulting from displacement of chloride ion. Both photostimulated and dark reactions of 4-chloro-2,6-dimethoxypyrimidine (1) with the carbanions of acetonitrile and propionitrile proceed exclusively by an ionic mechanism in liquid ammonia or THF to give a mixture of monosubstitution products resulting from displacement of chloride or the 6-methoxy substituent. With the acetonitrile carbanion the product resulting from displacement of methoxide was the major substitution product, while reaction of 1 with propionitrile carbanion afforded a preponderance of the product resulting from chloride displacement. Photostimulated reaction of 2-chlorothiazole (2) with the potassium enolate of pinacolone proceeds by a radical-chain mechanism to give the substitution product resulting from chloride displacement. However, when 2 is allowed to react with pinacolone enolate in the dark a completely unexpected product is formed. Under these conditions, the tertiary alcohol, 2-(2-chlorothiazol-4-yl)-3,3-dimethyl-2-hydroxybutane, was formed in which the pinacolone unit had been incorporated in an aldol fashion at the 4-position of 2. Both photostimulated and dark reactions of 2 with the enolate of diisopropyl ketone produced a similar carbinol, 3-(2-chlorothiazol-4-yl)-2,4-dimethyl-3-hydroxypentane, in good yields. Treatment of 2 with potassium amide in liquid ammonia or LDA in THF followed by addition of benzophenone afforded 2-chloro-4-(diphenylhydroxymethyl) thiazole in excellent yield. Trapping experiments with deuterium oxide and iodomethane provide evidence that such aldol-type reactions take place via initial metalation of 2 at position-4, followed by reaction of the resulting carbanion with the appropriate electrophile. This is the first example of direct metalation occurring at the 4-position of a thiazole derivative. / Ph. D.

This student is interested in reinvestigating the kinetics and mechanism of the bridged compound in l,2-dichloroethane with triisopropyl phosphite and in finding the reasons why the values of competition ratio k₋₂ /k₃ in this reaction are so large.

The substitution reaction of (DTN)W2 (CO)10 with P(OCH(CH3 )2 )3 is a stepwise reaction. The kinetics of step 1 follow the equation: -d[substrate] /dt = kld [substrate] + k la [substrate] [ligand]. Thus the mechanism of step 1 is expected to be a competition between dissociative and associative pathways. The kinetics of step 2 follow the equation: -d[(DTN)W(CO)5]/dt = k2dk3[(DTN)W(CO)5][ligand]/k-2[DTN] + k3[ligand] The plot of kobsd versus [ligand] thus is a hyperbolic curve and the plot of 1/kobsd versus 1/[L] exhibits linear behavior. A mechanism for step 2 in which (DTN)W(CO)5 dissociates to an intermediate, W(CO) 5 , and in which DTN and P(OCH(CH3 )2 )3 compete to associate with W(CO) 5 is favored. The dissociative rate constant of the first step, kld' is about 1.2 times of that of the second step, k2d. The dissociation of (DTN)W(CO) 5 from (DTN)W2 (CO) 1 0 is favored over the dissociation of DTN from (DTN)W(CO) 5 due to a combination of the steric, stoichiometric, charge repulsion and entropy effects of the reaction.

The kinetics and mechanism of the reactions of (tmpa)W-(CO)^ and (tmen)W(CO)^ (tmpa = N,N,N',N'-tetramethy1-1,3-diaminopropane and tmen = N,N,N1,N1-tetramethylethylenediamine) with four phosphorus donor ligands (triisopropyl phosphite, triphenyl phosphite, triphenylphosphine and "constrained phosphite", 4-methyl-2,6,7-trioxa-l-phosphabicylo[2.2.2]octane) in xylene have been investigated in detail. These reactions were found to take place by the ring-opening of the bidentate ligand in a reversible step which leads to the formation of a five-coordinate intermediate of the type [(h^-tmpa)W(CO)or [(h^-tmen)W(CO). The intermediate then reacts with one molecule of phosphorus ligand, L, to form a six-coordinate intermediate, which can either expel the bidentate ligand and react with another molecule of L leading to the formation of a new disubstituted tungsten tetracarbonyl or go through a ring-reclosure step to form a seven-coordinate activated com-2 2plex or intermediate of the type [(h -tmpa)W(CO)^(L)] or [(h - tmen)W(CO)^(L)] which then regenerates the substrate through the expulstion of the L molecule. This mechanism is consistent with the observed rate behavior in these systems. For the reaction of (tmpa)W(CO)^ with the "constrained phosphite", an intermediate of the type [(h1-tmpa)W(CO)4P(OCH2)3CCH3] was isolated and identified.

The second order rate constants for nucleophilic substitution by methoxide of (Z)- and (E)-O-methylbenzohydroximoyl fluorides [C6H4C(F)=NOCH3] with various substituents on the phenyl ring [p-OCH3 (1h, 2h), p-CH3 (1g, 2g), p-Cl (1f, 2f), p-H (1e, 2e), (3,5)-bis-CF3 (1i, 2i)] in 90:10 DMSO:MeOH have been measured. A Hammett plot of these rate constants vs σ values gave positive ρ values of 2.95 (Z isomer) and 3.29 (E isomer). Comparison of these rates with methoxide substitution rates for Omethylbenzohydroximoyl bromide [C6H4C(Br)=NOCH3] and Omethylbenzohydroximoyl chloride [C6H4C(Cl)=NOCH3] reveal an element effect for the Z isomers of Br:Cl:F(1e) = 2.21:1.00:79.7 and for the E isomers of Cl:F(2e) = 1.00:18.3. With the p-OCH3-imidoyl halides the following element effects are found: Br:Cl:F(1h) = 2.78:1.00:73.1 for the Z isomer and Br:Cl:F(2h) = 1.97:1.00:12.1 for the E isomer. Measurement of activation parameters revealed ∆S≠ = -17 eu for 1e and ∆S≠ = -9.9 eu for 2e. Ab initio calculations (HF/6-31+G*, MP2/6-31+G*//HF/6-31+G*, B3LYP/6- 31+G*//HF/6-31+G*, HF-SCIPCM/6-31+G*//HF/6-31+G*) were performed to define the reaction surface. These calculations demonstrate a relatively large barrier for nucleophilic attack in relation to halogen loss and support the experimental findings that this reaction proceeds by an addition-elimination mechanism (AN# + DN). The imidoyl fluorides have been used to synthesize highly functionalized O-methyloximes by reaction with enolate anions derived from malononitrile, ethyl cyanoacetate, and diethyl malonate. Acid-catalyzed isomerization of compounds containing the O-methyloxime moiety have been investigated with ab initio calculations (HF/6-31+G*, MP2/6- 31+G*//HF/6-31+G*, B3LYP/6-31+G*//HF/6-31+G*). Barriers for rotation around the C-N bond following protonation have been calculated. The calculated barriers are discussed in relation to an isomerization mechanism of protonation-rotation versus a nucleophilic catalysis.

Thesis--Ph. D., University of Hong Kong, 1983.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Adelaide, Dept. of Physical and Inorganic Chemistry, 1987. / Includes bibliographical references.