THESIS
1996
1 v. (various pagings) : ill. ; 30 cm
Abstract
Section 1 reports studies of the structure, bonding, isomerism and acetylene cyclotrimerization in the tris(alkyne) metal complexes [W(C
2R
2)
3(CO)]. Extended Hückel calculations show that the acetylenes are good π-acceptors and that carbons which are proximal to the axial CO ligand have a greater build up of negative charge.
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Section 1 reports studies of the structure, bonding, isomerism and acetylene cyclotrimerization in the tris(alkyne) metal complexes [W(C
2R
2)
3(CO)]. Extended Hückel calculations show that the acetylenes are good π-acceptors and that carbons which are proximal to the axial CO ligand have a greater build up of negative charge.
X-ray crystallographic studies of [W(PhC≡CPh)
3(CO)] (4) and [W(PhC≡CPh)
3(NCMe)] (8) indicate that the W-C alkyne bonding is stronger in (8) due to increased π-back donation to the acetylenes.
The compound [W(PhC≡CMe)
3(CO)] (5) has the potential for rotational isomerism.
1H and
13C NMR studies indicate the mixture with 3, 2 or 1 phenyl groups in the proximal position in the ratio 35 : 55 : 10. This is in general accord with our electronic prediction.
Cyclotrimers of PhC≡CPh and PhC≡CMe have been identified by X-ray crystallography from either oxidative reactions of (4) or in the thermal synthesis of (5). This is the first report of such trimerizations in the tungsten system.
Section 2 describes optimization of the oxidation of diphenylacetylene. by the catalyst system Co(OAc)
2/ NaBr/Mn(OAc)
2 in the ratio 1 : 1 : 0.02. Using 95% AcOH / H
2O at 70°C , a five fold excess of substrate gave 84% benzil within 7 hours. The rate of oxidation and the selectivity towards the dicarbonyl or cleavage products are strongly affected by the cobalt and manganese catalysts, sodium bromide, solvent and the concentration used. Oxidation of a variety of internal and terminal alkynes has also been investigated.
Of key interest is that oxidation of 4-octyne gave EtCOC≡CPr in 93% yield. Thus the C≡C triple bond is unmodified by the oxidation which converts one α-CH
2 group to a CO keto functionality. The excellent yield and remarkable selectivity of this reaction may render it useful in synthetic applications.
Two different mechanistic pathways are implied by our results on tolane and other substrates, one is thought to be REDOX based and the other to involve hydrogen atom abstraction.
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