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The catalytic decarbonylation of aldehydes using iron porphyrin complexes Belani, Ramesh


The aim of this project was to investigate the use of iron porphyrin complexes as potential homogeneous catalysts for the decarbonylation of aldehydes. Complexes of the type Fe(TPP)L₂ (where L = n-Bu₃P, PPh₃ or piperidine) were prepared and reacted with CO gas, or with aldehydes as sources of CO. Since the loss of coordinated CO from the Fe(TPP)(CO)(n-Bu₃P) complex was more facile, the bis(n-Bu₃P) phosphine system was studied in more detail. The X-ray structure of FeTPP(n-Bu₃P)₂ Is described, and this includes the first determination of an FeII-P bond distance for a metalloporphyrin. The study using Fe(TPP)L₂ complexes as decarbonylation catalysts was somewhat hindered by the extreme air-sensitivity of the porphyrin complexes in solution. UV/visible spectroscopy and gas chromatography were used to monitor the decarbonylation reactions. The reaction mixtures were analysed by GC/MS. The decarbonylation reactions were characterised by inconsistent turnover numbers and lack of reproducibility; during the decarbonylation of phenylacetaldehyde, bibenzyl was detected. Such factors are indicative of a free radical mechanism, similar to that proposed earlier for related Ru(II) porphyrin systems. The carbonylation of FeTPP(n-Bu₃P)₂ by CO gas was of interest with respect to the catalytic reaction, which must involve formation of a carbonyl complex. The reaction, FeTPP(n-Bu₃P)₂ + CO K [mathematical formula omitted] FeTPP(n-Bu₃P)(CO) + n-Bu₃P was found to have a K value of 0.72 at 29°C, while the temperature dependence of K was studied to obtain the thermodynamic parameters ΔS and ΔH for the equilibrium.

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