The Formation and Reactions of Metal to Carbon Bonds

Abstract

The interest in 3-methylthiophen-2-aldehyde as a precursor in the pharmaceutical context leads to our investigation of new methods of introducing the carbonyl group. A plausible approach to this problem was to first investigate the possibility of forming 2-thiophen aldehyde via a process which involves the metalation of thiophen followed by carbonylation and reductive elimination to give the desired aldehyde. Our studies on the trans-metalation reaction of 2-thienylmercuric chloride and tertiary phosphine complexes of palladium and platinum indicate the facile formation of the metal-carbon bond and the smooth carbon monoxide insertion into the metal-carbon bond. This stimulated us to investigate the interaction of other thiophen systems with low-valent transition metals. Tetrakis (triphenylphosphine) palladium(0) and tetrakis (triphenylphosphine) platinum(0) oxidatively add to 2-bromothiophen and subsequent carbonylation of the oxidatively added product afforded the acyl complex. However, when 2-bromo-3-methylthiophen was used instead of 2-bromothiophen, isomerization occurs during the carbonylation stage giving the 4-methyl and 3-methyl-acyl complex. The subsequent hydroformylation of 2-bromo-3-methylthiophen in the autoclave leads to the formation of 3-methylthiophen-2-aldehyde and 4-methylthiophen-2-aldehyde. The possible mechanisms of the above reactions are discussed. Having obtained some successful results using tertiary phosphine complexes of palladium and platinum, rhodium(I) was next considered. With [Rh(CO)(Ph₃P)₂C1], smooth oxidative addition reactions were not observed. Nevertheless, the reaction involving [Rh(CO)(Ph₃P)2C1] did proceed to give three main products, one of which is a Rh(I)-Rh(III) species. For the other two products, we are unable to identify the structure with absolute certainty using spectroscopic data since their X-ray crystal structures are unavailable. In the case of [Rh(CO)(PMe₃Ph)2Cl] and [Ir(CO)(Ph₃P)2Cl], we obtained the oxidatively added products, e.g. [IR(CO)(2-thienyl) (Ph₃P)₂C1Br]. After varying the metal ion in the tertiary phosphine complexes, attempts were made to alter the ligand coordinating to the metal. Since there is considerable interest in tellurium in our laboratory, we attempted to investigate the effect of tellurides on the metalation of thiophens. During these studies, a number of new complexes, e.g. [(R2Te)₂MX₀] (R = phenyl, p-ethoxylphenyl; M = palladium, platinum; X = chloride, bromide) were synthesized and characterised. However, we were unable to get these complexes to interact with the thiophen derivatives. Finally, we synthesized complexes of thiophen containing donor ligand atoms substituted at the 3-carbon atom, e.g. 3-(2-pyridyl) thiophen, 3-(2-pyrimidyl)thiophen 3-thenylidene-4-methylaniline and 3-thiophenaldoxime and then explore the possibility of ortho-metalating them ( i.e. forming a metal 2-carbon bond). In this area, new co-ordination and ortho-metalation complexes were obtained and characterised by spectroscopic methods.