Aldol condensation reactions of [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH 3}

Abstract

The aldol condensation reaction between [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH 3}] and a range of aromatic aldehydes [RCHO] and [RCH CH–CHO] gives a series of α,β-unsaturated ketones [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH CH–R}] and [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH CH–CH CH–R}] ( 3). The reaction is promoted by various bases: NaH proved to be the most effective whilst n BuLi gave [Co(η 4-C 4Ph 4){η 5-C 5H 4C(OH)( n Bu)CH 3}] as the major product. NaOH was ineffective, perhaps indicating that that the methyl protons in [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH 3}] are less acidic than those in [Fe(η 5-C 5H 5){η 5-C 5H 4C(O)CH 3}]. Compounds 3 were characterised spectroscopically. Their 1H NMR spectra are consistent with a trans configuration about their C C bond, and this was confirmed by X-ray crystallography in five cases, which showed that all have the same basic structure with parallel cyclobutadiene and cyclopentadienyl ligands, but they are not identical. The C 5H 4C(O)(CH CH) n –R ( n = 1 or 2) moieties show little evidence for delocalisation and often deviate from planarity. The UV/Vis spectra of those 3 with smaller aromatic rings (R = C 6H 5, 4-C 6H 4NMe 2, 2-C 4H 3S and 1-C 10H 7) suggest that these are donor–π-acceptor systems, but as the annellation of R increases (R = 9-C 14H 9, 1-C 16H 9 and 1-C 20H 11) the spectra increasingly resemble those of the parent polycyclic aromatic hydrocarbon, RH. Reduction of [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH CH–C 10H 7-1}] with DIBAL gives a mixture of [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH 2CH 2–C 10H 7-1}] and [Co(η 4-C 4Ph 4){η 5-C 5H 4CH(OH)CH CH–C 10H 7-1}]. A minor product from the preparation of [Co(η 4-C 4Ph 4){η 5-C 5H 4C(O)CH 3}] was shown by X-ray crystallography to be the η 4-butadiene complex [Co{η 4-Ph(H)C C(Ph)–C(Ph) C(H)Ph}{η 5-C 5H 4C(O)CH 3}].