Aldol condensation reactions of [Co(η4-C4Ph4){η5-C5H4C(O)CH3}]

The aldol condensation reaction between [Co(η4-C4Ph4){η5-C5H4C(O)CH3}] and a range of aromatic aldehydes [RCHO] and [RCHCH–CHO] gives a series of α,β-unsaturated ketones [Co(η4-C4Ph4){η5-C5H4C(O)CHCH–R}] and [Co(η4-C4Ph4){η5-C5H4C(O)CHCH–CHCH–R}] (3). The reaction is promoted by various bases: NaH proved to be the most effective whilst nBuLi gave [Co(η4-C4Ph4){η5-C5H4C(OH)(nBu)CH3}] as the major product. NaOH was ineffective, perhaps indicating that that the methyl protons in [Co(η4-C4Ph4){η5-C5H4C(O)CH3}] are less acidic than those in [Fe(η5-C5H5){η5-C5H4C(O)CH3}]. Compounds 3 were characterised spectroscopically. Their 1H NMR spectra are consistent with a trans configuration about their CC 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 C5H4C(O)(CHCH)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 = C6H5, 4-C6H4NMe2, 2-C4H3S and 1-C10H7) suggest that these are donor–π-acceptor systems, but as the annellation of R increases (R = 9-C14H9, 1-C16H9 and 1-C20H11) the spectra increasingly resemble those of the parent polycyclic aromatic hydrocarbon, RH. Reduction of [Co(η4-C4Ph4){η5-C5H4C(O)CHCH–C10H7-1}] with DIBAL gives a mixture of [Co(η4-C4Ph4){η5-C5H4C(O)CH2CH2–C10H7-1}] and [Co(η4-C4Ph4){η5-C5H4CH(OH)CHCH–C10H7-1}]. A minor product from the preparation of [Co(η4-C4Ph4){η5-C5H4C(O)CH3}] was shown by X-ray crystallography to be the η4-butadiene complex [Co{η4-Ph(H)CC(Ph)–C(Ph)C(H)Ph}{η5-C5H4C(O)CH3}].

Graphical abstractThe aldol condensation between the enolate anion of Co(η4-C4Ph4){η5-C5H4C(O)CH3} and various aromatic aldehydes RCHO or RCHCHCHO gives the α,β-unsaturated aldehydes Co(η4-C4Ph4){η5-C5H4C(O)CHCH–R} or Co(η4-C4Ph4){η5-C5H4C(O)CHCH–CHCHR} in which there is a trans arrangement across the CC. These are donor–acceptor–donor systems with only limited donor–acceptor electronic interactions.Figure optionsDownload full-size imageDownload as PowerPoint slide