Aldol condensation reactions of [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH 3 }] Christoph Behrendt a , Sven Dabek a , Jurgen Heck a, * , Donagh Courtney b , Anthony R. Manning b, * , Michael J. McGlinchey b, * , Helge Mueller-Bunz b , Yannick Ortin b a Institut fu ¨ r Anorganische und Angewandte Chemie, Universita ¨ t Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany b School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland Received 4 October 2005; received in revised form 18 November 2005; accepted 21 November 2005 Available online 4 January 2006 Abstract The aldol condensation reaction between [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH 3 }] and a range of aromatic aldehydes [RCHO] and [RCH@CH–CHO] gives a series of a,b-unsaturated ketones [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH@CH–R}] and [Co(g 4 -C 4 Ph 4 ){g 5 - C 5 H 4 C(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(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(OH)( n Bu)CH 3 }] as the major product. NaOH was ineffective, perhaps indicating that that the methyl protons in [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH 3 }] are less acidic than those in [Fe(g 5 -C 5 H 5 ){g 5 -C 5 H 4 C(O)CH 3 }]. Compounds 3 were characterised spectroscopically. Their 1 H 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 5 H 4 C(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 6 H 5 , 4-C 6 H 4 NMe 2 , 2-C 4 H 3 S and 1-C 10 H 7 ) suggest that these are donor–p-acceptor systems, but as the annellation of R increases (R = 9-C 14 H 9 , 1-C 16 H 9 and 1-C 20 H 11 ) the spectra increasingly resemble those of the parent polycyclic aromatic hydrocarbon, RH. Reduction of [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH@CH– C 10 H 7 -1}] with DIBAL gives a mixture of [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH 2 CH 2 –C 10 H 7 -1}] and [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 CH(OH)CH@ CH–C 10 H 7 -1}]. A minor product from the preparation of [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH 3 }] was shown by X-ray crystallography to be the g 4 -butadiene complex [Co{g 4 -Ph(H)C@C(Ph)–C(Ph)@C(H)Ph}{g 5 -C 5 H 4 C(O)CH 3 }]. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Cobalt; Tetraphenylcyclobutadiene; Aldol condensation; Unsaturated ketones 1. Introduction Although the chemistry of ferrocene and its derivatives has been studied extensively, less effort has been expended on their isoelectronic counterparts derived from [Co(g 4 - C 4 Ph 4 )(g 5 -C 5 H 5 )]. Here, we report a series of a,b-unsatu- rated ketones [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH@CH–R}] (Scheme 1) obtained from the base-promoted aldol condensation of [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH 3 }] with various aldehydes RCHO. The reduction of [Co(g 4 - C 4 Ph 4 ){g 5 -C 5 H 4 C(O)CH@CH–C 10 H 7 -1}] (3i), with DIBAL has also been studied with the intention of preparing allylic alcohols as precursors to cationic merocyanines such as [Co(g 4 -C 4 Ph 4 ){g 5 -C 5 H 4 CHCHCH–C 10 H 7 -1}][BF 4 ]. Acids and bases promote the reversible formation of aldols from enolisable aldehydes and ketones (Scheme 2, Step 1). The reaction was discovered independently by Borodin [1a] and Wurtz [1b] ca. 1870, and since then it has provided a versatile method for preparing new CAC bonds [1c]. Furthermore, as both of these carbon atoms may end up as chiral centres there has been much recent research into the asymmetric aldol addition reaction [1d]. However, aldols are b-hydroxy-aldehydes or ketones and are, therefore, susceptible to irreversible dehydration with the formation of a,b-unsaturated aldehydes or ketones (Scheme 2, Step 2). 0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2005.11.057 * Corresponding authors. E-mail address: Anthony.Manning@ucd.ie (A.R. Manning). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 1183–1196