Metallacycles 1 may be generated readily by C-H activation to give complexes formally containing a carbanion ligand, this having a large trans-effect (usually >> X). This increases the rate of substitution of one or more of the other ligands (L) on the metal which is frequently advantageous for improving catalytic activity. Application of C-H activation to a prochiral ferrocene substate generates planar chiral complexes 2. Such metallacycle-metallocenes contains two metal atoms, and interaction of these may lead to additional stereochemical and reactivity features.
A numer of diastereoselective and enantioslective methods have been developed to control the absolute configuration of the new element of planar chirality in metallacycles containing ferrocene, or a related bulky cobalt-sandwich complex. For example, an (S)-oxazoline auxilary results in either the Rp or Sp planar chiral configuration depending upon the identity of the oxazoline substituent (and in turn the identity of the amino acid from which it is derived).1,2
Two enantioselective methods have also been used for the synthesis of planar chiral palladacyles. Asymmetric transcyclopalladation was developed for the synthesis of (Sp)-5 in high ee using palladacycle (S,Rp)-3 as the source of palladium, a reaction in which the driving force is the greater strength of the product Pd-P bond over the Pd-N bond in the starting material.3
The second method builds upon earlier work from Sokolov,4 and uses readily available amino acid derivatives to control enantioselective C-H activation. This method, illustrated below for the highly enantioselective synthesis of (Sp)-6,5 has also been used to give (Sp)-76 and (Sp)-8, the latter [2.2]paracyclophane-based palladacycle obtained by highly selective kinetic resolution.7
Other metallacycles synthesised in the group include imidazole ligated complex 9,8 platinacycle 10,9 and also a series of C2-symmetric palladium 1110 and platinum 1211,12 pincer complexes obtained by selective C-H activation of 1,3-bisoxazolines. Taken together these provide a unique resource for exploring chiral metallacycle reactivity, as both catalysts and precatalysts, for use in asymmetric synthesis.
1. A. M. Stevens and C. J. Richards, Organometallics 1999, 18, 1346.
2. D. J. Cassar, H. Roghzai, D. Villemin, P. N. Horton, S. J. Coles and C. J. Richards, Organometallics 2015, 34, 2953.
3. F. X. Roca, M. Motevalli and C. J. Richards, J. Am. Chem. Soc. 2005, 127, 2388.
4. V. I. Sokolov, L. L. Troitskaya and O. A. Reutov, J. Organomet. Chem. 1979, 182, 537.
5. M. E. Günay and C. J. Richards, Organometallics 2009, 28, 5833.
6. D. J. Cassar, G. Ilyashenko, M. Ismail, J. Woods, D. L. Hughes and C. J. Richards, Chem. Eur. J. 2013, 19, 17951. [open access]
7. N. Dendele, F. Bisaro, A.-C. Gaumont, S. Perrio and C. J. Richards, Chem. Commun. 2012, 48, 1991.
8. G. Jones and C. J. Richards, Organometallics 2001, 20, 1251.
9. M. E. Günay, D.. L. Hughes and C. J. Richards Organometallics 2011, 30, 3901.
10. M. A. Stark, G. Jones and C. J. Richards, Organometallics 2000, 19, 1282.
11. J. S. Fossey and C. J. Richards, Organometallics 2004, 23, 367.
12. J. S. Fossey, G. Jones, H. V. Nguyen, C. J. Richards, M. A. Stark and H. V. Taylor, Tetrahedron: Asymmetry 2004, 15, 2067.