Information • Textbooks • Media • Resources
On Chirality in Substituted Metallocenes Bearing Identical Substituents Daisy de Brito Rezende Instituto de Química, Universidade de São Paulo, Cx. Postal 26077, CEP 05513-970 São Paulo, Brazil;
[email protected] Ivan P. de Arruda Campos Instituto de Ciências Exatas e Tecnologia, Universidade Paulista, Santana de Parnaíba, SP, 06500-000, Brazil
Most standard textbooks on stereochemistry in organic chemistry do not discuss compounds having a chiral plane (1–4 ), the less common chirality element. Indeed, probably because molecules with planar chirality are not very common, IUPAC has ignored up to now (5) the proposed Lemière– Alderweireldt convention (6 ) (an improvement upon the Cahn–Ingold–Prelog system [1–3]) for assigning their names. However, an introductory overview of the topic of planar chirality is included in the classic Stereochemistry of Organic Compounds by Eliel et al. (7). In Section 14.8 of Chapter 14, Metallocenes and Related Compounds, the authors state (pp 1175–1176): Metallocenes (metal “sandwich compounds”) and other aryl metal complexes display chirality when the arene is properly substituted (Fig. 14.79). The parent compounds may display average symmetry as high as Dnh … since the molecules generally pivot rapidly around the arene– metal axis. Yet an appropriate set of substituents on one of the aromatic rings (1,2,4-trisubstitution pattern for equal substituents and 1,2- for unequal ones) will destroy all symmetry planes and lead to chiral structures (Fig. 14.79…). Although, conceptually, this is presumably a case of planar chirality, it is, for purposes of nomenclature, treated in terms of chiral centers. A detailed dis[c]ussion of metallocenes is outside the scope of this book and the reader is referred to two reviews by Schlögl [8, 9].
Figure 14.79 of ref 7 (on p 1175, not reproduced here) is entitled “Chiral metallocenes”. It depicts 1,2,4-trimethylferrocene and 1-methyl-2-ethyl-di(benzene)chromium. While the latter compound is indubitably chiral, the same cannot be said of the former. In fact, as rotation around the arene– metal axis is rapid, it suffices to find one rotamer having Cs symmetry for 1,2,4-trimethylferrocene to be achiral, and such a rotamer in fact exists (see compound 1e, with R = Me, in Fig. 1). (Throughout this article, metallocenes will be represented by projections from above, akin to Newman projections, as depicted in Figure 2.) Having found this mistake in ref 7, and after checking refs 8 and 9 to verify whether Schlögl discussed the case of chiral metallocenes bearing identical substituents (he does not), we decided to write the present paper, having in mind the important catalytic and material applications that metallocenes have nowadays (10). As can be seen in Figure 1, for any dicyclopentadienylmetal structurally akin to ferrocene, the addition of any number of identical substituents to it will not eliminate the existence of at least one rotamer having a plane of symmetry. In 1130
Figure 1 we represent such a rotamer for all isomers of monoto tetrasubstituted dicyclopentadienylmetals that have identical substituents all in the same ring: each of them has a σh plane (for more about symmetry planes, see ref 7, pp 71–99). It follows that, if one imagines one of the rings as pentasubstituted, the hexa- to nonasubstituted dicyclopentadienylmetals also have a σh plane. Of course, the eclipsed rotamer of the
R R
R
R
1a
1b
R
R
R
1c
R
R
R
R 1d
R
R
R
1e
R 1f
Figure 1. All isomers of mono- to tetrasubstituted dicyclopentadienylmetals having all substituents (which are identical, being here represented by R) in the same ring. The symmetry plane σh contains the ring–metal bonds and is perpendicular to the plane of the page; owing to the way the formulas are drawn here, σh divides the left from the right side of each formula, being thus also a plane of bilateral symmetry.
M
Figure 2. A graphical explanation of the Newman-like metallocene projection. Here M represents any metal.
R
R
R
R
R
R
R
R
R
Figure 3. 1,2,4-Trisubstituted metallocenes having 6-, 7- (hypothetical), and 8- membered rings as ligands; all substituents (which are identical, being here represented by R) are bonded to the same (upper) ring, while the lower ring is not substituted.
Journal of Chemical Education • Vol. 78 No. 8 August 2001 • JChemEd.chem.wisc.edu
Information • Textbooks • Media • Resources
nonsubstituted, pentasubstituted (all identical substituents in the same ring), and decasubstituted (all substituents identical) dicyclopentadienylmetals have five σν symmetry planes. In fact, provided that the substituents are identical, all the other possible substituted dicyclopentadienylmetals present at least one rotamer having a σh plane of symmetry. In short: all substituted dicyclopentadienylmetals bearing identical substituents are achiral. This also holds for cyclopentadienylmetal carbonyls such as cymantrene (cyclopentadienylmanganesetricarbonyl), because it depends on a geometrical property of equilateral pentagons. On the other hand, for ligand rings of more than five carbon atoms, the 1,2,4-trisubstituted metallocenes bearing identical substituents always will be chiral, for they do not have any symmetry plane, as can be seen in Figure 3. In this figure we depict the 1,2,4-trisubstituted dicycloheptatrienylmetal, of which no example has been reported up to now; if prepared, however, it would be chiral. This also holds for 1,2,4-trisubstituted arylmetal carbonyls such as a derivative of benchrotrene (benzenechromiumtricarbonyl), as well as to metallocenes such as a 1,2,4-trisubstituted cycloheptatrienylmetalcyclopentadienyl, provided that the substituents are in the seven-membered ring. For substituted metallocenes having as ligands rings of more than six carbon atoms, such as uranocene (dicyclooctatetraenyluranium) derivatives, other substitution patterns of identical substituents can be envisaged that lead to chiral molecules.
Literature Cited 1. Cahn, R. S.; Ingold, C.; Prelog, V. Angew. Chem., Int. Ed. Engl., 1996, 5, 385. 2. IUPAC Commission on Nomenclature of Organic Chemistry. Pure Appl. Chem. 1976, 45, 11. 3. IUPAC Nomenclature of Organic Chemistry; Rigaudy, J.; Klesney, S. P., Eds.; Pergamon: Oxford, 1979. 4. Schlögl, K. Top. Curr. Chem. 1984, 125, 27. 5. A Guide to IUPAC Nomenclature of Organic Chemistry, Recommendations 1993; Panico, R.; Powell, W. H.; Richer, J.-C., Eds.; Blackwell: Oxford, 1993. Moss, G. P. Pure Appl. Chem. 1996, 68, 2193; http://www.chem.qmw.ac.uk/iupac/stereo/ (accessed May 2001). Compendium of Chemical Terminology, 2nd ed.; McNaught, A. D.; Wilkinson, A., Eds.; Blackwell: Oxford, 1997. Leigh, G. J.; Favre, H. A.; Metanomski, W. V. Principles of Chemical Nomenclature: a Guide to IUPAC Recommendations; Blackwell: Oxford, 1998. Favre, H. A.; Hellwich, K.-H.; Moss, G. P.; Powell, W. H.; Traynham, J. G. Pure Appl. Chem. 1999, 71 , 1327; http://www.chem.qmw.ac.uk/iupac/bibliog/errata.html (accessed May 2001). 6. Lemière, G. L.; Alderweireldt, F. C. J. Org. Chem. 1980, 45, 4175. 7. Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley: New York, 1994. 8. Schlögl, K. Top. Stereochem. 1967, 1, 39. 9. Schlögl, K. Pure Appl. Chem. 1970, 23, 413. 10. Long, N. J. Metallocenes; Blackwell: Oxford, 1998; Chapter 6, pp 227–272.
JChemEd.chem.wisc.edu • Vol. 78 No. 8 August 2001 • Journal of Chemical Education
1131
