Enantioselective CC bond formation in styrene dimerization

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Enantioselective CC bond formation in styrene dimerization
Enantioselective C-C bond formation in styrene dimerization with chiral ansa zirconocenebased catalyst.
Nunzia Galdi*
Dipartimento di Chimica, Università di Salerno, via S. Allende, I-84081 Baronissi (SA), Italy.
e-mail:[email protected]
In principle chiral C2 symmetric ansa metallocenes of group 4 present the opportunity to be applied in
asymmetric synthesis.1 Actually the greater amount of research on these systems deals with the stereospecific olefin
polymerization and for this purpose the racemic mixture of ansa metallocenes works as well as the optical active ones.2
However these complexes catalyze highly enantioselective reactions such as addition of alkylmagnesium halides to
alkenes, reduction of imines or ketones and asymmetric hydrosilation of ketones.3 Some interesting exploitations of
ansa zirconocenes in the asymmetric hydrogenation of tetrasubstituted olefins and imines were reported by Buchwald et
al.4,5 Pino and coworkers6 synthesized optically active low molecular weight compounds by propene
hydrooligomerization in order to obtain useful information on the stereocontrol mechanism of such polymerization
catalysts. More recently this approach has been reproduced with racemic catalysts using styrene as monomer.7 The
careful analysis of the products allows to obtain some information about the regiochemistry and stereochemistry of the
styrene polyinsertion. On the other hand this study suggests that the polyinsertion catalysis in presence of chiral ansa
metallocenes associated with chain transfer by molecular hydrogen could be useful for the convenient synthesis of
chiral molecules.
In the light of this observation, the aim of my work is the synthesis of
optically active molecules through styrene hydrooligomerization. In the presence
of hydrogen and of the chiral C2 symmetric ansa metallocene (R,R)-ethylenebis
tetrahydroindenylzirconium dichloride activated by methylalumoxane, the styrene
can be fully converted into a mixture of polyinsertion products. Among them, the
asymmetric hydrodimer arising from double primary insertion, 1,3diphenylbutane, was isolated and subjected to some reactions in order to evaluate
the enantioselectivity of the reaction. The enantiomeric excess was determined
through NMR and is 77% while the absolute configuration of the asymmetric
Enantiofacial selectivity of the
carbon allows inferring the enantiofacial preference for the styrene coordination to
ansa-zirconocene catalyst
the metal center (see figure).
The potential of this reaction is and also briefly discussed together with the regiochemistry and stereochemistry
of the styrene insertion.
References:
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7.
Schäefer, A.; Karl, E.; Zsolnai, L.; Huttner, G.; Brintzinger, H.-H. J. Organomet. Chem., 1987, 328, 87.
For reviews on the use of ansa zirconocenes in polymerization catalysis see: (a) Brintzinger, H.-H.; Fischer, D.;
Mülhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem. Int. Ed. Engl., 1995, 34, 1143. (b) Resconi, L.; Cavallo, L.;
Fait, A.; Piemontesi, F. Chem. Rev., 2000, 100, 1253.
Hoveyda, A. H.; Morken, J. P. Angew. Chem. Int. Ed. Engl., 1996, 35, 1262.
Troutman, M. V.; Appella, D. H.; Buchwald, S. L. J. Am. Chem. Soc., 1999, 121, 4916.
Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc., 1994, 116, 8952.
Pino, P.; Cioni, P.; Wei, J. J. Am. Chem. Soc., 1987, 109, 6189.
Izzo, L.; Napoli, M.; Oliva, L. Macromolecules, 2003, 36, 9340.