1116
N. Galdi et al. / Catalysis Communications 12 (2011) 1113–1117
2 2 2
Fig. 2. Gas-chromatograms of the mixture of hydrooligomers obtained in the presence of the chiral (Λ,R,R)-[OSSO]Zr(CH Ph) (1a) and of chiral (R,R)-(EBTHI)ZrCl (2).
hydrogen pressure observed (run 3, Table 1) can be explained
considering that at such hydrogen pressure a decomposition of the
catalyst due to the cleavage of the sulfur–carbon bond is not unlikely.
It is worth noting that the product 1,3-dpb holds a stereogenic
center, therefore should be possible in principle, by using the chiral
enantiopure catalyst 1a, to obtain this molecule as pure enantiomer.
From the stereochemical point of view, the analysis of the 1,3-dpb
arising from the styrene hydrodimerization conducted in presence
4. Conclusions
In this communication we have shown that the chiral non racemic
(Λ,R,R)-[OSSO]Zr(CH Ph) is able to catalyze the enantioselective
2
2
formation of a C–C bond through styrene hydrodimerization affording
the chiral (S)-1,3-diphenylbutane with good selectivity respect to the
achiral dimer 1,4-diphenylbutane. In particular the regioselectivity
toward this molecule is higher compared to the chiral ansa-
zirconocene compounds. In spite of the overall low yield (20%) it is
important to point out that styrene is a large available, low cost
commodity and that the main product of the reaction ethylbenzene is
the industrial precursor of the starting substrate. Furthermore the
analysis of the composition of the oligomer mixture and of the
configuration of the chiral carbon indicates the styrene secondary
insertion into the Zr–H bond in the pathway affording 1,3-dpb. As a
perspective, the nature of the coordinative framework of this complex
gives wide possibility of improving both chemoselectivity and
enantioselectivity towards the chiral hydrodimer, through the
increase of size of the substituents of the aromatic ring. It is worth
noting that with this system the stereocontrol works also without the
assistance of a hindered alkyl group bound to the metal, at difference
of what was observed with traditional as well homogeneous Ziegler–
Natta catalysts.
D
of the chiral non-racemic catalyst 1a displays an [α] =+10.6 with
an estimated optical purity of 68% [23]. Due to the absence of
functional groups into the 1,3-dpb molecule a direct separation by
chiral HPLC in order to evaluate the enantiomeric purity was
unsuccessful, therefore we transformed the 1,3-dpb into the 2-
methylglutaric acid by oxidation promoted by ruthenium tetraoxide
(
see Scheme 4).
The analysis of the obtained 2-methylglutaric by chiral HPLC gives
an enantiomeric excess of 71% very close to the value obtained from
optical rotation.
In addition, from the sign of the optical rotation, we can safely
attribute the absolute configuration of the carbon atom as S. This
clearly indicates that the Λ,R,R complex 1a prefers the coordination of
styrene enantioface re with secondary regiochemistry of insertion.
This result is in agreement with the molecular model based on a so-
called octant scheme by Pino et al. which take into account the steric
repulsion between the ligand sphere and the incoming monomer unit
Acknowledgments
[20,24,25].
The high enantioselectivity is further supported by the analysis of
Authors wish to acknowledge the financial support from Ministero
dell'Istruzione dell'Università e della Ricerca (MIUR, Roma — Italy;
FARB-2009), and Dr P. Oliva for the technical assistance to NMR
experiments.
the composition of the trimers 1,3,5-triphenylhexane (1,3,5-tphe)
arising from three consecutive regioregular insertions. As a matter of
fact Fig. 2 shows that the trimer arising by two consecutive insertions
of the monomer with same enantioface (iso) (i)-1,3,5-tphe is largely
prevailing with respect to the trimer produced by insertions with
opposite enantioface (syndio) (s)-1,3,5-tphe (iso/syndio≈12).
It is worth to note that in this polyinsertion process we observe the
stereocontrol for the first monomer insertion, that is for the insertion
into the Zr–H bond. On the contrary, the traditional Ziegler–Natta
catalysts exhibit good enantioselectivity for the 1-alkenes only when
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2011.03.037.
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(
3 2 4 3
Scheme 4. Reagents and conditions: i AcOEt, CH CN, H O, NaIO , RuCl , 17 h, rt.