Stereoselective Synthesis of Metalated Cyclobutyl Derivatives
COMMUNICATIONS
anhydrous MgSO4. Purification was performed by silica gel
chromatography with pure hexane.
Acknowledgements
This research was supported by the Israel Science Foundation
administrated by the Israel Academy of Sciences and Hu-
manities (70/08), and by the Fund for promotion of Research
at the Technion. I. M. is holder of the Sir Michael and Lady
Sobell Academic Chair.
References
Figure 1. Potential energy profile for the ring contraction of
17 to form 19.
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have further found that the ring contraction of 17 is
an exothermic process (15.7 kcalmolÀ1 are released),
which indeed affords a zirconium-substituted cyclobu-
tane species 19. The transformation feasibility is con-
firmed by a reasonable energetic barrier of 11.4 kcal
molÀ1 (Figure 1). A structural comparison between 17
À
and the TS17-18 shows a shortening of the Zr O bond
À
along with an elongation of the C1 O bond (Figure 1
vs. Scheme 3). The mechanism depicted in Scheme 2
is therefore coherent with the computational results.
In conclusion, the treatment of Z-vinyl carbamate
with Et2ZrCp2 leads to the cyclobutyl-zirconocene de-
rivatives in good yields and as a unique diastereoiso-
mer. The reaction proceeds through a carbometalative
ring-expansion followed by an intramolecular migra-
tory insertion. Applications to the synthesis of enan-
tiomerically pure cyclobutyl-zirconocene derivatives
are under study in our research group.
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Experimental Section
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General Procedure
A solution of ethylmagnesium bromide (2 equiv., 1N solu-
tion in Et2O) in diethyl ether was added slowly to a solution
of bis(cyclopentadienyl)zirconium dichloride (1 equiv.) in
dry diethyl ether at À788C. The reaction mixture was
warmed to 08C, stirred for 5 min and cooled again to
À788C. Z-Vinyl carbamate 2 (1 equiv.) was added slowly in
dry diethyl ether (5 mL). The solution was allowed to warm
to room temperature and stirred for 20 min. The formation
of zirconacyclobutane derivative 9 was checked by TLC
analysis after acidic hydrolysis of aliquots. The reaction was
quenched with an aqueous solution of HCl (1M). The
phases were separated and the aqueous phase was extracted
three times with ether. The combined organic layers were
washed with saturated solution of NaHCO3 and dried over
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with organozirconium species, see: S. Dixon, R. J.
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Functional Groups, (Eds.: Z. Rappoport, J. F. Lieb-
man), Wiley, Chichester, 2005.
Adv. Synth. Catal. 2009, 351, 1005 – 1008
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