C O M M U N I C A T I O N S
Supporting Information Available: Experimental procedures and
spectral data for all products. This material is available free of charge
References
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Singh, O. V.; Han, H. Org. Lett. 2007, 9, 4801. Widenhoefer described
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Figure 1. Crystallographic representation of iridium complex 4 and
schematic ChemDraw structure.
phosphoramidite 3 and LiI (relative to Ir(I)) in CDCl3, a color
change was observed from orange/red to yellow over the course of
15 min. At this time point, no free ligand or unreacted [Ir(cod)Cl]2
was noted by 1H and 31P NMR (see Supporting Information). The
most prominent features of the newly formed complex were a single
peak at 136.8 ppm in the proton decoupled 31P spectrum and the
1
diastereotopic H-Csp2 signals at 4.9 and 4.2 ppm in the H NMR,
indicating trans-complexation of two phosphoramidite ligands 3
via both the phosphorus and the double bond. Layering the crude
mixture of complex 4 with hexane led to crystals suitable for X-ray
analysis. The obtained crystal structure verified the proposed trans-
arrangement of ligands (see Figure 1) which is in contrast to related
structures reported by Gru¨tzmacher and Dorta.18 The bond lengths
of the complexed alkenes are elongated to 1.44 Å, compared to
1.32 Å in iminostilbene,19 and the Ir-C distances involving the
alkene carbons are 2.189 and 2.194 Å. The iridium-phosphorus
bonds are 2.28 Å long and together with the two Ir-C(8) bonds
form a nearly perfect plane orthogonal to the Ir-I bond. The other
alkene carbons, C(9), lie below this iridium-phosphorus-C(8)
plane (additional details for 4 may be found in the Supporting
Information).
(6) Arai, N.; Azuma, K.; Nii, N.; Ohkuma, T. Angew. Chem., Int. Ed. 2008,
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(8) (a) Wong, C.-H.; Whitesides, G. M. Enzymes in Organic Chemistry;
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Complex 4 is stable in solution (CDCl3) for 2 days and could be
purified by chromatography on silica gel with hexanes/diethyl ether
as eluent under standard isocratic conditions. The off-white complex
isolated may be employed in the reactions described in Table 2.
For example, when 4 is employed for the reaction of substrate 1a,
product 2a was isolated with excellent stereospecificity (>98% e.s.;
compare with entry 1, Table 2: >98% e.s.) and comparable yield
(65%; compare with entry 1, Table 2: 70%), confirming that 4 is
catalytically competent. It is comparable to the complex formed in
situ ([Ir(cod)Cl]2, 3, and LiI) as judged by 31P NMR spectroscopy
in CDCl3: the complex prepared in situ δ 136.920 versus a solution
of 4 δ 136.825.
In summary we have demonstrated the first general Ir-catalyzed
stereospecific allylic amination of optically active, unactivated,
allylic alcohols. The 1°-allylic amine can be isolated directly or
protected in situ as part of the workup. The reaction is made possible
because of the unique reactivity of a complex generated from Ir(I)
and 2 equiv of a phosphoramidite-alkene ligand whose reactivity
is modulated in the presence of LiI. Given the availability of
optically active allylic alcohols through a variety of methods, the
transformation we describe provides rapid entry to optically active
allylic amines. Spectroscopic studies have shed light on the catalyst
structure. Further studies are underway to clarify the pathway from
substrate to allylic amine product, which will be reported as results
become available.
Int. Ed. 2007, 46, 3139.
(11) Bartels, B.; Helmchen, G. Chem. Commun. 1999, 741.
(12) For additional mechanistic insight, see: (a) Helmchen, G.; Dahnz, A.; Du¨bon,
P.; Schelwies, M.; Weihofen, R. Chem. Commun. 2007, 675. (b) Takeuchi,
R.; Ue, N.; Tanabe, K.; Yamashita, K.; Shiga, N. J. Am. Chem. Soc. 2001,
123, 9525. For
a selection of fundamental studies on the rate of
isomerization of Ir-allyl complexes, see: (c) John, K. D.; Salazar, K. V.;
Scott, B. L.; Baker, R. T.; Sattelberger, A. P. Chem. Commun. 2000, 581.
(d) Tulip, T. H.; Ibers, J. A. J. Am. Chem. Soc. 1979, 101, 4201. The
conversion of racemic allylic acetates under Curtin-Hammet conditions,
in which equilibration of diastereotopic allyl-metal intermediates is rapid
relative to the rate of attack by nucleophile, has been described by Alexakis;
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K. Chem.sEur. J. 2006, 12, 3596. For an insightful discussion of
stereodynamic effects of allyl-Pd intermediates incorporating heterobidentate
ligands, see: (f) Faller, J. W.; Wilt, J. C. Organometallics 2005, 24, 5076.
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purity over the course of stereospecific reactions: Denmark, S. E.; Burk,
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(17) Reetz, M. T.; Bondarev, O. Angew. Chem., Int. Ed. 2007, 46, 4523.
(18) Gru¨tzmacher has described a related iridium-tropp complex that displays
both a cis and trans relation of the ligands; see: (a) Breher, F.; Ruegger,
H.; Mlakar, M.; Rudolph, M.; Deblon, S.; Schoenberg, H.; Boulmaaz, S.;
Thomaier, J.; Gruetmacher, H. Chem.sEur. J. 2004, 10, 641. Dorta reported
a Rh•((S)-P,olefin)2BF4 complex that shows an exclusive cis relation
between the ligands; see: (b) Drinkel, E.; Bricen˜o, A.; Dorta, R.; Dorta, R.
Organometallics 2010, 29, 2503. For a review of olefins as steering ligands,
see: (c) Defieber, C.; Gruetzmacher, H.; Carreira, E. M. Angew. Chem.,
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(19) Reboul, J. P.; Cristau, B.; Soyfer, J. C.; Estienne, J. Acta Crystallogr. 1982,
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Acknowledgment. We are grateful to Dr. W. B. Schweizer for
assistance with X-ray crystallographic analyses.
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