3254 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Communications to the Editor
Et2O/pentane cooled to -40 °C. This procedure provided 8a,b
in a 10:7 ratio in 35% yield. The relative stereochemistry of
diastereomers 8a,b was assigned (as with 7a and 7b) on the basis
of a 2D-NOESY spectrum of a 10:7 mixture of the two isomers.
We further characterized the 8a,b diastereomeric pair by convert-
ing them to the chlorides t-BuCpPIr(Cl)Ph (9a,b) by treatment with
CCl4 in C6H6. The air-stable complexes 9a and 9b were separated
by column chromatography and isolated in 51% combined yield.
The complex 9a (39% yield) was fully characterized and identified
as the (RR),(SS) isomer by a 2D-NOESY spectrum. Complex
elevated temperature. Similarly, thermolysis of 10a in C6H6 does
not result in significant conversion to 8a and 8b until the
temperature reaches 150 °C. Previously studied systems4,9
indicate that interconversion of similar alkyl- and aryliridium
hydride species occurs very slowly at temperatures below 130
°C. In the cyclohexyl case, both kinetics and thermodynamics
appear to favor the essentially exclusive formation of one
diastereomer.
Preliminary molecular modeling studies10 indicate that the
difference in kinetic selectivity for activation of the two substrates,
cyclohexane and benzene, appears to be due to the greater steric
demand of the cyclohexane ring during activation of the C-H
bond. When one examines the transition state that would be
required to activate a C-H bond of cyclohexane in the orientation
required to form diastereomer 10b, it is immediately apparent
that there is a large unfavorable interaction between the cyclo-
hexane molecule and the tert-butyl group of the cyclopentadienyl
ligand. The contrasting negligible kinetic selectivity and small
thermodynamic selectivity (3.5:1) observed in the benzene case
can be explained by an attack on the C-H bond by the Ir center
in the plane of the benzene ring, since an above-the-plane
approach would appear to generate much more steric repulsion.
If this interpretation of the data is correct, it would indicate that,
even if an η2 -benzene complex is formed as the initial
intermediate,8 in the subsequent C-H bond activation step the Ir
lies in the nodal plane of the π-system of the benzene ring.11
1
9b (12% yield) was identified by H NMR and EI-MS.
Cyclohexane also undergoes photolytic C-H activation with
dihydride 6, but the stereochemistry of this reaction contrasts
dramatically with that observed in the benzene reaction. Irradia-
tion of dihydride 1 in C6H12 at 15 °C results in the formation of
only one product, the cyclohexyliridium hydride (RR),(SS)-
t-BuCpPIr(H)Cy (10a, Scheme 2). The opposite diastereomer 10b
is not detected (5% detection limits). Complex 10a was subjected
to air-free, low-temperature chromatography (-80 °C, repeated
twice) and isolated in 11% yield. We could not obtain this
material completely pure, but we were able to characterize it
spectroscopically, including high-resolution mass spectrometry
of the molecular ion. As with the phenyl- and methyliridium
complexes described above, the stereochemical relationships
between the substituents on the cyclopentadienyl ligand and at
the iridium center were established by using 2D NMR experi-
ments.
Future experiments will be directed toward examining the
stereoselectivity of C-H activation of prochiral alkane substrates,
as well as studies aimed at preparing enantioenriched analogues
of the molecules described here.
When a C6H12 solution of t-BuCpPIr(H)Cy (10a) was heated at
150 °C for 72 h, no change in the 31P{1H} NMR spectrum (i.e.,
no formation of 10b) was observed. However, upon heating a
C6H6 solution of t-BuCpPIr(H)Cy (10a) at 150 °C for 120 h,
conversion to 8a and 8b was observed by 31P{1H} NMR
spectroscopy (Scheme 2). The equilibrium constant for the
interconversion of 8a and 8b was determined at 150 °C by 31P-
{1H} NMR spectrometry; this gave Keq ) 0.29 ( 0.03 at 423 K.
This value corresponds to a difference in free energy between 8a
and 8b of ∆G423 ) 1.1 ( 0.1 kcal/mol.
Acknowledgment. We thank Dr. F. Hollander of the University of
California at Berkeley CHEXRAY facility for the acquisition and solution
of the X-ray crystallographic data for complexes 4, 6, and 7b. This work
was supported by the Director, Office of Energy Research, Office of Basic
Energy Sciences, Chemical Sciences Division, U. S. Department of
Energy, under Contract No. DE-AC03-76SF00098.
In an attempt to determine whether the interconversion of 8a
and 8b is an intramolecular process, a 10:7 mixture of 8a and 8b
was thermolyzed in C6D6 at 150 °C. After 6 h of heating,
significant interconversion (10:3.5 ratio) of the diastereomers had
occurred; however, very little deuterium (<3%) had been
incorporated into 8a and 8b. After a total of 48 h of heating,
some deuterium incorporation into the hydride position of 8b was
evident in the 31P{1H} NMR spectrum by the appearance of a
small 1:1:1 triplet resonance that appeared slightly downfield from
the phosphorus resonance of the parent compound 8b. Integration
Supporting Information Available: Details of the synthetic proce-
dures for compounds 2-10 and details of the crystal structure determina-
tions (including ORTEP diagrams, tables of fractional atomic coordinates,
and crystal data) of compounds 4, 6, and 7b, and spectroscopic and
analytical data for 5 and 7-10 (19 pages, print/PDF). See any current
masthead page for ordering information and Web access instructions.
JA974323F
(9) (a) Mobley, T. A.; Schade, C.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 7822. (b) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J. Am. Chem.
Soc. 1986, 108, 1537.
1
of the ortho-proton resonances in the H NMR spectrum of 8a
and 8b, relative to one of the cyclopentadienyl resonances,
indicated that little deuterium had been incorporated into the
phenyl rings. This indicates, in agreement with earlier work in
achiral Rh systems,8 that the diastereomeric phenyl hydrides
interconvert with η2-benzene complexes substantially more rapidly
than benzene is displaced from the metal center.
On the basis of the above results, we believe that the
diastereomeric ratios observed in these photolytic reactions reflect
the kinetic selectivities for the C-H bond activation reaction.
This belief is based upon the fact that the equilibrium ratio of
the phenyliridium hydrides 8a and 8b (Keq ) 0.29 ( 0.03 at 150
°C) is reached only upon extended thermolysis even at this
(10) Modeling of the saturated compounds was accomplished by using PC-
Model. Gajewski, J. J.; Gilber, K. E.; McKelvey, J. AdVances in Molecular
Modeling; JAI Press Inc.: Greens Greenwich, CT 1990; Vol. 2. Program
version 5, with parameters for organometallic compounds updated by J. J.
Gajewski. The angles and distances from iridium to the hydrogen and carbon
in the model transition states for C-H bond activation were adapted from
the theoretically predicted transition states for CH4 activation by Cp(PH3)Ir.
Theoretical references: (a) Song, J.; Hall, M. B. Organometallics 1993, 12,
3118. (b) Margl, P.; Ziegler, T.; Blochl, P. E. J. Am. Chem. Soc. 1995, 117,
12625. (c) Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1995, 117, 799.
(11) We agree with a referee’s comment that on the basis of presently
3
available data, we cannot rule out a mechanism involving reversible η5 f η
conversion of the Cp* ring to open a coordination site at the Ir center. For an
analogous mechanism in a Tp*-substituted system, see: Bromberg, S. E.;
Yang, H.; Asplund, M. C.; Lian, T.; McNamara, B. K.; Kotz, K. T.; Yeston,
J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; Harris, C. B. Science 1997,
278, 260-263.
(8) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1986, 108, 4814.