C O M M U N I C A T I O N S
Scheme 5. Proposed Mechanism for Cleavage of the C-O Bond
(Dehydroaryloxation) of Ethyl Aryl Ethers by (PCP)Ir
Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (b) Shilov, A. E.; Shul’pin,
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(2) (a) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Vigalok, A.; Milstein,
D. Angew. Chem., Int. Ed. Engl. 1997, 36, 625. (b) van der Boom, M. E.;
Liou, S.-Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. J. Am. Chem.
Soc. 1998, 120, 6531. (c) Ueno, S.; Mizushima, E.; Chatani, N.; Kakiuchi,
F. J. Am. Chem. Soc. 2006, 128, 16516.
Scheme 6. Competition Experiment To Determine the Relative
Rates of Reaction of (PCP)Ir with Methyl versus Ethyl Aryl Ether
(3) Wolczanski has reported the oxidative addition to tantalum of the C-O
bond of strained and vinylic cyclic ethers. See: Bonanno, J. B.; Henry,
T. P.; Neithamer, D. R.; Wolczanski, P. T.; Lobkovsky, E. B. J. Am. Chem.
Soc. 1996, 118, 5132.
(4) (a) Ittel, S. D.; Tolman, C. A.; English, A. D.; Jesson, J. P. J. Am. Chem.
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Soc. 1993, 115, 2075. (b) Koo, K.; Hillhouse, G. L. Organometallics 1998,
17, 2924, and references therein.
(6) (a) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046. (b) Stambuli,
J. P.; Weng, Z.; Incarvito, C. D.; Hartwig, J. F. Angew. Chem., Int. Ed.
2007, 46, 7674, and references therein.
(7) (a) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. J. Am. Chem. Soc.
1997, 119, 6787. (b) Widenhoefer, R. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1998, 120, 6504, and references therein.
(8) (a) Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A. N. Organometallics
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Lam, Y.-F.; Vedernikov, A. N. J. Am. Chem. Soc. 2008, 130, 2174.
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127, 12790. (b) Racowski, J. M.; Dick, A. R.; Sanford, M. S. J. Am. Chem.
Soc. 2009, 131, 10974, and references therein.
(10) (a) Smythe, N. A.; Grice, K. A.; Williams, B. S.; Goldberg, K. I.
Organometallics 2009, 28, 277. (b) Williams, B. S.; Goldberg, K. I. J. Am.
Chem. Soc. 2001, 123, 2576. (c) Williams, B. S.; Holland, A. W.; Goldberg,
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addition are ∆Gq ) 44.4 kcal/mol for 4-ethoxy-2,3,5,6-tetrafluo-
rotoluene and 46.3 kcal/mol for ethoxybenzene (Table 1). In
contrast, we calculated barriers that are far lower for a pathway
involving addition of the terminal C-H bond followed by ꢀ-aryloxy
elimination and loss of ethylene (Scheme 5): ∆Gq ) 20.5 and 22.6
kcal/mol for 4-ethoxy-2,3,5,6-tetrafluorotoluene and ethoxybenzene,
respectively.
In accord with the much lower calculated barriers for the
ꢀ-aryloxy eliminations (Scheme 5; Table 1) than for the R-aryloxy
elimination pathway (Scheme 3), when a mixture of 4-methoxy-
2,3,5,6-tetrafluorotoluene and 7 was reacted with 1, complexes 5d
and 6 (the same products that are obtained from pure 7) were formed
exclusively (Scheme 6); i.e., there was no evidence of any reaction
with 4-methoxy-2,3,5,6-tetrafluorotoluene.12
In conclusion, we have presented two classes of reactions in
which the C-O bond of alkyl aryl ethers is cleaved. In the first
case, the net reaction is an unusual oxidative addition of the alkyl
(methyl) C-O bond. The second is a dehydroaryloxation reaction.
In both cases, the initial step is C(sp3)-H activation at the CH3 of
the methoxy and ethyl groups, respectively. It is noteworthy that
whereas the C-H bond has historically been characterized by its
inertness in contrast to “functional” groups, the results herein
suggest that C-H oxidative addition may provide an entry point
toward breaking bonds in a molecule that are significantly weaker24
than the C-H bond itself but kinetically less reactive toward
transition metals. In particular, this seems surprising in the case of
the C-O oxidative addition, wherein the C-H bond is re-formed
following the C-O bond cleavage step. Perhaps even less intuitive
is the conclusion, based on microscopic reversibility, that C-O
reductive elimination from complexes 4 would proceed via an initial
R-H migration. We are currently exploring ways to exploit both of
these reactions catalytically in either the direction observed in this
study or the reverse.
(11) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.; Goldman,
A. S. J. Am. Chem. Soc. 2000, 122, 11017.
(12) See the Supporting Information.
(13) Ben-Ari, E.; Cohen, R.; Gandelman, M.; Shimon, L. J. W.; Martin, J. M. L.;
Milstein, D. Organometallics 2006, 25, 3190.
(14) (a) Trovitch, R. J.; Lobkovsky, E.; Bouwkamp, M. W.; Chirik, P. J.
Organometallics 2008, 27, 6264. (b) Eisch, J. J.; Im, K. R. J. Organomet.
Chem. 1977, 139, C45. (c) Yamamoto, T.; Akimoto, M.; Yamamoto, A.
Chem. Lett. 1983, 12, 1725. (d) Yamamoto, T.; Akimoto, M.; Saito, O.;
Yamamoto, A. Organometallics 1986, 5, 1559.
(15) Davico, G. E. Org. Lett. 1999, 1, 1675.
(16) Secondary KIEs for SN2 reactions are generally small and typically even
inverse (<1). See: Kato, S.; Hacaloglu, J.; Davico, G. E.; DePuy, C. H.;
Bierbaum, V. M. J. Phys. Chem. A 2004, 108, 9887, and references therein.
(17) Lara, P.; Paneque, M.; Poveda, M. L.; Salazar, V.; Santos, L. L.; Carmona,
E. J. Am. Chem. Soc. 2006, 128, 3512.
(18) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.; Van Asselt, A.;
Bercaw, J. E. J. Mol. Catal. 1987, 41, 21.
(19) See: (a) Parkin, G. Acc. Chem. Res. 2009, 42, 315. (b) Janak, K. E.; Parkin,
G. J. Am. Chem. Soc. 2003, 125, 6889, and references therein.
(20) (a) Whited, M. T.; Grubbs, R. H. Organometallics 2008, 27, 5737. (b)
Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 5874. (c)
Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 16476. (d)
Whited, M. T.; Grubbs, R. H. Organometallics 2009, 28, 161. (e) Whited,
M. T.; Zhu, Y.; Timpa, S. D.; Chen, C.-H.; Foxman, B. M.; Ozerov, O. V.;
Grubbs, R. H. Organometallics 2009, 28, 4560.
(21) For other chemistry involving C-O cleavage and R-substituent elimination,
see: Ferrando, G.; Coalter, J. N., III; Gerard, H.; Huang, D.; Eisenstein,
O.; Caulton, K. G. New J. Chem. 2003, 27, 1451.
Acknowledgment. We thank the National Science Foundation
(Grant CHE-0719307) for support of this work.
(22) Gomez-Benitez, V.; Redon, R.; Morales-Morales, D. ReV. Soc. Quim. Mex.
2003, 47, 124.
Supporting Information Available: Experimental details and
procedures, details of DFT calculations, spectral data, and crystal-
lographic data for complexes 3a, 4b-d, 5a, and 5d (CIF). This material
(23) Analogous reactions of ethers with zirconocenes have been reported. See:
(a) Bradley, C. A.; Veiros, L. F.; Chirik, P. J. Organometallics 2007, 26,
3191. (b) Bradley, C. A.; Veiros, L. F.; Pun, D.; Lobkovsky, E.; Keresztes,
I.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 16600.
(24) The H3C-OPh bond dissociation energy (BDE) is 63.8 ( 1 kcal/mol. The
H-CH2OPh BDE has not been reported but is probably not very different
from the H-CH2OCH3 BDE, which is 93 ( 1 kcal/mol. See: McMillen,
D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33, 493.
References
(1) For some reviews of alkane C-H bond activation by organometallic
complexes, see: (a) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.;
JA906930U
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