Direct observation of β-aryl eliminations from Rh(I) alkoxides

Article abstract of DOI:10.1021/ja058550q

β-Aryl eliminations from a series of rhodium(I) alkoxides to form rhodium aryl complexes and free ketones are reported. Tertiary phenylmethoxide complexes [Rh(PEt₃)_n(OCPhRR′)] (n = 2, 3) were prepared via alcoholysis of {Rh(PEt₃)₂[N(SiMe₃)₂} by the corresponding alcohols HOCPhRR′ in the presence and absence of added PEt₃. Heating of these complexes in the presence of added PEt₃ generated the rhodium phenyl complex, (PEt₃)₃RhPh, and the corresponding ketones in good to high yields. Kinetic results are most consistent with irreversible β-phenyl elimination from a bisphosphine-ligated rhodium alkoxide complex. Such bisphosphine complexes result from ligand dissociation from the trisphosphine complexes and have been isolated in some cases. The bisphosphine complexes are stabilized by Rh-C_phenyl interactions, as evidenced by an X-ray structure, and this structure with a metal-aryl interaction likely illustrates the pathway for C-C bond cleavage. Copyright

Full text of DOI:10.1021/ja058550q

Published on Web 02/21/2006
Direct Observation of â-Aryl Eliminations from Rh(I) Alkoxides
Pinjing Zhao, Christopher D. Incarvito, and John F. Hartwig*
Department of Chemistry, Yale UniVersity, PO Box 208107, New HaVen, Connecticut 06520-8107
Received December 16, 2005; E-mail: john.hartwig@yale.edu
â-Hydrogen elimination from late transition-metal alkoxo and
ized by spectroscopic methods in solutions that contained added
PEt3 (∼5 equiv) to suppress ligand dissociation. Complexes 3c and
3d were stable enough as crystalline solids to allow satisfactory
elemental analysis to be obtained.
amido complexes is involved in many catalytic processes. The
analogous â-hydrocarbyl eliminations from metal alkoxides were
also suggested to occur in several catalytic processes.1-5 In contrast
to â-hydrogen eliminations from alkoxides, which have been
observed from isolated alkoxo complexes in several cases,6 few
â-hydrocarbyl eliminations from isolated alkoxo complexes have
been documented.7,8 We report â-aryl eliminations from a series
of isolated rhodium(I) alkoxides to generate ketones and a rhodium-
(I) aryl complex. One alkoxo complex of this series is coordinatively
unsaturated and allows a direct measurement of the rate of â-phenyl
elimination. The structure of the precursor to the elimination
contains a metal-aryl interaction that is likely to lie on the reaction
coordinate for C-C bond cleavage.
Heating of a mixture of 2a and PEt3 afforded (PEt3)3RhPh8 (4)
and benzophenone in good yields from â-phenyl elimination (eq
2).12 In contrast, the combination of complex 2c and added PEt3
generated the trisphosphine complex 3c. However, heating of 3c,
as well as complexes 3b and 3d, with 2-5 equiv of PEt3 for 90
min to 3 h at 40-70 °C led to â-phenyl eliminations to give
arylrhodium complex 4 and the corresponding ketones 5b-5d (eq
3). No products from ring opening of the alkoxides in complexes
3c or 3d were observed.13 Heating of trifluoro-tert-butoxide 3e gave
the free alcohol 1e as the major organic product and several
unidentified Rh species, as indicated by 31P NMR spectroscopy.
The contrast between the lack of â-methyl elimination from 3e and
the facile â-aryl elimination from complex 3b, which contains both
â-aryl and â-methyl groups, shows that â-aryl elimination from
these rhodium complexes is much more favorable than â-methyl
elimination.5
Triethylphosphine-ligated rhodium alkoxides were prepared by
the sequence in eq 1. Reaction of the Rh(I) silylamido precursor
{(PEt3)2Rh[N(SiMe3)2]}9 with tertiary alcohols 1a-e readily oc-
curred at room temperature to eliminate HN(SiMe3)2 and form
rhodium alkoxo complexes.10,11
Reactions of the silylamide with alcohols led to both bisphos-
phine and trisphosphine complexes. Reaction of the silylamide with
1 equiv of trityl alcohol (1a) and 9-phenyl-9-xanthenol (1c) formed
the bisphosphine complexes 2a and 2c. These alkoxide complexes
were stable enough to isolate and were obtained in analytically pure
form in 73 and 66% yields. Complex 2a remained as the
bisphosphine species in the presence of added PEt3 at room
temperature. In contrast, complex 2c coordinated added PEt3 and
formed the trisphosphine-ligated rhodium alkoxide 3c, which was
isolated in good yield.
The addition of 1,1-diphenylethanol (1b) and 9-phenyl-9-
fluorenol (1d) to the silylamide formed the bisphosphine complexes
2b and 2d, but these complexes were too unstable to isolate (see
Supporting Information for spectral data). However, addition of
these alcohols and addition of trifluoro-tert-butyl alcohol (1e) to
the silylamido complex in the presence of 2-5 equiv of added PEt3
formed more stable trisphosphine complexes 3b, 3d and 3e, which
were isolated as solids in good yields. Upon dissolution in the
absence of added PEt3, 3b-3e spontaneously dissociated PEt3 to
create an equilibrium mixture of the bisphosphine and trisphosphine
complexes that slightly favored the trisphosphine complex.
Complexes 2a and 2c were characterized by spectroscopic
methods and elemental analysis. Complexes 3b-3e were character-
The â-phenyl eliminations from trisphosphine alkoxide com-
plexes 3b-3d were inhibited by PEt3. The reactions in eq 3 were
conducted with 2 and 10 equiv of added PEt3, and the reaction in
the presence of the higher concentration of added ligand was 2-3
times slower. In addition, the yields of products from â-aryl
elimination were lower (27-50%) in the presence of added PEt3.14
Because the yields were lower in the presence of added PEt3,
quantitative rate data were not obtained. However, the inhibition
of the reaction by added PEt3 observed during qualitative experi-
ments is consistent with a reaction pathway that occurs by
dissociation of PEt3 and rate-limiting â-phenyl elimination from
the resulting bisphosphine rhodium complex.
Consistent with this assertion, quantitative kinetic studies showed
that the rate of â-phenyl elimination from bisphosphine complex
2a was unaffected by added PEt3. Reaction rate constants were
measured by 1H NMR spectroscopy at 50 °C with an initial 0.014
M concentration of 2a, concentrations of PEt3 varying from 0.028
to 0.14 M, and no added benzophenone or 0.070 M added
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J. AM. CHEM. SOC. 2006, 128, 3124-3125
10.1021/ja058550q CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
eliminations. The bisphosphine complexes undergo elimination
directly from the isolated species, and the trisphosphine complexes
undergo elimination after dissociation of a phosphine. The need
for an open coordination site for â-aryl elimination parallels the
need for an open coordination site in classic â-hydride elimination
pathways,20 and the structure of a bisphosphine â-aryl alkoxide
complex illustrates the interaction that is likely to precede C-C
bond cleavage. Studies on the relative reactivities of different
arylmethoxides and the potential reversibility of the elimination
process are underway.
Acknowledgment. Financial support for this work was provided
by the Department of Energy, Office of Basic Energy Sciences.
We thank Boehringer Ingelheim for an unrestricted gift.
Figure 1. ORTEP diagram of [Rh(PEt3)2(OCPh3)] (2a). Hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and bond angles (deg):
Rh-O ) 2.069(3), Rh-C(14) ) 2.350(4), Rh-C(15) ) 2.398(4), Rh-
P(1) ) 2.228(1), Rh-P(2) ) 2.222(1), O-C(13) ) 1.407(5), P(1)-Rh-
P(2) ) 97.21(5), O-Rh-P(2) ) 88.05(9), Rh-O-C(13) ) 101.4(2),
O-C(13)-C(14) ) 104.7(3), C(13)-C(20) ) 1.528(6), C(13)-C(26) )
1.530(6), C(13)-C(14) ) 1.552(6).
Supporting Information Available: Experimental details, kinetic
plots, and full structural characterization of 2a (CIF and PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Scheme 1
References
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Organomet. Chem. 2005, 14, 1. (b) Jun, C.-H.; Moon, C. W.; Lee, D.-Y.
Chem.sEur. J. 2002, 8, 2422. (c) Murakami, M.; Ito, Y. In ActiVation of
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(2) Ru-catalyzed â-allyl elimination: Kondo, T.; Kodoi, K.; Nishinaga, E.;
Okada, T.; Morisaki, Y.; Watanabe, Y.; Mitsudo, T.-a. J. Am. Chem. Soc.
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(3) Metal-catalyzed â-alkynyl eliminations: (a) Chow, H.-F.; Wan, C.-W.;
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(5) Pd-catalyzed â-aryl eliminations of tert-arylmethanols: (a) Terao, Y.;
Wakui, H.; Satoh, T.; Miura, M.; Nomura, M. J. Am. Chem. Soc. 2001,
123, 10407. (b) Terao, Y.; Wakui, H.; Nomoto, M.; Satoh, T.; Miura,
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benzophenone. A clear exponential decay of 2a indicated that the
reaction was first-order in rhodium (see Supporting Information).
The rate constants for reactions with the different concentrations
of PEt3 or added benzophenone were indistinguishable (Scheme
1). These results support a mechanism in which irreversible
â-phenyl elimination occurs from the bisphosphine-ligated Rh
alkoxide. Further, these data reveal the rate constant for C-C bond
cleavage in the absence of prior dissociation of ligand to generate
the requisite open coordination site. This cleavage of the C-C bond
occurs from an unstrained alkoxo complex with conventional
phosphine ligands within 1 h at only 50 °C.
The solid-state structure of 2a determined by X-ray diffraction
(Figure 1) hints at the pathway for the â-aryl elimination. This
structure consists of a pseudo-square planar rhodium center with
two cis-oriented triethylphospine ligands and one triphenylmethoxy
ligand that is bound through oxygen and the C(14)-C(15) carbons
of an η2-phenyl unit. The Rh-O distance is similar to those in
previously isolated Rh(I) alkoxides,15 and the Rh-C(14) and Rh-
C(15) distances are longer than the Rh-Colefin distances of a Rh-
(η1:η2-CH2CPh2CHdCH2) complex (2.12 and 2.16 Å).16,17 Most
relevant to the reaction mechanism, the C-C bond between the
alkyl carbon and the aryl group bound to rhodium is longer than
the other C-C bonds between the sp3 and ipso carbons. This
structural feature suggests that this complex with an η2-arene
interaction lies on the â-elimination pathway.
(6) (a) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, D. C.; Tam, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805. (b) Blum, O.; Milstein,
D. J. Am. Chem. Soc. 1995, 117, 4582. (c) Ritter, J. C. M.; Bergman, R.
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(8) â-Aryl eliminations from isolated Rh(I) iminyl complexes: Zhao, P.;
Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 11618.
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(10) Hill, M. S.; Hitchcock, P. B. J. Chem. Soc., Dalton Trans. 2002, 4694.
(11) Treatment of the silylamido complex with less acidic alcohols, such as
t-BuOH and HOC(Me)2Ph, at 20-80 °C led to no reaction.
(12) Heating 2a in the absence of ligand led to the formation of multiple
products due to the instability of the 14-electron [Rh(PEt3)2Ph] product.
(13) Both â-phenyl elimination and ring opening C-C cleavage have been
reported in relevant Pd-catalyzed processes; see refs 5a-c.
(14) See Supporting Information for details. Complex 4 underwent slow
aromatic C-H exchange with benzene as well as the aromatic ketone
products. See ref 8 for an analogous observation.
(15) See for example: Kegley, S. E.; Schaverien, C. J.; Freudenberger, J. H.;
Bergman, R. G. J. Am. Chem. Soc. 1987, 109, 6563.
(16) Nishihara, Y.; Yoda, C.; Osakada, K. Organometallics 2001, 20, 2124.
(17) A Pd-C distance of 2.34 Å was measured for a cationic Pd(II) complex
stabilized by an interaction with the ipso carbon of a phenyl group:
Ca´mpora, J.; Gutie´rrez-Puebla, E.; Lo´pez, J. A.; Monge, A.; Palma, P.;
del R´ıo, D.; Carmona, E. Angew. Chem., Int. Ed. 2001, 40, 3641.
(18) For a recent review, see: Scherer, W.; McGrady, G. S. Angew. Chem.,
Int. Ed. 2004, 43, 1782.
The potential intermediacy of 2a in the â-phenyl elimination
draws parallels with the role of â-agostic complexes in â-hydrogen
elimination.18 Just as an interaction of the metal center with a
â-hydrogen seems to precede â-hydrogen elimination, an interaction
of the rhodium with the â-aryl group of the alkoxide 2a seems to
precede the â-aryl elimination event.19 Simple lengthening of the
C-C bond and shortening of the Rh-C bond to the ipso carbon in
2a and its analogs would generate the initial arylrhodium ketone
complex that would react with phosphine to displace the ketone
and form the final arylrhodium product.
(19) Urtel, H.; Meier, C.; Eisentra¨ger, F.; Rominger, F.; Joschek, J. P.;
Hofmann, P. Angew. Chem., Int. Ed. 2001, 40, 781.
(20) (a) Cross, R. J. In The Chemistry of the Metal-Carbon Bond; Hartley, F.
R., Patai, S., Eds.; John Wiley: New York, 1985; Vol. 2, p 559. (b) For
a more recent study, see: Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J.
M.; Gregory, E. A.; Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L.
Organometallics 2004, 23, 5226. (c) For â-hydrogen elimination from
alkoxo and amido complexes by a migratory deinsertion mechanism, see
ref 6d and Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7010.
In summary, we have prepared a series of isolated rhodium(I)
tertiary phenylmethoxide complexes that undergo mild â-phenyl
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