β-aryl eliminations from Rh(I) iminyl complexes

Article abstract of DOI:10.1021/ja054132+

β-Aryl eliminations from a series of iminyl complexes to form rhodium aryl complexes and free nitriles are reported. Iminyl complexes [Rh(PEt₃)₃(N=CArAr′)] were prepared from [Rh(COE)Cl]₂, PEt₃, LiN(SiMe₃)₂, and the imines HN=CArAr′. One example of these complexes was characterized by X-ray diffraction. Heating of these complexes in cyclohexane generated the rhodium aryl complexes and free nitriles in high yields; heating in benzene formed the same products in slightly lower yields. Complexes with varied aryl groups on the imine were studied to assess the migratory aptitudes of the aryl groups. Migration of the o-anisyl group occurred much faster than migration of a phenyl group; migration of a phenyl group occurred slightly faster than migration of the more electron-rich p-anisyl group; and migration of a phenyl group occurred slightly faster than migration of the more hindered o-tolyl group. Kinetic studies showed that the reaction was inverse first-order in the concentration of added phosphine and zero-order in added nitrile. These results show that the β-aryl elimination most likely occurs by dissociation of phosphine from the starting complex and carbon-carbon bond cleavage of the resulting 14-electron intermediate. Copyright

Full text of DOI:10.1021/ja054132+

Published on Web 07/30/2005
â-Aryl Eliminations from Rh(I) Iminyl Complexes
Pinjing Zhao and John F. Hartwig*
Department of Chemistry, Yale UniVersity, PO Box 208107 New HaVen, Connecticut 06520-8107
Received June 22, 2005; E-mail: john.hartwig@yale.edu
â-Hydrogen elimination is a classic organometallic reaction of
late transition metal complexes. However, analogous reactions that
cleave carbon-carbon bonds are much less common.1 A few
examples of such â-carbon elimination reactions from late transition
metal complexes have been reported, but these reactions are more
common for early transition metal complexes. Directly observed
â-carbon elimination from late transition metal alkoxides,2 amides,
and related compounds with nondative metal-heteroatom bonds
are particularly rare, although several catalytic processes have been
suggested to occur by â-carbon eliminations from transition metal
alkoxides.3,4 We report the direct observation of â-aryl elimination
from the iminyl ligand of an isolated rhodium(I) iminyl complex.
This reaction generates a discrete rhodium(I) aryl complex and a
free aromatic nitrile.
Figure 1. ORTEP diagram of 2a. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Rh-N ) 2.039(2), Rh-
P(2) ) 2.268(1), Rh-P(1) ) 2.304(1), Rh-P(3) ) 2.321(1), Rh-N-C(19)
) 130.6(2), N-C(19)-C(20) ) 126.2(2), N-C(19)-C(26) ) 117.8(2).
Rhodium iminyl complexes were prepared by the sequence in
eq 1. [(COE)2Rh(µ-Cl)]2 was treated with 6 equiv of PEt3, followed
aryl product with solvent is likely to have complicated reactions in
benzene solvent.6,7 However, the reaction of the o-tolyl iminyl
complex 2c in the presence of added PEt3 in benzene formed the
rhodium aryl complex and free nitrile in excellent 96% and 83%
yields, respectively (eq 3). The rates were slower in the presence
of the added ligand, however (vide infra).
by 2 equiv of LiN(SiMe3)2 and 2 equiv of diarylimines 1a-g. This
procedure formed the silylamine and the rhodium iminyl complexes
in 48-78% isolated yields. These complexes were all formed in
moderate to high chemical yields; the isolated yields reflect the
solubility of the compounds more than the chemical yield of the
reaction sequence. The imine precursors 1a-g are either com-
mercially available or were formed by addition of the appropriate
Grignard reagent to a nitrile, followed by careful protonation of
the resulting anion.
Complexes 2a-g were characterized by spectroscopic methods
and elemental analysis, and the structure of 2a was determined by
X-ray diffraction (Figure 1). In the solid state, 2a adopts a square-
planar geometry. The sum of the angles around the Rh center was
found to be 360°. The Rh-N-C19 angle is bent (130.6°), and this
angle suggests weak NfRh π donation. Related square-planar
iminyl complexes also have bent M-N-C angles.5a The Rh-N
distance (2.04 Å) is comparable to that of another bent Rh iminyl
complex (2.02 Å),5b slightly shorter than that of a Rh amide (2.11
Å),5c and slightly longer than that in a linear Re iminyl complex
(1.90 Å).5d
Symmetrical iminyl complexes 2a-d underwent â-aryl elimina-
tion reactions to afford the corresponding rhodium aryl complexes
3a-d in 60-95% yields and benzonitrile derivatives 4a-d in 48-
88% yields (eq 2). In benzene, the rhodium complexes were formed
in somewhat lower 53-71% yields, and the nitriles were also
formed in similarly lower 31-67% yields. Reaction of the rhodium
The reactions of compounds 2b-2d were conducted to begin to
assess the electronic and steric effects on the reaction. The more
electron-rich p-tolyl and p-anisyl derivatives 2b and 2d reacted with
similar rates and in similar yields to the parent iminyl complex 2a.
In contrast, the o-tolyl complex 2c underwent â-aryl elimination
at much lower temperatures and in higher yields than the phenyl,
p-tolyl, or p-anisyl derivatives. This increase in reactivity could
result from a starting iminyl complex that is less stable with o-tolyl
groups because of the steric hindrance in the imine conformation
that creates overlap of the aryl π-system with the CdN bond. The
steric hindrance is less severe in the rhodium product that contains
an aryl group that lies perpendicular to the square coordination
plane.8
Trends in the migratory aptitudes of the aryl groups were
determined by conducting the reactions with the unsymmetrically
substituted iminyl complexes 2e-2g and determining the ratios of
the rhodium aryl complexes and nitrile products by a combination
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10.1021/ja054132+ CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
eliminations from isolated, isoelectronic iridium(I) amido and
alkoxo complexes.13
In summary, we have prepared a family of rhodium iminyl
complexes, all of which undergo C-C bond cleavage to form
rhodium aryl complexes and free aromatic nitriles. Such â-elimina-
tions that cleave C-C bonds in ligands bound to a transition metal
through a metal-nitrogen bond have little precedent, but kinetic
studies are most consistent with eliminations through a type of 14-
electron intermediate that is analogous to intermediates that undergo
more precedented â-hydrogen eliminations. Studies to probe the
thermodynamics, potential reversibility, and origins of the relative
rates of migration, as well as the potential of this reaction in catalytic
transformations, are underway.
Scheme 2
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.
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.
References
of NMR and GC methods with internal standards.9 These reactions
are summarized in Scheme 1. Complexes 2e-2g were prepared
by the method in eq 1 in 61-77% yields.
(1) For reviews on transition metal-mediated C-C activations, see: (a) Jun,
C.-H. Chem. Soc. ReV. 2004, 33, 610. (b) Murakami, M.; Ito, Y. Top.
Organomet. Chem. 1999, 3, 97. (c) Milstein, D.; Rybtchinski, B. Angew.
Chem., Int. Ed. 1999, 38, 870.
Most striking, o-anisyl phenyl iminyl complex 2e underwent
exclusive migration of the o-anisyl group at 60 °C, with a half-life
of roughly 200 s in C6D12 to afford 77% yield of (PEt3)3Rh(o-
anisyl) (3e) and 82% yield of benzonitrile (4a). At the same
temperature, o-tolyl phenyl iminyl complex 2f underwent competi-
tive migration of the two aryl groups. This reaction occurred with
a half-life of roughly 400 s in C6D12 to afford (PEt3)3RhPh (3a)
and (PEt3)3RhP(o-tolyl) (3c) in a roughly 2:1 ratio (79% overall
yield), slightly favoring phenyl migration. Thus, steric effects alone
do not control the relative rates for migration of the aryl groups.
Likewise, a simple electronic effect cannot account for the faster
migration of the o-anisyl complex. The phenyl p-anisyl iminyl
complex 2g underwent competitive migration of the two aryl groups
to form a roughly 2:1 ratio of rhodium aryl products 3a and 3d
(65% overall yield) in favor of complex 3a that results from
migration of the less electron-donating phenyl group. Thus, the
reactions of unsymmetrical iminyl complexes 2e-2g reveal the
unexpected relative migratory aptitudes: o-anisyl . phenyl >
p-anisyl ≈ o-tolyl. Studies to understand the origin of these
(2) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. Organometallics 1991,
10, 3344.
(3) (a) Kondo, T.; Kodoi, K.; Nishinaga, E.; Okada, T.; Morisaki, Y.;
Watanabe, Y.; Mitsudo, T.-a. J. Am. Chem. Soc. 1998, 120, 5587. (b)
Chow, H.-F.; Wan, C.-W.; Low, K.-H.; Yeung, Y.-Y. J. Org. Chem. 2001,
66, 1910. (c) Nishimura, T.; Araki, H.; Maeda, Y.; Uemura, S. Org. Lett.
2003, 5, 2997. (d) Nishimura, T.; Uemura, S. Synlett 2004, 201 and
references therein. (e) Nishimura, T.; Nishiguchi, Y.; Maeda, Y.; Uemura,
S. J. Org. Chem. 2004, 69, 5342. (f) Wakui, H.; Kawasaki, S.; Satoh, T.;
Miura, M.; Nomura, M. J. Am. Chem. Soc. 2004, 126, 8658.
(4) For C-C cleavage of an amine by a pathway other than â-elimination,
see: Zhang, X.; Emge, T. J.; Ghosh, R.; Goldman, A. S.; J. Am. Chem.
Soc 2005, 127, 8250.
(5) (a) Erker, G.; Fromberg, W.; Kruger, C.; Raabe, E. J. Am. Chem. Soc.
1988, 110, 2400 and references therein. (b) Esteruelas, M. A.; Lahoz, F.
J.; Olivan, M.; Onate, E.; Oro, L. A. Organometallics 1994, 13, 3315. (c)
Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 2694. (d) Ferreira,
C. M. P.; Guedes da Silva, M. F.; Kukushkin, V. Y.; Frausto da Silva, J.
J. R.; Pombeiro, A. J. L. J. Chem. Soc., Dalton Trans. 1998, 325.
(6) Traces of the corresponding biaryls and arenes were also observed. Heating
of isolated 3a-d formed the same byproducts.
(7) For a study on aromatic C-H activation by a PMe3-ligated square-planar
Rh(I) alkyl complex, see: Price, R. T.; Andersen, R. A.; Muetterties, E.
L. J. Organomet. Chem. 1989, 376, 407.
(8) For a discussion of R-substitution effects in Pd-catalyzed â-aryl elimination
via proposed alkoxide intermediates, see: Terao, Y.; Wakui, H.; Nomoto,
M.; Satoh, T.; Miura, M.; Nomura, M. J. Org. Chem. 2003, 68, 5236 and
references therein.
10
migratory aptitudes are ongoing.
The mechanism of the clean â-aryl elimination from iminyl
complex 2c with added PEt3 was investigated by kinetic methods.
(9) The rhodium aryl complexes 3a-e were prepared independently by the
reaction of (PEt3)3RhCl with the corresponding aryllithium or Grignard
reagents in 55-77% isolated yields (see Supporting Information).
(10) During the review of this manuscript, we obtained X-ray structural data
on complex 3e that shows the absence of a Rh-O bonding interaction.
Details will be reported in future publications.
1
Reaction rate constants were measured by H NMR spectroscopy
at 60 °C with an initial concentration of 2c of 0.020 M. Reactions
were conducted while varying the concentration of PEt3 from 0.020
to 0.40 M. A clear exponential decay of 2c indicated that the
reaction was first-order in rhodium. A plot of 1/kobsd vs [PEt3]
(Figure S2 in Supporting Information) showed that the reaction was
inverse first-order in added PEt3. The rate constants, as well as
reaction conversions, were unaffected by added o-tolunitrile (see
Supporting Information). These results are most consistent with a
pathway that occurs by reversible dissociation of PEt3 to form a
14-electron intermediate (Scheme 2), which undergoes â-aryl
elimination,11 presumably to form a nitrile-ligated, rhodium aryl
complex. Subsequent displacement of the nitrile by PEt3 would
afford tris-triethylphosphine complex 3c. This mechanism is
analogous to the mechanisms of â-hydrogen eliminations from d8
square-planar alkyl complexes12 and the mechanisms for â-hydrogen
(11) For catalytic processes that are proposed to occur by the insertion of a
nitrile into a late metal aryl or alkenyl linkage see: (a) Zhou, C. X.; Larock,
R. C. J. Am. Chem. Soc. 2004, 126, 2302. (b) Miura, T.; Nakazawa, H.;
Murakami, M. Chem. Commun. 2005, 2855. (c) Miura, T.; Murakami,
M. Org. Lett. 2005, 7, 3339.
(12) (a) Whitesides, G. M.; Gaasch, J. F.; Stedronsky, E. R. J. Am. Chem.
Soc. 1972, 94, 5258. (b) Evans, J.; Schwartz, J. Urquhart, P. W. J.
Organomet. Chem. 1974, 81, C37. (c) Ozawa, F.; Ito, T.; Yamamoto, Y.
J. Am. Chem. Soc. 1980, 102, 6457. (d) 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. (e) Miller, T. M.; Whitesides, G. M.
Organometallics 1986, 5, 1473.
(13) (a) Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7010. (b) Zhao, J.;
Hesslink, H.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 7220.
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