Imine Insertion into a Late Metal-Carbon Bond to Form a Stable Amido Complex

Article abstract of DOI:10.1021/ja038819a

An arylrhodium(I) complex containing a labile dative ligand was prepared, and its reactivity toward aryl imines was investigated. The arylrhodium(I) complex (DPPE)Rh(C₅H₅N)(p-tol), 2, was isolated in 65% yield from [(DPPE)Rh(μ-Cl)]₂, pyridine, and p-tolyllithium. Reaction of 2 with the aldimine (p-tol)CH=N(C₆H₄-p-CO₂Me) (3a-Tol) gave the Rh amide insertion product 4 in 88% isolated yield. The solid-state structure of 4 was determined by single-crystal X-ray diffraction. The reaction of 2 with the electron-neutral and electron-rich aldimines (Ph)CH=NPh (3b) and (p-tol)CH=N(C₆H₄-p-OMe) (3c) also appeared to involve insertion, but the amido complexes formed from these insertions were not stable. Thus, reaction of 2 with 3b, followed by addition of Et₃NHCl, gave the amine and ketimine products (Ph)(p-tol)CH?NHPh, 5, and (p-tol)(Ph)C=N(Ph), 6, in 25% and 50% yields. Several lines of data indicate that these products are formed by a sequence of transformations involving insertion of imine to give a Rh amide intermediate, β-hydrogen elimination, cyclometalation to form a bound imine and H₂, and protonolysis of the metallacycle upon addition of Et₃NHCl. Consistent with this proposal, the proposed metallacycle containing the ortho-metalated ketimine ligand (p-tol)₂C=N(C₆H₄-p-OMe) was isolated and characterized by single-crystal X-ray diffraction. Copyright

Full text of DOI:10.1021/ja038819a

Published on Web 02/12/2004
Imine Insertion into a Late Metal-Carbon Bond To Form a Stable Amido
Complex
Christopher Krug and John F. Hartwig*
Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107
Received October 1, 2003; E-mail: john.hartwig@yale.edu
Late transition metal complexes that bind and insert alkenes into
a metal-carbon bond are common and have been invoked as
intermediates in many catalytic processes.1 In contrast, late metal
complexes that insert imines into metal-carbon or -hydrogen2
bonds are limited. Arndtsen3 and Sen4 have described 2,1 insertions
of imines into metal-acyl bonds of cationic Ni and Pd complexes
to generate products with a new metal-carbon bond, but directly
observed 1,2 insertions of imines into late metal-carbon bonds to
form stable amido complexes are rare or unknown.
This difference may result from both thermodynamic and kinetic
factors. The insertion of an imine cleaves a C-X π-bond that is
stronger than the CdC bond in an olefin and forms a product with
an amido group that is matched less favorably with a soft metal
center than is an alkyl group from olefin insertion. In addition, stable
Figure 1. ORTEP diagram of 4. Selected bond lengths (Å) and bond angles
aldimines bear substituents at N and C and are, therefore, more
(deg): Rh(1)-N(1) 2.153; Rh(1)-N(2) 2.145; Rh(1)-P(1) 2.207; Rh(1)-
P(2) 2.205; N(2)-Rh(1)-N(1) 88.44; N(2)-Rh(1)-P(2) 91.09; N(1)-Rh-
(1)-P(2) 179.14; N(2)-Rh(1)-P(1) 170.71.
hindered than R-olefins. Finally, imines can bind to the metal
through the nitrogen lone pair, and complexes with such σ-bound
imines are less likely to undergo insertion than those with π-bound
imines.
We recently began to identify late metal complexes that would
insert aldehydes and imines to form metal alkoxo and amido
complexes. Although organorhodium(I) complexes of the general
formula [Rh(PPh3)2(CO)R] (R ) p-tol, o-tol, Me) inserted aromatic
aldehydes to form alkoxide intermediates, they did not react with
imines.5 We now report an arylrhodium complex that does insert
imines and gives rise to a stable amido complex from 1,2 insertion
of an N-aryl benzaldimine.
dissipation of the orange color of the arylrhodium complex and
generation of a red solution of amidorhodium complex 4 (eq 2).
This complex was isolated in 88% yield upon recrystallization.
X-ray diffraction of the orange microcrystalline material confirmed
the structure of the pyridine-ligated amide 4 (Figure 1). In the solid
state, 4 adopts a square planar geometry with the bulky diarylmethyl
and aryl substituents of the amide ligand positioned above and
below the plane of the molecule.
To promote insertions of imines, we sought to study complexes
that contain an aryl group located cis to a ligand more labile than
PPh3 or CO. To prepare complexes with such a coordination sphere,
we targeted arylrhodium complexes with one chelating phosphine
and one pyridine ligand. Such a complex, Rh(DPPE)(py)(p-tol) (2,
DPPE ) 1,2-bis(diphenylphosphino)ethane, py ) pyridine), was
31P NMR spectra of 4 at room temperature consisted of a broad
doublet at 68.0 ppm (JPRh ) 178 Hz), and 1H NMR spectra
contained broad resonances. Amide 4 undergoes reversible dis-
sociation of pyridine on the NMR time scale in solution. Spin
saturation transfer experiments on 4 at -20 °C with 1 equiv of
added pyridine showed saturation transfer between the ortho protons
of the bound pyridine and those of the free pyridine. We have not
yet distinguished between an associative and dissociative mecha-
nism for this exchange.
In contrast to NMR data obtained at room temperature, 1H NMR
spectra of 4 obtained at -20 °C were sharp and confirmed the
identity of this complex in solution. Two signals for the p-tolyl
methyl groups and four doublets corresponding to the aromatic
protons of the p-tolyl groups were observed. These data indicate
that the p-tolyl groups are diastereotopic because rotation about
the Rh-amide nitrogen bond is slow on the NMR time scale at
-20 °C. 31P NMR spectra of amide 4 acquired at -20 °C consisted
of a single doublet due to similar chemical shifts of the phosphines
and small PA-PB coupling.
6
prepared in two steps from the dimeric chloride [(DPPE)Rh(µ-Cl)]2
(eq 1). Addition of pyridine (15 equiv) to a THF suspension of
[(DPPE)Rh(µ-Cl)]2 generated a bright yellow solution of (DPPE)-
1
Rh(Cl)(NC5H5) (1), as determined by 31P and H NMR spectros-
copy, and reaction of the pyridine adduct with p-tolyllithium gave
2 in 65% isolated yield. The orange microcrystalline complex was
1
characterized by H, 13C, and 31P NMR spectroscopy, elemental
analysis, and single-crystal X-ray diffraction (see Supporting
Information for data).
Reaction of tolyl complex 2 with the N-aryl aldimine (p-tol)-
CHdN(C6H4-p-CO2Me) (3a-Tol) for less than 5 min led to
9
2694
J. AM. CHEM. SOC. 2004, 126, 2694-2695
10.1021/ja038819a CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
To determine if the high lability of the pyridine, which would
allow coordination of imine in the square plane, contributes to the
enhanced reactivity of arylrhodium 2, kinetic experiments on the
insertion reaction were conducted. Reactions of 0.026 M 2 with
0.13 M imine (Ph)CHdN(C6H4-p-CO2Me) (3a-Ph) were conducted
with concentrations of added pyridine ranging from 0.089 to 0.46
M. These kinetic data showed a clear first-order decay in rhodium
complex 2, first-order dependence on imine, and a clean inverse
first-order dependence on the concentration of added pyridine (see
Supporting Information). Although these data do not reveal the
coordination mode of the imine prior to insertion, they do imply
that coordination of imine to the site occupied by pyridine in 2
precedes the insertion process.
Tol was possible because of the greater stability of the amido group
containing an electron-poor N-aryl substituent.
The electron-neutral and electron-rich aldimines (Ph)CHdNPh
(3b) and (p-tol)CHdN(C6H4-p-OMe) (3c) also appeared to insert
into the aryl-rhodium bond of 2. However, the amido complexes
formed from these insertions were less stable than those formed
by insertion of the electron-poor imines 3a-Tol and 3a-Ph, and
this instability led to different final products. Addition of 3b
(5 equiv) to a C6D6 solution of arylrhodium complex 2 generated
diarylmethylamine (Ph)(p-tol)CH-NHPh, 5 (25%), and toluene
(15%), and subsequent addition of Et3NHCl (2-4 equiv) to the
reaction solution generated the E and Z isomers of ketimine (Ph)-
C-H bond cleavage to generate an iminoacyl complex and
reductive elimination of the p-tolyl and iminoacyl groups could
also account for the formation of ketimine. However, this mech-
anism is inconsistent with isolation of diarylmethylamido complex
4. It is also inconsistent with the formation of 5 as the only amine
product when the reaction of arylrhodium 2 with aldimine 3b was
conducted in the presence of added ketimine 7.
The regiochemistry of the insertion in the current study is distinct
from that of previous 2,1 insertions of imines into metal carbon
bonds of metal-acyl complexes.3,4 The electrophilicity of the acyl
group makes possible direct nucleophilic attack by the imine
nitrogen at the acyl carbon,7 while insertion into the rhodium aryl
bond is likely to occur by a path more akin to the migratory insertion
of olefins into late metal carbon bonds. Thus, the two types of
insertions are likely to occur by distinct mechanisms.
1
(p-tol)CdNPh, 6, in 50% total yield, as determined by H NMR
spectroscopy (eq 3).
The insertion of imine into a rhodium aryl linkage can be an
important step in catalytic transformations of imines. Several groups
have recently reported the rhodium-catalyzed reactions of imines8
with organotin, boron, and zirconium9 reagents to form diarylm-
ethylamines. These reactions could occur by well-precedented
nucleophilic attack on a coordinated imine10-14 or the previously
unknown insertion of imine into a rhodium aryl complex. Our
results suggest that the insertion of imine is a viable step in this
catalytic process and may allow the development of a series of
new catalytic transformations of imines.
Our data indicate that these products are formed from insertion
of imine, â-hydrogen elimination of the resulting amidorhodium
complex, and cyclometalation of the coordinated imine to form a
rhodium complex that releases ketimine upon protonation with
Et3NHCl. The diarylmethylamine and toluene formed prior to
addition of Et3NHCl is most likely generated from protonolysis of
the amidorhodium and arylrhodium complexes with residual water
or by reaction with a rhodium hydride. Consistent with the
formation of amine from the amido complex and not from free
ketimine, reaction of arylrhodium 2 with aldimine 3b in the presence
of the ketimine (Ph)(p-tol)CHdN(C6H4-p-CO2Me) (7) generated
amine 5 from arylation of the N-phenyl aldimine and no amine
from added ketimine 7.
Acknowledgment. This work was supported by the Department
of Energy, Office of Basic Energy Sciences.
Supporting Information Available: Experimental procedures and
full structural characterization of 2, 4, and 8 (PDF and CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Evidence for the formation of ketimines from arylrhodium
complex 2 and aldimines 3b and 3c was obtained from several
experiments. First, reaction of 2 with p-methoxy-substituted ald-
imine 3c for 3 h consumed 2 and formed a major rhodium product
8 (eq 4) containing a cyclometalated ketimine and a minor complex
in a 3.5:1 ratio. Major product 8 was identified by X-ray diffraction.
Further, addition of DCl in ether to the crude mixture generated
from arylrhodium 2 and aldimine 3c, followed by hydrolysis,
generated monodeuterio 4,4′-dimethylbenzophenone and unlabeled
p-anisidine, as determined by GC/MS analysis. We have not yet
definitively assigned the structure of the minor rhodium product.
Formation of ketimine from decomposition of a diarylmethyl-
amido complex was supported by the reactivity of amide 4. Heating
of 4 generated amine 9 and ketimine 10 in 50% and 45% yield
(eq 5), as was observed from reaction of 2 with aldimines 3b and
3c. Thus, the isolation of amido complex 4 from insertion of 3a-
References
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
(2) Fryzuk, M. D.; Piers, W. E. Organometallics 1990, 9, 986.
(3) Dghaym, R. D.; Dhawan, R.; Arndtsen, B. A. Angew. Chem., Int. Ed.
2001, 40, 3228 and references therein.
(4) Kacker, S.; Kim, J. S.; Sen, A. Angew. Chem., Int. Ed. 1998, 37, 1251.
(5) Krug, C.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 1674.
(6) Fairlie, D. P.; Bosnich, B. Organometallics 1988, 7, 936.
(7) Lin, Y.-S.; Yamamoto, A. Organometallics 1998, 17, 3466.
(8) Fagnou, K.; Lautens, M. Chem. ReV. 2003, 103, 169 and references therein.
(9) Kakuuchi, A.; Taguchi, T.; Hanzawa, Y. Tetrahedron Lett. 2003, 44, 923.
(10) Stark, G. A.; Arif, A. M.; Gladysz, J. A. Organometallics 1994, 13, 4523.
(11) Stark, G. A.; Gladysz, J. A. Inorg. Chim. Acta 1998, 269, 167.
(12) Harrowfield, J. M.; Sargeson, A. M. J. Am. Chem. Soc. 1979, 101, 1514.
(13) Martin, G. C.; Boncella, J. M. Organometallics 1989, 8, 2968.
(14) Martin, G. C.; Boncella, J. M.; Wucherer, E. J. Organometallics 1991,
10, 2804.
JA038819A
9
J. AM. CHEM. SOC. VOL. 126, NO. 9, 2004 2695