The reaction of [Cp*RhCl2]2, aniline, and a terminal alkyne: Formation of cyclometalated rhodium(III) complexes

Article abstract of DOI:10.1021/om300387q

The reaction of [Cp*RhCl₂]₂ with an aniline (R′NH₂) and a terminal alkyne (RCCH) afforded the N-containing cyclometalated rhodium complexes Cp*Rh(Cl)[N(R′)=C(CH ₂R)CH=CR] via a hydroamination and a 1,2-insertion of an alkyne. A reaction pathway has been proposed on the basis of deuterium labeling experiments and computational studies.

Full text of DOI:10.1021/om300387q

Article
pubs.acs.org/Organometallics
The Reaction of [Cp*RhCl2]2, Aniline, and a Terminal Alkyne:
Formation of Cyclometalated Rhodium(III) Complexes
Elumalai Kumaran and Weng Kee Leong*
Division of Chemistry and Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: The reaction of [Cp*RhCl2]2 with an aniline
(R′NH2) and a terminal alkyne (RCCH) afforded the N-containing
cyclometalated rhodium complexes Cp*Rh(Cl)[N(R′)C(CH2R)-
CHCR] via a hydroamination and a 1,2-insertion of an alkyne. A
reaction pathway has been proposed on the basis of deuterium
labeling experiments and computational studies.
Scheme 1
INTRODUCTION
■
N-containing cyclometalated compounds have been the focus
of much attention in the last few decades,1 due to their
interesting reactivity in C−C bond-forming reactions.2 More
specifically, cyclometalated compounds that incorporate an
internal imine functionality are proposed as intermediates in a
number of organic transformations.3 Cyclometalated rhodium
complexes that include a nitrogen donor atom are known to
possess excellent photoreduction properties,4 and reactivity
toward C−H bond functionalization reactions,5 and have also
been screened recently as catalysts for the transfer hydro-
genation of ketones and imines.6 The syntheses of such
complexes have included transmetalation,7 condensation of
keto metallacyclic complexes with amines,8 and ortho C−H
activation of the corresponding aromatic imine,9 or amine.10 In
contrast, such N-containing cyclometalated complexes with
Cp*Rh (Cp* = pentamethylcyclopentadienyl) have, with
possibly one exception,11 involved acetate-assisted orthometa-
lation of an imine, pyridine, or related species.5a,b,12
In an earlier study, we have found that [Cp*RhCl2]2, 1,
catalyzes the hydroamination of terminal alkynes with anilines
to afford ketimines.13 Since 1 can effectively cyclometalate
aromatic imines in the presence of acetate, we thought that the
two separate steps may be combined to yield N-containing
cyclometalated rhodium complexes in a single step. To the best
of our knowledge, there is only one report of a similar strategy,
which involves the single-step synthesis of alkenyl ketone
rhodium complexes using water and phenylacetylenes.14 We
wish to report here our attempt at this.
overall reaction requires an alkyne hydroamination as well as a
1,2-insertion of an alkyne. Typically, 10 equiv of the alkyne is
required; the use of 4 equiv gave incomplete reaction and the
aniline adduct [Cp*RhCl2NH2R′] (4).13 A change of the
solvent (to THF or DCE) did not give any improvement in
yield, and the reaction failed to proceed at ambient temper-
ature. Both electron-donating and electron-withdrawing sub-
stituents on the alkyne and the aniline are tolerated, and the
reaction proceeded smoothly with aliphatic amines as well.
Attempts at extending the substrate scope to aliphatic alkynes
(1-hexyne and 1-octyne) and internal alkynes (diphenylacety-
lene and 1-phenyl-1-propyne) were unsuccessful; only the
corresponding analogues of 4 were obtained.
The products 3 have all been characterized spectroscopically
and analytically, and the structures of 3c, 3k, and 3l have been
confirmed by single-crystal X-ray diffraction studies; the quality
of the structural determination for 3k is significantly poorer
than that of the other two. An ORTEP plot for 3c is shown in
Figure 1, and a common atomic numbering scheme with
selected bond parameters for 3c, 3k, and 3l are collected in
Table 1.
Similar cyclometalated rhodium complexes that have been
structurally characterized include [Cp*RhCl{C6H4-2-C(H)
N(CH2)2OCH3-κC,N}] (5a) and [Cp*RhCl{C6H4-2-C(H)
NPh-κC,N}] (5b).12f The Rh−N bond lengths (∼2.093 Å for 3
vs 2.089(2) and 2.115(3) Å for 5a and 5b, respectively) and the
chelate bite angle (∼78° vs 78.73(7)° and 78.33(12)° for 5a
and 5b, respectively) are similar to these related Cp*Rh
RESULTS AND DISCUSSION
■
As mentioned above, we thought that a one-pot reaction of 1,
aniline, and phenyl acetylene (PhCCH) may possibly lead to an
N-containing cyclometalated rhodium complex via the
formation and orthometalation of a ketimine (Scheme 1).
We have found, however, that the reaction in the presence of
additives (KPF6 and CH3COONa) failed to give 2. Instead,
cyclometalated rhodium complexes 3 were obtained, without
any additive, in 60−70% isolated yields (Scheme 2). The
Received: May 9, 2012
Published: June 28, 2012
© 2012 American Chemical Society
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Scheme 2
Table 1. Common Atomic Numbering Scheme and Selected
Bond Lengths (Å) and Angles (deg) for 3c, 3k, and 3l
bond parameter
3c
3k
3l
Rh−Cl
Rh−N
Rh−C3
N−C1
N−C(Ar′)
C1−C2
C2−C3
N−Rh−C3
C1−N−C(Ar′)
C1−C2−C3
2.4036(10)
2.093(4)
2.012(4)
1.303(5)
1.430(5)
1.433(6)
1.357(6)
78.14(15)
121.4(5)
117.1(4)
2.399(2)
2.096(7)
2.036(8)
1.301(10)
1.432(11)
1.452(12)
1.347(12)
78.7(3)
2.3910(8)
2.093(3)
2.021(3)
1.301(4)
1.434(3)
1.439(5)
1.354(5)
78.48(12)
122.5(3)
116.3(3)
122.6(7)
117.0(7)
Figure 1. ORTEP plot of 3c. All H atoms have been omitted, and only
one orientation of the disordered benzyl group is shown. Thermal
ellipsoids are plotted at the 50% probability level.
which tends to localize the π-electron density within the
aromatic ring.
cyclometalated complexes. Although the Rh−C(3) bond
lengths in 3 are not significantly shorter than those in 5a and
5b (2.027(2) and 2.032(3) Å, respectively), the C(1)−N and
C(2)−C(3) bond lengths appear to have significant double
bond character. In fact, the C(2)−C(3) bond (∼1.35 Å) is
significantly shorter than the corresponding bond in 5a and 5b
(1.407(3) and 1.399(5) Å, respectively) even though this is part
of an aromatic ring in the latter compounds. Consistent with
this are the C1−N−C(Ar′) and C1−C2−C3 bond angles,
which are indicative of sp2 hybridization at the N and C(2)
atoms. Although an NBO analysis on 3a and 5b did not show
any difference in the bond order (Wiberg bond index), the
13C{1H} NMR spectra of the complexes 3 exhibited a doublet
resonance at ∼216 ppm for C(3), typical of a carbene carbon,
in contrast to the ∼184 ppm reported for 5. Thus, there is
considerable carbene character in the Rh−C bond of 3, and
delocalization of the π electrons within the five-membered
metallacycle. Presumably, the difference between 3 and 5 lies in
the fused aromatic ring system to the metallacycle in the latter,
MECHANISTIC CONSIDERATIONS
■
The reactions afforded 4 in the absence of a large excess of the
alkynes, and 4 was also obtained in good yields from the
reaction of aniline with 1. With excess alkynes, 4 formed 3 in
similar yields to the one-pot reaction. Isotopic labeling
experiments employing (a) aniline and PhCCH in the presence
of D2O, (b) d7-aniline and PhCCH, and (c) aniline and
PhCCD yielded 3a with both, one and none, respectively, of
the diastereotopic CH2 protons being deuterated (Scheme 3).
These results are consistent with the source for each of these
protons being water and aniline. In case (c), the alkenic CH of
the metallacycle was also deuterated. A proposed reaction
pathway that accounts for these observations is shown in
Scheme 4; the energetics for the various steps (for aniline and
phenylacetylene) have also been computed with density
functional theory, and the computed energies (ΔGo, in kJ
mol−1) from B onward are also given.
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from B,15 but we have found that these structures that formally
contain a Rh(V) center could not be optimized (see also
below). The sequence depicted starts with a 1,2-insertion and
metallacycle formation from C to E, with a ΔGo of −103 kJ
mol−1; the presumed intermediate with the alkyne coordinated
(D) lies +46 kJ mol−1 above C. The binding of the alkyne in D
shows asymmetry, as has been observed earlier for a closely
related system.13 The deuterium labeling experiment (c) above
shows that the C−H bond is not cleaved in the insertion
step. The final step is an enamine-to-imine tautomerization,
which has been shown to involve an intramolecular 1,3-H
shift,16 and is consistent with the labeling experiment (b). The
free energy for this step is quite exergonic, larger than what may
be expected from the enamine−imine tautomerization. It,
therefore, suggests that a major contribution arises from
delocalization of the electron density within the metallacycle.
Although no kinetic barriers were computed, the negative or
small computed energies involved in the steps suggest that the
proposed reaction pathway is reasonable.
Scheme 3
One further aspect of this reaction that intrigued us is that,
although the alkyne insertion step is overall exergonic from C
to E, it does not appear clear why C (or its tautomer C′) could
not have undergone cyclometalation to an aminocarbene, as we
have observed in a similar reaction with the iridium analogue of
1, viz., [Cp*IrCl2]2, 1a.17 Indeed, such a reaction from C′ to 6 is
computed to have a ΔGo of −21 and −70 kJ mol−1 for rhodium
and iridium, respectively, with the same model chemistry
(Scheme 5). The difference in the reaction pathway taken is
thus not clear from the energetics. We believe that the
difference lies in the inability of C (or C′) to undergo oxidative
addition when M = Rh. We assume that this step proceeds
through E′, which would require the formation of an
unfavorable Rh(V) species; our attempts at optimization of a
structure E′ for M = Rh invariably failed. This may also be the
reason why the reaction to 2 failed. In contrast, the computed
ΔGo from C′ to E′, and from E′ to 6, for M = Ir are +56 and
−126 kJ mol−1, respectively.
Scheme 4
CONCLUDING REMARKS
■
In this study, we have reported that N-containing cyclo-
metalated rhodium complexes 3 can be obtained efficiently in a
one-pot reaction involving the dirhodium complex 1, an aniline,
and a terminal alkyne. The reaction pathway has been studied
experimentally via labeling experiments, as well as computa-
tionally, and we believe that the difference between 1 and its
iridium analogue 1a lies in the reduced tendency for rhodium
to orthometalate via a Rh(V) species.
Presumably, in the presence of an excess of the appropriate
alkyne, the coordinated aniline in the initial reaction product 4
is replaced. The alkyne species A rapidly rearranges to the
vinylidene complex B via an intermolecular 1,3-H shift. The
enamine intermediate C is formed by nucleophilic attack of
aniline at the vinylidene α-carbon atom and HCl elimination;
this is consistent with our earlier proposed pathway for the
alkyne hydroamination reaction.13 In the intermediate C, the N
atom of the enamine is also bonded to the metal center; this
lies ∼2 kJ mol−1 below the alternative in which the N atom is
not bound.
The steps from C through to the final product 3a involves a
1,2-insertion of an alkyne, an enamine−imine tautomerization,
and binding of the N atom to Rh, forming the metallacycle.
There are several alternatives for this sequence of events,
including pathways involving a four-membered metallacycle
EXPERIMENTAL SECTION
■
General. All reactions and manipulations, except for TLC
separations, were performed under argon by using standard Schlenk
techniques. The starting material 1 was prepared according to the
published method.18 All other chemicals were from commercial
sources and used as supplied without further purification. 1H and
13C{1H} NMR spectra were recorded in CDCl3 on a JEOL ECA400 or
ECA400SL spectrometer and were referenced to residual solvent
resonances. FAB mass and high-resolution mass spectra (HRMS) were
recorded in ESI mode on a Thermo Finnigan Mat95XP and Waters
UPLC-Q-TOF mass spectrometers, respectively. Elemental analyses
were performed by the microanalytical laboratory in NTU.
Reaction of 1 with Alkyne and Aniline. In a typical reaction, to
a solution of 1 (40 mg, 64 μmol) and phenylacetylene (130 μL, 20-
fold excess) in 1,2-dichloroethane (4 mL) was added aniline (12 μL,
100 μmol). The reaction mixture was then stirred at reflux overnight.
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Scheme 5
The solvent was then removed under reduced pressure, and the
residue obtained was dissolved in the minimum amount of
dichloromethane for chromatographic separation on silica gel TLC
plates. Elution with hexane/ethylacetate (4:1, v/v) yielded 3a as a
yellow solid.
labeling experiments. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Similar procedures were used with the other alkynes and anilines,
and these are given in the Supporting Information.
*E-mail: chmlwk@ntu.edu.sg.
Crystallographic Studies. Diffraction quality crystals were grown
either by slow diffusion of hexane into a dichloromethane solution (3c
and 3l) or by slow cooling from MeOH (3k) and then mounted onto
quartz fibers. X-ray data were collected on a Bruker X8 APEX system,
using Mo Kα radiation, with the SMART suite of programs.19 Data
were processed and corrected for Lorentz and polarization effects with
SAINT,20 and for absorption effects with SADABS.21 Structural
solution and refinement were carried out with the SHELXTL suite of
programs.22
The structures were solved by direct methods to locate the heavy
atoms, followed by difference maps for the light, non-hydrogen atoms.
Hydrogen atoms were placed in calculated positions and refined with a
riding model. The crystal of 3c exhibited disorder of the benzyl group,
which was modeled with two alternative sites, with occupancies
summed to unity. A disordered methanol solvate was found in the
crystal of 3k. This was modeled with two alternative sites, with their
occupancies summed to unity. Appropriate restraints on the bond and
thermal parameters were placed on the disordered parts. All non-
hydrogen atoms were given anisotropic displacement parameters in
the final model.
Computational Studies. The reaction energetics were studied
using DFT theory utilizing the Becke’s three parameter hybrid
function23 and Lee−Yang−Parr’s gradient-corrected correlation
function24 (B3LYP). The LanL2DZ (Los Alamos Effective Core
Potential Double-ζ) basis set, together with polarization functions, is
employed for the Rh atom, and the 6-311+G(2d,p) basis set for all the
other atoms. Spin-restricted calculations were used for geometry
optimization, and harmonic frequencies were then calculated to
characterize the stationary points as equilibrium structures with all real
frequencies, and to evaluate zero-point energy (ZPE) corrections.
Bond-order analysis was carried out with the NBO program
implemented within Gaussian 03W.25 All calculations were performed
using the Gaussian 03 suite of program.26
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Nanyang Technological University
and the Ministry of Education (Research Grant No.
T208B1111). Assistance on the crystallographic studies by
Drs. Yongxin Li and Rakesh Ganguly is acknowledged, and E.K.
thanks the university for a Research Scholarship.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF format, experimental details and
characterization for the complexes, and details of deuterium
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Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
4853
dx.doi.org/10.1021/om300387q | Organometallics 2012, 31, 4849−4853