Palladium(0)-catalyzed intermolecular amination of unactivated Csp 3-H bonds

Article abstract of DOI:10.1002/anie.201102880

Filling out the space: The title reaction of unactivated Csp³-H bonds using aryl amines as the nitrogen source is disclosed (see scheme; dba=dibenzylideneacetone, Tf=trifluoromethanesulfonyl). Either the C-N cross-coupling product or the C-H am

Full text of DOI:10.1002/anie.201102880

DOI: 10.1002/anie.201102880
À
C H Amination
Palladium(0)-Catalyzed Intermolecular Amination of Unactivated
À
Csp3 H Bonds**
Jun Pan, Mingjuan Su, and Stephen L. Buchwald*
Nitrogen-containing compounds are ubiquitous among bio-
logically active molecules.[1] Consequently, the development
of efficient methods to form carbon–nitrogen bonds is of great
importance. From a synthetic standpoint, a strategy involving
À
transition-metal-catalyzed C H bond activation followed by
À
À
C N bond formation represents an extremely attractive
Our study commenced by examining the C H amination
approach for installing nitrogen functional groups.[2] In fact,
great achievements have been made based on amination of
Csp2 H bonds, as well as activated Csp3 H bonds.
of 1a to afford the corresponding N-(2-methyl-2-phenyl-
propyl)aniline, 3a, using palladium catalysts having different
ligands. While the biarylphosphane ligands developed in our
laboratory led to catalysts that exhibited modest activities
(Table 1, entries 1–7),[8] an examination of alternative ligand
classes revealed that the utilization of an N-heterocyclic
[3]
[3h,4]
À
À
À
However, the activation of a simple Csp3 H bond followed
À
by C N bond formation remains a challenge, especially in an
intermolecular fashion.[5] To the best of our knowledge, the
À
À
intermolecular C H amination of unactivated Csp3 H bonds
has only been reported using in situ generated, highly reactive
nitrene intermediates.[6] Thus, the development of comple-
mentary methods is strongly desired. Herein, we report on the
carbene ligand (SIPr·HBF4) provided
improved reaction efficiency to afford 3a in 80% yield
a
significantly
Table 1: Ligand evaluation.[a,b]
À
palladium(0)-catalyzed intermolecular C H amination of
À
unactivated Csp3 H bonds using aryl amines as the nitrogen
source.
During our investigation of Suzuki–Miyaura cross-cou-
pling processes,[7] we disclosed that the reaction of 1-bromo-
2,4,6-tri-tert-butylbenzene (1a) with phenylboronic acid pro-
duced the a,a-dimethyl-b-phenyl hydrostyrene, 2, in 95%
yield, instead of the desired biaryl [Eq. (1); dba = dibenzyli-
deneacetone]. This transformation likely proceeds by a
Entry Ligand
Yield [%][e] Entry Ligand
Yield [%][e]
0
1
2
3
4
5
6
7
XPhos
SPhos
RuPhos
DavePhos
CPhos
30
23
32
7
8[c]
PCy3·HBF4
PtBu3·HBF4 59
IMes·HCl
IPr·HCl
9[c]
À
pathway involving a tandem C H activation/Suzuki–Miyaura
10[c]
11[c]
12[c]
13[c]
14[d]
0
30[f]
cross-coupling reaction. On the basis of these results, we
postulated that a related transformation involving an inter-
23
0
0
SIPr·HCl
SIPr·HBF4
SIPr·HBF4
72
86 (80)
88 (83)
À
À
molecular tandem Csp3 H activation/C N coupling might be
feasible [Eq. (2)].
BrettPhos
Cy-JohnPhos
[a] Reaction conditions: 1a (0.5 mmol), PhNH2 (0.6 mmol), NaOtBu
(0.75 mmol), [Pd2(dba)3] (5 mol%), ligand (20 mol%), dioxane (5 mL),
1208C, 40 h. [b] The reaction reached 100% conversion, unless other-
wise noted. The mass balance consists of product, reduced starting
material, and benzocyclobutene by-product. [c] Reaction was run at
1108C for 12 h. [d] Reaction was performed in toluene with 11 mol%
ligand at 1108C for 4 h. [e] Determined by GC, with dodecane as an
internal standard. Yield of isolated 3a (1 mmol scale reaction) in
parentheses. [f] The reaction reached 58% conversion.
[*] Dr. J. Pan, M. Su, Prof. Dr. S. L. Buchwald
Department of Chemistry, Room 18-490
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
E-mail: sbuchwal@mit.edu
[**] Generous financial support from the National Institutes of Health
(GM-46059) is gratefully acknowledged. The Bruker 400 MHz
instrument used in this work was purchased with funding from the
National Institutes of Health (GM 1S10RR13886-01). We are
grateful to Georgiy Teverovskiy (MIT) for initial studies on DFT
calculations and to Sophie Rousseaux (MIT) for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102880.
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Communications
[a]
(Table 1, entry 13).[9] Further optimization of the solvent
À
Table 3: Amination of unactivated Csp3 H bonds with aniline.
system led to an 83% yield of the isolated 3a (Table 1,
entry 14).
Entry
Substrate
Product
Yield
[%][b]
With optimized reaction conditions in hand, we then
À
evaluated the scope of the C H amination of 1a with respect
1
4a
5a
6a
4b 94
5b 75
6b 77
to the aryl amine component (Table 2). Both electron-rich
and electron-deficient anilines gave the expected products in
good to excellent yield (3a–3 f), as well as anilines containing
an ortho-alkyl substituent (3e). We were pleased to find that
heteroaryl amines such as 3-aminopyridine and 3-amino-
quinoline also provided the corresponding products in good
yields (3g, 3h). Unfortunately, N-substituted anilines and
alkyl amines do not work under the current reaction
conditions. Notably, for reactions of 1a with aryl amines, no
diaryl amines were observed despite the fact that SIPr·HBF4
2
3
À
is an efficient ligand for palladium-catalyzed C N cross-
coupling reactions.[10] We reasoned that this was likely due to
the steric effects of the two ortho tert-butyl groups of 1a.
We next examined the reactivity of less sterically hindered
substrates (Table 3). The reaction of 4a with aniline produced
the diaryl amine 4b as the sole product (Table 3, entry 1). It is
likely that the ortho-methyl group does not possess the steric
4
5
7a
8a
7b 81
8b 82
À
bulk necessary to suppress the direct C N cross-coupling.
Replacing the methyl group with a bulkier isopropyl, cyclo-
pentyl, or cyclohexyl group led to complete suppression of the
9b 40
9c 41
10b 70
À
À
C N cross-coupling pathway, thus affording the desired C H
amination products exclusively in 75-81% yields (Table 3,
À
entries 2–4). No C H amination of the isopropyl, cyclopentyl,
6
7
9a
[a]
À
Table 2: C H amination of 1a with aryl amines.
10a
Product
Yield Product
[%][b]
Yield
[%][b]
8
11a
12a
11b 80
12b 70
83
84
76
76
9[c]
13b 37
13c 35
10[c]
13a
86
73
75
84
[a] Reaction conditions: substrate (1.0 mmol), PhNH2 (1.2 mmol),
NaOtBu (1.5 mmol), [Pd2(dba)3] (5 mol%), SIPr·HBF4 (11 mol%),
toluene (10 mL), 1108C, 4 h. [b] Yield of isolated product is based on an
average of two runs. [c] LiOtBu (2.5 mmol) was used. Tf=trifluoro-
methanesulfonyl, TIPS=triisopropylsilyl, TMS=trimethylsilyl.
[a] Reaction conditions: 1a (1.0 mmol), ArNH2 (1.2 mmol), NaOtBu
(1.5 mmol), [Pd2(dba)3] (5 mol%), SIPr·HBF4 (11 mol%), toluene
(10 mL), 1108C, 4 h. [b] Yield of isolated product is based on an average
of two runs.
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or cyclohexyl group was observed, thereby indicating that the
amination is highly selective for only the methyl groups of the
tert-butyl group. The steric influence on the outcome of this
reaction could be further illustrated when using the diol-
protected benzaldehyde substrates 8a, 9a, and 10a. In the
reaction of the ethylene-glycol-protected substrate 8a with
À
aniline, only the direct C N cross-coupling product 8b was
observed (Table 3, entry 5). However, using a more sterically
hindered pinacol protecting group led to the formation of a
À
À
1:1 ratio of the C N cross-coupling product 9b and the C H
amination product 9c (Table 3, entry 6). An additional
increase in size of the diol protecting group resulted in
À
exclusive formation of the C H amination product 10b
(Table 3, entry 7). Thus, a simple switch of the diol from
ethylene glycol to 2,4-dimethyl-2,4-pentanediol allows access
À
À
to both the C N cross-coupling product and the C H
amination product selectively. In addition, substrate 11a
À
bearing an ortho-OTIPS group underwent the C H amina-
tion smoothly giving the desired product 11b in 80% yield
(Table 3, entry 8). Notably, the reaction was not restricted to
aryl bromide substrates. Starting from aryl triflate 12a, the
À
corresponding C H amination product 12b was also pro-
duced in good yield when LiOtBu was employed as base
À
instead of NaOtBu (Table 3, entry 9). C H amination of the
TMS group was not observed. Employing 13a under the
optimized reaction conditions provided the desired product
13b along with the olefin product 13c (Table 3, entry 10). The
À
by-product 13c possibly arose from the C H activation of the
À
À
Scheme 2. Proposed mechanism of the tandem C H activation/C N
cross-coupling.
ethyl group followed by b-hydride elimination.[11] Interest-
ingly, the tert-amyl group in the para position plays a crucial
role in producing the desired product, as 14a failed to yield
À
any C H amination product under the same reaction
cross-coupling (side reaction A), as well as the benzocyclo-
butene formation (side reaction B).[12] Therefore, it dimin-
ishes the formation of the undesired by-products 21 and 22. In
addition, as suggested by the results of the reaction of 14a
with aniline, a bulky R2 group seems critical for minimizing
the formation of the by-product 24 that most likely arises
conditions. Instead, a mixture of olefin 14b and benzocyclo-
butene 14c[12] was obtained in a ratio of 1:1.4 and in an 81%
combined yield (Scheme 1). It is worth noting that the
reactive benzylic and ethereal hydrogen atoms are tolerated
À
from the intramolecular Csp2 H activation of 18 followed
by reductive elimination (side reaction C).[12]
To gain additional insight into the steric influence of the
À
substrates 8a, 9a, and 10a on direct C N cross-coupling
À
versus C H amination, we performed a computational
study at the density functional theory (DFT) level with the
hybrid functionals B3LYP.[13] The oxidative addition inter-
mediates of 8a, 9a, and 10a were evaluated (Table 4). The
Scheme 1. Reaction of 14a with aniline.
in the reaction (Table 3, entries 1–7). Therefore, it
provides an orthogonal approach to the existing
Table 4: DFT calculations of the oxidative addition intermediates.
nitrene methods.[2]
Based on the results described above, we propose
a reaction mechanism as shown in Scheme 2. The
oxidative addition of Pd0 to aryl bromide 15 gives
À
intermediate 16, which would undergo C H activa-
À
tion of one of the Csp3 H bonds to form the pallada-
À
cycle 17. Protonation of the Csp2 Pd bond of 17
affords the alkyl PdII species 18, which then under-
goes transmetalation with aniline to give 19. Finally,
reductive elimination occurs to yield the product 20
OA1a
OA2a
OA3a
2.962 ꢀ
2.480 ꢀ
2.277 ꢀ
with concomitant regeneration of LPd0. A sterically
Pd-C1-C2
134.78
119.48
116.98
1
À
hindered R group helps to suppress the direct C N
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www.angewandte.org 8649

Communications
intermediates (OA1 a, OA2 a, and OA3 a) with the carbene
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ligand trans to the aromatic ring are found to be more stable.
II
À
The calculated distances between the Pd atom and the C H
s bond of the tert-butyl group and the bond angles, Pd-C1-C2,
are listed in Table 4. Notably, the distance decreases as the
size of diol protecting group increases; that is, the Pd is being
“pushed” toward the tert-butyl group as indicated by the
decrease in the bond angle. In addition, the calculated
distances are consistent with a three-center two-electron,
À
[2] For recent reviews on C H amination, see: a) A. Armstrong,
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agostic interaction between the PdII atom and the C H
À
s bond in OA2a and OA3a.[12,14] As recently demonstra-
ted,[14c,15] an agostic interaction increases the acidity of the C
À
À
H bond that is geminal to the agostic C H bond. This increase
in acidity is supported by the computed natural atomic
charges. For OA3a, the agostic hydrogen atom has a less
positive charge (+ 0.203) than either of the geminal hydrogen
atoms (+ 0.227 and + 0.225). Similar results were found for
OA2a (agostic H: + 0.150; geminal H: + 0.209, 0.211). The
shorter distance in OA3a suggests that the agostic interaction
is likely stronger than that in OA2a. This stronger agostic
interaction in OA3 confers a more acidic character on the
geminal hydrogen atom to be deprotonated. Consequently,
À
[3] For selected examples of C N bond formation following
À
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À
the tendency for the subsequent C H activation rises from
OA1a to OA3a (OA1a < OA2a < OA3a), which is indeed
consistent with our experimental observations.
In summary, we have developed a conceptually novel
À
palladium(0)-catalyzed intermolecular C H amination of
À
unactivated Csp3 H bonds using aryl amines as the nitrogen
source. We have also demonstrated selective access to both
À
À
the C N cross-coupling product and the C H amination
product by adjusting the steric environment of the substrate.
To the best of our knowledge, this reaction is the first
À
À
intermolecular unactivated Csp3
H bond activation/C N
bond-forming process that does not involve nitrenes. Addi-
tional investigations to increase the generality of this process
and to better understand its mechanism are currently under-
way in our laboratory.
Experimental Section
À
À
Typical procedure: In a nitrogen-filled glovebox, aryl bromide
(1.0 mmol, 1.0 equiv), [Pd2(dba)3] (46 mg, 5 mol%), SIPr·HBF4
(53 mg, 11 mol%), NaOtBu (144 mg, 1.5 mmol, 1.5 equiv), aryl
amine (1.2 mmol, 1.2 equiv), and toluene (10 mL) were added to an
oven-dried test tube containing a magnetic stir bar. The test tube was
sealed with a Teflon-lined septum, removed from the glovebox, and
heated at 1108C in a preheated oil bath for 4 h. After the reaction was
complete, the reaction mixture was cooled to room temperature,
filtered through a plug of silica gel, and eluted with diethyl ether. The
filtrate was concentrated in vacuo and the crude product was purified
by flash chromatography on silica gel (see the Supporting Information
for details).
[4] For selected examples of C H amination of activated Csp3 H
À
À
bonds which include allylic, benzylic C H bonds, and C H
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Received: April 26, 2011
Revised: May 30, 2011
Published online: August 2, 2011
À
Keywords: amines · C H amination · homogeneous catalysis ·
palladium
.
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À
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À
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[13] See the Supporting Information for details.
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