Simple Amine-Directed Meta-Selective C-H Arylation via Pd/Norbornene Catalysis

Article abstract of DOI:10.1021/jacs.5b02809

Herein we report a highly meta-selective C-H arylation using simple tertiary amines as the directing group. This method takes advantage of Pd/norbornene catalysis, offering a distinct strategy to control the site selectivity. The reaction was promoted by commercially available AsPh₃ as the ligand and a unique "acetate cocktail". Aryl iodides with an ortho electron-withdrawing group were employed as the coupling partner. A wide range of functional groups, including some heteroarenes, are tolerated under the reaction conditions. In addition, the amine directing group can be easily installed and transformed to other common versatile functional groups. We expect this C-H functionalization mode to have broad implications for developing other meta-selective transformations beyond this work.

Full text of DOI:10.1021/jacs.5b02809

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Communication
Simple Amine-Directed meta-Selective C-
H Arylation via Pd/Norbornene Catalysis
Zhe Dong, Jianchun Wang, and Guangbin Dong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b02809 • Publication Date (Web): 24 Apr 2015
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Simple AmineꢀDirected metaꢀSelective C−H Arylation via
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Pd/Norbornene Catalysis
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Zhe Dong, Jianchun Wang, and Guangbin Dong*
Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
Supporting Information Placeholder
ABSTRACT: Herein we report a highly metaꢀselective C−H
arylation using simple tertiary amines as the directing group. This
method takes advantages of Pd/norbornene catalysis offering a
distinct strategy to control the siteꢀselectivity. The reaction was
promoted by commercially available AsPh3 ligand and unique
“acetate cocktail”. Aryl iodides with an orthoꢀelectron withdrawꢀ
ing group were employed as the coupling partner. A wide range of
functional groups, including some heteroarenes, can be tolerated
under the reaction conditions. In addition, the amine directing
group can be easily installed and transformed to other common
versatile functional groups. We expect this C−H functionalization
mode should have broad implications for developing other metaꢀ
selective transformations beyond this work.
Scheme 1. metaꢀSelective Arene C−H functionalization via
Pd/NBE catalysis
Siteꢀselective arene functionalization has been continuously
impacting the fields of drug discovery, material science and
1
chemical industry. While numerous orthoꢀselective arene funcꢀ
tionalization methods have been extensively developed, the metaꢀ
selective functionalization of an electronically unbiased arene
2
remains a difficult task. Recently, a number of elegant approachꢀ
es have been developed, including stericꢀsensitive borylation and
3
4
silylation, clusterꢀtemplated metalation, diaryliodonium saltꢀ6
5
mediated arylation, use of a traceless directing group (DG),
orthoꢀmetalationꢀtriggered ArS , use of a Uꢀshape template, and
others. Despite these seminal efforts, a broadly applicable C−H
7
8
E
9
functionalization strategy, that is completely metaꢀselective, overꢀ
rides intrinsic steric and electronic preference of the arene subꢀ
strates and tolerates a broad range of functional groups (FGs),
remains highly sought after.
We foresaw the potential of employing the Pd/norbornene(NBE)
10
catalysis, namely the Catellani reaction, to control the siteꢀ
selectivity for arene C−H functionalization. The Catellani reaction
generally uses aryl halides as the substrates allowing ipso and
ortho diꢀfunctionalization of arenes, through a NBEꢀmediated
We postulated that the NBEꢀbridged fiveꢀmembered palladacyꢀ
cle, the key intermediate in the Catellani reaction, could also be
14
generated via a C−H metalationꢀinitiated pathway. A proposed
metaꢀarylation is depicted in Scheme 1c, guided by a DG, orthoꢀ
metalation should give a Pd(II) intermediate (step a), which
should be able to undergo an analogous Catellani reaction pathꢀ
way (steps b and c). The fiveꢀmembered palladacycle is expected
to react with an electrophile, e.g. an aryl halide, through either a
1
c,f,10
vicinal C−H metalation reaction (Scheme 1a).
Seminal work
by Catellani and Lautens has shown that various carbon FGs can
1
1
be installed at the orthoꢀposition of the arene, which has been
1
1j,k
employed to prepare metaꢀsubstituted arenes.
Our group reꢀ
cently demonstrated the first ortho C−N bond forming transforꢀ
mations with an electrophilic amination reagent, and further illusꢀ
trated a twoꢀstep metaꢀamination sequence via electrophilic haloꢀ
genation followed by reductive orthoꢀamination (Scheme 1b). In
this communication, we extend our efforts towards developing a
15
Pd(IV) intermediate or a transmetalation pathway, to generate a
metaꢀsubstituted complex (step d). The resulting Pd(II) intermediꢀ
ate would then undergo βꢀcarbon elimination followed by reꢀ
protonation at the orthoꢀposition to furnish the desired metaꢀ
product (steps e and f). However, the challenges of the proposed
pathway are threeꢀfold. First, both the orthoꢀmetalationꢀ
deprotonation (step a), its reverse step (step f, demetalationꢀ
12
direct approach for catalytic metaꢀfunctionalization of arenes
1
3
based on the unique features of the Pd/NBE catalysis.
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protonation) and the metalation at the meta-position (step c) must
cient Pd(0) scavenger to minimize Pd(0)ꢀmediated reactions. It
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be accommodated under the same reaction conditions. Second, the
electrophile can possibly react with either the orthoꢀpalladated
arene species or the intermediate after metaꢀmetalation. Thus,
finding an appropriate ligand to control the relative reactivity of
these arylꢀpalladium intermediates is nontrivial. Finally, for broad
applicability purposes, it would be more attractive to utilize a
common and versatile FG as the DG.
should be noted that in the absence of the “acetate cocktail”, the
desired metaꢀproducts can be still be formed albeit in lower yields
(entry 10). Interestingly, while in the absence of acetic acid the
reaction became more sluggish, its use as solvent completely shut
down the reaction (entry 11). Finally, shortening the reaction time
or using less NBE led to incomplete conversion and more monoꢀ
arylation product (entries 12 and 13). Further control experiments
showed that no ortho and paraꢀarylation products were observed
Stimulated by the aforementioned challenges, we sought the
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during the reaction with 1a. One major side reaction arises from
use of a simple tertiary amine, i.e. dimethyl amine, as the DG.
A
1
1j,21
the selfꢀdimerization of aryl ioide 2a with NBE.
number of benefits with this type of DG can be envisioned: 1) it
was demonstrated three decades ago by Ryabov that the dimethylꢀ
amine can direct a reversible orthoꢀmetalation under mildly acidic
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Table 1. Control experiments for the metaꢀarylation
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conditions; 2) amines are widely available and frequently found
in bioꢀactive and pharmaꢀinteresting molecules; 3) it is small and
light (the MW of the Me N group is only 44); 4) it can be easily
2
installed from a number of common starting materials, e.g. benꢀ
1
8
zaldehydes and halides, etc (Scheme 2); 5) it is also known that
the dimethylamino group at the benzylic position can be easily
16b,c,19
removed under hydrogenolysis conditions
or converted to
other FGs (vide infra, Scheme 3).
Scheme 2. Availability and versatility of the amine DG
a
Determined by GC using dodecane as the internal standard.
Benzylamine 1a was employed as the initial model substrate.
After extensively examining a range of Pd, ligand, additive and
solvent combinations (for detailed optimization studies, see SI), to
our delight, the desired metaꢀarylation can be obtained when usꢀ
ing Pd(OAc) /AsPh as the metal/ligand choice, aryl iodide 2a as
With the optimal conditions established, we first examined the
scope of benzylamine substrates (Table 2). When substituted benꢀ
zylamines were employed as the substrate, the amount of aryl
iodide and AgOAc can be reduced to 2 equiv and 2.5 equiv reꢀ
spectively. To our delight, both electronꢀrich and deficient arenes
provided the desired metaꢀarylated products in moderate to good
yields. In particular, the highly electronꢀrich 3ꢀmethoxy substrate,
with a strong electron bias on the ortho and paraꢀpositions, still
afford the metaꢀproduct (3e) in 80% yield. Substitutions on variꢀ
ous positions of the arenes can be tolerated. Moreover, this transꢀ
formation is compatible with a number of functional groups, inꢀ
cluding tertiary amines, aryl fluorides, chlorides, bromides, aniꢀ
soles, trifluoromethyl, methylenedioxy groups and esters. In addiꢀ
tion, the substrate with a methyl substituent at the benzylic posiꢀ
tion still provided the desired mono and diarylation products (3j
and 3j’). When enantiopure 1j was used, no racemization was
observed. Besides the dimethylamino group, other tertiary
2
3
the aryl source, and chlorobenzene as the solvent. Given that subꢀ
strate 1a has two metaꢀpositions, both mono and diarylation are
possible; nevertheless, by controlling the amount of the aryl halꢀ
ide used, the diarylation product (3a’) can be formed as the domiꢀ
nate product (72% yield, entry 1, Table 1).
A series of control experiments were subsequently conducted to
understand the role of each reactant (Table 1). In the absence of
either Pd precatalyst or NBE, no desired product was observed
(entries 2 and 4). The commercially available AsPh proved to be
3
the most efficient ligand for promoting the arylation (entry 3); in
contrast, use of more electronꢀrich ligands, such as PPh or SꢀPhos,
3
gave significantly lower yields. The silver salt was employed to
accelerate the oxidative addition of aryl iodide. In the absence of
AgOAc, the monoꢀarylation product (3a) can still be obtained in
22
amines can also be employed as the DG (e.g. 3k) albeit with a
lower efficiency. In contrast, use of Acꢀprotected benzylamines
did not lead to the desired products. Further investigation of this
transformation with other DGs is ongoing in our laboratory. It is
noteworthy that heteroarenes, such as protected pyrrole and pyriꢀ
dineꢀderived substrates, are also amenable for this transformation
6
% yield (entry 5). We further discovered that the reaction rate
can be improved by using an interesting “acetate cocktail” conꢀ
taining LiOAc·2H O, CsOAc and Cu(OAc) ·H O (1: 3: 0.5) in
2
2
2
acetic acid (entries 6ꢀ8). Although the exact reason remains to be
further explored, we speculate that HOAc and LiOAc would help
dechelation of the DG from the palladium (through protonation or
complexation) after the initial orthoꢀC−H activation, which is
required for the second metalation (vide supra, step b, Scheme 1c);
CsOAc might work as a stronger acetate source to assist the
deprotonation/metalation step; the Cu(II) salt may act as an effiꢀ
(
3l and 3m).
a
Table 2. Substrate scope with different benzylamines
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a
b
All yields are isolated yields. The reaction time was 36h; 4.0
c
equiv of 2a and 4.5 equiv of AgOAc were used. The reaction
was run at 130°C. 3.0 equiv of 2a and 3.5 equiv of AgOAc were
used.
a
b
c
d
All yields are isolated yields. The reaction time was 36h. The
reaction was run at 130°C with 4.0 equiv of the aryl iodide and
.5 equiv of AgOAc.
4
The scope of the aryl halides was investigated next (Table 3).
Finally, as an example to demonstrate the potential usage of
Aryl halides containing an ortho electronꢀwithdrawing group
this metaꢀarylation reaction, we showed the triaryl product 3a’
can be easily transformed to a benzyl chloride or an aldehyde by
using established procedures. Both moieties are well known as
versatile synthetic precursors, and can be readily transformed to
various other FGs.
(EWG) proved to be most efficient, which is consistent with the
previous observation in the typical Catellani arylation
reaction.
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5
11g,23
It is likely that the EWGs can accelerate oxidative
addition of the aryl iodide (step d, Scheme 1c) through a comꢀ
bined electronic and weakꢀdirecting effect. Aryl iodides with only
a para or metaꢀsubstituent proved to be much less reactive. For
example, when methyl 4ꢀiodobenzoate 2u was used, a low conꢀ
version was observed (<10%) but still giving the desired metaꢀ
arylation product (3u). Nevertheless, besides ester group, amide,
Weinreb amide, alkyl and aryl ketone and nitro groups were all
found suitable for this transformation.
Scheme 3. Derivatization of the metaꢀarylation product
2
4
a
Table 3. Substrate scope with different aryl iodides
In summary, we have developed a distinct highly metaꢀ
selective arylation using Pd/NBE catalysis. This transformation
uses a simple tertiary amine as the DG and commercially availaꢀ
ble AsPh as the ligand. In addition, arenes with various electronic
3
and steric properties can be used as the substrates, and a signifiꢀ
cant number of FGs, including some heteroarenes, can be toleratꢀ
ed. We expect this mode of reactivity should have high potential
to be generalized allowing other metaꢀfunctionalization, e.g.
forming C−X (X≠C) bonds at the metaꢀposition. Efforts towards
expanding the reaction scope and detailed mechanistic studies to
understand the C–C bond formation step (e.g. whether it reacts
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through a Pd(IV) intermediate or a transmetalation pathway) are
underway.
(9) (a) Zhang, Y.ꢀH.; Shi, B.ꢀF.; Yu, J.ꢀQ. J. Am. Chem. Soc. 2009, 131,
072; (b) Zhang, J.; Liu, Q.; Liu, X.; Zhang, S.; Jiang, P.; Wang, Y.;
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Luo, S.; Li, Y.; Wang, Q. Chem. Commun. 2015, 51, 1297.
10) Catellani, M. Top. Organomet. Chem. 2005, 14, 21.
(
(
ASSOCIATED CONTENT
11) (a) Catellani, M.; Frignani, F.; Rangoni, A. Angew. Chem., Int. Ed.
1997, 36, 119; (b) Motti, E.; Rossetti, M.; Bocelli, G.; Catellani, M. J.
Organomet. Chem. 2004, 689, 3741; (c) Mariampillai, B.; Alliot, J.;
Li, M.; Lautens, M. J. Am. Chem. Soc. 2007, 129, 15372; (d) Bressy,
C.; Alberico, D.; Lautens, M. J. Am. Chem. Soc. 2005, 127, 13148; (e)
Ferraccioli, R.; Carenzi, D.; Rombolà, O.; Catellani, M. Org. Lett.
2004, 6, 4759; (f) Deledda, S.; Motti, E.; Catellani, M. Can. J. Chem.
Supporting Information Experimental procedures; specꢀ
tral data. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
2
005, 83, 741; (g) Motti, E.; Della Ca, N.; Deledda, S.; Fava, E.;
Panciroli, F.; Catellani, M. Chem. Commun. 2010, 46, 4291; (h)
Maestri, G.; Della Ca, N.; Catellani, M. Chem. Commun. 2009, 4892;
(i) Faccini, F.; Motti, E.; Catellani, M. J. Am. Chem. Soc. 2004, 126,
78. (j) Deledda, S.; Motti, E.; Catellani, M. Can. J. Chem. 2005, 83,
741. (k) Wilhelm, T.; Lautens, M. Org. Lett. 2005, 7, 4053.
Corresponding Author
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gbdong@cm.utexas.edu
Notes
The authors declare no competing financial interests.
(12) Dong, Z.; Dong, G. J. Am. Chem. Soc. 2013, 135, 18350.
(
13) When this manuscript was in preparation, Yu group reported a related
metaꢀselective arene alkylation and arylation enabled by a highly
optimized pyridine ligand: Wang, X.ꢀC.; Gong, W.; Fang, L.ꢀZ.; Zhu,
R.ꢀY.; Li, S.; Engle, K. M.; Yu, J.ꢀQ. Nature 2015, 519, 334.
14) For an N−H activation initiated functionaliation of 2ꢀposition of
indoles and pyrroles via Pd/NBE catalysis, see: (a) Jiao, L.; Bach, T.
J. Am. Chem. Soc. 2011, 133, 12990; (b) Jiao, L.; Herdtweck, E.;
Bach, T. J. Am. Chem. Soc. 2012, 134, 14563; (c) Jiao, L.; Bach, T.
Angew. Chem. Int. Ed. 2013, 52, 6080.
ACKNOWLEDGMENT
We thank CPRIT for a startꢀup fund and the Welch Foundation
(
(Fꢀ1781) for research grants. G.D. is a Searle Scholar. We thank
Dr. Zhou and Dr. Young for proofreading the manuscript. Mr.
Sorey and Ms. Spangenberg are acknowledged for their NMR
advice. Johnson Matthey is thanked for a donation of Pd salts.
(
15) (a) Cárdenas, D. J.; MartínꢀMatute, B.; Echavarren, A. M. J. Am.
Chem. Soc. 2006, 128, 5033; (b) MartínꢀMatute, B.; Mateo, C.;
Cárdenas, D. J.; Echavarren, A. M. Chem. Eur. J. 2001, 7, 2341; (c)
Maestri, G.; Motti, E.; Della Ca’, N.; Malacria, M.; Derat, E.;
Catellani, M. J. Am. Chem. Soc. 2011, 133, 8574.
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2 2 2
CH NMe )
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3
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(
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