Palladium-catalyzed N-(2-pyridyl)sulfonyl-directed C(sp3)-H γ-arylation of amino acid derivatives

Article abstract of DOI:10.1039/c2sc21162a

The direct Pd-catalyzed γ-arylation of amino acid esters bearing a removable N-(2-pyridyl)sulfonyl directing group is described. A variety of N-(2-pyridyl)sulfonamide amino acid derivatives, including α-quaternary amino acid and β-amino acid substrates, react with iodoarenes in the presence of Pd(OAc)₂ to provide γ-arylated products in synthetically useful yields. An unprecedented remote C(sp³)-H arylation of dipeptides is presented, illustrating the compatibility of the method with the presence of the peptidic bond. The process occurs without racemization at the Cα center and the auxiliary controlling group can be easily installed and removed in the amino acid backbone. A bimetallic Pd ^II γ-metalated complex has been isolated and characterized showing the key role exerted by the (2-pyridyl)sulfonyl unit.

Full text of DOI:10.1039/c2sc21162a

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Palladium-catalyzed N-(2-pyridyl)sulfonyl-directed
C(sp3)–H g-arylation of amino acid derivatives†‡
Cite this: Chem. Sci., 2013, 4, 175
´
*
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Nuria Rodrıguez, Jose A. Romero-Revilla, M. Angeles Fernandez-Ibanez
*
and Juan C. Carretero
The direct Pd-catalyzed g-arylation of amino acid esters bearing a removable N-(2-pyridyl)sulfonyl directing
group is described. A variety of N-(2-pyridyl)sulfonamide amino acid derivatives, including a-quaternary
amino acid and b-amino acid substrates, react with iodoarenes in the presence of Pd(OAc)2 to provide
g-arylated products in synthetically useful yields. An unprecedented remote C(sp3)–H arylation of
dipeptides is presented, illustrating the compatibility of the method with the presence of the peptidic
bond. The process occurs without racemization at the Ca center and the auxiliary controlling group can
be easily installed and removed in the amino acid backbone. A bimetallic PdII g-metalated complex has
been isolated and characterized showing the key role exerted by the (2-pyridyl)sulfonyl unit.
Received 3rd August 2012
Accepted 13th September 2012
DOI: 10.1039/c2sc21162a
www.rsc.org/chemicalscience
biotechnology.8 In this regard, Corey et al. described the rst
pioneering examples of b- and g-functionalization of N-phtha-
Introduction
Development of efficient methods for the catalytic functionali-
zation of C–H bonds is a fundamental challenge in current
organometallic and synthetic organic chemistry.1 In recent
years, a great progress has been made in the direct and selective
metal-catalyzed transformation of arene C(sp2)–H bonds into a
variety of functional groups through the formation of new
C(sp2)–C or C(sp2)–X bonds.1,2 However, the activation of the
less reactive C(sp3)–H bonds has now entered the mainstream
and it is still at its infancy.3 In many cases this type of process
deals with the functionalization of C(sp3)–H bonds at benzylic
positions or those next to heteroatoms.4 To date, catalytic
processes for the functionalization of unactivated simple
C(sp3)–H bonds are relatively rare and, consequently, any
progress in this eld is of great synthetic interest.5
In 2005, Daugulis et al. described the Pd-catalyzed b-arylation
of carboxylic acid and the g-arylation of amine derivatives by
using the 8-aminoquinoline and the picolamide auxiliary
directing groups.6,7 This type of Pd-catalyzed chelating strategy,
although at an early stage, has been also applied to the synthesis
of non-natural amino acids. The development of methods for the
synthesis of amino acids, especially from native amino acids, is
highly important due to their broad use in drug discovery and
loyl-a-amino amides by employing the 8-aminoquinoline auxil-
iary.9 This coordinating group and the 2-thiomethylaniline
group have been recently applied by Daugulis et al. in the effi-
cient b-arylation of N-phthaloylalanine derivatives with iodoar-
enes.10 On the other hand, in the elegant study of He and Chen on
the Pd-catalyzed picolinamide-directed C(sp3)–H bond func-
tionalization, some examples of g-arylation of natural amino
acids were described, although with moderate chemical yields.11
In spite of this progress in the b- and g-functionalization of
natural amino acids, there is plenty of room for further
improvements both regarding the overall chemical efficiency of
the process and the scope at the amino acid structure, as well as
the study of the presumed palladacycle intermediates12 and the
nal removal of the key directing group. In the last few years,
our research group has demonstrated the efficiency of the easily
removable N-(2-pyridyl)sulfonyl auxiliary in the selective func-
tionalization of C(sp2)–H bonds.13 We report herein the appli-
cation of this auxiliary group to the efficient g-arylation of the
readily available N-(2-pyridyl)sulfonamide derivatives of simple
amino acid methyl esters, and the rst examples of remote
C(sp3)–H arylation in dipeptide derivatives. Some mechanistic
studies dealing with the palladation step, including the isola-
tion of a palladacycle species, are also described.
´
´
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Departamento de Quımica Organica, Facultad de Ciencias, Universidad Autonoma de
Madrid, Cantoblanco, 28049 Madrid, Spain. E-mail: tati.fernandez@uam.es;
juancarlos.carretero@uam.es; Fax: +34 914973966; Tel: +34 914973925
Results and discussion
† Electronic supplementary information (ESI) available: data for new compounds,
experimental procedures, crystallographic details, copies of HPLC
chromatograms and 1H and 13C NMR spectra. CCDC 887847 and 887848. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2sc21162a
Aer screening a wide variety of reaction conditions, we found
that the reaction of the N-(2-pyridyl)sulfonamide of L-valine
methyl ester (1) with 4-iodotoluene (2.5 equiv.), Pd(OAc)2
(10 mol%) and AgOAc (1.5 equiv.) in HFIP (1,1,1,3,3,3-hexa-
uoro-2-propanol)14 at 150 ꢀC, resulted in 95% conversion of the
‡ In memory of Dr. Christian G. Claessens.
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starting chiral amino acid derivative, providing a nearly 1 : 1
In agreementwith thehighly stereoselective monoarylation of
mixture of the g-monoarylated derivative 2a and the g-bisary- the L-valine derivative 1, a very different reactivity prole was
lated product 3a. Aer chromatographic purication both observed in the reaction of the isoleucine derivative 4 and the
products were isolated in 44% and 42% yield, respectively allo-isoleucine diastereomer 6 (Table 2, entries 1 and 2). Thus,
(Table 1, entry 1). In addition to the chemical efficiency of this while substrate 4 afforded only 25% of the g-monoarylated
C(sp3)–H functionalization reaction, three issues are worth isoleucine derivative 5,17 the diastereomeric allo-isoleucine
noting: (a) the g-monoarylated product 2a was obtained with derivative 6, having the methyl group at the same stereochemical
very high diastereoselectivity (dr $ 20 : 1), with the arylation position as the pro-S methyl in the valine substrate 1, provided 7a
occurring exclusively at the pro-S methyl valine group. (b) The in 72% yield under standard reaction conditions. Similar
Pd-catalyzed process takes place preserving the chiral integrity stereochemical effects, likely based on the accessibility of the
at the a position, as demonstrated by the very high enantio- C(sp3)–H bond in the palladation step and stability of the
purity of product 3a ($99% ee determined by HPLC). (c) No resulting palladacycle, have been previously described.9a It is also
gem-diarylated products were observed, presumably because of worth mentioning that the reaction takes place exclusively over
steric factors.
the methyl g C–H bond and not at the methylene g0 C–H bond.
With this result in hand, we next tried to improve the mono/
Extending the scope at the amino acid structure, the reaction
bisarylation selectivity. As expected, the bisarylated product 3a of the protected threonine derivative 8 provided the g-mono-
could be isolated in excellent yield (92%, entry 2) by simply arylated product 9 in 70% yield (entry 3). The more conforma-
increasing the amount of 4-iodotoluene (5 equiv.) and AgOAc (3 tionally exible homoalanine derivative 10 led to the
equiv.). On the other hand, the use of only a slight excess of 4- corresponding g-arylated product 11 in 44% yield (entry 4),
iodotoluene (1.5 equiv.) resulted in incomplete conversion (70% providing a direct method for the synthesis of homoar-
aer 4 h) and moderate mono/bisarylation selectivity (2a/3a ¼ ylalanines, important intermediates in medicinal chemistry.18
67/33). The g-monoarylated product 2a could be isolated in 70% Additional examples include the arylation of the g-C(sp3)–H
yield by performing the reaction at a slightly lower temperature bond at a a-quaternary amino acid derivative, the a-methylva-
(140 ꢀC for 4 h,15 entry 4) using 2.5 equiv. of 4-iodotoluene. line substrate 12, and the b-amino acid derivative 14 (entries 5
According with the importance of steric effects, the arylation of and 6). While the a-methylvaline substrate 12 revealed a strong
1
with the more sterically demanding 2,4-dimethoxy- trend to provide the corresponding bisarylated product 13 (60%
iodobenzene provided the monoarylated product 2b with yield), the b-amino acid derivative 14 furnished the g-mono-
reasonably good selectivity (2b/3b ¼ 80 : 20, 68% yield 2b, entry arylated product 15 in 43% yield under standard reaction
5). The relative and absolute conguration of 2b was deter- conditions or the bisarylated product 16 (64% yield) by
mined by single-crystal X-ray diffraction analysis.16
increasing the amount of 4-iodotoluene and AgOAc.
Table 1 Selectivity in the g-CH3 arylation of L-valine derivative 1a
Yieldc
Entry
ArI (equiv.)
t/h
T/ꢀC
Conv.b/% (ratio 2 : 3)
2/%
3/%
1
4-Me–C6H4–I (2.5)
4-Me–C6H4–I (5.0)
4-Me–C6H4–I (1.5)
4-Me–C6H4–I (2.5)
2,4-MeO–C6H3–I (2.5)
4
4
4
4
3
150
150
150
140
150
95 (53 : 47)
100 (8 : 92)
70 (67 : 33)
90 (80 : 20)
88 (80 : 20)
44
6
—
70
68
42
92
—
18
17
2d
3
4
5
a
Reaction conditions: 1 (0.25 mmol), 10 mol% Pd(OAc)2, 1.5 equiv. AgOAc, HFIP (1 M). b Conversions determined by 1H NMR on the crude mixtures.
Isolated yields aer chromatographic purication. d 3.0 equiv. of AgOAc.
c
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Table 2 Pd-catalyzed g-arylation of methyl C(sp3)–H bonds of N-(2-pyridyl)
sulfonyl amino acid esters
Entry
1a
Substrate
Product
Scheme 1 Scope of the g-CH3 arylation of the allo-isoleucine derivative 6.
2a
ortho substituted 2,4-dimethoxyiodobenzene also proved to be a
suitable substrate (72% yield for 7g).
To highlight the synthetic utility of this process in non-
natural amino acid synthesis, the efficient three-step sequence
for the diarylation of L-valine methyl ester is shown in Scheme 2:
introduction of the key 2-pyridylsulfonyl directing group, the
Pd-catalyzed bisarylation and the nal reductive removal of the
sulfonyl moiety.20 Importantly, the reductive elimination of
the sulfonyl moiety took place with less than 2% of racemiza-
tion as determined by HPLC (98% ee for 17a). Although the Pd-
catalyzed reactions were typically run on a 0.25 mmol scale, a
2.00 mmol scale reaction provided 3a with similar high yield
(98%), thus demonstrating the robustness of the reaction.
Having thus succeeded in the arylation of different amino
acid esters, we next undertook the remote C(sp3)–H arylation of
dipeptides, which as far as we know it is unprecedented.21
Satisfyingly, both the arylation of the valine–glycine 18 and the
allo-isoleucine-glycine dipeptide 20 occurred with good selec-
tivity at the g-methyl group providing the monoarylated dipep-
tides 19 and 21 in synthetically useful yields (60% and 43% yield,
respectively) under the usual reaction conditions (Scheme 3).
3b
4c
5d
6
a
4 h. b HFIP (0.2 M), 120 ꢀC, 4 h. c 20 mol% Pd(OAc)2, 3.0 equiv. AgOAc,
16 h. d 3.0 equiv. p-Tol-I, 2.0 equiv. AgOAc, 8 h. e 6 h. f 5.0 equiv. p-Tol-I,
3.0 equiv. AgOAc, 6 h.
Next, we evaluated the scope of the process with regard to the
substitution at the aryl iodide and using the allo-isoleucine
derivative 6 (Scheme 1). Electron-donating and electron-with-
drawing substituents were well tolerated (71–75% yield),
including uorinated aryl iodides that have an important role in
agrochemicals and pharmaceuticals.19 Interestingly, bromide is
also tolerated opening the access to scaffolds that may be later
functionalized using standard Pd0 coupling processes. The
Scheme 2 Introduction and removal of the N-(2-pyridyl)sulfonyl group.
Chem. Sci., 2013, 4, 175–179 | 177
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Scheme 5 Unreactive substrates.
Scheme 3 Palladium-catalyzed C(sp3)–H g-arylation of dipeptides.
derivative 22 with Pd(OAc)2 (Scheme 4). To our delight, the
reaction in acetonitrile at 60 ꢀC cleanly provided a palladium
containing compound, which aer recrystallization and X-ray
diffraction analysis proved to be the bimetallic ve-membered
ring palladacycle complex 23, metalated at one carbon of the
methyl tert-leucine groups (Fig. 1) (see ESI† for crystallographic
details).22 The Pd(1)–Pd(2) distance is 3.1276 (12) A, which is
˚
23
˚
within the sum of van der Waals radii (3.26 A), and the Pd–C
˚
distance (1.997 (11) A) are similar to the distances found in
ve-membered ring metalated complexes.6a,12 Interestingly, in
this dimeric complex the slightly distorted square planar PdII
atom is bonded to the sulfonamide nitrogen and the g-palla-
dated carbon, while the other two coordination positions are
occupied by the pyridine nitrogen and one of the sulfonylic
oxygens. In agreement with this type of palladium coordina-
tion mode, either the p-tolylsulfonamide derivative 26 (lacking
the key pyridine nitrogen), or the N-methyl sulphonamide 27
(lacking the easily palladated acidic NH group), or the alanine
and leucine derivatives 28 and 29 (both lacking the g-methyl
group required in the formation of
a ve-membered
Scheme 4 Synthesis and reactivity of the bimetallic complex 23.
palladacycle) proved to be completely unreactive under the
standard arylation conditions (Scheme 5). Finally, we
conrmed that palladacycle 23 was able to react with 4-iodo-
ꢀ
toluene (60 C in HFIP) leading to a nearly equimolar mixture
of the monoarylated and bisarylated products 24 and 25 (79%
yield).
Conclusions
In summary, the N-(2-pyridyl)sulfonyl group behaves as an
efficient controlling auxiliary for the Pd-catalyzed g-C(sp3)–H
methyl activation in amino acid esters. A variety of N-(2-pyridyl)
sulfonamide amino acid derivatives, including a-quaternary
amino acids, b-amino acids, and dipeptide substrates, react
with iodoarenes in the presence of Pd(OAc)2 to provide g-ary-
lated products in synthetically useful yields. The process occurs
without racemization at the Ca center and the auxiliary
controlling group can be easily installed and removed in the
amino acid backbone. A bimetallic PdII g-metalated complex
has been isolated and characterized showing the key role exer-
ted by the (2-pyridyl)sulfonyl unit. Further synthetic applica-
tions and mechanistic work are in progress.
Fig. 1 ORTEP view of 23, hydrogen atoms have been removed for simplicity.
This result shows the compatibility of the Pd-catalyzed arylation
conditions with the presence of the peptidic bond.
To obtain some mechanistic insights into the course of this
C(sp3)–H activation process and the coordination mode of the
palladium catalyst, we focused our attention on the stoichio-
metric reaction of the N-(2-pyridyl)sulfonyl tert-leucine
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Acknowledgements
´
Financial support from the Ministerio de Economıa y Com-
petitividad (MINECO, project CTQ2009-07791) and the Con-
´
´
sejerıa de Educacion de la Comunidad de Madrid (programme
AVANCAT; S2009/PPQ-1634) are gratefully acknowledged.
M.A.F.-I., N.R. and J.R.R. thank the MINECO for a Juan de la
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´
Cierva contract, a Ramon y Cajal contract and a predoctoral
fellowship, respectively.
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