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electrophiles, such as maleimide, a-halocarbonyl, 3-arylpropio-
lonitrile,[5] and pyridyldisulfide. Thiyl radical can also be utilized
for bioconjugation through thiol-ene coupling chemistry.
Whilst these approaches offer advances in this growing field of
bioconjugation, they suffer from some drawbacks. Usually, the
reagents used for bioconjugation can also modify other side-
chain residues, particularly lysine residues of peptides and pro-
teins.[6] Moreover, although thiol conjugation with maleimides
is broadly useful, the resulting adducts are often unstable in
biological environments due to reversible thiol exchange and
other side reactions.[7] More recently, rhodium-catalyzed modi-
fication of cysteine with diazo reagents has been reported[8] as
an alternative method, although potential side reactions with
tryptophan have been noted as a limitation. In addition, diazo
reagents are not usually commercially available and need to
be prepared through multistep reaction sequences. In this con-
text, the development of novel methods to efficiently function-
alize cysteine-containing biomolecules under mild conditions is
highly desirable. We are particularly interested in the direct
palladium-catalyzed coupling of cysteine-containing peptides
and proteins with various (hetero)aryl halides at ambient tem-
perature. Developing a method that can generate a nonlabile
C(sp2)ÀS bond that is more stable than the C(sp3)ÀS bonds
formed in previously reported reactions would be a major ad-
vance in the field of bioconjugation chemistry. Recently, Buch-
wald et al.[9] reported the use of a palladium(II) complex as a re-
agent for efficient cysteine bioconjugation. The reaction is
rapid and robust and proceeds under a range of biocompat-
ible reaction conditions. The utility of the bioconjugation plat-
form was further established by the synthesis of new classes of
antibody–drug conjugates. However, the main limits of this
methodology are: (i) the necessity to prepare the oxidative ad-
dition complex partners in a glovebox for each coupling, and
(ii) the necessity to use an excess of the palladium(II) complex
(2–10 equiv) in all reactions.
Table 1. Optimization of coupling reaction of 1a with 2a under various
conditions.[a]
Entry
Solvent
t [min]
Yield[b] [%]
1
2
4
THF
THF/H2O (8:2)
H2O
5
10
90
99
99
99
[a] Reaction conditions:
A solution of 1a (0.673 mmol) and 2b
(0.612 mmol) in solvent (2.4 mL) was added dropwise to the Pd-G3-Xant-
phos complex and Et3N and the mixture was stirred at 258C. [b] Yield of
isolated 3b.
Phos (2 mol%) and NEt3 (1.5 equiv) in THF for 5 min at room
temperature furnished the desired product 3b in 99% yield
(entry 1). The reaction rate was slightly lower (10 min), but the
same yield of 3b was obtained (99%) when the reaction of 1a
and 2b was performed in THF/H2O (8:2) (entry 2). Surprisingly,
conducting the model reaction in water only under otherwise
identical conditions led to the desired product 3b within
90 min. These results clearly indicated that the use of water as
a solvent might decrease the rate of reaction. Nevertheless,
the yield of isolated product 3a remained at 99% (entry 3).
With a viable coupling procedure in hand, we turned our at-
tention to the generality of the process by studying the cou-
pling of various structurally diverse aryl, alkenyl, and alkynyl
halides with cysteine derivatives 1a–d (Scheme 2). Remarkably,
this C(sp2)ÀS coupling reaction appeared to be quite general
with respect to different electrophilic partners and tolerated
various functional groups on the aryl halide (e.g., -Br, -Cl, -OAc,
-OH, -CN, -N3, -CO2Me, -NHBoc). N-Acetylcysteine 1a was readi-
ly coupled with aryl iodides bearing para and meta electron-
donating or electron-withdrawing substituents to give S-arylat-
ed products 3c–o in excellent yields. In addition, the sterically
demanding ortho substitution pattern was tolerated in the
coupling reaction of 1a, leading to ortho-substituted S-aryl cys-
teine derivatives 3b and 3p–r in excellent yields, regardless of
the electronic nature of the substituents. As also shown in
Scheme 2, the presence of a hydroxyl group on the aryl halide
partner did not interfere with the outcome of the present reac-
tion (compound 3k). Extending this method to heteroarylation
of 1a also proved to be successful. 3-Iodopyridine and 5-iodo-
NH-free indole were good partners with 1a under our opti-
mized conditions, furnishing the desired products 3s,t in excel-
lent yields. It is particularly notable that the amino acid func-
tion on phenylalanine is well tolerated, which should prove in-
In 2015, our group reported an efficient protocol for the pal-
ladium-catalyzed coupling of (hetero)aryl halides with various
thiols,[10] including polyglycosyl thiols,[11] thiophenols, and al-
kylthiols. Using the Buchwald palladacycle pre-catalyst G3-
XantPhos (1 mol%), the CÀS bond-forming reaction was ach-
ieved rapidly (5 min) at room temperature in the presence of
NEt3 (1 equiv) in THF. In addition, we demonstrated preliminari-
ly that the use of these conditions also enabled, for the first
time, arylation of the amino acid derivative N-acetylcysteine 1a
to provide 3a in 98% yield (Table 1).
Results and Discussion
As part of our continued interest in adapting the tools of or-
ganic chemistry to synthetic biology, we considered it worth-
while to explore this protocol in peptide and protein cross-
couplings. Taking into account that suitable conditions for pep-
tide and protein functionalization include aqueous media and
ambient temperature, we decided to initially explore the feasi-
bility of the coupling of N-acetyl cysteine 1a with 1-iodonaph-
thalene 2b as a model reaction in different aqueous media.
Thus, reaction of 1a with 2b in the presence of Pd-G3-Xant-
Chem. Eur. J. 2016, 22, 11365 – 11370
11366
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Al-Shuaeeb, Riyadh Ahmed Atto
