Transient protection of strained alkynes from click reaction via complexation with copper

Article abstract of DOI:10.1021/ja507660x

A transient protection method of cyclooctynes from a click reaction with an azide through 1:1 complexation with a cationic copper(I) salt is reported. The application of the method to a cyclooctyne bearing a terminal alkyne enabled the selective copper-catalyzed click conjugation with an azide at the terminal alkyne moiety, which made cyclooctyne derivatives readily accessible.

Full text of DOI:10.1021/ja507660x

Communication
pubs.acs.org/JACS
Transient Protection of Strained Alkynes from Click Reaction via
Complexation with Copper
Suguru Yoshida,*,† Yasutomo Hatakeyama,† Kohei Johmoto,‡ Hidehiro Uekusa,‡
and Takamitsu Hosoya*,†
†Laboratory of Chemical Bioscience, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10
Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan
‡Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
S
* Supporting Information
Scheme 1. Synthetic Plan for Functionalized Cyclooctynes
via Click Conjugation
ABSTRACT: A transient protection method of cyclo-
octynes from a click reaction with an azide through 1:1
complexation with a cationic copper(I) salt is reported.
The application of the method to a cyclooctyne bearing a
terminal alkyne enabled the selective copper-catalyzed
click conjugation with an azide at the terminal alkyne
moiety, which made cyclooctyne derivatives readily
accessible.
he click reaction,1 notably the copper(I)-catalyzed azide−
T
alkyne cycloaddition (CuAAC), has become one of the
most reliable methods for connecting molecules in broad
disciplines including materials chemistry and chemical biology.2
Moreover, copper-free click reactions that exploit function-
alized strained alkynes, and particularly cyclooctyne derivatives
that spontaneously react with azides,3 have enabled the
chemical modification of azido-incorporated biomolecules in
cultured cells and in living animals, greatly expanding the utility
of click chemistry.4 To make the method more practical,
various types of cyclooctynes with improved properties, such as
increased azidophilicity and reduced hydrophobicity, have been
developed, and a variety of derivatives bearing functional
moieties, such as fluorescent or biotinyl groups, have been
prepared.5 Some of these cyclooctynes are commercially
available, but on-demand synthesis is required for those with
a new functionality, which is not always easy because of the
high reactivity of the strained alkyne moiety.6
Scheme 2. Attempted Protection of Cyclooctyne 5 with a
Dinuclear Cobalt Reagent
used reagent for alkyne protection due to its stable complex-
ation ability with alkynes and the availability of known and
reliable decomplexation methods.7 While the protection/
deprotection of some medium-sized cycloalkynes via cobalt
complexation has been demonstrated, there is, to the best of
our knowledge, no report on the successful regeneration of the
alkyne moiety via the decomplexation of cycloalkyne−cobalt
complexes when the ring size is less than nine.8 Most of these
complexes were used directly in valuable transformations, such
as the Pauson−Khand reaction, without isolation of demeta-
lated cycloalkynes.9 In our case, even the complexation of 5
with Co2(CO)8 was unsuccessful (Scheme 2).
Although cyclooctynes readily coordinate to metals and are
known to form complexes with transition metals, including
platinum, gold, silver, and copper,10 the synthetic utility of
these complexes has not been explored. We therefore searched
for an appropriate metal that forms a complex with cyclo-
Versatile functionalized cyclooctynes will be readily available
if the CuAAC reaction can be performed selectively at the
terminal alkyne moiety of diyne compounds such as 1 (Scheme
1). However, the catalyst-free click conjugation of diyne 1 with
an azide proceeds only at the higher azidophilic strained alkyne
side to afford undesired triazole 2. To address this issue, the
protection of the strained alkyne to allow the CuAAC reaction
to proceed at the unprotected terminal alkyne moiety of 3
should be an effective solution for the preparation of
functionalized cyclooctyne 4. In this study, we present a
novel method for the protection of strained alkynes that
enables the facile preparation of cyclooctyne derivatives.
Our initial attempt to protect the strained alkyne moiety of
dibenzo-fused cyclooctyne 5 with Co2(CO)8 was unfruitful
(Scheme 2). This dinuclear cobalt complex is the most widely
Received: August 3, 2014
Published: September 18, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
octynes, sufficiently stable to prevent the reaction with an azide,
and also capable of dissociating under mild conditions to
regenerate the highly reactive strained alkyne moiety.
After extensive studies, the complexation of cyclooctyne 5
with a copper salt was found to meet these criteria (Table 1).
Table 1. Screening of Metals To Mask the Clickability of
Cyclooctyne 5
a
a
entry
MtlX
x (equiv)
6 (%)
8 (%)
Figure 1. NMR studies of cyclooctyne 5 following the addition of a
copper salt. (A) 1H NMR spectra of cyclooctyne 5, azide 6, and 5 with
an equimolar amount of copper salt in the presence and absence of 6.
(B) 13C chemical shifts of the alkyne carbons in 5 following the
addition of the copper salt.
b
1
2
3
4
5
6
7
8
none
−
0
97
AgBF4
1.0
1.0
1.0
1.0
1.0
2.0
0.5
18
11
40
12
77
61
50
78
0
AuCl
AuCl + AgBF4
CuCl
(MeCN)4CuBF4
(MeCN)4CuBF4
quant.
quant.
46
the copper reagent. This result indicated that complex 9 and
uncomplexed alkyne 5 rapidly exchanged the copper, which is
similar to the behavior observed for the previously reported
cyclooctyne−coinage metal complexes.10f,11 Furthermore, a
gradual upfield shift of the signals for the alkyne carbons of 5
was observed in the 13C NMR spectrum as the amount of
added copper salt was increased from 0.1 to 1.0 equiv (Figure
1B). Further addition of copper, however, did not induce this
13C chemical shift, supporting the 1:1 complexation of the
cyclooctyne 5 with the copper salt.
X-ray crystallographic analysis also suggested a 1:1 complex-
ation. The recrystallization of the cycloalkyne−copper complex
from CH2Cl2−hexane provided a single crystal that was found
to be the tricoordinate cycloalkyne−copper(I) complex 10,
containing acetonitrile and water as ligands (Figure 2).12 The
C−CC angle of 10 was decreased by 4° compared to that of
0
(MeCN)4CuBF4
47
a
1
Yields based on H NMR analysis by using 1,1,2,2-tetrachloroethane
as an internal standard, unless otherwise noted. Isolated yield.
b
Transition metal salts were screened by pretreating them with 5
in methanol for 30 min, followed by addition of azide 6. After
stirring each of the mixtures for 12 h, the quantities of the click
product 8 and recovered 6 were determined. Without the
addition of metal under these conditions, all of the azide 6 was
consumed to furnish triazole 8 nearly quantitatively (entry 1).
Several coinage metal salts were found to be effective at
inhibiting the triazole formation (entries 2−6). In particular,
the cationic copper(I) salt (MeCN)4CuBF4 provided an
excellent result: the cycloaddition reaction was completely
inhibited and azide 6 was completely recovered (entry 6). This
result suggested the in situ formation of cycloctyne−metal
complex 7. Notably, the use of one-half of a molar equivalent of
the copper reagent led to the protection of only half of the
starting cyclooctyne 5 (entry 8), indicating that more than an
equimolar amount of the copper reagent is required to
completely protect the strained alkyne from cyclization with
the azide.
NMR studies then provided valuable insights into the
formation of the cycloalkyne−copper complex (Figure 1).
Addition of an equimolar amount of (MeCN)4CuBF4 to a
methanol-d4 solution of cyclooctyne 5 caused a slight downfield
1
shift in the signals for the aromatic protons in the H NMR
spectrum, suggesting the formation of complex 9 (Figure 1A).
Addition of azide 6 to this mixture clearly demonstrated the
independent existence of 9 and 6, which was in good agreement
with the experimental results (Table 1, entry 6). In addition, a
single set of peaks was observed in the H and 13C NMR
spectra when cyclooctyne 5 was treated with 0.1−2.0 equiv of
1
Figure 2. X-ray structure of cyclooctyne−copper complex 10.
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Journal of the American Chemical Society
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uncoordinated cyclooctyne 5 as found for its optimized
structure using the density functional theory (DFT) method
at the B3LYP/6-31G(d) level.13 Notably, this result is in good
agreement with the angles observed for the previously reported
cyclooctyne−copper complexes.10c
moiety, which was achieved using a simple one-pot three-step
procedure (Scheme 6). Diyne 15 was first treated with an
Scheme 6. Synthesis of a Click-Modified Cyclooctyne via
Transient Protection of a Strained Alkyne
Fortunately, the treatment with aqueous ammonia14 led to
the successful dissociation of the cycloalkyne−copper complex
9 and cyclooctyne 5 was regenerated with high efficiency
(Scheme 3). These results clearly confirmed the smooth
Scheme 3. Regeneration of Cyclooctyne from the Copper
Complex
excess of the cationic copper salt for complexation, and then
azide 6 and a catalytic amount of tris(benzyltriazolylmethyl)-
amine (TBTA)16 were added to perform the CuAAC reaction.
Finally, aqueous ammonia was added to dissociate the copper,
furnishing the desired terminal alkyne-modified cyclooctyne 16
in high overall yield.17 The key to the success of this reaction
must be the moderate strength of the coordination bond
between the strained alkyne and the copper,11 which must be
sufficiently strong to withstand the CuAAC conditions but
readily dissociable under specific mild conditions. Note that the
same copper salt played a contrasting double role for the two
alkynes in this transformation: the prevention of the clickability
of the strained alkyne as a protective group and the promotion
of the click reaction of the terminal alkyne as a catalyst.2
In conclusion, we demonstrated that the strained alkyne
moiety of a cyclooctyne can be transiently masked via 1:1
complexation with a cationic copper(I) salt. Furthermore, this
method enabled the facile modification of a cyclooctyne
derivative bearing a terminal alkyne moiety via the CuAAC
reaction. Investigation of the scope of alkyne substrates,
including acyclic ones,18 that can be protected by this method,
evaluation of the stability of the strained alkyne−copper
complex under various reaction conditions, and application of
the method to the synthesis of diverse cyclooctyne derivatives
with various functional groups are now in progress.
reversibility of the complexation of the strained alkyne by the
copper salt and indicated that this approach should be suitable
as a protection method for cycloalkynes.
Furthermore, we demonstrated that complexation using the
copper salt prevents the Sondheimer diyne15 (11) from
participating in click reactions (Scheme 4). We previously
Scheme 4. Masking the Clickability of Sondheimer Diyne
with Copper
reported that diyne 11 spontaneously reacts with two azides to
afford the bistriazole in high yield.6a However, the pretreatment
of 11 with 2 molar equiv of the copper salt completely
protected the two strained alkyne moieties from cyclization
with the azides. In addition, the assumed diyne−dicopper
complex 12 was uneventfully returned to diyne 11 by treatment
with aqueous ammonia.
In addition to dibenzo-fused cyclooctynes, bicyclo[6.1.0]-
nonyne (BCN) derivative5e 13 was similarly protectable with
copper, expanding the scope of the method (Scheme 5).
The utility of copper complexation as a method for the
protection of strained alkynes was then demonstrated in the
selective click conjugation of diyne 15 at the terminal alkyne
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization for new com-
pounds including copies of NMR spectra and the X-ray
crystallographic data for cycloalkyne−copper(I) complex 10
(CCDC 1016209). This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
s-yoshida.cb@tmd.ac.jp
thosoya.cb@tmd.ac.jp
Scheme 5. Protection of BCN with Copper
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Number
24310164 (T.H.) and 26350971 (S.Y.); the Naito Foundation
(S.Y.); and the Platform for Drug Discovery, Informatics, and
Structural Life Science of MEXT, Japan. The authors thank Dr.
Tomoya Hirano and Prof. Hiroyuki Kagechika at Tokyo
Medical and Dental University for their assistance with the
HPLC system.
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Journal of the American Chemical Society
Communication
compare the results with the experimental data obtained for the crystal
structure of cycloalkyne−copper(I) complex 10. See the Supporting
Information.
(14) Fields, E. K. Arynes. In Organic Reactive Intermediates;
McManus, S. P., Ed.; Academic Press: New York, 1973; p 475.
(15) Wong, H. N. C.; Garratt, P. J.; Sondheimer, F. J. Am. Chem. Soc.
1974, 96, 5604−5605.
(16) Rodionov, V. O.; Presolski, S. I.; Díaz, D. D.; Fokin, V. V.; Finn,
M. G. J. Am. Chem. Soc. 2007, 129, 12705−12712.
(17) The structure of triazole 16 was confirmed by further
transformation. The reaction of 16 with ethyl azidoacetate without
the copper catalyst afforded bistriazole in 89% yield, which was
different from the product obtained from unprotected diyne 15 by the
sequential double-click reaction, i.e. strain-promoted cycloaddition
with azide 6 followed by CuAAC reaction with ethyl azidoacetate. See
the Supporting Information for details.
(18) Preliminary studies suggested the applicability of the protection
method to unstrained internal alkynes. For example, pretreatment of a
mixture of dimethyl acetylenedicarboxylate (DMAD) and phenyl-
acetylene with (MeCN)4CuBF4 in THF followed by the addition of
benzyl azide and TBTA and heating the reaction mixture at 65 °C for
24 h afforded the CuAAC product, 1-benzyl-4-phenyl-1H-1,2,3-
triazole (quant.), with recovery of DMAD (52%). In this case,
Huisgen cycloaddition between DMAD and benzyl azide did not
proceed, although the cycloadduct was obtained in 61% yield when the
reaction was performed without premixing with copper. See the
Supporting Information for details.
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