Metal-catalyzed one-pot synthesis of tetrazines directly from aliphatic nitriles and hydrazine

Article abstract of DOI:10.1002/anie.201201117

Paving the way: The lack of convenient synthetic methods is a significant roadblock to the broader use of 1,2,4,5-tetrazines in bioorthogonal chemistry and functional materials. Lewis acid metal catalysts-most notably divalent nickel and zinc salts-are described to catalyze the one-pot synthesis of 1,2,4,5-tetrazines directly from aliphatic nitriles (see scheme). Copyright

Full text of DOI:10.1002/anie.201201117

Angewandte
Chemie
DOI: 10.1002/anie.201201117
Synthetic Methods
Metal-Catalyzed One-Pot Synthesis of Tetrazines Directly from
Aliphatic Nitriles and Hydrazine**
Jun Yang, Mark R. Karver, Weilong Li, Swagat Sahu, and Neal K. Devaraj*
There is rapidly growing interest in the use of 1,2,4,5-
tetrazines as bioorthogonal coupling agents.[1–3] Recent appli-
cations of tetrazine cycloadditions include intracellular small-
molecule imaging, genetically targeted protein tagging, post-
synthetic DNA labeling, nanoparticle-based clinical diagnos-
tics, and in-vivo imaging.[4–7] In addition, tetrazines have seen
significant use in materials science,[8,9] coordination chemis-
try,[10,11] and specialty explosives research.[12,13] They are also
valuable synthetic intermediates, and have been elegantly
deployed on route to several natural product syntheses.[14–16]
Despite the promise of tetrazines, the lack of convenient
synthetic methods is a significant roadblock to their broader
use and study by the scientific community.[17] Here we report
that Lewis acid transition metal catalysts, most notably
divalent nickel and zinc salts, can catalyze the formation of
1,2,4,5-tetrazines directly from nitriles. To our knowledge, this
is the first method utilizing homogenous catalysis to directly
synthesize tetrazines from a wide range of unactivated
aliphatic nitriles and hydrazine. Symmetric and asymmetric
tetrazines were conveniently prepared from multiple precur-
sors including alkyl nitriles, aromatic nitriles, and formami-
dine salts. This methodology should greatly improve the
accessibility of tetrazines and lead to further exploration of
their applications, particularly with respect to bioorthogonal
conjugations.
such as imidates, amidine salts, and aldehydes, but these
methods suffer from low yields, limited substrate scope, and
the requirement of additional synthetic steps.[22–24] For these
reasons, a general and robust method to prepare symmetric
and asymmetric 1,2,4,5-tetrazines directly from unactivated
nitriles would be highly desirable.
Though the mechanism of tetrazine synthesis has been
debated, it is generally agreed that reaction begins with
nucleophilic attack of the nitrile by hydrazine forming an
amidrazone.[25,26] We speculated that the addition of Lewis
acid catalysts might promote this reaction by binding to the
nitrile and/or hydrazine. Metal ions have long been known to
activate nitriles to nucleophilic addition.[27–29] However, there
has not been a report of using homogenous transition metal
catalysis to promote the formation of 1,2,4,5-tetrazines.[21,30]
We used the reaction of benzyl cyanide with neat hydrazine to
survey a range of Lewis acid catalysts at 5 mol% loading
(Table 1).[31] In the absence of catalyst, tetrazine products
could not be isolated.[20] Remarkably, the addition of 5 mol%
Table 1: Survey of metal catalysts.
The most convenient route to 1,2,4,5-tetrazines is by
addition of hydrazine to aromatic nitriles followed by
oxidation of the 1,2-dihydrotetrazine product.[18] Unfortu-
nately, this strategy is not viable for producing tetrazines from
unactivated nitriles such as alkyl nitriles. Earlier reports
claiming to access tetrazines directly from alkyl nitriles have
proven difficult to reproduce, likely due to confusion between
the intermediate 1,2-dihydrotetrazines and isomeric 4-amino-
1,2,4 -triazoles.[19–21] There have been several reported meth-
ods to access dialkyl tetrazines from alternative precursors,
Catalyst
Yield
[%][a]
Catalyst
Yield
[%][a]
Catalyst
Yield
[%][a]
none
Zn(OAc)2
ZnCl2
ZnBr2
ZnI2
Zn(OTf)2
Cu(OTf)2
MgBr2
0
Cu(OAc)2
MnBr2
CuBr2
CoCl2·6H2O
MgCl2
Yb(OTf)3
Sc(OTf)3
Ni(acac)2
59
55
23
13
63
31
26
10
NiCl2
NiI2
Ni(OTf)2
CuOAc
CuCl
CuBr
CuI
Cu(OTf)
73
93
95
53
12
42
50
57
38
11
46
68
70
11
15
[a] Yields reported after isolation by silica flash chromatography.
[*] Dr. J. Yang, W. Li, S. Sahu, Prof. N. K. Devaraj
Chemistry and Biochemistry, University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
E-mail: ndevaraj@ucsd.edu
nickel triflate (Ni(OTf)2) led to near quantitative yield of 3,6-
dibenzyl-1,2,4,5-tetrazine. Zinc salts also gave good yields,
with addition of 5 mol% zinc triflate (Zn(OTf)2) leading to
70% yield of the desired tetrazine. Nickel and zinc salts
possessing stronger coordinating anions gave lower yields,
possible due to the lowered solubility of these salts in aprotic
media and the decreased Lewis acid strength compared to the
triflates.[32]
Given the high yields obtained with nickel and zinc
triflates, we tested their effect on the yields of several other
tetrazine syntheses where at least one component was an alkyl
nitrile (Table 2). In each instance we tested either Ni(OTf)2 or
Homepage: http://devarajgroup.uscd.edu
Dr. M. R. Karver
Massachusetts General Hospital
185 Cambridge Street, Boston, MA 02114 (USA)
[**] The authors gratefully acknowledge Ralph Mazitschek, Carlos
Guerrero, and Scott Hilderbrand for helpful discussions. This
material is based upon work supported in part by NIH grant
K01EB010078, the University of California, San Diego, and the NSF
under CHE-0741968.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201117.
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Table 2: Synthesis of 1,2,4,5-tetrazines directly from nitriles catalyzed by
Ni(OTf)2 or Zn(OTf)2.
impressive given that all previous attempts to synthesize
the molecule have only led to trace isolated yield.[24,33]
The scope of this method extends to asymmetric 3,6-
disubstituted 1,2,4,5-tetrazines, which are among the most
challenging tetrazines to synthesize.[17,25,34] Metal ions
could readily promote the formation of 6-methyl-termi-
nated tetrazines from acetonitrile and aromatic nitriles in
40–70% yield (entries 5–10). Alkyl nitriles and acetoni-
trile could also combine with hydrazine to yield 6-methyl-
terminated alkyl tetrazines in 36–40% yield (entries 11–
14). Several of these tetrazines possess functional group
handles to facilitate their use in biological applications.
For instance, it has recently been demonstrated that
methyl-terminated tetrazines are highly stable partners in
bioorthogonal cycloadditions and can be used in a mutu-
ally orthogonal fashion with azide–alkyne cycloaddi-
tions.[6,35] 6-Methyl-terminated tetrazines were previously
only accessible from reactive precursors such as imidates
and amidine salts and in lower yield.[23,35] Dialkyl
asymmetric tetrazines with bulkier substituents are
extremely difficult to isolate, even from imidates and
amidines. In contrast, we were able to isolate 3-benzyl-6-
pentyl-1,2,4,5-tetrazine (entry 15) from benzyl cyanide
and excess hexanenitrile, albeit in lower yield (12%).
Several groups have measured the rate of cycloaddi-
tion between various tetrazines and strained dienophiles
such as norbornene and trans-cyclooctene.[1,2,5,36] The
substituents on the 3 and 6 positions of 1,2,4,5-tetrazines
have a significant effect on the kinetics of the reaction.[35]
While 6-methyl-terminated tetrazines benefit from sta-
bility, tetrazines terminated with hydrogen at the 6
position react much faster and have proven utility in
live cell and live animal applications where lowered
concentrations of labeling agent are typically used.[2,7,36]
With this in mind, we examined if metal catalysis could
improve synthetic routes to hydrogen-terminated mono-
aryl tetrazines. We found that an excess of trimethylsilyl
cyanide can be used along with an aromatic nitrile to yield
a hydrogen-terminated asymmetric tetrazine (entry 16).
This is the first example of using trimethylsilyl cyanide to
synthesize tetrazines and is possible due to the addition of
nitrile-activating metal catalysts. Additionally, we
explored the effect of Ni(OTf)2 and Zn(OTf)2 on the
synthesis of tetrazine from aromatic nitriles and forma-
midine salts (Table 3). Although these reactions do not
require catalysis, yields are typically low, between 10–
20%.[35] Interestingly, we found that metal ions could
promote the reaction and significantly increase the yield
of tetrazine, which was 60–74% depending on the precursors
and catalyst used. This improved methodology will be highly
useful to researchers interested in performing rapid bioor-
thogonal couplings.
Entry R1
R2
Cat. Product
Yield
[%][a]
1
2
3
4
Ni
Zn
Zn
Zn
95
59
24
32
5[b]
Ni
58
6
7
8
9
Ni
Ni
Ni
Zn
68
66
41
43
10
Ni
70
11
Zn
Zn
40
36
12[c]
13
14
Ni
36
40
Zn
15
Zn
Zn
12
30
16[b]
[a] Yields reported after isolation by silica flash chromatography. [b] The
protective groups were lost during oxidative workup. [c] Reaction required
36 h.
Zn(OTf)2 for catalytic effect. In general we observed that zinc
salts gave higher yields for less active nitriles such as those
that were sterically hindered or affected by electron-donating
groups. On the other hand, more reactive nitriles benefited
from the use of nickel salts. However, there were exceptions
and it is suggested that both catalysts be tried when
attempting synthesis of new tetrazines. For the synthesis of
symmetric 3,6-dialkyl 1,2,4,5-tetrazines, yields ranged from
95% for 3,6-dibenzyl-1,2,4,5-tetrazine (entry 1) to 24% for
the sterically hindered 3,6-di-tert-butyl-1,2,4,5-tetrazine
(entry 3). The moderate yield for the latter tetrazine is
Given the lack of clarity on the mechanism of tetrazine
formation, and the large number of metal-coordinating
species involved such as hydrazine, nitriles, and amidrazones,
it is difficult to confidently propose the precise role of the
metal ion. It is likely that the metal acts as a Lewis acid by
coordinating to the nitrile and promoting nucleophilic
addition by hydrazine.[28,29,37,38] It is also plausible that the
2
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Table 3: Metal-catalyzed synthesis of tetrazines from aromatic nitriles
and formamidine.
Keywords: bioorthogonal synthesis · cycloaddition ·
heterocycles · metal catalysis · tetrazines
.
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Received: February 9, 2012
Published online: && &&, &&&&
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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Chemie
Communications
Synthetic Methods
Paving the way: The lack of convenient
synthetic methods is a significant road-
block to the broader use of 1,2,4,5-
tetrazines in bioorthogonal chemistry and
functional materials. Lewis acid metal
catalysts—most notably divalent nickel
and zinc salts— are described to catalyze
the one-pot synthesis of 1,2,4,5-tetrazines
directly from aliphatic nitriles (see
scheme).
J. Yang, M. R. Karver, W. Li, S. Sahu,
N. K. Devaraj*
&&&& — &&&&
Metal-Catalyzed One-Pot Synthesis of
Tetrazines Directly from Aliphatic Nitriles
and Hydrazine
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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