Tetrazine Assists Reduction of Water by Phosphines: Application in the Mitsunobu Reaction

Article abstract of DOI:10.1002/chem.201600913

Reaction of 3,6-disubstituted-1,2,4,5-tetrazines with water and PEt₃forms the corresponding 1,4-dihydrotetrazine and OPEt₃. Thus PEt₃, as a stoichiometric reductant, reduces water, and the resulting two reducing equivalent

Full text of DOI:10.1002/chem.201600913

DOI: 10.1002/chem.201600913
Full Paper
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Water Reduction |Hot Paper|
Tetrazine Assists Reduction of Water by Phosphines:
Application in the Mitsunobu Reaction
Alexander V. Polezhaev,[a] Nicholas A. Maciulis,[a] Chun-Hsing Chen,[a] Maren Pink,[a]
Richard L. Lord,[b] and Kenneth G. Caulton*[a]
Abstract: Reaction of 3,6-disubstituted-1,2,4,5-tetrazines
with water and PEt3 forms the corresponding 1,4-dihydrote-
trazine and OPEt3. Thus PEt3, as a stoichiometric reductant,
reduces water, and the resulting two reducing equivalents
serve to doubly hydrogenate the tetrazine. A variety of pos-
sible initial interactions between electron-deficient tetrazine
and electron-rich PR3, including a charge transfer complex,
were evaluated by density functional calculations which re-
vealed that the energy of all these make them spectroscopi-
cally undetectable at equilibrium, but one of these is never-
theless suggested as the intermediate in the observed redox
reaction. The relationship of this to the Mitsunobu reaction,
which absorbs the components of water evolved in the con-
version of alcohol and carboxylic acid to ester, with desirable
inversion at the alcohol carbon, is discussed. This enables
a modified Mitsunobu reaction, with tetrazine replacing
EtO2CN=NCO2Et (DEAD), which has the advantage that dihy-
drotetrazine can be recycled to tetrazine by oxidation with
O2, something impossible with the hydrogenated DEAD. For
this tetrazine version, a betaine-like intermediate is unde-
tectable, but its protonated form is characterized, including
by X-ray structure and NMR spectroscopy.
Introduction
(Scheme 1a,b). A positive application of this is the use of tetra-
zines as click reagents by reverse-electron-demand Diels–Alder
chemistry,[6] which condenses alkynes with tetrazines, with ex-
pulsion of N2 (Scheme 1c,d).[7] Thus every “vulnerability” can be
reimagined as a useful capability, for different purposes. Degra-
dation of coordinated tetrazines has been reported, including
hydrogenation[8] and replacement of N2 by ethylene.[9] Bis-pyri-
dyltetrazine is dramatically transformed by a mixture of CuII
and copper turnings, in the presence of water and ethylene, to
give CuI complexes of dihydrotetrazine. In addition, the ethyl-
ene also underwent exchange with the tetrazine, to effect the
click reaction and form the dihydropyridazine analogue, with
release of N2. The role of the ferrocene also present in these re-
actions was not established.[9] Ring-opening cleavage of tetra-
zines can follow four-electron reduction, in the presence of
protons, and the resulting imine can be hydrolyzed
Catalysis turnover number, that is, moles of products produced
per mole of catalyst, is often determined by catalyst degrada-
tion, and there is increasing recognition that catalyst degrada-
tion can often be prevented by recognition of functionality in
a ligand that is incompatible with catalytic reaction conditions.
Ligands will ultimately be oxidized by even weakly (e.g. N2O)
oxidizing reaction conditions,[1] alkane dehydrogenation will fi-
nally dehydrogenate ligand sp3 carbons,[2] or imine functionali-
ty will be attacked under nucleophilic reaction conditions.[3]
Catalyst degradation is already the subject of a review article[4]
and thus becomes subject to increasingly rational understand-
ing.
We have been exploring the electron-withdrawing function-
ality of the tetrazine ring as a component of metal complexes,
with a goal of using these rings to temporarily store electrons
in the ring p* orbitals during reductive substrate conversions.[5]
We begin to recognize that low-lying p* orbitals intended to
store electrons may also be subject to nucleophilic attack
[a] Dr. A. V. Polezhaev, N. A. Maciulis, Dr. C.-H. Chen, Dr. M. Pink,
Prof. Dr. K. G. Caulton
Department of Chemistry
Indiana University Bloomington
47405, Bloomington, IN (USA)
E-mail: caulton@indiana.edu
[b] Dr. R. L. Lord
Department of Chemistry
Grand Valley State University
49401, Allendale, MI (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600913.
Scheme 1. Scope of tetrazine reactivity.
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(Scheme 1e). These conversions have been observed in the
presence of metal ions, to stabilize the developing negative
charge on nitrogen, and those metals are the source of the re-
ducing equivalents.[8c]
Results
Reaction of dimethyltetrazine (Me2Tz, 1) with PEt3 and water in
a 1:2:3 mole ratio shows complete consumption of the dark
red tetrazine after 12 h at 708C in THF. The products are the di-
hydrodimethyltetrazine (H2Me2Tz, 2), as established by its
proton and 13C NMR spectra, and OPEt3. At the end of the
1:2:3 reaction, tetrazine is gone and PEt3 and water remain.
The phosphine oxide identity was established by comparison
Regarding tetrazine/nucleophyle compatibility it is perhaps
a surprise that the conversion of Equation (1) in Figure 1, a re-
duction of water, is thermodynamically favorable: phosphines
have the ability to reduce water (Equation (2), which has
DHo =À21.9 kcalmolÀ1). This reduction of water by phosphine
is then formally analogous to the water–gas shift reaction,
which converts CO and water to CO2 and H2. PR3 and CO are
the reducing agents, respectively. Indeed the generic reaction
in Equation (1) will be favorable if the sum of the two bond
dissociation energies of SÀH (S=substrate to be hydrogenat-
ed) merely exceeds 80.3 kcalmolÀ1; this depends on the fact
that the PO bond dissociation enthalpy is large (124.5 kcal
molÀ1).[10]
to an authentic sample (from PEt3 and O2) by 31P, H and GC/
1
mass spectroscopies. The H2Me2Tz is essentially colorless, and
1
its methyl H NMR signal is shifted to d=1.88 ppm from d=
1
2.92 ppm for Me2Tz, and it shows a H NMR signal, broadened
by quadrupolar nitrogen, for two protons at d=6 ppm. When
the dimethyltetrazine:PEt3:H2O mole ratio is changed to 2:1:3,
all PEt3 is consumed, but water and tetrazine remain; this clear-
ly establishes that dimethyltetrazine is now in excess and, by
integration of the products and unconsumed excess, the reac-
tion stoichiometry is established as 1:1:1 (Scheme 2).
Figure 1. Oxidation of phosphines with water.
For example, the use of these reducing equivalents is also
evident in the reaction[11] of Equation (3) in Figure 1, where
water is the source of the hydrogens in reduction of a disulfide.
The problem with accomplishing Equation (1) is that there is
no attractive mechanism: both PR3 and H2O are nucleophiles,
so there is no reasonable collision geometry to progress to-
wards products, and thus the transition state must lie very
high. However, the thermodynamic conclusion about Equa-
tion (2) is consistent with the fact that H2 does not reduce
R3PO. Instead a catalyst is required for Equation (2), or an inter-
mediate which can be attacked by either R3P or H2O to start
the reaction and perhaps itself be the substrate where the two
reducing hydrogens (Equation (1)) are deposited. In the course
of the study of possible reactions of phosphines with tetra-
zines, nitrogen heterocycles whose very low-lying p* orbital
energy is readily reducible by electron transfer, we discovered
reaction chemistry which reflects on the above reasoning, and
offers a new application for tetrazines.
Scheme 2. Reaction of Me2Tz with Et3P and water.
Control experiments show that there is no reaction between
either of the two components of this three-component reac-
tion under conditions identical to those employed above. In
this reaction tetrazine is reduced, the reducing equivalents
come from phosphine, and the hydrogens stabilize the re-
duced tetrazine. The aromaticity of the tetrazine is destroyed,
1
as is evident from the change in the methyl H NMR chemical
shift from the benzylic region in Me2Tz to the aliphatic region
in the H2Me2Tz. In spite of lost aromaticity, the reaction is ther-
modynamically favorable; this is Equation (1) in Figure 1,
where the 2 “H” reduce Me2Tz. It is established[12] that 1,4-dihy-
drotetrazine is more stable than 1,2-dihydrotetrazine, but iso-
merization between them (proton transfer) is facile.
The hydrogenated tetrazine 2 was established to be the 1,4
isomer by single-crystal X-ray diffraction (Figure 2). Colorless
crystals form from the reaction solution and a single-crystal X-
ray determination of their structure shows that the low solubil-
We report here an exploration of reactivity of tertiary phos-
phines with tetrazines, which takes some unusual turns, due to
participation by available active protons, and reveals the more
general reactivity of tetrazines as oxidants, but simultaneously
Brønsted bases. We then go beyond these observations to test
this capability as a potentially useful element for water absorp-
tion in the metal-free Mitsunobu reaction. Finally, this is then
followed by a more general perspective on what reaction con-
ditions should not be damaging to tetrazine ligands, hence
better defining overall reaction conditions which can be con-
sidered for reactivity applications by complexes of tetrazine-
containing ligands.
Figure 2. Mercury view of one molecule in the asymmetric unit of com-
pound 2. Selected structural parameters [ꢁ]: N1–N2 1.440(5), N3–N4
1.4502(10)- N1–C2 1.273(6), N4–C3 1.285(6), N2–C3 1.394(6), N3–C2 1.386(5).
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ity originates from intermolecular hydrogen bonding. The hy-
drogens were located, and freely refined, on nitrogens in para
positions, which then leaves the two imine functionalities
linked by NH groups. Curiously, from a single solution, two dis-
tinct polymorphs of this same compound crystallize separately,
and structure determinations on each show two molecules in
each asymmetric unit. In all four cases, the molecule has
a boat conformation with bow and stern being the amine ni-
trogens, and the amine hydrogens are cis. The six-membered
ring is distinctly nonplanar; the two planar imine functionali-
ties are folded at the two NH groups which they share. Hydro-
gen bonds go from NH to imine nitrogens in neighboring mol-
ecules. As shown in Figure S31 and S34 in the Supporting In-
formation, a least-squares fit of the two molecules in the asym-
metric unit shows only negligible deviations from each other.
Figure 3. Mercury view (50% probabilities) of the H4btzpMe·OPEt3 unit in
2H4btzpMe·2OPEt3·THF, with selected atom-labelling; only the NH hydrogens
are shown and methyl groups of OPEt3 are omitted. Unlabeled atoms are
carbon. Selected structural parameters [ꢁ]: N6–N7 1.441(3), N8–N9 1.431(3),
N6–C8 1.272(4), N9–C9 1.279(3), N7–C9 1.389(3), N8–C8 1.405(4), N7–H3
0.861(18), N8–H4 0.863(18).
Reduction of water with a bis-tetrazinyl pincer construct
Since our envisioned ligand application involved a pincer in-
corporating tetrazines at both ortho pyridine carbons, we also
investigated the reduction of a 2,6-di(6-methyltetrazin-3-yl)pyr-
idine (btzpMe, 3) (Scheme 3) with PEt3 and H2O. Two equiva-
lents of PEt3 and H2O were mixed with one equivalent of
btzpMe in THF and heated at 808C for 1 h, which effects com-
plete consumption of btzpMe. This reaction is thus considerably
faster than that with Me2Tz, showing the effect of an electron-
withdrawing pyridinyl substituent on the reactivity of the tetra-
zine. When the reaction is intentionally run at lower tempera-
ture, to detect intermediates, a species with only one tetrazine
produced, and instead reflects the stoichiometric demands of
hydrogen bonding. The two product molecules crystallize to-
gether, with tetrahydrofuran also in the unit cell. The dihydro-
tetrazine rings all adopt the boat conformation, as does
H2Me2Tz (Figure 2). The two NH functionalities directed inward
all hydrogen bond to either a phosphine oxide (Figure 3) or
a THF oxygen (see Supporting Information), consistent with
participation by two oxygen lone pairs on each oxygen. N···O
distances to the THF oxygen are 2.965 ꢁ and to phosphine
oxide oxygen are 3.004 ꢁ. The second phosphine oxide does
not sit within the pincer interior, but instead bridges outwardly
directed NH hydrogens, linking the two crystallographically in-
dependent H4btzpMe into a polymeric chain, with O/N distances
of 2.867 ꢁ and 2.898 ꢁ, respectively. Distances N9–C9 (1.276 ꢁ)
and C9–N7 (1.390 ꢁ) are consistent with double- and single-
bond character, respectively. The O2–P2 (1.474 ꢁ) bond length
is consistent with double-bond character. H4btzpEt was synthe-
sized by using the same procedure and isolated as an orange
oil in 89% yield after column chromatography.
1
reduced is readily detected by H NMR spectroscopy. A system-
atic study showed that tetrazines are more easily reduced by
outer sphere electron transfer (reversible CV) than are triazines,
and both nitrogen heterocycles show their reduction poten-
tials to be subject to traditional substituent effects, and linear
in their Hammett parameters.[13]
Scheme 3. Reduction of btzpMe (3).
A related reaction: two H and O scavenged by an N=N bond
We next discuss a reaction with clear relevance to the results
described above. The Mitsunobu reaction (Scheme 4) accom-
plishes the conversion of alcohol and a carboxylic acid to ester,
but the liberated water is diverted to a different fate: produc-
tion of R3PO and the product of hydrogenation of an N=N
bond.[14] Without consumption of water, the esterification is re-
versible. The utility of the Mitsunobu reaction is that it occurs
with cleavage of the alcohol CÀO bond, and with inversion at
that carbon, but the favorable thermodynamics of the reaction
rely on the logic described above in Equations (1) and (2) in
Figure 1. PR3 and the N=N bond make ester formation thermo-
dynamically even more favorable than when water is the prod-
uct.
Crystallization of the fully hydrogenated product (H4btzpMe
,
4) (from THF/pentane) yields a yellow-orange solid which con-
tains both products of the reaction in a single homogeneous
solid. Single-crystal X-ray diffraction establishes (Figure 3) the
chemical formula 2H4btzpMe·2OPEt3·THF, which is thus different
from the 1:2 product stoichiometry in Scheme 3.
The asymmetric unit of the solid contains two independent
phosphine oxides and two independent hydrogenated pincer
moieties. Each phosphine and each H4btzpMe is bisected by
a crystallographic mirror plane of symmetry perpendicular to
the pyridyl plane. The arms of both bis-tetrazinyl pyridines
have been doubly hydrogenated and these NH functionalities
all participate in hydrogen bonding in the unit cell. Stoichiom-
etry in the crystal lacks half of the phosphine oxide actually
The mechanism of the Mitsunobu reaction is still a subject
of debate[14] but multiple intermediates were detected and
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characterized by 31P NMR spectroscopy[15] and the main path
to the product is clear (Scheme 4). Briefly, the first step is for-
mation of the Morrison–Brunn–Huisgen (MBH) betaine B,
whose identity has been established by multinuclear NMR
spectroscopy and ESI-MS.[14] Next, betaine deprotonates the
carboxylic acid to form the ionic species C, which, upon react-
ing with the alcohol, forms the key alkoxyphosphonium salt D,
liberating the hydrazine. Alternatively, betaine B can react first
with the alcohol (depending on the order of addition) to lead
to the pentacoordinate phosphorane E; this phosphorane may
also react with acid forming alkoxyphosphonium salt D. Finally,
alkoxyphosphonium salt D undergoes SN2 displacement to
produce phosphine oxide and ester. Alkylphosphines were also
explored, in place of PPh3.[15] This mechanism clearly has rele-
vance to our tetrazine observations here.
Scheme 5. Proposed mechanism for tetrazine reduction.
the nucleophile PMe3 (as a simplified model for PEt3). Geome-
try optimizations and frequency calculations were performed
at the B3LYP-D3[16]/6-311+G(d,p) level of theory, modeling sol-
vation (THF) with the SMD model[17] as implemented in Gaussi-
an09.[18] The trends were also confirmed with M06-2X[19]/6-
311+G(d,p) calculations; see the Supporting Information for
details. A starting structure for geometry optimization with
five-coordinate P at covalent bond lengths from the two ring
carbons optimized to the p-complex F that is best character-
ized as a van der Waals complex. The p-complex is approxi-
mately Cs symmetric, with the mirror plane bisecting the ring
and containing the phosphorus atom (Figure 4, left). The free
tetrazine bond lengths of CÀN=1.34 ꢁ and NÀN=1.32 ꢁ do
not change significantly upon forming F.
Scheme 4. Mechanism of Mitsunobu reaction; d is the 31P chemical shift.[14, 15]
Proposed mechanism of phosphine-assisted tetrazine reduc-
tion with water
Since combining of trialkylphosphine or Ph3P with a tetrazine
in very dry conditions fails to change the 31P NMR spectrum
(shows only free phosphine), we sought evidence for a mecha-
nism proceeding through an endergonic pre-equilibrium
adduct, which is then trapped by acid. We propose that the re-
action starts from the phosphine lone pair attacking the p* of
the tetrazine ring with formation of p-complex F that rearrang-
es into s-complex G with the phosphorus atom attacking one
of the tetrazine nitrogens (Scheme 5). The amide charge in
complex G is stabilized by two imine functionalities.
Figure 4. Optimized structures of F (left) and G (right) from Scheme 5 with
important bond lengths [ꢁ] labeled.
Thermodynamically, the p-complex is enthalpically favored
by 3.2 kcalmolÀ1 compared to free PMe3 and Me2Tz. The en-
tropic penalty makes the reaction disfavored, however, with
DG= +7.7 kcalmolÀ1. This adduct F should therefore not
reach spectroscopically detectable concentration. As an alter-
native structure to F, we investigated s-complexes with the
phosphine attacking a ring C or N. The P-C attack product is
Protonation of the negatively charged nitrogen with water
protons leads to structure H and then conjugate base OHÀ at-
tacks phosphorus and finally forms dihydrotetrazine and Et3PO.
No intermediates were observed for Me2Tz or btzpMe reduction
1
using H and 31P NMR spectroscopy and we used DFT calcula-
tions to investigate the nature of possible intermediates F and
G. We also synthesized analogues of intermediate H by proto-
nation of tetrazine phosphine adduct G.
always disfavored, DH=5.4 kcalmolÀ1 and DG=20.6 kcalmolÀ1
,
making this species unlikely to be mechanistically relevant. The
P-N attack structure G is enthalpically favored by 1.8 kcalmolÀ1
but the entropic penalty of PMe3 and Me2Tz combining leads
to DG=13.1 kcalmolÀ1. This structure (Figure 4) has two short
C=N bonds (compare intermediate G Scheme 5), the nitrogen
bonded to P is distinctly pyramidal (consistent with localization
of a lone pair on that N). Both the van der Waals adduct and
DFT calculations
Regarding the mechanism in Scheme 5, we calculated mini-
mum energy structures for three possible 1:1 products from
the simple binary combination of the electrophile Me2Tz and
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the PÀN bonded structures lie within 5 kcalmolÀ1 of each
other, and represent possible intermediates in the phosphine-
mediated reduction of Me2Tz, but neither is predicted or ob-
served to form to a spectroscopically detectable extent. We
now describe how known synthetic organic methodology en-
lightens the mechanism downstream from this adduct, primari-
ly because the kinetically significant zwitterion is subject to
easy protonation at a ring nitrogen atom in the zwitterionic P-
N adduct G.
Tetrazinium salts
Since intermediates F and G (Scheme 5) do not reach detecta-
ble concentration we attempted to trap intermediate G. If
water is used as acid, strong nucleophilic OHÀ forms after pro-
tonation, making G undetectable because of fast conversion to
final products. Accordingly, we decided to use an acid that has
a non-nucleophilic conjugated base. When 1 equiv of strong
acid (Et2O·HBF4) was added to the 1:1 mixture of PPh3 and 3,6-
diphenyltetrazine (Ph2Tz, 5) in CD3CN, fast bleaching of tetra-
zine color was observed. 31P NMR spectroscopy shows only
one peak at d=38 ppm and confirmed complete conversion
Figure 5. Mercury view (50% probabilities) of the non-hydrogen atoms of
[Ph3PTzH]BF4·THF (6); THF and hydrogen atoms are omitted for clarity. Unla-
beled atoms are carbons. Selected structural parameters [ꢁ]: P1–N1
1.6575(15), N1–C2 1.415(2), N1–N2 1.461(2), N2–C1 1.294(2), N3–C2 1.278(2),
N3–N4 1.402(2), N4–C1 1.372(3).
1
of PPh3 into a single product. By H NMR spectroscopy, all sig-
carbons appear at d=163.6 and 142.7 ppm as a singlet and
a doublet with JP,C =8.1 Hz, respectively, because of splitting by
31P through two bonds. The Mitsunobu DEAD phosphine be-
taine (Scheme 4) was never characterized by X-ray diffraction
but a few examples of its protonated form are known.[21]
In contrast to the above, after addition of 1 equiv of
Et2O·HBF4 to an acetonitrile solution of a 1:1 mixture of PEt3
and compound 5, no color change was observed (Scheme 7).
Proton NMR spectroscopy shows unchanged signals of 3,6-di-
phenyltetrazine and formation of Et3PH+, whose acidic proton
appears as a doublet at d=5.8 ppm with a large JH,P =480 Hz;
the 31P NMR spectrum shows a peak at d=22.3 ppm, in agree-
ment with the literature value.[22] However, heating at 1008C
for 12 h in a sealed tube bleached the tetrazine color.
nals of both 3,6-diphenyltetrazine and PPh3 are gone and
a new set of aromatic signals appears (d=7.0–8.0 ppm) to-
gether with singlet at d=9.5 ppm due to the N-H proton of
product 6.
Removal of the solvent and recrystallization from Et2O/THF/
MeCN results in light yellow crystals suitable for X-ray analysis
in nearly quantitative yield (Scheme 6). The structure of the
compound is shown in Figure 5 and confirmed that phospho-
rus is attached to N1 in the tetrazine ring. A single-crystal
structure determination of this product shows it to have the
Scheme 6. Formation of tetrazinium salt with Ph3P.
composition [Ph3PTzH]BF4·THF, with the doubly derivatized
C2N4 ring in a boat conformation with H equatorial on N, and
with two C=N bonds in the ring. The nitrogen carrying phos-
phorus is coplanar with its three attached substituents (angle
sum 358.58), consistent with P=N double-bond character.[20]
Scheme 7. Formation of tetrazinium salt 7 with Et3P.
NMR spectroscopy after heating confirmed the absence of
starting material signals and formation of one major product
with a 31P NMR chemical shift of d=73 ppm. Equimolar acid
thus annihilates the nucleophilic character of the phosphine
by protonation of Et3P; this problem can be overcome by
using a “buffered” solution of phosphonium and conjugate
base phosphine. When 2 equiv of PEt3 was used (but only
1 equiv of acid), reaction was complete in 1 h at 258C with for-
À
The NH proton is involved in hydrogen bonding to one BF4
fluoride (N/F distance 2.863(2) ꢁ; angle NHF 160(2)8), and to
the THF. This fully confirms all conclusions from the NMR data.
The four nitrogen atoms are not coplanar so aromaticity of the
tetrazine ring is lost; the C1–N2 and C2–N3 bond lengths are
more than 0.1 ꢁ shorter than the C2–N1 and C1–N4 lengths,
confirming tetraza-1,4-cyclohexadiene character of the central
ring. 13C NMR spectroscopy shows chemical shifts consistent
with the proposed structure. Two inequivalent tetrazine ring
1
mation of a single product 7. H NMR spectroscopy shows an
acidic NH proton at d=9.31 ppm, a set of Ph ring protons at
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d=7.5–8 ppm, and CH2 and CH3 protons signals at d=2.1 and
1.3 ppm respectively. 13C NMR spectroscopy shows 14 carbon
signals, proving breaking of the symmetry of the tetrazine
ring; as in the PPh3 case, one of the tetrazine ring carbons ap-
pears as a doublet with JP,C =5.4 Hz. A NOESY experiment
shows a cross-peak between the N-H hydrogen atom and the
ortho-H atom of one close aryl ring, CH2 protons of ethyl
groups show strong coupling with ortho protons of the oppo-
site ring and a weak cross-peak with the same ortho protons
as N-H. No cross-peaks are observed between NH and CH2 hy-
drogens, proving that the phosphorus atom and NH protons
are in the 1 and 4 positions of the tetrazine ring, as in case of
PPh3.
Scheme 9. Reaction of btzpEt 8 with tBuNC.
Tetrazines as reagents for the Mitsunobu reaction
We have clear evidence that tetrazines can behave in a similar
manner to EtO2CN=NCO2Et (DEAD) in reaction with water and
phosphines, so we tested the ability of tetrazines to couple al-
cohol with acid under Mitsunobu conditions. The Mitsunobu
reaction, discovered in 1967,[24] is an important and well-devel-
oped process in SN2 chemistry.[14] This reaction is widely used
in organic synthesis and medicinal chemistry because of its
scope, stereospecificity, and mild reaction conditions. Disad-
vantages of the reaction are production of waste phosphine
oxide and hydrazine. In addition, DEAD is explosive, photosen-
sitive, toxic, shock sensitive, thermally unstable, and therefore
commercially available as a solution.[14,25] The final challenge is
isolation of the desired product from this three-component
mixture. Multiple attempts to solve these problems have been
reported.[16b,26] Phosphine-containing resins allow easy separa-
tion of phosphine oxide side products by filtration[26c] and
these could be regenerated by silanes, but a stable, easy sepa-
rable and recyclable alternative to DEAD is an important goal.
For example, Lipshutz and co-workers reported di-p-chloroben-
zylazodicarboxylate (DCAD), which in CH2Cl2 generates a readily
separable hydrazine byproduct.[27] Several aryldiazo com-
pounds were reported as effective alternatives to DEAD, and
produce readily separable arylhydrazine byproducts that can
be regenerated by oxidation with iodosobenzene diacetate[26a,b]
or even by oxygen, in the presence of porphyrin-based iron
catalyst.[26d]
Deprotonation of tetrazinium salt
Treatment of the above tetrazinium salt 6 with para-dimethyla-
minopyridine (DMAP) led to deprotonation with formation of
DMAPH+ and recovery of free phosphine and tetrazine
(Scheme 8). In contrast, no reaction is observed for the triethyl-
phosphonium analogue 7. The cation is clearly a stronger acid
when R=Ph vs. Et, and so DMAP can deprotonate only the
former cation. This means that stronger nucleophilic and basic
character of the phosphorus atom in Et3P vs. Ph3P translates to
stronger basic character of the tetrazine nitrogen atom in the
R3P/Tz adduct.
Scheme 8. Deprotonation of tetrazinium salts 6 and 7; n.r.: no reaction.
Tetrazines are potentially attractive for several reasons: they
are easily synthesized in one step from inexpensive starting
materials such as nitriles and hydrazine,[28] both electronic and
steric properties could be finely tuned by variation of ring-
carbon substituents, and dihydrotetrazine side products could
be easily reoxidized to tetrazines, sometimes with atmospheric
oxygen[12] or with NaNO2 and acid[28b,29] with high yields.
Tetrazine attack by a second nucleophile: tBuNC
The lack of binary (i.e. anhydrous) reaction of PEt3 with Me2Tz
prompted us to think of another potential ligand in this role,
to establish whether its conversion thermodynamics is better
than that of an electron-rich phosphine. Reaction of btzpEt (8)
with tBuNC (1:2 mole ratio), which contains oxidizable carbon,
was carried out in THF (Scheme 9). Reaction occurs immediate-
ly at 258C to give a product with C2v symmetry, containing
a new pyridyl AX2 spin system, new ethyl signals, and a new
tBu signal. Both 13C NMR and mass spectroscopies are consis-
tent with a structure in which the isonitrile has displaced two
N2 units, to yield two five-membered rings with formation of
a single diastereoisomer. Thus, under these mild conditions,
there is a low-energy mechanism which effects the analogous
[4 + 1] cycloaddition, then displacement of N2. This is thus
a “click” reaction,[23] which must be considered in future
chemistry involving free isocyanide with coordinated tetrazines.
Optimizing Mitsunobu applications
We tested several tetrazine/phosphine pairs in esterification of
2,2-diphenylacetic acid with 1-hexanol (Scheme 10, see Table 1
for reaction conditions). Consistent with all tetrazine/phos-
phine pairs we tested above, no betaine reaches detectable
concentration.
We find that activity in Mitsunobu esterification depends on
both the phosphine and the tetrazine substituents. PPh3 is uni-
formly less reactive than PEt3, probably because of the reversi-
bility of formation of intermediates F and G, as was shown
above by reaction with DMAP (Schemes 5, 8). If Me2Tz or Ph2Tz
are used in combination with PPh3 (Table 1, entries 1 and 2),
more than a 150 h at 808C are necessary for completion (no
decomposition or side products were detected under these
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Scheme 10. Tetrazine-assisted Mitsunobu reaction; R=Et, Ph; 1 R1 =R2 =Me;
5 R1 =R2 =Ph; 11 R1 =R2 =2-Pyridyl (Py2Tz); 12 R1 =Me, R2 =2-Pyridyl
(MeTzPy).
Table 1. Substituent effects on the rate of Mitsunobu reaction of 2,2-di-
phenylacetic acid with 1-hexanol.[a]
Scheme 11. Proposed mechanism of tetrazine-assisted Mitsunobu esterifica-
tion, showing observed 31P NMR chemical shifts.
Entry
R1
R2
R
Time
[h]
Temp
[oC]
Yield
[%]
1[b]
Me
Ph
Ph
Py
Py
Py
Py
Me
Ph
Ph
Me
Me
Py
Ph
Ph
Et
Ph
Et
150
150
12
1.5
12
80
80
20
80
20
80
20
92
40
89
60
86
84
85
2[b]
D* and H*, respectively. For PEt3/Py2Tz and PEt3/MeTzPy,
3[b,d]
4[c,e]
5[b,e]
6[c]
bleaching of tetrazine color occurs in 10 min and 1 h at room
1
temperature, respectively. H NMR spectroscopy confirms com-
plete reduction of the tetrazine ring and 31P NMR spectroscopy
shows the most intense signals at d=102 ppm assigned as in-
termediate D*,[15c,e] together with free PEt3 (d=À19 ppm) and
Et3P=O (d=49 ppm) and H* (d=69 ppm) (Figure 6); two addi-
tional hours were needed to accomplish disproportionation of
D*. This indicates that in the case of Et3P, alcoholysis of the tet-
razinium salt is fast and disproportionation of alkoxyphospho-
nium salt D* has become the rate-limiting step; this makes the
activity of PEt3/Py2Tz and PEt3/MeTzPy comparable with the
traditional reagent DEAD/PPh3. If the reaction is performed in
MeCN, H2Py2Tz precipitates from the reaction mixture, enabling
separation and reoxidation to Py2Tz in 85% yield based on
Py2Tz used.
Ph
Et
3
12
7[c,g]
Py
[a] Reactions were performed in a sealed NMR tube by mixing all four re-
agents in the appropriate solvent. NMR spectra were recorded immediate-
ly after mixing and monitored over time, yields were calculated from inte-
gral intensity of residual hexanol a-CH2 protons and hexyl ester a-CH2
signal. [b] In [D8]THF. [c] In CD3CN. [d] Bleaching of tetrazine color ob-
served after 1 h. [e] After 65 h at 258C, 5% conversion observed, then
heated. [f] Bleaching of tetrazine color observed after 10 min. [g] Bleach-
ing of tetrazine color; complete precipitation of dihydrotetrazine observed
after 1 h.
harsh conditions (see Figures S67–S70 in the Supporting Infor-
mation); however, for the Ph2Tz/PEt3 pair, bleaching of tetra-
1
zine color was observed in 1 h (Table 1, entry 3). H NMR spec-
troscopy shows complete conversion of tetrazine to dihydrote-
trazine, and 31P NMR spectroscopy shows disappearance of the
PEt3 signal at d=À19 ppm and new signals at d=102 ppm,
assigned as alkoxyphosphonium salt D* (Scheme 11) together
with an Et3P=O signal at d=49 ppm; however, another 1 h
was necessary for conversion of D* to Et3PO and ester
(Scheme 11, see Figures S71 and S72 in the Supporting Infor-
mation).
A pyridyl substituent on tetrazine dramatically increases the
reaction rate, even with PPh3 (Table 1, entries 4 and 6), the re-
action is complete in 3 h at 808C (10% conversion after 65 h
at 258C) for PPh3/Py2Tz compared to more than 150 h for
PPh3/Ph2Tz. We suggest that pyridine substituents act not only
as electron-withdrawing group, but also as a Brønsted base in
formation of H*. 31P NMR monitoring of the reaction mixture
for PPh3/Me2Tz and PPh3/Ph2Tz shows only signals of free Ph3P
at d= À5 ppm and Ph3P=O at d=25 ppm; no intermediates
were detected, showing formation of tetrazinium salt H* is the
rate-limiting step (see Figures S77 and S78 in the Supporting
Information).
Figure 6. 31P NMR monitoring of Mitsunobu reaction assisted with MeTzPy
(12) and Et3P. bottom) After 10 min at room temperature; middle) after 1 h
at room temperature; top) after 3 h at room temperature.
For PPh3/MeTzPy after 65 h at 258C, 31P NMR singlets at d=
62 and 38 ppm were detected and assigned as intermediates
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Impact of tetrazine ring substituent on phosphine attack re-
giochemistry
reason for its lower selectivity; the bulk of PPh3 does not ex-
plain this preference.
3-Methyl-6-pyridyltetrazine 12 was treated under the same
conditions. For both PEt3 and PPh3, two isomers were formed
with isomer I as the major form. Formation of an intramolecu-
lar hydrogen bond between the NH proton and the pyridine
nitrogen could play a role in the formation of significant
amounts of isomer J for PPh3 (consistent with the low basicity
of the Ph3P/Tz- vs. the Et3P/Tz-betaine), but H-bonded J re-
mains the minor product in each case. Unequivocal assign-
ment of isomer spectra were made using 2D NMR spectrosco-
py (Figure 7) (see Figures S81–S92 in the Supporting Informa-
tion).
To test the effect of substituents on regiochemistry of phos-
phine attack at the tetrazine ring, two unsymmetrical tetra-
zines 3-methyl-6-pyridyltetrazine (MeTzPy, 12) and (3-methyl-6-
phenyltetrazine (MeTzPh, 14) were synthesized. Two possible
regioisomers, structure I and J in Scheme 12, could form after
addition of phosphine and H+ (both isomers have two tauto-
meric forms, 1,2- and 1,4- depending on the position of the
NÀH bond, but we consider the 1,4-tautomer to be the more
stable, based on the discussion above).
Discussion
We were initially drawn into this study since use of tetrazines
as ligands[5] calls into question their compatibility with nucleo-
philes, such as phosphines and isonitriles. We envisioned that
phosphines might be oxidized by tetrazines, as they are by a-
diketones or ortho-quinones,[30] and just as they are in the use
of tetrazines to react with alkynes or olefins to expel N2. Our
work here shows that 1:1 adduct formation between tetrazine
and phosphine is at best thermoneutral; while such adducts
do not reach spectroscopically detectable concentrations, they
nevertheless are kinetically viable for subsequent reactions.
The polarized, even zwitterionic character of the adduct makes
it reactive towards protic reagents, and the oxophilicity of
phosphorus then plays a role in consuming the conjugate
base of any acid EO-H. More generally, tetrazines must be
thought of as oxidizing agents, a feature not typically associat-
ed with a nitrogen lone pair donor (i.e. a Lewis base). The tet-
razine in the reactions reported here thus serves as a mediator
of redox chemistry, necessary because PR3 and H2O alone have
no low-energy mechanism to reach their thermodynamic desti-
ny: OPR3 and H2 (Equation (2) in Figure 1). Finally, as envisioned
in the use of tetrazines as recyclable replacements for (stoichio-
metrically consumed) DEAD in the Mitsunobu methodology,
the hydrogenated tetrazine, when it is a ligand, can be consid-
ered as a place to temporarily store reducing equivalents. Stor-
age of protons at pendant amines is a currently attractive ap-
proach for the electrocatalytic reduction of water to H2.[31] Stor-
age of reducing hydrogen equivalents at tetrazines (i.e. imine
to amine) offers something very different: subsequent hydro-
genation of coordinated substrate.
Scheme 12. Reaction of phosphines and Et2O·HBF4 with an unsymmetrical
tetrazines. R=Ph, Et; 12 R1 =2-Pyridyl; 14 R1 =Ph; 15 R=Et, R1 =2-Pyridyl;
16 R=Ph, R1 =2-Pyridyl; 17 R=Et, R1 =Ph; 18 R=R1 =Ph. Ratio of isomers:
15I:J=4.6:1; 16I:J=2.6:1; 17I:J=2:1; 18I:J=1:0.
When 1 equiv of Et2O·HBF4 was added to the 1:1 mixture of
PPh3 and 3-methyl-6-phenyltetrazine 14 in CH3CN, fast bleach-
ing of tetrazine color was observed. 31P NMR spectroscopy
shows complete disappearance of the PPh3 signal and a signal
1
at d=36.2 ppm for one product. The methyl H NMR singlet is
shifted upfield from d=3.02 to 1.97 ppm, consistent with lost
tetrazine aromaticity. 2D NOESY NMR spectroscopy shows
a NOE contact between the NH hydrogen at d=8.89 ppm and
the protons of the methyl group, confirming that NH is ortho
to the tetrazine CH3. HMBC and HMQC are also consistent with
structure 18I, proving that phosphine selectively attacks the
more electrophilic tetrazine nitrogen (ortho to phenyl) (see Fig-
ures S99–S104 in the Supporting Information). Using PEt3 in-
stead of PPh3, two new 31P NMR signals in a 2:1 ratio appear at
d=72.14 and 72.18 ppm. Proton spectra show doubled signals,
the most characteristic being NH at d=8.83 and 8.81 ppm and
CH3 groups at d=2.23 and 2.10 ppm. HSQC and HMBC were
used for assignment of signals of two isomers (key HMBC con-
tacts are shown on Figure 7band then NOESY confirms that
the major isomer has NOE contact between the NH hydrogen
at d=8.83 ppm and protons of the methyl group at d=
2.10 ppm (Figure 7a, see Figures S93–S98 in the Supporting In-
formation). Stronger nucleophilicity of PEt3 could be the
Overall there is a remarkable similarity between the reactivi-
ty of the Group 5 representatives from transition series versus
main group series, V[5b] versus P: each trivalent Group 5 repre-
sentative is a reducing agent and each becomes a monoxide
(Scheme 13). The tetrazines are hydrogenated in each case.
The difference is that element–halogen bonds, which are pres-
ent only in the vanadium case, have their own characteristic
reactivity. Formal elimination of HCl between vanadium and
a dihydrotetrazine NH bond, followed by intramolecular redox
redistribution between doubly oxidized vanadium and doubly
reduced tetrazine, yields the observed closed shell singlet vana-
dium complex of tetravalent vanadium and tetrazinyl radical.
1
Figure 7. Selected 1H/1H NOE contacts (a) and HMBC H/13C contacts (b) for
isomers I and J.
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As summarized above[9] reducing agents PR3, Cu0, and perhaps
even ferrocene, in the presence of protons, can reduce tetra-
zines to dihydrotetrazines. The similarity is clearly dictated by
the thermodynamic preference for diverse reducing agents to
hydrogenate tetrazines to hydrotetrazines.
Scheme 14. Resonance stabilization of betaine.
to favor acid/alcohol condensation by trapping the water
product by the Mitsunobu mechanism.
Efficiency of the tetrazine/phosphine-assisted Mitsunobu re-
action strongly depends on the phosphine and tetrazine sub-
stituents. Regarding H-acceptor recycling in a Mitsunobu appli-
cation, donor groups (methyl) accelerate oxidation of dihydro
species to tetrazine by air but oxidizing the dihydro species
containing electron-withdrawing groups (phenyl and pyridyl)
requires stronger oxidizing agents such as NaNO2/H3CCO2H in
order to avoid long reaction times with air as the recycling oxi-
dant. Pyridyl-containing tetrazines in combination with Et3P are
the most effective. After completion of the reaction, dihydrodi-
pyridyltetrazine product can be filtered from the reaction mix-
ture and re-oxidized to the corresponding tetrazine with
NaNO2/AcOH, offering an easy and inexpensive way to recycle
one of the reagents.
Scheme 13. Reduction of tetrazine in vanadium complex.
Analogous results were reported[32] wherein Ph2Tz does not
react with a dihydroacridine hydride transfer reagent unless
stoichiometric Sc3+ is added, to bind to anionic nitrogen in the
product. Similarly, electron transfer from CoII to Ph2Tz does not
take place until Sc3+ is added, to stabilize the radical anion of
Ph2Tz. In every case, an electrophile is needed to stabilize the
reduced product in spite of the already highly electrophilic
character of Ph2Tz.
Conclusion
Experimental Section
The results reported here allow the following insights:
All reagents were commercial samples and used without further
purification. CDCl3, C6D6, CD3CN, and [D8]THF were purchased from
Cambridge Isotope Laboratory and dried over CaH2. Diethyl ether
and silica gel grade 62, 60–200 mesh, were purchased from EMD
Tetrazine is a redox mediator that enables transfer of PR3 re-
ducing power to the protons of water. The reaction sequence
continues with formation of R3PO and the dihydrotetrazine.
Molecular structures of H2Me2Tz (2) and H4btzpEt·Et3PO (4) un-
ambiguously prove the connectivity of the redox products.
A phosphorus nucleophile is converted into an amide nucle-
ophile (G in Scheme 5), thus a soft into a hard nucleophile,
and a weaker Brønsted base into a stronger one. Phosphine
does not react significantly with water, but the amide of be-
taine does. In the broadest sense, the betaine zwitterion con-
verts the basicity and nucleophilicity of the phosphine into
that of amide nitrogen. Only when electrophile (H+) is present,
formation of the protonated betaine (tetrazinium salt) was ob-
served. The molecular structure of Ph3P(Ph2Tz)H+ (Figure 5)
proves that phosphorus forms a double bond with N and the
positive charge is mostly on the trigonal-planar N atom of the
tetrazine ring.
1
and used as received. H, 13C, and 31P NMR spectra were collected
on a Varian 400 MHz instrument. 31P NMR was referenced against
85% H3PO4 (0.0 ppm). Mass spectra were collected using an Agi-
lent 1200 HPLC-6130 MSD and an Agilent 6890N Gas Chromato-
graph with 5973 Inert Mass Selective Detector. BtzpMe was synthe-
sized according to the published procedure.[5a] 3,6-Disubstituted
tetrazines 5, 11–13 were synthesized by using original procedures,
spectral data are in agreement with those in the literature.[29]
Unless otherwise noted, all manipulations were performed under
a nitrogen or argon atmosphere using standard Schlenk and glove-
box techniques or in J-Young NMR tubes (JY) with resealable
Teflon valve closure.
Synthesis of 3,6-dimethyl-s-tetrazine (Me2Tz) (1)
Following literature procedures,[28a] a 250 mL round-bottom flask
was loaded with 10.0 g of N2H4·H2O (0.200 mol) and 60.0 mL of ace-
tonitrile (1.4 mol). Then 10.0 mL of ethanol was added to make the
two liquids miscible. The solution was refluxed at 908C for 1 h,
giving a clear solution with a light-pink hue. The solution was
cooled to 508C and 1.02 g S8 (0.004 mol) was added, resulting in
a cascade of colors from blue to green to red-brown. The solution
was allowed to reflux at 908C for 3 h open to air. The solvent was
removed in vacuum, to yield an off-white solid that turned pink on
admission to air. Acetic acid (15 mL) and 10 mL of water were
added to the flask and it was cooled in an ice bath. To the cold
stirring solution was added 27 g NaNO2 (0.400 mol) in small por-
tions and it was allowed to stir for 1 h, yielding a bright pink solu-
tion. The remaining acidic solution was neutralized with NaHCO3
and the pink product was extracted with chloroform. Silica gel was
The rate of Mitsunobu reaction depends strongly on the
substituents in 3,6 positions of the tetrazine ring. Donor
groups (Me) disfavor reactivity by raising the LUMO of the tet-
razine. Nucleophilic attack of PR3 on DEAD is more favorable
than the corresponding attack on a tetrazine since the latter
costs the aromatic resonance energy of tetrazine. DEAD has
ester groups to stabilize and delocalize the amide charge local-
ization, so a fairer comparison might be to tetrazines bearing
electron-withdrawing groups. In the tetrazine-derived betaine
zwitterion G, the amide nitrogen carries two electron-with-
drawing imine groups, one of which is positioned suitably to
delocalize negative charge (Scheme 14). The thermodynamic
energy yield of the above redox reaction of PIII can be “spent”
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added to the chloroform solution and the solvent was removed in
vacuum. The dry, pink silica gel containing crude dimethyl-s-tetra-
zine was loaded onto a short column of silica and eluted with di-
ethyl ether to isolate dimethyl-s-tetrazine from unreacted S8. The
first colorless fraction was discarded (2,5-dimethyl-1,3,4-thiadia-
zole[33]) and the dark pink fraction containing dimethyl-s-tetrazine
was collected in a round-bottom flask. A yellow band correspond-
ing to S8 remained on the column. The concentrated dimethyl-s-
tetrazine solution was transferred to a sublimation apparatus and
the ether was removed in vacuo. Caution is required in this process
to avoid losing highly volatile dimethyl-s-tetrazine with diethyl
ether. The pink solid was carefully sublimed under static vacuum at
508C over 2 h, yielding dark pink crystals (0.395 g, 2%). 1H NMR
(258C, 400 MHz, [D8]THF): d=2.92 ppm (s, 6H, C-CH3); 13C NMR
(258C, 400 MHz, [D8]THF): d=20.73 (C-CH3), 167.88 ppm (C-CH3).
column and eluted with dry acetonitrile to remove OPEt3. The sub-
sequent orange–yellow band was collected and the solvent was re-
moved on a rotary evaporator to afford a red–orange oil (89%
yield). H NMR (258C, 400 MHz, CD3CN): d=1.12 (t, JH,H =8 Hz, 6H,
CH2CH3), 2.20 (q, JH,H =8 Hz, 4H, CH2CH3), 7.24 (s, 2H, N-H), 7.82 (t,
1
J
H,H =8 Hz, 1H, p-C-H), 7.97 (d, JH,H =8 Hz, 2H, m-C-H), 8.31 ppm (s,
2H, N-H); 13C NMR (258C, 125 MHz, CD3CN): d=10.73, 24.21,
122.90, 138.69, 147.06, 148.00, 152.95 ppm; MS: (APCI+): m/z: 300
(M + H).
Synthesis of 5,5’-(pyridine-2,6-diyl)bis(N-(tert-butyl)-3-ethyl-
4H-pyrazol-4-imine) (9)
An NMR tube was loaded with 0.020 g btzpEt (0.0678 mmol) and
0.6 mL of [D8]THF. To this was added 15 mL tBuNC (0.136 mmol).
After shaking the NMR tube for 45 s the color changed from fu-
schia to bright orange. Conversion of starting material to product
General procedures for investigating tetrazine and phos-
phine-assisted reduction of H2O
1
in quantitative yield was observed by NMR spectroscopy. H NMR
(500 MHz, 258C, [D8]THF): d=1.16 (br s, 18H), 1.25 (t, JH,H =5 Hz,
In a glovebox, stock solutions of 0.201m PEt3 in [D8]THF were pre-
pared by adding 0.0489 g PEt3 (0.414 mmol) to a 2 mL volumetric
flask. A stock solution of 0.201 Me2Tz in [D8]THF was prepared by
adding 0.0456 g Me2Tz (0.414 mmol) to a 2 mL volumetric flask.
The appropriate amount of solution was dispensed into the JY
tube for study of 2:1 to 1:2 of phosphine:tetrazine stoichiometry.
The solution was allowed to mix and stand for 12 h at room tem-
perature, followed by NMR assay. To the JY tube was next added
1 mL of water by syringe and the solution was freeze-pump-
thawed to remove dissolved oxygen. The JY tube was placed on
a mechanized rotator and allowed to mix for 12 h at room temper-
ature, followed by NMR assay. The JY tube was next placed in an
oil bath and heated to 708C for 12 h, during which time the red–
pink solution turned colorless, followed by NMR assay.
6H), 2.56 (br s, 4H), 7.79 (d, JH,H =5 Hz, 2H), 8.03 ppm (t, JH,H
=
10 Hz, 1H); 13C (125 MHz, 258C, [D8]THF): d=11.6, 20.4, 30.9, 61.8,
125.5, 139.1, 152.8, 153.8, 160.8, 166.0 ppm; MS: (APCI+): 406 (M+
H); MS: (ESI+): 428 (M+Na); MS: (ESI+): m/z: 933 (2M+Na).
Synthesis of 3,6-diphenyltetrazine (Ph2Tz, 5)[29a]
In a 50 mL round-bottom flask, benzonitrile (2.1 mL, 20 mmol) was
dissolved in 15 mL of absolute ethanol. Hydrazine monohydrate
(1 mL, 20 mmol) and sulfur (0.4 g) were quickly added, and the so-
lution was refluxed for 4 h. Volatiles were removed and the remain-
ing orange solid was washed with cold ethanol (2ꢂ10 mL), filtered,
and dried, giving the crude dihydrotetrazine. The crude flaky
orange solid of was placed in a 50 mL beaker and dissolved in
15 mL of acetic acid/water (3:1) at room temperature with stirring.
Solid NaNO2 was added portionwise to the solution at room tem-
perature until color changes stopped and then stirred for another
1 h. Purple cloudiness as well as the evolution of brown nitric
oxide gas signifies the completion of the reaction. The deep
purple solid was filtered using a glass frit and recrystallized from
75 mL of CH2Cl2/hexane 1:10. The first recrystallization gave
230 mg of 2,5-diphenyl-1,3,4-thiadiazole as a white solid (1H NMR
(400 MHz, CDCl3): d=8.06–7.96 (m, 4H), 7.55–7.44 (m, 6H), MS
(ESI+): m/z: 239.3 (M+H).[16c] The mother liquor was concentrated
to give 1.25 g (51%) of pure 3,6-diphenyltetrazine 5 as purple nee-
Procedure for NMR assay of reaction of btzpMe + 2H2O +
2PEt3 in [D8]THF
In a glovebox, 0.0112 g btzpMe (0.0379 mmol) was added to a vial.
To this was added 0.4 mL of [D8]THF and the mixture was dis-
pensed into a JY tube. To 0.0090 g PEt3 (0.0756 mmol) weighed
out in a 20 mL vial, was added 0.4 mL of [D8]THF. This solution was
dispensed into the JY tube containing the btzpMe solution. The JY
tube was capped, taken out of the glovebox and opened to add
1.4 mL of deionized H2O (0.0756 mmol). The solution was freeze–
pump–thawed to remove any dissolved oxygen and was heated at
808C for 1 h, during which time the heterogeneous pink solution
became homogeneous and turned yellow–orange in color. The so-
lution was poured into a 20 mL scintillation vial, layered with an
equal amount of pentane, and stored in the dark at À358C to
1
dles. H NMR (258C, 400 MHz, [D8]THF): d=7.46 (m, 6H), 8.48 ppm
(m, 4H); 13C NMR (258C, 101 MHz, [D8]THF): d=163.9, 132.4, 132.1,
123.0. 127.5 ppm; MS: (ESI+): m/z: 235.3 (M+H).
Synthesis of 2,6-bis(6-ethyl-1,2,4,5-tetrazin-3-yl)pyridine (8)
1
yield orange crystals. H NMR (258C, 400 MHz, [D8]THF): d=1.80 (s,
6H, C-CH3), 7.74 (t, JH,H =8.0 Hz, 1H, p-CH), 7.87 (s, 2H, N-H), 7.95
(d, JH,H =8.0 Hz, 2H, m-CH), 8.86 ppm (s, 2H, N-H).
A 2000 mL two-neck round-bottom flask equipped with a stir bar
and a reflux condenser was loaded with 4.00 g dicyanopyridine
(31.0 mmol) and 200 mL of ethanol. To the stirring heterogeneous
solution was added 80 mL of N2H4·H2O followed by an additional
80 mL of ethanol. The stirring solution became homogeneous and
was heated to 558C for 2 h during which time a white precipitate
formed. To the heterogeneous solution was added 200 mL of pro-
pionitrile and an additional 20 mL of ethanol to rinse out residual
propionitrile into the reaction flask. To this was added 1.20 g S8
(4.69 mmol) and the solution instantly turned green/brown. The
solution was heated at 808C for 16 h during which time the solu-
tion turned an orange–red color with sulfur subliming on the con-
denser. The ethanol, hydrazine, and water were distilled off using
a rotary evaporator. The flask containing the red oil was immersed
Synthesis of 2,6-bis(6-ethyl-1,4-dihydro-1,2,4,5-tetrazin-3-yl)-
pyridine H4btzpEt (4)
To a 100 mL Schenk flask equipped with a stir bar under an argon
atmosphere was added 0.10 g btzpEt (0.34 mmol), 45 mL of THF,
and 0.012 g H2O (0.68 mmol). To the stirring solution was added
0.080 g PEt3 (0.68 mmol) by syringe. The reaction flask was heated
to 808C for 1 h during which time the red–pink solution turned
yellow–orange. Following vacuum removal of all volatiles, includ-
ing unreacted PEt3 and water, at 808C for 30 min, the remaining
red–orange oil was dissolved in acetonitrile and run on a silica
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Full Paper
in an ice bath and to the flask was added 25 mL of acetic acid.
This was diluted with 15 mL of deionized water. To the orange stir-
ring solution was added NaNO2 in small portions until the bub-
bling stopped, during which time the solution turned dark red.
The remaining acid was quenched with a sodium bicarbonate solu-
tion. The crude product was extracted with chloroform and the sol-
vent was distlled off using a rotary evaporator. The remaining red
silica gel was loaded onto a column filled with 30 cm of silica pre-
pared in dichloromethane. The first 200 mL of dichloromethane
was discarded as it contained sulfur. The red-colored, crude prod-
uct was washed from the column with acetonitrile and collected
into a 1000 mL round-bottom flask. The solvent was removed in
vacuo and redissovled in a minimum amount of dichloromethane
followed by layering with hexane to crash out btzpEt. The red solu-
tion containing diethyltetrazine was decanted off and the remain-
ing pink solid was placed under vacuum overnight to remove any
residual diethyltetrazine resulting in 2.8 g of btzpEt 8 (29% yield).
1H NMR (258C, 500 MHz, [D8]THF): d=8.76 (d, JH,H =8 Hz, 2H), 8.32
(t, JH,H =8 Hz, 1H), 3.42 (q, JH,H =8 Hz, 4H), 1.55 ppm (t, JH,H =8 Hz,
6H); 13C NMR (258C, 125 MHz, [D8]THF): d=173.07, 165.92, 154.20,
140.24, 127.26, 30.01, 13.20 ppm; MS: (ESI+): m/z: 292.2; (APCI+):
m/z: 292.2.
crystallization from hexane 340 mg (10%) of 3-phenyl-6-methylte-
trazine 14 were obtained as violet crystals. 1H NMR (258C,
400 MHz, CD3CN): d=8.51 (m, 2H), 7.65 (m, 3H), 3.01 ppm (s, 3H);
13C NMR (258C, 101 MHz, [D8]THF): d=167.2, 164.1, 132.5, 131.8,
129.2, 127.9, 21.1 ppm; MS: (ESI+): m/z: 173.2 (M+H).
Synthesis of 3,6-diphenyl-1-(triphenyl-l5-phosphanylidene)-
1,4-dihydro-1,2,4,5-tetrazin-1-ium tetrafluoroborate (6)
Triphenylphosphine (26.2 mg, 0.1 mmol) and 3,6-diphenyltetrazine
(23.4 mg, 0.1 mmol) were mixed in 5 mL of CH3CN to give a violet–
pink solution. Et2O·HBF4 (13.7 mL, 0.1 mmol) was added to the mix-
ture at room temperature and bleaching was observed. After
10 min the solution turned light yellow. After stirring for an addi-
tional hour, volatiles were removed under high vacuum and a vis-
cous light-yellow oil was formed. The oil was dissolved in a mini-
mum amount of THF and diethyl ether was added to cause precip-
itation. The solution was left overnight in a fridge at À358C to
give a light-yellow powder, liquids were decanted, and the residue
was dried in vacuum. Yield: 95%. Crystals suitable for X-ray analysis
were grown by slow diffusion of ether vapors into THF:MeCN.
1H NMR (258C, 400 MHz, CD3CN): d=9.52 (s, 1H, N-H), 7.93 (m,
6H), 7.82 (t, JH,H =6.6 Hz, 3H), 7.64 (m, 7H), 7.45 (m, 4H), 7.26 (t,
J
H,H =7,5 Hz, 1H), 7.22 (d, JH,H =7.6 Hz, 2H), 7.09 ppm (t, JH,H =
Synthesis of 3-(2-pyridyl)-6-methyltetrazine (MeTzPy, 12)
and 3,6-di(2-pyridyl)tetrazine (Py2Tz, 11)[29b]
7.5 Hz, 2H); 31P NMR (258C, 163 MHz, CD3CN): d=38.0 ppm;
19F NMR (258C, 376 MHz, CD3CN): d=À151.6 (s, 1F), À151.7 ppm
(s, 3F); 13C NMR (258C, 101 MHz, CD3CN): d=163.6, 142.7 (d, JC,P
=
To 22.08 g (20 mmol) of 2-cyanopyridine dissolved in 25 mL of eth-
anol, was added 40 mL of hydrazine monohydrate while stirring.
The solution was heated to reflux for 4 h. The solution was cooled
to 508C and 100 mL of MeCN and 1.5 g of sulfur powder was
added rapidly and an immediate color change to greenish/brown-
ish, as well as bubbling, was observed. The reaction was allowed
to stir at reflux for 12 h, after which time a dark orange solution
was observed. The solution was cooled to room temperature and
the solvent was removed under vacuum. To the residual slurry was
added 100 mL of a 3:2 mixture (v/v) of CH3COOH:H2O and stirred
to dissolution. NaNO2 was added in small spatula amounts and the
solution turned red then dark pink with evolution of brownish gas.
The solution was allowed to stir for 1 h to allow for gas evolution.
Residual acetic acid was neutralized by dropwise addition of a con-
centrated solution of NaHCO3 in H2O until evolution of CO2 ceased.
The resulting solution was extracted with 3ꢂ80 mL CHCl3. The
crude product was placed on a SiO2 column (60–230 mesh) packed
with hexane. The first fraction was collected with CHCl3/hexane 1:1
and discarded. After this collection, elution with CHCl3 allowed for
collection of the 3-(2-pyridyl)-6-methyltetrazine 12, then elution
with CHCl3/CH3CN 10:1 afforded 3,6-di(2-pyridyl)tetrazine 11. Both
compounds obtained were recrystallized from CH2Cl2/hexane. Yield
1.6 g (46%) of MeTzPy 12 as shiny violet flakes and 0.26 g (18%) of
8.1 Hz), 136.1, 134.8 (d, JC,P =11.3 Hz), 133.16, 131.99, 130.8, 130.4
(d, JC,P =13.6 Hz), 129.2 (d, JC,P =24.2 Hz), 127.7 (d, JC,P =6.4 Hz),
127.57, 118.55 ppm; MS: (ESI+): m/z: 497.6; (APCI+): m/z: 497.6.
Synthesis of 3,6-diphenyl-1-(triethyl-l5-phosphanylidene)-
1,4-dihydro-1,2,4,5-tetrazin-1-ium tetrafluoroborate (7)
3,6-Diphenyltetrazine (23,4 mg, 0.1 mmol) and triethylphosphine
(29 mL, 0.2 mmol) were mixed in 5 mL of CH3CN to give a violet–
pink solution. Et2O·HBF4 (13.7 mL, 0.1 mmol) was added to the mix-
ture at room temperature and bleaching was observed. After
30 min the solution turned light-yellow. After stirring for an addi-
tional one hour, the volatiles were removed under high vacuum
and a viscous light-yellow oil formed. The oil was dissolved in
a minimum amount of THF, and diethyl ether was added to cause
precipitation. The solution was left overnight in a fridge at À358C
to give a light-yellow sticky solid. The liquids were decanted and
the residue was dried in vacuum to afford a light-yellow powder.
1
Yield: 92%. H NMR (258C, 400 MHz, CD3CN): d=9.31 (s, 1H), 7.87
(d, JH,H =7.4 Hz, 2H), 7.67 (d, JH,H =6.5 Hz, 2H), 7.63–7.52 (m, 6H),
2.26 (dq, JP,H =11.8, JH,H =7.6 Hz, 6H), 1.25 ppm (dt, JP,H =19.7, JP,H
=
7.6 Hz, 9H); 31P NMR (258C, 163 MHz, CD3CN): d=73.1 ppm;
19F NMR (258C, 376 MHz, CD3CN): d=À151.6 (s, 1F), À151.7 ppm
(s, 4F); 13C NMR (258C, 101 MHz, CD3CN): d=163.3 (s), 143.0 (d,
1
Py2Tz 11 as deep red crystals. 12: H NMR (258C, 400 MHz CDCl3):
d=8.92 (m, 1H), 8.60 (t, JH,H =7.8 Hz, 1H), 7.95 (dt, JH,H =8.0 Hz,
J
C,P =5.4 Hz), 133.1 (s), 131.9 (s), 131.6 (s), 129.9 (s), 129.4 (s), 128.1
J
H,H =2.0 Hz, 1H), 7.53 (dd, JH,H =7.5 Hz, JH,H =4.7 Hz, 1H), 3.13 ppm
(s, 3H); 13C NMR (258C, 101 MHz, CDCl3): d=168.1, 163.6, 150.8,
150.2, 137.4, 126.3, 123.8, 21.3 ppm; MS (ESI+): m/z: 174.2 (M+H),
196.1 (M+Na). 11: 1H NMR (258C, 400 MHz CDCl3): d=8.97 (d,
(s), 127.8 (s), 127.2 (s), 14.8 (d, JP,H =55.8 Hz), 5.08 ppm (d, JP,H
5.1 Hz); MS: (ESI+): m/z: 353.2; (APCI+): m/z: 353.2.
=
Synthesis of 3-methyl-6-(2-pyridyl)-1-(triethyl-l5-phosphany-
lidene)-1,4-dihydro-1,2,4,5-tetrazin-1-ium tetrafluoroborate
(15)
J
H,H =4.8 Hz, 2H), 8.73 (d, JH,H =7.9 Hz 2H), 7.99 (t, JH,H =7.9 Hz, 2H),
7.57 ppm (dd, H,H =7.7 Hz,
101 MHz, CDCl3): d=163.7, 150.9, 149.9, 137.3, 126.4, 124.4 ppm;
MS: (ESI+): m/z: 237.1 (M+H), 259.1 (M+Na).
J
J
H,H =4.8 Hz, 2H); 13C NMR (258C,
3-(2-Pyridyl)-6-methyltetrazine (17.3 mg, 0.1 mmol) and Et2O·HBF4
(13.7 mL, 0.1 mmol) were mixed in 5 mL of CH3CN to give a deep
red solution. Triethylphosphine (29 mL, 0.2 mmol) was added to the
mixture at room temperature and bleaching was observed. After
2 h solution turned to light-yellow. After stirring for additional
Synthesis of 3-phenyl-6-methyltetrazine (MeTzPh, 14)[28b]
Following the procedure outlined above, starting from 2.06 g
(20 mmol) of benzonitrile, after elution with CH2Cl2/hexane and re-
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12 h, volatiles were removed under high vacuum and a light-
yellow viscous oil was formed. The oil was dissolved in minimum
amount of diethyl ether, and pentane was added to cause precipi-
tation. The solution was left overnight in a fridge at À358C to give
a light-yellow sticky solid, the mother liquor was decanted and the
residue was dried under vacuum to yield a yellow gum. Yield:
a fridge at À358C, the mother liquid was decanted, and a light-
yellow gum on the walls was dried under vacuum affording a light
yellow powder. Yield: 85%. 1H NMR (258C, 500 MHz, CD3CN):
major: d=8.83 (s, 1H), 7.64 (m, 2H), 7.56 (m, 3H), 2.21 (dq, JP,H
11.3, JH,H =7.6 Hz, 6H), 2.10 (d, JP,H =0.9 Hz, 3H), 1.24 ppm (dt, JP,H
=
=
19.7, JP,H =7.6 Hz, 9H); minor: d=8.81 (s, 1H), 7.82 (m,2H), 7.55 (m,
2H), 7.46 (m, 1H), 2.59 (dq, JP,H =11.7, JH,H =7.6 Hz, 6H), 2.23 (s,
3H), 1.33 ppm (dt, JP,H =19.5, JP,H =7.6 Hz, 9H); 31P NMR (258C,
163 MHz, CD3CN): major: d=72.14 ppm; minor: d=72.18 ppm;
19F NMR (258C, 376 MHz, CD3CN): d=À151.7 ppm; 13C NMR (258C,
126 MHz, CD3CN): major: d=164.63, 141.65, 132.53, 131.60. 129.63,
127.73, 14.67, 14.39 (d, JP,H =56.0 Hz), 4.89 ppm (d, JP,H =5.1 Hz);
minor: d=(158), 140.81, 131.89, 130.4, 129.18, 127.4, 18.37, 15.24
(d, JP,H =30.4 Hz), 5.01 ppm (d, JP,H =5.3 Hz); MS: (ESI+): m/z: 291.2;
(APCI+): m/z: 291.2.
90%. 1H NMR (258C, 500 MHz, CD3CN): major: d=8.68 (d, JH,H
=
4.8 Hz, 1H), 8.65 (s, 1H), 8.05 (JH,H =8.0 Hz, 1H), 7.99–7.95 (m, 1H),
7.59 (dd, JH,H =7.2, 5.6 Hz, 1H), 2.47 (dq, JP,H =11.7, JH,H =7.6 Hz,
6H), 2.08 (s, 3H), 1.25 ppm (dt, JP,H =19.3, JH,H =7.6 Hz, 9H); minor:
d=9.17 (s, 1H), 8.13 (d, JH,H =7.9 Hz, 1H), 7.95 (m, 1H), 7.63–7.61
(m, 1H), 2.62 (dq,
JP,H =11.8, JH,H =7.6 Hz, 6H), 2.25 (s, 3H),
1.34 ppm (dt, JP,H =19.8, JH,H =7.6 Hz, 9H), 31P NMR (258C, 163 MHz,
CD3CN): major: d=71.66 ppm; minor: d=73.47 ppm; 19F NMR
(258C, 376 MHz, CD3CN): d=À151.7 ppm; 13C NMR (258C,
126 MHz, CD3CN): major: d=159.87 (d, JP,C =5.5 Hz), 149.29, 148.28,
138.39, 126.34, 123.17, 16.64 (d, JP,C =59.7 Hz), 14.89 (d, JP,C
10.3 Hz), 5.31 ppm (d, JP,C =5.4 Hz); selected signals for minor: d=
147.96, 145.31, 140.29, 137.92, 127.19, 122.44, 18.40. 14.93 (d, JP,C
56.2 Hz), 5.00 ppm (d, JP,C =5.2 Hz); MS (ESI+): m/z: 292.2; (APCI+):
m/z: 292.2.
=
Synthesis of 3-methyl-6-phenyl-1-(triphenyl-l5-phosphanyli-
dene)-1,4-dihydro-1,2,4,5-tetrazin-1-ium tetrafluoroborate
(18)
=
3-Phenyl-6-methyltetrazine (17.2 mg, 0.1 mmol) and triphenylphos-
phine (26.2 mg, 0.1 mmol) were mixed in 5 mL of CH3CN to give
a violet–pink solution. Et2O·HBF4 (13.7 mL, 0.1 mmol) was added to
the mixture at room temperature and bleaching was observed.
After 30 min the solution turned light-yellow. After stirring for an
additional 12 h, volatiles were removed under high vacuum and
a viscous light-yellow oil had formed. The oil was dissolved in
a minimum amount of THF and diethyl ether was added to cause
precipitation. The solution was left overnight in a fridge at À358C
to give a light-yellow sticky solid, the mother liquor was decanted,
and the residue was dried in vacuum to give a light-yellow
powder. Yield: 90%. 1H NMR (258C, 500 MHz, CD3CN): d=8.99 (s,
1H), 7.97–7.81 (m, 9H), 7.67 (m, 6H), 7.24 (m, 3H), 7.13–7.07 (m,
2H), 1.97 ppm (d, JP,H =1.0 Hz, 3H); 31P NMR (258C, 163 MHz,
CD3CN): d=36.2 ppm; 19F NMR (258C, 376 MHz, CD3CN): d=
À151.7 ppm; 13C NMR (258C, 101 MHz, CD3CN): d=165.19, 141.35,
Synthesis of 3-methyl-6-(2-pyridyl)-1-(triphenyl-l5-phospha-
nylidene)-1,4-dihydro-1,2,4,5-tetrazin-1-ium tetrafluorobo-
rate (16)
3-(2-Pyridyl)-6-methyltetrazine (17.3 mg, 0.1 mmol) and triphenyl-
phosphine (26.2 mg, 0.1 mmol) were mixed in 5 mL of CH3CN to
give a violet–pink solution. Et2O·HBF4 (13,7 mL, 0.1 mmol) was
added to the mixture at room temperature and bleaching was ob-
served. After 30 min the solution turned yellow–orange. After stir-
ring for an additional 12 h, the volatiles were removed under high
vacuum to give a light-yellow viscous oil. The oil was dissolved in
a minimum amount of diethyl ether, and pentane was added to
cause precipitation. The solution was left overnight in a fridge at
À358C to give a light-yellow sticky solid, the mother liquor was
decanted, and the residue was dried under vacuum yielding an
1
orange gum. Yield: 90%. H NMR (258C, 500 MHz, CD3CN): d=7.4–
136.00. 134.76 (d, JC,P =11.2 Hz), 132.12, 130.62, 130.33 (d, JC,P
=
8.0 ppm (m); selected signals for major: d=9.45 (s, 1H,NH), 8.67,
(ddd, JH,H =0.9, JH,H =1.5, JH,H =4.9, 1H), 1.66 ppm (s, 3H, CH3);
minor: d=8.94 (s, 1H, NH), 1.99 ppm (s, 3H, CH3), 31P NMR (258C,
163 MHz, CD3CN): major: d=37.69 ppm; minor: d=38.28 ppm;
19F NMR (258C, 376 MHz, CD3CN): d=À151.6 ppm; 13C NMR (258C,
126 MHz, CD3CN): major: d=156.17, 149.13, 139.97, 138.57, 136.38,
135.17, 134.73 (d, JP,H =11.2 Hz), 130.60 (d, JP,H =13.6 Hz), 127.48,
122.40. 118.11 (d, JP,H =103.1 Hz), 19.06 ppm (s); selected signals for
15.7 Hz), 128.91, 127.44, 118.56, 15.37 ppm; MS: (ESI+): m/z: 435.2;
(APCI+): m/z: 435.2.
Deprotonation of tetrazinium salts
Triphenylphosphine (26.2 mg, 0.1 mmol) and 3,6-diphenyltetrazine
(23,4 mg, 0.1 mmol) were mixed in 0.5 mL of CD3CN to give
a violet–pink solution. Et2O·HBF4 (13.7 mL, 0.1 mmol) was added to
the mixture at room temperature and bleaching was observed for
minor: d=147.00. 144.63, 134.45 (d, JP,C =10.8 Hz), 129.72 (d, JP,C
=
13.8 Hz), 125.59, 122.76, 120.19 (d, JP,C =106.5 Hz), 14.83 ppm; MS:
(ESI+): m/z: 436.2; (APCI+): m/z: 436.2.
1
10 min; H and 31P NMR spectroscopy confirmed the formation of
6. Then 12.2 mg (0.1 mmol) of DMAP was added to the NMR tube
Synthesis of 3-methyl-6-phenyl-1-(triethyl-l5-phosphanyli-
dene)-1,4-dihydro-1,2,4,5-tetrazin-1-ium tetrafluoroborate
(17)
1
and a violet–pink color was immediately observed. H and 31P NMR
spectra show the appearance of free Ph3P and tetrazine (60% con-
version), after 3 h another 6 mg (0.05 mmol) was added to the
tube and NMR confirmed completion of the reaction. Free diphe-
nyltetrazines crystallyzed out from the solution as deep-violet nee-
dles which were separated from the mother liquor, washed with
pentane, and dried. Overall 15 mg (64%) of 5 was recovered.
3-Phenyl-6-methyltetrazine (17.2 mg, 0.1 mmol) and triethylphos-
phine (29.2 mL, 0.2 mmol) were mixed in 5 mL of CH3CN to give
a violet–pink solution. Et2O·HBF4 (13.7 mL, 0.1 mmol) was added to
the mixture at room temperature and bleaching was observed.
After 30 min the solution turned light yellow. After stirring for an
additional 12 h, the volatiles were removed under high vacuum
and a viscous light-yellow oil formed. The oil was dissolved in
a minimum amount of diethyl ether and pentane was added to
give a milky-white solution. The solution was left overnight in
3,6-Diphenyl-1-(triethyl-l5-phosphanylidene)-1,4-dihydro-1,2,4,5-
tetrazin-1-ium tetrafluoroborate (20 mg) was dissolved in 0.5 mL of
CD3CN than 1 equiv of DMAP was added to the mixture. No color
1
changes were observed. H and 31P NMR confirmed no reaction in
48 h.
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General conditions for Mitsunobu reaction between 2,2-di-
phenylacetic acid and 1-hexanol catalyzed with tetrazine/
phosphine
Phosphine (0.1 mmol) and tetrazine (0.1 mmol) were mixed in
0.6 mL of dry and degased [D8]THF or CD3CN in an NMR tube than
2,2-Diphenylacetic acid (22.1 mg, 0.1 mmol) and 1-hexanol (12.5 mL,
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Regeneration of Py2Tz 11 after Mitsunobu reaction
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Triethylphosphine (15 mL, 0.1 mmol) and 3,6-di(2-pyridyl)-s-tetrazine
11 (23 mg, 0.1 mmol) were mixed in 0.6 mL of dry and air-free
CH3CN, then 2,2-diphenylacetic acid (22.1 mg, 0.1 mmol) and 1-
hexanol (12.5 mL, 0.1 mmol) were added. The reaction mixture was
bleached after 1 h and stirring was continued for another 24 h re-
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decanted, the yellow precipitate was washed with pentane, and
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water/AcOH mixture and several crystals of NaNO2 were added.
The reaction mixture turned from yellow to deep violet and dark
precipitate was formed. The reaction mixture was diluted with
4 mL of water, neutralized with conc. NaHCO3, and extracted with
dichloromethane. Phases were separated and evaporation of vola-
tiles afforded 19.5 mg of Py2Tz 11 (85% based of Py2Tz).
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Acknowledgements
The authors are grateful to the Indiana University, Bloomington
NMR Facility and Dr. Eric Twum for his assistance in 2D NMR
experiments.
Keywords: Mitsunobu reaction · phosphine oxidation · redox-
noninnocent ligands · tetrazines · water reduction
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Received: February 25, 2016
Published online on && &&, 0000
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FULL PAPER
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Water Reduction
A. V. Polezhaev, N. A. Maciulis, C.-H. Chen,
M. Pink, R. L. Lord, K. G. Caulton*
&& – &&
Tetrazine Assists Reduction of Water
by Phosphines: Application in the
Mitsunobu Reaction
Tetrazine is a redox mediator that ena-
bles the transfer of the PR3 reducing
power to the protons of water. The re-
action sequence continues with the for-
mation of R3PO and dihydrotetrazine. In
the presence of strong acid phosphine/
tetrazine pairs, tetrazinium salts are
formed. This enables a modified Mitsu-
nobu reaction, with tetrazine replacing
EtO2CN=NCO2Et (DEAD), with the ad-
vantage that dihydrotetrazine can be re-
cycled to tetrazine by oxidation with
HNO2.
Chem. Eur. J. 2016, 22, 1 – 15
www.chemeurj.org
15
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ