Selective N1/N4 1,4-Cycloaddition of 1,2,4,5-Tetrazines Enabled by Solvent Hydrogen Bonding

Article abstract of DOI:10.1021/jacs.0c09775

An unprecedented 1,4-cycloaddition (vs 3,6-cycloaddition) of 1,2,4,5-tetrazines is described with preformed or in situ generated aryl-conjugated enamines promoted by the solvent hydrogen bonding of hexafluoroisopropanol (HFIP) that is conducted under mild reaction conditions (0.1 M HFIP, 25 °C, 12 h). The reaction constitutes a formal [4 + 2] cycloaddition across the two nitrogen atoms (N1/N4) of the 1,2,4,5-tetrazine followed by a formal retro [4 + 2] cycloaddition loss of a nitrile and aromatization to generate a 1,2,4-triazine derivative. The factors that impact the remarkable change in the reaction mode, optimization of reaction parameters, the scope and simplification of its implementation through in situ enamine generation from aldehydes and ketones, the reaction scope for 3,6-bis(thiomethyl)-1,2,4,5-tetrazine, a survey of participating 1,2,4,5-tetrazines, and key mechanistic insights into this reaction are detailed. Given its simplicity and breath, the study establishes a novel method for the simple and efficient one-step synthesis of 1,2,4-triazines under mild conditions from readily accessible starting materials. Whereas alternative protic solvents (e.g., MeOH vs HFIP) provide products of the conventional 3,6-cycoladdition, the enhanced hydrogen bonding capability of HFIP uniquely results in promotion of the unprecedented formal 1,4-cycloaddition. As such, the studies represent an example of not just an enhancement in the rate or efficiency of a heterocyclic azadiene cycloaddition by hydrogen bonding catalysis but also the first to alter the mode (N1/N4 vs C3/C6) of cycloaddition.

Full text of DOI:10.1021/jacs.0c09775

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Selective N1/N4 1,4-Cycloaddition of 1,2,4,5-Tetrazines Enabled by
Solvent Hydrogen Bonding
Zixi Zhu, Christopher M. Glinkerman, and Dale L. Boger*
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ABSTRACT: An unprecedented 1,4-cycloaddition (vs 3,6-cycloaddition) of 1,2,4,5-
tetrazines is described with preformed or in situ generated aryl-conjugated enamines
promoted by the solvent hydrogen bonding of hexafluoroisopropanol (HFIP) that is
conducted under mild reaction conditions (0.1 M HFIP, 25 °C, 12 h). The reaction
constitutes a formal [4 + 2] cycloaddition across the two nitrogen atoms (N1/N4) of
the 1,2,4,5-tetrazine followed by a formal retro [4 + 2] cycloaddition loss of a nitrile
and aromatization to generate a 1,2,4-triazine derivative. The factors that impact the
remarkable change in the reaction mode, optimization of reaction parameters, the
scope and simplification of its implementation through in situ enamine generation
from aldehydes and ketones, the reaction scope for 3,6-bis(thiomethyl)-1,2,4,5-
tetrazine, a survey of participating 1,2,4,5-tetrazines, and key mechanistic insights into
this reaction are detailed. Given its simplicity and breath, the study establishes a
novel method for the simple and efficient one-step synthesis of 1,2,4-triazines under
mild conditions from readily accessible starting materials. Whereas alternative protic solvents (e.g., MeOH vs HFIP) provide
products of the conventional 3,6-cycoladdition, the enhanced hydrogen bonding capability of HFIP uniquely results in promotion of
the unprecedented formal 1,4-cycloaddition. As such, the studies represent an example of not just an enhancement in the rate or
efficiency of a heterocyclic azadiene cycloaddition by hydrogen bonding catalysis but also the first to alter the mode (N1/N4 vs C3/
C6) of cycloaddition.
preferential formation of two C−C versus two C−N bonds and
the subsequent release of N2 rather than a nitrile.
INTRODUCTION
■
The inverse electron demand Diels−Alder reaction of electron-
deficient heterocyclic azadienes is an effective method for the
synthesis of highly functionalized heterocycles widely used in
organic synthesis,1 medicinal chemistry, and chemical biology.2
Previously, we have reported systematic explorations and
applications of the cycloaddition reactions of 1,2,4,5-
tetrazines,3 1,2,4-triazines,4 1,3,5-triazines,5 1,3,4-oxadiazoles,6
1,2-diazines,3b,7 1,2,3-triazines,8 and most recently a 1,2,3,5-
tetrazine.9 Among all heterocyclic azadienes, the readily
available 1,2,4,5-tetrazines are the most widely used due to
their superb cycloaddition reactivity with an unusually broad
range of dienophiles (Figure 1A).1,2 In the >60 years since its
first disclosure and among the now countless examples, a single
cycloaddition mode is observed that occurs across the two
carbon atoms (3,6-cycloaddition) independent of the 1,2,4,5-
tetrazine substitution pattern or nature of the dienophile.10 To
the best of our knowledge, no example of the alternative [4 +
2] cycloaddition across two nitrogen atoms (1,4-cyclo-
addition) of a 1,2,4,5-tetrazine has been disclosed. Best defined
and articulated by Houk in computational studies,11 the
remarkable rate of cycloaddition, the preferential 3,6-cyclo-
addition mode, and lack of 1,4-cycloaddition can be attributed
to orbital interactions and differential distortion energies en
route to the transition state along with the energetically
Until recently and although examined for decades, no
general approach to catalysis of the inverse electron demand
Diels−Alder reactions of heterocyclic azadienes had been
described.12,13 Typically, additives such as Lewis acids lead to
nonproductive consumption of the electron-rich dienophiles
without productive activation of the electron-deficient
heterocyclic azadienes. We found that heterocyclic azadienes
can be activated for cycloaddition by H-bonding14 with the
non-nucleophilic solvents hexafluoroisopropanol (HFIP) and
trifluoroethanol (TFE).11 The H-bonding was established in
1
mechanistic H NMR studies, the catalysis was found to be
unique to HFIP and TFE versus other protic solvents due to
the reduced basicity of such heterocyclic azadienes, and our
conclusions were verified in subsequent computational studies
by Houk.15 Since its discovery, we have continued to
investigate the scope of solvent H-bonding assisted inverse
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Figure 2. Selected biologically active 1,2,4-triazines.
RESULTS AND DISCUSSION
■
Reaction Discovery, Key Parameters, and Optimiza-
tion. In order to better understand the role of each factor,
especially the solvent, a series of experiments was conducted
(Figure 3). An increase in the reaction temperature to 60 °C
Figure 3. Reaction parameters. aUnless otherwise noted, all the
b
reactions were run with 1a (0.05 mmol) in 0.5 mL solvent. Isolated
yield, other yields were established by NMR.
Figure 1. Cycloadditions of 1,2,4,5-tetrazines (C3/C6 cycloaddition)
and the unprecedented formal 1,4-cycloaddition (N1/N4) detailed
herein.
(entry 2, vs 25 °C), an increase in the reaction time to 22 h
(entry 3, vs 13 h), and an increase in the reaction
concentration (entry 4) had little impact on the yield of
product. Importantly, only the conventional 3,6-cycloaddition
product 5a, without elimination of pyrrolidine, was generated
when the reaction was conducted in nonfluorinated solvents,
including CHCl3 (entry 5) and methanol (entry 6). The
reaction in methanol was found to generate the aromatic
pyridazine product 5a′ (66%) with elimination of pyrrolidine
when it was further warmed to 50 °C for 24 h (eq 1). The use
electron demand Diels−Alder cycloadditions.13 In these
studies, which have focused on defining the rate accelerations,
improved conversions, regioselectivity enhancements or
alterations, and expansion of the productive reactive diene/
dienophile pairs, we discovered an unprecedented formal 1,4-
cycloaddition of 3,6-bis(methylthio)-1,2,4,5-tetrazine (1a)
with the enamine 4a, 1-styrylpyrrolidine. When HFIP was
used as the solvent at room temperature open to air, the
reaction provided the 1,2,4-triazine 3a in 75% yield, the
structure of which was established with a single-crystal X-ray
structure determination (Figure 1C).16 Given the unprece-
dented nature of the reaction coupled with the importance of
the 1,2,4-triazine core in drugs and biologically active
molecules (Figure 2),17 we examined and herein detail features
that impact this change in the cycloaddition mode,
optimization of the reaction parameters, the scope and further
simplification of the reaction as it relates to the 1,2,4,5-
tetrazine and preformed or in situ generated enamine, and
mechanistic insights into this remarkable reaction. Given its
breath, it establishes a new simple method for the efficient one-
step synthesis of 1,2,4-triazines under mild reaction conditions
from easily accessible starting materials.
of HFIP as solvent was found to be essential for the altered 1,4-
cycloaddition, although use of mixed solvent of CHCl3−HFIP
(1:1) led to the same product 3a, albeit in lower yield (entry
7). Finally, addition of strong acid, TFA (1 equiv), and
conducting the reaction in CHCl3 resulted in no cycloaddition
products, likely due to protonation of the enamine 4a (entry
8). The unique behavior of HFIP arises from its ability to H-
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bond the 1,2,4,5-tetrazine, thereby activating it for reaction,
and yet not consume either the starting 1,2,4,5-tetrazine
because of the attenuated nucleophilic character of HFIP or
the conjugated enamine through protonation because of its
weakly acidic nature (pKa = 9.3).
Scheme 1. Screen of Secondary Amines
A more refined solvent survey for the reaction of 1a with
enamine 4b in a series of perfluoroalcohols was conducted.
Despite the variations in yields, the ratio of formal 1,4-
cycloaddition versus the conventional 3,6-cycloaddition that
provides 5b and pKa of the perfluoroalcohols, which is a
measure of their H-bonding capability,18 were found to
correlate exceptionally well (Figure 4). In fact, a clean switch
used or in the absence of an added secondary amine, and both
the 1,2,4,5-tetrazine 1a and aldehyde 2a were recovered
unchanged. Use of 0.5 equiv of pyrrolidine (with 1 equiv of
1a/3 equiv of 2a) as above provided similar results (79% yield)
indicating productive turnover, although use of ≤0.25 equiv
pyrrolidine resulted in lower yields even with extended
reaction times (Supporting Information Figure S4). This
substoichiometric use of pyrrolidine with 2b improved with the
faster reaction of in situ generated 4b, where good conversion
was observed even with 0.25 equiv and dropped off only at 0.1
equiv of pyrrolidine (Supporting Information Figure S4). The
limited pyrrolidine turnover is possibly due to acid-promoted
self-condensation of the in situ generated enamine (Supporting
Information Figure S5).
Substrate Scope. The carbonyl substrate scope for this
transformation was explored (Scheme 2). With the 1,2,4,5-
tetrazine 1a as the diene, 2-arylacetaldehyde substrates bearing
either electron-rich (3b, 3f, 3h, 3i, 3k, 3l) or electron-deficient
(3c−e, 3g, 3j, 3m−o) arenes are well tolerated, all
participating in the reaction effectively, although electron-
deficient arenes were found to display a lower reactivity. As a
result of the benign reaction conditions, a wide range of
functional groups are expected to be well tolerated, including
those illustrated herein, consisting of methoxy (3b), halides
(3c−e, 3j), phenyl (3f), trifluoromethyl (3g), ester (3m), and
nitrile (3o) substituents. Arenes with ortho-substitution (3i,
3k, 3n) that might suffer steric issues also provide the 1,2,4-
triazines in satisfactory yields. Heterocyclic as well the all-
carbon arenes are also compatible, including thiophene (3h),
indole (3k), and quinoline (3n). Nonconjugated enamines
(e.g., 1-pyrrolidinocyclopentene) did not react with 1a under
current reaction conditions likely due to their protonation by
HFIP (for unreactive substrates, see Supporting Information
Figure S6). Although the ketone 1-phenylacetone was
unreactive, conjugated cyclic ketones (see below) and two
related substrates containing five-membered heterocycles, 1-
(thiophen-2-yl)propan-2-one (2p) and 1-(furan-2-yl)propan-
2-one (2q), were found to be suitable substrates under the
current reaction conditions, providing the 1,2,4-triazines 3p
(79%) and 3q (26%), respectively. This differentiated behavior
can be attributed to a lower steric barrier (Me/O repulsion vs
Me/C−H repulsion) to achieving a conjugated coplanar
enamine conformation, increasing the enamine stability toward
nonproductive HFIP protonation and self-condensation
(Figure 5). This conclusion was further supported by
computation (AM1, Gaussian 09) of the C1−C2−C3−C4
Figure 4. Refined solvent survey. Yields were established by NMR.
from exclusive 3,6-cycloaddition to exclusive formal 1,4-
cycloaddition was observed as the pKa of the solvent decreased
from 15.5 (MeOH) to 9.3 (HFIP). As such, the results
highlight the unique behavior of HFIP and indicate that the
extent of the H-bonding interaction between the tetrazine and
solvent is the feature controlling the 1,4- versus 3,6-
cycloaddition selectivity. Experimentally, we observed that
the HFIP alcohol proton exhibits a pronounced downfield
chemical shift upon titration with 3,6-bis(thiomethyl)-1,2,4,5-
tetrazine (1a, Δ0.53 ppm, 0−2 equiv) consistent with this H-
bonding interaction with 1a (Supporting Information Figure
S1). It is possible that the selectivity is altered due to change in
the LUMO molecular orbital distribution that is induced by
solvent H-bonding, leading to a conjugated nitrogen now more
susceptible to nucleophilic attack than carbon. Simple AM1
computation of the LUMO energy of free and protonated 1a
(−1.93 eV vs −6.17 eV) and sum of squared coefficients as
they relate to tetrazine carbons (C3, C6) and nitrogens (N1,
N2, N4, N5) (free 1a, 0.62 for C and 0.26 for N; protonated
1a, 0.43 for C and 0.39 for N; see Supporting Information
Figure S2 for details) supports both the enhanced reactivity
(rel ELUMO) and a shift from C to N for attack of a nucleophile.
On the basis of the precedent that we first introduced4b and
with recognition that a preformed enamine may not always be
readily available, easily prepared and stored, or stable in open
air, we examined whether the enamine could be generated in
situ from the corresponding aldehyde and amine. To our
delight, replacement of enamine 4a (3 equiv) with phenyl-
acetaldehyde (2a, 3 equiv) and pyrrolidine (1 equiv) provided
3a in an improved yield (88%) under otherwise identical
conditions (0.1 M HFIP, 25 °C, 13 h, open flask) (Scheme 1).
A screen of alternative secondary amines revealed that the in
situ generated pyrrolidine enamine provided the highest
conversion to 3a of those examined (Scheme 1). No reaction
was observed either when a tertiary amine such as Et3N was
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dihedral angle of 1-phenylacetone (62.8°) and C1−C2−C3−
O dihedral angle of 2p (10.0°).
Scheme 2. Acyclic Substrates
Significantly, conjugated cyclic ketones of which 2-tetralones
and 2-indanones are prototypical members participate
effectively in the reaction under the current reaction conditions
(Scheme 3). In addition to the parent 2-tetralone (3r) and 2-
Scheme 3. Conjugated Cyclic Ketone Substrates
indalone (3s), derivatives bearing methoxy (3t) and bromine
(3u) substituents were also suitable substrates for this reaction.
Cyclic ketones that form conjugated enamines with fused
heterocycles, such as pyrrole (3v), indole (3w), pyridine (3x,
3y), and thiophene (3z), all provided the 1,2,4-triazine
products in good to excellent yield. The structures of 3r and
3v were confirmed by single-crystal X-ray structure determi-
nations.16
a
Reaction conducated at 60 °C versus 25 °C.
The substrate scope as it relates to symmetrical 1,2,4,5-
tetrazines was also briefly explored (Scheme 4). This was not
done with the intent of comprehensively defining the scope
and was conducted without alteration of the reaction
parameters or optimization of the conditions (temp, time)
including choice of perfluoroalcohol. Rather, it was done to
establish whether the 1,4-addition extends beyond 1a and to
define the range of productive substituents, and the results
portend a broad tetrazine scope. 3,6-Bis(benzylthio)-1,2,4,5-
tetrazine (1a′) displayed a reactivity similar to that of 1a
toward the in situ generated pyrrolidine enamine of
phenacetaldehyde, providing the corresponding 1,2,4-triazine
6a in excellent yield (78%). Notably, the nonvolatile
benzylthiocyanate was observed and characterized by NMR
(76%) as a released product in this reaction. As such and while
the reaction is unlikely to represent a true cycloaddition across
the tetrazine N1/N4, the products are the same as those
expected of such a reaction. More remarkable and without an
effort at optimization, 1,2,4,5-tetrazines that are less reactive or
unreactive in traditional cycloaddition reactions also provided
the corresponding 1,2,4-triazines, including 3,6-dimethoxy
Figure 5. Steric repulsion of the pyrrolidine enamine of 1-
phenylacetone versus the in situ generated enamine of 2p.
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Scheme 4. Brief Survey of 1,2,4,5-Tetrazine Scope
Figure 6. Reaction of unsymmetrical 1,2,4,5-tetrazines. aYield
established by NMR. Others are isolated yields.
removal of a substituent, 3a is exclusively (7a,c,d,f) or
predominately (7e) formed in all cases with the exception of
7b, bearing a methoxy substituent.
Mechanistic Insights. The unique 1,4-cycloaddition was
examined in greater detail to gain insights into the potential
reaction mechanism. Given that no prior example of
cycloaddition across two nitrogen atoms (N1/N4) of 1,2,4,5-
tetrazines has been described, it is unlikely that a concerted [4
+ 2] cycloaddition followed by retro [4 + 2] nitrile extrusion is
operative in this transformation. Moreover, analysis of the
reaction indicates that not only is a concerted cycloaddition
unlikely but even a stepwise addition−cyclization prior to
nitrile loss is not able to account for formation of a single 1,2,4-
triazine product rather than a mixture of two isomeric products
(Figure 7). Further reinforcing this analysis is the observation
(6b), diamino (6c), dimethyl (6d), and diphenyl (6e) 1,2,4,5-
tetrazines, although in more modest conversions under the
present reaction conditions. Unsubstituted 1,2,4,5-tetrazine (s-
tetrazine) and dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate
were found to undergo the conventional 3,6-cycloaddition
without observation of the formal 1,4-cycloaddition product
under current conditions (Supporting Information Figure S7).
A series of unsymmetrical 1,2,4,5-tetrazines were also
examined as is shown below, some of which displayed a
remarkable regioselectivity for the 1,4-cycloaddition. Repre-
sentative of this, the reaction of 4-methyl-1-thiomethyl-1,2,4,5-
tetrazine (7d) with the in situ generated pyrrolidine enamine
of phenylacetaldehyde under the standard reaction conditions
(HFIP, 25 °C, 12 h) provided exclusively the 1,2,4-triazine 3a
(71%) with retention of the thiomethyl group and loss of
acetonitrile.
In addition to the symmetrical 1,2,4,5-tetrazine substrates
presented in Scheme 4, a larger representative series of
unsymmetrical 1,2,4,5-tetrazines (7a−f) were prepared and
examined, with one site substituted with a methylthio group
and the remaining site bearing an electron-withdrawing (7a,
S(O)Me), electron-donating (7b and 7c, OMe and NHAc),
neutral (7d, Me), conjugated (7e, Ph), or no substituent (7f,
H). They were allowed to react directly with enamine 4a in
HFIP (0.1 M, 25 °C) to further probe factors impacting the
mode of cycloaddition, its regioselectivity, and plausible
mechanisms (Figure 6). The results revealed that only two,
tetrazine 7b (R = OMe, entry 3, 1:1) and 7e (R = Ph, entry 6,
5:1), led to two 1,2,4-triazine products that bear each
substituent (3a:6) in reactions that display the 1,4 mode of
cycloaddition but represent two regioselectivities for the formal
1,4-cycloaddition. Like the symmetrical tetrazine 1a, the
remaining tetrazines provided the single 1,2,4-triazine 3a in a
regioselective 1,4-cycloaddition. In addition, the reactions of
the electron-deficient tetrazine 7a (R = S(O)Me, entry 2, 2:1)
and the unsubstituted tetrazine 7f (R = H, entry 7, 1:3)
exhibited competitive 1,4-cycloaddition and 3,6-cycloaddition.
Aside from the observation that an electron-withdrawing
substituent increases competitive 3,6-cycloaddition as does
Figure 7. Plausible but improbable mechanisms.
that room temperature retro [4 + 2] nitrile extrusion from an
initial bicyclic intermediate is unlikely since related isolated
bicyclic intermediates obtained from the cycloadditions of
2,4,6-tris(methylthio)-1,3,5-triazine require elevated reaction
temperatures, extended reaction times, and often additional
acid catalysis for nitrile extrusion.19
First and foremost, to determine whether a fragmented
tetrazine, potentially releasing an acyclic azadiene, might be
responsible for the observed reaction, we examined the fate of
1,2,4,5-tetrazine 1a in HFIP alone (25 °C, 12 h), in the
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presence of pyrrolidine (1 equiv), and in the presence of
phenylacetaldehyde (2a, 1 equiv) (eq 2). Under all conditions,
tetrazine 1a was recovered quantitatively without change,
verifying that 1a, and not a more reactive fragment, is the
compound undergoing reaction.
monitored by 1H NMR (Supporting Information Figure S8) as
was that with 4b (4-methoxyphenacetaldehye pyrrolidine
enamine). For 4a, the consumption of 1a and accumulation
of product 3a were directly observed as the reaction
proceeded, monitoring the emerging diagnostic C6−H signal
of 3a and a discernible chemical shift in the SMe signal of 3a
versus 1a. In addition, a diagnostic signal for 3a′ (aminal CH, δ
5.48 in HFIP-d2) was observed to rise as the reaction
progressed. No other prominent signals attributable to a
thiomethyl group of other potential intermediates were
observed to accumulate over the course of the reaction.
Notably, enamine 4a was not observed at any stage of the
reaction. Instead, its deuterated iminium ion (4a-iminiun) was
observed, suggesting that the enamine is in rapid equilibration
with the protonated iminium ion and the former rapidly reacts.
With the concentration of all major components able to be
1
We then undertook the detection, trap, or isolation of a
reaction intermediate to help clarify the origin of the altered
cycloaddition mode and the overall reaction process. After
extensive attempts on intermediate capture, compound 3a′ was
isolated in 36% yield when 1a and enamine 4a were allowed to
react in a MeOH/HFIP (1:1) mixed solvent system for 2 h
(Figure 8A). The structure of 3a′ was unambiguously
quantified by H NMR (Supporting Information Table S1), a
reaction time course profile was plotted and is depicted in
Figure 9. As the reaction proceeds and as the concentration of
Figure 8. Isolation and studies of compound 3a′.
established with a single crystal X-ray structure determina-
tion.16 Compound 3a′ converts to product 3a in 78% yield
when subjected to the standard reaction conditions (HFIP, 25
°C, 13 h; Figure 8B). A crossover experiment where 3a′ was
mixed with the enamine 4b under the standard reaction
conditions afforded 3a as the sole product without observation
of 3b, indicating that at least one step in the generation of 3a′
from starting materials is not reversible (Figure 8C). Notable
in this structure is a connectivity that indicates enamine attack
on a tetrazine nitrogen (N1) as well as the necessary cleavage
of a N−N bond (N1/N2) for 1,2,4-triazine formation prior to
methylthiocyanate loss and that it is the alkylated, not distal,
N−N bond (N4/N5) that is cleaved.
Figure 9. Reaction profile over time for reaction of 1a with 4a (top)
and 4b (bottom).
starting 1a and 4a-iminiun decrease, the concentration of 3a′
increases until it reached the maximum concentration of 0.015
M, from where it slowly decreases. The appearance of product
3a increases faster than the appearance of 3a′ for the first 200
min (2.2- to 2.7-fold), at which point nearly all 1a is
consumed, followed by a slower increase in product that
now nearly matches the rate of disappearance of 3a′. The
decrease in the 4a-iminiun was found to be faster than 1a,
likely attributable to nonproductive competitive consumption
The reaction between 1a (0.1 M, HFIP-d2) and enamine 4a
(2 equiv, phenacetaldehyde pyrrolidine enamine) was also
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of 4a-iminium by acid-promoted enamine self-condensation. A
more refined treatment of the data, plotting the relative
concentration of 3a and 3a′ over time for the first 20 min,
revealed that the 3a′:3a ratio (0.45, 1:2.2) was time-
independent. This conclusively establishes that 3a and 3a′
are formed in parallel reactions derived from a mutual
intermediate (Supporting Information Figure S9). This is
consistent with 3a′ serving as an off route reversibly generated
compound rather than being a primary reaction intermediate.
Further consistent with this, the reaction profile of the
reaction of 1a with enamine 4b (4-methoxyphenacetaldehyde
pyrrolidine enamine) was much more straightforward (Figure
9). The analogous intermediate 3b′ was not observed in
detectable amounts, indicating that it is not generated in
significant amounts under the conditions of the reaction or is
much more rapidly converted to product precluding detection.
To address this question, we identified conditions that allowed
the isolation of 3b′ (Figure 10) and measured the rate at which
of product 3b, indicating that essentially no nonproductive
consumption of tetrazine 1a occurs in the first 160 min.
With these key observations in hand, a plausible pathway
from 1a and 4a to 3a and 3a′ is summarized in Figure 11. H-
Figure 11. Plausible reaction pathway.
Bonding activation of 3,6-dimethylthio-1,2,4,5-tetrazine and
irreversible enamine nucleophilic attack at N1, either para (N4,
shown) or ortho (N2) to the site of H-bonding (interconver-
tible by tautomerization), generate intermediate a or its 2H
tautomer. Regeneration of the enamine from the iminium ion
and 6π electrocyclic rearrangement with required cleavage of
the N1/N2 bond lead to c,20,21 followed by facile cis to trans
azo-isomerization that provides d. Intermediate d may undergo
a 6π electrocyclic cyclization (C8−N4 bonding) to generate f
or a reversible 5-endo-trig cyclization22 (C7−N4 bonding)
followed by reversible 5-exo-trig ring closure to generate
compound 3a′ (red arrow). Final aromatization of f with loss
of pyrrolidine and MeSCN through a six-membered transition
state, of which the latter two steps both may benefit from an
intramolecular H-bond, provides the products of the reaction
(3a and MeSCN). Direct generation of f from 3a′ via e
through a migrative ring expansion (blue arrows) is also
possible. In this mechanism, 3a′ is also eventually converted to
3a, although the possibility of direct transformation of 3a′ to
3a without passing through e cannot be ruled out. A related
alternative route to intermediate c involves an initial addition
to provide a and its stepwise cyclization to the [2 + 2] adduct23
followed by a strain induced 6π electrocyclic rearrangement
with cleavage of the N1/N2 bond, followed by aminal cleavage
and conversion to c (Figure 10). In addition to accounting for
Figure 10. Related studies of compound 3b′.
it and 3a′ convert to the triazine products under the reaction
conditions (25 °C, HFIP-d2). Both 3a′ and 3b′ converted to
the triazine products at slow and near equivalent rates
(apparent first order k(obs) = 2.0 × 10−3 min−1 (3a′) and 4.5
× 10−3 min−1 (3b′)), requiring approximately 25.0 h (3a′, t1/2
= 5.8 h) and 11.1 h (3b′, t1/2 = 2.6 h) to run to completion
(reach 95% yield). Thus, the lack of detection of 3b′ in the
reaction of 1a with 4b is not due to a much more rapid
conversion to 3b. Rather, it can be attributed to 3b′ not being
generated in detectable amounts under the conditions of the
faster direct reaction of 1a with 4b to provide 3b. Together,
the combined observations indicate that compound 3a′ is and
3b′ would be reversibly generated off route compounds that
ultimately also convert to the triazine products but do not
appear to be on the direct pathway to the products. Finally, the
extent of consumption of 1a nearly matches the accumulation
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the generation of single 1,2,4-triazine product (3-methylthio-5-
phenyl-1,2,4-triazine but no 3-methylthio-6-phenyl-1,2,4-tria-
zine, Figure 5), key elements of the mechanism are a solvent
H-bonding activation of the 1,2,4,5-tetrazine for enamine
nucleophilic addition to N1 and a 6π electrocyclic rearrange-
ment for the needed cleavage of the N1/N2 bond, representing
a pathway to 3a where overall stepwise 1,4-cycloaddition
occurs prior to elimination of thiomethylcyanate (MeSCN).
Application Potential. The unprecedented 1,4-cycoladdi-
tion has provided rapid access to a suite of analogs of 3a, which
itself has been used both to access functionalized 1,2,4-triazines
and as a key intermediate in the synthesis of biologically active
compounds (Figure 12). Thus, the thiomethyl group in the
under mild reaction conditions (0.1 M HFIP, 25 °C, 12-14 h)
with a broad scope of in situ generated acyclic and cyclic aryl-
conjugated enamines to provide 5-aryl-1,2,4-triazines in good
yields. Factors that impact this unprecedented change in the
reaction mode (N1/N4 vs C3/C6 cycloaddition), optimization
of the reaction conditions, the substrate scope of the reaction
partners, simplification of its implementation with preformed,
in situ formed, or substoichiometrically generated enamines,
and mechanistic insights into this remarkable cycloaddition are
detailed. Given its breath, it establishes a new method for the
simple and efficient one-step synthesis of 1,2,4-triazines under
mild conditions from readily accessible starting materials.
Mechanistic studies revealed several important features of the
reaction and highlight the unique behavior of HFIP, where the
strength or extent of its H-bonding interaction with the
tetrazines is responsible for the alteration of the cycloaddition
mode from the typical 3,6-cycloaddition to 1,4-cycloaddition.
As such, the studies represent the first example of not just an
enhancement in the rate and efficiency of a heterocyclic
azadiene cycloaddition by H-bonding catalysis but an
alteration in the mode of cycloaddition as well. Key elements
of a plausible stepwise addition−cyclization mechanism that
accounts for the generation of a single 1,2,4-triazine product
(3-methylthio-5-phenyl-1,2,4-triazine and no 3-methylthio-6-
phenyl-1,2,4-triazine) and other observations to date are a
solvent H-bonding activation of the 1,2,4,5-tetrazine that
promotes enamine nucleophilic addition to N1 and a 6π
electrocyclic rearrangement for the needed cleavage of the N1/
N2 bond, providing a pathway to the products where overall
1,4-cycloaddition occurs prior to elimination of a nitrile.
Continued efforts that examine and further expand solvent
hydrogen bonding catalysis of other heterocyclic and acyclic
azadienes are ongoing and will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c09775.
■
sı
Materials and methods, full experimental details for
studies of substrate scope, mechanistic insights, AM1
1
studies, and H, 13C, and 19F NMR spectra (PDF)
Figure 12. Representative diversification of 3a and its use in the
synthesis of biologically active compounds.
Crystallographic information (CIF)
Crystallographic information (CIF)
Crystallographic information (CIF)
Crystallographic information (CIF)
product 1,2,4-triazines 3 can be used as an effective functional
group for further diversification of the product 1,2,4-
triazine.24,25 This serves to complement the use of alternatively
substituted 1,2,4,5-tetrazines (e.g., 6c). An example that we
would like to especially highlight is the use of 3a as a precursor
for a second heterocyclic azadiene Diels−Alder reaction,26
setting the foundation for sequential cycloaddition strategies
for rapidly accessing a diverse set of highly substituted or fused
pyridines that are challenging to prepare by other means.1b
Similarly, representative examples of the direct use of 3a in the
preparation of biologically active compounds can be high-
lighted with the preparation of inosine monophosphate
dehydrogenase (IMPDH) inhibitors27 and adenosine A2A
antagonists,28 for which the work herein provides a simple
synthesis of a suite of 3a analogs.
AUTHOR INFORMATION
Corresponding Author
■
Dale L. Boger − Department of Chemistry and The Skaggs
Institute for Chemical Biology, The Scripps Research Institute,
La Jolla, California 92037, United States; orcid.org/
0000-0002-3966-3317; Email: dale.boger@outlook.com
Authors
Zixi Zhu − Department of Chemistry and The Skaggs Institute
for Chemical Biology, The Scripps Research Institute, La
Jolla, California 92037, United States
Christopher M. Glinkerman − Department of Chemistry and
The Skaggs Institute for Chemical Biology, The Scripps
Research Institute, La Jolla, California 92037, United States
CONCLUSIONS
An unprecedented 1,4-cycloaddition of 1,2,4,5-tetrazines with
enamines has been discovered. The reaction is conducted
■
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09775
H
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(9) Wu, Z.-C.; Boger, D. L. Synthesis, characterization, and
cycloaddition reactivity of a monocyclic aromatic 1,2,3,5-tetrazine. J.
Am. Chem. Soc. 2019, 141, 16388−16397.
Notes
The authors declare no competing financial interest.
(10) Carboni, R.; Lindsey, R., Jr. Reactions of tetrazines with
unsaturated compounds. A new synthesis of pyridazines. J. Am. Chem.
Soc. 1959, 81, 4342−4346.
ACKNOWLEDGMENTS
■
We are especially grateful to the National Institutes of Health
for financial support (Grant CA042056, D.L.B.) and a JITRI
Fellowship (Z.Z.). We thank Dr. Milan Gembicky and Dr.
Arnold Rheingold of the Crystallography Facility at the
University of California, San Diego, for the X-ray structure
determinations of 3a, 3r, 3v, and 3a′.
(11) Yang, Y.-F.; Liang, Y.; Liu, F.; Houk, K. Diels−Alder reactivities
of benzene, pyridine, and di-, tri-, and tetrazines: The roles of
geometrical distortions and orbital interactions. J. Am. Chem. Soc.
2016, 138, 1660−1667.
(12) (a) Wegner, H. A.; Kessler, S. N. Bidentate Lewis acids as
catalysts for the activation of 1,2-diazenes in organic synthesis. Synlett
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2012, 699−705. (b) Turkmen, Y. E.; Montavon, T. J.; Kozmin, S. A.;
Rawal, V. H. Silver-catalyzed formal inverse electron-demand Diels−
Alder reaction of 1,2-diazines and siloxy alkynes. J. Am. Chem. Soc.
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