Intermediacy of a Persistent Urazole Radical and an Electrophilic Diazenium Species in the Acid-Catalyzed Reaction of MeTAD with Anisole

Article abstract of DOI:10.1021/acs.joc.5b02520

The reaction of N-methyl-1,2,4-triazoline-3,5-dione (MeTAD) with anisole in the presence of trifluoroacetic acid affords unexpected disubstituted urazole products instead of the expected monosubstituted urazole as typically observed in the reactions of MeTAD with other substituted benzenes. Our investigation into the mechanism of formation of these disubstituted products suggests that MeTAD is capable of further reaction with the initially formed monosubstituted urazole to afford a persistent urazole radical. The identity of this radical has been established by UV-vis spectroscopy, the nature of its self-dimerization reaction, and via independent generation. Electrochemical oxidation of this radical was carried out, and the resulting diazenium ion was demonstrated to be reactive with added substituted benzenes, including anisole. When oxidation was carried out chemically using thianthrenium perchlorate in the presence of anisole it was shown to produce the same disubstituted products (and in the same ratio) as observed in the acid-catalyzed reaction. A common diazenium species is proposed to be active in both cases. We also report the synthesis and characterization of three interesting tetrazane dimers resulting from unstable urazole radicals.

Full text of DOI:10.1021/acs.joc.5b02520

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Article
Intermediacy of a Persistent Urazole Radical and an Electrophilic
Diazenium Species in the Acid-Catalyzed Reaction of MeTAD with Anisole
Gary W Breton, and Alice H. Suroviec
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b02520 • Publication Date (Web): 09 Dec 2015
Downloaded from http://pubs.acs.org on December 13, 2015
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Intermediacy of a Persistent Urazole Radical and an Electrophilic Diazenium
Species in the Acid-Catalyzed Reaction of MeTAD with Anisole
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Gary W. Breton* and Alice H. Suroviec
Department of Chemistry, Berry College, Mount Berry, GA 30149, United States
*Corresponding Author: gbreton@berry.edu
Table of Contents Graphic
ABSTRACT
The reaction of Nꢀmethylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione (MeTAD) with anisole in the
presence of trifluoroacetic acid affords unexpected disubstituted urazole products instead
of the expected monosubstituted urazole as typically observed in the reactions of MeTAD
with other substituted benzenes. Our investigation into the mechanism of formation of
these disubstituted products suggests that MeTAD is capable of further reaction with the
initiallyꢀformed monosubstituted urazole to afford a persistent urazole radical. The
identity of this radical has been established by UVꢀVis spectroscopy, the nature of its
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selfꢀdimerization reaction, and via independent generation. Electrochemical oxidation of
this radical was carried out and the resulting diazenium ion was demonstrated to be
reactive with added substituted benzenes, including anisole. When oxidation was carried
out chemically using thianthrenium perchlorate in the presence of anisole it was shown to
produce the same disubstituted products (and in the same ratio) as observed in the acidꢀ
catalyzed reaction. A common diazenium species is proposed to be active in both cases.
We also report the synthesis and characterization of three interesting tetrazane dimers
resulting from unstable urazole radicals.
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INTRODUCTION
We recently reported our studies on the trifluoroacetic acid (TFA) catalyzed
reaction of Nꢀmethylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione (MeTAD, 1) with electronꢀrich aromatic
rings to afford Nꢀaryl urazoles (2) in good yields (Scheme 1).1 This route provides for
the synthesis of substituted urazoles, compounds that have promising pharmaceutical
Scheme 1. Reaction of MeTAD (1) with substituted benzenes in the presence of
trifluoroacetic acid (TFA) to afford substitututed urazoles, 2.
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applications, in a single synthetic step versus the threeꢀstep method that has traditionally
been utilized.1 While most of the aromatic compounds we studied behaved in a
straightforward manner to afford the expected monosubstituted urazoles 2, reaction of 1
with anisole afforded only small amounts of the monosubstituted urazole 3 (< 6% yield,
Scheme 2). Instead, the major products (44% yield) were the unexpected regioisomeric
disubstituted urazole compounds 4a and 4b produced in a ~60:40 ratio, respectively. Nꢀ
Methylurazole 5, the reduced form of 1, was also isolated in 40% yield. We suggested at
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Scheme 2. Reaction of MeTAD (1) with anisole to afford substitututed urazoles 3 and
disubstituted urazoles 4a,b.
the time that formation of disubstituted urazoles 4 could be rationalized if initially
produced 3 was subject to further oxidation to afford an electrophilic species such as
diazenium intermediate 6 which could then react with anisole via a standard electrophilic
aromatic substitution (EAS) process (Scheme 3).1 In this work we present our studies on
the mechanism of this reaction that led to some interesting observations and evidence for
the intermediacy of a persistent urazole radical as well as the earlierꢀproposed diazenium
electrophile.
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Scheme 3. Possible route for formation of disubstituted urazoles 4a,b from initially
formed urazole 3 via a proposed intermediate diazenium species, 6.
RESULTS AND DISCUSSION
A. Involvement of a Persistent Urazole Radical
Control experiments confirmed that urazole 3 is unable to react on its own with
anisole to afford compounds 4a,b in the presence of TFA. Similarly, addition of MeTAD
(1) alone to urazole 3 in the absence of acid catalyst did not afford disubstituted urazoles
4a,b. However, almost immediately upon mixing of the characteristically red colored
solution of 1 with colorless 3 in the absence of TFA, a deep purple colored solution
resulted. Curious as to the source of this newly formed color, we mixed solutions of 1
and 3 in a quartz cuvette and observed the rapid formation of a new species by UVꢀVis
spectroscopy (Figure 1A). Subtraction of the spectrum of 1 from that of the mixture
revealed strong absorptions for this species at 345 and 629 nm and weaker absorptions at
423, 451 and 485 nm.
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Figure 1. A) UVꢀVis spectra of MeTAD (red) alone, a mixture of MeTAD and urazole 3
(black), and the result of subtracting the two spectra (purple); B) UVꢀVis spectrum of
independently generated urazole radical 10.
Several urazole radicals have previously been reported to be persistent (see Chart
1).2 The 1ꢀαꢀcumyl (7) and 1ꢀtertꢀbutylurazole (8) radicals are indefinitely persistent,
while others, such as 1ꢀphenylurazole (9) are transient, with 9 decomposing in solution
within 24 h. Furthermore, the radicals have been reported to be in equilibrium with the
corresponding tetrazane dimers (Scheme 4).2c The tetrazane dimers of 7 and 8 can be
isolated and purified as colorless solids. When these solids are dissolved in a solvent
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they provide deeply colored solutions of the corresponding free radicals. Pirkle reported
that urazole radicals could be generated via treatment of the corresponding urazoles with
oxidants such as tertꢀbutyl hypochlorite, NBS, or (most cleanly) PbO2.2
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Chart 1. Structures of some persistent urazole radicals
Scheme 4. Dimerization of urazole radicals to form tetrazane dimers
We found that treatment of urazole 3 with the commerciallyꢀavailable
heterogenous oxidant Ni2O3 in CH2Cl2 followed by filtration afforded a deep blue
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solution of radical 10 (see Chart 1). The UVꢀVis spectrum of 10 was identical to that of
the species generated via admixture of 3 and 1 as described earlier (Figure 1B). Radical
10 proved to be more persistent than 9, requiring nearly a week for complete loss of the
blue color. The structure of 10 was further explored computationally at the UB3LYP/6ꢀ
31G* level of theory (Figure 2A). The urazole ring remains coplanar with the aromatic
ring which allows for delocalization of the unpaired electron not only within the urazole
unit but extending into the aromatic ring. Spin density is, therefore, observed not only on
the two nitrogen atoms and adjacent carbonyl group oxygens of the urazole ring, as might
be anticipated, but also at the ortho and para positions of the benzene ring as well as the
oxygen atom of the OMe group (Figure 2B). Unlike unsubstituted urazole radical 9,
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Figure 2. A) Minimized structure of urazole radical 10 as calculated at UB3LYP/6ꢀ31G*
level of theory including some relevant bond distances (Å) and dihedral angles; B)
Resulting spin density map for urazole radical 10 (isovalue = 0.004).
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however, which has been reported to decompose to an “uncharacterized precipitate”,2c
radical 10 undergoes clean dimerization to form compound 11 (Scheme 5). A proposed
mechanism for the dimerization is provided in Scheme 5. Attack of one molecule of
radical 10 onto a second is directed by bond formation between radical spin density on
the nitrogen of one with the radical spin density at the ortho position of the benzene ring
on the other. Hydrogen atom transfer from within the initial intermediate leads to the
observed product, 11. When TFA was added to a solution of radical 10, dimerization to
form 11 took place much more rapidly (within 45 min) in the presence of the acid catalyst
( a small amount of urazole 3 [6%] was also observed to form under these conditions).
Finally, when a solution of 10 was allowed to react in the presence of an equivalent of
urazole 3, uninterrupted dimerization of 10 to form 11 occurred over a period of 2 days
and 3 was left untouched. The increase in the rate of dimerization in the presence of 3
can be attributed to the ability of 3 to act as a weak acid catalyst (pKa = 4ꢀ5).3
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Scheme 5. Dimerization of urazole radical 10 to form 11
Mixing a solution of 10 with 1 in the presence of anisole did not produce any of
the disubstituted products 4. Thus, 1 alone is incapable of oxidizing urazole radical 10 in
the absence of an acid catalyst. Interestingly, however, addition of trifluoroacetic acid to
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a solution of 10 alone in the presence of anisole resulted in the formation of a 60:40
mixture of 4a and 4b in 25% yield in addition to an equivalent amount of urazole 3
(Scheme 6). Dimer 11 was also formed in 16% yield. The significance of this
experimental result with respect to the reaction mechanism will be revisited later.
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Scheme 6. Acidꢀcatalyzed reaction of urazole radical 10 in the presence of anisole to
form disubstituted products 4.
B. Evidence for the Intermediacy of a Diazenium Species
Electrochemical generation of diazenium 6
Knowing that radical 10 was readily generated by reaction of 1 with 3, it seemed
plausible that 10 might be subject to further oxidation to afford diazenium 6, an
electrophilic species which could be responsible for formation of the observed
disubstituted compounds 4a,b via a straightforward EAS reaction. To determine if
generation of 6 from 3 was feasible, we conducted cyclic voltammetry experiments on
acetonitrile solutions of 10 in the presence and absence of aromatic substrates. All
experimental potentials are reported relative to SCE reference and at a scan rate of 50
mV/s unless otherwise specified.
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Figure 3. A) Cyclic votammogram (CV) of a solution of 10 (4.5 mM) in acetonitrile
containing TBAF (0.1 M) as supporting electrolyte, starting at low potential (oxidative
events are positive); B) CV of a solution of 10 immediately after the addition of 5
equivalents of anisole to the solution.
Oxidation of 10 to diazenium 6 was observed to occur at Ep[ox]=1.17 V followed
by reduction back to the radical at Ep[red]=1.08 V (O1 and R1, respectively, in Figure 3A).
The peakꢀtoꢀpeak separation (Ep[ox]– Ep[red]) was 90 mV at 50 mV/s but increased
gradually as scan rate was increased (to 140 mV at 1,000 mV/s). Additionally, the ratio of
the currents measured at the two electrochemical events, Ip[ox]/ Ip[red]= 1.42, is greater than
unity (i.e. the value consistent with a reversible process).4 Given these observations, we
judge this process to be only partially reversible most likely because of competing
chemical reaction of the oxidized species (vide infra). When the cyclic voltammogram
was conducted in the presence of anisole (5 equivalents), although the oxidation event
(O1) was unaffected (Ep[ox] = 1.14 V in the presence of anisole), the reduction event was
no longer observed (Figure 3B). This indicates that anisole is capable of trapping the
generated diazenium species 6, presumably via an EAS process. A similar result was
observed in the presence of electronꢀrich mesitylene. Toluene is considerably less active
towards electrophilic aromatic substitution than either mesitylene or anisole.5 Hence,
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when the oxidation was conducted in the presence of toluene, both the reduction and
oxidation events were unaffected (Ip[ox]/ Ip[red]= 1.49) suggesting the generated diazenium
species is not sufficiently electrophilic to react with toluene.
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The parent urazole 3 was also subjected to analysis by cyclic voltammetry.
Unlike radical 10, urazole 3 has very limited solubility in acetonitrile and a saturated
solution of unknown concentration was analyzed. A twoꢀelectron oxidation event at
E
p[ox]=1.18 V (i.e., essentially identical to the oxidation potential for 10) was observed
(O1 in Figure 4), followed by an approximately oneꢀelectron reduction at Ep[red] = 1.09 V
5.00%
O1
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Poten; al%(V)%vs%SCE%
Figure 4. CV of urazole 3 in acetonitrile and TBAF (0.1M) as supporting electrolyte,
starting at low potential (oxidative events are positive).
(R1 in Figure 4) as suggested by Ip[ox]/ Ip[red]= 2.55. Thus, a single electron oxidation of
the urazole to form radical cation 12 appears to be rapidly followed by deprotonation to
10 which is already at an electrode potential sufficient to effect a second oxidation to 6
(see Scheme 7). Single electron reduction of the generated 6 to afford 10 then occurs
uneventfully.
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Scheme 7. Proposed sequence of electrochemical events during CV analysis of urazole
3.
Figure 5. A) Appearance of the platinum working electrode during collection of the CV
of urazole 3 prior to the oxidation event (O1 in Figure 4) as well as past the reduction
event (R1). B) Electrode during the oxidation event.
Interestingly, upon oxidation of 3, a deep purple colored species, most likely that of 6,
was observed to coat the working electrode (Figure 5). Upon reduction, the color was
lost. Presumably the same process occurs during formation of 6 from radical 10, but the
deep blue color of solutions of 10 prevents its observation. When the electrode potential
was held just positive of that of Ep[ox], a purple film appeared on the electrode, but then
dissipated within a few seconds, and the reduction event was no longer observed. These
observations suggest that diazenium 6 is not a stable species. When the cyclic
voltammogram was conducted in the presence of anisole, while the oxidation remained
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unaffected, the reduction was no longer observed (as was the case for 10) and there was
no indication of color formation on the working electrode suggesting rapid quenching of
generated 6.
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The instability of 6 is easily rationalized by the close proximity of the two
electronꢀwithdrawing carbonyl groups relative to the positively charged nitrogen atoms.
The most closely related diazenium species we could find in the literature is that of 13,6
which lacks the second carbonyl group adjacent to the azo group. Interesting, diazenium
13 is reported to be isolable as the tetrafluoroborate salt. Unfortunately, however, neither
its electrochemical properties nor its chemical reactivity have been described.
Chemical generation of diazenium 6
Given the apparent success of electrochemical generation of diazenium 6, we
were keen to test whether we might be able to chemically oxidize radical 10 to diazenium
6 in solution. We selected thiathrenium perchlorate, 14, as a suitable oneꢀelectron
oxidant because of its ready solubility in acetonitrile and the ease with which the reduced
thianthrene form could be separated from our expected products.7
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Our initial experiment involved dropwise addition of a solution of 14 to a
preformed solution of 10 in the presence of anisole. We realized quite early, however,
that addition of even a catalytic amount of oxidant was sufficient to deplete the deep blue
color of 10 in just a few minutes. Disubstituted compounds 4a,b (60/40 ratio) were
observed in a yield (32%) strikingly similar to that observed for the trifluoroacetic acidꢀ
catalyzed reaction of 10 with anisole described earlier. Similarly, an equivalent amount
of urazole 3 (29% yield) was also isolated. Evidently, small amounts of acid generated
from the initial EAS process further catalyzed the reaction in a manner similar to that of
TFA.
To prevent the acidꢀcatalyzed reaction route we added an equivalent of the nonꢀ
nucleophilic base 2,6ꢀlutidine to act as a proton sponge. In the presence of 2,6ꢀlutidine,
under otherwise identical conditions, it required the dropwise addition of 0.72 equivalents
of 14 to deplete the deep blue color of 10, suggesting that the radical was effectively
formed in 72% yield from Ni2O3 oxidation of the urazole. A mixture of compounds 4a,b
was isolated from the reaction in a ratio of ~60:40 in 55% yield (or 76% yield based on
72% yield for the radical formation process). No other products were observed apart
from that of the thianthrene byꢀproduct. The observed ratio of products 4a,b was
essentially identical to that observed from the direct reaction of MeTAD with anisole in
the presence of TFA suggesting that the diazenium ion 6 is most likely an intermediate in
both reactions. Furthermore, the same product ratio is observed from the acidꢀcatalyzed
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reactions of 10 with anisole, implicating 6 as a common intermediate in all of these
reactions.
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In a separate experiment, a solution of 14 was added dropwise to a solution of 10
and 2,6ꢀlutidine in the initial absence of anisole. Immediately upon consumption of the
deep blue color of the radical (again requiring only 0.75 equivalents of 14), anisole was
immediately introduced into the stirring reaction mixture. Under these conditions,
however, a complex reaction mixture was obtained, and there was no indication of
formation of compounds 4a,b. This experiment confirmed what the cyclic voltammetry
experiments had previously suggested, that diazenium 6 is an unstable species.
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C. A Proposed Reaction Mechanism
As put forward in our previous work,1 initial reaction of 1 with anisole in the
presence of an acid catalyst to afford urazole 3 is expected to proceed via a typical EAS
reaction. The UVꢀVis experiments reported in this work have established that 1 is
capable of further reaction with 3 via hydrogen atom abstraction to generate readily
observable concentrations of radical 10 and presumably urazole radical 15 (Scheme 8).
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Scheme 8. Proposed mechanism for the formation of disubstituted urazoles 4a,b from
initially formed urazole 3 via intermediate diazenium 6.
Protonation of 15 by TFA would afford radical cation 16 that is capable of oxidizing
radical 10 to form intermediate diazenium 6. Cyclic voltammetry experiments
demonstrated that oxidation of Nꢀmethylurazole (5) to form radical cation 16 occurs at
higher oxidation potential (Ep=1.52 V) than does oxidation of 10 (Ep = 1.17 V) rendering
16 a viable oxidant for 10.8 Thus, the formation of 5 as a byproduct from the reaction
(40% yield) is accounted for by this mechanism as well as a nearly equivalent amount of
disubstituted compounds 4a,b (44% yield) resulting from subsequent reaction of 6 with
anisole. Radical 10 could also potentially be oxidized to diazenium 6 by protonated 10
(i.e. 12) in addition to oxidation by 16. However, the low yields of urazole 3 (the
expected byproduct of oxidation) derived from the acidꢀcatalyzed reaction of MeTAD
with anisole suggest this is a minor reaction route at best.
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As described earlier, treatment of solutions of radical 10 with an acid catalyst
(either TFA or acid byproduct from introduction of a catalytic amount of 14) in the
presence of anisole produced 4a,b in the same 60:40 ratio as the MeTAD/TFA reaction
suggesting the common intermediacy of diazenium 6. Under these circumstances it must
be that protonated 10, i.e., 12, is acting as the oxidant for 10 to generate urazole 3 and
diazenium 6, which then reacts with anisole to form 4a,b (see Scheme 9). This accounts
for the formation of equivalent yields of 4a,b and 3 from the acidꢀcatalyzed reactions.
Additionally, only small amounts of dimer 11 were formed under these conditions,
suggesting that trapping of diazenium 6 by anisole is a faster reaction than is acidꢀ
catalyzed formation of dimer 11 (i.e. k2 > k1 ). Recall, however, that when TFA alone
was added to a solution of 10, the predominant reaction route was dimerization to 11, and
only a minor amount of urazole 3 (6% yield) was obtained. Any diazenium 6 formed
from oxidation of 10 by 12 in the absence of anisole must be uninterested in reacting with
the aromatic rings of either radical 10 or dimer 11, most likely for steric reasons.
Therefore, under these conditions, any 6 formed is most likely reduced back to 10 (by 3)
via an equilibrium process (Scheme 9). Consistent with these observations is that acidꢀ
catalyzed consumption of 10 in the presence of anisole is considerably faster (3 min) than
reaction in its absence (45 min).
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Scheme 9. Proposed processes between active species in solutions of 10 in the presence
of an acid catalyst.
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Finally, we attempted to determine if 12 could act as an independent electrophile
in its reaction with anisole. To this end, a solution of 10 was added dropwise over an
hour to a solution of anisole and TFA in an attempt to keep the concentration of 10 (and
therefore 12) low in order to prevent oxidation of 10 by a molecule of 12. The deep blue
color of the radical was initially rapidly discharged with each drop of solution. As further
addition occurred, however, while the blue color continued to be rapidly discharged, a
pale emerald green color gradually developed which remained at reaction’s end.
Interestingly, we obtained results nearly identical to those of the reaction in which TFA
was added directly to a mixture of 10 and anisole, i.e. a 35% yield of 4a,b (60/40 ratio)
and a 29% yield of 3 were isolated. Hence, 12 is apparently not capable of reacting with
anisole alone via an EAS reaction or a yield of 4a,b significantly greater than that of 3
would be expected.
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One question remains unanswered at this point, why were disubstituted products
only observed in the acidꢀcatalyzed reaction of 1 with anisole and not with the other
substituted benzene substrates investigated in the original study (e.g., 1,3ꢀ
dimethoxybenzene, 1,3,5ꢀtrimethoxybenzene, mesitylene, etc.)?1 A clue to this mystery
is found in the earlier described UVꢀVis experiment in which 1 was mixed with urazole 3
to rapidly form appreciable concentrations of radical 10. Interestingly, when 1 was
similarly mixed with urazoles 17aꢀc (i.e. the urazoles formed from EAS reaction of 1
with the aromatic substrates above) the UVꢀVis spectra remained unaffected and the
production of new species, including the expected corresponding radicals 18a-c, was not
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Scheme 10. Urazole radicals (18a-c) potentially formed via hydrogen abstraction from
urazoles 17a-c.
observed to occur (Scheme 10). One source of stability for radical 10 derives from its
ability to maintain coplanarity of the urazole ring with the aromatic system that allows for
extended delocalization of the unpaired electron as discussed earlier (Figure 2). We
suspected that the orthoꢀsubstituents on the aromatic rings of 18a-c might prevent similar
coplanarity of the two ring systems and thereby lessen their stabilities. Thus, as for 10,
we minimized the structures of the radical species 18a-c (see Figure 6) at the B3LYP/6ꢀ
31G* level of theory. As expected, the urazole rings were incapable of attaining
planarity with the benzene rings due to steric hindrance. The dihedral angles between the
two rings were found to be 62.9° for 18a, 69.9° for 18b, and 44.7° for 18c. Thus, the
extent of delocalization of the unpaired spin, and expected commensurate stabilization, is
attenuated relative to that of 10 (compare Figures 2 and 6).
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Figure 6. Spin density maps calculated at the UB3LYP/6ꢀ31G* level of theory (isovalue
= 0.004) for urazole radicals A) 18a, B) 18b, and C) 18c.
When urazoles 17a-c were treated with Ni2O3 in the same manner as described for
3, colored solutions characteristic of urazole radicals were not observed to form even
though the starting urazole was consumed as judged by TLC. Upon filtration and
concentration of each of these solutions, white solid products were obtained which did
not exhibit NꢀH stretches in the IR spectrum ruling out dimerized products akin to the
formation of 11 from 10. Furthermore, unlike radical 10 for which NMR data could not
be obtained due to its paramagnetic nature, the 1H and 13C NMR spectra of solutions of
the products isolated from these oxidations were readily obtained. These observations, in
addition to the lack of formation of characteristic colors upon dissolution in organic
solvents, suggests that for these three systems the tetrazane dimers are considerably more
stable than the corresponding urazole radical species (see Scheme 4). While similar
tetrazane dimers are known,9 to our knowledge this is the first report of urazole radicals
that are heavily weighted to the tetrazane dimer side of the equilibrium while in solution.
The 1H NMR spectra for the tetrazane dimer derived from urazole radical 18a was well
behaved but showed unique chemical shifts for all three methyl groups as well as both
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protons on the aromatic ring (see copies of all of these spectra in the Supporting
Information). One of the methyl groups was significantly upfield (1.48 ppm) of the
remaining two (2.23 and 2.24 ppm). Additionally, the 13C NMR spectrum displayed six
individual carbon atoms for the aromatic rings in addition to three unique carbon atoms
for the benzene methyl groups. The NMR spectra for the tetrazane dimer from 18b
exhibited similar characteristics. The reason for this behavior most likely derives from
the crowded structure required for these dimers that heavily restrict free rotation about
the C–N bond between the urazole nitrogen atom and the aromatic ring (Figure 7). A
conformation such as this would account for the single upfield shift of one of the CH3
groups of the tetrazane dimer of 18a (and one of the OCH3 groups from the tetrazane
dimer of 18b) as one group is necessarily suspended within the shielding cone of the
aromatic ring of the other urazole unit. Further support for restricted rotation derives
from the NMR spectra of 18c. Having a single orthoꢀsubstituent on the aromatic ring
allows for more rotational freedom in this tetrazane dimer than those of 18a,b. The 1H
and 13C NMR spectra display complex, broadened signals even though the solutions
remain colorless suggesting that the signal broadening derives from restricted rotation
and not from the presence of free urazole radicals. Heating the sample to 60 °C in the
NMR probe sharpened the signals in the 1H NMR spectrum somewhat (see the
Supporting Information). Further heating resulted in the formation of a dark green
colored solution as would be expected from the generation of free urazole radicals via
dissociation of the tetrazane dimer. Unfortunately, however, this led to decomposition of
the sample so that collecting a suitable 13C NMR spectrum was rendered impossible.
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Hence, as to why disubstituted products were observed only from the acidꢀ
catalyzed reaction of 1 with anisole, it is most likely that the relative instability of urazole
radicals 18a-c inhibits their formation via reaction of 1 with initiallyꢀformed urazoles
17a-c thereby preventing further oxidation to a diazenium species which would be
required to form disubstituted products.
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Figure 7. Minimized geometry for the tetrazane dimer derived from urazole radical 18a
at the B3LYP/6ꢀ31G* level of theory
CONCLUSION
The mechanism by which disubstituted urazole products 4a,b were formed from
the acidꢀcatalyzed reaction of MeTAD (1) with anisole has been elucidated. Rapid
hydrogen atom abstraction from initiallyꢀformed urazole 3 by 1 generates a particulary
stable and persistent radical 10. This was confirmed via UVꢀVis studies and corroborated
by generation of 10 by independent means (i.e. Ni2O3 oxidation of 3). Oxidation of 10 by
either radical cation 12 or 16 generates electrophilic diazenium species 6. Diazenium 6
could be independently generated via electrochemical as well as chemical (thianthrenium
perchlorate) oxidation of 10 and demonstrated to react with electronꢀrich anisole to form
products 4a,b in a ratio equivalent to that of the MeTAD/TFA/anisole reaction suggesting
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identical electrophiles are active in both cases. The reluctance of other urazoles (e.g.,
17a-c) to engage in a similar series of reactions leading to disubstituted products has been
traced to the apparent instability of the required urazole radicals (18a-c).
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EXPERIMENTAL
General Methods. Column chromatography was conducted on silica gel (234ꢀ
400 mesh) using appropriate mixtures of hexanes and EtOAc as eluent as specified.
Thinꢀlayer chromatography was performed on precoated silica gel plates (250 mm) and
visualized by ultraviolet light. H and 13C NMR spectra were obtained on a 400 MHz
1
NMR spectrometer. Chemical shifts are reported in units of parts per million downfield
from TMS. Highꢀresolution mass spectra (HRMS) were acquired via electron spray
ionization on an LTQꢀFTMS hybrid mass spectrometer. NꢀMethylꢀ1,3,5ꢀtriazolineꢀ3,5ꢀ
dione (2) was synthesized via oxidation of Nꢀmethylurazole10 with N2O4 according to the
literature,11 and sublimed before use. Thianthrenium perchlorate was synthesized
according to the literature procedure.7 CH2Cl2 was dried over 3 Å molecular sieves prior
to use. All other chemicals and solvents were obtained from commercial sources and
used without further purification.
Reaction of MeTAD (1) with Anisole. To a stirring solution of 0.113 g (1
mmol) of MeTAD (1) in 10 mL of dry CH2Cl2 was added 0.162 g (1.5 mmol) of anisole
followed by 160 ꢀL (2 mmol) of TFA. The solution rapidly turned blood red in color.
After stirring overnight, the resulting pale pink solution was diluted with 25 mL of
CH2Cl2 and washed 1 × 15 mL of 1 M NaOH. The aqueous phase was backwashed with
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1 × 10 mL of CH2Cl2 and the combined organic layers dried over Na2SO4, filtered and
concentrated. Column chromatography (50:50 hexanes: EtOAc) afforded 0.145 g (44%
yield) of a mixture of compounds 1,2ꢀbis(4ꢀmethoxyphenyl)ꢀ4ꢀmethylꢀ1,2,4ꢀtriazolidineꢀ
3,5ꢀdione (4a) and 1ꢀ(2ꢀmethoxyphenyl)ꢀ2ꢀ(4ꢀmethoxyphenyl)ꢀ4ꢀmethylꢀ1,2,4ꢀ
triazolidineꢀ3,5ꢀdione (4b) in a 60:40 ratio respectively as a white solid (ratio determined
by relative integrations in the 1H NMR spectrum). The aqueous phase was acidified with
conc. HCl and washed 6 × 10 mL of CH2Cl2. The combined organic layers were dried
over Na2SO4, filtered and concentrated to afford 12.8 mg (6% yield) of 1ꢀ(4ꢀ
methoxyphenyl)ꢀ4ꢀmethylꢀ1,2,4ꢀtriazolidineꢀ3,5ꢀdione (3) as a white solid. Finally, the
acidified aqueous layer was concentrated to a white solid. The solid was washed 2 × 20
mL of boiling nꢀpropanol followed each time by filtration. The combined washes were
concentrated to afford 46 mg (40% yield) of 4ꢀmethylꢀ1,2,4ꢀtriazolidineꢀ3,5ꢀdione (5) as
a white solid. All of the compounds isolated have been fully characterized elsewhere.1
Note that the yield of compounds 4a/4b reported in this report (44%) differs from that
reported earlier (62%).1 However, the currently reported value is considered to be
reliable.
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Dimerization of Urazole Radical 10 to Afford 1-[5-methoxy-2-(4-methyl-3,5-
dioxo-1,2,4-triazolidin-1-yl)phenyl]-2-(4-methoxyphenyl)-4-methyl-1,2,4-
triazolidine-3,5-dione (11). To a stirring mixture of 60 mg (0.3 mmol) of urazole 3 in
20 mL dry CH2Cl2 was added 300 mg of Na2SO4 followed by 170 mg of Ni2O3. The
solution rapidly turned deep blue in color. After stirring for 20 min, the reaction mixture
was filtered under N2 pressure through a fine glass frit into a flask. The solution was
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deoxygenated with a stream of dry N2, sealed, and stored in the dark until the radical
color was lost (8 days). The resulting pale green solution was concentrated and the
residue column chromatographed (100% EtOAc) to afford 18 mg (30% yield) of dimer
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1
11 as a pale yellow film. H NMR (CDCl3) δ 3.20 (s, 3H), 3.21 (s, 3H), 3.70 (s, 3H),
3.74 (s, 3H), 6.72 (d, J = 2 Hz, 1H), 6.88 (d, J = 9.2 Hz, 2H), 6.89 (dd, J = 9.0, 2.9 Hz,
1H), 7.34 (d, J = 9.2 Hz, 2H), 7.36 (d, J = 9.0 Hz, 1H), 8.57 (br s, 1H). 13C NMR
(CDCl3) δ 161.2, 158.5, 155.2, 153.7, 153.4, 152.5, 136.1, 131.4, 127.9, 125.2, 123.9,
115.1, 114.7, 109.3, 55.8, 55.4, 26.1, 25.5. HRMS (ESI) m/z: [M+H]+ Calcd for
C20H21N6O6: 441.1523; Found 441.1516.
Reaction of Radical 10 with Anisole as Catalyzed by TFA. A deep blue
solution of radical 10 was generated by stirring 30 mg (0.14 mmol) of urazole 3 with 150
mg of Na2SO4 and 85 mg of Ni2O3 in 10 mL of CH2Cl2 for 20 min followed by filtration
through a fine glass frit under N2 pressure. To this stirring solution was added 150 ꢀL
(10 eq) of anisole followed by 21 ꢀL (2 eq) of TFA via syringe. The solution decolorized
within 3 min time upon which it was diluted with 25 mL CH2Cl2 and washed 1 × 25 mL
of 1 M NaOH. The aqueous phase was backwashed 1 × 10 mL of CH2Cl2 and the
combined organic layers dried over Na2SO4, filtered and concentrated. Column
chromatography of the residue (50:50 hexanes: EtOAc) afforded 11 mg (25% yield) of a
mixture of 4a and 4b in a 60:40 ratio, respectively. The aqueous layer was then acidified
with conc. HCl and washed 6 × 10 mL of CH2Cl2. The combined organic layers were
dried over Na2SO4, filtered and concentrated to afford 12.5 mg of a mixture of urazole 3
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and dimer 11 in 25% and 16% yields, respectively, as determined by relative integrations
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in the 1H NMR spectrum.
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Reaction of Radical 10 with TFA (in the Absence of Anisole). A deep blue
solution of radical 10 was generated by stirring 30 mg (0.14 mmol) of urazole 3 with 150
mg of Na2SO4 and 85 mg of Ni2O3 in 10 mL of CH2Cl2 for 20 min followed by filtration
through a fine glass frit under N2 pressure. To this stirring solution was added 21 ꢀL (2
eq) of TFA via syringe. The deep blue solution turned pale green over 45 min upon
which it was diluted with 25 mL CH2Cl2 and washed 1 × 25 mL of 1 M NaOH. The
aqueous phase was backwashed 1 × 10 mL of CH2Cl2. The aqueous layer was then
acidified with conc. HCl and washed 6 × 10 mL of CH2Cl2. The combined organic layers
were dried over Na2SO4, filtered and concentrated to afford 22.8 mg of a mixture of
urazole 3 and dimer 11 in 6% and 74% yields, respectively, as determined by relative
integrations in the 1H NMR spectrum.
Reaction of Radical 10 in the Presence of Urazole 3. A deep blue solution of
radical 10 was generated by stirring 60 mg (0.27 mmol) of urazole 3 with 300 mg of
Na2SO4 and 170 mg of Ni2O3 in 20 mL of CH2Cl2 for 20 min followed by filtration
through a fine glass frit under N2 pressure. To this stirring solution of radical 10 was
added 45 mg (0.20 mmol, approximately 1 eq relative to generated radical) of urazole 3.
The solution was briefly deoxygenated with N2, sealed with a septum, and allowed to stir
in the dark until the color of the solution turned a pale green (2 days). The reaction
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mixture was concentrated to afford a mixture of dimer 11 and unreacted 3 as determined
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by relative integrations in the 1H NMR spectrum.
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Reaction of Radical 10 with Anisole as Catalyzed by Catalytic
Thianthrenium Perchlorate. A deep blue solution of radical 10 was generated by
stirring 30 mg (0.14 mmol) of urazole 3 with 150 mg of Na2SO4 and 85 mg of Ni2O3 in
10 mL of CH2Cl2 for 20 min followed by filtration through a fine glass frit under N2
pressure. To this stirring solution was added 30 ꢀL (2 eq) of anisole followed by ~20
drops of a prepared solution of 37 mg (0.12 mmol) of thianthrenium perchlorate in 6 mL
of acetonitrile (~5% of the ThClO4 was added as determined by mass difference after
removing acetonitrile from the remaining solution). The solution decolorized within 5
min time upon which it was diluted with 25 mL CH2Cl2 and washed 1 × 25 mL of 1 M
NaOH. The aqueous phase was backwashed 1 × 10 mL of CH2Cl2 and the combined
organic layers dried over Na2SO4, filtered and concentrated. Column chromatography of
the residue (50:50 hexanes: EtOAc) afforded 14 mg (32% yield) of a mixture of 4a and
4b in a 60:40 ratio, respectively. The aqueous layer was then acidified with conc. HCl
and washed 6 × 10 mL of CH2Cl2. The combined organic layers were dried over Na2SO4,
filtered and concentrated to afford 16 mg of a mixture of urazole 3 and dimer 11 in 29%
and 24% yields, respectively, as determined by relative integrations in the 1H NMR
spectrum.
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Generation of Diazenium 6 via Oxidation of Radical 10 by Thianthrenium
Perchlorate in the Presence of Anisole and 2,6-Lutidine. A deep blue solution of
radical 10 was generated by stirring 30 mg (0.14 mmol) of urazole 3 with 150 mg of
Na2SO4 and 85 mg of Ni2O3 in 10 mL of CH2Cl2 for 20 min followed by filtration
through a fine glass frit under N2 pressure. To this stirring solution was added 30 ꢀL (2
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eq) of anisole and 10 ꢀL (1 eq) of 2,6ꢀlutidine. A solution of 41 mg (1 eq) of
thianthrenium perchlorate in 6 mL of acetonitrile was added dropwise until the blue color
of the radical was consumed (~72% of the ThClO4 was added as determined by mass
difference after removing acetonitrile from the remaining solution). The resulting pale
yellow solution was then diluted with 25 mL of CH2Cl2 and washed 1 × 25 mL of 1 M
HCl and 1 × 25 mL of 1 M NaOH. The aqueous phases were each backwashed 1 × 10
mL of CH2Cl2 and the combined organic layers dried over Na2SO4, filtered and
concentrated. Column chromatography of the residue (50:50 hexanes: EtOAc) afforded
24.3 mg (55% yield based on starting 3) of a mixture of 4a and 4b in a 60:40 ratio,
respectively.
Slow Addition of Radical 10 to a Solution of Anisole and TFA. A deep blue
solution of radical 10 was generated by stirring 30 mg (0.14 mmol) of urazole 3 with 150
mg of Na2SO4 and 85 mg of Ni2O3 in 10 mL of CH2Cl2 for 20 min followed by filtration
through a fine glass frit under N2 pressure. This solution was taken up in a syringe and
added dropwise to a stirring solution of 30 ꢀL (2 eq) of anisole and 21 ꢀL (2 eq) of TFA
over an hour as controlled by a syringe pump. The first part (~30%) of the solution
resulted in rapid decolorization of the blue radical color to afford a colorless solution, but
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The Journal of Organic Chemistry
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as addition continued the color of the solution turned pale green and progressed to an
emerald green by the end of the hour. The resulting solution was stirred for an additional
0.5 hr without any change in color. It was then diluted with 25 mL CH2Cl2 and washed 1
× 25 mL of 1 M NaOH. The aqueous phase was backwashed 1 × 10 mL of CH2Cl2 and
the combined organic layers dried over Na2SO4, filtered and concentrated. Column
chromatography of the residue (50:50 hexanes: EtOAc) afforded 16 mg (35% yield) of a
mixture of 4a and 4b in a 60:40 ratio, respectively. The aqueous layer was then acidified
with conc. HCl and washed 6 × 10 mL of CH2Cl2. The combined organic layers were
dried over Na2SO4, filtered and concentrated to afford 16 mg of a mixture of urazole 3
and dimer 11 in 29% and 25% yields, respectively, as determined by relative integrations
in the 1H NMR spectrum.
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Oxidation of Urazole 17a to 4-methyl-1-[4-methyl-3,5-dioxo-2-(2,4,6-
trimethylphenyl)-1,2,4-triazolidin-1-yl]-2-(2,4,6-trimethylphenyl)-1,2,4-triazolidine-
3,5-dione (i.e. Dimer of Radical 18a). To a stirring solution of 32 mg (0.14 mmol) of
urazole 17a in 10 mL dry CH2Cl2 was added 150 mg of Na2SO4 followed by 85 mg of
Ni2O3. The mixture was stirred for 1 hr, filtered through a fine glass frit under N2
pressure, and concentrated to afford 32 mg (quantitative) of a white solid, m.p. 160 °C
(decomp). IR (ATR) cmꢀ1 2921, 1689, 1467. 1H NMR (CDCl3) δ 1.48 (s, 3H), 2.23 (s,
3H), 2.25 (s, 3H), 3.22 (s, 3H), 6.36 (s, 1H), 6.81 (s, 3H); 13C NMR (CDCl3) δ 154.2,
149.9, 140.5, 140.2, 137.7, 129.6, 128.8, 127.5, 26.1, 21.2, 18.2, 16.8. HRMS (ESI) m/z:
[M+H]+ Calcd for C24H29N6O4: 465.2250; Found 465.2245.
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