Azo synthesis meets molecular iodine catalysis

Article abstract of DOI:10.1039/d1ra00369k

A metal-free synthetic protocol for azo compound formation by the direct oxidation of hydrazine HN-NH bonds to azo group functionality catalyzed by molecular iodine is disclosed. The strengths of this reactivity include rapid reaction times, low catalyst loadings, use of ambient dioxygen as a stoichiometric oxidant, and ease of experimental set-up and azo product isolation. Mechanistic studies and density functional theory computations offering insight into this reactivity, as well as the events leading to azo group formation are presented. Collectively, this study expands the potential of main-group element iodine as an inexpensive catalyst, while delivering a useful transformation for forming azo compounds.

Full text of DOI:10.1039/d1ra00369k

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Azo synthesis meets molecular iodine catalysis†
*
Rozhin Rowshanpour and Travis Dudding
Cite this: RSC Adv., 2021, 11, 7251
A metal-free synthetic protocol for azo compound formation by the direct oxidation of hydrazine HN–NH
bonds to azo group functionality catalyzed by molecular iodine is disclosed. The strengths of this reactivity
include rapid reaction times, low catalyst loadings, use of ambient dioxygen as a stoichiometric oxidant, and
ease of experimental set-up and azo product isolation. Mechanistic studies and density functional theory
computations offering insight into this reactivity, as well as the events leading to azo group formation are
presented. Collectively, this study expands the potential of main-group element iodine as an inexpensive
catalyst, while delivering a useful transformation for forming azo compounds.
Received 15th January 2021
Accepted 2nd February 2021
DOI: 10.1039/d1ra00369k
rsc.li/rsc-advances
From this precedent along with other works,21,22 and with it,
a heavy reliance upon transition metals and/or hazardous
Introduction
reagents, the prospect of employing the oxidative capacity of
main-group elements for azo group construction captured our
attention. This possibility was especially appealing in light of
a growing interest in replacing, or at the very least, com-
plementing metal-based procedures with sustainable main group
element-catalyzed protocols.23 Driving this interest, the key
parameters of cost, dwindling supplies of precious metals and
associated political factors, and importantly global sustainability
enabling productive harmony, stability and resilience to support
present and future generations. Towards this end, we report
herein the rst metal-free, synthetic protocol for azo compound
Azo compounds have captivated chemists ever since their debut
in the 1860's and use as an industrial manufactured color,
shortly aer Peter Griess's synthesis of Manchester Bismarck
brown.1 Nowadays, the commercial, academic and industrial uses
of azo compounds are widespread, including applications as
organic dyes and pigments,2 free-radical initiators,3 acid–base
4
5
´
indicators and chemosensors. Beyond this melange of usages,
the switchable and spatiotemporal (light/thermal)-driven control
of E- vs. Z-azo group double geometry of many azo compounds
continues to prompt their use in photoresponsive so materials
functioning as liquid crystals,6 smart polymers,7 photochromic
ligands for optochemical genetics8 and photoswitches.9
As to be expected, this revered status has ushered in the
advancement of various synthetic strategies for accessing
(hetero)aryl functionalized azo compounds, yet practical
synthetic methods for preparing these valuable compounds is
needed. Among the most widely used methods for forming azo
compounds are oxidative dimerization of aromatic amines,10
reductive dimerization of nitrobenzenes,11 Mills reaction,12 diazo-
nium coupling13 and oxidative dehydrogenation14 (Fig. 1, top);
however, each of these synthetic methods have inherent drawbacks,
such as employing toxic and expensive transition metals and/or
hazardous reagents. For instance, the oxidative dimerization of
aromatic amines frequently relies on the use of stoichiometric
AgO,15 ferrate salts,16 tert-butyl hypoiodite17 or environmentally
unfriendly heavy-metal oxidants BaMnO4,18 Pb(OAc)4,19 and HgO.20
Moreover, the Mills reaction12 and diazonium coupling13 require the
use of toxic nitrosoarenes or explosive diazonium salt substrates.
Similarly, reductive dimerization of nitroarenes11 is plagued by the
use of toxic and oen carcinogenic nitro-containing compounds.
Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, L2S 3A1, Canada.
E-mail: tdudding@brocku.ca
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00369k
Fig. 1 Previous reported procedures for azo synthesis (top) and
current iodine catalyzed synthesis of azo compounds (bottom).10–14
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formation by the direct oxidation of hydrazine HN–NH bonds to foreshadowing reactivity, the highest occupied molecular orbital
azo group functionality catalyzed by substoichiometric amounts (HOMO, ꢀ0.26 eV) of this halogen bond complex was localized
of molecular iodine (Fig. 1, bottom). The strengths of this reac- upon the hydrazine fragment, while the lowest unoccupied
tivity include rapid reaction times, low catalyst loadings, use of molecular orbital (LUMO, ꢀ0.09 eV), dominated by anti-bonding
ambient dioxygen as a stoichiometric oxidant, and ease of s*-character, resided on I2 (Fig. 2, middle and right-hand side).
experimental set-up and azo product isolation.
Optimization studies
With this insight, yet cautiously aware of the lack of a reported
metal-free molecular iodine-catalyzed oxidation of hydrazine
HN–NH bonds to azo N]N double bonds,27 we commenced our
Results and discussion
At the outset of this study with an eye towards using easy-to-
study by investigating the reaction of diphenyl hydrazine (1a) in
various solvents using catalytic iodine loadings (2–10 mol%) as
summarized in Table 1. To this end, performing the reaction
with a tenth of an equivalent of molecular iodine at room
temperature in the presence of light and dry air as a stoichio-
metric co-oxidant resulted in very little azo product (2a)
formation in polar protic solvent MeOH (Table 1, entry 1).
Further, nonpolar solvents hexane and toluene proved even
worse (Table 1, entries 2 and 3), whereas polar aprotic solvents,
including acetone, acetonitrile, ethyl acetate, and tetrahydro-
furan (THF) resulted in negligible conversions (Table 1, entries
4–7). Suspecting this poor reactivity was not a systemic problem,
but rather an intrinsic feature of (1) hydroxyl, nitrile or carbonyl
binding to I2, that is, a solvent–solute sequestering effect or (2)
insolubility of I2 in nonpolar solvents, we performed the reac-
tion in dichloromethane (DCM) and chloroform (CHCl3). The
logic for choosing these solvents stemming from their weak
ability to engage in halogen–halogen or hydrogen-halogen
bonding with iodine, while offering good solubility. Indeed,
handle, environmentally friendly and relatively inexpensive
solid molecular iodine (e.g., over the last 5 years the bulk price
of iodine has been $19–27 per kg) as a catalyst for azo double
bond formation, we sought to delineate to what degree, if any,
halogen bonding might have in triggering hydrazine HN–NH bond
oxidation. Prompting this interest, among other factors, was the
prominence of iodine-catalyzed reactions, many of which catalyt-
ically pivot around the formation of a “halogen bond” (X–B)—
dened by IUPAC as the association between the electrophilic
region of a polarized halogen atom and a Lewis basic unit.24 The
electrophilic region, in this case, commonly being referred to as
a sigma hole (s).25 For molecular iodine this feature is clearly
visible from the calculated electrostatic potential surface, display-
ing signicant anisotropic electron distribution with positive
polarization on the iodine atoms (Fig. 2, le-hand side, top).
With this bearing in mind, and in reecting upon the history
of the halogen bond dating back more than two-centuries to the
rst reported synthesis of a halogen bond complex NH3/I2 by
Colin in 1814,26 it occurred to us that the formation of a hydrazine–
I2 complex should be a facile process. Indeed, formation of
computed n-type halogen bond complex XB from diphenyl
hydrazine and I2 was computed to be exergonic by ꢀ0.1 kcal molꢀ1
at the (IEFPCMDCM)UAPFD/def2-TZVP//UAPFD/DGDZVP level of
theory. This complex displayed an almost linear N/I–I angle of
175.9ꢁ with nitrogen-iodine and iodine–iodine bond distances of
Table 1 Screening iodine-catalyzed oxidation conditions for azo-
benzene formationa
ꢀ
ꢀ
2.77 A and 2.76 A (Fig. 2, bottom le-hand side). Moreover, in
Catalyst Reaction
loading conditions
Reaction
time
Yieldb
(%)
Entry Solvent
1
2
3
4
5
6
7
8
9
MeOH
10 mol% Room light
10 mol% Room light
10 mol% Room light
10 mol% Room light
2 hours
2 hours
2 hours
2 hours
2 hours
2 hours
2 hours
1 hour
>5
0
0
0
0
Toluene
Hexane
Acetone
Acetonitrile 10 mol% Room light
Ethyl acetate 10 mol% Room light
Dry THF
Dry DCM
Dry
0
10 mol% Room light
10 mol% Room light
10 mol% Room light
>5
100
92
1 hour
chloroform
Dry DCM
Dry DCM
Dry DCM
Dry DCM
10
11
12
13
5 mol% Room light
2 mol% Room light
2 mol% Dark
1 hour
1 hour
20 min
48 hours
100
100
100
0
Fig. 2 Electrostatic potential surface of I2 (electronegative (red) and
electropositive (blue) regions (left-hand side, top)) computed at the
B3LYP-D3/lacv3p**++ level of theory (isovalue ¼ ꢀ0.003 (max ¼
0.3, min ¼ ꢀ0.3)). Halogen bond complex XB (left-hand side, bottom)
computed at the (IEFPCMDCM)UAPFD/def2-TZVP//UAPFD/DGDZVP
level of theory. Chemdraw and computed HOMO and LUMO of
complex XB with orbital energies in eV (middle and right-hand side).
—
Dark
a
Reaction conditions: 1a (1 mmol), solvent (5.0 mL) and I2 (2–10 mol%)
at room temperature under a dry air atmosphere in the absence/
presence of ambient light. Yields are determined by quantitative 1H
b
NMR using CDCl3 as the internal standard.
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this hypothesis proved productive as full conversion to product target azo compounds in good-to-excellent yields (2a–u, Table
azobenzene 2a occurred in an hour with DCM as the solvent and 2). For instance, the reaction of o- and/or p-alkyl substituted aryl
high conversion was observed in CHCl3 (Table 1, entries 8 and hydrazines (1a–i) successfully afforded aromatic azo
9). Moreover, decreasing the loading of catalyst I2 afforded high compounds 2a–i in good-to-high yields, though sterically
conversion (Table 1, entries 10 and 11). Meanwhile, the use of hindered hydrazines 1i provided a slightly reduced yield of azo
dry air in the absence of light dramatically reduced the reaction compound 2i. Meanwhile, the presence of inductively-
time (Table 1, entry 12). Lastly, demonstrating the importance withdrawing and weak electron-donating p-substituted halo-
of iodine as a catalyst, no conversion was observed in the gens on the hydrazine aryl rings had little effect on the reaction
absence of I2 (Table 1, entry 13).
efficiency. For instance, p-iodo-, p-uoro-, and p-chloro-
substituted aryl hydrazines (1j–l) smoothly reacted to afford
azo compounds 2j–l. Similarly, o-chloro-substituted aryl hydra-
zine 1m was compatible, furnishing azo compound 2m in high
yield. Moreover, p-Alkyl and m-chloro- or m- or p-bromo-
substituted aryl hydrazines (1n–p) underwent efficient oxida-
tion to provide azo compounds 2n–p. Further, the presence of
electron-withdrawing triuoromethyl- and triuoromethyl
ether groups had no effect on the reaction with azo compounds
2q–s formed in high yields. Likewise, the reaction of electron-
donating p-methoxy-substituted aryl hydrazine 1t afforded azo
product 2t in high yield. Nitro functionality was also tolerated
with azo compound 2u formed in high yield. Lastly, to
demonstrate the synthetic utility of this methodology, we per-
formed a gram-scale reaction using 2.15 grams of 1a to afford 2a
in 94% isolated yield (Table 2, bottom).
Substrate scope
With the optimal reaction conditions established, we briey
examined the scope of this azo-forming protocol. To this end,
various 1,2-disubstituted hydrazines with electron-rich and
electron-poor aromatic groups smoothly reacted to furnish the
Table 2 Table of substrates for iodine-catalyzed oxidationa,b
Control studies
Our interest next shied to understanding the mechanistic
details of this simple iodine-catalyzed azo N]N double bond
forming reaction. An initial reaction using catalytic iodine
(2 mol%) in the absence of air negatively affected azo product
formation, suggesting the oxidation of molecular iodine or
iodide ion to higher oxidation states might have a role in the
reaction (Table 3, entry 1). To explore this aspect further, the role of
higher iodine oxidation states were investigated using hydrogen
peroxide (entry 2) resulting in quantitative conversion of 1a to
azobenzene (2a). Moreover, when we added DMSO (5 mol%, entry
3), a very good iodonium binding solvent,23 to the original iodine
(2 mol%) catalyzed reaction conditions, a good conversion was
obtained, thus suggesting iodonium is not involved in the reac-
tion. On the other hand, this nding is not at all surprising and
can be explained by the fact that DMSO is known to oxidize iodine,
thus implicating oxidative pathway(s). On the other hand, in the
presence of 10 mol% KOH (entry 4), which could form potassium
hypoiodite (KOI) in situ, no azo product was formed suggesting
that a hypoiodous/hypoiodite pathway is unlikely.
Further, addition of radical scavenger 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) to the reaction had little effect, hint-
ing that a radical pathway was not involved (entries 5 and 6).28
Next, exchanging elemental iodine for HI still provided product
suggesting that hydrogen iodide might be an intermediate in
the reaction (entry 7). Conversely, the use of HCl resulted in no
conversion establishing the essential role of iodine ion in this
reaction (entry 8). Meanwhile, the addition of potassium iodide
(10 mol%), negatively affected the reaction outcome (entry 9),
which might be due to the formation of potassium triiodide.
The use of molecular bromine instead iodine as a catalyst was
a
Reactions conditions: 1a–u (1 mmol), solvent (5.0 mL) and I2 (2 mol%)
at room temperature under a dry air atmosphere in the absence of light
for 20 minutes. b Yields refer to the isolated material.
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Table 3 Control studies of iodine-catalyzed oxidationa
Entry
Catalyst (2 mol%)
Additive
Conditions
Reaction time
Conversionb (%)
1
2
3
4
5
6
7
8
I2
I2
I2
I2
I2
I2
—
—
—
Br2
I2
—
H2O2
N2, dark
2 hours
20 min
20 min
2 hours
2 hours
1 hour
20 min
2 hours
2 hours
2 hours
1 hour
1 hour
0
100
100
0
74
100
100
0
8
0
40
68
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
Dry air, dark
DMSO (5 mol%)
KOH (10 mol%)
TEMPO (5 mol%)
TEMPO (100 mol%)
HI (10 mol%)
HCl (10 mol%)
KI (10 mol%)
—
9
10
11
12
Water (5 mol%)
Water (10 mol%)
I2
a
Reaction conditions: 1a (1 mmol), dry DCM (5.0 mL) and catalyst I2 (2 mol%) in presence/absence of additives at room temperature, in the absence
b
1
of light and under dry air atmosphere or N2(g)
.
Yields were determined by H NMR.
also attempted resulting in no observed azo product formation
Next, following slight structural reorganization subsequent rst-
and clearly demonstrating the importance of I2 as a catalyst order saddle point TS1 with a Gibbs free activation barrier (DG‡) of
(entry 10). Lastly, moisture had a negative effect on reactivity, 13.9 kcal molꢀ1 is surmounted. Dening this transition state
suggesting in situ derived hypoiodous acid (HOI) formed by involving Hydrogen Atom Transfer (HAT) and electron transfer type
ꢀ
disproportionation of molecular iodine and water had a nega- events were bond-breaking and bond-making distances of 1.44 A and
+
ꢀ
tive effect on azo product formation (entries 11 and 12).
1.81 A. The resulting cationic azo intermediate 2a-H of TS1 then
reacts by proton transfer transition state TS2 with an activation
barrier of 19.9 kcal molꢀ1 and bond-breaking and bond-making
Mechanistic studies
ꢀ
ꢀ
distances of 1.67 A and 1.93 A. Desired azo product (2a) formation
follows concomitant with the reaction of dioxygen and in situ derived
HI to afford water and catalytic I2. Collectively, these transformations
result in an overall exergonic process of ꢀ34.9 kcal molꢀ1. Mean-
while, in terms of the energetic span model and turnover frequency
(TOF), halogen bond complex XB corresponds to the TOF
As a last tool for understanding this reactivity we turned to DFT
computations at the (IEFPCMDCM)UAPFD/def2-TZVP//UAPFD/
DGDZVP level of theory using diphenyl hydrazine as a repre-
sentative substrate. Ultimately, this led to the tentative
computed pathway depicted in Fig. 3 initiating from hydrazine
1a that upon binding to I2 forms halogen bond complex XB.
Fig. 3 Free energy profile for I2 catalyzed azo formation computed at the (IEFPCMDCM)UAPFD/def2-TZVP//UAPFD/DGDZVP level of theory. Free
energies are given in kcal molꢀ1
.
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determining intermediate (TDI), while the TOF determining transi- of yield and Reaction Mass Efficiency (RME) all three procedures
tion state (TDTS) is TS2, thus resulting in an overall energy span of are high yielding with respectable RME values. Though the re-
26.0 kcal molꢀ1 (TON ¼ 5.5 ꢂ 10ꢀ8, TOF ¼ 2.9 ꢂ 10ꢀ9 sꢀ1).29
ported quantitative isolated product yield for the dehydrohaloge-
nation procedure of Okumura et al.17 is remarkable given the need
for several purication steps, including silica gel ash chroma-
tography. Meanwhile, our iodine-catalyzed oxidation procedure
beneted from signicantly higher carbon efficiency (CE) compare
to the two other reported procedures, in addition to having a much
lower E-factor making it a greener chemistry.
Finally, in speaking to sustainability and in light of the new
twelve green chemistry principles promulgated recently by
Zimmerman et al.,33 it is notably our reported molecule iodine
catalyzed synthesis of azo compounds fulls the majority of
these twelve principles.
Greenness measurements of the procedure
Lastly, to highlight the overall greenness and efficiency of our
molecular iodine catalyzed azo forming protocol we employ
green metrics.30 In this regard, as no one green metric is capable
of accounting for every possible parameter inuencing the
environmental impact of a process, we use several green metrics
and the standard reaction metric yield for this analysis. Further,
to showcase the favorability of our protocol comparisons are made
to previous literature reports for oxidizing hydrazine HN–NH
bonds to azo groups. For this analysis two metrics relating to mass
efficiency were selected; (1) Sheldon's E-factor was selected as an
easy measure of relative waste and (2) GlaxoSmith-Klein's (GSK)
metric was used to gauge reaction mass efficiency. Carbon effi-
ciency (CE) was also calculated to determine the efficiency of the
transfer of organic material. E-Factors and RME were calculated by
eqn (1) and (2) respectively, such that msm is the mass of starting
materials and mp is the mass products.31
Conclusion
To recap, in harnessing the oxidative power of molecular iodine,
we have reported an innovative catalytic method for converting
hydrazine HN–NH bonds to azo group N]N double bond
functionally. The signicance of this work is manifold,
including the use of commercially available, cheap, and easy-to-
handle molecular iodine as a non-metal catalyst. In addition,
the synthetic protocol described in this letter allows for the
preparation of privileged azo group functionality from plentiful
hydrazine precursors, while offering underlying mechanistic
understanding into iodine catalysis. Ongoing efforts in our lab
are expanding this unique reactivity to a repertoire of uses and
related ndings will be reported in due course.
msm ꢀ mp
E ¼
(1)
mp
mp
RME ¼
ꢂ 100
(2)
msm
The mass of all consumable starting reagents and catalysts
were incorporated, while solvents and silica gel were considered
recoverable and aqueous solutions were considered benign, and
therefore excluded from the calculation. These assumptions
were made for the sake of comparing bench scale chemistries,
where solvent choices, quantities and lifecycles can be expected
to change signicantly during scale-up to an industrial process.
CE was determined according to eqn (3), such that np is the
moles of product, nsm is the moles of each reagent, Csm is the
number of carbons of that reagent and Cp is the number of
carbons in the product.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
T. D. acknowledges nancial support from the Natural Science
and Engineering Research Council (NSERC) Discovery grant
(2019-04205). Computations were carried out using facilities at
SHARCNET (Shared Hierarchical Academic Research
Computing Network: http://www.sharcnet.ca) and Compute/
Calcul Canada. T. D. and R. R. thank the “Brock Library Open
Access Publishing Fund”.
np ꢂ Cp
P
CE ¼
ꢂ 100%
(3)
Csm ꢂ nsm
Overall, the green metrics and yield of our reaction are high,
if not, excellent as seen from Table 4. This is clearly seen by
comparison with reported t-BuOCl and t-BuOK procedures for
oxidizing hydrazine HN–NH bonds to azo groups.17,32 In terms
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