Selective Single-Step Oxidation of Amine to Cross-Azo Compounds with an Unhampered Primary Benzyl Alcohol Functionality

Article abstract of DOI:10.1021/acs.orglett.8b02829

This is the first report of a single-step synthesis of primary benzyl alcohol containing different cross-azo compounds (14 examples) by Cu(II) in the presence of a newly synthesized amino-ether heteroditopic macrobicycle cage. Interestingly, even with ext

Full text of DOI:10.1021/acs.orglett.8b02829

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Selective Single-Step Oxidation of Amine to Cross-Azo Compounds
with an Unhampered Primary Benzyl Alcohol Functionality
Sayan Sarkar, Piyali Sarkar, and Pradyut Ghosh*
School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Kolkata 700032,
India
S
* Supporting Information
ABSTRACT: This is the first report of a single-step synthesis
of primary benzyl alcohol containing different cross-azo
compounds (14 examples) by Cu(II) in the presence of a
newly synthesized amino-ether heteroditopic macrobicycle
cage. Interestingly, even with extreme conditions, the benzyl
alcohol remains unoxidized by the Cu(II) catalyst due to the
protective etherial pocket of the cage.
he development of synthetic strategies for selective
oxidation of a particular group among multiple oxidizable
macrobicyclic cage as a host that is suitable for forming a Cu(II)
complex as well as cascade complexation toward the syntheses of
a series of cross-azo compounds containing BOH functionality
(Scheme 1b).
T
functionalities is always a challenging task. In this direction,
syntheses of primary benzyl alcohol containing cross-azo
compounds are important in biology and chemistry.1−5 For
example, azo functionality itself plays a very important role in
photosensitive biological systems due to their cis−trans
isomerization,1 and the hydroxymethyl group incorporation
eases the progress of azo compounds toward the formation of
self-assembled monolayers,2 chain-shattered or end-capped for
self-immolative polymers,3 copolymers,4 dendritic organogels,5
etc. In the literature, many of the synthetic strategies for
benzylic−OH (BOH) containing azo-benzene (BOCAB)
formations involve oxidation of one amine to a nitroso group
followed by the condensation with another amine moiety
(Scheme 1a).1h,6 Generally, direct single-step syntheses of these
To catalyze a selective group in the presence of different labile
groups, an appropriately designed structured framework could
be very useful as a selective host for a specific group(s), which
eventually determines the reactivity of a particular group toward
catalysis. Thus, it may be articulated that employment of a cage-
type 3D moiety, having its well-defined functional groups, can
surround a guest with a greater proximity and greater binding
capability.9 Such a concept has indeed been utilized previously
by different groups toward catalysis.10 Also, the capability of the
cage toward protection of buried functions provides a general
approach to selective reactions.11 Keeping all these facts in mind
here, we have designed a new oxy-ether tris-amino heteroditopic
macrobicyclic cryptand (L) that consists of (i) tris-amine
functionality as the metal-chelating site, (ii) three parallel
benzene moieties for aromatic π−π stacking interactions for the
incoming aromatic guest molecules, and (iii) an oxy-ether
pocket for hydrogen bonding interactions. In fact, the
tetradentate metal chelating site (N4 site) of L can easily engulf
a transition metal ion such as Cu(II),12 which is prone to
forming a trigonal bipyramidal (TBP) structure; thus, this can
further allow coordination of another monodentate guest
molecule.13 Here, in our present cage design, an aromatic
amine guest can be chosen to occupy the vacant coordination
site of the metal center in the Cu(II) complex of L. On the other
hand, the oxy-ether pocket can act as a well-defined 3D cavity
which can also participate in hydrogen bonding interaction with
the BOH group of the substrate.14
Scheme 1. Comparative BOCAB Synthesis
compounds could not be achieved, as most of the oxidative
catalysts (metal as well as organic catalysts) responsible for N
N bond formation from amines7 also oxidize the benzyl alcohols
either to aldehydes or to acids.8 To the best of our knowledge,
selective single-step oxidation of amine to azo compound,
keeping the primary BOH substitution intact, has not been
reported to date. To achieve such partial oxidation by the Cu(II)
catalyst here, we introduce a new amino-ether heteroditopic
The L is synthesized in five steps starting from oxy-ether (1)
formation by reacting 4-hydroxybenzaldehyde with 2-(2-
chloroethoxy)ethanol followed by stepwise modifications of
Received: September 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02829
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
the leaving group to achieve 3 via 2. Then, the reaction between
3 and phloroglucinol provides the benzene platform-based
trialdehyde 4. In the final step, L is prepared in good yield (65%)
by reacting 4 with tris(2-aminoethyl)amine in dichloro-
methane−methanol solvent system via a high-dilution techni-
que followed by NaBH4 reduction (Scheme 2a). Character-
ization of L is carried out by 1H NMR, 13C NMR, DOSY, COSY
NMR, and HRMS (Figures 9S−12S). The ROESY spectrum
(Figure 11S) justifies the 3D spatial arrangement of L.
Table 1. Optimization of the Reaction Condition
temp
time
(h)
yield
b
entry
catalyst
CuSO4
Cu(NO3)2
Cu(OTf)2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
L-Cu(II) cryptate
solvent
(°C)
(%)
1
2
3
4
5
DMF
DMF
DMF
DMF
DMSO
CH3CN
C2H5OH
DMF
100
100
100
80
24
12
12
2.5
8
45
59
64
73
61
58
34
66
72
70
70
74
Scheme 2. Synthesis of New Heteroditopic Macrobicycle L
80
6
7
8
9
80
80
70
80
12
12
24
8
DMF
10
11
12
DMF
DMF
DMF
90
100
80
12
24
2.5
a
Reaction was performed with 4-aminobenzyl alcohol 5a (1.0 mmol),
p-toluidine 6a (1.0 mmol), catalyst (10 mol %), and macrobicycle L
b
(10 mol %) in 2 mL of solvent under aerobic conditions. Isolated
yields.
To ascertain the loading of the Cu(II)−L mixture in the reaction
of 5a and 6a, various amounts of a 1:1 mixture of Cu(II) and L
were attempted (Figure 14S). An effective percentage of the
mixture shows a linear increase in yield up to 10 mol %. Further
increase in the loading does not show any improvement in the
yield.
Employment of the acyclic analogue, L′ (condensation
product of 1 and tris(2-aminoethyl)amine, Scheme 2b), in
place of L under the optimized reaction condition does not
produce any trace of 7a. This was noticed from TLC and more
To establish the role of L in the oxidative coupling reaction,
we have chosen 4-aminobenzyl alcohol (5a) and p-toluidine
(6a) as model substrates. We have utilized L as a host having a
defined cavity as well as functionalities to perform the reaction of
5a and 6a in the presence of Cu(II) catalyst. Interestingly, in this
case, cross-coupled product with an unperturbed BOH group is
obtained as the major product (60−73% yields, depending on
the counteranions).
1
importantly from the H NMR of the crude, which shows no
characteristic peak of the benzylic proton of 7a (Figure 45S). It
is probably due to unbound orientations of the arms which are
incapable of providing protection to the benzylic−OH group.
Also, when Cu(II) salt alone is used as a catalyst (Scheme 3a),
cross-coupled product with oxidation of the BOH group is
obtained as the minor product (∼10%) along with predominant
homocoupled products (overall ∼77%) obtained from HPLC
Thus, this approach could activate the amine group by
keeping the BOH group intact, which eventually results in the
synthesis of the product BOCAB, 7a, in high yield, even in
different experimental conditions (Table 1). The catalytic
activity of various Cu(II) salts such as CuSO4, Cu(OTf)2,
Cu(NO3)2, and Cu(ClO4)2 was screened with different time and
temperature (Table 1). Due to relatively noninteracting nature
of the counteranions for Cu(OTf)2 and Cu(ClO4)2, better
throughputs of yields (60−70%) have been achieved (Table 1,
entries 3 and 4). When different solvent systems (Table 1,
entries 4−7) are explored, DMF provides the highest yield
(73%). Interestingly, even in extreme conditions, such as at
higher temperature and with greater reaction time (Table 1,
entries 9−11), benzylic alcohol functionality remains intact with
comparable yields. The catalytic activity of the isolated L−
Cu(II) cryptate is also investigated, but no mentionable
improvement in the yield is observed (Table 1, entry 12),
indicating that the in situ formed Cu(II) cryptate is equally
efficient to catalyze the above reaction. Due to the presence of a
single metal binding site (N4 cavity) in L, an equimolar mixture
(1:1) of Cu(II) and L is used as a catalyst in the above studies.
Scheme 3. Oxidative Coupling Using (a) Cu(II) Salt as
Catalyst and (b) Cu(II) Salt in the Presence of L as Catalyst
B
DOI: 10.1021/acs.orglett.8b02829
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
analysis. These results eventually prove that the cage moiety of L
is essential for this selective oxidation.
Scheme 4. List of Synthesized Compounds
The beauty of the reaction here is the formation of the cross-
azo compound as the major product over the corresponding
homo products, which is established by HPLC data of the crude
product for the reaction of 5a and 6a, using hexane as eluent
(Figure 1). The second peak from the left corresponds to the
Figure 1. HPLC data of the crude product for the reaction of 5a and 6a.
hetero product, and other two peaks represent the homo
products, which indicates that the formation of a cross-coupled
product highly outweighs the homo products in the presence of
L.
At the optimized reaction conditions, several aromatic amines
and aminobenzyl alcohols have been explored to produce 14
various unsymmetrical azo derivatives. Each of those products is
characterized by 1H NMR, 13C NMR, IR, and HRMS
spectroscopy (Figures 15S−42S), which clearly shows the
presence of the BOH functionality.
a
All the reactions were carried out using amines (1 mmol each),
Cu(ClO4)2 (10 mol %), and L (10 mol %) in DMF (2.0 mL) at 80 °C
for 2−3 h under aerobic conditions. Isolated yields.
b
Scheme 4 clearly demonstrates that this reaction method is
favorable for a library of aniline systems containing different
substituents. Electron-donating substituents in the nucleophilic
amine part advance yields of desired products (7a, 7b, 7j, 7l, and
7m), whereas, for electron-withdrawing groups, the yields are
reduced diminutively (7c−7i, 7k, and 7n). Diversely substituted
aminobenzyl alcohols also provide excellent yields, which
demonstrate the stability of meta-BOH and substituted BOH
in the cavity of L. Interestingly, we have successfully crystallized
compounds 7e and 7n. The single-crystal X-ray structures of
these two compounds (Figure 43S) confirm the presence of the
unhampered BOH moiety in this class of molecules.
L/Cu(ClO4)2/PABA (λmax ∼ 440 nm, Figure 2a, spectrum 2)
and (ii) L/Cu(ClO4)2/p-toluidine (λmax ∼ 412 nm, Figure 2a,
spectrum 3) reveals that the λmax of L/Cu(ClO4)2/PABA
matches well with the spectrum of the L/Cu(ClO4)2/PABA/p-
toluidine mixture. This indicates the formation of a new five-
coordinated Cu(II) complex13 preferably with PABA in the
reaction mixture. The preference of PABA over the p-toluidine
could be due to possible hydrogen bonding interactions between
the BOH group and the oxy-ether pocket of L. Further, the ESI-
MS spectrum of this reaction mixture shows a peak at m/z
1085.4, which strongly supports this 1:1:1 ternary complex [II]
formation (Figure 2c). After the ternary complex formation, one
e− transfer takes place from the amine of PABA to the Cu(II)
center that produces a radical cation [III],7b,c,g,i,j and
subsequently Cu(II) is converted to Cu(I). Then Cu(I) oxidizes
to Cu(II) by donating an electron to aerial O2 to form
superoxide radical (O2•−). Simultaneously, nucleophilic attack
of p-toluidine to the radical cation [III] generates 3e− σ bond
Here, we propose a plausible mechanism for this oxidative
coupling, taking 4-aminobenzyl alcohol (5a) and p-toluidine
(6a) as model substrates (Scheme 5). When L and Cu(ClO4)2
are mixed in DMF, the UV−vis data recorded after 5 min of
mixing show a d−d band at 560 nm (Figure 2a, spectrum 1),
which indicates the formation of a L−Cu(II) complex. A
− +
characteristic peak of [L−Cu(II) + ClO4 ] at m/z 962.31
(Figure 2b) in the ESI-MS study confirms the above complex
formation. Upon addition of 4-aminobenzyl alcohol (PABA)
and p-toluidine into L−Cu(II), the UV−vis spectrum (taken
after 10 min, Figure 2a, spectrum 4) of the reaction mixture
shows a new peak at around λmax 438 nm along with the peak at
560 nm. Comparison of this spectrum with a 1:1:1 mixture of (i)
species [IV].15 Then 1e− transfer from 3e− σ bond to O2
•−
generates a peroxide anion (O22−) which takes up two protons
from species [IV] that produces the hydrazine derivative [V]
along with H2O2. Finally, hydrazine [V] is oxidized via similar
two-electron oxidation procedure by aerial oxygen,7b,i,j which
C
DOI: 10.1021/acs.orglett.8b02829
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In conclusion, we have developed a new oxidative coupling
route for the synthesis of the unaffected primary benzylic alcohol
group containing cross-azo compounds from aromatic amines,
by using Cu(ClO4)2 in the presence of a well-designed cryptand
having both an oxy-ether moiety and a metal chelating site as the
catalyst. The selectivity of the particular oxidation of the amine
part is imparted by the oxy-ether moiety of macrobicyclic
cryptand through H-bonding with benzylic alcohol. Moreover,
the cross-selectivity is also maintained via the same macrobi-
cycle. Formation of ternary copper complex tuned the formation
of unsymmetric azo compounds with reliable yields. This novel
approach of regulation of selectivity using a macrobicycle moiety
may improve modern synthetic chemistry in the near future.
Scheme 5. Proposed Mechanism for Single-Step Catalytic
Aerial Oxidation
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02829.
Experimental procedures and spectroscopic data (PDF)
Accession Codes
CCDC 1860527−1860528 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: icpg@iacs.res.in.
ORCID
Pradyut Ghosh: 0000-0002-5503-6428
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.S. thanks the Council of Scientific and Industrial Research
(CSIR), New Delhi, for his fellowship (SRF). P.S. thanks the
SERB project PDF/2016/001529 for funding. The authors
acknowledge Somnath Bej for his suggestion during the initial
stage of work. The authors also thank IACS for the
Instrumentation Facility.
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Figure 2. (a) UV−vis spectra of L−Cu(ClO4)2 (pink); 1:1:1 mixture of
L, Cu(ClO4)2, and 4-aminobenzyl alcohol (PABA) (brown); 1:1:1
mixture of L, Cu(ClO4)2, and p-toluidine (green); 1:1 mixture of PABA
and Cu(ClO4)2 (orange); PABA (blue) at 1 × 10−3 M each in DMF
solvent and crude reaction mixture (violet) after 10 min. (b) ESI-MS of
L−CuII(ClO4)2. The inset picture depicts the enlarged portion of the
region for the corresponding monopositive ion (blue) and its isotopic
distribution pattern (black). (c) ESI-MS of the 1:1:1 ternary complex,
■
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