Visible-Light-Induced Difunctionalization of Styrenes: Synthesis of N-Hydroxybenzimidoyl Cyanides

Article abstract of DOI:10.1021/acs.orglett.0c01235

A visible-light-induced synthesis of N-hydroxybenzimidoyl cyanides from aromatic terminal alkenes is achieved by using Eosin Y as an organic photoredox catalyst. The process goes via a radical pathway with successive incorporation of two nitrogen atoms, one each from tert-butyl nitrite and ammonium acetate. The final product is achieved by the concomitant installation of an oxime and a nitrile group. DFT calculation supports a biradical pathway and all the proposed steps. A few useful synthetic transformations of N-hydroxybenzimidoyl cyanide are also illustrated.

Full text of DOI:10.1021/acs.orglett.0c01235

pubs.acs.org/OrgLett
Letter
Visible-Light-Induced Difunctionalization of Styrenes: Synthesis of
N‑Hydroxybenzimidoyl Cyanides
Tipu Alam, Amitava Rakshit, Pakiza Begum, Anjali Dahiya, and Bhisma K. Patel*
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ABSTRACT: A visible-light-induced synthesis of N-hydroxybenzimidoyl cyanides from aromatic terminal alkenes is achieved by
using Eosin Y as an organic photoredox catalyst. The process goes via a radical pathway with successive incorporation of two
nitrogen atoms, one each from tert-butyl nitrite and ammonium acetate. The final product is achieved by the concomitant installation
of an oxime and a nitrile group. DFT calculation supports a biradical pathway and all the proposed steps. A few useful synthetic
transformations of N-hydroxybenzimidoyl cyanide are also illustrated.
isible-light-mediated photocatalytic functionalization have
in a variety of compounds having numerous biological
Vattracted considerable attention in modern organic activity.12 The presence of these two important functional
groups in a single molecule may further augment its activity
against various targets (Figure 1).13 Conventionally, nitrile
synthesis, because of sustainable energy source to promote
chemical reactions.1 Usually, the photoredox process via a
single-electron transfer (SET) under mild reaction conditions
is utilized for the synthesis of numerous organic compounds.2
In this context, Ru- and Ir-based complexes are well-explored
photocatalysts for several organic transformations mediated by
visible light.3 However, despite their extraordinary photo-
physical properties, their use is discouraged, because of their
scant availability, toxicity, and high cost.4 Lately, organic dyes
such as Rose Bengal, Eosin Y, Eosin B, etc. have been used in
lieu of transition metals, since they are inexpensive, less toxic,
and easy to handle. In particular, Eosin Y is used as an
organophotoredox catalyst in many radical-based organic
transformations.5 Of late difunctionalization of alkenes has
become a popular and competent chemical transformation by
which two functional groups can be introduced simultaneously
across the double bond.6 Normally, difunctionalization of
alkenes requires transition-metal catalysts and proceeds via
radical-mediated processes. This increases the molecular
complexity in a step-economical fashion.7 Among various
methods, the radical-mediated difunctionalization of alkenes
has made great progress recently, particularly the visible-light-
mediated photoredox SET process.8,9
Figure 1. Representative biologically active oximes and N-
hydroxybenzimidoyl cyanides.
group is introduced by Sand Meyer and Rosenmund-von
Braun reactions, as well as by the use of traditional cyanating
reagents such as NaCN, CuCN, KCN, TMSCN, Zn(CN)2,
and K4[Fe(CN)6].14 However, many of the cyanation
processes suffer from certain drawbacks, because of the direct
use of the toxic cyanide anion, which releases fatal and volatile
HCN under the reaction conditions.15
An elegant synthesis of oxime has been reported by the
Beller group in 2009 from styrene and tert-butyl nitrite in the
On the other hand, oxime is a privileged scaffold for the
preparation of amines, amides, and nitrogen-containing
heterocyclic compounds.10 Oxime functionality is found in
compounds having anti-inflammatory, antibiotics, and pestici-
dal activities.11 Similarly, the nitrile serves as a synthetic
precursor of many functional groups such as amine, amide,
aldehyde, tetrazole, and carboxylic acid, which are also found
Received: April 7, 2020
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© XXXX American Chemical Society
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presence of Fe(II) catalyst [Scheme 1(i)].16 Later, the Wang
group introduced a CN group to an allyl benzene using Pd(II)
photocatalyst, NH4OAc (4 equiv), N-hydroxyphthalimide
(NHPI) (40 mol %) as a cocatalyst in the presence of 2 ×
10 W white LEDs in acetonitrile at room temperature. A new
product was isolated in 40% yield. Standard spectroscopic
analysis of the isolated product and subsequent single-crystal
X-ray analysis reveal its structure to be N-hydroxybenzimidoyl
cyanide (2a) (Figure S1 in the Supporting Information). To
the best of our knowledge, this is a unique concomitant
oximation−cyanation of alkene using tert-butyl nitrite as the
N−O and ammonium acetate as the nitrogen (N1) synthon.
Thrilled by this visible-light-mediated synthesis of hydrox-
yiminoacetonitrile subsequent tuning of reaction parameters
were attempted to enhance the productivity taking p-Me
styrene (2) as the prototypical substrate. The details
optimization of various reaction parameters are summarized
in Table S1 in the Supporting Information. Finally, the ideal
condition for the synthesis of 2a was the use of p-
methylstyrene (2) (0.25 mmol), tert-butyl nitrite (a) (1.25
mmol), Eosin Y (2 mol %) NH4OAc (4 equiv), and NHPI (50
mol %) in 2 mL CH3CN under irradiation of 2 × 10 W white
LEDs.
Scheme 1. Oximation and Cyanation of Terminal Alkenes
The substrate scope of the methodology was applied to
various aromatic terminal alkenes adopting the optimized
reaction condition (Scheme 2). Simple styrene (1), having no
substituent in the phenyl ring, provided 61% yield of the
difunctionalized product (1a). Styrenes having electron-
donating groups, namely, p-Me (2), m-Me (3), o-Me (4),
p-tBu (5), p-OMe (6), and p-CH2Cl (7) yielded their
corresponding N-hydroxybenzimidoyl cyanides (2a, 65%),
(3a, 62%), (4a, 59%), (5a, 67%,), (6a, 69%) and (7a, 63%),
respectively (see Scheme 2). Styrenes possessing substituents
such as p-F (8), m-F (9), p-Cl (10), m-Cl (11), o-Cl (12), and
p-Br (13), having moderate electron-withdrawing ability, all
underwent successfully difunctionalizations to produce N-
hydroxybenzimidoyl cyanides (8a, 60%), (9a, 58%), (10a,
59%), (11a, 55%), (12a, 52%), and (13a, 57%), respectively,
in acceptable yields (see Scheme 2). A meta-substituted
nitrostyrene (14) having a strongly electron-withdrawing
group also reacted smoothly, affording the corresponding
product (14a, 48%) (see Scheme 2).
Trisubstituted and disubstituted aromatic terminal alkenes
such as 2,4,6-trimethylstyrene (15), 2,4-dimethylstyrene (16),
and 3-methoxy-4-benzyloxystyrene (17) underwent efficient
conversion to their desired N-hydroxybenzimidoyl cyanides
(15a, 51%), (16a, 56%), and (17a, 62%), respectively (see
Scheme 2). A biphenyl styrene (18) reacted smoothly, giving
hydroxyaminoacetonitrile (18a) in 71% yield. Other biphenyl
styrenes having an electron-donating substituent (p-tBu (19)),
electron-withdrawing substituents (p-F (20) and p-Cl (21)),
and disubstituted such as 3-Me-4-OMe (22) all were
successfully converted to their hydroxyaminoacetonitriles
(19a, 70%), (20a, 69%), (21a, 63%), and (22a, 70%),
respectively. Unlike terminal aromatic alkenes, aliphatic
terminal alkenes, such as ally benzene (23), 1-hexene (24),
4-bromo-1-butene (25), and heteroaryl alkene (26) all failed
to provide any product, which is possibly due to the instability
of the radicals produced in the medium (see Scheme 2). A
successful large-scale reaction was performed using 4-
methylstyrene (2) (5 mmol, 590 mg), tert-butyl nitrite (25
mmol, 2.57g) ammonium acetate (20 mmol, 1.54 g), and
NHPI (50 mol %, 407 mg), which provided product (2a) in
52% yield. No significant polymerization of styrene was
observed during the reaction.
catalyst and tert-butyl nitrite serving as the nitrogen source in
the presence of N-hydroxyphthalimide as the co-catalyst
[Scheme 1(ii)].17 However, only a limited method is available
for the synthesis of hydroxyiminoacetonitriles. Bohle and co-
workers demonstrated the synthesis of 2-hydroxyimino-2-
phenylacetonitrile from benzyl cyanide, potassium methoxide,
and nitric oxide.18 Hydroxyiminoacetonitriles can also be
synthesized from aldoxime via chlorination, followed by
reaction with alkali cyanide.19 The synthesis of 2-hydrox-
yimino-2-phenyl acetonitrile starting from styrene was first
reported by the Kunai group via a three-step process involving
nitration of styrene to 4-nitro-3-phenylfurazan-2-oxide, an
azidation step followed by photolytic decomposition [Scheme
1(iii)].20 Although the above processes provide access to
hydroxyiminoacetonitriles, these methods have inherent
limitations, such as the requirement of prefunctionalized
starting materials, multistep process, limited substrate scope,
and harsh reaction conditions. Therefore, designing routes for
the synthesis of hydroxyiminoacetonitriles that are efficient,
cost-effective, atom economy is highly desirable. In recent
times, our group is actively involved in the functionalization of
alkenes using tert-butyl nitrite, which gave a variety of
functionalized products under different reaction conditions.21
The reagent tert-butyl nitrite (TBN) is emerging as a versatile
synthon in organic synthesis.21a TBN serves as an “N−O”
synthon during the construction of isoxazolines from terminal
aryl alkenes21b and as an “N1” synthon in the construction of
imidazo[1,2-a]quinolines.21c Interestingly, TBN serves the
dual role of “N1” and “N−O” synthons in the construction
of 1,2,4-oxaziazole-5(4H)-ones from terminal aryl alkenes.21d
In continuation of our efforts toward functionalization of
alkene, we were curious to see the reactivity of an alkene with
tert-butyl nitrite in the presence of NH4OAc under 2 × 10 W
white LEDs mediated by an organophotoredox catalyst
[Scheme 1(iv)].
The visible-light-mediated difunctionalization of alkenes
were initiated taking p-Me styrene (2) (0.25 mmol), tert-
butyl nitrate (a), (4 equiv), Eosin Y (1 mol %) as the
B
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times, a logical mechanism is suggested (see Scheme 3). In the
presence of white LEDs light, Eosin Y (EY) is first
Scheme 2. Substrates Scope for the Synthesis of N-
Hydroxybenzimidoyl Cyanides from Aryl Alkenes
a b
,
Scheme 3. Proposed Reaction Pathway for the Formation of
N-Hydroxybenzimidoyl Cyanides
photoexcited. This is followed by photo-oxidation of NHPI
to a PINO radical by the activated Eosin Y via HAT (hydrogen
atom transfer) mechanism. This process regenerates the Eosin
•
Y back, producing a HO2 radical, which, upon disproportio-
nation, produces oxygen and hydrogen peroxide.22a,b The
PINO radical then attacks at the terminal sp2-carbon of alkene
(1) to generate a benzylic radical intermediate (A). Photo-
chemical decomposition of tert-butyl nitrite generates NO and
tert-butoxyradicals.22c The benzylic position of the intermedi-
ate (A) is attacked by the NO radical to generate intermediate
(B). Although both PINO and NO radicals exist in the
reaction medium, only the PINO attacks at the terminal
carbon of styrene (1) and the NO at the benzylic carbon. This
type of preferential attack of one of the radicals over others
(NO and NO2) has been observed earlier.21b−d To get an
insight into this preferential attack by these radicals on styrene,
DFT calculations were performed and modeled the reaction
profile at the M06/6-31+G(d) level of theory; no symmetry
constraints were imposed during geometry optimization.
Furthermore, single-point energy calculations have been
perfomed using the M06/cc-pVTZ method and subsequently
used these energies in the analyses of chemical mechanisms.
Transition states were established by their characteristic solo
imaginary frequency in normal vibrational mode. As can be
a
Reaction conditions: (i) 1−26 (0.25 mmol), tBuONO (1.25 mmol),
NH4OAc (1 mmol), NHPI (0.125 mmol), Eosin Y (0.005 mmol),
and CH3CN (2 mL) for 24 h. Yield reported for 5 mmol scale.
b
After an excellent demonstration of a useful synthesis of N-
hydroxybenzimidoyl cyanides, it is time to suggest a plausible
mechanism. In order to ascertain the radical nature of the
reaction, two independent reactions were performed: one in
the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO,
2 equiv) and the other with 2,6-di-tert-butyl-4-methyl phenol
(BHT, 2 equiv). While in the former case, no formation of the
product (1a, 00%) was observed but the latter scavenger
provided <10% yield of (1a), thereby approving the radical
nature of the reaction [see Scheme S2(i) in the Supporting
Information]. Based on the control experiments and
intermediates identified by the HRMS analysis (see the
Supporting Information) of the reaction aliquots at various
C
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a
Scheme 4. Energy Profile Diagram of the Reaction
a
Parts 1 and 2 shows calculated energy profile diagrams for the photolytic difunctionalization of alkene. The relative energies from DFT
calculations are in kcal mol−1 and bond lengths in Å, done at M06/6-31g+(d,p) level of theory. Single-point calculations done using the M06/cc-
pVTZ method and these energies were subsequently used in the analyses of chemical mechanisms. The relative energies are shown in blue color,
the activation barrier is shown in italic bold font, and the stabilization energy is shown in normal font; all are given in units of kcal mol−1. The cross
sign in red color (X) indicates the unfavorable reaction with higher activation barrier and lower stabilization.
D
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seen from Scheme 4, the attack of NO radical at the terminal
sp2-carbon goes through a high activation barrier of 13.75 kcal/
mol, generating an unstable intermediate (A′). On the other
hand, an identical attack of PINO radical required 8.53 kcal/
mol less energy, generating a comparatively stable radical
intermediate (A). The in-situ-generated nitroso intermediate
(B) is tautomerized to an oxime intermediate (C). The tert-
butoxyl radical sequentially abstracts two H atoms from the
oxime intermediate (C) to first generate a mono radical (D1),
followed by a 1,4-biradical intermediate (D). An intra-
molecular coupling of biradical (D) generates a four-
membered cyclic intermediate (E). The hydrogen peroxide is
produced in situ and decomposes to a hydroxyl radical in the
presence of light.22d,e The strained cyclic intermediate (E)
undergoes ring-opening via attack of an OH radical to form a
hemiacetal radical intermediate (F). The N−O radical
intermediate (F) abstracts a proton from tert-butanol to
generate a neutral hemiacetal intermediate (G). The neutral
hemiacetal intermediate loses NHPI providing an oxime
aldehyde (H). Condensation between ammonia (generated
from ammonium acetate) and the aldehydic intermediate (H)
form an iminium intermediate (I).23 Abstraction of an iminium
N−H from the intermediate (I) by the tBuO radical produce a
nitrogen-centered radical (J). Finally, the abstraction of the
aldehydic proton from intermediate (J) by tert-butoxy radical
provided the desired cyano functionality. As can be seen from
the DFT calculation (see Scheme 4).
tert-butyl nitrite and ammonium acetate, respectively. This
reaction proceeds through a biradical reaction path. The
proposed mechanism is supported by the detection of some of
the reaction intermediates and each of the steps is supported
by DFT calculation. Some useful post-synthetic modifications
were performed to show the synthetic utility of the
difunctionalized hydroxyaminoacetonitrile, thereby expanding
the further scope of this methodology.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01235.
Experimental procedures, spectral and analytical data of
all products; X-ray data for 2a and DFT calculations
(PDF)
Accession Codes
CCDC 1979124 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
The difunctionalized hydroxyaminoacetonitrile having an
oxime (=NOH) and a reactive nitrile (CN) group can be
transformed to a variety of other functionalized products, as
shown in Scheme 5. When (2a) was heated in an aqueous
Bhisma K. Patel − Department of Chemistry, Indian Institute of
Technology Guwahati, North Guwahati 781039, India;
orcid.org/0000-0002-4170-8166; Email: patel@iitg.ac.in
Scheme 5. Post-Synthetic Applications*
Authors
Tipu Alam − Department of Chemistry, Indian Institute of
Technology Guwahati, North Guwahati 781039, India
Amitava Rakshit − Department of Chemistry, Indian Institute of
Technology Guwahati, North Guwahati 781039, India
Pakiza Begum − Department of Chemistry, Indian Institute of
Technology Guwahati, North Guwahati 781039, India
Anjali Dahiya − Department of Chemistry, Indian Institute of
Technology Guwahati, North Guwahati 781039, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01235
*
Reaction conditions: (a) H2SO4(aq), 130 °C, 15 h; (b) H2SO4(1
mL), rt, 3 h; (c) Ph-B(OH)2, Pd(OAc)2, 80 °C, 8 h; and (d) NaN3,
NH4Cl, 120 °C, 14 h.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
H2SO4, the nitrile group was converted to an acid functionality
(2b).24 On the other hand, when (2a) was treated with
concentrated H2SO4, the nitrile functionality was converted to
an amide (2c).25 Furthermore, the treatment of (2a) with a
phenylboronic acid in the presence of Pd(II) catalyst, both the
oxime and cyano functionality were converted to keto group
(2d) with concurrent incorporation of a phenyl group.26 The
cyano substrate (2a) on treatment with NaN3 is transformed
to a tetrazole moiety (2e) without affecting the oxime
functionality.27
In conclusion, a visible-light-mediated synthesis of N-
hydroxybenzimidoyl cyanides have been accomplished from
styrenes in the presence of organic photoredox catalyst. The
bifunctionalized moiety possesses two important function-
alities, viz, an oxime and a nitrile group, which originate from
■
B.K.P. acknowledges the support of this research by the
Department of Science and Technology (DST/SERB) (No.
EMR/2016/007042), CSIR (No. 0333/NS-EMRII). T.A,
A.R., and A.D. thank IIT Guwahati for a fellowship. P.B.
thanks the Param-Ishan supercomputing facility at IIT
Guwahati. CIF data provided by IIT Guwahati is highly
acknowledged.
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