Visible-Light-Driven Neutral Nitrogen Radical Mediated Intermolecular Styrene Difunctionalization

Article abstract of DOI:10.1021/acs.orglett.9b01362

A neutral nitrogen radical-mediation strategy, wherein the existing N-H moiety of substrates serves as a neutral nitrogen radical precursor to enable room-temperature intermolecular radical difunctionalization of styrenes under photoredox catalysis, is reported. The reaction shows high functional group tolerance and substrate scope with respect to both components, giving the corresponding products with generally good yields. Preliminary control experiments and DFT calculations are performed to explain the reaction mechanism.

Full text of DOI:10.1021/acs.orglett.9b01362

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Visible-Light-Driven Neutral Nitrogen Radical Mediated
Intermolecular Styrene Difunctionalization
Quan-Qing Zhao,† Man Li,‡ Xiao-Song Xue,*,‡ Jia-Rong Chen,*,† and Wen-Jing Xiao*,†
†CCNU-uOttawa Joint Research Centre, Hubei International Scientific and Technological Cooperation Base of Pesticide and Green
Synthesis, Key Laboratory of Pesticides & Chemical Biology Ministry of Education, College of Chemistry, Central China Normal
University, 152 Luoyu Road, Wuhan, Hubei 430079, China
‡State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: A neutral nitrogen radical-mediation strategy,
wherein the existing N−H moiety of substrates serves as a
neutral nitrogen radical precursor to enable room-temperature
intermolecular radical difunctionalization of styrenes under
photoredox catalysis, is reported. The reaction shows high
functional group tolerance and substrate scope with respect to
both components, giving the corresponding products with
generally good yields. Preliminary control experiments and DFT calculations are performed to explain the reaction mechanism.
Scheme 1. Design of Neutral Nitrogen Radical (NCR)-
Mediated Styrene Difunctionalization
he use of the existing functional group of substrate as a
Tcatalyst or activator precursor to in situ generate an
transient reactive species to trigger otherwise inaccessible or
challenging reactions is an ideal strategy.1 Given the unique
capability of the photoredox catalysis in generation of various
highly reactive species,2 we hypothesized that application of
photoredox catalysis to these substrates containing a photo-
sensitizing group would probably offer an opportunity for new
chemical reaction design. With the evolution of the photo-
catalysis,2 the chemistry of nitrogen-centered radicals (NCRs)
has witnessed remarkable advances in recent years and emerged
as a key synthetic platform for the creation of C−N bonds and
nitrogen-containing molecules.3,4 More recently, increasing
attention has also been devoted to application of this radical
species class to inert C−H bond activation because of their
unique HAT properties5 as well as C−C bond cleavage,6 thus
enabling a diverse range of remote site-selective functionaliza-
tions. These versatile reactivity modes of NCRs are attributed to
their different configurations and nucleophilic or electrophilic
properties caused by the N-hybridization and substituents.
Despite these significant advances, to the best of our knowledge,
use of NCRs to mediate intermolecular radical reaction has
never been reported. In our efforts to develop NCR-mediated
radical functionalization of alkenes, we found that the protecting
groups on nitrogen atom had an obvious effect on their reactivity
modes.4a,6b
new complementary access to diversely substituted anilines and
sulfonyl-containing compounds.9 To our knowledge, there is no
precedent for such a radical transformation.
Our studies commenced with an examination of reaction
conditions to test the reaction of model substrates 1a and 2a
(Table 1). An extensive screening of commonly used photo-
catalysts, solvents, and bases identified that the designed
reaction did indeed occur to furnish product 3aa in 87%
NMR yield when Ir(ppy)2bpyPF6 was used as catalyst and
K2CO3 as base in DMF under 7 W blue LED illumination at
room temperature (entry 1).10 In the control experiments, no
product 3aa was detected in the absence of photocatalyst, base,
or light, and large amounts of both starting materials remained
intact (entries 2−4). A slightly prolonged reaction time
increased the NMR yield to 92% (entry 5), and 88% isolated
yield of 3aa was obtained when the reaction was carried out on a
0.2 mmol scale (Scheme 2). In contrast to many precedents that
typically used allyl sulfones as ally radical precursors,8 the
current protocol allows convergent incorporation of both allyl
and sulfonyl moieties into the final products.9
Moreover, considering the extensive studies on photocatalytic
activation and transformation of amines, we speculated that the
existing amine moiety of certain substrates might serve as a NCR
radical precursor. For proof of concept, we focused on 2-
vinylanilines as suitable candidates (Scheme 1).7 Herein, we
reported that in situ generated neutral NCRs by photoredox
catalysis enabled an efficient intermolecular difunctionalization
of 2-vinylanilines using allyl sulfones.8 This protocol provides a
Received: April 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01362
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Letter
a
Table 1. Condition Optimization
Scheme 2. Scope of Neutral NCR-Mediated Intermolecular
Difunctionalization of 2-Vinylanilines
a b
,
b
entry
variation from standard conditions
yield (%)
1
2
3
4
5
none
87
<5
<5
<5
92
no Ir(ppy)2bpyPF6
no K2CO3
no light
4 h
a
Reaction conditions: 1a (0.1 mmol), 2a (0.3 mmol), Ir(ppy)2bpyPF6
(0.002 mmol, 2 mol %), K2CO3 (0.2 mmol), DMF (2 mL), 7 W blue
b
1
LEDs, rt, 3 h. Yields were determined by H NMR using 1,3,5-
trimethoxybenzene as an internal standard.
The optimized reaction conditions show broad substrate
scope and high functional group tolerance with respect to both
reaction components. Scheme 2 provides a series of
representative examples. For instance, a wide variety of
functional groups such as methyl, fluoro, and chloride could
be incorporated into the phenyl ring of the arylsulfone moiety in
2-vinylanilines, and products 3ba−3da are obtained in 80−90%
yields. As shown in the synthesis of product 3ea, sterically
congested substrate was also compatible with the reaction. The
reactions with substrates 1f and 1g bearing fused aromatic or
heteroaromatic groups also proceeded smoothly to afford 3fa
and 3g with good yields. Notably, substrate 1h with a
methylsulfonyl group on the nitrogen atom reacted well with
2a to give 3ha in 53% yield. To demonstrate the scalability of
this protocol, a gram-scale reaction of 1b and 2a was also
performed, and product 3ba was still obtained in 90% yield.
We then proceeded to examine the effect of substitution
pattern of the phenyl ring of 2-vinylanilines on the reaction. A
series of N-tosyl-substituted 2-vinylanilines 1i−1x with electron-
donating (e.g., MeO, Me) or electron-withdrawing (e.g., CF3,
COMe, CHO, CO2Me, F, Cl, Br) groups at the different
positions were all well tolerated. The corresponding products
3ia−3xa were obtained in high yields in most cases. In particular,
these biologically relevant functionalities such as ketone,
aldehyde, and ester also highlighted the potential medicinal
use of the products. Moreover, halogen atoms, including F
(3ra−3ua), Cl (3va−3wa), and Br (3xa), provided a convenient
handle for further synthetic elaborations at the halogenated
positions. N-Tosyl-3-vinylnaphthalen-2-amine 1y also reacted
well with 2a to furnish product 3ya in acceptable yield. However,
the reaction is very sensitive to the substitution pattern of the
vinyl moiety. As demonstrated in the case of substrate 1z, no
alkene difunctionalization product can be detected under the
standard conditions. We attribute the poor reaction efficiency to
the steric hindrance. In good agreement with our design plan, it
is found that the N−H moiety of the substrate is critical to the
desired reaction. For example, the substrate 4 with a tertiary
amine proved to be ineffective in the transformation. This result
also suggests the possible involvement of nitrogen radical
mediation strategy in activation of substrates.
a
Conditions: 1 (0.2 mmol), 2 (0.6 mmol), Ir(ppy)2bpyPF6 (0.004
mmol, 2 mol %), K2CO3 (0.4 mmol), DMF (4 mL), 7 W blue LEDs,
rt, 4 h. Isolated yields. Gram-scale reaction. Using CH3CN as
solvent.
b
c
d
from a sequential alkene radical difunctionalization and thermal
aza-addition reaction.
To further display the synthetic potential of this protocol, we
also extended the current catalytic system to include 3-
vinylaniline 5a and 4-vinylaniline 5b (eqs 1 and 2). Interestingly,
both substrates reacted smoothly with 2a to give highly
functionalized anilines 6a and 6b in good yields, which resulted
B
DOI: 10.1021/acs.orglett.9b01362
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Letter
To gain some insight into the mechanism, we first carried out
several control experiments with substrates 1b and 2a (Scheme
3a). Upon addition of commonly used radical scavengers
detected by HRMS, which is indicative of the nitrogen radical
engaged in the process. In accordance with the optimization
studies (Table 1, entry 3), luminescence-quenching experiments
of 1a also revealed that only in the presence of base K2CO3 can
substrate 1a efficiently quench the excited-state photocatalyst.
Scheme 3. Mechanistic Studies
1
Moreover, the H NMR analysis of 1a disclosed that the
addition of K2CO3 led to complete deprotonation of 1a and its
full conversion to nitrogen anion intermediate 1a-A.10 In
principle, both the vinyl and nitrogen anion moieties of 1a-A
would undergo visible-light-induced SET oxidation by the
excited-state photocatalyst.4,11 The computed ionization
potentials (IP) of styrene 1a-A-a and N-phenylbenzenesulfona-
mide anion 1a-A-b are 193.1 and 71.8 kcal mol−1, respectively
(Scheme 3b),12 indicating that the nitrogen anion moiety is a
stronger electron donor and thus should be the most likely site of
SET oxidation. This is corroborated by the computed spin-
density distribution in radical 1a-B in which the spin density is
mainly localized at the nitrogen atom. Taken together, these
results suggest that the nitrogen anion moiety should be more
easily oxidized than the vinyl group and N-radical is involved as a
mediator.
Allyl sulfones are a well-established class of radical acceptors;8
thus, we hypothesized that the in situ formed N-radical might
initially activate the allyl sulfones by radical addition to the
alkene moiety. We then carried out the crossover experiment by
subjecting 1b, 2a, and two equimolar amounts of presynthesized
aniline 8 to the standard conditions (Scheme 3c). In addition to
the formation of routine coupling product 3ba resulting from 1b
and 2a, we also isolated product 3bb in 21% yield, lending
support to the intermediacy of 8 in the process. This result also
suggests the involvement of sulfonyl radical 2a-A in the reaction.
We then examined the possible application of this strategy in
the intermolecular reaction of simple styrene 9 and allyl sulfone
2a by using aniline substrate 1b as a radical catalyst (eq 3). In
addition to the formation of normal product 3ba (58% yield),
TEMPO or PhSeSePh, significant or complete inhibition of
target reaction was observed. Notably, selenide adduct 7 was
Figure 1. Free energy diagram of the reaction of N-radical 1a-B and substrate 2a calculated at the SMD-M06-2X/[6-311++G(2d, p)-SDD(Ir)]//
SMD-M06-2X/[6-31G(d)-LANL2DZ(Ir)] level of theory.
C
DOI: 10.1021/acs.orglett.9b01362
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
pleasingly, we could also isolate the addition product 10 in 37%
yield, which implied that substrate 1b-derived N-radical 1b-B
could also mediate the reaction between styrene 9 and allyl
sulfone 2a.
development of a wide range of radical transformations using
this strategy as well as nitrogen radical catalysis.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b01362.
Experimental procedures, full analysis data for new
compounds, and copies of NMR spectra (PDF)
Accession Codes
To better understand the mechanism of the reaction, we
conducted DFT calculations at the SMD-M06-2X/[6-311+
+G(2d, p)-SDD(Ir)]//SMD-M06-2X/[6-31G(d)-LANL2DZ-
(Ir)] level of theory.12 As shown in Figure 1, addition of the in
situ formed N-radical 1a-B to the olefinic double bond in 2a via
transition state TS1 to afford intermediate 1a-C requires an
activation free energy of 20.9 kcal mol−1. The radical
intermediate 1a-C could readily release a sulfonyl radical 2a-A
to generate 1a-D, which is located 4.0 kcal mol−1 below the
intermediate 1a-C. Notably, the control experiment of 1a and 2a
without photocatalyst and visible light irradiation demonstrated
that thermal SN2′ substitution among them did not occur, and
no 1a-D or sulfinate was observed.10 Thus, an alternative
pathway involving thermal SN2′ substitution between 1a and 2a
and photocatalytic SET oxidation should not be involved in our
reaction system.13 Then, radical addition of 2a-A to the olefin
moiety of 1a-C via TS2 leads to a benzyl radical 1a-E with a
barrier of 11.7 kcal mol−1 relative to 1a-D. Subsequently,
intermediate 1a-E could either undergo radical cyclization
(pathway a) or SET reduction by the reduced-state photo-
catalyst [Ir(ppy)2bpy]2+ (pathway b). The calculation shows
that SET reduction of 1a-E by [Ir(ppy)2bpy]2+ is more facile
than the radical cyclization. Intermediate 1a-F undergoes an
intramolecular Michael addition via TS4 to give a seven-
membered-ring anion 1a-H, which has a barrier of 11.3
kcalmol−1. Notably, 1a-H is more stable than 1a-F by 13.1
kcalmol−1 as the negative charge is stabilized by the adjacent
electron-withdrawing ester group. A retro-Michael addition
reaction via TS5 to form a nitrogen anion intermediate 1a-I has a
barrier of only 5.8 kcal mol−1 with respect to 1a-H. Intermediate
CCDC 1870689−1870690 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 Authors
■
*E-mail: xuexs@nankai.edu.cn.
*E-mail: chenjiarong@mail.ccnu.edu.cn.
*E-mail: wxiao@mail.ccnu.edu.cn.
ORCID
Xiao-Song Xue: 0000-0003-4541-8702
Jia-Rong Chen: 0000-0001-6054-2547
Wen-Jing Xiao: 0000-0002-9318-6021
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the NNSFC (91856119, 21622201,
21820102003, and 21772053), the Science and Technology
Department of Hubei Province (2016CFA050 and
2017AHB047), and the Program of Introducing Talents of
Discipline to Universities of China (111 Program, B17019) for
support of this research.
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