Alkene Synthesis by Photo-Wolff-Kischner Reaction of Sulfur Ylides and N-Tosylhydrazones

Article abstract of DOI:10.1002/chem.202102671

A visible-light-driven and room temperature photo-Wolff-Kischner reaction of sulfur ylides and N-tosylhydrazones has been developed for the first time to provide modular access to alkene synthesis. The high functional group tolerance and broad substrate scope were demonstrated by more than 60 examples. Both E- and Z-olefinic stereochemistry in the products could be controlled with excellent stereoselectivity. A series of mechanistic studies support that the reaction should proceed through a radical-carbanion crossover pathway, specifically involving addition of photo-generated sulfur ylide radical cations to N-tosylhydrazones to form carbanions and subsequent Wolff-Kischner process.

Full text of DOI:10.1002/chem.202102671

Accepted Article
Accepted Article
Title: Alkene Synthesis by Photo-Wolff-Kischner Reaction of Sulfur
Ylides and N-Tosylhydrazones
Authors: Pan-Pan Gao, Dong-Mei Yan, Ming-Hang Bi, Min Jiang, Wen-
Jing Xiao, and Jia-Rong Chen
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202102671
Link to VoR: https://doi.org/10.1002/chem.202102671
01/2020

10.1002/chem.202102671
Chemistry - A European Journal
COMMUNICATION
Alkene Synthesis by Photo-Wolff-Kischner Reaction of Sulfur
Ylides and N-Tosylhydrazones
Pan-Pan Gao, Dong-Mei Yan, Ming-Hang Bi, Min Jiang, Wen-Jing Xiao, and Jia-Rong Chen*
Abstract:
A
visible-light-driven
and
room
temperature
trans-1,2,3-benzoylcyclopropane II in 90% yield with 30%
conversion via 1,2-dibenzoylethylene intermediate I, which is
formed through nucleophilic attack of sulfur ylide radical cation by
another molecule of DMSY [Eq. (1)]. By contrast, the reaction of
benzylmethylsulfur phenacylide (BMSY) under the otherwise
same conditions produced mixtures of 1,2-(dibenzoyl)vinyl
methyl sulfide III (52%), trans-1,2,3-benzoylcyclopropane II
(26%), and minor benzylmethyl sulfide (18% yield), indicative of
different pathway of BMSY-derived sulfur ylide radical cation [Eq.
(2)]. Specifically, BMSY sulfur ylide radical cation can undergo
hemolytic C-S bond cleavage to form cation BMSY-I
predominantly with release of benzylic radical; then the cation
reacts with another molecule of sulfur ylide to lead to major
1,2-(dibenzoyl)vinyl methyl sulfide III. Despite these inspiring
studies on the chemical properties of phenacyl sulfur ylide radical
cations, surprisingly, the synthetic applications of such radical
species have long been unexplored.
photo-Wolff-Kischner reaction of sulfur ylides and N-tosylhydrazones
has been developed for the first time to provide modular access to
alkene synthesis. The high functional group tolerance and broad
substrate scope were demonstrated by more than 60 examples. Both
E- and Z-olefinic stereochemistry in the products could be controlled
with excellent stereoselectivity. A series of mechanistic studies
support that the reaction should proceed through a radical-carbanion
crossover pathway, specifically involving addition of photo-generated
sulfur ylide radical cations to N-tosylhydrazones to form carbanions
and subsequent Wolff-Kischner process.
Sulfur ylides, firstly reported in 1930 by Ingold and Jessop,[1] are
a class of zwitterionic compounds characterized by a carbanion
and a neighboring positively charged sulfur atom. Since the
pioneering establishment of the synthetic prowess of these
compounds in the Johnson−Corey−Chaykovsky epoxidation and
cyclopropanation reactions,[2] the application of sulfur ylides as
versatile one-carbon synthons in organic chemistry has
undergone tremendous growth over the past decades, owing to
their inherent unique chemical properties.[3] Many of these
classical reactions have become textbook knowledge.
Surprisingly, in sharp contrast to these ionic chemistry, the use of
sulfur ylide radical cations, generated from the corresponding
sulfur ylides by one-electron oxidation, in radical synthetic
chemistry has been largely ignored.[4,5] The main reason may be
attributed to the fact that the chemical properties of sulfur ylide
radical cation depend primarily on the nature of the substituents
on the sulfur, which often results in the formation of complex
mixtures through various decomposition pathways. For example,
Schuster was the first to investigate the reactivity modes of
phenacyl sulfur ylides under photosensitization (λ> 400 nm) with
9,10-dicyanoanthracene (DCA) as the photocatalyst.[6] It was
found that the outcome of the reactions depends remarkably on
the structures of sulfur ylides because sulfur ylide radical cations
were involved as the same intermediates (Scheme 1A). For
instance, the DCA-sensitized photolysis of dimethylsulfur
phenacylide
(DMSY)
led
to
formation
of
[*]
P.-P. Gao, D.-M. Yan, M.-H. Bi, M. Jiang, Prof. W.-J. Xiao, Prof. J.-R.
Chen
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
E-mail: chenjiarong@mail.ccnu.edu.cn
Scheme 1. Chemical properties of sulfur ylide radical cation and new reaction
design. DMSY = dimethylsulfur phenacylide, BMSY = benzylmethylsulfur
phenacylide, DCA = 9,10-dicyanoanthracene, PC = photoredox catalysis.
Prof. M. Jiang
College of Materials, Chemistry and Chemical Engineering,
Hangzhou Normal University, 2318 Yuhangtang Road, Hangzhou
310036, China
Prof. J.-R. Chen
School of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang, Henan 453007, China
Our research group is actively involved in developing
valuable photoredox-catalyzed synthetic methods proceeding
through radical pathways.[7,8] Furthermore, our continued interest
in photocatalytic transformation of hydrazones[9] and inspiration
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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from the chemical and structural properties of sulfur ylide radical
cations led to a novel route being envisioned for the modular
access to alkenes by photo-Wolff-Kischner process (Scheme 1B). the condition optimization (Table 1).[11] Pleasingly, the target
We commenced the study by employing sulfur ylide 1a and
acetone-derived N-tosylhydrazone 2a as model substrates for
We assumed that adding a photo-generated sulfur ylide radical
cation A to a hydrazone, and subsequent fragmentation of an
arenesulfonyl radical could provide a diazene intermediate B.
reaction indeed worked to afford the desired product 3aa in 46%
NMR yield, when using cost-effective Eosin Y as a photocatalyst
under irradiation of 7 W blue LEDs at room temperature (entry
1).[12] In agreement with Schuster’s study,[6] a significant amount
of byproduct trans-1,2,3-benzoylcyclopropane 3aa-II was also
observed, which should be formed from three molecules of sulfur
ylides. In contrast, irradiation of the reaction mixture with 40 W
green LEDs in the presence of DMAP as a base resulted in a
lower yield of 3aa, while 1,4-diketone 3aa-I was observed as the
major byproduct instead (entry 2). With these challenges in mind,
we then examined a range of organic bases (entries 3-6), and
found that the base has played a crucial role in the reaction, with
TMG being the best of choice (entry 5). Though the formation of
byproduct 3aa-I could not be fully suppressed, full conversion of
2a and almost quantitative yield of 3aa were achieved. A brief
screening of solvents revealed that CH2Cl2 and THF were
optimal, compared to MeCN and acetone (entries 8 and 9).
When the equivalent ratio of 1a to 2a was decreased to 1.2:1, no
loss of efficiency was observed and 3aa was isolated in 81%
yield (entry 10). Without addition of TMG, the conversion of 2a
dropped substantially, leading to 3aa in only 29% yield (entry 11).
As expected, in the absence of either the Eosin Y or visible-light
irradiation, no desired product 3aa was detected (entries 12 and
13). These results confirmed that each component is essential
for the reaction. When performed on a 0.2 mmol scale reaction,
comparable results were obtained (entry 14). Notably, when the
reaction was performed at 10.0 mmol, product 4aa can still be
obtained in 68% isolated yield (1.3 g), highlighting the scalability
of this protocol (entry 15). Decrease of the light intensity from 40
W to 10 W under otherwise same conditions led to lower
conversion, giving 3aa in 74% yield (entry 16). In order to explore
the reproducibility and potential of this new photo-Wolff-Kischner
reaction, we also performed condition-based sensitivity
assessment developed by Glorius.[13] The intuitive, standardized
presentation of the results in a radar diagram revealed that the
reaction is only medium sensitive to light intensity, temperature
and oxygen concentration (see Section 4 in the Supporting
Information).
Then, intermediate
B
undergoes Wolff-Kischner-type N2
extrusion to form carbanion C, which would undergo an E1cB
elimination to afford alkene with loss of dimethyl sulfide. The
feasibility of our design plan is corroborated by a recent study of
König and co-workers, who demonstrated that trap of
sulfur-centered radicals by N-sulfonyl hydrazones allowed
formation of α–functionalized carbanion via a Wolff-Kischner
process, thus leading to its subsequent intermolecular coupling
with electrophilic CO2 and aldehydes.[10] However, several
competing reactions need to be overcome to achieve the desired
reaction, such as 1) a range of sulfur ylide di- and trimerizations
well established by Schuster;[6] 2) possible formation of
hydrazonyl radicals observed in previous reports.[9]
Table 1: Optimization of the reaction conditions.[a]
Conv [%][b]
Yield [%][b]
3aa-I/3aa-II
Yield [%][b]
Entry
Base
Solvent
2a
3aa
1[c]
2
K2CO3
DMAP
DIPEA
Et3N
TMG
DBU
TMG
TMG
TMG
TMG
-
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
83
37
<5/25
20/<5
33/<5
13/<5
21/<5
37/<5
19/<5
42/<5
44/<5
18/<5
<5/<5
<5/<5
<5/<5
17/<5
N.D.
46
36
3
<5
<5
4
<5
<5
5
>95
64
95
Following optimization, the generaltiy of the reaction was
firstly explored by reacting 1a with various N-tosylhydrazones on
a 0.2 mmol scale (Table 2). As summarized in Table 2A, a range
of acyclic linear and α-branched aliphatic ketone-derived
N-tosylhydrazones 2b-e reacted smoothly with 1a, giving the
corresponding trisubstituted alkenes 3ab-ae in reasonable yields
6
60
7
>95
78
95
8
MeCN
Acetone
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
78
9
90
80
10[d]
11[d]
12[d,e]
13[d,f]
14[d,g]
15[h]
16[d,g,i]
>95
68
95 (81)
29
with moderate E/Z ratios. Moreover,
a
series of
TMG
TMG
TMG
TMG
TMG
<5
<5
N-tosylhydrazones 2f-m, easily prepared from the related cyclic
and heterocyclic ketones with various sizes, were all well
accommodated. Products 3af-am were afforded with moderate
to good yields. As shown in the synthesis of 3ag and 3ak, the
reaction efficiency could be significantly enhanced when
performed in continuous-flow reactors,[14] increasing the yields to
62% and 68%, respectively.[11] Compared with the Vedejs’ ionic
strategy of alkene synthesis that relied on condensation of
aldehyde-derived N-tosylhydrazones with stablilized carbanions
in the presence of strong base,[15] good tolerance of these
enolizable ketone-derived N-tosylhydrazones demonstrates
obvious advantages of the current reaction system.
<5
<5
>95
>95
80
95 (83)
(68)
(74)
35/<5
[a] Reaction conditions: 1a (0.15 mmol), 2a (0.1 mmol), Eosin Y (3 mol%), base
(0.15 mmol, 1.5 equiv), solvent (1.0 mL), 40 W green LEDs (λ = 525 nm), Ar, rt,
12 h. [b] NMR yields with 1,3,5-trimethoxybenzene as an internal standard;
values in parentheses are isolated yield. [c] Under irradiation of 7 W blue LEDs
(λ = 455 nm). [d] Use of 1.2 equiv of 1a. [e] Without Eosin Y. [f] Without
visible-light irradiation. [g] 0.2 mmol scale. [h] Gram-scale: 1a (15.0 mmol), 2a
(10.0 mmol, 1.5 equiv), 24 h. DMAP = 4-Dimethylaminopyridine. DIPEA =
N,N-Diisopropylethylamine. TMG
= 1,1,3,3-Tetramethylguanidine. DBU =
1,8-Diazabicyclo[5.4.0]undec-7-ene. N.D. = Not determined. [i] Use of 10 W
green LEDs (λ = 525 nm).
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Table 2: Scope of the N-tosylhydrazones.[a,b]
Remarkably, these products bearing halogen atoms (Cl, Br) on
the phenyl ring also provide an easy handle for further synthetic
elaboration at the respective positions. Again, substrates 4v and
4w with 3,4-disubstituted phenyl ring were also compatible with
reaction, affording 5av and 5aw with good yields.
N-Tosylhydrazones 4x having sterically demanding 1-naphthyl
group and 4y-aa with heteroaromatic ring such as quinoline,
thiophene, and furan all participated in the reaction well, with the
expected products 5ax-aaa being obtained in satisfactory yields.
Encouraged by these results, we continued to simply
evaluate the generality of this photo-Wolff-Kischner reaction by
employing a representative range of sulfur ylides, which can be
easily prepared from the α–bromo carbonyl compounds over two
steps. As shown in Table 3, the reaction demonstrated a broad
substrate scope and good functional group tolerance. Aside from
1a, sulfur ylides 1b-h with electron-neutral, electron-donating and
electron-withdrawing substituent at the para-, meta- or
ortho-position of the phenyl ring all reacted well with 2f. The
desired alkene products 6bf-if were produced with moderate to
good yields. Though the substrates with electron-donating
groups resulted in higher yields, as shown in some cases (e.g.,
6bf, 6df-ef, 6hf), application of the technology of continuous-flow
photochemistry to the substrates with electron-withdrawing
groups at the aromatic ring also substantially improved the
reaction efficiency compared to these performed in batch reactor.
It is noteworthy that the reaction of 2-naphthyl-substituted sulfur
ylide 1j as well as 3-thiophenyl- and 2-furanyl-substituted sulfur
ylides 1k and 1l also proved to be viable substrates for the
reaction, giving 6jf-lf with modest yields.
Table 3: Scope of the sulfur ylides.[a,b]
[a] Reaction conditions: 1a (0.3 mmol), 2 or 4 (0.2 mmol), Eosin Y (3 mol%),
base (0.15 mmol, 1.5 equiv), solvent (1.0 mL), 40 W green LEDs (λ = 525 nm),
Ar, rt, 12 h. [b] Isolated yields. [c] Performed in continuous-flow reactors.
Gratifyingly, the catalytic system could be further extended to
a wide variety of aldehyde-derived N-tosylhydrazones (Table 2B).
For instance, the reactions of substrates 4a-c prepared from
linear, α-substituted and cyclic aliphatic aldehydes all worked
well to produce 1,2-disubstituted alkenes 5aa-ac with useful
yields (66-71%). N-Tosylhydrazone 4d derived from
α,β-unsaturated aldehyde also proved to be suitable partner for
the synthesis of valuable 1,3-dienes.[16] Then, we proceeded to
[a] Refer to footnote [a] of Table 2 with 1.5 equiv of 1b-l. [b] Isolated yield. [c]
Performed in continuous-flow reactors.
To further demonstrate the practicality of this protocol, we
attempted to use sulfonium salts directly as precursors for in situ
generation of the relevant sulfur ylides (Scheme 2). For example,
upon slight increase of the base loading (5 equiv), the reaction
between sulfonium salt 1a’ and 2a also worked well to give
product 3aa with comparable yield [Scheme 2, Eq. (1)]. As a
result, this one-pot process would allow the direct use of their
precursors of certain otherwise-difficult-to-access sulfur ylides.
As such, we further reacted sulfonium salts 7 and 9 that were
prepared from 1-bromopinacolone and tert-butyl bromoacetate
with N-tosylhydrazone 2h; and the desired (E)-α,β–unsaturated
examine
an
array
of
aromatic
aldehyde-derived
N-tosylhydrazones. It was found that a variety of electron-neutral,
electron-donating (e.g., Me, Ph, OMe, SMe), and
electron-withdrawing (e.g., Br, Cl, CF3, CO2Me) functional groups
at the para-, meta-, or ortho-position of the phenyl ring were well
tolerated. The desired 1,2-disubstituted alkene products 5ae-5au
were isolated with yields ranging from 62-92% and excellent E/Z
selectivity. These examples also revealed that the substitution
pattern of the phenyl ring has no obvious effect on the reaction.
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ketone 8 and ester 10 were obtained exclusively with high yields
[Scheme 2, Eq. (2) and (3)]. Notably, bromoacetonitrile-derived
sulfonium salt 11 also proved to competent substrate, giving the
corresponding valuable (E)-arylacrylonitrile 12 in 71% yield
[Scheme 2, Eq. (4)].
[a] Conditions of Step 1: 8 (0.3 mmol), 2a (0.2 mmol), Eosin Y (3 mol%), TMG
(1.0 mmol, 5.0 equiv), CH2Cl2 (2.0 mL), 40 W green LEDs (λ = 525 nm), Ar, rt,
12 h. Conditions of Step 2: 13 (0.2 mmol), Ir(dFppy)2(dtbbpy)PF6 (1 mol%),
CH2Cl2 (2.0 mL), 7 W blue LEDs, 12 h. [b] Isolated yield. [c] E/Z ratio was
determined by 1H NMR analysis of the crude mixture.
Scheme 2. Photo-Wolff-Kischner reaction of sulfonium salts and
N-tosylhydrazones. Use of 5 equiv of sulfonium salts.
In recent years, visible-light-driven photosensitization has
been established as a robust tool for switching of the cis/trans
selectivity of alkenes.[17] In particular, isomerization-based
strategies to enable stereodivergent construction of (E)- and
(Z)-isomers of target alkenes from the identical set of starting
materials remain conspicuously underdeveloped. Thus, we
turned our attention to the development of a sequential
photo-Wolff-Kischner reaction and geometrical isomerization for
synthesis of (Z)-alkenes by merger visible-light-driven
single-electron-transfer (SET) and energy transfer catalysis.[18]
To our delight, as shown in Table 4, sulfonium salt 9 reacted well
with a series of sterically congested arylalkyl ketone-derived
N-tosylhydrazones 13a-d under photo-Wolff-Kischner reaction
conditions. The corresponding trisubstituted α,β-unsaturated
esters 14a-d were produced in good yields (60-73%) with
E-selectivity. Notably, when these alkenes were further subjected
to a catalytic amount of photocatalyst [Ir(dF-ppy)2(dtbbpy)]PF6 (1
mol%) under irradiation of 7 W blue LEDs for 12 h, the
corresponding thermodynamically unfavored α,β-unsaturated
esters (Z)-14a-d were obtained with 80-91% yields and excellent
Z-selectivity (Z/E>20:1). These results showed that both E- and
Z-olefinic stereochemistry in the products could be accessible by
simple combination of photocatalytic systems. When we tried to
use photocatalyst [Ir(dF-ppy)2(dtbbpy)]PF6 alone to promote both
steps, surprisingly, it was found that such a catalyst showed
lower overall reaction efficiency (47% yield, Z/E = 5/1) (results
not shown).
Table 4: Visible-light-driven sequential Photo-Wolff-Kischner reaction and
isomerization for synthesis of Z-alkenes.[a,b,c]
Scheme 3. Mechanistic studies. (A) Radical trapping experiments. (B) EPR
experiments. (i) EPR spectra of 1a-I. (ii) EPR spectra of 1a-I and 2a-I.
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To gain some insight into the mechanism, we conducted a
series of experimental mechanistic investigations (Scheme 3).
When stoichiometric radical acceptor, 1,1-diphenylethylene 15,
was added to the model reaction of 1a and 2a under the standard
conditions, the yield of product 3aa dropped drastically [Scheme
3A, Eq. (1)]. However, tosyl-substituted 1,1-diphenylethylene 16
and cyclopropane 17 were detected by HMRS, which implied that
sulfonyl radical 2a-A and sulfur ylide radical cation 1a-A might be
involved in the process. We postulated that cyclopropane 17
should be formed by radical addition of 1a-A to 15, followed by
SET reduction and intramolecular cyclization. Addition of
stoichiometric TEMPO as radical scavenger inhibited the
formation of 3aa completely, further suggesting that the reaction
may involve a radical process (see Section 6.2 in the Supporting
Information).[11]
with release of arylsulfonyl radical.[21] Upon base-mediated
deprotonation, 1-C further undergoes a Wolff−Kishner type N2
extrusion process to give zwitterionic species 1-D. Another SET
transfer from the reduced form of the photocatalyst to arylsulfonyl
radical to furnish a sulfinate anion with regeneration of the
ground state photocatalyst Eosin Y, closing the photocatalytic
cycle. Finally, an intramolecular E1cB elimination of 1-D occurred
to give the alkene products with release of dimethyl sulfide.
Taken together, key to the success of the reaction was the
modulating the reactivity mode of photo-generated sulfur ylide
radical cations
and the radical-carbanion crossover
mechanism.[22] Notably, the overall reaction is redox-neutral
process and does not need any external oxidant.
To further confirm a radical pathway that might be engaged in
this photo-Wolff-Kischner reaction, we then exploited electron
paramagnetic resonance (EPR) in the presence of a radical
spin-trap agent, 5,5-dimethyl-pyrroline N-oxide (DMPO), to trap
radical species possibly involved in the process (Scheme 3B). In
a mixture of 1a and DMPO performed under standard conditions
for 2 min, the formation of persistent radical adduct 1a-I was
identified according to EPR signals with six lines (g = 2.0041; AN
= 1.45 mT, AH = 2.21 mT), implying the involvement of radical
intermediate, sulfur ylide radical cation 1a-A [Scheme 3B, Eq.
(2)]. Moreover, with addition of DMPO to the model reaction of 1a
and 2a that was firstly irradiated for 30 min, two sets of EPR
signals were observed [Scheme 3B, Eq. (3)]. Specifically, signals
with six lines (g = 2.0041; AN =1.45 mT, AH = 2.21 mT) were
identified as EPR signals of adduct 1a-I, while signals with four
lines (g = 2.0044; AN =1.45 mT, AH = 1.42 mT) were observed
and identified as adduct 2a-I, respectively. All of these results
confirmed the formation of sulfur ylide radical cation 1a-A and
sulfonyl radical 2a-A. Moreover, Schuster previously reported
that aryl sulfur ylides demonstrated an irreversible one-electron
wave with a peak at around +0.5 V,[6] which means that the
Scheme 4. Proposed mechanism for the photo-Wolff-Kischer alkene synthesis.
In summary, we have developed for the first time
a
visible-light-driven photo-Wolff-Kischner reaction for the practical
and modular access to diverse alkenes, starting from readily
available and bench-stable sulfur ylides and N-tosylhydrazones.
The high functional group tolerance and broad substrate scope
were demonstrated by more than 60 examples. Both E- and
Z-olefinic stereochemistry in the products could be controlled
with excellent stereoselectivity. A series of mechanistic studies
support that the reaction was enabled by control the chemical
property of sulfur ylide radical cations and a radical-carbanion
crossover mechanism. Thus, this strategy opened a new way for
the exploration of sulfur ylides in synthetic radical chemistry.
Studies in this area are currently underway in our laboratory.
red
excited-state *Eosin Y (E1/2 = +0.83 V vs SCE) is sufficiently
oxidizing to undergo SET reduction by the sulfur ylides to give
the corresponding radical species. We also carried out
Stern−Volmer luminescence quenching experiments of the
photocatalyst Eosin Y using each reaction component (see
Figure S3 in the Supporting Information).[11] It was found that that
it is only the sulfur ylide that can efficiently quench the excited
photocatalyst. Collectively, these observations suggested that
the photocatalytic cycle should commence with SET oxidation of
sulfur ylide in a reductive quenching photocatalytic cycle. The
quantum yield of the model reaction was determined to be ϕ =
0.341, which suggested a truly photocatalytic pathway though a
short radical chain cannot be excluded.[11]
Acknowledgements
On the basis of the above experimental evidence and
mechanistic pathways reported about the reactivity mode of
We are grateful to the NNSFC (91856119, 21971081,
21820102003, 91956201, and 21901080), the Open Research
Fund of School of Chemistry and Chemical Engineering, Henan
Normal University (2021YB02), and the Program of Introducing
Talents of Discipline to Universities of China (111 Program,
B17019) for support of this research. We also wish to thank Prof.
Frank Glorius (Westfꢀlische Wilhelms-Universitꢀt Mꢁnster,
Mꢁnster) and Prof. Burkhard König (University of Regensburg)
for their valuable discussions and suggestions.
carbanion in Wolff−Kishner reaction,[10,19] we postulated
a
plausible mechanism as depicted in Scheme 4 for the current
photo-Wolff-Kischer reaction. Initially, the reaction begins with a
SET-mediated oxidation of sulfur ylide 1 by the photoexcited
*Eosin Y, affording the radical species sulfur ylide radical cation
1-A and the reduced form of the photocatalyst (Eosin Y·–),
respectively. Subsequent radical addition of electrophilic sulfur
ylide radical cation 1-A to the C=N bond of N-tosylhydrazones
forms the aminyl radical intermediate 1-B.[20] Then, the
intermediate 1-B undergoes a homolytic N-S bond fragmentation
to generate leaving group-containing diazene intermediate 1-C
Conflict of interest
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The authors declare no conflict of interest.
[17] For a recent review, see: a) J. J. Molloy, T. Morack, R. Gilmour,
Angew. Chem. Int. Ed. 2019, 58, 13654-13664; b) H. Zhang, S. Yu,
Chin. J. Org. Chem. 2019, 39, 95-108; for selected examples, see: c)
J. B. Metternich, R. Gilmour, J. Am. Chem. Soc. 2016, 138,
1040-1045; d) J. J. Molloy, M. Schafer, M. Wienhold, T. Morack, C.
G. Daniliuc, R. Gilmour, Science 2020, 369, 302-306; e) F. Song, F.
Wang, L. Guo, X. Feng, Y. Zhang, L. Chu, Angew. Chem. Int. Ed.
2020, 59, 177-181; f) X. Shen, C. Huang, X.-A. Yuan, S. Yu, Angew.
Chem. Int. Ed. 2021, 60, 9672-9679.
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Keywords: photoredox catalysis • hydrazones • radical reactions
• sulfur ylides • photo-Wolff-Kischner reaction
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Photocatalysis
P.-P. Gao, D.-M. Yan, M.-H. Bi, M. Jiang,
W.-J. Xiao, J.-R. Chen*
Page No. – Page No.
Alkene Synthesis by Photo-Wolff-Kischner
Reaction of Sulfur Ylides and
N-Tosylhydrazones
A visible-light-driven and room temperature photo-Wolff-Kischner reaction of
sulfur ylides and N-tosylhydrazones has been developed for the first time to
provide modular access to alkene synthesis. The high functional group
tolerance and broad substrate scope were demonstrated by more than 60
examples. Both E- and Z-olefinic stereochemistry in the products could be
controlled with excellent stereoselectivity. A series of mechanistic studies
support that the reaction should proceed through a radical-carbanion crossover
pathway, specifically involving addition of photo-generated sulfur ylide
radical cations to N-tosylhydrazones to form carbanions and subsequent
Wolff-Kischner process. PC = photoredox catalysis. SET = single-electron
transfer.
This article is protected by copyright. All rights reserved.