Visible-Light-Driven α-Allenylic C-O Bond Cleavage and Alkenyl C-S Formation: Metal-Free and Oxidant-Free Thiolation of Allenyl Phosphine Oxides

Article abstract of DOI:10.1021/acs.orglett.7b03052

A visible-light photoredox cleavage of α-allenylic C-O bond and alkenyl C-S formation is disclosed for the first time. The thiolation of allenyl phosphine oxides with diaryl disulfides occurs smoothly in metal-free and mild conditions, affording novel S,P-bifunctionalized butadienes with moderate to excellent yields. Mechanistic studies explain the cleavage of a C(sp³)-O(Ar) bond in initiating a key alkenyl radical intermediate.

Full text of DOI:10.1021/acs.orglett.7b03052

Letter
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
pubs.acs.org/OrgLett
Visible-Light-Driven α‑Allenylic C−O Bond Cleavage and Alkenyl C−S
Formation: Metal-Free and Oxidant-Free Thiolation of Allenyl
Phosphine Oxides
†
†
†
,†,‡
and Wei-Hua Zhang*,†
Ling Zhang, Jie Zhu, Jing Ma, Lei Wu,*
†Jiangsu Key Laboratory of Pesticide Science and Department of Chemistry, College of Sciences, Nanjing Agricultural University,
Nanjing 210095, P. R. China
‡
Beijing National Laboratory for Molecular Sciences and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R.
China
*
S Supporting Information
ABSTRACT: A visible-light photoredox cleavage of α-
allenylic C−O bond and alkenyl C−S formation is disclosed
for the first time. The thiolation of allenyl phosphine oxides
with diaryl disulfides occurs smoothly in metal-free and mild
conditions, affording novel S,P-bifunctionalized butadienes
with moderate to excellent yields. Mechanistic studies explain
3
the cleavage of a C(sp )−O(Ar) bond in initiating a key
alkenyl radical intermediate.
lkenyl sulfides are fundamental building blocks in synthetic1
relied on the visible-light-generated sulfur radicals in the early
stage, or single-electron oxidation of in situ generated sulfur
radical species.
A
intermediates, naturally occurring molecules, and drugs.
Not surprisingly, considerable effort has been focused toward the
synthesis of alkenyl sulfides, which undoubtedly represents an
Our group has advanced a series of studies on the reaction of
allenylphosphine oxides (1) via palladium-catalyzed cleavage of
1b,h
active research area in recent decades.
Typically, the most
6
powerful methods for constructing alkenyl sulfides rely on the
an α-allenylic C−O bond (Scheme 1c). Later, acetic acid
2
thiolation of alkynes and transition-metal-catalyzed cross-
mediated sulfonylation of the allenylphosphine oxide offered
divergent sulfonylated allenes and butadienes, exhibiting the
special reactivity of the α-allenylic C−O bond with acid
coupling of vinyl precursors with sulfur sources, where palladium,
copper, nickel, iron, and other metals have been successfully
1b,3
7
employed as catalysts. Given the increasing demands in green
and sustainable chemistry, contributions have also been extended
activation. Inspired by recent work on photocatalytic cleavage
8
of C−C and C−X bonds, we envisioned that an alkenyl radical
2
to oxidative cross-couplings between C(sp )−H and RS−H/
9
species might form via a photocatalytic C−OAr cleavage,
4
RSSR nucleophiles, most often with molecular iodine and/or
rendering new chemistry for alkenyl C−S formation (Scheme
d). From our continuing explorations on photoredox catalysis
and radical chemistry, we herein disclose an unprecedented
organic-dye-sensitized photocatalytic α-allenylic C−OAr bond
cleavage and alkenyl C−S formation.
synthetic oxidants. Although many achievements have been
made, the necessity of expensive ligand, a narrow substrate scope,
harsh reaction conditions, and stoichiometric oxidants often
limits its application.
1
10
Alternatively, visible-light-driven reactions have emerged as
Taking into account that sulfur radicals, generated from thiol
or dialkyl disulfide in the early stage, would alter the proposed
reaction pathway from allenylphosphine oxide (1), diaryl
disulfide was selected as the sulfur source. This choice was
based on the recent reports that diaryl disulfides exhibited
difficulty during photolysis under visible light from the LEDs, but
2
wonderful strategies for C(sp )−S formations, exhibiting great
5
superiority for both reaction conditions and substrate tolerance.
For recent elegant studies, Lei and co-workers reported an
intramolecular aromatic C−H thiolation by dual cobalt and
visible-light photoredox catalysis under oxidant-free conditions
5
c
(
Scheme 1a). Lately, Fu et al. developed an efficient visible-
11
were good as radical acceptors. We initially performed the
visible-light photoredox thiolation of allenylphosphine oxide
light photoredox system for arylation of thiols with aryl halides,
5g
including less reactive aryl chlorides. However, alkenyl C−S
formation by visible light photocatalysis has been under-
developed. This process has been dominantly confined within
12
(
1a) with diphenyl disulfide (2a) as a model reaction to
optimize the conditions, including photocatalyst, base, solvent,
atmosphere, and reaction time. No conversion could be observed
when the reactions were conducted in the dark or without a
5
k−m
the synthesis of benzothiophene derivatives to date.
In 2012,
2
̈
Konig revealed the first example of alkenyl C(sp )−S formation
with organic dyes, in which the radical cyclization was initiated by
the decomposition of diazonium salt upon irradiation by green
Received: September 29, 2017
5k
light (Scheme 1b). Inherently, these achievements mainly
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03052
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2
Scheme 1. (a and b) Representative Studies on C(sp )−S
Formation via Photoredox Catalysis; (c) Our Previous Work;
Information (SI) for details, Table S1). Afterward, several
commercial organic dyes as photocatalysts (PCs) were tested
(entries 4−7), among which eosin Y outperformed the others
leading to the highest yield. In the absence of a base, only <5%
yield of 3aa was observed by 31PNMR (entry 8). When the
reaction was conducted in the presence of stronger bases, such as
DBU, DABCO, Cs CO , and t-BuOK, the yields of 3aa increased
(d) This Work
2
3
substantially compared to case of NaHCO (entry 9). DBU
3
enabled the reaction with an excellent yield of 98% (entries 11).
Notably, the yield remained high even when using 0.5 equiv of
DBU, giving the target product 3aa in the same yield (entry 13).
As a comparison, lower loadings of the inorganic base led to a
substantial yield decrease to 39% (entry 14), suggesting a distinct
reaction mechanism depending on the base type. A dramatic
decrease in yield was observed when the reaction was carried out
under air, with only a trace amount of product detected (entry
1
5). These results indicate that light, eosin Y, a base, and a
nitrogen atmosphere are all essential to achieve the reaction with
high efficiency.
Encouraged by the preliminary results, various allenyl-
phosphine oxides (1b−1q) were evaluated using the optimal
conditions, with the results summarized in Scheme 2. In general,
Scheme 2. Substrates Scope on Allenylphosphine Oxides
photocatalyst (Table 1, entries 1−2). By employing a photo-
sensitizer under weak alkaline conditions, the desired product
S,P-bifunctionalized 1,3-butadiene (3aa) was obtained smoothly,
with no other additives. Screening different solvents revealed that
aprotic polar solvents afford higher yields and DMSO is the best
solvent for this process with a yield of 95% (see the Supporting
a
Table 1. Optimization of the Reaction Conditions
b
entry
1
photocatalyst (PC)
base
yield (%)
−
PivONa
PivONa
PivONa
PivONa
PivONa
PivONa
PivONa
−
N.R.
N.R.
93(86)
63
c
2
eosin Y
eosin Y
fluorecein
rhodamine B
eosin B
alizarin Red S
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
3
4
5
6
7
8
9
67
83
91
<5
allenylphosphine oxides with terminal alkyl, cyclic, aromatic, or
heterocyclic substitutions afforded the corresponding thioether
products with moderate to good yields. With aromatic
substitutions, both electron-rich groups (such as p-MeO) and
NaHCO3
50
1
1
1
1
1
1
0
1
2
3
Cs CO
2
97
3
DBU
98
t-BuOK
DBU
97
d
electron-deficient groups (such as p-Cl, p-F, and p-CF ) on the
3
98(92)
43(39)
trace
d
phenyl moiety proceeded smoothly in the system, without a
distinct electronic effect on the reactivity being observed. For
alkyl, alicyclic substituted allenes or endmost aromatics with
electron-neutral and -rich substituents, however, two isomers
could be isolated with slight preferences of E-selectivity in ratios
of 1.2:1−3.4:1 (3ba, 3ca, 3da, and 3oa). It is worthy to mention
that, owing to a combined effect of the stabilization of electron-
4
5
PivONa
DBU
e
a
Reaction conditions: allenylphosphine oxide (1a, 0.2 mmol),
diphenyl disulfides (2a, 0.2 mmol), photocatalyst (PC, 5 mol %),
b
31
base (1 equiv), N , 8 h. Yields based on PNMR, N.R. = no reaction,
isolated yields in bracket. Under dark. 0.5 equiv of base. Under air.
2
c
d
e
B
DOI: 10.1021/acs.orglett.7b03052
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
deficient groups to radical intermediates and π−π stacking
Scheme 4. Mechanism Studies
between the aromatic substitutions and P(O)Ph moiety, E-
2
isomers of 3ea, 3fa, 3ga, 3ia were obtained exclusively. The
stereoselectivity was elucidated by the X-ray structure of 3ia
(
CCDC 1573645). Allene without terminal substitution (1k)
produced a lower yield, which might be rationalized by relative
differences in the stability and reactivity of radical intermediates
when there were substitution groups on allenes. Cyclopentyl-,
cyclohexyl-, and cycloheptyl-derived allenes were also effective in
furnishing the corresponding products (3la−3na), giving similar
yields ranging from 59% to 64%. In addition, O- and S-
heterocyclic terminated allenes (1p, 1q) were applicable with
yields of 56% and 85%, respectively.
Next, the nature of diaryl disulfides was examined to verify the
generality of this photoinduced approach. As shown in Scheme 3,
Scheme 3. Substrates Scope on Disulfides
+
·
proposed [DBU] and radical intermediates [A] or [B] could
be detected, as well as the DBU cation radical and TEMPO (see
HR-MS details in the SI). The mechanistic studies clearly
support the photoredox cleavage of an α-allenylic C−O bond to
initiate radical intermediates instead of a sulfur radical.
On the basis of the experimental facts, quenching experiments
11,13
(
in SI), and previous reports,
a plausible reaction mechanism
is presented in Scheme 4. Initially, eosin Y changes to its excited
state (eosin Y*) under visible light irradiation, which donates a
single electron to allenylphosphine oxide (1) to form the ArO
anion as well as an α-allenylic radical species ([A]) and turned
into the eosin Y radical cation itself. Subsequently, [A]
tautomerizes to an alkenyl radical [B], which reacts with
diphenyl disulfides to deliver the S,P-bifunctionalized 1,3-
butadiene (3) and a sulfur radial PhS·. In addition, the PhS·
could be trapped by another alkenyl radical to furnish 3. There
was evidence that DBU played a role in the transference of an
electron to eosin Y radical cation to regenerate eosin Y.
In summary, we revealed a practical and efficient method for
alkenyl C−S bond construction through photocatalytic thio-
lation of allenyl phosphine oxides with diaryl disulfides. Under
visible light irradiation, successive cleavage of an α-allenylic C−O
bond and alkenyl C−S formation were involved, delivering a
series of novel S,P-bifunctionalized butadienes with moderate to
excellent yields. The reactions exhibited many advantages
including mild conditions, eco-benign procedures, and good
functional group compatibility. We expect this new and
operationally simple protocol to provide novel scaffolds for
building potential bioactive compounds.
high compatibility was exhibited with functional groups such as
methyl, methoxy, halo, and nitro substituents. Ortho-, meta-, and
para-substituted diaryl disulfides were all well tolerated, but
meta-substituted methyl impaired the reactivity somehow, giving
a lower yield compared with the others. Notably, heteroaromatic
disulfides, including thienyl, furyl, pyridinyl, and benzothiazolyl
disulfides, performed well, providing the corresponding thiolated
product in medium to excellent yields, which offered further
functionalization of the products. Furthermore, benzyl disulfide
underwent the reaction successfully to give product 3am in a 52%
yield.
To gain some insight into the reaction mechanism, several
control experiments were conducted, as described in Scheme 4.
On one hand, in the presence of 5 equiv of TEMPO, the coupling
reaction of 1a and 2a was entirely inhibited (Scheme 4, eq a),
indicating a radical pathway. On the other hand, as shown in eq b,
an attempt to capture the radical species generated from diphenyl
disulfides failed under current conditions, which was in line with
ASSOCIATED CONTENT
Supporting Information
11a
■
the previous reports. Intriguingly, from monitoring the high-
*
S
resolution mass spectrum (HR-MS), allenic alcohol (5) and
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03052.
2
,2,6,6-tetra-methylpiperidine (6) were detected in the reaction
of allene 1a with 5 equiv of TEMPO. These two compounds
were probably produced from the decomposition of a radical
trapping product (7). Fortuitously, a coupling product of
Experimental procedures; spectral data for all new
compounds (PDF)
C
DOI: 10.1021/acs.orglett.7b03052
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
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CCDC 1573645 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 Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
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*
E-mail: rickywu@njau.edu.cn.
E-mail: zhwh@njau.edu.cn.
*
5
1
906. (j) Liu, B.; Lim, C.-H.; Miyake, G. M. J. Am. Chem. Soc. 2017, 139,
nig, B.
ORCID
3616. Alkenyl C−S formation: (k) Hari, D. P.; Hering, T.; Ko
̈
Lei Wu: 0000-0001-9130-6619
Notes
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The authors declare no competing financial interest.
(
n) Shi, Q.; Li, P.; Zhang, Y.; Wang, L. Org. Chem. Front. 2017, 4, 1322.
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ACKNOWLEDGMENTS
This project is supported by the Fundamental Research Funds
for the Central Universities (NJAU, Grant No. KYTZ201604).
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