Bronsted Acid/Organic Photoredox Cooperative Catalysis: Easy Access to Tri- and Tetrasubstituted Alkenylphosphorus Compounds from Alcohols and P-H Species

Article abstract of DOI:10.1021/acs.orglett.8b01173

A Bronsted acid/organic photoredox cooperative catalytic system toward P-C bond formation from alcohols and P-H species is developed. With the assistance of visible light and TBHP, the reactions proceeded smoothly in an environmentally benign manner to give various alkenylphosphorus compounds in high efficiency.

Full text of DOI:10.1021/acs.orglett.8b01173

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Bronsted Acid/Organic Photoredox Cooperative Catalysis: Easy
Access to Tri- and Tetrasubstituted Alkenylphosphorus Compounds
from Alcohols and P−H Species
,
†
†
†
†
†
,†,‡,§
Peizhong Xie,* Jing Fan, Yanan Liu, Xiangyang Wo, Weishan Fu, and Teck-Peng Loh*
†School of Chemistry and Molecular Engineering, Institute of Advanced Synthesis, Jiangsu National Synergetic Innovation Center for
Advanced Materials, Nanjing Tech University, Nanjing 211816, P. R. China
‡
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371
§
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
*
S Supporting Information
ABSTRACT: A Bronsted acid/organic photoredox coopera-
tive catalytic system toward P−C bond formation from
alcohols and P−H species is developed. With the assistance
of visible light and TBHP, the reactions proceeded smoothly in
an environmentally benign manner to give various alkenyl-
phosphorus compounds in high efficiency.
4
a
4a
lkenylphosphorus compounds, due to their manifold
alkynes (Scheme 1a). The need to use additives, excess
1
5c
4
A
applications in organic synthesis, distribution in material
base, and well-defined ligands in addition to problems of
stereo- and regioselectivity control encourages further research
in this area. Considering the easy accessibility of alkenes,
transition-metal-catalyzed reactions of alkenes and P−H species
have also been reported. However, addition products rather
than oxidative cross-coupling was predominant, irrespective of
the transition-metal promoters or radical initiators. Until now,
only terminal styrenes were identified as suitable partners with
H-phosphonates/diarylphosphine oxides in the transition-
2
3
science, and biological activity, have received increasing
attention in the past few years. Therefore, tremendous effort
has been directed toward the development of efficient methods
to access this skeleton. In this context, transition-metal-
catalyzed P−C bond formation reactions of P−H species
6
,7
4
6
7
with alkynes (Scheme 1a) or specifically functionalized
5
alkenes (Scheme 1b) have been established as the dominant
strategies. For example, the pioneering research of Han has
realized the transition-metal-catalyzed direct phosphorylation of
8
metal-catalyzed oxidative cross-coupling (Scheme 1c).
Although much progress has been made in this area, problems
associated with narrow substrate scope, the need to use
expensive transition metals, a stoichiometric amount of
additives, and poor regio- and stereocontrol have encouraged
researchers to search for new and more efficient methods.
We envisaged that the reaction of the P−H species using the
Scheme 1. Strategies for the Preparation of
Alkenylphosphorus Compounds
9
cooperative catalysis of Bronsted acid and photocatalysts
under environmentally friendly conditions with readily available
10
feedstock such as alcohols will provide an attractive and
practical approach to access alkenylphosphorus compounds.
Although photocatalysis has been selected as promoting a
myriad of synthetic transformations with a range of applications
11
in a green manner, to the best of our knowledge, no suitable
organic photoredox catalysis has been developed toward
alkenylphosphorus compounds from alcohols. Furthermore,
12
organic photoredox catalysis might be compatible with
Bronsted acids due to its acidic nature. In this paper, we
report an efficient and environmentally friendly method for the
construction of P−C bonds via a Bronsted acid/organic
Received: April 19, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01173
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Organic Letters
Letter
a
photoredox cooperative catalysis reaction between alcohols and
Scheme 2. Substrate Scope of Alcohols
1
3
P−H species. This metal-free system was carried out under
green solvent (2-propanol) to yield t-BuOH and water as the
only byproducts will provide an effective method for the
synthesis of alkenylphosphorus compounds (Scheme 1d).
We initiated our investigation by reacting 1,1-diphenyletha-
nol with diphenylphosphine oxide in the presence of TsOH (20
+
mol %) and rhodamine B (RhBH ) (2 mol %) (Supporting
Information (SI), Figure 1) under air with the expectation of
obtaining (2,2-diphenylvinyl)phosphine oxides 3a. To our
delight, the desired 3a, together with addition product 3a′ was
generated in 11% yield with good selectivity (SI Table 1, entry
1
). Oxidants were then screened with the intention of
improving the overall performance of the reaction (SI Table
1
, entries 2−5). TBHP (2-hydroperoxy-2-methylpropane) gave
the best result associated with yield and selectivity between 3a
and 3a′ (SI, entry 5). Other Bronsted acids, such as benzoic
acid, trifluoroacetic acid (TFA), and HNTf , delivered the
2
product in lower yield and poor selectivity (SI Table 1, entries
6−8). The organic photoredox catalysts (SI Figure 1) also have
great influence on the reaction, and no positive results were
obtained when Eosin Y or Eosin B was employed (SI Table 1,
entries 9−11). The effect of solvent was also investigated (SI
Table 1, entries 12−17). Acetonitrile, dichloromethane, DMF,
toluene, as well as other alcohols disfavored this reaction,
affording 3a in lower yield and poor selectivity. The cooperative
catalytic system is crucial for this transformation. Either TsOH
+
or RhBH itself could not catalyze the reaction well (SI Table 1,
entries 18 and 19). Decreasing the loading of TsOH resulted in
a lower yield of the desired product (SI Table 1, entry 20). In
contrast, a lower amount of RhBH also can give the product in
a
Unless otherwise specified, reactions were carried out using 1 (0.2
mmol) and 2a (0.6 mmol) in i-PrOH (2.0 mL) at 80 °C for 30 min.
TBHP (2.0 equiv, 5.5 mol/L in decane) was then added. After 8 h,
+
good yield, but with a decline in selectivity (SI Table 1, entry
TBHP (2.0 equiv, 5.5 mol/L in decane) was added again. Isolated
21). A temperature of 80 °C was identified as the best
31
yields (3 and 3′). The ratio of 3/3′ and E/Z was determined by
P
temperature to obtain the product in terms of both yield and
selectivity (SI Table 1, entries 22 and 23).
NMR. All of the reactions were performed under white LED (23 W).
Under the optimized conditions, the scope of this P−C bond
construction reaction was then investigated. As shown in
Scheme 2, the desired products 3 can be isolated in high yield
with good to excellent selectivity. The position and electronic
properties of the aryl substituents on the 1,1-diarylethanols
have a limited effect on the overall performance. Unsym-
metrical 1,1-diarylethanols also delivered the corresponding 3h
in high yields with excellent chemoselectivity (3h/3h′),
although the E/Z selectivity is poor. Importantly, tetrasub-
stituted alkenylphosphine oxides, which were a challenge to
prepare due to steric hindrance, could be conveniently obtained
by using our developed method (3i−k). Thus, we can safely
come to the conclusion that this was a powerful complementary
method for alkenylphosphorus compounds. The reaction also
can be performed well on gram scale, and 3a can be obtained in
a comparable result. Unfortunately, 1-alkyl-1-arylethanols such
as 2-phenylpropan-2-ol were not suitable substrates for this
reaction.
We also examined the feasibility of extending this reactivation
to other kinds of dialkylphosphine oxides from diphenylphos-
phine oxide. To the best of our knowledge, the former P−H
species has not been successfully incorporated into alkenyl-
phosphorus compounds via oxidative cross-coupling. The
results are summarized in Scheme 3. Herein, we found that
our method could well tolerate diverse dialkylpshosphine
oxides, irrespective of their chain length (3l,o) or cyclic alkyl
substitutions (3j,k), affording the corresponding products in
moderate to high yields with excellent selectivity. The scope of
diarylphosphine oxides bearing various functional groups was
also checked. The electronic effect has a limited effect on the
yield and chemical selectivity (3p−u). Even the hindered
diarylphosphine oxides proceed well with the generation of
desired product in high yield, although the selectivity was lower
in some cases (3s,t). Di-1-naphthylphosphine oxides and di-2-
naphthylphosphine oxides can be successfully incorporated in
the reaction (3v,w). 6H-Dibenzo[c,e][1,2]oxaphosphinin-6-ol
was a suitable substrate for this reaction, which generated 3x in
moderate yield with high selectivity. Moreover, this protocol
could also be used in the preparation of an enantiomerically
pure P-stereogenic alkenylphosphorus skeleton (3y−aa)
species from optically pure H-phosphinate. Only one isomer
was isolated, which was proposed with the retention of
14
configuration at the phosphorus (see the SI for details).
With an attempt to gain more insight into the reaction
mechanism, a series of control experiments were then
conducted. As shown in Scheme 4, under identical reaction
conditions, 3a cannot be generated from 3a′ (Scheme 4a). This
result indicates that 3a′ was not the intermediate for generating
3a, and the reaction pathways for 3a and 3a′ were competitive.
Comparable results were obtained when alcohol 1a was
replaced by the corresponding ethene-1,1-diyldibenzene
(Scheme 4b, middle). Thus, the alkene should be the
intermediate for generating alkenylphosphine oxides. In
contrast, much lower yields and selectivities were observed
when the reaction was conducted without TsOH (Scheme 4b,
B
DOI: 10.1021/acs.orglett.8b01173
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Letter
a
Scheme 3. Substrate Scope for Phosphine Oxidants
conditions, no reaction occurred when 2.0 equiv of TEMPO
was added (Scheme 4c). This outcome, to some extent, means
the reaction proceeded via a radical process.
The (2,2-diarylvinyl)phosphines have been identified as key
ligand skeletons in palladium-catalyzed aromatic amination
15
reactions. However, transformation and tedious purification
processes were generally required for their preparation.
Theoretically, the (2,2-diarylvinyl)phosphines oxides prepared
in this investigation can be used as key feedstocks for the
aforementioned (2,2-diarylvinyl)phosphines. We selected alke-
nylphosphine oxides 3a as the model substrate. As is shown in
Scheme 5, the desired (2,2-diphenylvinyl)phosphines 4 can be
conveniently obtained in excellent yield.
Scheme 5. Reduction of 3a to Phosphine 4
To explore the mechanism of the visible-light-mediated P−C
bond construction, we conducted the Stern−Volmer fluo-
rescence quenching experiments. A 581 nm fluorescence
launched by rhodamine B was observed when the sample was
excited at 571 nm. The fluorescence intensity dramatically
decreased when diphenylphosphine oxide 2a was added (see
the SI for details). This investigation indicated the single-
electron transfer between rhodamine B and 2a. Based upon the
aforementioned investigations, we proposed a plausible reaction
pathway, as shown in Scheme 6. The reaction started with the
a
Unless otherwise specified, reactions were carried out using 1a (0.2
mmol) and 2 (0.6 mmol) in i-PrOH (2.0 mL) at 80 °C for 30 min,
TBHP (2.0 equiv, 5.5 mol/L in decane) was then added. 8h later,
TBHP (2.0 equiv, 5.5 mol/L in decane) was added again. Isolated
31
yields (3 and 3′). The ratio of 3/3′ was determined by P NMR. All
Scheme 6. Proposed Mechanism
of the reactions were performed under white LED (23 W).
Scheme 4. Control Experiments
single electron oxidation of diphenylphosphine oxide by
photoexcited rhodamine B (PC*) to give radical cation A
•
−
and reduced rhodamine B (PC ). The deprotonation occurred
with the generation of phosphinoyl radical B, which was
followed by the anti-Markovnikov addition to the in situ
•−
generated alkenes to deliver radical D. The PC was quenched
•
−
−
by TBHP to yield rhodamine B (PC), t-BuO , and OH . OH
might be captured immediately by TsOH with the generation
−
•
top). These results indicate that TsOH not only promotes the
initial dehydration process, but also influences the whole
catalytic cycles. Oxides 3a were obtained in a lower yield and
poor selectivity when the reaction was performed at room
temperature with TsOH (Scheme 4b, bottom). This result
reveals that a higher temperature was required for the overall
transformation, beyond dehydration. Under the optimized
of TsO . In the presence of proton, t-BuO reacted with radical
D by SET to give the cation intermediate E. With the assistance
of TsO , the deprotonation process then occurred to give the
final product 3.
In summary, we have developed an efficient and practical
method for the construction of the P−C bond under Bronsted
acid/organic photoredox cooperative catalysis. A variety of
−
C
DOI: 10.1021/acs.orglett.8b01173
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Organic Letters
Letter
alkenylphosphine oxides could be obtained in moderate to high
yields with good to excellent selectivities. It should be noted
that the challenging tetrasubstituted alkenylphosphorus com-
pounds could also be successfully accessed by using our
developed method. This protocol proceeded in an environ-
mentally benign manner, generating water and tert-butyl
alcohol as the only byproducts. The Bronsted acid/organic
photoredox cooperative catalysts systems will shed light on
related cooperative catalyst systems designed toward oxidative
cross-coupling reactions involving alcohols, even though the
alcohols are currently limited to α,α-diaryl alcohols.
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AUTHOR INFORMATION
■
(
d) Peng, P.; Lu, Q.; Peng, L.; Liu, C.; Wang, G.; Lei, A. Chem.
Commun. 2016, 52, 12338.
8) (a) Mao, L.-L.; Zhou, A.-X.; Liu, N.; Yang, S.-D. Synlett 2014, 25,
Corresponding Authors
*E-mail: peizhongxie@njtech.edu.cn.
E-mail: teckpeng@ntu.edu.sg.
(
*
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ORCID
Teck-Peng Loh: 0000-0002-2936-337X
Notes
(a) Zhang, F.; Wei, Y.; Wu, X.; Jiang, H.; Wang, W.; Li, H. J. Am.
Chem. Soc. 2014, 136, 13963. (b) Sako, M.; Takeuchi, Y.; Tsujihara, T.;
Kodera, J.; Kawano, T.; Takizawa, S.; Sasai, H. J. Am. Chem. Soc. 2016,
The authors declare no competing financial interest.
1
38, 11481. (c) Lin, Z.; Zhang, Z. M.; Chen, Y. S.; Lin, W. Angew.
Chem., Int. Ed. 2016, 55, 13739.
ACKNOWLEDGMENTS
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Van der Eycken, E. V. Chem. Soc. Rev. 2013, 42, 1121. Some selected
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■
We greatly acknowledge financial support by the National
Natural Science Foundation of China (21702108), the State
Key Program of the National Natural Science Foundation of
China (21432009), the Natural Science Foundation of Jiangsu
Province, China (BK20160977), Nanjing Tech University, and
the SCIAM Fellowship by the Jiangsu National Synergetic
Innovation Center for Advanced Material.
2
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