Visible light induced hydrophosphinylation of unactivated alkenes catalyzed by salicylaldehyde

Article abstract of DOI:10.1039/d1gc00374g

An air and water insensitive visible light induced hydrophosphinylation of unactivated alkenes is reported. A small amount of a simple and cheap compound, salicylaldehyde, is used as a photosensitizer. The reaction is carried out in a basic aqueous solution which enables the deprotonated salicylaldehyde to show visible light absorption.

Full text of DOI:10.1039/d1gc00374g

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Visible light induced hydrophosphinylation of
unactivated alkenes catalyzed by salicylaldehyde†
Cite this: DOI: 10.1039/d1gc00374g
Zeqin Yuan, Simin Wang, Miaomiao Li, Tian Chen, Jiaye Fan, Fuying Xiong,
Received 1st February 2021,
Accepted 16th April 2021
Qianggen Li, Ping Hu, Bi-Qin Wang, Peng Cao and Yang Li
*
DOI: 10.1039/d1gc00374g
rsc.li/greenchem
An air and water insensitive visible light induced hydrophosphiny- equivalent catalytic ability. The mechanism involves the gene-
lation of unactivated alkenes is reported. A small amount of a ration of a phosphonyl radical directly through reductive
simple and cheap compound, salicylaldehyde, is used as a photo- quenching from the photoredox catalytic cycle (Scheme 1b;
sensitizer. The reaction is carried out in a basic aqueous solution photocatalyst: PC).9a–c,g,j Inspired by this work, the phosphonyl
which enables the deprotonated salicylaldehyde to show visible radical addition across unsaturated substrates was expanded
light absorption.
to the hydrophosphinylation of alkynes,12 oxidative phosphiny-
lation,13
cascade
phosphoryl
radical
cyclization,14
Phosphorus containing organic compounds are of great
importance owing to their unique functional activities. They
have wide applications in the fields of medicine, agriculture,
materials science and catalysis.1 One of the most important
strategies for the construction of organophosphorus com-
pounds relies on C–P bond formation through transition
metal mediated coupling, nucleophilic substitution, addition
reaction and so on.2 Among them, the hydrophosphinylation
of unsaturated bonds represents one of the simple, efficient
and atom-economical approaches for the synthesis of novel
tertiary phosphine oxides.3 Reactions with unactivated alkene
substrates are difficult to carry out because they require tran-
sition metals,4 radical initiators,5 mw,6 high temperature,4–7 or
UV light8 for activation (Scheme 1a).
oxyphosphorylation,11,15 migratory carbophosphinylation16
and phosphonocarboxylation.17 In these reactions, noble
In recent years, visible light induced photocatalysis has
become a powerful strategy which renders the reaction con-
ditions greener and milder.9 This strategy has also been suc-
cessfully applied in the synthesis of organophosphorus com-
pounds through C–P bond formation.10 In 2013, Kobayashi
reported an elegant hydrophosphinylation of unactivated
alkenes induced by visible light.10a,11 This reaction proceeded
exceedingly well when an iridium complex was used as the
catalyst in the initial study, and the author managed to replace
the metal complex with the organic dye Rhodamine B with an
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu
610068, China. E-mail: yangli@sicnu.edu.cn
†Electronic supplementary information (ESI) available: Experimental details,
NMR spectra, light spectra and computational studies. See DOI: 10.1039/
d1gc00374g
Scheme 1 Reaction design.
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Table 1 Screening resultsa
metal-based complexes or organic molecules with a relatively
big π-conjugated ligand or a structural scaffold were commonly
used as the catalyst (Scheme 1b).
To further reduce the cost of visible light induced reactions,
simplifying the photocatalyst to smaller and cheaper organic
compounds has received a lot of attention.9g,h,18 In 2017,
Mathé and coworkers used a simple aromatic ketone, 2,2-
dimethoxy-2-phenylacetophenone (DPAP), as a photoinitiator
in P–H addition to unsaturated carbon–carbon bonds.8c
However, higher energy light (UV-A, 365 nm, 60 W) was
required for this reaction. Several examples show that simple
aromatic aldehydes are able to catalyze addition reactions to
alkenes under light irradiation.19 A representative mechanism
includes an energy transfer (EnT) process9i–l from the excited
state catalyst to the substrate (energy accepter: EA), following
the generation of reactive radical species (Scheme 1c).19a,c
Again, due to the nature of the small π-conjugated system of
these simple catalysts, high energy of light activation is often
needed (mainly in the UV region).20 Last year, Kang and co-
workers presented a visible light induced reaction catalyzed by
salicylaldehyde.21 The authors discovered that the light
absorption of salicylaldehyde in DMSO solution shows a red
shift from 324 to 417 nm when deprotonated by K3PO4. A
visible light induced EnT process was shown to activate the
corresponding substrates (Scheme 1c). We were interested in
exploring versatile C–P coupling reactions, and we envisioned
that this strategy could be used to trigger phosphonyl
radical additions across alkenes. Herein, we report an air
insensitive and visible light induced hydrophosphinylation of
unactivated alkenes in a salicylaldehyde-base aqueous solution
(Scheme 1d).
Entry
Deviation from the standard conditions
NMR yieldb
1
2
3
4
5
6
7
8
None
No cat. 1
No Na2CO3
No blue LED
No blue LED, 60 °C
Under Ar
Under O2
Cat. 2 instead of cat. 1
Cat. 3 instead of cat. 1
Cat. 4 instead of cat. 1
Cat. 5 instead of cat. 1
Cat. 6 instead of cat. 1
Cat. 7 instead of cat. 1
Ru(bpy)3Cl2·6H2O instead of cat. 1
Ir(ppy)2(bpy)·BF4 instead of cat. 1
95%
<5%
7%
NR
NR
>99%
69%
<5%
47%
24%
68%
30%
66%
52%
95%
9
10
11
12
13
14
15
As shown in Table 1, diphenylphosphine oxide 1a
(0.1 mmol) and 1-hexene 2a (2.0 equivalent) were chosen as
standard substrates. They reacted smoothly in the presence of
2.5 mol% salicylaldehyde, 2.5 equivalents Na2CO3 in H2O
under air atmosphere and irradiation with 30 W blue LEDs.22
The NMR yield reached 95% after overnight reaction (Table 1,
entry 1). It was later found that the reaction reached com-
a All reactions were performed using 1a (0.1 mmol), 2a (0.2 mmol),
2.5 mol% catalyst, 2.5 equivalent Na2CO3 and 0.1 mL distilled H2O
under 30 W blue LED irradiation under an air atmosphere overnight
(20–24 hours). b Determined by crude 1H NMR using 1,2-dichlor-
oethane (ClCH2CH2Cl) as an internal standard.
pletion after 3 hours. Control experiments showed that the ortho-hydroxyl substituted heteroaromatic aldehyde core struc-
presence of the catalyst, base and blue LED is indispensable ture.23 We tested its derivative pyridoxal hydrochloride in this
(Table 1, entries 2–5). These satisfactory results were further reaction, and the product was generated in 66% NMR yield
improved when the reaction was carried out under an argon (Table 1, entry 13). Metal based photoredox catalysts were also
atmosphere with degassed H2O, although extra experimental active under these reaction conditions (Table 1, entries 14 and
complications were introduced (Table 1, entry 6). When the 15), and Ir(ppy)2(bpy)·BF4 showed comparative results to
reaction was carried out under an oxygen atmosphere and with salicylaldehyde.
oxygen saturated H2O, the NMR yield decreased to 69%
For a better understanding of the function of salicylalde-
(Table 1, entry 7). Benzaldehyde was not able to promote this hyde, we measured the UV-visible absorption spectra of the
reaction (Table 1, entry 8). Changing the position of substi- aldehyde catalysts cat. 1–4 and cat. 7 (see the ESI† for details).
tution of the hydroxyl group on the catalyst reduced the cata- Without any base, these catalysts show no absorption in the
lytic ability (Table 1, entries 9 and 10). Other organo-photoca- visible light region. In the presence of Na2CO3 (100 times com-
talysts were also tested to replace salicylaldehyde. The com- pared to the aldehyde catalyst, the same ratio as under the
monly used organic dye Solvent Red 43 promoted the reaction standard reaction conditions), cat. 1 shows the best red-shift
and gave the product in 68% NMR yield (Table 1, entry 11). absorption in the λ > 400 nm region. The spectral irradiance of
DPAP showed a much lower catalytic ability under the stan- the blue LED was tested. It shows a light distribution that
dard reaction conditions (Table 1, entry 12).8c Pyridoxal phos- starts from 400 nm (λmax = 459 nm) and is in accordance with
phate (PLP), a co-factor of the vitamin B6 family, contains an the visible light absorption of the catalysts. Under the standard
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reaction conditions, the product was generated in 56% NMR phosphine oxide with a para Cl- or F-substituent was used, 3fa
yield after 1 hour and the yield was not increased (stirred over- and 3ga were obtained in 71% and 80% yields. Using bis(3,5-
night) after the irradiation was stopped. This observation indi- bis(trifluoromethyl)phenyl)phosphine oxide, the product 3ha
cates that visible light is required for the whole reaction was generated in lower yield. An SPO with a bigger π-system
process.
such as di(2-naphthalenyl)phosphine oxide reacted smoothly
With the best reaction conditions in hand, the scope of the under these reaction conditions and the product 3ia was
secondary phosphine oxide (SPO) was evaluated (Scheme 2). obtained in high yield (85%). Secondary phosphine sulfide
Using the standard substrate, the product 3aa was isolated in was shown to be compatible in this reaction, despite a lower
90% yield. Following this good result, both electron-rich and yield of the product being obtained (3ja, 50% yield).
-poor SPOs reacted smoothly and produced the products in Hydrophosphorylation with ethyl phenylphosphinate or
generally high yields. Di-p-tolylphosphine oxide gave 3ba in diethyl phosphonate was unsuccessful under these reaction
94% yield. Di-m-tolylphosphine oxide and bis(3,5-dimethyl- conditions. To our delight, high reactivity was observed when
phenyl)phosphine oxide showed similar reactivities, giving methyl(phenyl)phosphine oxide was used, affording 3ka in
both the corresponding products in 77% yield (3ca and 3da). 67% yield.3e,24
Di-o-tolylphosphine oxide was also tested. However, no reactiv-
Next, a variety of unactivated alkenes were evaluated by
ity was observed. This might be due to the negative steric reacting with diphenylphosphine oxide 1a (Scheme 3). More
effect on the P radical generation. The more electron rich aliphatic external alkenes were tested, and the products were
diphenylphosphine oxide with the methoxyl substituent pro-
vided 3ea in 86% yield. When electron poor diphenyl-
Scheme 3 The scope of unactivated alkenes. Reaction conditions: 1a
(0.2 mmol), 2 (0.4 mmol), cat. 1 (0.005 mmol), Na2CO3 (0.5 mmol), dis-
tilled H2O (0.2 mL), 30 W blue LED, air atmosphere. a 4.0 equivalents of
the corresponding alkene are used. b Regioselectivity: hexan-2-yldiphe-
Scheme 2 The scope of secondary phosphine oxides. Reaction con-
ditions: 1 (0.2 mmol), 2a (0.4 mmol), cat. 1 (0.005 mmol), Na2CO3
(0.5 mmol), distilled H2O (0.2 mL), 30 W blue LED, air atmosphere.
nylphosphine oxide/hexan-3-yldiphenylphosphine oxide
=
1.1 : 1.
c Regioselectivity: hexan-2-yldiphenylphosphine oxide/hexan-3-yldi-
phenylphosphine oxide = 1.5 : 1.
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generated in high yields (3ab–3ad). Functional groups such as might indicate that a phosphonyl radical addition mechanism
phenyl and trimethylsilyl groups were tolerated under these is involved in the generation of vinyl radical intermediates.26
reaction conditions (3ae and 3af). It has been shown that SPO
To further prove the utility of this method, a gram scale
was able to undergo nucleophilic substitution with haloalk- reaction was carried out (Scheme 5a). The reaction was easily
anes under basic reaction conditions.25 We were happy to find scaled up to 5.0 mmol and performed well under the standard
out that substrates bearing a bromide group were also toler- reaction conditions, with the product being obtained with
ated in this reaction, although a lower yield was obtained and slightly improved yield (92%). The reaction of SPO 1a with
no substituted side product was observed (3ag). Not much diallyl ether 2o was performed, affording the cyclic product
negative effect due to the base was observed when an alkene 3ao in 40% yield and 4.7 : 1 dr (Scheme 5b). The standard reac-
with a hydroxyl group was used, and the product 3ah was gen- tion was carried out in the presence of 2.0 equivalents of
erated in 69% yield. Two terminal alkenes with an ether group TEMPO and no reaction was observed (Scheme 5c). These
were examined. When allylic phenyl ether was used as the sub- results indicated that the reaction mechanism involves the
strate, 74% yield of 3ai was obtained. When active terminal generation of the phosphonyl radical species.
alkene ethoxyethene was used, the reaction was able to gene-
To explain the mechanism of phosphonyl radical gene-
rate the anti-Markovnikov adduct 3aj in 45% yield. Cyclic ration, several possibilities were discussed. The cyclic voltam-
internal alkenes showed high reactivities in this reaction, fur- metric behavior of salicylaldehyde in buffers with differing pH
nishing 3ak and 3al in 85% and 90% yield, respectively. values has been studied by He and coworkers.27 According to
Finally, linear internal alkenes were tested, and the alkenes this work we expect that the anodic peak potential of the cata-
with both (E)- and (Z)-configurations were able to undergo the lyst under our standard reaction conditions (2.5 M Na2CO3
reaction smoothly. In the case of (E)-2-hexene and (Z)-2- solution, around 12.32 pH value) is approximately 0.995 V vs.
hexene, the product 3am was obtained in 75% and 62% yield, SHE. The oxidative potential of 1a could be estimated to be
respectively, although the regioselectivities were low. Using (E)- around 1.245 vs. SHE.10a,28 The excited state of the deproto-
3-hexene and (Z)-3-hexene, 3an was generated as a single nated catalyst might not be able to oxidize 1a to the phospho-
isomer in 50% and 72% yield, respectively.
nyl radical (Scheme 1b).
This methodology was expanded to unactivated terminal
The energy difference between the ground state and excited
alkynes (Scheme 4). The alkynes with both linear and triplet state of the deprotonated salicylaldehyde was calculated
branched R5 groups reacted well with 1a, and substrates (ΔE = 54.7 kcal mol−1), and it was lower than the calculated
bearing chloro or hydroxyl substituents were tolerated. In the triplet energies of 1a and its isomer hydroxydiphenylpho-
case of substrates where the R5 group is less hindered, the sphane (1a′) (ΔE = 85.0 kcal mol−1 and 65.6 kcal mol−1
,
adducted products were generated mainly in the (Z)-configur- respectively). The direct P–H and O–H bond dissociation
ation, with the products 5aa, 5ab and 5ac being obtained in energy values of 1a and 1a′ were also calculated to be relatively
good yields (63%, 63% and 66%) and with moderate (Z) : (E) high (ΔE = 87.7 kcal mol−1 and 86.5 kcal mol−1 respectively).
selectivities (2.9 : 1, 2.4 : 1 and 5 : 1). In the substrate with the On the other hand, there is no overlap of the fluorescence
bulky R5 group (t-Bu), the product 5ad was generated mainly emission spectrum of deprotonated cat. 1 and the absorption
in the (E)-configuration (2 : 1) in 71% yield. These results spectrum of 1a (see the spectra in the ESI†). The absolute fluo-
rescence quantum yield of deprotonated cat. 1 is also relatively
low.29 All these results indicate that the phosphine oxide sub-
strate could not be excited by the excited catalyst through EnT
Scheme 4 The scope of unactivated alkynes. Reaction conditions: 1a
(0.2 mmol), 4 (0.4 mmol), cat. 1 (0.005 mmol), Na2CO3 (0.5 mmol), dis-
tilled H2O (0.2 mL), 30 W blue LED, air atmosphere. Total yield of the
(Z)- and (E)-configurations was reported. The (Z) : (E) or (E) : (Z) ratio was
determined from the isolated yield. a The (Z) : (E) ratio was determined by
crude 1H NMR.
Scheme 5 Scale up reaction and radical trapping reactions.
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calculation studies and previous research, photoredox and EnT
processes are excluded, and HAT activations are proposed for
the mechanism.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant Nos. 21901177, 21572218 and 21772135) for financial
support.
Scheme 6 Mechanism proposal.
Notes and references
1 For a recent book, see: Organophosphorus Chemistry: From
Molecules to Applications, ed. V. Iaroshenko, Wiley-VCH,
Verlag GmbH & Co. KGaA, 2019.
processes (neither the Dexter EnT process nor the fluorescence
resonance EnT process)9i–l (Scheme 1c).
In 2019, Kokotos and coworkers published a photochemical
hydroacylation of alkenes using 4-cyanobenzaldehyde as the
photoinitiator.19b,e They proposed that the triplet state of 4-cya-
nobenzaldehyde interacts with a ground state molecule of
4-cyanobenzaldehyde and generates a benzoyl radical and a
hydroxybenzyl radical, and the benzoyl radical subsequently
undergoes a hydrogen atom transfer (HAT) process to generate
the substrate radical. Based on this, a similar mechanism was
proposed (Scheme 6). Upon visible light irradiation, the depro-
tonated cat. 1 was activated to its excited state cat. 1*. Cat. 1*
interacts with the ground state cat. 1, furnishing the benzoyl
radical cat. 1′ and the hydroxybenzyl radical cat. 1″. The
benzoyl radical cat. 1′ then undergoes a HAT process with SPO
1 to generate the phosphonyl radical I and reform deproto-
nated cat. 1. Radical I undergoes an anti-Markovnikov addition
with alkene 2, generating radical II. Radical II abstracts the H
radical from the hydroxybenzyl radical cat. 1″, producing the
product 3 and reforming deprotonated cat. 1.30 The base is
proposed to be used for not only deprotonating the catalyst,
but also facilitating the HAT processes.
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Conclusions
In conclusion, a visible light induced hydrophosphinylation of
unactivated alkenes catalyzed by deprotonated salicylaldehyde
is presented. This reaction has two advantages: low energy
excitation (visible light: blue LED, 30 W) and a cheap catalyst
source (salicylaldehyde: 55.0 CNY per 100 g, reagent grade
98%, from the company Energy Chemical). The reactions are
easily performed in water and under an air atmosphere, and
the products are produced in generally high yields. Better
results can be obtained when the reaction is performed on a
larger scale which further shows the potential utility of this
methodology in industry. Based on the experimental results,
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