Visible light promoted hydration of alkynes catalyzed by rhodium(III) porphyrins

Article abstract of DOI:10.1039/c5cc04015a

Visible light promoted hydration of a wide scope of alkynes to ketones catalyzed by rhodium(iii) porphyrin complexes was described. The key intermediate β-carbonyl alkyl was observed and independently synthesized. The rate of photolysis is over two orders of magnitude faster than that of the thermal process.

Full text of DOI:10.1039/c5cc04015a

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DOI: 10.1039/C5CC04015A
Journal Name
COMMUNICATION
Visible Light Promoted Hydration of Alkynes Catalyzed by
Rhodium(III) Porphyrins
Received 00th January 20xx,
Accepted 00th January 20xx
Xu Liu, Lianghui Liu, Zikuan Wang and Xuefeng Fu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Visible light promoted hydration of a wide scope of alkynes to hydration of alkynes with 100% atom efficiency will be obtained.
ketones catalyzed by rhodium(III) porphyrin complexes was Thus, the most challenging step is the hydration of the Rh-C bond in
described. The key intermediate β-carbonyl alkyl was observed (por)Rh-CH2C(O)R complex to form CH3C(O)R and (por)Rh-OH.
and independently synthesized. The rate of the photolysis is over
two orders of magnitude faster than that of the thermal process.
Although migration-insertion reaction or reductive elimination
reaction, typical product formation pathway for organometallic
catalytic systems, is usually unfavourable for the planar aromatic N4
ligand, our recent study in catalytic hydroxylation of a Si-C(sp3)
bond by porphyrin rhodium(III) in water indicated that irradiation
with visible light triggered the hydration of Rh-C bonds to form Rh-
OH and alkanes, which afforded a novel strategy to accomplish
unusual catalytic processes (eq3).17
Hydration of alkynes provides a perfect approach for synthesis of
carbonyl compounds with 100% atom efficiency. 1Mercury(II) salt
catalyzed hydration of acetylene was used to produce acetaldehyde
and acetic acid before 1970s which gradually became obsolete due
to the toxicity of mercurial waste.2 Recently, replacement of
mercury by more efficient metal catalysts has been developed, such
as Ag,3 Au,4 Co,5 Fe,6 In,7 Ir,8 Pd,9 Pt,10 Rh,11 Ru,12 and Sn-W.13 The
metal ketonyl complexes were generally suggested as the key
intermediates14 and the cleavage of the metal-carbon bonds might
be the rate determining step (eq 1).
Photo-chemical processes attracted increasing attentions which
provide promising pathways to obtain both thermodynamic and
kinetic challenging reactions.18 We turned our attention to photo
induced organometallic transformations by taking advantage of
porphyrin ligand as a dye through incorporation of valuable
organometallic intermediates of organo-metal porphyrin complexes
into novel photo catalysis. Herein we report on visible light
Our group has accomplished aerobic oxidation of alkenes to
ketones in water catalyzed by porphyrin rhodium(III).16 Mechanistic
studies indicated that β-hydroxy alkyl rhodium porphyrins were the
key intermediate and the β-carbonyl alkyl rhodium porphyrins were
side products which interrupted the catalytic cycle. However, it is
worth noting that reactions of porphyrin rhodium(III) with alkynes
might produce the β-carbonyl alkyl rhodium porphyrins, the key
intermediate for hydration of alkynes, through the same pathway
as the reaction with alkenes (eq 2) but without forming β-hydroxy
promoted
hydration
of
alkynes
using
tetra(p-
sulfonatophenyl)porphyrin rhodium(III) ((TSPP)RhIII) as the catalyst,
where the porphyrin rhodium ketonyl complex (TSPP)Rh-CH2COPh
was observed as the reactive intermediate (Scheme 1).
alkyl rhodium porphyrins.16a
.
Scheme 1. (TSPP)RhIII catalyzed hydration of alkynes
More importantly, if the β-carbonyl alkyl rhodium porphyrins
could be converted to porphyrin rhodium (III) and ketone, catalytic
The catalytic hydration of phenylacetylene was achieved at room
temperature under visible light irradiation. And 10% yield of
acetophenone was observed (Table 1, entry 1) after 12h irradiation
of the methanol solution containing 0.2 mmol of phenylacetylene
and 2 μmol of (TSPP)RhIII in acidic conditions ([HCl] = 0.01 mol/L).
The yield increased to 38% with addition of one equivalent of H2SO4
Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth
Materials Chemistry and Applications, College of Chemistry and Molecular
Engineering, Peking University, Beijing, China. E-mail: fuxf@pku.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000x
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instead of HCl (Table 1, entry 2). Various organic acids including
Journal Name
Under the optimized reaction conditions, the scope of the alkyne
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DOI: 10.1039/C5CC04015A
benzene sulfonic acid, trifluoromethane sulfonic acid, substrates was examined. Phenylacetylene derivatives containing
methanesulfonic acid and trifluoromethane sulfonimide were electron rich and electron withdrawing groups on the phenyl ring
tested, and trifluoromethane sulfonimide showed the highest were all converted to the corresponding methyl ketones (Table 2,
selectivity (Table 1, entry 3-6). Control experiments indicated that entry 1-7). The methoxy, methyl, ethyl and tert-butyl group
acid, (TSPP)RhIII and light were all necessary (Table 1, entry 1, 7-8).
remained stable during the photo reaction (Table 2, entry 2-5). The
reactivity of aliphatic alkynes was relatively low so the hydration
reaction of 1-heptyne and 1-decyne were performed at 50 oC (Table
2, entry 8-9)
Table 1. Visible Light Promoted Hydration of Phenylacetylene Catalyzed by
(TSPP)RhIII at Room Temperaturea
Subsequently, the reaction mechanism was investigated.
According to previous observations, the first step in catalytic
hydration of alkynes could be the reactions of porphyrin rhodium(III)
with alkynes producing the β-carbonyl alkyl rhodium porphyrins
through the same pathway in analogous to the reaction of
(TSPP)RhIII complex with olefins to produce β-methoxyalkyl
complexes.16a
Heating the methanol solution containing 0.2 mmol of
phenylacetylene and 2 μmol of (TSPP)RhIII in acidic conditions ([HCl]
= 0.01 mol/L) at 60 oC for 48 hours resulted in the formation of
(TSPP)Rh-CH2COPh with a yield of 50%. When performed in
methanol-d4 solution, (TSPP)Rh-CD2COPh was obtained and
confirmed by ESI-MS(Figure 2S). The (TSPP)Rh-CH2COPh was also
independently prepared by reaction of (TSPP)RhI with PhCOCH2Br.
(eq5 and Figure 3S).
aReaction conditions: phenylacetylene 0.20 mmol and (TSPP)RhIII 2.0 μmol mixing in
1.0 ml methanol, stirred at room temperature under visible light irradiation (500 W
Hg lamp, 420-780 nm filter, 25 cm distance, 27 mW/cm2 ) for 12 hours. bGC results.
cIn the dark. dNo (TSPP)RhIII was added
Table 2. Scope of the Alkyne Substratesa
-6.0
-6.5
-7.0
-7.5
-8.0
A
-8.5
0
100 200 300 400 500 600 700 800 9001000
t/s
t/h
4
0
2
6
8
-5.5
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
-8.5
-6.0
-6.5
-7.0
-7.5
-8.0
-8.5
l n
c
(
R h
I I I
- R
)
/
M
B
0
10 20 30 40 50 60 70 80 90 100
t/s
Figure 1. A: Photo production of ketonyl from (TSPP)Rh-CH2COPh complex at
various pH value; : pH ~ 4.5, 298 K, kobs.
: pH ~ 6.5, kobs. = 0.00143 s-1,
0.00172 s-1, ●: pH ~ 3.5, kobs. = 0.00239 s-1, : pH ~ 1.2, kobs. = 0.0497 s-1; B:
▼
▲
=
■
Thermal and photo production of ketonyl from(TSPP)Rh-CH2COPh complex;
▼:
pH ~ 1.2, dark, kobs. =7.94 x 10-5 s-1(0.286 h-1),
■: pH ~ 1.2, visible light, kobs. =
aReaction conditions: alkyne 0.20 mmol, (TSPP)RhIII 2 mmol and methanol 1 ml,
stirred at room temperature under visible light irradiation (500 W Hg lamp, 420-
780 nm filter, 25 cm distance, 27 mW/cm2 ) for 12 hours. bGC results. c50oC
0.0497 s-1
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Notes and references
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Scheme 2. Photo induced hydration of alkynes catalyzed by rhodium porphyrin
In acidic methanol-d4 solution (pH=6.5), (TSPP)Rh-CH2COPh was
completely converted to acetophenone and (TSPP)RhIII at 298 K
within 20 min under visible light irradiation (500 W Hg lamp, 420-
780 nm filter, 25 cm distance, 27mW/cm2) (Figure 1A). The rate
enhancement was observed with the increase of proton
concentration, 50 seconds to reach completion at pH= 1.2 under
visible light irradiation in comparison to without the irradiation of
visible light which took 8 hours to reach completion at pH= 1.2
(Figure 1B). The photo irradiation largely enhanced the rate of Rh-C
bond cleavage, and the photo irradiation process was much more
efficient than that of thermal process for the hydration of Rh-C
bonds. All of these reactions showed first-order kinetics on the
(TSPP)Rh-CH2COPh and the rate constants were listed in Figure 1.
The hydration of alkynes catalyzed by (TSPP)RhIII was envisioned
to proceed through a four-step cycle involving: a) coordination of
alkynes to (TSPP)RhIII(MeOH)2 which was the dominant species at
pH<3,19 and then methanol functioned as the nucleophile to attack
the alkynes to form β-acetal alkyl rhodium porphyrins; b) the
interconversion between ketone and ketal to give the intermediate
(TSPP)Rh-CH2COPh; c) photolysis of the (TSPP)Rh-CH2COPh complex
yielded PhCOCH2 radical and (TSPP)Rh(II); d) subsequently,
(TSPP)Rh(II) reacted with methanol rapidly to produce (TSPP)Rh-H
and (TSPP)Rh-OMe to complete the catalytic cycle20, and
meanwhile, hydrogen atom transfer occurred from (TSPP)Rh-H to
PhCOCH2 radical to yield the final product (Scheme 2), completing
the catalytic cycle.
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Conclusions
In summary, we have, to the best of our knowledge,
developed the first catalytic visible light promoted hydration of
alkynes. The rhodium ketonyl intermediates were observed
and independently prepared which underwentfast photo
cleavage of Rh-C bonds in acidic solution to produce ketone
products. Combination of alkyne hydration with visible light
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