Photo induced alkyne hydration reactions mediated by a water soluble, reusable Rhodium (I) catalyst

Article abstract of DOI:10.1016/j.catcom.2018.07.007

Under photochemical irradiation, the hydration of aryl alkynes in the presence of water proceeds in good yields to afford the corresponding ketone, using a water-soluble rhodium (I) catalyst. The catalyst is effective for internal and terminal alkynes and showed high functional group compatibility. A low catalyst loading, low temperature and shorter duration of photolysis are ideal feature of reaction. The catalyst can be reused several times under aerobic and aqueous conditions, exemplifying the robust nature of the catalyst. In comparison with known Rh and other metal catalysts, the present reaction provides a remarkable green approach for alkyne hydration reactions.

Full text of DOI:10.1016/j.catcom.2018.07.007

Accepted Manuscript
Photo induced alkyne hydration reactions mediated by a water
soluble, reusable Rhodium (I) catalyst
Munsaf Ali, Avinash K. Srivastava, Shepherd Siangwata, Gregory
S. Smith, Raj K. Joshi
PII:
DOI:
Reference:
S1566-7367(18)30251-6
doi:10.1016/j.catcom.2018.07.007
CATCOM 5447
To appear in:
Catalysis Communications
Please cite this article as: Munsaf Ali, Avinash K. Srivastava, Shepherd Siangwata,
Gregory S. Smith, Raj K. Joshi , Photo induced alkyne hydration reactions mediated
by a water soluble, reusable Rhodium (I) catalyst. Catcom (2018), doi:10.1016/
j.catcom.2018.07.007
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ACCEPTED MANUSCRIPT
Photo induced alkyne hydration reactions mediated by a water soluble, reusable
Rhodium (I) catalyst
Munsaf Alia, Avinash K. Srivastavaa, Shepherd Siangwatab, Gregory S. Smithb, Raj K.
Joshia,*
aDepartment of Chemistry, Malaviya National Instituteof Technology, Jaipur 302017, Rajasthan, India
bDepartment of Chemistry, University of Cape Town, Rondebosch 7701, Cape Town, South Africa

ACCEPTED MANUSCRIPT
2
ABSTRACT
Under photochemical irradiation, thehydration of aryl alkynes in the presence of water proceeds in good yields to afford thecorresponding
ketone, using a water-soluble rhodium (I) catalyst. The catalyst is effective for internal and terminal alkynes and showed high functional group
compatibility. A low catalyst loading, low temperature and shorter duration of photolysis areideal feature of reaction. Thecatalyst can be
reused several times under aerobic and aqueous conditions, exemplifying therobust nature of the catalyst. In comparison with known Rh and
other metal catalysts, thepresent reaction provides a remarkable green approach for alkyne hydration reactions.
Keywords:
Acetylene
Hydration
Rhodium (I)
Ketone
Catalysis
* Corresponding authors at: Department of Chemistry, Malaviya National Instituteof Technology Jaipur, JLN Marg, Jaipur 302017, Rajasthan,
India. E-mail addresses : rkjoshi.chy@mnit.ac.in,(R.K Joshi), Ph.+91 1412713279, Gregory.smith@uct.ac.za, (G. S. Smith Email)Ph.+27-216505279

ACCEPTED MANUSCRIPT
Chemical transformations that use water as a reagent or reaction medium continue to attract attention in organic synthesis.1 This stems fromthe
non-toxic nature of water to both the user and the environment, its relative abundance compared to other solvents, as well as thecompatibility
with various organic substrates. Theatom-economical hydration of alkynes (with all of the reactant atoms found in thedesired products)
represents one of the most important reactions that utilize water in organic synthesis. This well-established reaction is classically catalyzed by
the toxic and environmentally-destructive metal-containing mercuric sulfate or a strong acid catalyst. In the presence of sulfuric acid, the
hydration of alkynes leads to industrially relevant carbonyl compounds, used in thebulk and fine chemical industries.2 Mercury promoted
hydration of alkynes often suffer a major drawback where certain acid-labile functional groups are not compatible with the harsh acidic
conditions. Moreover, the reaction proceeds under stringent conditions, i.e. high temperatures (> 100 °C), prolonged reaction time as well as
requiring a large excess of acid and water. To circumvent these drawbacks, various transition-metal-based catalysts have been developed as
environmentally benign alternatives to mercuric catalysts in the hydration of alkynes, and these include Ag,3 Pt,4 Cu,5 Au,6 Fe,7-12 Ir,13 In,14
Ru15-17 and Rh18-23 complexes. These metals provide a much easier, mild, greener and efficient protocolfor the hydration of simple alkynes. In
selected cases, aryl alkynes posed
several challenges giving reduced yields, lower selectivity or requiring higher catalyst loading and elevated temperatures. The hydration of
terminal alkynes usually occurs via Markovnikov addition, giving methyl ketones, with symmetric internal alkynes selectively yielding single
ketone products. It is generally accepted that the hydration proceeds via an enol which immediately transforms into the more stable ketone via
tautomerism, with the metal ketonyl complexes being the key intermediates, in the presence of metal catalysts.24 Rhodium-mediated addition of
water to alkynes is attractive owing to the well-known good functional group tolerance of the metal, as well as the high activity and selectivity
demonstrated under mild reaction conditions in various homogeneous catalytic transformations.25 Complexing the metal to chelating Schiff
base ligands is a very popular approach because of the good stability and versatility afforded by the ligands, often leading to excellent catalytic
activity.26 Herein, we report the photolytic hydration of various alkynes using a previously synthesized, reusable, water-soluble N,O-chelate
Rh(I)–Schiff base catalyst precursor bearing ferrocene as an ancillary support.27 Using this rhodium catalyst, the hydration of alkynes
proceeded in 30 minutes under irradiation, yielding methyl ketones.
Based on previous reports of water-soluble Rh salts19 and Rh complexes20 used for hydration reactions, various methods were attempted to
improve the yield of these transformations. However, these methods have significant limitations which include slow processes, long reaction
time (12-24 hr), and the loss of volatile alkynes at high temperatures during the long reaction hours. However, after studying a series of
reported reactions with our Rh catalyst, we found insignificant changes to the product yield; hence it was decided to adopt a different strategy.
This entailed the preliminary screening of hydration reactions in a UV reactor (containing 125 W, Hg vapor lamp ≈ 289 nm) which produced
some unprecedented results.
Scheme 1 depicts our preliminary reaction in which the hydration of phenyl acetylene (1) resulted in the formation of acetophenone (2).
When a methanol solution of phenylacetylene containing a catalytic amount of Rh catalyst was exposed to UV radiation for 30 min. at -5˚C, the
formation of acetophenone was unequivocally observed.
Scheme 1: Photochemical hydration of phenylacetylene with a Rh (I) catalyst in methanol
Phenylacetylene was used as a model reactant for the optimization of various reaction parameters in different reaction conditions. The
feasibility of the catalytic reaction is summarized in Table 1. At a very low catalyst loading (0.2 mol%, entry 2), the yield of phenyl acetylene
was very poor. However, the yield was proportionally improved with an incremental catalyst loading up to

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Table 1: Optimization of different reaction parameters with phenylacetylene
Solvent Ratio
Time
(min.)
Temp
(˚C)
Yielda
(%)
Entry
mol (%) Catalyst
(MeOH: Water)
1.
-
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
0:1
1:1
1:2
2:1
1:0
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
30
30
30
30
30
30
30
30
30
30
30
30
30
15
20
25
30
35
40
30
30
30
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
-5
0
nd
30
50
60
70
78
78
78
nd
70
78
40
08
20
37
60
78
77
75
53
78
77
2.
0.2
0.5
0.7
1.0
1.5
2
3.
4.
5.
6.
7.
8.
2.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
-5
-10
aisolated Yields
1.5 mol% (entries 3-6). Moreover, a further increase of catalyst loading to 2.5 mol% (entry7,8) does not show any significant effect on the
product yield. For the solvent optimizations, when water was used as a solvent, the substrate does not undergo any chemical reaction. This may
be ascribed to the insolubility of phenylacetylene in water (entry 9). However, a water and methanol (1:1) solvent mixture substantially
increase the yield up to 70% of the desired acetophenone (entry 10,11). In an attempt to optimize the yield of the desired product, various other
combinations of water and methanol were investigated for this reaction. Furthermore, increasing the water in the solvent mixture does not bring
any changes to the yield of product. A higher ratio of methanol in the solvent mixture decreased the yield of acetophenone to a certain extent,
while 1:2 ratio of methanol and water produced the highest yield (entry 12,13). Other alcohols like ethanol, propanol and butanol in
combination with water were also studied as a solvent, but unfortunately low-yielding transformations were obtained. Moreover, reactions in
the presence of branched alcohols, isopropanol and tert-butanol also failed to result in a favorable transformation. Nonetheless, the obtained
results from various trial experiments reveal a new strategy to pursue hydration reactions, which gave us impetus to explore a wider scope of
the hydration reaction. In optimizing the parameters for alkyne hydration, a set of trial reactions were carried out at various times and
temperatures. Increasing the reaction time from 15 to 30 min (entries 14-18) consistently affords an increased yield, while longer periods of
photolysis of the reaction mixture does not affect the course of the reaction. In assessing the effect of temperature, a series of reactions in the
range -10 ˚C to 10 ˚C was tested and it was noted that the product yield was higher at lower temperatures. Moreover, the yield of the desired
product was almost constant between -5 to -10 °C, while increasing the temperature to 0 °C drastically deaccelerates the rate of hydration
(entry 210-22). Under the optimized reaction parameters (2 mol%, 1:2 MeOH-H2O, 30 min, -5 °C), a selective formation of acetophenone was
recorded for the hydration of phenyl acetylene.
Using the established optimised reaction conditions, we examined the hydration of a variety of alkynes and the results are summarised in
Table 2. Both terminal and internal alkynes are efficiently and selectively converted to their corresponding ketones in high yields, in the
presence of a range of electron-donating or withdrawing functional groups, demonstrating a high tolerance for a variety of functional groups.
Substituted phenylacetylenes, such as 2- and 4-methoxyphenylacetylene gave the corresponding ketones in 82%, and 84% yields respectively
(Table 2, entries 8, 9). Similarly, ortho-, meta-, and para-substituted methylphenylacetylene reacted smoothly; producing the corresponding
methyl ketones in 83%, 82%, and 83% yields respectively (Table 2, entries 4-6). The highest yield (85%) was achieved using 4-tert-
butylphenylacetylene (Table 2, entry 3). The 4-ethylphenylacetylene and 3-aminophenylacetylene also reacted favourably and formed the
corresponding ketone in 82% and 79% yields respectively (Table 2, entry 7, 14). The hydration of ferrocenylacetylene also reacted successfully
and yielded 80% of keto product (entry 2). The same reaction conditions were applied to halo-substituted phenylacetylenes, such as 4-fluoro-,
4-chloro-, 4-bromo and 2-chloro phenylacetylene to give 81%, 73%, 75% and 74% yields (Table 2, entry 10-13) respectively. The hydration of
internal alkynes yielded the corresponding diphenylacetylene in yield of about 76, 81% of the corresponding hydrated product (Table 2, entry
15 - 17). However, this reaction was not found feasible for the hydration of aliphatic alkynes under the present reaction conditions.

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Table 2: Hydration products of various terminal and internal alkynes
Entry
Reactant
Product
Yielda
1.
78
2.
80
85
3.
4.
83
82
5.
6.
7.
8.
83
82
82
9.
84
10.
11.
75
74

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12.
13.
14.
15.
16.
17.
73
81
79
76
77
81
Conditions: Rh-Catalyst 1.5 mol%, Solvent (80mL) MeOH: Water (1:2 ratio), Temp. -5˚C, hʋ, 0.5h. aIsolated Yield
The recyclability of the catalyst was studied, using phenylacetylene as a model reactant under the optimized reaction conditions. In a
typical experiment, the water-soluble rhodium complex was recovered at the end of the reaction by liquid-liquid extraction using
dichloromethane and water. The separated aqueous phase was directly used in a new catalytic run. The catalyst showed good recyclability and
could be used up to 4 times before a decrease in product yield was observed. The catalyst was rendered ineffective after the seventh cycle.
Previous Rh metal based catalysts does not show such promising catalysis and recyclability , with such a less amount of catalyst and short
duration of reaction ( SI table 3).
No. of Cycles
Product Yielda
1
2
3
4
5
6
7
8
78
78
72
70
60
50
20
Trace
Condition: Catalyst 1.5 mol%, Solvent (80mL) MeOH: Water (1:2), Temp. -5˚C, hʋ, 30 min, aIsolated Yields
A plausible mechanism is envisioned with the aid of previous reports in the literature (Figure 1). It is suggested that hydration of alkynes
can be conducted in UV20, MW28 and Thermal29 conditions. The methanol20 and water30 are found suitable for alkyne hydrations. In general,
metal catalysts are employed with polar aprotic solvents with a stoichiometric amount of water in the reaction. There are multiple intermediates
with water and methanol that have been investigated in previous reports. This suggests the formation of intermediates b to d and it is possible
in the presence water as well as methanol20. Various possible pathways for the formation of d as an intermediate complex with water and
methanol are depicted and discussed in the Supporting Information (Figure S2). Further homolytic cleavage of Rh–C bond under the influence
of UV radiation produces a PhCOCH2 radical20. Formation of complexes e and f occurs at high concentration of methanol29, 31-33. Complex e
and f behaves as H radical carrier which is transferred to the PhCOCH2 radical.20
In conclusion, we have demonstrated a novel and green strategy for the hydration of terminal and internal alkynes to produce the ketones,
using a water-soluble rhodium (I) catalyst. This represents the first example of a UV-assisted alkyne hydration in the presence of a water-
soluble and reusable rhodium (I) catalyst. The catalytic conversion occurs under aqueous, aerobic conditions and the catalyst can be reused up
to 4 cycles without any regeneration, which reflects the green and robust nature of the catalyst. Furthermore, this catalyst produces excellent,
selective, high-yielding results with terminal, internal and substituted aryl alkynes via a one-pot hydration in 30 minutes, which is far quicker in
comparison with earlier reports.

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Figure 1 Plausible reaction mechanism for Rh (I) catalysed alkyne
hydration
Reaction temperatures of -5 ˚C make this reaction a more favorable
approach when working with low boiling alkynes. Hence, this
methodology has shown its ability to overcome the challenges of earlier
metal mediated and metal free methods reported for the hydrations of
alkynes.28, 34
Experimental
To a 100 mL capacity borosilicate immersion well of UV reactor,
80 mL aqueous methanol MeOH: H2O (1:2), alkyne (1 mmol), Rh
catalyst (1.5 mol %) was added, and the reaction mixture was cooled to
-5 °C. It was irradiated using a Hg vapour UV lamp, 125W, 289nm for
30 min with continuous stirring. After completion of the reaction, the
reaction mixture was left to warm to room temperature and then
concentrated in vacuo. The residue was extracted with dichloromethane
and water, the organic phase was collected, dried with anhydrous
Na2SO4, concentrated at reduced pressure, and purified by flash column
chromatography using hexane/ethyl acetate as eluent to obtain the
corresponding product.
The catalyst was synthesized as per the procedure reported in reference 15.
Aacknowledgments
R. K Joshi and G.S. Smith acknowledged the DST India (DST/INT/South Africa/P-09/2014) and NRF South Africa (UID:90673) for financial
aid. M. Ali and A. K. Srivastava thanks to MANF-UGC and MNIT Jaipur for fellowships.
References
1. R. N. Butler, A. G. Coyne, Chem Rev. 2010, 110, 6302–6337.
2. R. Mello, A. Alcalde-Aragonés, M. E. González-Núñez, Tetrahedron Lett. 2010, 51, 4281–4283.
3. Q. Dong, N. Li, R. Qiu, J. Wang, C. Guo, X. Xu, J. Organomet. Chem. 2015, 799–800, 122–127.
4. L. W. Francisco, D. A. Moreno, J. D. Atwood, Organometallics 2001, 20, 4237– 4245
5. T. FeiNiu, D. Yun Jiang, S. yuan Li, X. Ge Shu, H. Li, A. Ling Zhang, J. Yu Xu, and B. Ni, Tetrahedron Lett. 2017, 58, 1156–1159.
6. T. Chen, C. Cai, Catal. Commun. 2015, 65, 102–104.
7. M. Bassetti, S. Ciceri, F. Lancia, C. Pasquini, Tetrahedron Lett. 2014, 55, 1608– 1612;
8. P. Mathur, B. Jha, A. Raghuvanshi, R. K. Joshi, S. M. Mobin, J. Organomet. Chem. 2012, 712, 1 – 14;
9. P. Mathur, R. K. Joshi, D. K. Rai, B. Jha, S. M. Mobin, Dalton Trans. 2012, 41, 5045 – 5054;
10. P. Mathur, R. K. Joshi, B. Jha, A.K. Singh, S. M. Mobin, J. Organomet. Chem. 2010, 695, 2687 – 2694;
11. B. Jha, A. Raghuvanshi, R. K. Joshi, S. M. Mobin, P. Mathur, appl. Organomet. Chem. 2017, 31, 3805 –3811.
12. R. K. Joshi, N. Satrawala, Tetrahedron Lett. 2017, 58, 2931 – 2935.
13. S. Ogo, K. Uehara, T. Abura, Y. Watanabe, S. Fukuzumi, J. Am. Chem. Soc. 2004, 126, 16520–16527;
14. M. Zeng, R. X. Huang, W. Y. Li, X. W. Liu, F. L. He, Y. Y. Zhang, F. Xiao, Tetrahedron 2016, 72, 3818–3822;
15. M. Tokunaga, Y. Wakatsuki, Angew. Chemie Int. Ed. 1998, 37, 2867–2869;
16. F. Boeck, T. Kribber, L. Xiao, L. Hintermann, J. Am. Chem. Soc. 2011, 133, 8138–8141;
17. D. B. Grotjahn, C. D. Incarvito, A. L. Rheingold, Angew. Chemie - Int. Ed. 2001, 40, 3884–3887.
18. Y. Zhou, Q. Liu, W. Lv, Q. Pang, R. Ben, Y. Qian, J. Zhao, Organometallics 2013, 32, 3753–3759;
19. A. Lumbroso, N. R. Vautravers, B. Breit, Org. Lett. 2010, 12, 5498–5501;
20. X. Liu, L. Liu, Z. Wang, X. Fu, Chem. Commun. 2015, 51, 11896–11898;
21. M. Setty-Fichman, Y. Sasson, J. Blum, J. Mol. Catal. A Chem. 1997, 126, 27–36;
22. J. Blum, J. Mol. Catal. 1992, 75, 153–160;
23. J. P. Young, B. I. Kwon, J. A. Ahn, H. Lee, C. H. Jun, J. Am. Chem. Soc. 2004, 126, 13892–13893.
24. H. Kanemitsu, K. Uehara, S. Fukuzumi, S. Ogo, J. Am. Chem. Soc. 2008, 130, 17141–17147.
25. I. Ojima, A. T. Vu, D. Bonafoux, in Science of Synthesis, eds. I. Ojima, A. T. Vu, D. Bonafoux, Georg Thieme Verlag KG 2001, 531–616;
26. K. C. Gupta, A. K. Sutar, Coord. Chem. Rev. 2008, 252, 1420–1450.
27. S. Siangwata, N. Baartzes, B. C. E. Makhubela, G. S. Smith, J. Organomet. Chem. 2015, 796, 26–32.
28. A. Vasudevan, M. K. Verzal, synlett. 2004, 4, 0631–0634.
29. J. Zhang, S. Li, X. Fu, B. B. Wayland Dalton Trans. 2009, 3661–3663.
30. A. Leyva, A. Corma, J. Org. Chem. 2009, 74, 2067 – 2074.
31. J. Zhang, B. B. Wayland, L. Yun, Shan Li, X. Fu, Dalton Trans., 2010, 39, 477–483;
32. S. Li, W. Cui, B. B. Wayland, Chem. Commun. 2007, 4024–4025;
33. S. Li, S. Sarkar, B. B. Wayland, Inorganic Chemistry 2009, 48, 8550-8558.
34. Munsaf Ali, Avinash K. Srivastava, Raj K. Joshi, Tetrahedron Lett. 2018, 59, 2075–2078.

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Graphical abstract

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Highlights
 N,O-ChelateRh(I)-Schiff base catalyst with ferrocene as an ancillary ligand.
 A unique water soluble, reusable homogeneous Rh(I) Catalyst.

Photochemical assisted hydration of internal and terminal acetylenes.
 Shortest route for one pot synthesis for strongly feasible hydration of acetylenes.