The hydration of nitriles catalyzed by water-soluble rhodium complexes

Article abstract of DOI:10.1016/S0040-4039(00)00743-7

The water-soluble rhodium complex generated in situ from [Rh(COD)Cl]₂ and P(m-C₆H₄SO₃Na)₃ has been found to be a very effective catalyst for the hydration of nitriles, under basic conditions. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00743-7

Tetrahedron Letters 41 (2000) 4845±4849
The hydration of nitriles catalyzed by water-soluble
rhodium complexes
Marie Charlotte Ko-Bie Djoman and Abdelaziz Nait Ajjou*
Department of Chemistry and Biochemistry, University of Moncton, Moncton, New-Brunswick E1A 3E9, Canada
Received 5 May 2000
Abstract
The water-soluble rhodium complex generated in situ from [Rh(COD)Cl]2 and P(m-C6H4SO3Na)3 has
been found to be a very e€ective catalyst for the hydration of nitriles, under basic conditions. # 2000
Elsevier Science Ltd. All rights reserved.
Keywords: hydration; nitriles; amides; water-soluble catalyst; acetronitrile.
The application of metal catalysts in aqueous systems that enables an easy recovery of the
catalyst by decantation, has recently attracted attention.1 Sulfonated phosphines are widely used
as ligands in such processes. Triphenylphosphine trisulfonate (TPPTS), owing to its higher solu-
bility in water, is generally the ligand of choice for carrying out commercial reactions in water
and has been incorporated into several industrial processes, e.g. the hydroformylation of propylene
3
by RhCl(TPPTS)3,2 the telomerization of dienes with Pd(TPPTS)4 and the functionalization of
dienes by cis-PtCl2(TPPTS)2.4
Carboxamides are classically prepared by the hydration of the corresponding nitriles in the
presence of a strong base or strong acid. These processes, however, lead to the formation of
carboxylic acids as side products.5 Several attempts have been made to develop catalytic methods
which can give the desired amides selectively.6 To the best of our knowledge, only two water-
soluble catalysts have been used for the hydration of nitriles and no attempts have been made to
recycle the catalysts.6a c The water-soluble catalysts PdCl(OH)(bipy)(H2O)6a,b and IrH(CO)-
6c
(TPPTS)3 gave low activities. In this paper we present our preliminary results concerning the
hydration of nitriles, especially acetonitrile, catalyzed by the water-soluble [Rh(COD)Cl]2/TPPTS
system.
No reaction occurred when acetonitrile (10 mL) was treated with water (10 mL) at 90ꢀC in the
presence of TPPTS (0.3 mmol). When the reaction was conducted with [Rh(COD)Cl]2 (0.01
mmol)/TPPTS (0.3 mmol), acetamide was selectively obtained with the turnover frequency of
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00743-7

4846
50.8 mol of acetamide per mol of Rh per hour. The activity of the catalytic system was found to
be a function of the pH of the solution. Rates of catalysis were independent of the base con-
centration at a pH greater than ꢁ11.7. At lower [OH^] a linear dependence of the catalytic rates
was found (Fig. 1). From plots of [OH^] versus the rate of catalysis, the rate constant of 36.3 M^1
s^1 was determined for nucleophilic attack of OH^ on the coordinated nitrile. At a pH of ꢁ11.7
the e€ectiveness of our system for the hydration of acetonitrile relative to the fastest known
catalytic systems is given in Table 1. [Rh(COD)Cl]2/TPPTS is ꢁ3.4 times more reactive than the
water-insoluble analog system [Rh(COD)Cl]2/PPh3 (Table 1, entries 1 and 3). Our system is the
best water-soluble catalyst and the second best catalyst reported for the hydration of acetonitrile
with an initial turnover frequency of 295.3 at 90ꢀC and pH of ꢁ11.7 (Table 1, entry 2). A maximum
turnover number of 934.2 is reached after 24 h with the comcomitant decrease of the turnover
frequency to 38.9 turnovers/h (Fig. 2).
Figure 1. Dependence of the rate of acetonitrile hydration on hydroxide concentration at 90ꢀC7
Table 1
Comparison of catalytic activities for hydrolysis of acetonitrile to acetamide at pH of ꢁ11.77

4847
Figure 2. Time resolved-evolution of acetamide formation during the hydration of acetonitrile at 90ꢀC and pH of
ꢁ11.77
The rate of catalysis is temperature dependent (Table 2). [Rh(COD)Cl]2/TPPTS was found to
have superior reactivity at 25ꢀC (31 turnovers/h) than the platinum system trans-PtHCl(PMe3)2/
NaOH.6e The activation enthalpie calculated from the Eyring plot (Fig. 3) was 8.58 kcal mol^1.
An important aspect of the aqueous-phase catalysis is the possibility to separate the catalyst
easily from the reaction medium and to recycle it in further experiments. Table 3 displays the
results obtained for the hydration of acetonitrile after several consecutive runs with
[Rh(COD)Cl]2/TPPTS. A decrease in the catalytic activity was observed between each of the ®ve
recycling experiments. This decrease in activity is mainly attributed to the dissolution of rhodium
complex in acetamide organic solutions which are slightly yellow. Indeed, we observed that
[Rh(COD)Cl]2/TPPTS is very soluble in molten acetamide. It is relevant to point out that no
metallic rhodium formation was observed after the reactions and the aqueous phases remained as
yellow solutions, indicating no decomposition of the catalyst. As has been previously demon-
strated,6g the separation of acetamide from the catalyst by sublimation is expected to lead to the
decomposition of the catalyst with a subsequent severe decrease of the catalytic activity during
the catalyst recycling.
Table 2
Hydration of acetonitrile to acetamide catalyzed with [Rh(COD)Cl]2/TPPTS at pH of ꢁ11.77
The hydration mechanism is thought to involve an initial substitution of chloride by hydroxide
on treatment of [Rh(COD)Cl]2 with NaOH. Indeed, when [Rh(COD)(OH)]2, prepared from
[Rh(COD)Cl]2 and KOH,8 was used as a substitute for [Rh(COD)Cl]2, identical catalytic activities
were obtained.

4848
Figure 3. Eyring plot of temperature dependence of rate for the hydration of acetonitrile catalyzed by [Rh(COD)Cl]2/
TPPTS
Table 3
Catalyst recycling for the hydration of acetonitrile to acetamide at 90ꢀC and pH of ꢁ11.7a
[Rh(COD)Cl]2/TPPTS also catalyzes the hydration of water insoluble nitriles such as phenyl-
acetonitrile and benzonitrile. Under the same catalytic conditions as for acetonitrile,7 except that
the amount of nitrile was reduced to 5 mmol, phenylacetonitrile and benzonitrile were completely
converted to the corresponding amides after 24 h at 90ꢀC and pH of ꢁ11.7.
We are currently examining the scope of our catalytic system using various nitriles. Attempts to
improve the catalytic activities and to study the hydration mechanism are in progress.
Acknowledgements
We are grateful to the FESR and the Chemistry and Biochemistry Department for ®nancial
support of this research. We also thank Michel Guay for his assistance.

4849
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7. Typical procedure for the hydration of acetonitrile: In a glass liner of a 45 mL autoclave and under an atmosphere
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containing the desired amount of NaOH, at room temperature. Then acetonitrile (10 mL) degassed with N2 was
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degassed acetonitrile (10 mL) and water (10 mL). In the case of benzonitrile and phenylacetonitrile, ethyl acetate
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