Catalytic hydration of benzonitrile and acetonitrile using nickel(0)

Article abstract of DOI:10.1002/adsc.200505382

The homogeneous catalytic hydration of benzo- and acetonitrile under thermal conditions was achieved using nickel(0) compounds of the type [(dippe)Ni(η²-NCR)] with R = phenyl or methyl (compounds 1 and 2, respectively), as the specific starting intermediates. Alternatively, the complexes may be prepared in situ by direct reaction of the precursor [(dippe)NiH]₂ (3) with the respective nitrile. Hydration appears to occur homogeneously, as tested by mercury drop experiments, producing benzamide and acetamide, respectively. Addition of Bu₄NI did not lead to catalysis inhibition, suggesting the prevalence of Ni(0) intermediates during catalysis. Hydration using analogous complexes of 3, such as [(dtbpe)NiH] ₂ (4) and [(dcype)NiH]₂ (5) was also addressed.

Full text of DOI:10.1002/adsc.200505382

FULL PAPERS
DOI: 10.1002/adsc.200505382
Catalytic Hydration of Benzonitrile and Acetonitrile using
Nickel(0)
Marco G. Crestani, Alma Ar e´ valo, Juventino J. García*
Facultad de Química, Universidad Nacional Aut o´ noma de M e´ xico, M e´ xico, D.F. 04510, M e´ xico
Phone: (þ52)-555-622-3514, e-mail: juvent@servidor.unam.mx
Received: October 1, 2005; Accepted: February 7, 2006
Abstract: The homogeneous catalytic hydration of and acetamide, respectively. Addition of Bu NI did
4
benzo- and acetonitrile under thermal conditions not lead to catalysis inhibition, suggesting the preva-
was achieved using nickel(0) compounds of the type lence of Ni(0) intermediates during catalysis. Hydra-
2
[
(
(dippe)Ni(h -NCR)] with R¼phenyl or methyl tion using analogous complexes of 3, such as
compounds 1 and 2, respectively), as the specific [(dtbpe)NiH] (4) and [(dcype)NiH] (5) was also ad-
2 2
starting intermediates. Alternatively, the complexes dressed.
may be prepared in situ by direct reaction of the pre-
cursor [(dippe)NiH] (3) with the respective nitrile.
2
Hydration appears to occur homogeneously, as tested Keywords: amides; homogeneous catalysis; hydration;
by mercury drop experiments, producing benzamide nickel; nitriles
Introduction
In fact, the fast hydrolysis of the amide function under
these conditions appears as a disadvantage when the
The direct hydration (or hydrolysis, which is often used preparation of amides by direct hydration of nitriles is
as a synonym) of nitriles to access amides [Eq. (1)] is a intended. Still, a number of procedures for stopping at
very important transformation both on the laboratory the amide stage have been reported; these being, for ex-
[
1]
scale and in industry.
ample, the use of concentrated H SO , acetic acid and
2 4
ꢀ
BF , H O and OH , two equivalents of chlorotrime-
3
2
2
ð1Þ
thylsilane followed by H O, heating on neutral alumina,
2
ꢀ
[5]
Oxone and the use of different metallic ions. More re-
cently, an efficient single-step, indirect, acid-catalyzed
Amides exhibit a quite broad range of industrial applica- hydration of both aliphatic and aromatic nitriles, into
tions (foam boosting and stabilization, emulsification, the corresponding amides, using TFA-H SO or
mixtures, was also published. However,
2
4
[
6]
detergency, viscosity modification, lubricity, antistatic AcOH-H SO
2 4
[
2]
properties, corrosion inhibition, wetting, etc.), and as from the industrial point of view, the acid- or base-cata-
such, the search for reagent systems that perform this lyzed reactions also suffer from the general restriction,
conversion selectively and efficiently, under mild condi- apart from selectivity and yield, that the final neutraliza-
tions, continues to be a challenge. Moreover, amides in tion of either base or acid, leads to an extensive salt for-
general constitute excellent intermediates and raw ma- mation with inconvenient product contamination and
[
3]
[1]
terials for synthetic organic chemistry. In particular, pollution effects. In this context, a variety of homoge-
[8]
[7]
the lability of the amide CꢀN bond accounts for its fast neous transition metal-based catalysts using Pd, Pt,
[
4]
[4,9]
[10]
[11]
[12]
[13]
hydrolysis under basic or acid catalysts, regenerating Co, Rh, Ir, Ru and Mo, as well as heteroge-
[
14]
[15]
[16]
the parent carboxylic acid and an amine, if the amide neous and enzymatic hydrolyses, among others,
[
3]
is N-substituted, or ammonia if it is not [Eq. (2)].
have been reported. In all these cases, the catalytic con-
versions of nitriles to amides have been regarded to oc-
cur through an initial h -coordination of the terminal ni-
1
trogen in the nitrile function to the metal center (end-on
coordination), prior to the nucleophilic attack of a hy-
droxide ion and further protonation of the nitrogen
[
7a,8a, e,f,13b]
ð2Þ
atom to give the respective amides.
Similarly,
732
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Catalytic Hydration of Benzonitrile and Acetonitrile using Nickel(0)
FULL PAPERS
in the stoichiometric cases of nitrile hydration in which Results and Discussion
amide complexes are ultimately formed, the same kind
of end-on coordination is also found; in this light end-
on coordination of the nitrile may appear as a general
Catalysis of Nitrile Hydration
feature, ultimately leading to its hydrolysis, in aqueous
[1]
media. This is important considering that nitriles can The catalytic hydration of benzo- and acetonitrile was
interact with metal centers in two more ways, other selectively achieved by heating aqueous solutions of
1
than the terminal s-bonded, h -NCR mode already the corresponding nitriles, at temperatures ranging
2
mentioned, namely the p-bonded (side-on), or h - from 140 to 1808C, in the presence of mononuclear nick-
NCR coordination, and the bridging s,p-bonded, m- el(0), [(dippe)Ni], moieties, which initially bind the ni-
1
2
2
h ,h -NCR coordination, often found in clusters trile substrates in the h -mode that is typical for these
(
[17]
[19]
Scheme 1). A relevant series of tungsten-nitrile com- Ni(0) complexes. Hydration catalysis for both nitrile
plexes, showing either s- or p-bonding, has been recent- substrates appears to occur homogeneously, as con-
[
18]
ly reported by Templetonꢂs group.
firmed by mercury drop experiments performed for
[23]
each; the catalytic hydrations likely occurring through
2
neutral intermediates, of which the [(dippe)Ni(h -
NCR)] complex is probably the first. The prevalence
2
of such an h -coordinated nitrile complex in aqueous
3
1
1
media was verified for both 1 and 2, by P{ H} NMR
spectroscopy of these complexes in D O solution. The
2
3
1
1
P{ H} NMR spectrum in both cases exhibits two broad
asymmetric doublets, characteristic of two types of
phosphorus environments, with resonances at d¼79.25
[17]
Scheme 1. s-, p- and s,p-modes of coordination of nitriles.
2
and 67.25 and J coupling constants of 68 Hz in the
P, P
case of 1 and resonances at d¼82.18 and 66.78 with
2
J
coupling constants of 77 Hz in the case of 2; which
P, P
In this context, our group has also published a series of are consistent with the reported values for these Ni(0)
[
19]
reports regarding the activation of aryl, heteroaryl and complexes. The results are important since they dem-
alkyl cyanides using the nickel(0) fragment [(dippe)Ni], onstrate that the oxidation state of the nickel center re-
2
that yield p-bonded h -NCR complexes, such as [(dip- mains apparently unchanged in water media; a fact that
2
pe)Ni(h -NCR)] (R¼phenyl or methyl; 1 and 2), re- was also verified during hydration catalysis of both ben-
[19]
spectively. In the cases of aryl and heteroaryl cya- zo- and acetonitrile, as no inhibition of the catalytic
nides, oxidative addition of the CꢀCN bond of the nitrile processes was found to occur in the presence of the
to the nickel(0) proceeds with a mild warming, forming non-coordinating iodide salt, tetrabutylammonium io-
[
19b, c]
the corresponding nickel(II) complexes.
In the case dide (Bu NI). Inhibition of the catalytic processes is ex-
4
of the alkyl cyanides, such an oxidative addition is pected to occur if the metal center is oxidized at some
strongly dependent on the chain length and eventually point of the catalytic cycle which, in these cases, would
[19a]
can be inhibited.
As part of our ongoing studies in have been attributed to the formation of the nickel(II)
this field, we decided to extend the chemistry of some iodide compounds derived from the poisoning of the
2
of the nickel(0) nitrile complexes in order to investigate [(dippe)Ni(h -NCR)] catalyst by reaction with Bu NI,
4
their reactivity towards nucleophilic substitution which, but which did not happened. In this context, a similar ex-
[
8a]
in the case of the nitrile hydration process, is accompa- periment performed by Ghaffar and Parkins, the ad-
nied by the presence of water. The fact that one starts dition of potassium iodide to a preheated solution of
2
[20]
from p-bonded h -NCR complexes raises questions the neutral [PtH(PMe OH)(PMe O) H], in a mixture
2
2
2
regarding the feasibility of such a hydration, considering of acrylonitrile, water and ethanol, has validated the ex-
the large number of reports that imply the necessity for istence of a cationic species responsible for the respec-
1
h -NCR coordination. We report herein that hydration tive nitrile hydration catalysis, this being presumably
þ
was achieved under catalytic conditions by a nickel(0) [Pt(S)(PMe OH)(PMe O) H] , with S¼solvating mol-
2
2
2
2
system, using the h -NCR coordination. To the best of ecule. Catalysis was stopped after addition of the potas-
our knowledge, only a few stoichiometric examples of sium salt, due to the poisoning of the cationic catalyst
the hydration process exist with this metal, all of which and could be restarted after further addition of silver tet-
In this instance, the rafluoroborate. In Parkinsꢂ catalysis example (see Fig-
use of nickel-catalyzed transformations – hereby exem- ure 1, in ref. ), the nitrile substrate is coordinated in a
1
[21]
exhibit h -NCR coordination.
[8a]
1
plified in the case of nitrile hydrations – results in added h -mode to the platinum(II) center of the catalyst,
value, as nickel is inexpensive and presents environ- through the terminal nitrogen. The nitrile is proposed
mental advantages over other metals, namely those in to undergo an intramolecular nucleophilic attack of
[22]
the same group.
the PMe OH hydroxy placed in an armchair position
2
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FULL PAPERS
Marco G. Crestani et al.
[
24]
1
in cis-coordination, leading to an oxo-bound, five- pe)Ni(h -NCR)(H)(OH)], by oxidative addition of wa-
membered ring imide complex which, after dihydro- ter} or through an alternative general base catalysis
oxo-biaddition of a water molecule at the unsaturated {for example, by in situ formation of square planar [(dip-
1
þ
C¼N bond of the imide ligand, produces the final oxo- pe)Ni(h -NCR)(OH)] species}, by analogy to the ex-
[
8a]
[7a]
[13b]
bound hydroxy-carbamide complex. Metathesis of the amples by Parkins, Kostic, and Tyler would be
oxo substituent for hydroxy at the PMe armchair ligand expected. However, this is does not seem to be the
2
would be likely to assist the decoordination of the amide case. Therefore, the amide formation is expected to oc-
and the regeneration of the catalyst, completing the cat- cur through direct dihydro-oxo-biaddition (in this case,
[
8a]
[5a]
alytic cycle.
N,N-dihydro-C-oxo-biaddition ) of a water molecule
2
A closely related example of catalytic nitrile hydra- to the unsaturated CN bond of a h -coordinated nitrile,
tion proceeding through direct hydroxo nucleophilic at- in a similar way as it occurs over the five-membered
1
tack over h -NCR coordinated nitrile to the molybdenu- ring imide intermediate in Ghaffar and Parkinsꢂ mecha-
1
þ
[8a]
m(IV) complex [(Me-Cp) Mo(h -NCR)(OH)] , has nism, as mentioned above. Other examples of dihy-
2
[
13b]
also been reported by Tylerꢂs group.
However, direct dro-oxo-biaddition of water to unsaturated bonds such
intramolecular nucleophilic attack of hydroxo-metal as the triple bonds in alkynes, in the presence of mercu-
1
bound ligands, over h -coordinated nitriles does not ric ions with the corresponding production of ketones,
[
25]
seem to be the sole mechanism available for this process have also been reported [Eq. (3)].
to occur. In fact, an intermolecular alternative in which a
palladium(II) coordinated hydroxy ligand may interact
with a water molecule from the bulk, through a general
ð3Þ
base catalysis assisting for the nitrile hydration, has also
[
7a]
been proposed by Kaminskaia and Kostic. In this
work, the kinetics for the catalytic hydration of various
nitriles using palladium(II) aqua complexes were also The first step of the mechanism of alkyne hydration in-
2
studied. At a pK value of 5.1ꢁ0.1 in D O, the aqua co- volves the formation of a mercuric complex [Hg(h -
a
2
2
þ
ordinated Pd(II) complexes may undergo deprotona- C,C-alkyne)] , which is then attacked by bulk water
tion, forming the corresponding hydroxo complexes, in an S 2-type process to give a water-bound adduct
N
which are somewhat better catalysts for the nitrile hy- that is rearranged into an enol. Further tautomerism
dration reaction than the aqua precursors; the hydroxo of the organic function enables the final formation of
[
25]
ligand being a stronger nucleophile and general base the respective ketone. By analogy with this mecha-
[
5a]
than the aqua ligand. Kinetic experiments, however, nism, N,N-dihydro-C-oxo-biaddition of water mole-
could not distinguish between the aqua ligand-assisted cules may also be expected to occur over the [(P-
2
hydration (intramolecular) and the hydroxo general P)Ni(h -N,C-nitrile)] complexes, with the correspond-
base catalysis (intermolecular), therefore not ruling ing formation of the water adducts that further rear-
[
7a]
out either of these. A final example of nitrile hydra- range into N-protonated hydroxy imine complexes. Tau-
tion in basic media, reported by Jensen and Trogler in tomerism of the coordinated imines would produce, in
[
26]
1
986, does seem to favor intermolecular nucleophilic at- every case, saturated CꢀN amide bonds which are
1
tack of bulk hydroxide ions over platinum(II) h -coordi- likely to be readily decoordinated from the nickel(0)
nated nitriles. In all these examples, the high oxida- center, giving free amides as the overall products of cat-
tion state of the metal centers determines the preference alysis and the regeneration of the starting nickel(0) cat-
[8e]
1
2
for h -coordination of the nitriles.
alyst species, [(dippe)Ni(h -NCR)] (Scheme 2).
Contrasting with the latter information, the hydra-
Several attempts were performed for the follow-up of
tions of acetonitrile and benzonitrile in the present the reaction under pseudo-order conditions at different
work seem to occur directly over Ni(0) complexes exhib- nitrile-to-water molar proportions, temperatures rang-
2
iting h -coordination; a conclusion that is drawn from ing from 80 to 1608C and a number of solvents (THF-
the fact that no inhibition of the catalytic cycle of nitrile d , toluene-d and dioxane-d ), all of which confirm the
8
8
8
2
hydration was observed in the presence of Bu NI, as stability of the [(dippe)Ni(h -NCR)] complexes in the
4
mentioned above. This result can be interpreted under presence of water, which had already been corroborated
the premise of the nickel center maintaining the same in D O. The imine intermediate, however, proved to be
2
reduced oxidation state during catalysis and, therefore, highly elusive and we were unable to properly establish
the same nitrile coordination mode. A change in the ox- its formation. at least on the NMR timescale, our experi-
idation state of the nickel center during the respective ments concluding with mixtures of the starting 1 or 2
hydrations would imply a shift in the coordination complexes and the respective free organics, either reac-
1
mode of the nitriles, likely to acquire the h -mode. If tants or products. Interestingly, when heating was per-
2
so, a hydration mechanism probably proceeding formed at a less than 50 water-to-[(dippe)Ni(h -NCR)]
through Ni(II) intermediates, either through direct mol proportion, the formation of CꢀCN oxidative addi-
[
19a]
aqua ligand interaction {e.g., the formation of [(dip- tion products of the type [(dippe)Ni(R)(CN)],
which
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Catalytic Hydration of Benzonitrile and Acetonitrile using Nickel(0)
FULL PAPERS
at the proton transfer at the imine intermediate, prior
to the amide formation and, in fact, deuterium-labeling
experiments performed by Jensen and Trogler have veri-
fied that rapid proton transfer from the solvent presents
itself as a limiting condition for the catalytic hydration to
[
8e]
occur. From this publication it follows that the TOFin
the hydration experiment under neutral conditions
would be expected to be higher, due to the equimolarity
of protons and hydroxide ions: the initial and final pH of
the aqueous phase in all experiments was always meas-
ured during this work and no significant variations of
pH were observed in any of the experiments starting
with neutral water. The TON found for the added base
experiment was 354, with a TOF of 5 cycles per hour;
while the TON found for the neutral pH experiment
was 984, with a TOF of 14 cycles per hour; the final pH
value of the aqueous phase in the added base experi-
ment had lowered to a value of 7, thus confirming the
fact that the added base had indeed been consumed dur-
ing the hydration catalysis. However, the fact that the
TOF value is significantly lower than the TOF under
neutral conditions supports a rate-limiting proton trans-
fer step and ,therefore, the premise of a concerted N,N-
dihydro-C-oxo-biaddition to the nickel(0) bound CN
bond is reasonable.
Scheme 2. Suggested mechanism for the hydration of nitriles
2
by the Ni(0)-phosphine catalyst [(dippe)Ni(h ꢀNCR)].
Catalytic Hydration of Benzonitrile using
[
(dtbpe)NiH] (4) and [(dcype)NiH] (5): Effect of
2
2
Steric Hindrance of the Phosphine Ligands on the
are rather unlikely to lead to the amides, was verified. Catalysis
This result is consistent with further observations in cat-
alysis, which show that the turnover number and fre- Alternatively to the catalytic hydration of benzonitrile
quency of catalysis are increased when the water-to- using [(dippe)NiH] (3), the hydration of this substrate
2
2
[
(dippe)Ni(h -NCR)] mol proportion is increased (vide was also attempted using [(dtbpe)NiH] (4) and [(dcy-
2
infra); thus it proves that the presence of water acts to pe)NiH] (5) under the conditions found for highest ef-
2
quench the CꢀCN oxidative addition reactions and ficiency and yield using complex 3 {1808C, 0.04mole
2
therefore increases the efficiency of the nickel(0) cata- percent of [(dippe)Ni(h -NCPh)] and 20% water vol-
lyst.
ume under neutral conditions; TON¼984and TOF of
1
4cycles per hour}. Ayield of 23% of benzamide was ob-
tained with the use of complex 4, with a TON of 651 and
Hydration Catalysis in Neutral Conditions and in the
Presence of Added Base: Evaluation of Catalysis
Efficiency
a TOFof 9 cycles per hour, calculated against the formal
2
mole amount of the respective [(dtbpe)Ni(h -NCPh)]
catalyst species, while complex 5 produced benzamide
in 17% yield, with a TON of 467 and a TOF of 6 cycles
[
5]
Since N,N-dihydro-C-oxo-biaddition of water to the per hour {also calculated with respect to the mole
2
unsaturated CN bond of the coordinated nitriles is ex- amount of [(dcype)Ni(h -NCPh)]}. The results in terms
pected to occur through a concerted S 2 mechanism, of TOF are illustrated in Figure 1.
N
[
25]
by analogy with the alkyne hydration reaction, an ex-
The results depict a decreasing trend in the hydration
periment adding a source of hydroxide (NaOH solution rate as the alkyl substituents on the phosphine ligands
at pH¼10) to a benzonitrile/[(dippe)NiH] mixture was become bulkier. The results are consistent with Jensen
2
performed and the turnover frequency (TOF) of hydra- and Troglerꢂs observation regarding the steric bulk of
[
8e]
tion compared to the respective hydration using neutral the substituents, since increased steric hindrance low-
water, under otherwise the same conditions {1808C, 0.04 ers the probability of effective substrate binding to the
2
mole percent of [(dippe)Ni(h -NCPh)] and 20% water catalyst metal center and, due to their alkyl nature, these
volume}. A smaller value in the hydration TOF under may also increase the hydrophobicity of the entire cata-
excess free hydroxide would imply a rate-limiting step lyst, therefore reducing the catalysis rate. The hydro-
Adv. Synth. Catal. 2006, 348, 732 – 742
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FULL PAPERS
Marco G. Crestani et al.
nitrile at 1608C, however, did not give a large increase in
the production of benzamide (14to 40 % yield), at the
expense of a significant decrease in catalyst activity
(
TON¼360 to 40; entries 2 and 3, Table 1) and thus,
the system was further heated to 1808C. Yield and
TON at 1808C, at 1 mole percent both peaked with re-
spect to former experiments at large water amounts (en-
try 4, Table 1); which were originally used in this work
inspired from Tylerꢂs nitrile hydration system employing
[
13b]
a molybdenum catalyst.
Decreasing both the catalyst
concentration and the water volume percent were found
to be useful in order to increase the catalyst efficiency
2
Figure 1. Activity of [(P-P)Ni(h -NCPh)] catalysts in cycles
(
vide infra), which reached a TON value of 984for ben-
per hour, relative to benzonitrile hydration.
zonitrile hydration and 99 for acetonitrile; both systems
2
using 0.04mole percent of [(dippe)Ni( h -NCR)] and
phobicity of alkyl substituents such as isopropyl and eth- 20% water volume (entry 5, Table 1 and entry 3,Table 2,
yl in platinum catalysts has also been commented by respectively). In the case of acetonitrile, further pressur-
[
8f]
Yoshidaꢂs group in an early publication.
izing the system with 100 psi of argon, to avoid acetoni-
trile evaporation, led to an improved catalysis of 257
turnovers (entry 4, Table 2); which is actually invariant
upon additional pressure over 100 psi, since further
pressurizing the autoclave to 500 psi did not produce
an additional improvement (entry 5, Table 2) and there-
Optimization of the Hydration Catalysis using
2
[
(dippe)Ni(h -NCR)]
As mentioned earlier in the text, the hydration of benzo- fore implying that 100 psi is already a large enough pres-
and acetonitrile was achieved at a range of temperatures sure.
going from 140 to 1808C; hydration attempts performed
The latter results contrast with the hydration temper-
at lower temperatures were unsuccessful. The results for atures (T<1008C) and the corresponding activities
benzonitrile hydration on going from 140 to 1808C are found with some palladium and platinum catalysts,
summarized in Table 1 and those of acetonitrile hydra- which are intrinsically more nucleophilic metal centers
tion are depicted in Table 2; the processes were opti- {TON for acetonitrile hydration¼34.9 using [trans-
[
8e]
mized either in terms of activity, yield or both.
PtH(H O)(PEt ) ][OH]; 89.2 using [trans-PtH(H O)-
2 3 2 2
[8e]
The results depicted show that at least in terms of tem- (PMe ) ][OH]; 369, using [PtH(PPh OH)(PPh O) -
405, using Pt[P(i-Pr) ] ;
40 to 1608C, when the catalyst percent is the same and Pt[P(c-C H ) ] ; 4000, using LPd (CH CONH) with
3
2
2
2
2
[
8a]
[8f]
perature, catalysis efficiency is improved on going from H];
1
520, using
3
3
[
8f]
6
11 3 2
2
3
[
7b]
the water volume percentage is kept constant at 90% L¼a binucleating ligand; and 5700 using [PtH(PMe2-
[
8a]
(
entries 1 and 2, Table 1, and entries 1 and 2, Table 2). OH)(PMe O) H] }; the respective hydrations using
2 2
2
Further concentrating the catalyst in the case of benzo- the nickel(0) catalyst, [(dippe)Ni(h -NCMe)], require
Table 1. Catalytic hydration of benzonitrile.
Entry
T [ 8C]
Catalyst mole percent
Water volume [%]
90
TON
148
TOF [TON/h]
2
Yield [%]
6
1
2
3
140
160
160
1.00
1801
0.04
0.0490
1.00
90
372
90
5
14
40
1
1
40
4
5
180
67
68
0.0420
4
14
984
Table 2. Catalytic hydration of acetonitrile.
Entry
T
Catalyst mole
percent
Water volume
[%]
Added argon pressure
[psi]
TON
17
TOF
[TON/h]
Yield
[%]
[
8C]
1
2
3
140
160
180
0.04
180
0.04
0.0490
0.0420
20
0.0420
90
–
–
0.2
3
4
1
68
99
1
1
10
104
–
4
5
180
100
257
4
500
257
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Catalytic Hydration of Benzonitrile and Acetonitrile using Nickel(0)
FULL PAPERS
a higher temperature, a fact that is also consistent with
general observations regarding the catalytic activity of
first row late-transition metals with respect to their re-
[
27]
spective group members. However, the overall activ-
ity for benzonitrile hydration was in fact improved to
over 1000 turnovers by proper tuning of the catalytic
conditions, although for acetonitrile this number is sig-
nificantly smaller (TON¼250 under argon pressure).
This dramatic difference in activity, on going from the
aryl to the alkyl nitrile could also imply differences in
the thermal behavior of the coordinated nitrile com-
plexes 1 and 2; the alkyl nitrile complex 2 being more
likely to undergo irreversible oxidative addition to the
2
Figure 2. Activity of the [(dippe)Ni(h -NCPh)] catalyst to-
[19a]
CꢀCN bond.
If this were to occur under the reaction
wards benzonitrile hydration, with respect to water volume
conditions to some extent, then it seems likely that the percentage.
catalyst half life would be significantly reduced by for-
mation of [(dippe)Ni(CN)(Me)].
ty, corresponds in fact to the equimolar benzonitrile-to-
water situation. Thus, hydration could not be further op-
timized in terms of substrate proportion variation, un-
less substoichiometric proportions were to be chosen.
This, however, would require the water amount to be
Catalysis Optimization Based on Variation of the
Water Volume Percentage
As depicted in Eq. (1), the process of nitrile hydration the limiting reactant, and since the activity of the cata-
presumes an equimolar relationship between nitrile lyst is decreased whenever the water percentage is in-
and water, such that the addition process would con- creased over the nitrile substrate; the use of such strat-
sume either reactants to the same extent. Under such a egy for the optimization of the catalytic system would
basis, questions are posed regarding the eventual insol- also lead to difficulties in maintaining the thermal stabil-
2
ubility of the catalysts and therefore a potential draw- ity of the [(dippe)Ni(h -NCPh)] catalyst at the tempera-
back in the overall yield and conversion efficiency, ture required for the process. Thus, a different alterna-
once the system has reached a certain stage of reactant tive, employing a further diluted catalyst amount, was
consumption. To solve this, the use of unreactive addi- addressed at the low end of the water volume propor-
tional solvents is a reasonable alternative that presents tionality of 20%. In this instance, catalysis at this water
the additional possibility of achieving milder thermal volume, using the same total amount of nickel pre-cata-
conditions for the process and, indeed, this was attempt- lyst [(dippe)NiH] (80 mg), actually leads to a lower
2
ed during the course of the present work, using THF as Ni(0) catalyst concentration, relative to the nitrile sub-
the solvent {1808C, 0.04mole percent of [(dip-
strate (0.1 mole percent), versus that which results
pe)Ni(h -NCPh)] and 20% water volume, pH¼7 and when the same total amount of nickel(0) is present in
0 mL of THF}. However, catalysis under these condi- the 90% water volume mixture (1 mole percent). In
2
5
tions did not take place; quantitative thermal dispropor- this instance, Figure 3 shows the results on going from
tionation of the nickel(0) catalyst to metallic nickel was 0.1 mole percent [(dippe)Ni(h -NCPh)] to a further di-
2
detected after the reaction time. The result may be ex- luted 0.01 mole percent nickel(0) concentration.
plained on the basis of a lack of quenching by water of
the oxidative addition reactions, due to the higher solu-
[
28]
bility of the catalyst in the organic solvent. Thus, var-
iation of the proportion of nitrile and water substrates,
using them as neat solvents, was therefore explored in
an attempt to determine the highest catalyst efficiency
conditions. Figure 2 shows the linear correlation of cat-
alyst activity in terms of TON, found by varying the wa-
ter volume percentage from 20 to 90%, with respect of
benzonitrile, maintaining the same catalyst load and
the temperature of the processes at 1808C.
As depicted, the catalyst activity drops when the
amount of water is increased. From these results, it fol-
lows that hydration is best achieved at low water volume
percentages. Incidentally, the lowest water volume per- wards benzonitrile hydration, with respect to catalyst mole
centage at which the catalyst exhibits the highest activi- percentage.
2
Figure 3. Activity of the [(dippe)Ni(h -NCPh)] catalyst to-
Adv. Synth. Catal. 2006, 348, 732 – 742
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
737

FULL PAPERS
Marco G. Crestani et al.
Figure 3 illustrates that the catalytic activity is indeed from [(dtbpe)NiCl
] and [(dcype)NiCl ]. All the complexes
2
2
were purified by crystallization or column chromatography
increased when the concentration of the catalyst is de-
creased to 0.01 mole percent, at the same water-to-nitrile
volume proportionality of 20%. The yield at this situa-
tion, however, does not increase proportionally with the
TON, at the same reaction time, 13% isolated yield of
benzamide being obtained with the respective 0.01 cata-
lystmole percent, versus 41%, onusing the 0.04 mole per-
cent of the nickel(0) catalyst. As such, the 0.04mole per-
1
and their integrity was verified by the corresponding H and
P{ H} NMR spectra of all complexes, before their use. The
3
1
1
[32]
[33]
chelating bisphosphine ligands, dippe and dtbpe were syn-
thesized from 1,2-bis(dichlorophosphino)ethane (Strem) and
the corresponding Grignard or lithium alkyls (Aldrich), and
dcype was purchased from Strem. Neutral alumina used for
the preparation of complexes 3–5, was heated to 2008C under
vacuum for two days and stored in the glovebox. All other
cent concentration of the nickel(0) catalyst was selected chemicals such as Bu NI (Aldrich) and filter aids were reagent
4
instead of the 0.01, this showing the best compromise be- grade and were used as received. The catalytic experiments
were performed by in situ preparation of the nickel(0) com-
plexes 1 or 2, unless otherwise stated. Both complexes were
tween TON and yield. The low activity of the catalyst at
high water concentrations or the otherwise similar condi-
tions of high mole percent at constant water amount, con-
cluded from these series of experiments, are useful in
terms of a rationale regarding the close relationship
that possibly exists between the hydrophobic nature of
thecatalystandthewaterfromthebulk,thatparadoxical-
prepared by reaction of [(dippe)NiH] (3) with a neat solution
2
of the corresponding nitrile, prior to the addition of water, sim-
[19]
ilarly to the reported procedures. The total amount of nitrile
used was calculated on the basis of the mole percentage re-
quired for each experiment, relative to the corresponding nick-
2
el(0) catalyst, [(dippe)Ni(h -NCR)]. All catalytic mixtures
lypreventsthedisproportionationoftheformerwhenthe were loaded in the reactor inside the glovebox. All the hydra-
mixture is heated to 1808C, but in excess water drastically tion products were recovered from the reactor washings in
reduces the hydration rate.
ethanol solution, filtered through a frit packed with alumina
ꢀ
4
and dried in the vacuum line (P<10 mmHg) for 4hours, pri-
or to their quantification or characterization; the latter of
which, was made by direct comparison of the respective melt-
Conclusions
[
34] 1
13
1
ing points,
H and C{ H} NMR spectra with those of com-
[
13b,35]
ꢀ
mercially available amides and literature reported values.
The catalytic hydration of benzo- and acetonitrile using
nickel(0) catalysts of the type [(dippe)Ni(h -NCR)] was
achieved under heating of the respective mixtures, opti-
The pH measurements were performed using a Corning
Scholar 425 pH meter. All NMR spectra of complexes and
products in this work, were recorded at ambient temperature
2
1
31
1
mized at 1808C, via an N,N-dihydro-C-oxo-biaddition, on a 300 MHz Varian Unity spectrometer. The H, P{ H}
using low catalyst loadings and low water content; the NMR spectra of the complexes were obtained from concen-
equimolar amount of nitrile and water being the most trated solutions (30 mg) of the pure compounds in THF-d , un-
8
less otherwise stated, and all samples were handled under ar-
convenient proportion between the two substrates, at
which the nickel(0) catalyst exhibits the highest efficien-
cy for both hydration systems. Experiments are under-
way to further extend this chemistry to other nitriles.
ꢀ
gon, using thin wall-0.38 mm-WILMAD NMR tubes with J
1
13
1
Young valves. H and C{ H} NMR spectra of the hydration
products were obtained either in CDCl (benzamide) or
3
1
13
1
CDCl þDMSO-d (acetamide). H and C{ H} chemical
3
6
shifts (d, ppm) are reported relative to the residual proton res-
3
1
1
onance in the deuterated solvent and P{ H} NMR spectra are
reported relative to external 85% H PO . Turnover numbers
Experimental Section
3
4
(
TON) for the catalytic hydrations were calculated on the basis
General Considerations
of mole of isolated product per mole of catalyst and respective
turnover frequencies (TOF) are calculated as TON over reac-
tion time, typically 72 h, unless otherwise specified. All argon
used (Praxair) was supplied in high purity grade 99.998%.
Unless otherwise noted, all manipulations were performed un-
der an argon atmosphere in an MBraun glovebox (<1 ppm
H O and O ) or using standard Schlenk techniques. A 300-mL,
2
2
ꢀ
stainless steel Parr Series 4560 Bench Top Mini Reactor was
used for all experiments of catalysis. Acetonitrile was pur-
chased from J. T. Baker and benzonitrile was purchased from Reaction of [(dippe)NiH] (3) with Benzo- and
2
2
Aldrich. Both liquid substrates were reagent grade and were
Acetonitrile in D O: Stability of [(dippe)Ni(h -
NCPh)] (1) and [(dippe)Ni(h -NCMe)] (2) in
2
dried and distilled from calcium hydride (CaH ) prior to their
2
2
[29]
use;
the water used for all hydrations was distilled and
Aqueous Media
purged with argon for at least 30 min before any experiment.
All other solvents used were dried and distilled from sodium/
benzophenone ketyl. Deuterated solvents for NMR experi-
ments were purchased from Cambridge Isotope Laboratories
and stored over 3 Š molecular sieves in the glovebox. The nick-
The stability of complex 1 and complex 2 in aqueous media was
explored by addition of the respective nitrile (benzonitrile:
0.010 mL, 0.098 mmol, acetonitrile: 0.005 mL, 0.095 mmol) to
a slurry of [(dippe)NiH] (0.030 g, 0.046 mmol) in D O. Reac-
tants were mixed at room temperature in the glovebox and
the resulting mixture was led to react for 5 min under constant
stirring with mild evolution of hydrogen gas, that was entirely
vented out to the glovebox.
2
2
ꢀ
el(I) complex 3, was prepared from Super-Hydride and [(dip-
[30]
pe)NiCl2] suspended in hexane, similarly to the literature
[31]
procedure. Complexes 4 and 5, also used in some additional
hydration experiments, were prepared analogously, starting
738
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ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 732 – 742

Catalytic Hydration of Benzonitrile and Acetonitrile using Nickel(0)
FULL PAPERS
1
NMR spectra for 1 in D O: H, d¼0.87–2.22 (dippe), 7,29– Attempted Inhibition of the Nitrile Hydration
2
31
1
2
7
J
.77 (phenyl); P{ H}, d¼79.25 (br d, J ¼68 Hz), 67.25 (br d,
P, P
Processes in the Presence of Bu NI: Proof of Neutral
4
2
¼68 Hz).
P, P
intermediates
1
NMR spectra for 2 in D O: H, d¼1.01–1.25 (m, 24H, CH ),
2
3
4
The reactor vessel was charged in separate runs with [(dip-
1
4
6
.5–1.65 (m, 4H, CH ), 1.94–2.13 (m, 4H, CH), 2.42 (d, J
¼
2
H,P
3
1
1
2
(0.026 g, 0.04mmol), benzonitrile (20 mL,
.8 Hz, 3H, CH CN); P{ H}, d¼82.18 (br d, J ¼77 Hz),
pe)NiH]2
3
P,P
2
0
0
.20 mol), water (5 mL, 0.28 mol) and Bu NI (0.0612 g,
.165 mmol), or with [(dippe)NiH] (0.0498 g, 0.077 mmol),
6.78 (br d, J ¼77 Hz).
4
P, P
Similar results are obtained when 3 is dissolved in THF-d ,
2
8
acetonitrile (20 mL, 0.38 mol), water (5 mL, 0.28 mol) and
Bu NI (0.1172 g, 0.32 mmol). Either mixture was heated at
toluene-d or dioxane-d , in the presence of water and the cor-
8
8
responding nitrile.
4
1
808C for 72 h, as described for previous experiments. Yield
of benzamide after work-up: 41% (9.7846 g, 0.081 mol), with
a TON of 1000 and a TOF of 14cycles per hour. Yield of acet-
amide after work-up: 4% (0.9142 g, 0.015 mol), with a TON of
Control Experiments: Attempted Nitrile Hydration in
the Absence of Nickel(0) Catalysts
1
00 and a TOF of 1 cycle per hour. Both series of results are
The reactor vessel was charged in separate runs with nitrile and
water mixtures: 20 mL benzonitrile (0.20 mol) and 5 mL water
consistent values with those obtained in the absence of
Bu NI, under otherwise the same conditions.
4
(
(
0.28 mol), or 20 mL acetonitrile (0.38 mol) and 9.5 mL water
0.53 mol), and these were heated at 1808C under vigorous stir-
ring for 72 h. No benzamide or acetamide was formed.
Evaluation of the Effect of Basic Media on the
Catalytic Hydration of Benzonitrile
Attempted Inhibition of the Nitrile Hydration
Processes in the Presence of Metallic Mercury: Proof
of Homogeneity
The reactor vessel was charged with [(dippe)NiH] (0.0264g,
2
0.04mmol), benzonitrile (20 mL, 0.20 mol), and 5 mL of an un-
tampered aqueous solution of NaOH at pH¼10. The mixture
was then heated at 1808C for 72 h as described for previous ex-
periments. Yield of benzamide after work-up: 15% (3.5133 g,
0.029 mol), with a TON of 354and a TOF of 5 cycles per
hour. The pH of remaining aqueous phase after reaction was 7.
The reactor vessel was charged in separate runs with [(dip-
pe)NiH]2
(0.026 g, 0.04mmol), benzonitrile (20 mL,
0
.20 mol) and water (5 mL, 0.28 mol) or [(dippe)NiH]2
(
(
0.05 g, 0.078 mmol), acetonitrile (20 mL, 0.38 mol) and water
5 mL, 0.28 mol) and 2 drops of elemental mercury were added
to each mixture. Both mixtures were heated at 1808C under
vigorous stirring for 72 h, after which time the reactor vessel
Hydration Process Optimization at Large Water
was left to cool down and opened in a hood prior to work-up. Content (90% Water Volume) upon Temperature
Yield of benzamide after work-up: 41% (9.7105 g, Variation
0
.080 mol), with a TON of 993 and a TOF of 14cycles per
hour. Yield of acetamide after work-up: 4% (1.0012 g,
.017 mol), with a TON of 109 and a TOF of 2 cycles per
The reactor vessel was charged in separate runs with a constant
loading of [(dippe)NiH] (0.0058 g, 0.009 mmol), benzonitrile
2
0
(
5 mL, 0.05 mol) and water (37.5 mL, 2.08 mol), or [(dip-
pe)NiH]2 (0.004g, 0.006 mmol), acetonitrile (1.5 mL,
.03 mol) and water (11 mL, 0.61 mol). Each resulting mixture
hour. Both series of results are consistent with the values ob-
tained in the absence of metallic mercury, at otherwise the
0
same conditions. NMR spectra for acetamide in CDCl þ
3
1
(implying 0.04mole percent of the catalyst) was then heated at
a different temperature (140 or 1608C) during 72 h. Benzoni-
trile hydration: yield of the benzamide produced at 1408C, af-
ter work-up: 6% (0.3342 g, 0.003 mol), with a TON of 148 and a
TOF of 2 cycles per hour. Yield of benzamide produced at
DMSO-d : H, d¼6.39 (br s, NH), 5.8 (br s, NH), 1.76 (s, 3H,
6
13
1
CH ); C{ H}, d¼172.78 (carbonyl), 22.23 (CH ).
3
3
Catalytic Hydration of Benzonitrile using
1
608C, after work-up: 14% (0.8393 g, 0.007 mol), with a TON
2
[
(dippe)Ni(h -NCPh)] (1) Independently Prepared:
of 372 and TOF of 5 cycles per hour. Acetonitrile hydration:
yield of acetamide produced at 1408C: 1% (0.0122 g,
Proof of Mononuclear Intermediates
0
.0002 mol), with a TON of 17 and a TOF of 0.2 cycle per
hour. Yield of acetamide produced at 1608C, after work-up:
% (0.05 g, 0.0008 mol), with a TON of 68 and a TOFof 1 cycle
per hour.
The reactor vessel was charged in the glovebox with a mixture
2
[19b, c]
of [(dippe)Ni(h -NCPh)] prepared independently
0.1116 g, 0.26 mmol), benzonitrile (2.8 mL, 0.03 mol) and wa-
3
(
ter (21 mL, 1.17 mol). The mixture was heated at 1608C under
vigorous stirring for 72 h, similarly to the procedure described
above. Yield of benzamide after work-up: 39% (1.3022 g,
0
.011 mol), with a TON of 41 and a TOF of 1 cycle per hour. Hydration Process Optimization at Large Water
The same result is obtained when 85 mg of [(dippe)NiH]2
Content (90% Water Volume): Assessment of the
Effect of Increased Catalyst Concentration (1.0 Mole
Percent), on going from 160 to 1808C
[0.13 mmol; yielding in situ 0.26 mmol of the nickel(0) catalyst]
are used under the same conditions. NMR spectra for benza-
1
mide in CDCl : H, d¼7.8 (d, J¼6.89 Hz, 2H, o-Ar), 7.51 (t,
3
J¼7.19 Hz, 2H, m-Ar), 7.42 (t, J¼7.19 Hz, 1H, p-Ar), 6.13
The reactor vessel was charged in separate runs with [(dip-
pe)NiH]2 (0.080 g, 0.11 mmol), benzonitrile (2.5 mL,
0.02 mol) and water (20 mL, 1.11 mol). The resulting mixtures
1
3
1
(
1
br s, NH), 3.13 (br s, NH); C{ H}, d¼170.28 (carbonyl),
32.96 (ipso), 132.58 (o-Ar), 128.9 (m-Ar), 127.72 (p-Ar).
Adv. Synth. Catal. 2006, 348, 732 – 742
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
739

FULL PAPERS
Marco G. Crestani et al.
both produce 1.0 mole percent of nickel(0) catalyst loading,
and as such were tested at 160 and 1808C for 72 h, as described
in the previous experiments. Yield of benzamide produced at
and 500 psi of additional argon pressure after work-up: 10%
(2.35 g, 0.04mol), with a TON of 257 and a TOF of 4cycles
per hour.
1608C after work-up: 40% (1.3423 g, 0.011 mol), with a TON
of 40 and a TOF of 1 cycle per hour. Yield of benzamide pro-
duced at 1808C after work-up: 68% (2.0222 g, 0.017 mol),
with a TON of 67 and TOF of 1 cycle per hour.
Benzonitrile Hydration usingDifferent Catalysts:
Assessment of the Hydrophobic Effect of the Ligands
on Catalyst Efficiency
The reactor vessel was charged in separate runs with
Benzonitrile Hydration Process Optimization at 1808C
upon Water Volume Percent Variation
[
(dtbpe)NiH]2, 4, (0.0262 g, 0.035 mmol), benzonitrile
20 mL, 0.20 mol) and water (5 mL, 0.28 mol), or [(dcy-
pe)NiH] , 5, (0.035 g, 0.036 mmol), benzonitrile (20 mL,
(
The reactor vessel was charged in separate runs with a constant
2
loading of [(dippe)NiH] (0.0800 g, 0.12 mmol), while varying
0.20 mol) and water (5 mL, 0.28 mol). The catalyst loading
that results in each mixture is 0.04mole percent, similar to
the conditions described for the nickel(0) catalyst using dippe,
described above. Both mixtures were heated at 1808C for 72 h.
Yield of benzamide on using 4 after work-up: 23% (5.4621 g,
0.045 mol), with a TON of 651 and a TOF of 9 cycles per
hour. Yield of benzamide on using 5 after workup: 17%
(4.1079 g, 0.034 mol), with a TON of 467 and a TOFof 6 cycles
per hour.
2
the volume proportions of substrates so as to achieve mixtures
of 50 or 20 water volume percent, respectively: 10 mL of ben-
zonitrile (0.1 mol) and 10 mL of water (0.56 mol) or 20 mL of
benzonitrile (0.2 mol) and 5 mL of water (0.28 mol). The cata-
lyst loading that results in each mixture corresponds to 0.25
mole percent and 0.1 mole percent, respectively. Each mixture
was heated to 1808C for 72 h, as described for previous experi-
ments. Yield of benzamide produced on using 50% water vol-
ume, after work-up: 26% (3.099 g, 0.026 mol), with a TON of
1
03 and a TOF of 1 cycle per hour. Yield of benzamide pro-
duced on using 20% water volume, after work-up: 19%
4.5905 g, 0.038 mol), with a TON of 153 and TOF of 2 cycles
Acknowledgements
(
We thank DGAPA-UNAM (IN-205603) and CONACYT
per hour.
(
C02–42467) for their support of this work, the last also for a
Ph.D. grant for M. G. C.
Benzonitrile Hydration Process Optimization at Low
Water Content (20% Water Volume), upon Catalyst
Mole Percent Variation at 1808C
References and Notes
The reactor vessel was charged in separate runs with constant
loadings of benzonitrile (20 mL, 0.20 mol) and water (5 mL,
[1] A recent review on the metal-mediated and metal-cata-
lyzed hydrolysis of nitriles, that points out the impor-
tance of this reaction, has recently been published:
V. Y. Kukushkin, A. J. L. Pombeiro, Inorg. Chim. Acta
2005, 358, 1–21.
0.28 mol), while varying the amount of starting nickel dimer
[
(dippe)NiH] , so as to achieve low nickel(0) catalyst concen-
2
trations of 0.04and 0.01 mole percent, respectively. The
amounts used were 0.0262 g (0.04mmol) and 0.006 g
[
[
[
2] I. Johansson, in: Kirk-Othmer Encyclopedia of Chemical
(
7
0.009 mmol). Each mixture was heated at 1808C during
2 h. Yield of benzamide produced on using 0.04mole percent
Technology: Amides, Fatty Acid, (Ed.: J. I. Kroschwitz),
th
5
edn., Wiley, Hoboken, NJ, 2004, V. 2, pp. 442–463.
of nickel(0) catalyst after work-up: 41% (9.6902 g, 0.080 mol),
with a TON of 984and a TOF of 14cycles per hour. Yield of
benzamide produced on using 0.01 mole percent of nickel(0)
catalyst: 13% (3.0471 g, 0.025 mol), with a TON of 1350 and
TOF of 19 cycles per hour.
3] B. C. Challis, J. A. Challis, Reactions of the carboxamide
group, in: The Chemistry of Amides (Ed.: J. Zabicky),
Wiley-Interscience, New York, 1970, pp. 733–848.
4] For example, the second-order rate constants for the hy-
droxide-ion catalyzed hydrolysis of acetonitrile and acet-
amide, have been included. The respective constants are
ꢀ
6
ꢀ5
ꢀ1 ꢀ1
1
.6ꢂ10 and 7.4ꢂ10
M
s , indicating that the sec-
Catalytic Acetonitrile Hydration at 1808C, 0.04 Mole
Percent of Nickel(0) Catalyst and Low Water Content
ond step of the hydrolysis for the base-catalyzed reaction
is faster than the first one, therefore pushing the reaction
to the carboxylic salt formation, rather than stopping at
the amide stage, see: J. Chin, Acc. Chem. Res. 1991, 24,
(
20% Water Volume)
The reactor vessel was charged in separate runs with constant
1
45–152.
loadings of [(dippe)NiH] (0.050 g, 0.078 mmol), acetonitrile
2
[
5] a) M. B. Smith, J. March, Marchꢀs Advanced Organic
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(
20 mL, 0.38 mol) and water (5 mL, 0.28 mol).The resulting
th
mixtures provide 0.04mole percent of the corresponding nick-
el(0) catalyst loading, and as such, were heated at 1808C for
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(
100 and 500 psi). Yield of acetamide produced without added
argon pressure after work-up: 4% (0.909 g, 0.015 mol), with a
TON of 99 and a TOF of 1 cycle per hour. 38 mol) and water
[6] J. N. Moorthy, N. Singhal, J. Org. Chem. 2005, 70, 1926–
(
5 mL, 0.28 mol). Yield of acetamide produced both at 100
1929.
740
asc.wiley-vch.de
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 732 – 742

Catalytic Hydration of Benzonitrile and Acetonitrile using Nickel(0)
FULL PAPERS
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