Kinetic studies on the selective hydrogenation of phenylacetylene catalyzed by [Rh(NBD)(PPh3)2]BF4 (NBD=2,5-norbornadiene)

Article abstract of DOI:10.1016/S0022-328X(97)00426-9

The complex [Rh(NBD)(PPh₃)₂]BF₄ catalyzes the selective hydrogenation of phenylacetylene to styrene. In dichloromethane solution at 25°C, selectivities close to 80% are achieved. The reaction is first order in catalyst and

Full text of DOI:10.1016/S0022-328X(97)00426-9

Journal of Organometallic Chemistry 551 Ž1998. 49–53
Kinetic studies on the selective hydrogenation of phenylacetylene
1
w ž
/ ž
/ x
ž
/
catalyzed by Rh NBD PPh BF NBDs2,5-norbornadiene
3
2
4
b
a
a,)
b,2
Miguel A. Esteruelas , Inmaculada Gonz ´a lez , Juana Herrero , Luis A. Oro
a
Departamento de Qu ´ı mica, UniÕersidad de Cantabria, 39005-Santander, Spain
b
Departamento de Qu ´ı mica Inorg a´ nica-Instituto de Ciencia de Materiales de Arag o´ n, UniÕersidad de Zaragoza-CSIC, Zaragoza 50009, Spain
Received 19 April 1997; received in revised form 8 June 1997
Abstract
The complex wRhŽNBD.ŽPPh . xBF catalyzes the selective hydrogenation of phenylacetylene to styrene. In dichloromethane solution
3
2
4
at 258C, selectivities close to 80% are achieved. The reaction is first order in catalyst and substrate, and second order in hydrogen
pressure, while the addition of triphenylphosphine inhibits the reduction. The mechanism deduced for this hydrogenation, on the basis of
the above mentioned kinetic results, suggests that the reduction of the diolefin of the catalyst does not occur during the hydrogenation of
the alkyne. q 1998 Elsevier Science S.A.
1
. Introduction
q
Cationic rhodium and iridium complexes with the general formula wM
Ž
diene
.
L x
as2 or 3 have been found to
Ž .
a
be active catalysts for the hydrogenation of olefins w1–7x, dienes w8,9x, internal alkynes w10x and ketones w11x. In
solvents such as acetone, ethanol or acetonitrile, these compounds react with molecular hydrogen to give dihydrido-
q
metal complexes wMH S L x
Ž
MsRh, Ir; Sssolvent
.
, which under catalytic conditions are in equilibrium with
Eq. 1
Ž Ž ... Both the dihydrido and monohydrido species are the active
a
2
x
a
the corresponding monohydridos MHS L
y
catalysts w12x.
q
q
MH S L
|MHS L qH
1
Ž .
2
x
a
y
a
The reduction of terminal alkynes with these systems is not easy. In this sense, it has been proposed that the
terminal alkynes, which are fairly acidic, destroy the active species by formation of alkynyl derivatives, according to
Eq.
Ž
2
.
w10x.
MHS L qRC[CH™M
Ž
C[CyR
.
S L qH
Ž
2
.
y
a
y
a
2
In spite of these difficulties, 1-hexyne can be successfully and selectively reduced to 1-hexene using 2-
q
methoxyethanol and wRh
Ž
NBD.ŽPPh Me
.
x
Ž
NBDs2,5-norbornadiene
.
as solvent and catalyst respectively w10x, or
TFBstetrafluorobenzobarrelene
type complexes w13x.
COD.Žh - Pr PCH CH OMe xBF
CODs1,5-
2
2
Ž
q
using dichloromethane and wRh
Ž
TFB
.
Ä
P
C H -4-R
. 4
x
Ž
.
6
4
3 2
2
i
Recently, it has been also observed that the iridium complexes wIr
Ž
.
Ž
4
2
2
2
)
Corresponding authors.
1
Dedicated to Prof. Peter M. Maitlis, author of major advances in the understanding of homogeneous catalysis, on the occasion of his 65th
birthday.
2
Also corresponding author.
0
022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

5
0
M.A. Esteruelas et al.rJournal of Organometallic Chemistry 551 (1998) 49–53
cyclooctadiene
.
catalyzes the selective hydrogenation of phenylacetylene to styrene in dichloromethane as solvent
w14x. Kinetic and spectroscopic investigations indicate that the diolefin is not reduced during the reaction, and
2
i
although the hydrido-akynyl complex wIrH
Ž
C[CPh.ŽCOD.Žh - Pr PCH CH OMe
.
xBF is the main species under
2
2
2
4
2
catalytic conditions, the hydrogenation proceeds via the dihydrido intermediate wIrH
Ž
COD.Žh -
2
i
Pr PCH CH OMe
.
xBF according to Eqs.
Ž
3
.
–
Ž
5
.
, where the last equation is the rate-determining step w14x.
2
2
2
4
2
i
2
i
Ir
Ž
COD
.
Ž
h y Pr PCH CH OMe
.
BF qH | IrH
Ž
COD
.
Ž
h y Pr PCH CH OMe
.
BF4
Ž
Ž
Ž
3
4
5
.
.
.
2
2
2
4
2
2
2
2
2
2
i
IrH2
Ž
COD
IrH
.
Ž
h y Pr PCH CH OMe
.
BF qPhC[CH
2
2
2
4
2
i
|
Ž
CH5CHPh. ŽCOD
.
Ž
h y Pr PCH CH OMe
.
BF4
2
2
2
2
i
IrH
Ž
CH5CHPh. ŽCOD
.
Ž
h y Pr PCH CH OMe
.
BF qH
2
2
2
4
2
2
i
™
IrH2
Ž
COD
.
Ž
h y Pr PCH CH OMe
.
BF qPhCH5CH
2
2
2
4
2
In order to investigate the generality of this mechanism for rhodium cationic complexes, we have carried out a
kinetic study about the hydrogenation of phenylacetylene, catalyzed by wRhŽNBD.ŽPPh3.2 xBF . In this paper, we
4
report the results from this study.
2
. Results and discussion
Complex wRh NBD.ŽPPh3.2 xBF efficiently catalyzes the sequential hydrogenation of phenylacetylene in
Ž
4
dichloromethane solution. At 258C and atmospheric pressure, selectivities close to 80% are achieved for the
hydrogenation of the alkyne to the alkene, as illustrated in Fig. 1. Reduction of the double bond only begins to take
place when most of the alkyne has been consumed. In the absence of the alkyne, styrene is hydrogenated to
ethylbenzene at slower rates than those observed in the reduction of the acetylene triple bond. During the reduction of
the C[C triple bond the formation of norbornene or norbornane was not observed.
Initial hydrogenation rates were obtained from gas uptake experiments at 258C. In order to determine the rate
dependence on the various reaction components, hydrogenation runs were performed at different catalyst and substrate
concentrations and at different hydrogen pressures. The data collected in Table 1 indicate that the reaction is first
order in catalyst and substrate. Plots of log
cases straight lines of slope 0.9. An analogous plot of log
.0, suggesting that the reduction of the alkyne is second order in hydrogen pressure. In addition, it should be
Ž
ydVrdt
.
versus log
Ž
wCatalystx
.
and versus log
ŽwPhC[CHx.
give in both
Ž
ydVrdt
.
versus log
P
Ž Ž
H2 .. yields a straight line of slope
2
mentioned that the hydrogenation of phenylacetylene was found to be inhibited by the addition of triphenyl phosphine
y3
Fig. 1. Hydrogenation of phenylacetylene to styrene catalyzed by wRhŽNBD.ŽPPh . xBF in dichloromethane at 258C, 1 atm H , 1.6=10
M
3
2
4
2
wCatalystx, 0.17 M PhC H. 'Phenylacetylene; Bstyrene; vethylbenzene.
2

M.A. Esteruelas et al.rJournal of Organometallic Chemistry 551 (1998) 49–53
51
Ž
Ž
Fig. 2
.
. In light of these results, a similar mechanism to that described by Eqs.
.
Ž
3
.
–
Ž
5
.
can be proposed
Ž
Eqs.
6
.
–
Ž
10..
K6
Rh
Ž
NBD. ŽPPh3 .2 BF qH | RhH
Ž
NBD. ŽPPh3 .2 BF4
Ž
Ž
Ž
Ž
6
7
8
9
.
.
.
4
2
2
1
2
K7
RhH2
RhH2
Ž
Ž
NBD. ŽPPh3 .2 BF | RhH
Ž
NBD. ŽPPh3
.
BF qPPh
4
2
4
3
2
3
K8
NBD. ŽPPh33
.
BF qPhC[CH | RhH
Ž
CH5CHPh. ŽNBD. ŽPPh3
.
BF4
4
4
k9
RhH
Ž
CH5CHPh. ŽNBD. ŽPPh3
.
BF qH ™ RhH
Ž
NBD. ŽPPh35
.
BF qCH 5CHPh
.
.
4
2
2
4
2
4
RhH2
Ž
NBD. ŽPPh3
.
BF qPPh ™ RhH
Ž
NBD. ŽPPh3 .2 BF4
Ž
10
4
3
2
5
1
With regard to the kinetic data, there is no doubt that Eq. 9 is the rate-determining step. Thus, the rate of
Ž .
formation of styrene is
d styrene rdtsk 4 P
The concentration of the key intermediate 4 can be determined as follows:
Ž
H2
.
Ž
Ž
11
.
.
9
Rh Tot s 1 q 2 q 3 q 4
12
since K s w2xrw1xP
Ž
H 2
.
,
K s w3xwPPh xrw2x and K s w4xrw3xwPhC Hx, we have w1x s
6
7
3
8
2
w4xwPPh xrK K K wPhC HxP
Ž
H2
.
, w2xsw4x wPPh xrK K wPhC Hx and w3xsw4xrK wPhC Hx and finally
3
6
7
8
2
3
7
8
2
8
2
K K K Rh
PhC H P
Ž
H2
.
6
7
8
Tot
2
4
s PPh
Ž
13
.
Ä
1qK P
Ž
H2
.
qK P P
Ž
H2
.
qK K K PhC H P
Ž
H2
.
4
3
6
6
7
6
7
8
2
Complex 1 is the main species under catalytic conditions. Thus, 1 4 K P H2
K K K wPhC HxPŽ . Ž .F1 and wPPh x equilibrium(wPPh x added. Therefore w4x can be written as
H , when P H
2 2
Ž
q K K P H2 q
. Ž .
6
6
7
6
7
8
2
3
3
follow:
K K K Rh
PhC H P
Ž
H2
.
6
7
8
Tot
2
4
(
Ž
14
.
PPh3
and 14
Combining Eqs.
Ž
11
.
Ž
.
, we obtained Eq.
Ž
15
.
, where wRhxTot and wPPh x are the concentrations of catalyst
3
and added phosphine, respectively.
2
d styrene
k K K K Rh
PhC H
Ä
P
Ž
H2
.
4
9
6
7
8
Tot
2
(
Ž
15
.
dt
PPh3
Inspection of Eq.
Ž
15
.
shows that the rate of the catalytic reaction is aproximately proportional to the concentra-
Table 1
Kinetic data for the hydrogenation of phenylacetylene to styrene catalyzed by wRhŽNBD.ŽPPh . xBF in dichloromethane at 258C
3
5
2
4
wCatalystx 104
wPhC Hx 10 M
PŽH . atm
wPPh x 10 M
M
ydVrdt mlrmin
2
2
3
1.14
1.55
2.07
2.41
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
1.55
0.76
0.76
0.76
0.76
0.62
1.25
1.47
1.89
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
1
1
1
1
1
1
1
1
0.61
0.72
0.81
0.84
1
1
1
1
y
y
y
y
y
y
y
y
y
y
y
y
0.9
1.9
2.6
3.0
1.20
1.38
2.10
2.34
1.30
2.20
2.55
3.30
0.52
0.80
0.90
1.17
1.30
1.14
1.06
1.05

5
2
M.A. Esteruelas et al.rJournal of Organometallic Chemistry 551 (1998) 49–53
y3
Fig. 2. Plot of the rate of hydrogenation of phenylacetylene to styrene versus 1rwPPh x in dichloromethane at 258C, 1 atm H , 1.55=10
M
3
2
wCatalystx, 0.067 M PhC H.
2
tions of catalyst and substrate, second order with respect to hydrogen pressure, and inversely proportional to the
concentration of added phosphine, which agrees well with the experimental data.
The mechanism summarized by Eqs.
6 – 10 merits some additional comment, the reaction of
Ž . Ž .
wRh
Ž
NBD.ŽPPh3.2 xBF with molecular hydrogen most likely should give a short-lived intermediate, cis-
4
wRhH
Ž
NBD.ŽPPh3
.
xBF , which must have the stereochemistry shown in Fig. 3. In agreement with this proposal
2
2
4
q
Crabtree and co-workers have observed that the iridium complexes wIr
Ž
COD.ŽPR
.
x
Ž
PR sPPh , PPh Me
.
react
x , which have
3
2
3
3
.
2
q
with molecular hydrogen at y808C in dichloromethane to give the dihydrido cis-wIrH
Ž
COD.ŽPR
2
3
2
the stereochemistry shown in Fig. 3. When the dichloromethane solutions of these complexes are warmed to y208C,
q
in the absence of an excess of H , hydrogen is partly lost to give wIr
Ž
COD.ŽPR
.
x . In acetone, the reactions lead to
2
3 2
q
the cations wIrH
Ž
.
acetone. ŽPR .
x
w15x. In contrast to these compounds, the also dihydrido derivative cis-
2
2
3 2
wIrH
Ž
COD.Ždppm
xBF
Ž
dppmsbis
Ž
diphenylphosphino
molecular hydrogen nor transferring it to the coordinated 1,5-cyclooctadiene w16x.
NBD.ŽPPh . xBF is six-coordinate and thus coordinatively saturated. So, one arm of the
3 2
methane is stable, even at room temperature, neither losing
. .
2
4
Intermediate cis-wRhH
Ž
2
4
chelating NBD ligand or triphenylphosphine must be dissociated from the coordination sphere of the metal before the
reaction with the alkyne. In this context, it should be mentioned that spectroscopic studies on the addition of
q
molecular hydrogen to the also saturated complex wRh
Ž
NBD.ŽPPhMe2
.
x
suggest that it is PPMe , rather than one
3
2
arm of the diene ligand, which dissociates prior to addition of hydrogen w12x. Furthermore, in the complex
cis-wRhH
Ž
NBD.ŽPPh
. xBF , the dissociation of the phosphine trans disposed to the hydrido ligand should be
3 2
2
4
favored due to high trans effect of the hydrido and the large steric hindrance experienced by the triphenylphosphine
groups, which are mutually cis disposed.
If the alkyne fills the site of the dissociated phosphine, the alkyne would then be cis to one hydride but trans to the
other. So, if the cis-hydrido migrates to the alkyne and the stereochemistry does not change further, the resulting
hydrido-alkenyl intermediate would have trans stereochemistry. This trans coordination would not facilitate the
reductive elimination of styrene in the next step, explained why a new reaction with molecular hydrogen is necessary
for the formation of the olefin.
The selectivity observed in the reduction of phenylacetylene may have kinetic reasons. Fig. 1 shows that
phenylacetylene is hydrogenated at a rate which is much greater than the rate at which styrene is reduced. The same
q
behavior has been observed for the hydrogenation of 2-hexyne to cis-2-hexene catalyzed by wRh
Ž
NBD.ŽPR3
.
x
w10x,
2
2
i
and for the hydrogenation of phenylacetylene to styrene catalyzed by wIrŽCOD.Žh - Pr PCH CH OMe.xBF w14x. In
these cases, it has been argued that the alkyne competes strongly with the olefin for coordination sites at the metal
center. Consequently, the higher coordination ability of phenylacetylene, in comparison with styrene, could favor the
2 2 2
4
displacement of triphenylphosphine from cis-wRhH2ŽNBD.ŽPPh3.2 xBF and therefore the alkyne hydrogenation.
4
In contrast to these cationic systems, the selectivity observed for the hydrogenation of phenylacetylene catalyzed by
i
OsHCl
Ž
CO.ŽP Pr3.2 appears to be thermodynamically controlled w17x. The independent study of the reduction of C[C
Fig. 3.

M.A. Esteruelas et al.rJournal of Organometallic Chemistry 551 (1998) 49–53
53
and C5C bonds indicates that the latter is kinetically favored. However, the vinyl complex Os
Ä
Ž
E
.
-
i
CH5CHPh
4
Cl
Ž
CO.ŽP Pr3.2 w18x is the main species under catalytic conditions; this complex represents a thermody-
namic sink that causes virtually all the osmium present in solution to be tied up this form, and consequently, the
kinetically unfavored pathway becomes the only one available in the presence of phenylacetylene w17x. However,
kinetic reasons seems to be operative in the above mentioned rhodium and iridium complexes.
3
. Conclusions
This study has revealed that in dichloromethane as solvent the mechanism for the selective hydrogenation of
phenylacetylene to styrene catalyzed by wRhŽNBD.ŽPPh3. xBF is similar to that previously descrited for the same
2
4
2
i
reaction catalyzed wIr
ŽCOD.Žh - Pr PCH CH OMe.xBF . These reactions do not imply the reduction of the dienes of
2 2 2
the catalysts, which remain coordinated during the processes, in contrast to that previously reported for related
4
q
systems in coordinating solvents, where dihydrido and monohydrido derivatives of the types wMH SxŽPR3.2
x
and
2
MHS
ŽPR . ŽMsRh, Ir; Sssolvent.
are the active species.
y
3
4
. Experimental details
All manipulations were conducted with rigorous exclusion of air. Solvents were dried by known procedures and
distilled under argon prior to use. Phenylacetylene Merck was purified by distillation. Complex
wRh NBD.ŽPPh3.2 xBF was prepared as described in literature w1x.
Ž
.
Ž
4
4
.1. Kinetic studies
The catalytic reactions were followed, at constant pressure, by measuring the hydrogen consumption as a function
of time on a gas buret Afora 516256 . The analysis of the products of the catalytic reactions was carried out on a
Perkin–Elmer 8500 gas chromatograph with a flame ionization detector and an FFAP on Chromosorb 6HP 80r100
mesh 3.6 m=1r8 in. column at 2208C. In a typical procedure, the substrate dissolved in dichloromethane 20 ml
was added to a solution of the catalyst in dichloromethane 20 ml , under argon atmosphere. This solution was
Ž
.
Ž
.
Ž
.
Ž
.
syringed through a silicon septum into a 50-ml flask attached to a gas buret, which was in turn connected to a Schlenk
manifold and had been previously evacuated and refilled with hydrogen three times. The flask was then immersed in a
2
58C bath and the mixture was vigorously shaken during the run.
Acknowledgements
We thank the DGICYT Projects PB95-0806 and PB94-1186, Programa de Promoci o´ n General del Conocimiento
Ž .
and the Comisi o´ n Mixta Caja Cantabria-Universidad de Cantabria for financial support.
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1