Nickel-catalyzed isomerization of 2-methyl-3-butenenitrile

Article abstract of DOI:10.1021/om049206p

The isomerization of the branched 2M3BN to the linear 3PN by a DPEphosNi species has been investigated by means of variable-temperature NMR spectroscopy, and activation parameters have been determined. An intermediate in this reaction, which is formed via

Full text of DOI:10.1021/om049206p

Organometallics 2005, 24, 13-15
13
Nickel-Catalyzed Isomerization of
2-Methyl-3-butenenitrile†
Jos Wilting,‡ Christian Mu¨ller,‡ Alison C. Hewat,‡ Dianne D. Ellis,§
Duncan M. Tooke,§ Anthony L. Spek,§ and Dieter Vogt*,‡
Laboratory of Homogeneous Catalysis, Eindhoven University of Technology,
Eindhoven, The Netherlands, and Department for Crystal and Structural Chemistry,
Utrecht University, Utrecht, The Netherlands
Received October 13, 2004
Scheme 1
Summary: The isomerization of the branched 2M3BN
to the linear 3PN by a DPEphosNi species has been
investigated by means of variable-temperature NMR
spectroscopy, and activation parameters have been de-
termined. An intermediate in this reaction, which is
formed via C-C bond activation, could be trapped by
addition of ZnCl2, and the molecular structure of the
corresponding Ni(II) complex has been determined by
X-ray crystallography.
Among C-C bond formation reactions, the hydrocya-
nation of alkenes is an attractive, yet challenging, route
for the functionalization of carbon-carbon double
bonds.1,2 The isomerization of 2-methyl-3-butenenitrile
(2M3BN) to 3-pentenenitrile (3PN), for example, is
closely related to hydrocyanation and is at the same
time an important step in the industrial adiponitrile
process.3 In combination with Ni(cod)2, all classes of
bidentate phosphorus ligands (phosphines,4,5 phosphin-
ites,6 phosphonites,4,7 and phosphites8) catalyze the
isomerization, without the addition of Lewis acids.
However, the influence of ligand parameters on this
conversion still remains unknown. The isomerization is,
in fact, a suitable reaction to study reductive elimination
to the final product 3PN without the deactivation of the
catalyst usually caused by HCN during hydrocyana-
tions.9,10 We started a spectroscopic investigation of the
isomerization of 2M3BN to 3PN catalyzed by DPEphos/
Ni(cod)2 to study the reductive elimination of nitriles,
which is the crucial step in catalytic hydrocyanation
reactions. In fact, we were moved to report on our
results because of recent contributions in this field by
Santini et al.11 and Chaudret et al.5
Upon addition of 1 equiv of DPEphos to Ni(cod)2 in
toluene-d8 the 31P NMR spectrum displays a singlet at
δ 33.1 ppm for the species (DPEphos)Ni0(cod) (1)
(Scheme 1). This signal disappears on addition of
approximately 3 equiv of 2M3BN at -35 °C, and two
2
doublets appear at δ 21.3 and 24.2 ppm, with JPP
)
38 Hz. These resonances correspond to (DPEphos)Ni0-
(2M3BN) (2), with 2M3BN coordinating most likely
through its alkene bond; the η2-alkene complex (DPE-
phos)Ni0(styrene) gives a very similar 31P NMR spec-
trum, which shows two doublets at δ 20.2 and 22.9 ppm
* To whom correspondence should be addressed. E-mail: d.vogt@
tue.nl.
2
with JPP ) 44 Hz. When the temperature is raised to
† Dedicated to Professor Wilhelm Keim on the occasion of his 70th
birthday.
+25 °C, the two doublets of 2 disappear and a signal
could no longer be detected in the 31P NMR spectrum.
‡ Eindhoven University of Technology.
1
§ Utrecht University.
The H NMR spectrum shows broad signals at δ 4.5
(CHCH2), 3.4 (CHCH3), 2.4 (1H, CHsynHanti), 1.6 (CH3),
and 1.2 (1H, CHsynHanti) ppm. Increasing the tempera-
ture to +35 °C causes slow isomerization of 2M3BN to
(1) Huthmacher, K.; Krill, S. Reactions with hydrogen cyanide
(hydrocyanation). In Applied Homogeneous Catalysis with Organome-
tallic Compounds; VCH: Weinheim, Germany, 1996; Vol. 1, pp 465-
486.
(2) Goertz, W.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M.; Vogt,
D. Chem. Eur. J. 2001, 7(8), 1614-1618.
1
3PN, indicated by the methyl signals in the H NMR
spectrum. The 3PN analogue (DPEphos)Ni0(3PN) (5)
could not be detected with VT NMR spectroscopy when
approximately 5 equiv of 3PN was added to 1. However,
1 disappears and the same broad signals in the 1H NMR
spectrum occur which were detected upon warming of
2. We ascribe this spectrum to (DPEphos)NiII(C4H7)(CN)
(3), which is formed by oxidative addition of 2M3BN or
(3) Tolman, C. A.; McKinney, R. J.; Seidel, W. C.; Druliner, J. D.;
Stevens, W. R. Adv. Catal. 1985, 33, 1-46.
(4) van der Vlugt, J. I.; Hewat, A. C.; Neto, S.; Sablong, R.; Mills,
A. M.; Lutz, M.; Spek, A. L.; Mueller, C.; Vogt, D. Adv. Synth. Catal.
2004, 346(8), 993-1003.
(5) Chaumonnot, A.; Lamy, F.; Sabo-Etienne, S.; Donnadieu, B.;
Chaudret, B.; Barthelat, J. C.; Galland, J. C. Organometallics 2004,
23(14), 3363-3365.
(6) Siegel, W. (BASF AG, 67063 Ludwigshafen, Germany) Ger.
Offen. DE10150286, 2004.
(7) Lenges, C. P. (E. I. Du Pont de Nemours & Co.) Phosphonite
ligands and their use in hydrocyanation. WO03/076394, 2003.
(8) Foo, T.; Garner, J. M.; Tam, W. (E. I. Du Pont de Nemours &
Co., USA) Hydrocyanation of diolefins and isomerization of noncon-
jugated 2-alkyl-3-monoalkenenitriles. WO99/06357, 1999.
(9) Casalnuovo, A. L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T.
H. J. Am. Chem. Soc. 1994, 116(22), 9869-9882.
(10) Goertz, W.; Keim, W.; Vogt, D.; Englert, U.; Boele, M. D. K.;
Van der Veen, L. A.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M. J.
Chem. Soc., Dalton Trans. 1998(18), 2981-2988.
(11) Vallee, C.; Valerio, C.; Chauvin, Y.; Niccolai, G. P.; Basset, J.
M.; Santini, C. C.; Galland, J. C.; Didillon, B. J. Mol. Catal. A: Chem.
2004, 214(1), 71-81.
10.1021/om049206p CCC: $30.25 © 2005 American Chemical Society
Publication on Web 12/09/2004

14 Organometallics, Vol. 24, No. 1, 2005
Communications
Figure 2. Crystal structure of 4. Hydrogen atoms and a
solvent molecule have been omitted for clarity. Bond
lengths (Å) and angles (deg): Ni(1)-P(1) ) 2.3080(6),
Ni(1)-P(2) ) 2.2031(6), P(1)-Ni(1)-P(2) ) 105.89(2),
Ni(1)-C(17) ) 1.893(2), C(17)-N(1) ) 1.150(3), Ni(1)-
C(13) ) 2.067(3), Ni(1)-C(14) ) 2.000(3), Ni(1)-C(15) )
2.148(3), N(1)-Zn(1) ) 1.985(2).
Figure 1. 31P{1H} VT NMR spectra on a sample of 1 equiv
of DPEphos and Ni(cod)2 in toluene-d8 to which 3 equiv of
2M3BN was added at -78 °C.
Scheme 2
3PN to the Ni center (Scheme 1). Chaudret et al.
reported similar 1H NMR data for (dppb)NiII(C4H7)-
(CN).5 Both complexes display distinct signals for the
syn and anti protons. The substitution of 1,5-cycloocta-
diene by 2M3BN occurs more quickly compared to the
substitution by 3PN, as is expected for a terminal versus
an internal alkene.12
Cooling the NMR tube from +35 to -85 °C results in
three sets of signals, as displayed in Figure 1.13 One
set of very low intensity is similar to the signals for 2.
The other two sets most likely correspond to the syn
2
and anti isomers of 3 (Scheme 2), with JPP ) 49 and
64 Hz. Isomerization between syn and anti is indeed
possible via the η1-methylallyl species I and II, which
can undergo rotation of the C2-C3 bond without
exchange of the syn and anti protons of the CH2 group.14
The other η1-methylallyl species III and IV would give
interconversion of the syn and anti protons upon rota-
tion of the C1-C2 bond. Although this interconversion
is not observed, we strongly believe that formation of
3PN proceeds via III and IV. Slowly warming the NMR
tube shows the dynamic behavior: first the two sets of
doublets become one set of broad signals (-40 °C),
indicating fast exchange between the syn and anti
isomers without rotation of the allyl group. Only one
position seems to be favored, as only two species were
detected at low temperature. Rotation of the allyl group,
indicated by one broad signal, is observed at 0 °C. This
signal disappears after warming the NMR tube to
temperatures above 20 °C. At these temperatures the
complex becomes catalytically active: i.e., reductive
elimination to form the isomerized product occurs.
The addition of ZnCl2 to 3 caused precipitation of
complex 4. The 1H NMR spectrum is similar to the
spectrum of 3. An alternative route to complex 4 is
addition of either 2M3BN or 3PN to DPEphosNiCl2 in
the presence of zinc dust. However, from a mixture of
2M3BN and 3PN (1:1) only 2M3BN gave oxidative
addition to Ni(0) at 20 °C; no isomerization is observed,
due to precipitation of complex 4. This demonstrates the
large difference in reaction rates for coordination of
2M3BN over 3PN to the Ni center (vide infra).
Single crystals of 4 suitable for X-ray diffraction could
be obtained.15 The molecular structure of 4 is shown in
Figure 2 and is best described as pseudo-tetrahedral
around the nickel center. This has also been observed
in the recently reported structure for (dppb)NiII(C4H7)-
(15) Crystal data for 4: (C43H41Cl2NNiO2P2Zn)‚1/2C6H6, Mr ) 899.74,
monoclinic, space group C2/c, a ) 24.9627(4) Å, b ) 12.0804(2) Å, c )
28.9802(5) Å, â ) 106.7861(7), V ) 8366.9(2) Å3, Z ) 8, Dcalcd ) 1.429
Mg m-3, F(000) ) 3720, λ(Mo KR) ) 0.710 73 Å, µ ) 1.267 mm-1, T )
150(2) K, crystal dimensions 0.18 × 0.12 × 0.08 mm. Of 36 646
reflections measured, 9590 unique reflections were used in the refine-
ment. Final R ) 0.0379 (Rw ) 0.0881); S ) 1.03.
(12) Tolman, C. A. Organometallics 1983, 2(5), 614-621.
(13) The small sharp singlet at δ 23 ppm is an impurity.
(14) Pregosin, P. S.; Salzmann, R. Coord. Chem. Rev. 1996, 155, 35-
68.

Communications
Organometallics, Vol. 24, No. 1, 2005 15
Scheme 3
order kinetics in substrate would suggest an associative
mechanism for the reductive elimination, as observed
in the hydrocyanation of ethylene.18
The activation parameters of the isomerization reac-
tion were determined to be ∆Hq ) 60.5 ( 5.2 kJ mol-1
and ∆Sq ) -112 ( 15 J mol-1 K-1. An investigation on
ligand differentiations on the isomerization reaction, by
measurement of the activation parameters, is currently
being carried out in our laboratories, and the results
will be published elsewhere.
However, the data indicate that the reductive elimi-
nation is the rate-determining step in the isomerization
of 2M3BN. It has been proposed that the allyl species 3
is in equilibrium with its hydrido cyano butadiene
species 6 with k-4 . k4 (Scheme 3), which explains the
labeling experiments described by Druliner19 and Ra-
janBabu.9 This hypothesis was rejected by Chaudret et
al. on the basis of DFT calculations for a Ni-PH3 model
system without Lewis acids. The substitution reaction
(5 to 2) most likely follows an associative mechanism,
avoiding the unstable 14-electron complex.
The isomerization of 2M3BN to 3PN can be used to
study the influence of different ligand parameters on
the reductive elimination, which is currently being
performed in our group. It should be mentioned here
that the formation of 2M3BN is not included in the rate
of isomerization. The activation parameters were de-
termined for the DPEphos/Ni(cod)2 catalyst system, and
the individual reaction steps were studied by NMR. The
reversibility of the reductive elimination reaction was
demonstrated. Furthermore, the C-C activation prod-
uct 4, which was synthesized independently and char-
acterized by single-crystal X-ray diffraction, is an
intermediate in the isomerization of 2M3BN to 3PN.
(CN).5 Both molecular structures are very similar, i.e.
the Ni-CN bond length is 1.893 Å in both structures,
and thus the Lewis acid ZnCl2 has little influence. This
is also observed during catalysis, which shows similar
rates for 0, 1, and 2 equiv of ZnCl2 in DMSO. However,
the molecular structure reported for (dippe)NiII(C3H5)-
(CN)16 has a square-pyramidal geometry, with the CN
in the apical position.
The structural difference originates from the short
bridge between the phosphorus atoms, resulting in
different P-Ni-P angles (105.0° in 4 vs 88.7° in (dippe)-
NiII(C3H5)(CN)).
Upon analysis by GC, the isomerization of 2M3BN
to 3PN (with and without ZnCl2) was determined to be
zero order in substrate (eq 1 and Scheme 3), which has
been observed in the hydrocyanation of 6-methoxy-2-
vinylarene as well.9
Supporting Information Available: Text and figures
giving details of kinetic measurements and a CIF file giving
full characterization of 4 including crystal structure analysis.
This material is available free of charge via the Internet at
http://pubs.acs.org. The crystallographic data have also been
deposited with the Cambridge Crystallographic Database
(CCDC No. 252435).
d[3PN]/dt ) k[catalyst]
(1)
This result is in contradiction to the report by Santini
et al. They concluded the system to have first-order
kinetics in substrate for the isomerization of 2M3BN
to 3PN in ionic liquids with monodentate phosphines
in combination with Ni(cod)2.11 However, their results
could equally well point to zero-order kinetics.17 First-
OM049206P
(17) We considered the possibility of a different mechanism for
monodentate ligands. Thus, we carried out the isomerization of 2M3BN
to 3PN with Ni(cod)2 and 10 equiv of PPh3 as the catalyst system in
toluene and followed the product formation in time by GC. The results
clearly showed zero-order kinetics.
(18) McKinney, R. J.; Roe, D. C. J. Am. Chem. Soc. 1985, 107(1),
261-262.
(19) Druliner, J. D. Organometallics 1984, 3(2), 205-208.
(16) Brunkan, N. M.; Jones, W. D. J. Organomet. Chem. 2003,
683(1), 77-82.