Coordinated olefins as incipient carbocations: Catalytic codimerization of ethylene and internal olefins by a dicationic Pt(II)-ethylene complex

Article abstract of DOI:10.1021/ja0263386

The coordinated ethylene molecule in the dicationic complex [(PNP)Pt(CH₂=CH₂)](BF₄)₂ (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine) undergoes nucleophilic attack by free internal alkenes, giving the isolable c

Full text of DOI:10.1021/ja0263386

Published on Web 07/10/2002
Coordinated Olefins as Incipient Carbocations: Catalytic Codimerization of
Ethylene and Internal Olefins by a Dicationic Pt(II)-Ethylene Complex
Christine Hahn, Maria E. Cucciolito, and Aldo Vitagliano*
Dipartimento di Chimica, UniVersita` degli Studi di Napoli Federico II, Complesso UniVersitario di Monte Sant’
Angelo, Via Cintia, I-80126 Napoli, Italy
Received March 27, 2002
The nucleophilic addition reaction at Pd(II)1 or Pt(II)2 coordinated
alkenes has been widely studied because of the great importance
for the transformation or functionalization of hydrocarbons.1,2 An
important fact which has been revealed experimentally3 and
theoretically4 is that the electrophilicity of the coordinated alkene
can be enhanced by increasing the positive charge on the metal
ion. A pictorial way to account for the reactivity enhancement is
to look at the olefin (which during the nucleophile approach is
slipping along its axis into an η1 coordination mode4,5) as a metal-
stabilized incipient carbocation (A), whose stability and fractional
charge would increase with increasing positive charge on the metal
ion. Accordingly, positively charged Pt(II) and Pd(II) complexes
have been occasionally reported to promote carbocationic reactivity
of alkenes,6 such as ionic oligomerization7 and polymerization8
reactions, and, more recently, numerous catalyzed cyclizations of
diynes and enynes.9 Metal-stabilized carbocations have also been
proposed as initiators for the polymerization of electron-rich
monomers by [Cp*TiMe2]+.10 In no case, however, has a reaction
between an olefin complex and a free alkene been observed, giving
an isolable complex of the dimeric olefin.
We have recently described the first class of dicationic Pd(II)11
and Pt(II)12 complexes of simple monoalkenes, showing that their
reactivity toward oxygen or nitrogen nucleophiles is increased (both
kinetically and thermodynamically) by several orders of magnitude
in comparison to neutral or monocationic species.11b,12 We now
report that the ethylene complex 1 (eq 1) can react also with
moderately electron-rich alkenes, giving rise to a coupling reaction
which turns into a nicely selective catalytic process.13
hand it is beneficial in that it precludes a successive oligomerization.
In the presence of an excess of the substrates, it is therefore possible
to achieve a selective catalytic process, via displacement of the
coordinated product by free ethylene and further reaction. The
reaction appeared to be very selective when run in CH2Cl2/MeNO2,
10/1, at a catalyst concentration of 0.01 mol/L, forming the
codimerization product 3,4-dimethyl-1-pentene almost exclusively.
In plain nitromethane and high concentration of the catalyst the
reaction rate was higher,15 but ca. 10% of the homodimer 2,3,4,4-
tetramethyl-1-hexene was formed.
The latter decene product was also rapidly formed (without any
codimerization product) in a “blank” reaction run in absence of 1
but in the presence of etherated HBF4 (0.1 mol %) as a catalyst.
Therefore, the formation of the decene byproduct in the Pt-catalyzed
reaction is very likely to be promoted by small amounts of acidic
species formed upon degradation of the catalyst.16
The catalytic codimerization was comparably efficient with
Me2CdCMe2 as substrate (giving 3,4,4-trimethyl-1-pentene), but
much slower (ca. 5 ton in 24 h) with Z-2-butene (giving 3-methyl-
1-pentene). The geminally disubstituted olefin 2-methyl-1-butene
also gave the codimer 3,4-dimethyl-1-pentene. By monitoring the
1
latter reaction by H NMR we could observe the rapid formation
of a small amount of the internal isomer 2-methyl-2-butene, whose
concentration, however, remained small and approximately station-
ary while the codimerization proceeded. This indicates that complex
1 is able to catalyze the interconversion between the two isomeric
pentenes, but since their equilibrium ratio would be largely in favor
of 2-methyl-2-butene,17 it also indicates that the latter was the
reacting species, selectively consumed by the successive reaction
with 1 (eq 1) at the same overall rate of its production. Isobutylene
did not give any hydrovinylation product, being instead dimerized
mainly to CH2dC(Me)CH2CMe3, most likely via an acid-catalyzed
pathway like that leading to the decene homodimer of 2-methyl-
2-butene (see above).
Although the catalytic runs were performed using “anhydrous”
solvents, we could not exclude the presence of traces of water in
the system. However, when water was deliberately added to the
solution (1 mol/mol catalyst), the effect on the substrate conversion
was deleterious, confirming the initial assumption (eq 1) that the
reaction takes place via direct activation of ethylene by coordination
to the doubly positive metal center.18
To get further mechanistic insight, we performed the reactions
with C2D4. In all the cases one deuterium atom was selectively
transferred from the ethylene molecule to the adjacent C(3) carbon
atom of the product CD2dCD-CD(Me)C(Me)RR′, while the vinyl
group retained its original deuterium content. A proposed mecha-
nism for the catalytic cycle is shown in Scheme 1.
In an initial experiment, when the isolated complex 1 (fluoro-
borate salt)12 was reacted with the stoichiometric amount of the
trisubstituted olefin 2-methyl-2-butene in CH2Cl2/MeNO2 solution
(10/1), a clean reaction took place in a few minutes at room
temperature, giving the complex 2b in nearly quantitative yield (eq
1). The product was obtained as an equimolar mixture of two
diastereomeric pairs, differing by the relative configurations of the
C(3) carbon atom and of the coordinated olefinic diastereoface.14
Complexes of propene and 1-butene were surprisingly not
reactive. While the lack of reactivity of R-olefins higher than
ethylene can be a limit to the scope of the reaction, on the other
The carbocation (B) generated from the nucleophilic attack on
the “slipping” coordinated ethylene (A) could in principle react
along various paths, among which the capture of the R group is
* To whom correspondence should be addressed. E-mail: alvitagl@unina.it.
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J. AM. CHEM. SOC. 2002, 124, 9038-9039
10.1021/ja0263386 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
2893. (c) Ba¨ckvall, J.-E.; Bystro¨m, S. J. Org. Chem. 1982, 47, 1126. (d)
Oshima, N.; Hamatani, Y.; Fukui, H.; Suzuki, H.; Moro-Oka, Y. J.
Organomet. Chem. 1986, 303, C21. (e) McDaniel, K. F. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, A., Wilkinson, G., Eds.;
Pergamon Press: London 1995; Vol. 12, pp 601-622. (f) Collman, J. P.;
Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Application of
Organotransition Metal Chemistry; University Science Books, Mill Valley,
CA, 1987; pp 410-415.
(2) (a) Young, G. B. ComprehensiVe Organometallic Chemistry II; Abel, E.
W., Stone, A., Wilkinson, G., Eds.; Pergamon Press: London, 1995; Vol.
9, pp 533-588. (b) Palumbo, R.; De Renzi, A.; Panunzi, A.; Paiaro, G.
J. Am. Chem. Soc. 1969, 91, 3874. (c) Panunzi, A.; De Renzi, A.; Palumbo,
R.; Paiaro, G. J. Am. Chem. Soc. 1969, 91, 3879. (d) Green, M.; Sarhan,
J. K. K.; Al-Najjar; I. M. J. Chem. Soc., Dalton Trans. 1981, 1565. (e)
Kaplan, P. D.; Schmidt, P.; Orchin, M. J. Am. Chem. Soc. 1968, 90, 4175.
(3) (a) Maresca, L.; Natile, G.; Rizzardi, G. Inorg. Chim. Acta 1980, 38, 53.
(b) Annibale, G.; Maresca, L.; Natile, G.; Tiripicchio, A.; Tiripicchio
Camellini, M. J. Chem. Soc., Dalton Trans. 1982, 1587. (c) Maresca, L.;
Natile, G. Comments Inorg. Chem. 1994, 16, 95 and references therein.
(4) Sasaki, S.; Maruta, K.; Ohkubo, K. Inorg. Chem. 1987, 26, 2499.
(5) (a) Eisenstein, O.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4308. (b)
Blo¨chl, P. E.; Togni, A. J. Am. Chem. Soc. 2000, 122, 4098.
(6) The reactivity of cationic acetylene complexes of Pt(II) was interpreted
in terms of platinum-induced carbonium ions, see: Chisholm, M. H.;
Clark, H. C. J. Am. Chem. Soc. 1972, 94, 1532. Acc. Chem. Res. 1973, 6,
202.
(7) (a) De Renzi, A.; Panunzi, A.; Vitagliano, A. Chem. Commun. 1976, 47.
(b) Sen, A.; Lai, T.-W. J. Am. Chem. Soc. 1981, 103, 4627.
(8) Albietz, P. J.; Yang, K.; Lachicotte, R. J.; Eisenberg, R. Organometallics
2000, 19, 3543.
(9) Oi, S.; Tsukamoto, I.; Miyano, S.; Inoue, Y. Organometallics 2001, 20,
3704 and references therein.
(10) (a) Quyoum, R.; Wang, Q.; Tudoret, M.-J.; Baird, M. C. J. Am. Chem.
Soc. 1994, 116, 6435. (b) Ewart, S. W.; Baird, M. C. Top. Catal. 1999,
7, 1. (c) Baird, M. C. Chem. ReV. 2000, 100, 1471.
(11) (a) Hahn, C.; Vitagliano, A.; Giordano, F.; Taube, R. Organometallics
1998, 17, 2060. (b) Hahn, C.; Morvillo, P.; Vitagliano, A. Eur. J. Inorg.
Chem. 2001, 419.
reasonably preferred since it produces a carbocation at least as stable
as B.19 At this point the capture of a vicinal hydride to form the
delocalized cation C, with final slipping to the ground-state product
2, is the expected downhill evolution of the system. The final
displacement of the coordinated olefinic product giving back the
starting complex 1 is straightforward, being thermodynamically
favored (Keq ) 4 ( 1, as determined by H NMR on the isolated
complexes 2) and kinetically smooth (half-life 1-2 min under the
conditions of the catalytic reaction).
In the above mechanism the Pt2+ ion plays the double role of
activating the ethylene molecule in the transition state A and
stabilizing the transition state C, thereby reducing the occurrence
of possible side reactions of the carbocation B. The latter are most
likely responsible for the deactivation of the catalyst, accompanied
by the release of protons16 and the formation of stable11 and
unreactive Pt-C σ-bonded species.15 The mechanism in Scheme 1
is also consistent with the failure of isobutylene to give an
hydrovinylation product, since the first step in the evolution of B
(the capture of R ) H) would produce a secondary carbocation
and thus be disfavored. Acid-catalyzed homodimerization was
therefore the prevailing reaction.
Although many issues need to be addressed by further experi-
ments (including the very existence of the postulated intermediate
B and the possible role of solvent and counterion), our results have
shown that ethylene, when coordinated to a highly electrophilic
metal center, displays a carbocationic reactivity that gives rise to
a catalytic hydrovinylation process. This newly found reaction
appears to be alternative or complementary to known metal-
catalyzed codimerizations occurring through different pathways.13
Its scope and potentiality are currently under investigation.
(12) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano, A. Organometallics
2002, 21, 1807.
(13) Although the metal-catalyzed hydrovinylation of olefins is known by far
(for a leading reference, see: Jolly, P. W.; Wilke, G. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W. A., Eds.; VCH: New York, 1996; Vol. 2, pp 1024-1048),
the known cases appear to involve insertion-elimination steps into M-H
bonds rather than direct attack of an external olefin on a coordinated double
bond, which seems to be unprecedented (see also Nomura, N.; Jin, J.;
Park, H.; RajanBabu, T. V. J. Am. Chem. Soc. 1998, 120, 459 and
references therein).
1
1
(14) Crystallized from CH2Cl2/Et2O. H NMR (400 MHz, CD2Cl2/CD3NO2):
diastereomer 2b′ δ 0.33 (d, 3H, CH3), 0.36 (d, 3H, CH3), 0.52 (d, 3H,
CH3), 1.55 (m, 1H, CH), 1.74, (m, 1H, CH), 4.24 (1H, m, dCHH), 4.52
(1H, m, dCHH), 4.75 (d ps t, 2H, PCHaHb), 5.07 (d ps t, 2H, PCHaHb),
5.4 (m, 1H, dCH), 7.6-8.0 (m, 22H, Ph, py), 8.08 (t, 1H, py).
Diastereomer 2b′′ δ 0.40 (d, 3H, CH3), 0.43 (d, 3H, CH3), 0.90 (d, 3H,
CH3), 1.18 (m, 1H, CH), 1.85, (m, 1H, CH), 4.42 (1H, m, dCHH), 4.56
(1H, m, dCHH), 4.66 (d ps.t, 2H, PCHaHb), 5.15 (br m, 2H, PCHaHb),
5.4 (m, 1H, dCH), 7.6-8.0 (m, 22H, Ph, py), 8.16 (t, 1H, py). Anal.
Calcd for C38H41B2F8NP2Pt: C, 48.43; H, 4.38; N, 1.49. Found: C, 48.17;
H, 4.49; N, 1.65.
(15) Unoptimized conditions: a mixture of 1 (87 mg, 0.10 mmol), anhydrous
MeNO2 (1.0 mL, free from nitriles), 2-methyl-2-butene (3.2 mL, 30 mmol),
giving two liquid phases, was stirred at 20 °C under ethylene at
atmospheric pressure. The reaction was monitored (1H NMR) by repeated
sampling (10-µL portions) of the upper hydrocarbon phase. After 7 h (85%
conversion), the upper layer was collected and the main product separated
by fractional distillation (3,4-dimethyl-1-pentene, 1.9 g, 65% yield based
on 2-methyl-2-butene). The lower layer (nitromethane phase) was
evaporated to dryness and the solid residue analyzed by 1H and 13C NMR
spectroscopy. The major component of the residue (ca. 40% of the total)
was identified as complex 2b. Although the second component (ca. 30%
of the total) was not identified, it was inferred to be a σ-bonded derivative
by a peak in the 13C NMR spectrum at δ ) -4.9, flanked by 195Pt satellites
(1JC-Pt ) 625 Hz).
(16) In one experiment, the catalytic reaction was run in CD3NO2 and directly
monitored by 1H NMR. A broad signal (due to traces of water) was
detectable, which slowly moved downfield (in the range 2-4 ppm),
suggesting a progressive protonation by slow degradation of the catalyst.
(17) The catalytic isomerization reaction was separately monitored by 1H NMR,
after adding a large excess (20 equiv) of 2-methyl-1-butene to an NMR
sample of 1. After 1 h at room temperature, a 90% conversion to the
internal isomer 2-methyl-2-butene was observed.
Acknowledgment. We thank the Deutsche Akademie der
Naturforscher Leopoldina (BMBF-LPD 9801-4) (C.H.) and the
MURST (PRIN 9803243241) for financial support. We also thank
the CIMCF, Universita` di Napoli Federico II, for the access to NMR
facilities.
(18) We can therefore exclude that water might interact with the metal ion
and promote the formation of carbocationic active species by acting as a
Brønsted acid.8,10
(19) Fry, J. L.; Karabatsos, G. J. In Carbonium Ions; Olah, G. A., von Schleyer,
References
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