Reversible cleavage of the C-H bond of aldimine with platinum complexes

Article abstract of DOI:10.1021/om020638e

The reversible cleavage of the C-H bond of aldimine with platinum complexes was studied. For the C-H oxidative addition to occur a heteroatom capable of coordinating to the metal center was necessary in the aldimine molecule. Systematic examinations on the thermodynamics and kinetics of both C-H bond oxidative addition and reductive elimination and catalytic application were presented.

Full text of DOI:10.1021/om020638e

5464
Organometallics 2002, 21, 5464-5466
Communications
Rever sible Clea va ge of th e C-H Bon d of Ald im in e w ith
P la tin u m Com p lexes
Atsushi Ohtaka, Naohiro Kato, and Hideo Kurosawa*
Department of Applied Chemistry & Frontier Research Center, Faculty of Engineering,
Osaka University, Suita, Osaka 565-0871, J apan
Received August 6, 2002
Summary: The intramolecular oxidative addition of the
C-H bond of N-(2-(diphenylphosphino)benzylidene)-2-
phenylethylamine (PN) in Pt(PN)3 (1) to give cis-Pt[η2-
o-P(Ph)2C6H4CdN(CH2)2Ph](H)(PN) (3) proceeded re-
versibly under mild conditions. The enthalpy and the
entropy of the equilibrium between 1 and (3 + PN)
(∆H° ) 38.0(8) kJ / mol, ∆S° ) 23.2(6) eu) suggested that
the oxidative addition of the imine C-H bond to Pt(0)
is unfavorable in terms of the enthalpy but is driven by
the contribution of the entropy.
monitored by 31P NMR spectroscopy (eq 1). The forma-
The scission of a C-H bond by transition-metal
complexes1 has received much interest, with oxidative
addition of alkyl-H and aryl-H bonds being among the
key issues.2-5 For the cleavage of the C-H bond of
aldimine, some examples have been reported in catalytic
reactions,6-9 but only a limited number of reports have
been published on stoichiometric reactions.10-12 Accord-
ing to these reports, for the C-H oxidative addition to
occur a heteroatom capable of coordinating to the metal
center is necessary in the aldimine molecule. In this
paper we report the cleavage of the imine C-H bond of
N-(2-(diphenylphosphino)benzylidene)-2-phenylethyl-
amine (PN)13 with a Pt(0) complex under mild condi-
tions. Particularly notable is that the present system
provides the rare case where C-H oxidative addition
and reductive elimination proceed reversibly, thereby
allowing access to thermodynamic and kinetic features
of the C-H activation process.
tion of three complexes was confirmed after 2 h; there
were observed two singlets at δ 42.5 (J Pt-P ) 4395 Hz)
and δ 29.8 (J Pt-P ) 3537 Hz) and two doublets centered
at δ 20.5 (J Pt-P ) 1891 Hz, J P-P ) 8 Hz) and δ 47.5
(J Pt-P ) 2078 Hz, J P-P ) 8 Hz). The first singlet was
determined to be due to Pt(PN)3 (1) by the elemental
analysis of a sample isolated separately14 and compari-
son of the 31P NMR spectrum with that of Pt(PPh3)3 (δ
49.9, J Pt-P ) 4438 Hz).15 The second singlet was
assumed to be due to Pt(nb)(PN)2 (2) by the comparison
of the 31P NMR spectrum with that of Pt(nb)(PPh3)2 (δ
33.7, J Pt-P ) 3550 Hz).16 It is also assumed that in 1
and 2 the N atom of PN does not coordinate to platinum.
The yields of 1 and 2 (32 and 64% based on platinum)
decreased gradually to 21 and 33%, respectively, after
11 days. In contrast, the amount of the complex showing
Initially, the reaction of Pt(nb)3 (nb ) norbornene)
with 3 equiv of PN at 25 °C was performed and
(1) Kakiuchi, F.; Murai, S. Topics in Organometallic Chemistry;
Springer: Berlin, Germany, 1999; p 47.
(2) J anowicz, A. H.; Bergman, R. G. J . Am. Chem. Soc. 1982, 104,
352.
(3) Rabinovich, D.; Parkin, G. J . Am. Chem. Soc. 1990, 112, 5381.
(4) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; J ones,
W. D.; Perutz, R. N. J . Am. Chem. Soc. 1993, 115, 7685.
(5) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J . L. J .
Am. Chem. Soc. 2001, 123, 12724.
(6) J un, C.-H.; Lee, H.; Park, J .-B.; Lee, D.-Y. Org. Lett. 1999, 1,
2161.
(7) J un, C.-H.; Hong, J .-B. Org. Lett. 1999, 1, 887.
(8) J un, C.-H.; Lee, D.-Y.; Lee, H.; Hong, J .-B. Angew. Chem., Int.
Ed. 2000, 39, 3070.
(9) J un, C.-H.; Chung, K.-Y.; Hong, J .-B. Org. Lett. 2001, 3, 785.
(10) Suggs, J . W. J . Am. Chem. Soc. 1979, 101, 489.
(11) Arce, A. J .; Sanctis, Y. D.; Hernandez, L.; Marquez, M.;
Deeming, A. J . J . Organomet. Chem. 1992, 436, 351.
(12) Yeh, W.-Y.; Yang, C.-C.; Peng, S.-M.; Lee, G.-H. J . Chem. Soc.,
Dalton Trans. 2000, 1649.
(14) The reaction of Pt(COD)2 with PN was used for the isolation of
1 and 3. This reaction proceeded faster than eq 1 to give 1 (80%) and
3 (20%) in 2 h, but no 31P signal assignable to Pt(COD)(PN)x (x ) 1, 2)
was observed in the course of the reaction. The complex 1 was obtained
by repeated recrystallization from the mixture of Pt(COD)2 and 3 equiv
of PN (reaction time 30 min). The complex
3 was obtained by
recrystallization from a reaction mixture of Pt(COD)2 and 3 equiv of
PN, which had been heated at 60 °C for 24 h. See the Supporting
Information for more details.
(13) Shirakawa, E.; Yoshida, H.; Takaya, H. Tetrahedron Lett. 1997,
38, 3759.
(15) Sen, A.; Halpern, J . Inorg. Chem. 1980, 19, 1073.
(16) Cobley, C. J .; Pringle, P. G. Inorg. Chim. Acta 1997, 265, 107.
10.1021/om020638e CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/09/2002

Communications
Organometallics, Vol. 21, No. 25, 2002 5465
F igu r e 2. van’t Hoff plot for the equilibrium between 1
and (3 + PN).
equilibrium constant from 25 to 55 °C (Figure 2) allowed
the determination of the enthalpy and the entropy as
∆H° ) 38.0(8) kJ /mol and ∆S° ) 23.2(6) eu. Thus, it is
evident that the oxidative addition of the imine C-H
bond to Pt(0) is unfavorable in terms of the enthalpy20
but is driven by the contribution of the entropy.
A precise kinetic study on the conversion of 1 into 3
in the presence of added PN was difficult, because the
solubility of PN was not sufficiently high to maintain
PN concentrations constant during the course of the
reaction. Nevertheless, it was found that the initial rate
of the disappearance of 1 (2.0 × 10-5 mol/(L‚min)
without added PN) decreases with the increase of added
PN (5.0 × 10-6, 2.8 × 10-6, and 1.7 × 10-6 mol/(L‚min)
at 8.3 × 10-3 mol/L of 1 and 8.3 × 10-3, 1.7 × 10-2, and
2.5 × 10-2 mol/L of PN, respectively). This fact suggests
that the dissociation of PN from 1 would occur to
produce Pt(PN)2 (4) before the oxidative addition of the
imine C-H bond of PN to Pt(0) takes place.21,22
F igu r e 1. ORTEP diagram of 3.
the two doublets (4% at 2 h) increased gradually and
reached 46% after 11 days. Its 1H NMR spectrum
showed a doublet of doublets centered at δ -1.7 with
satellites due to coupling to 195Pt (J H-P ) 28, 172 Hz,
J H-Pt ) 1242 Hz), suggesting formation of a Pt-H
bond.17 An analytically pure sample of this complex was
isolated14 and structurally characterized as cis-Pt[η2-o-
P(Ph)2C6H4CdN(CH2)2Ph](H)(PN) (3, ν(Pt-H) 2043
cm-1). A crystal of 3 suitable for X-ray diffraction was
obtained after recrystallization from toluene/hexane,
and Figure 1 confirmed that the oxidative addition of
the imine C-H bond of PN occurred.18
The reaction of 3 with dibenzylideneacetone (dba)
resulted in quantitative formation of Pt(dba)(PN)2 (5)
(eq 3). This reaction obeyed first-order kinetics with
When an isolated sample of 1 was dissolved in C6D6,
it gradually afforded 3 (50% yield at 25 °C for 2 h)
accompanied by the liberation of PN. After 2 days, no
further change of the ratio of 1, 3, and PN was observed,
suggesting that an equilibrium between 1 and (3 + PN)
is attained (eq 2). To confirm the equilibrium, we
respect to 3, and the rate constant (3.3 × 10-4 min-1
)
was independent of the amount of the added dba
(0.025-0.10 mol/L, initial concentration of 3 0.005-
0.015 mol/L). This suggests the rate-determining step
of eq 3 to be the reductive elimination from 3 to form 4,
followed by rapid quenching by dba to give 5 (Scheme
1). We assume the oxidative addition and the reductive
elimination involving the C-H bond take place through
a planar transition state, as shown in Chart 1, in accord
with a theoretical prediction.23
monitored the reaction of 3 with 1 equiv of PN at 25 °C
by 31P NMR spectroscopy. The formation of 1 was
observed in 7% and 28% yields (based on 3) after 5 h
and 8 days, respectively. The equilibrium constant (K)
of eq 2 was determined to be K ) 2.2 × 10-2 mol/L (25
°C in C6D6).19 The temperature dependence of the
(20) Halpern, J . Acc. Chem. Res. 1982, 15, 332.
(21) The oxidative addition of methyl iodide to Pt(PPh3)2 is more
facile than to Pt(PPh3)3: Pearson, R. G.; Rajaram, J . Inorg. Chem. 1974,
13, 246.
(22) The dependence of the rate of the disappearance of 1 on [PN]
may be similar for two kinetic schemes; one involves a rapid preequi-
librium for dissociation of PN from 1 to form 4 followed by slow C-H
oxidative addition, and the other, steady-state formation of 4. No
discrimination is possible at the moment.
(17) For comparison, the hydride peak of Pt(H)(Ph)(DPPE) was
observed at δ -1.0 (J H-P ) 16, 195 Hz, J H-Pt ) 1272 Hz): Crespo, M.;
Sales, J .; Solans, X.; Altaba, M. F. J . Chem. Soc., Dalton Trans. 1988,
1617.
(18) Crystal data of 3: space group P21/n (No. 14) with a ) 9.2410-
(2) Å, b ) 19.6169(4) Å, c ) 24.4580(5) Å, â ) 96.9851(8)°, Z ) 4, F )
1.48 g/cm3, R ) 0.032, Rw ) 0.060.
(19) The average of three measurements for mixtures containing
different concentrations of 1 and PN.
(23) Sakaki, S.; Mizoe, N.; Musashi, Y.; Biswas, B.; Sugimoto, M.
J . Phys. Chem. A 1998, 102, 8027.

5466 Organometallics, Vol. 21, No. 25, 2002
Communications
Ch a r t 1
examinations on the thermodynamics and kinetics of
both C-H bond oxidative addition and reductive elimi-
nation and catalytic application using 3 are now under
way.
Ack n ow led gm en t. Partial support of this work
through a Grant-in-Aid for Scientific Research, Ministry
of Education, Science and Culture, the J SPS research
fellowship for young scientists, and CREST of J apan
Science and Technology Corp. is gratefully acknowl-
edged.
Sch em e 1
Su p p or tin g In for m a tion Ava ila ble: Text, figures, and
tables giving experimental procedures, analysis data for the
compounds, and X-ray crystal data for 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
In summary, the oxidative addition of an imine C-H
bond to Pt(0) and the reductive elimination from 3
proceeded under mild conditions. Further systematic
OM020638E