Intermolecular propargyl/allenyl group transfer from Pd(II) to Pt(0) and Pt(II) to Pd(0). Key reaction in metal-catalyzed isomerization between propargyl and allenyl metal complexes

Article abstract of DOI:10.1016/S0022-328X(00)00787-7

Isomerization of phenyl-substituted propargylplatinum(II) complex, trans-Pt(CH₂C≡CPh)(Cl)(PPh₃)₂ (1) to allenyl complex, trans-Pt(CPh=C=CH₂)(Cl)(PPh₃)₂ (2) was found to be catalyzed by zerovalent complex Pd(PPh₃)₄. The reaction was proposed to proceed through the transfer of the propargyl/allenyl ligand both from Pt(II) to Pd(0) and Pd(II) to Pt(0). The former transfer, which seemingly has a thermodynamic disadvantage, has unambiguously been confirmed to take place; treatment of 1 with Pd(PPh₃)₄ or a mixture of Pd₂(dba)₃ and PPh₃ resulted in the formation of Pd(I) complex, Pd₂(μ-PhCCCH₂)(μ-Cl)(PPh₃)₂ which lies in equilibrium with a mixture of propargyl/allenylpalladium(II) and Pd(0) complexes.

Full text of DOI:10.1016/S0022-328X(00)00787-7

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Journal of Organometallic Chemistry 620 (2001) 190–193
Intermolecular propargyl/allenyl group transfer from Pd(II) to
Pt(0) and Pt(II) to Pd(0). Key reaction in metal-catalyzed
isomerization between propargyl and allenyl metal complexes
Sensuke Ogoshi *1, Takuma Nishida, Yoshiaki Fukunishi, Ken Tsutsumi,
Hideo Kurosawa *2
Department of Applied Chemistry, Faculty of Engineering, Osaka Uni6ersity, Suita, Osaka 565-0871, Japan
Received 24 August 2000; received in revised form 21 September 2000; accepted 5 October 2000
Abstract
Isomerization of phenyl-substituted propargylplatinum(II) complex, trans-Pt(CH2CꢀCPh)(Cl)(PPh3)2 (1) to allenyl complex,
trans-Pt(CPhꢁCꢁCH2)(Cl)(PPh3)2 (2) was found to be catalyzed by zerovalent complex Pd(PPh3)4. The reaction was proposed to
proceed through the transfer of the propargyl/allenyl ligand both from Pt(II) to Pd(0) and Pd(II) to Pt(0). The former transfer,
which seemingly has a thermodynamic disadvantage, has unambiguously been confirmed to take place; treatment of 1 with
Pd(PPh3)4 or a mixture of Pd2(dba)3 and PPh3 resulted in the formation of Pd(I) complex, Pd2(m-PhCCCH2)(m-Cl)(PPh3)2 which
lies in equilibrium with a mixture of propargyl/allenylpalladium(II) and Pd(0) complexes. © 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Intermolecular group transfer; Metal-catalyzed isomerization; Propargyl and allenyl metal complexes
1. Introduction
and more surprisingly, could involve a thermodynami-
cally unfavorable [4] redox transmetalation between
propargyl/allenylplatinum(II) and Pd(0) complexes. Ev-
idence for occurrence of this unique transmetalation
giving the propargyl complex of the Pd(I)–Pd(I) unit
will be described. A thermoneutral redox process in-
volving Co(III) to Co(I) transfer of a propargyl ligand
accompanying linkage isomerization [2a] and Pd(II) to
Pd(0) transfer of an allyl ligand accompanying configu-
rational inversion at the allylic sp3 carbon [5], or a
downhill redox process involving Pd(II) to Pt(0) allyl
transfer [5a,b] have been described.
Interconversion between h1-propargyl and h1-allenyl-
metal linkages has received special attention due to its
inherent importance in catalytic transformations pro-
ceeding through propargyl/allenylmetal intermediates
[1,2]. In the course of our recent study on spontaneous
reversible isomerization of trans-Pt(CH2CꢀCPh)-
(Cl)(PPh3)2 (1) to trans-Pt(CPhꢁCꢁCH2)(Cl)(PPh3)2 (2)
[3a], we found that this isomerization can be catalyzed
by Pt(0) complexes with considerable ease [3b]. The
catalyzed isomerization may proceed through the trans-
fer of the propargyl/allenyl ligand from Pt(II) to Pt(0)
with a concomitant shift of the metal-binding site with
regard to this ligand. We describe here a superficially
analogous catalyzed isomerization between 1 and 2 in
the presence of Pd(0) complexes, which however pro-
ceeds much more rapidly than the Pt(0)-catalyzed one,
2. Results and discussion
In contrast to the modest acceleration of the isomer-
ization of 1 to 2 on addition of 10 mol% of Pt(PPh3)4 to
a C6D6 solution of 1 (rate constant for the catalyzed
reaction, kcat=1.6×10−5 s−1 at 70°C in C6D6;
E-mail
address:
kurosawa@ap.chem.eng.osaka-u.ac.jp
(H.
k
uncat=1.6×10−6 −1) [3], the catalytic efficiency of
s
Kurosawa).
1 *Corresponding author.
the Pd(0) complex, Pd(PPh3)4 was much larger. For
example, conversion of 1 to an equilibrium mixture
2 *Corresponding author. Fax: +81-6-68797394
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00787-7

S. Ogoshi et al. / Journal of Organometallic Chemistry 620 (2001) 190–193
191
1/2=5/95 was completed within 1 h in C6D6 at 70°C in
the presence of 10 mol% of Pd(PPh3)4. This result can
be compared with the half-life (120 h) of the uncata-
lyzed reaction. Moreover, the degree of the acceleration
um(II) complex [Pd(h3-PhCCCH2)(PPh3)2]BF4 [5a,7]
failed to undergo clean redox transmetalation with
Pt(C2H4)(PPh3)2.
In all of the redox transmetalations involving h1-
propargyl or h1-allenyl ligand described above, the first
step would be the p-complexation of the CꢀC or CꢁC
bond with Pt(0), followed by the release of Pd(0) which
had originally formed the h1-Pd–C bond. Alterna-
tively, an SN2% path without the p-complexation could
complete the redox transmetalation.
was estimated to amount to ca. 104; compare kcat
=
2.8×10−4 s−1 in the presence of 5 mol% (5.7×10−4
M) of Pd(PPh3)4 with kuncat=6×10−8 −1, both at
s
40°C in C6D6. As shown in Scheme 1, we initially
thought that the Pd(0)-catalyzed isomerization of 1 to 2
(1)
The remaining problem is a feasibility and mecha-
nism of the first step of Scheme 1, i.e. Pt(II)–Pd(0)
transmetalation which appears to possess a thermody-
namic disadvantage. For example, the cationic h3-al-
lylplatinum(II) complex, [Pt(h3-C3H5)(PPh3)2]BF4 did
not react at all with Pd(PPh3)4 [5b]. Furthermore, theo-
retical calculations suggested [4] that different ground
state electronic configurations of the Pd atom (d10) and
Pt atom (d9s1) lead to the much larger exothermicity of
reductive elimination of Pd(CH3)2 to Pd(0) and ethane
than that of Pt(CH3)2 to Pt(0) and ethane. Thus, ac-
cording to these results [4] we can estimate that a
proceeded via three steps, (1) redox transmetalation
between 1 and Pd(0), without regard to the mechanism,
to give allenylpalladium(II), trans-Pd(CPhꢁCꢁCH2)-
(Cl)(PPh3)2 (4), (2) interconversion between 4 and
propargylpalladium(II),
trans-Pd(CH2CꢀCPh)(Cl)-
(PPh3)2 (3), and (3) redox transmetalation between 3
and Pt(0) to give allenylplatinum(II).
Among these three steps, the second and the third
ones can be well expected to occur with a sufficiently
low activation energy. For example, linkage isomeriza-
tion in propargyl/allenylpalladium(II) systems is known
to be quite rapid [6]. We further confirmed rapid trans-
fer of propargyl/allenyl ligands from Pd(II) to Pt(0); a
mixture of 3 and 4 (3/4=25/75) was treated with
Pt(PPh3)4 or Pt(C2H4)(PPh3)2, giving rise to 1/2 (5/95)
in 97% or 83% yield, respectively, within 5 h at room
temperature in C6D6 (Eq. (1)). Likewise, treatment of
trans-Pd(CHꢁCꢁCH2)(Cl)(PPh3)2 with Pt(C2H4)(PPh3)2
gave 84% yield of trans-Pt(CHꢁCꢁCH2)(Cl)(PPh3)2.
These are similar to the allyl group transfer from Pd(II)
to Pt(0) [5a,b]. It should also be noted that in contrast
to the case of the allyl transfer where cationic h3-allyl-
palladium(II) complexes were far more reactive than
the neutral analogs [5a,b], cationic h3-propargylpalladi-
hypothetical
redox
transmetalation
between
Pt(CH3)2(PH3)2 and Pd(PH3)2 giving rise to
Pd(CH3)2(PH3)2 and Pt(PH3)2 is an extremely endother-
mic (−34 kcal mol−1) process. Nevertheless, the fol-
lowing fact encouraged us to carry out a detection or
the trapping experiment for propargyl/allenylpalladiu-
m(II) species if they formed even in a very small
amount in the first step of Scheme 1; that is, in the
reaction mixture from 3/4 and Pt(PPh3)4 described
above, the propargyl/allenyl ligand still remained asso-
ciated with Pd in the form of Pd(I) dinuclear complex,
Pd2(m-PhCCCH2)(m-Cl)(PPh3)2 (5), though in a very
small amount, which is known [8] to lie in equilibrium
with 3/4. Then, we examined reactions of 1 with some
Pd(0) complexes in detail.
When heated in C6D6 with two equivalents Pd(PPh3)4
at 70°C in 2 h, 1 transferred 9% of its organic ligand to
palladium to give 5, with the balance of the ligand
remaining in the equilibrium mixture of 1 and 2 (Eq.
(2)). The formation of Pt(0) species was confirmed by
31P-NMR measurements in the form of Pt(PPh3)3. In
addition, 1H- and 31P-NMR analysis of the reaction
mixture from 1, Pd2(dba)3 (one equivalent) and PPh3
(three equivalents) in C6D6 at 70°C revealed the forma-
tion of both 5 (33%) and Pt(dba) (PPh3)2 (50%). The
discrepancy between amounts of these two products
could have been due to unknown decomposition
Scheme 1.

192
S. Ogoshi et al. / Journal of Organometallic Chemistry 620 (2001) 190–193
path(s) of 3/4 or 5 under the reaction condition. A total
of 43% of the propargyl/allenyl ligand was recovered in
1/2. Unfortunately, in the above two experiments the
presence of 3 or 4 was not confirmed. In view of the
ready conversion of 3/4 to 5 which bears considerably
increased thermal stability compared to the former [8],
the propargyl/allenylpalladium(II) intermediates 3 and
4, even if formed from 1 and Pd(0), would have been
trapped by another Pd(0) unit rapidly and efficiently to
give 5.
Alternatively, it is possible that the direct ligand
transfer from Pt(II) to Pd(0) to give 3 and 4 does not
actually take place. In such a case, the formation of 5 in
Eq. (2) might have occurred via initial attack of Pd(0)
at the Pt atom of 1 affording the Pd–Pt bonded
transient intermediate PdPt(m-PhCCCH2)(m-Cl)(PPh3)2,
followed by replacement of the Pt atom in this complex
by another Pd(0). However, we gained no spectral
evidence for the existence of such a heterodinuclear
complex [9]. In any case, as far as 3 and 4 lie in
equilibrium with 5, though in very small amounts, they
could participate as intermediates in Scheme 1. Or it is
also possible that the dinuclear complex 5 substitutes
for 3 and 4 as intermediate in Scheme 1 since 5 also
reacted readily with Pt(PPh3)4 to give the equilibrium
mixture of 1 and 2.
shifts are given in ppm using TMS or H3PO4 as a
standard.
3.2. Preparation of trans-Pd
(CH2CꢀCPh/CPhꢁCꢁCH2)(Cl)(PPh3)2 (3/4)
To a suspension of 2.82 g (2.44 mmol) of Pd(PPh3)4
in 120 ml of THF was added 523.8 mg (3.48 mmol) of
PhCꢀCCH2Cl at room temperature (r.t.) and the mix-
ture changed to a yellow solution within 10 min. After
40 min, addition of pentane to the solution and filtra-
tion yielded yellow solids (1.17 g, 62%). M.p. (dec.)
136–140°C; 1H-NMR (C6D6) l 1.89 (brs, 2H), 3.78
(brs, 2H); 31P-NMR (C6D6) l 24.45 (s), 27.79 (brs);
Anal. Calc. for C45H37ClP2Pd: C, 69.15; H, 4.77.
Found: C, 69.05; H, 5.01%.
3.3. Preparation of trans-Pt(CHꢁCꢁCH2)(Cl)(PPh3)2
The procedure was similar to that for 1 reported
before [3b]. Yield 13%; m.p. (dec.) 205–210°C; 1H-
NMR (C6D6) l 3.20 (dt, JPH=3.5 Hz, JHH=6.5 Hz,
J
PtH=56.7 Hz, 2H), 5.31 (tt, JPH=4.3 Hz, JHH=6.5
Hz, JPtH=117.5 Hz, 1H); 31P-NMR (C6D6) l 23.91 (s,
PtP=3035 Hz); Anal. Calc. for C39H33ClP2Pt: C,
J
58.98; H, 4.19. Found: C, 58.75; H, 4.33%.
3.4. Preparation of trans-Pd(CHꢁCꢁCH2)(Cl)(PPh3)2
The procedure was similar to that for 3/4 (at −15°C,
in CH2Cl2). Yeld 80%; m.p. (dec.) 147–152°C; 1H-
NMR (C6D6) l 3.48 (dt, JPH=2.4 Hz, JHH=6.2 Hz,
2H), 5.01 (tt, JPH=6.2 Hz, JHH=6.2 Hz, 1H); 31P-
(2)
In summary, we have demonstrated facile transfer of
propargyl/allenyl ligands not only between Pd(II) and
Pt(0), but Pt(II) and Pd(0) (two equivalents). The redox
transmetalation is expected to be more commonly en-
countered during zerovalent metal-catalyzed transfor-
mations of propargyl and allenyl substrates and its
facility might have a crucial influence on the regiochem-
ical and stereochemical outcome of the catalytic reac-
tions [10]. Further efforts related to this problem are
under way in this laboratory.
NMR (C6D6)
l
24.45 (s); Anal. Calc. for
C39H33ClP2Pd: C, 66.40; H, 4.71. Found: C, 66.11; H,
4.82%.
3.5. Kinetic measurements of isomerizations
A solution of 9.3 mg of 1 (0.0107 mmol) and 0.6 mg
of Pd(PPh3)4 (0.000519 mmol) in 0.9 ml of degassed dry
C6D6 was heated at 40°C. The isomerization was moni-
tored by 1H-NMR spectroscopy measured at 25°C.
Plots of ln 95/(p−5) versus time gave straight lines (p:
molar percent of the propargyl isomer), from whose
slope k1+k−1 can be obtained.
3. Experimental
3.1. General procedures and measurements
Most of the commercially available reagents were
used without further purification. All reactions and
manipulations of air- and moisture-sensitive com-
pounds were carried out under an atmosphere of dry
Ar by use of standard vacuum line techniques. Melting
points were determined on a Yanagimoto 1493 micro-
melting-point apparatus. NMR spectra were obtained
on a JEOL GSX-270 and JEOL GSX-400. Chemical
3.6. Reaction of trans-Pd(CH2CꢀCPh/CPhꢁCꢁCH2)-
(Cl)(PPh3)2 (3/4) with Pt(C2H4)(PPh3)2
A total of 8.7 mg (0.0111 mmol) of 3/4 and 9.3 mg
(0.0124 mmol) of Pt(C2H4)(PPh3)2 in an NMR tube
were dissolved in 0.9 ml of degassed dry C6D6. The
NMR tube was sealed under vacuum, and left at r.t.
1
The reaction was followed by H-NMR. Compound 1

S. Ogoshi et al. / Journal of Organometallic Chemistry 620 (2001) 190–193
193
was obtained after 1.5 h (83%) accompanied with 5
31P-NMR. Compound 5 was obtained after 1 h (33%)
(17%).
accompanied with Pt(dba)(PPh3)2 (50%).
3.7. Reaction of trans-Pd(CH2CꢀCPh/CPhꢁCꢁCH2)-
(Cl)(PPh3)2 (3/4) with Pt(PPh3)4
Acknowledgements
Partial support of this work through Grants-in-Aid
for Scientific Research from Ministry of Education,
Science and Culture, Japan is gratefully acknowledged.
A total of 8.8 mg (0.013 mmol) of 3/4 and 15.3 mg
(0.0123 mmol) of Pt(PPh3)4 in an NMR tube were
dissolved in 0.7 ml of degassed dry C6D6. The NMR
tube was sealed under vacuum, and left at r.t. The
reaction was followed by H-NMR. Compound 1 was
obtained after 5 h (97%) accompanied with 5 (3%).
1
References
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3.8. Reaction of trans-Pd (CHꢁCꢁCH2)(Cl)(PPh3)2
with Pt(C2H4)(PPh3)2
A
total of 8.0 mg (0.0113 mmol) of trans-
Pd(CHꢁCꢁCH2)(Cl)(PPh3)2 and 9.5 mg (0.0127 mmol)
of Pt(C2H4)(PPh3)2 in an NMR tube were dissolved
under an atmosphere of argon in 0.6 ml of dry C6D6.
The reaction was followed by 1H-NMR. trans-
Pd(CHꢁCꢁCH2)(Cl)(PPh3)2 was obtained after 10 min
(84%).
3.9. Reaction of trans-Pt(CH2CꢀCPh)(Cl)(PPh3)2 (1)
with Pd(PPh3)4
A total of 8.7 mg (0.0100 mmol) of 1 and 24.3 mg
(0.0210 mmol) of Pd(PPh3)4 in an NMR tube were
dissolved in 0.8 ml of degassed dry C6D6 and an
appropriate amount of trioxane was added as internal
standard. The NMR tube was sealed under vacuum,
and heated at 70°C in an oil bath. The reaction was
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rapid isomerization between propargylpalladium and allenylpal-
ladum complexes, the results clearly indicate the occurrence of
this step.
1
followed by H-NMR. Compound 5 was obtained after
1 h (9%). In a similar reaction in toluene-d8 Pt(PPh3)3
(31P-NMR: l 50.3 in toluene-d8) was observed at
−80°C.
3.10. Reaction of trans-Pt(CH2CꢀCPh)(Cl)(PPh3)2 (1)
with Pd2(dba)3 and PPh3
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of PPh3 in an NMR tube were dissolved in 0.8 ml of
degassed dry C6D6 and an appropriate amount of
trioxane was added as internal standard. The NMR
tube was sealed under vacuum, and heated at 70°C in
an oil bath. The reaction was followed by 1H- and
[8] S. Ogoshi, K. Tsutsumi, M. Ooi, H. Kurosawa, J. Am. Chem.
Soc. 117 (1995) 10415.
[9] A similar allyl heterodinuclear (Pd–Pt) complex was reported.
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Y. Kai, S. Sakaki, K. Takeuchi, Organometallics 15 (1996) 2089.
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