Mechanism of catalytic chalcogen atom replacement of phosphine chalcogenides and separation of the intermediate phosphine

Article abstract of DOI:10.1246/cl.2009.18

The reaction mechanism of the chalcogen atom replacement of phosphine chalcogenides has been revealed to be dissociative from a kinetic investigation for the reaction of phosphine selenide with sulfur. The dissociation is enthalpically promoted by the catalysis of Pd⁰. For a bidentate phosphine chalcogenide, the intermediate phosphine was separated as the Pd ^II complex by the catalysis and oxidative addition of Pd⁰ complex. The novel catalytic replacement and dissociation of chalcogen atom are applicable to regeneration of phosphines from their oxides via the phosphine sulfides. Copyright

Full text of DOI:10.1246/cl.2009.18

18
Chemistry Letters Vol.38, No.1 (2009)
Mechanism of Catalytic Chalcogen Atom Replacement of Phosphine Chalcogenides
and Separation of the Intermediate Phosphine
Sen-ichi Aizawa,ꢀ Arpi Majumder, Daisuke Maeda, and Akina Kitamura
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555
(Received September 29, 2008; CL-080935; E-mail: saizawa@eng.u-toyama.ac.jp)
The reaction mechanism of the chalcogen atom replacement
via a three-membered cyclic transition state and Mechanism 2
is an X-philic attack via a linear transition state. In the present
work, however, we have revealed from kinetic experiments
that the chalcogen atom replacement (R3P=X1 + X2 ꢀ
R3P=X2 + X1) proceeds via dissociation of X in conflict with
the above theoretical investigation. We have also attempt to sep-
arate the intermediate phosphine as evidence of the dissociative
mechanism and show the applicability of the catalytic dissocia-
tion to regeneration of phosphines from their oxides.
Kinetic measurements for the chalcogen atom replacement
reaction were carried out by monitoring the reaction of triphen-
ylphosphine selenide with excess sulfur in DMF in the presence
and absence of [Pd(dba)2] (dba = dibenzylideneacetone).4 By
following an increase in the 31P NMR signal intensity of phos-
phine sulfide formed, it was confirmed that the observed rates
are first-order with respect to the phosphine-selenide concentra-
tion.5 Since the rates were substantially slow especially in the ab-
sence of Pd0, the rate constants were obtained by the initial slope
method. The observed rate constants were independent of the
sulfur concentration and the temperature dependence of the rate
constants in Figure 1 gave the activation parameters: ꢀHz ¼
110 ꢁ 1 kJ molꢂ1 and ꢀSz ¼ 26 ꢁ 2 J molꢂ1 Kꢂ1 in the pres-
ence of Pd0 and ꢀHz ¼ 159 ꢁ 1 kJ molꢂ1 and ꢀSz ¼ 103 ꢁ 2
J molꢂ1 Kꢂ1 in the absence of Pd0.
of phosphine chalcogenides has been revealed to be dissociative
from a kinetic investigation for the reaction of phosphine sele-
nide with sulfur. The dissociation is enthalpically promoted by
the catalysis of Pd0. For a bidentate phosphine chalcogenide,
the intermediate phosphine was separated as the PdII complex
by the catalysis and oxidative addition of Pd0 complex. The
novel catalytic replacement and dissociation of chalcogen atom
are applicable to regeneration of phosphines from their oxides
via the phosphine sulfides.
Since phosphines have been regarded as effective ligands for
the Pd0 catalysts, phosphine-assisted Pd0 catalyses have been
widely employed in various coupling reactions for organic syn-
theses. However, while the substrate adducts of PdII formed dur-
ing the catalytic cycle are relatively stable, phosphines are usu-
ally susceptible to oxidation to give inactive Pd sediment after
the catalytic reactions are completed.1 It is difficult to practically
regenerate phosphines from the phosphine oxides for recycling,
so that even elaborate and valuable phosphines are unavoidably
discarded as phosphine oxides. Recently, we proposed that phos-
phine-sulfide groups can stabilize the Pd0 center thermodynami-
cally to form air-stable Pd0 catalysts even after consumption of
the substrates. By way of example, we reported recyclable air-
stable Pd0 complexes with tetradentate phosphine sulfide and
polymer-supported phosphine sulfide.2 In the course of this
study, we have found a new catalytic activity of Pd0 that pro-
motes chalcogen atom replacement of phosphine chalcogenides
(R3P=X, X = O, S, and Se). So far, two associative mecha-
nisms have been proposed for the chalcogen transfer from phos-
phine chalcogenides to phosphines by theoretical calculations as
shown in Scheme 1.3
The sulfur-concentration independency of the observed rate
constants and the positive ꢀSz values indicate dissociative acti-
vation. The smaller positive ꢀSz value in the presence of Pd0 is
attributed to the interaction between the phosphine chalcogenide
and Pd0 in the activation state in which the P=X bond breaking
is promoted enthalpically. The catalytic activation may be
caused by ꢀ back donation from Pd0 to the ꢀꢀ orbital of the
−36
−38
Mechanism 1 involves a nucleophilic attack of the phos-
phine phosphorus on the phosphine-chalcogenide phosphorus
Mechanism 1
−40
-
T
X
X
R
R
R'
R'
R'
R
R
R'
R'
R'
R
R
R'
R'
+
k
+
R
P
P
R
P
X
P
R
P
P
R'
−42
kh
-
X
R'
R
R
R'
R'
R'
+
R
R
P
R
P
R'
R'
R
P
X
+
P
−44
−46
Mechanism 2
R
R
R'
R'
R'
R
R
R
R'
R'
R'
R'
R'
+
-
P
X
X
P
R
P
X
P
+
R
P
X
P
R'
R
0.0026
0.0028
0.003
0.0032
R
T −1 K −1
/
R
R
R'
R'
R'
R
R'
+
-
R
P
X
P
R
P
+
P
R'
R'
Figure 1. Temperature dependence of the rate constants for the
chalcogen atom replacement reaction in the presence ( ) and
absence ( ) of the Pd0 catalyst.
R
Scheme 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.1 (2009)
19
P=X bond,6 which is supported by the fact that the ꢀ back-do-
nating Pt0 center of the corresponding Pt0 complex, [Pt(dba)2],7
also showed similar catalytic activity.8
2
3
4
S. Aizawa, M. Tamai, M. Sakuma, A. Kubo, Chem. Lett.
2007, 36, 130.
M. Kullberg, J. Stawinski, J. Organomet. Chem. 2005, 690,
2571.
Because phosphine-sulfide groups can stabilize the Pd0 cen-
ter electronically, the phosphine-sulfide Pd0 catalysts can be
reused in air.2 However, by a number of repetitions of the reac-
tion, the sulfur atoms in the phosphine-sulfide groups are grad-
ually replaced with oxygen atoms by the catalysis of Pd0 to give
the phosphine oxides, which deactivate the Pd0 catalysts by pre-
cipitation of Pd sediment. This problem can be solved by the
present catalytic replacement of chalcogen atoms. For example,
though the phosphine-sulfide groups in 1,2-bis(diphenylphosphi-
no)ethane disulfide (p2S2) of the substantially air-stable Pd0 cat-
alyst, [Pd(p2S2)(dba)],9 were gradually converted to phosphine
oxides by heating under reflux in DMF for several hours without
any substrate, the phosphine sulfide was regenerated by the ad-
dition of a large excess of sulfur to the DMF solution followed
by heating at 125 ꢃC for 2 h under N2. In the absence of Pd0,
the conversion between phosphine-sulfide and phosphine-oxide
groups can hardly proceed under the above conditions.
The phosphine intermediate, 1,2-bis(diphenylphosphino)-
ethane (p2), can be successfully separated as the chelate com-
pound of PdII by employing the novel catalysis of Pd0. The p2S2
complex, [Pd(p2S2)(dba)],9 was reacted with an excess of iodo-
benzene in DMF at 125 ꢃC for 48 h under N2. About one third of
p2S2 was converted to p2, which is coordinated to PdII showing a
31P NMR singlet at 65.9 ppm. The PdII complex was separated
chromatographically with an SiO2 column. The 31P NMR spec-
trum of the isolated PdII complex in chloroform was in agree-
ment with that of [PdI2(p2)]10 exactly, showing the singlet at
61.9 ppm. The formation of iodo PdII complex is attributed to
oxidative addition of the Pd0 complex with iodobenzene fol-
lowed by disproportionation.11 Almost quantitative formation
of p2 from p2S2 and isolation as the PdII complex was achieved
without the chromatographic separation by the subsequent addi-
tion of two equiv of [Pd(dba)2], which acts as a catalyst and a
source of [PdI2(p2)]. This result gave us evidence of the dissoci-
ation of the chalcogen atom from the phosphine chalcogenide
and revealed that the phosphine can be regenerated easily from
the phosphine sulfide by the catalysis of Pd0 and its oxidative
addition.12
The concentrations of triphenylphosphine selenide, sulfur,
and [Pd(dba)2] were 0.0212, 0.212–0.636, and 0.0106 or
0 mol kgꢂ1, respectively. The reaction temperature was
changed from 305 to 333 K for the catalytic reaction and
from 333 to 373 K for the considerably slow noncatalytic
reaction. The catalytic reaction rate was unchanged in
chloroform.
5
6
7
8
It was confirmed that the 31P NMR signal intensity is propor-
tional to the phosphine-chalcogenide concentration.
´
J. A. Dobado, H. Martınez-Garcıa, J. M. Molina, M. R.
Sundberg, J. Am. Chem. Soc. 1998, 120, 8461.
´
W. J. Cherwinski, B. F. G. Johnson, J. Lewis, J. Chem. Soc.,
Dalton Trans. 1974, 1405.
By similar kinetic experiments, the sulfur-concentration in-
dependency of the observed rate constant and the activation
parameters, ꢀHz ¼ 116 ꢁ 1 kJ molꢂ1 and ꢀSz ¼ 32 ꢁ 2
J molꢂ1 Kꢂ1, similar to those for the Pd0 catalysis were
obtained for the Pt0 complex.
9
The phosphine sulfide Pd0 complexes, [Pd(p2S2)(dba)], was
obtained by the reaction of p2S2 with equimolar [Pd(dba)2]
in chloroform at room temperature followed by the addition
of diethyl ether. 31P{1H} NMR (CHCl3): ꢁ (relative to
1
D3PO4 in external D2O) 44.2 (s). H NMR (CHCl3): ꢁ 1.25
3
(t, CH3– of diethyl ether, JH{H ¼ 2:8 Hz), 2.72 (d,
–CH2CH2– of p2S2), 3.72 (q, –CH2– of diethyl ether,
3
3JH{H ¼ 2:8 Hz), 7.10 (d, Ph–CH=CH– of dba, JH{H
¼
16 Hz), 7.26 (s, CHCl3), 7.40–7.43 and 7.61–7.65 (m, Ph
of dba) 7.43–7.50 and 7.77–7.83 (m, Ph of pp3S4), 7.75 (d,
Ph–CH=CH– of dba, 3JH{H ¼ 16 Hz). The catalytic activity
for the cross-coupling reactions is comparable to or better
than that of commonly used [Pd(PPh3)4], which is unstable
under aerobic conditions.
10 W. Oberhauser, C. Bachmann, T. Stampfl, R. Haid, P.
Bruggeller, Polyhedron 1997, 16, 2827.
¨
11 The phosphine oxide was mainly formed without iodoben-
zene because p2 was not trapped by PdII and consequently
oxidized. When the PdII complex such as [Pd(CH3CN)4]-
(BF4)2 was used to trap the phosphine instead of adding
iodobenzene, the conversion from p2S2 to p2 was hardly
observed probably due to the formation of the PdII complex
with p2S2 prior to the catalytic reaction of p2S2 with Pd0.
12 It is difficult to obtain pure phosphine directly from the phos-
phine oxide because the P=O bond is much stronger than the
P=S bond and the interaction between the phosphine-oxide
group and Pd0 is extremely weak.
We can conclude that the chalcogen atom replacement of
phosphine chalcogenides proceeds by dissociation of the chal-
cogen atoms, which is enthalpically promoted by the catalytic
interaction of Pd0. Taking advantage of this catalysis, it is pos-
sible to regenerate phosphines from phosphine oxides via phos-
phine sulfide formation. This catalytic conversion will be signif-
icant especially for elaborate and valuable phosphines.13
13 The regeneration of valuable optically active diphosphines,
2,20-bis(diphenylphosphino)-1,10-binaphthyl (BINAP) deriv-
atives, is under investigation and will be successful retaining
optical purity.
14 Supporting Information is also available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-
lett/index.html.
References and Notes
1
I. P. Beletskaya, A. V. Cheprakov, Chem. Rev. 2000, 100,
3009; A. Suzuki, J. Organomet. Chem. 1999, 576, 147;
R. F. Heck, Palladium Reagents in Organic Synthesis,
VCH, Weinheim, 1996; J. Tsuji, Palladium Reagents and
Catalysts, Wiley, Chichester, 1995.
Published on the web (Advance View) November 29, 2008; doi:10.1246/cl.2009.18