Chemoselectivity control in the reaction of a dinuclear chloro-bridged cyclopalladated complex with potassium diphenylphosphide

Article abstract of DOI:10.1016/j.jorganchem.2010.10.020

Reactions of KPPh₂ with the dimeric cyclopalladated complex {Pd(N∩C)(μ-Cl)}₂ 1 derived from N,N-dimethylbenzylamine were studied using different reagent ratios, temperature, reaction time, concentrations and solvents. Complex 3, the μ-Cl-μ-PPh₂ analog of 1, was obtained in 76% yield in the one-hour reaction using the 1:1 ratio of dimer 1 and KPPh₂ in THF at rt. ortho-(Diphenylphosphino)benzylamine 4 was synthesized in 58% yield by reacting complex 1 with 4.5 equiv. of KPPh₂ in THF at rt. A possible mechanism of the aminophosphine formation is proposed on the basis of the experimental data including ³¹P{¹H} NMR spectra of reaction mixtures.

Full text of DOI:10.1016/j.jorganchem.2010.10.020

Journal of Organometallic Chemistry 696 (2011) 871e878
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Chemoselectivity control in the reaction of a dinuclear chloro-bridged
cyclopalladated complex with potassium diphenylphosphide
,
Valeria A. Stepanova a, Valery V. Dunina b, Irina P. Smoliakova a
*
a Chemistry Department, University of North Dakota, 151 Cornell St., Stop 9024, Grand Forks, ND 58202-9024, USA
b Chemistry Department, M. V. Lomonosov Moscow State University, 1 Leninskie Gory, Moscow 119991, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Reactions of KPPh2 with the dimeric cyclopalladated complex {Pd(NXC)(m-Cl)}2 1 derived from N,N-
Received 20 July 2010
Received in revised form
3 September 2010
Accepted 7 October 2010
Available online 14 October 2010
dimethylbenzylamine were studied using different reagent ratios, temperature, reaction time, concen-
trations and solvents. Complex 3, the -Cl- -PPh2 analog of 1, was obtained in 76% yield in the one-hour
m
m
reaction using the 1:1 ratio of dimer 1 and KPPh2 in THF at rt. ortho-(Diphenylphosphino)benzylamine 4
was synthesized in 58% yield by reacting complex 1 with 4.5 equiv. of KPPh2 in THF at rt. A possible
mechanism of the aminophosphine formation is proposed on the basis of the experimental data
including 31P{1H} NMR spectra of reaction mixtures.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Cyclopalladated complexes
KPPh2
Aminophosphines
Palladium(II) phosphido complexes
1. Introduction
method, (ii) the presence of byproducts, such as LiCl and PhLi, (iii)
solvent polarity, (iv) concentration and (v) age of the phosphide
One of the most promising applications of cyclopalladated
complexes (CPCs) is ligand transformation using reactions of the
PdeC bond [1,2]. The most studied reagents in these reactions are
alkynes,allenes, alkenes,andcarbonmonoxide, althoughisocyanides,
acyl halides, halogens and a few other types of compounds have been
used as well [1,2]. Our group has been interested in reactions of CPCs
with alkali metal phosphides because of possible transformations of
cyclopalladated complexes into aminophosphines, diphosphines and
related compounds containing a PR2 group and another functionality.
Previously, brief studies by Sokolov et al. [3,4], Bolm et al. [5] and
Dunina et al. [6] revealed that PPh3 adducts of C,N-cyclopalladated
complexes react with LiPPh2 or KPPh2 to yield either the corre-
sponding aminophosphines or their Pd(0) complexes. Recently, we
reported a detailed investigation of the LiPPh2 reactions with the
dinuclear chloro-bridged CPC 1 and showed that three products are
formed in these transformations: (1) the corresponding mononuclear
derivative 2 with Ph2P(CH2)4OH as an auxiliary ligand, (2) complex 3,
reagent. Transformations of complex 1 upon the action of the
LiPPh2 reagents were also sensitive to the addition order and
reaction time. In spite of preparing aminophosphine 4 in excellent
yields, the reactions were difficult to control because even minor
changes provided different results. It is noteworthy that the use of
three LiPPh2 solutions in THF purchased from SigmaeAldrich Co.
provided varied and often irreproducible data.
One can expect that, due to different coordination properties of
lithium and potassium cations, replacing the former for the latter in
the phosphide reagent should significantly decrease the problem of
controlling the reagent’s structure and, therefore, its reactivity [8].
Monomeric, dimeric, trimeric and polymeric structures have been
reported for LiPR2 reagents in solid state and solution [9e16];
however, KPPh2 in solution is most likely to have a monomeric
form, though other structures cannot be completely ruled out.
While organolithium reagents, in general, can readily form mixed
aggregates, related species are less feasible for KPPh2. Coordination
of KPPh2 to THF and other ethereal solvents is expected to be
significantly weaker [17e19]; therefore, in common solvents
including THF and toluene, this reagent is likely to have the same
structure. By replacing lithium for potassium, possible catalysis by
LiCl, reported for some reactions with organolithium reagents,
would be impossible [20]. In addition, due to the high sensitivity of
the LiPPh2 reactions with CPCs to minor changes, we were unable
which is the m-Cl-m-PPh2 analog of dimer 1, and (3) ortho-(diphenyl-
phosphino)benzylamine 4 (Scheme 1) [7].
Compounds 2e4 were formed selectively by varying the struc-
ture of the LiPPh2 reagent, which was affected by (i) the preparation
* Corresponding author. Tel.: þ1 701 777 3942; fax: þ1 701 777 2331.
E-mail address: ismoliakova@chem.und.edu (I.P. Smoliakova).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.020

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V.A. Stepanova et al. / Journal of Organometallic Chemistry 696 (2011) 871e878
Scheme 1.
to obtain reproducible data while monitoring the transformations
using 31P NMR spectroscopy. With the goal of investigating the
possibility and practical convenience of using KPPh2 in reactions
with dinuclear chloro-bridged CPCs, we undertook the study of the
reactions of this reagent with complex 1, and herein we report our
findings.
For comparison, in our previous study, it was determined that the
reaction of complex 3 with LiPPh2 in THF leads to the amino-
phosphine; the same reaction in toluene provided a complex
mixture of unidentified products [7]. These results suggest that the
m-PR2-m-Cl complexes can be intermediates in the formation of
aminophosphines from the corresponding
m-Cl dimeric CPCs and
metal phosphides in a coordinating solvent.
2. Results and discussion
2.4. 31P{1H} NMR spectra
2.1. The KPPh2 reagent
To shed more light on a possible mechanism of the described
transformations, reactions of CPC 1 with KPPh2 were monitored
using 31P{1H} NMR spectroscopy. Particularly, two reaction
mixtures containing the 1:4.5 ratio of CPC 1 and KPPh2 in d8-THF
were monitored: one sample at ꢀ42 ꢁC over 5 h (Fig. 1) and the
other during a slow warm-up from ꢀ40 ꢁC to rt. In the first
experiment, the spectra of the reaction mixture taken after mixing
the reagents and in 5 h were very similar and contained signals of
In this study, two 0.5 M solutions of KPPh2 in THF were used:
one purchased from SigmaeAldrich Co. and the other prepared in
our lab from K metal and ClPPh2 using a known procedure [21]. The
31P{1H} NMR spectra of both reagents in d8-THF were identical and
exhibited a broadened peak at
d
ꢀ20.5 ppm. The structure of the
KPPh2 reagents remained unchanged over a period of a year as
determined by the identical broadness and chemical shift of the 31P
signal. In reactions with CPC 1, the purchased and synthesized
KPPh2 solutions provided essentially the same results (vide infra)
and were used interchangeably.
aminophosphine 4 (
addition, there were three more significant signals: a broadened
singlet at ꢀ12.8 and
ꢀ50.6 ppm (B) and two doublets at
ꢀ13.5 ppm with JPP ¼ 80 Hz (C, Fig. 1). No signal of complex 3
30.2 ppm) was detected in the spectra of this reaction mixture.
d
ꢀ26.0 ppm) and Ph2PPPh2 (
d
ꢀ26.4 ppm). In
d
d
2.2. Factors affecting reactions of complex 1 with KPPh2
(d
Relative intensities of the major signals, C: aminophosphine 4:
Ph2PPPh2: B, in the spectra taken ca. 30 min after mixing the
First, reactions of the dimeric CPC 1 with KPPh2 were studied
using different ratios of the reagents (for details, see
Supplementary Material). The standard conditions included the use
of THF as a solvent, addition of KPPh2 to the CPC solution, a complex
concentration of 10 mg/mL (Co) and stirring for 18 h at rt. The
reaction with the 1:1 ratio of dimer 1 and KPPh2 (the 2:1 ratio of
reagents and in
5 h were similar: 1.00:0.73:0.20:1.36 and
1.00:0.77:0.20:1.14, respectively. These data suggest that at ꢀ42 ꢁC,
the composition of the 1:4.5 reaction mixture was practically the
same after 30 min and 5 h after mixing the reagents.
In the second experiment (see Supplementary Material for the
spectra), the initial spectrum resembled the ones obtained at
ꢀ42 ꢁC. When the sample was warmed up to ꢀ20 ꢁC, two new
Pd:PPh2) resulted in for formation of the m-Cl-m-PPh2 complex 3,
with no traces of aminophosphine 4. An increase in the reagent
ratio to 1:2 led to the same product, although in a reduced yield.
The further increase in the reagent ratio resulted in a change of
chemoselectivity and formation of aminophosphine 4 with no
traces of complex 3. The highest yield of 58% was obtained using
the 1:4.5 ratio. It is noteworthy that in the reactions furnishing
compound 4, tetraphenyldiphosphine monoxide was isolated as
well. Its formation is a result of oxidation of Ph2PPPh2, a byproduct
of aminophosphine 4 (see below).
The effects of reaction time and temperature were also evalu-
ated (for details, see Supplementary Material). The best yield of
complex 3, 76%, was obtained after stirring the 1:1 reaction mixture
for 1 h at rt. The highest yield of aminophosphine 4 was obtained at
rt after 18 h, while at ꢀ43 ꢁC this compound was isolated in less
than 5% yield.
doublets showed up at
d
ꢀ16.4 and ꢀ16.0 ppm and JPP ¼ 80 Hz.
When the temperature reached ꢀ10 ꢁC, the intensity of signal B at
d
ꢀ50.6 ppm decreased, and later, at 0 ꢁC, it disappeared. Signal B
must belong to an intermediate of this reaction containing either
one P atom or two of them with a very small coupling constant. Two
doublets at
doublets at
d
ꢀ13.4 and ꢀ12.8 ppm appeared first and the other two
d
ꢀ16.4 and ꢀ16.0 ppm showed up later; therefore,
these signals belong to two different compounds, with two P atoms
in each structure. The J value of the two sets of two doublets
suggests that two P atoms are likely to be separated by two bonds
and cis to each other. The two sets of two doublets disappeared
after warming the reaction mixture to rt. Therefore, the signals are
most likely to belong to intermediates, not impurities or byprod-
ucts. It is also notable that no signal of complex 3 (d 30.2 ppm) was
detected in the spectra of the reaction mixture. The signals of
Ph2PPPh2 (ca. ꢀ25.7 ppm) and aminophosphine 4 (ca. ꢀ25.5 ppm)
were observed in the spectrum taken at ꢀ20 ꢁC. However, the
Ph2PPPh2 signal was gradually vanishing during the warm-up: its
signal was overlapped with the singlet of aminophosphine 3 in the
spectrum taken at 0 ꢁC and became invisible at 20 ꢁC [22]. Two low-
intensity singlets at ca. 131 and 14 ppm in the spectrum recorded at
ꢀ20 ꢁC became more prominent at 0 and 20 ꢁC. We suggest that
these singlets may belong to the (Ph2P)2Pd(II) species formed as
a result of oxidative addition of Ph2PPPh2 to Pd(0) (see below).
2.3. Reactions of complex 3 with KPPh2
To determine whether complex 3 is an intermediate of the
aminophosphine synthesis, reactions of the former with 3.5 equiv.
of KPPh2 were performed at rt for 18 h in THF and toluene. The
reaction in THF yielded 25% of the aminophosphine, while in
toluene 52% of the desired compound was isolated along with 22%
of the unreacted complex 3. In the latter reaction, traces of THF
were present because KPPh2 was impossible to dissolve in toluene.

V.A. Stepanova et al. / Journal of Organometallic Chemistry 696 (2011) 871e878
873
31
Fig. 1. The P{1H} NMR spectrum of the 1:4.5 (dimer 1: KPPh2) reaction mixture in d8-THF at ꢀ42 ꢁC taken in ca. 30 min after mixing the reagents.
The 31P{1H} NMR spectrum of the reaction mixture with the 1:1
ratio of dimer 1 and KPPh2, taken at rt in ca. 30 min after mixing the
Pd:PPh2). In an attempt to observe intermediate A, the 31P NMR
spectrum of the 1:2 (dimer 1:KPPh2) reaction mixture was taken
(Fig. 3a). The spectrum contained a number of signals, including the
reagents, contained an intense singlet at
complex 3, another significant singlet at
doublets with the coupling constant, J, of 41.5 Hz at
d
30 ppm assigned to
d
ꢀ78 ppm, two peculiar
one at
singletsat
with J ¼ 40 Hz at
d
30 ppm assigned to complex 3. The most intense signals were
85andꢀ82ppm. Othernotablesignalsweretwodoublets
5 and 100 ppm. No signalofaminophosphine 4 was
d
107 and 6 ppm
d
and several low-intensity singlets (Fig. 2).
d
Previously, we suggested [7] that reactions of dimeric
m-Cl CPCs
detected in this experiment. Another spectrum of the same 1:2
reaction mixture was taken in 3 h and showed no notable changes.
When excess KPPh2 (more than 2 equiv.) was added to the same
reaction mixture (overall more than 4 equiv. of the phosphide per
1 equiv. of dimer 1), the spectrum changed significantly (Fig. 3b). The
with LiPPh2 leading to aminophosphines take place through the
formation of either intermediate A (in toluene without traces of
coordinating THF)or D(inTHF;seestructuresof A and D inScheme 2).
Formation of intermediate D would require 2 equiv. of KPPh2 per one
dimeric complex 1 (a 1:1 ratio of Pd:PPh2), while 4 equiv. of the
phosphide are necessary to obtain intermediate D (a 1:2 ratio of
singlet of aminophosphine 4 at
most prominent signal. Also, the singlets at 85 and ꢀ82 ppm and two
d
ꢀ29 ppm appeared and became the
Fig. 2. The 31P{1H} NMR spectrum of a 1:1 reaction mixture (dimer 1:KPPh2) in d8-THF at rt taken in ca. 30 min after mixing the reagents.

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V.A. Stepanova et al. / Journal of Organometallic Chemistry 696 (2011) 871e878
Scheme 2.
doublets at 5 and 100 ppm disappeared. In addition, two sets of two
low-intensity doublets, around 12e20 ppm showed up (Fig. 3b).
Either one of the sets or both of them were also observed in the
spectra of the 1:4.5 mixtures (see Fig.1 and Supplementary Material).
Related Pd(II) and Pt(II) complexes with two bridging diphenyl-
phosphido groups are known and the 31P NMR data for represen-
tative compounds of this kind (IeIV) are presented in Chart 1
d
[23e26]. Among
a
variety of reported diphosphido-bridged
-PPh2)}2, contains a cyclo-
complexes, only one of them, {LPt(
m
metallated ligand (HL ¼ benzo[h]quinoline) [27]. According to the
X-ray analysis, this complex has the anti geometry in the solid state.
Unfortunately, no other data are known for this compound because
it is insoluble in common solvents [27]. Intermediate A may have
either anti or syn geometry; the former complex is expected to be
2.5. Possible mechanism of the aminophosphine formation
On the basis of the NMR data and the results of the experiments
described above, we suggest a mechanism for the formation of
aminophosphine 4, which is depicted in Scheme 2. We propose that
the reaction between CPC 1 and KPPh2 begins with the formation of
more thermodynamically favorable considering
a significant
difference in ligands’ trans influence for both isomers [28]. The anti
m
-Cl-
m
-PPh2 complex 3. Reaction of this complex with a sec-
isomer is expected to give an upper-field singlet in the 31P NMR
ond equiv. of KPPh2 furnishes the di-
m-phosphido intermediate A.
Fig. 3. a) The 31P{1H} NMR spectrum of the 1:2 reaction mixture of dimer 1 and KPPh2 in d8-THF in 30 min after mixing at rt; and b) the spectrum after addition of excess KPPh2 to
the same 1:2 mixture at rt.

V.A. Stepanova et al. / Journal of Organometallic Chemistry 696 (2011) 871e878
875
Chart 1. Known representative Pd complexes containing the PPh2 group and their 31P NMR spectroscopy data [23e26,29e31]. (Coupling constants, J, are given in Hz; chemical shift
values, , are reported in ppm relative to 85% H3PO4).
d
spectrum (cf. Chart 1), while the syn dimer, if formed at all, is to
aminophosphine 4 in PhMe, while the same reaction in the coor-
give two doublets with significantly different chemical shift values.
dinating solvent THF took place with 4.5 equiv. of LiPPh2. Trans-
We suggest that the singlet at
mixture (Fig. 3a, cf. the singlet at
the anti isomer of intermediate A. The chemical shift of this inter-
mediate is close to those reported for the -PPh2 group in a number
d
ꢀ82 ppm in the spectrum of the 1:2
formation of the neutral intermediate A, {(NXC)Pd(
the anionic intermediate D, {(NXC)Pd(
1-PPh2)2}ꢀ, in THF contain-
ing byproduct KCl is likely to occur through the mononuclear
complex E, {(NXC)Pd(
1-PPh2)Q}, where Q is Clꢀ or THF. Due to the
m-PPh2)}2, into
d
ꢀ78 ppm in Fig. 2) may belong to
k
m
k
of Pd(II) and Pt(II) diphosphido-bridged complexes, such as IeIV
(Chart 1) [23e26].
absence of the literature data for closely related complexes, it is
impossible to reliably assign the proposed intermediates D and E to
particular signals in the 31P NMR spectra of the 1:4.5 reaction
mixture of dimer 1 and KPPh2 in THF (Fig. 1). For example, reported
chemical shifts for Pd(II) complexes with a terminal PAr2 group vary
(Chart 1, structures VI and XII) [30,31]; in addition, structures of
these complexes are significantly different from the proposed
intermediates D and E. Nevertheless, the peculiar singlet at
The 31P NMR signal of compound A (
d
ca. ꢀ80 ppm) was prom-
inent in the spectra of both 1:1 and 1:2 reaction mixtures at rt (dimer
1:KPPh2; Figs. 2 and 3a) [32]. However, the signal of A disappeared
upon addition of excess KPPh2 inTHFat rt (Fig. 3b). It is reasonable to
suggest that when a 1:4.5 ratio of dimer 1 and KPPh2 is used in THF,
intermediate {(NXC)Pd(k
1-PPh2)2}K (D) is formed. For comparison,
a number of Pt(II) and Pd(II) complexes with a terminal PR2 ligand
have been reported or proposed as reaction intermediates
d
ꢀ50.7 ppm (signal B in Fig. 2) maybelong to intermediate E. During
the 31P NMR monitoring of the 1:4.5 reaction mixture at ꢀ42 to rt in
d8-THF, this signal appeared in the very first scan and was the most
intense at the lower temperature, which suggests high reactivity of
the intermediate giving rise tothis broadened singlet. One of the two
sets of two doublets between ꢀ11 and ꢀ17 ppm (JPP ¼ 80 Hz)
observed in the 31P{1H} NMR spectra of the reaction mixtures (signal
[29,33e38]. Complexes {(C6F5)2M(
k
1-PPh2)2}2ꢀ (M ¼ Pt or Pd) were
obtained in situ by reacting cis-{(C6F5)2M(PHPh2)2} with 2 equiv. of
n-BuLi in THF, although no spectral data were reported for the
compounds [26]. Previously, we reported that 2 equiv. of LiPPh2 was
necessary in the reaction of dimer
1 in order to obtain

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V.A. Stepanova et al. / Journal of Organometallic Chemistry 696 (2011) 871e878
C in Fig. 1) may belong to intermediate D. It is noteworthy that
neither signal B nor C was observed in 31P NMR spectra of the 1:4.5
reaction mixtures in C6D6-PhMe at rt.
Recently, evidence of Pd(III) intermediates in the Pd(OAc)2-cata-
lyzed CeH arylation and acetoxylation was reported [50,51]. These
reactions presumably proceed through the formation of m-AcO-CPCs.
The reductive elimination is to be the last step of the trans-
formation, which provides aminophosphine 4, Ph2PPPh2 and Pd(0)-
containing species (Scheme 2). Signals of the free aminophosphine
4 and Ph2PPPh2 were observed in the spectra of the 1:4.5 reaction
mixtures (Fig. 1).
In our case, formation of Pd(III) intermediates is unlikely. The Pd(III)e
Pd(III) P-containing intermediates are expected to give 31P NMR
signals in a very low field (cf. data for compounds VIII, Chart 1) [25].
No signals above 130 ppm were observed in the 31P NMR spectra of
the studied reactions. Also, the “open-book” geometry of dimeric
As was noted above, the amount of Ph2PPPh2 formed in the
studied transformations was steadily decreasing during the warm-
up of the 1:4.5 reaction mixture. Yet, some amount of Ph2P(O)PPh2
was practically always isolated during chromatographic separations
of the 1:4.5 reaction mixtures. The analysis of the 1:4.5 reaction
mixture after 18 h at rt using analytical two-dimensional TLC [39]
indicated that aminophosphine 3 was present in the reaction
mixture not as a complex with Pd(0) or Pd(II) but as a free ligand (its
spot was on the diagonal), while Ph2PPPh2, at least partly, was
forming during the chromatography (the spot of the diphosphine
was off the diagonal). We suggest that Ph2PPPh2 reacts with the Pd
(0)Ln species formed in the final step of the reaction sequence to
furnish Ph2PePd(II)L2ePPh2, Ph2PePd(II)L3 and/or related
complexes. Then, during the chromatographic separation on silica
gel, these complexes are converted back to Pd(0) and Ph2PPPh2. (Pd
(0) black was clearly seen on preparative TLC, but was not visible in
the reaction mixtures before chromatographic separations.) Previ-
ously, Kawaguchi et al. reported that both Pd(II) and Pd(0) are
capable of catalyzing hydrophosphination reactions of Ph2PPPh2
[40,41]. In these reactions, the formation of PdL2(PPh2)2 species was
proposed. It was also reported that Pd(PhCN)2Cl2 reacted with
Ph2PPPh2 to yield a complex of the formula Pd(P2Ph4)Cl2 [42]. The
composition of this complex was supported by elemental analysis
[42]. The 31P NMR spectrum of this complex in CDCl3 contained
a singlet at 76.4 ppm [42]. On the other hand, in the Pd(0)-catalyzed
allylic phosphination reactions of HPPh2, Ph2PPPh2 was isolated in
1e40% yield depending on the reaction conditions [43]. Similar
observations were reported for hydrophosphination of styrenes by
Pd(PhCN)2Cl2 [44]. High lability of the PeP bond in R2PPR2 was
observed in the metathesis reactions of diphosphines in CH2Cl2:
Ph2PPPh2 þ Me2PPMe2 % 2Me2PPPh2 [45]. The hypothesis that Pd
(0)Ln complexes formed in the studied transformations are capable
of reacting with Ph2PPPh2 to yield Ph2PePd(II)Lm and/or related Pd
(II) species can be supported by the fact that, in our trial experiment,
the Pd(II)-catalyzed reaction of N,N-dimethylbenzylamine with
KPPh2 furnished aminophosphine 4 in 25% yield. This resultsuggests
that Pd(II) is likely to be converted to Pd(0) and then back to Pd(II) to
begin a new catalytic cycle.
m
-AcO-CPCs with a short distance between two Pd atoms is quite
different from -di-PPh2 and -Cl- -PPh2 complexes with a relatively
m
m
m
planar geometry of the {PdP2Pd} [27] and {PdClPPd} [6] core. For the
Pd-catalyzed acetoxylation and a few related reactions proceeding
through the formation of cyclopalladated complexes, the involve-
ment of the Pd(IV) intermediates were also proposed [52e54].
Unfortunately, we could not find relevant 31P NMR data for Pd(IV)
complexes to compare them with our spectra. However, formation of
some Pd(0) black in all reactions during chromatographic separations
on silica gel makes the hypothesis of the Pd(IV) intermediates in the
reactions of CPCs with metal phosphides unlikely.
3. Conclusions
Both the purchased and lab synthesized KPPh2 reagents provided
practically identical and reproducible results in the reactions with
complex 1 making KPPh2 a more reliable reagent for reactions with
CPCs than LiPPh2. By controlling the reagent ratio and temperature, it
was possible to selectively obtain either the m-Cl-m-PPh2 complex 3
or aminophosphine 4. Longer reaction times benefit the formation of
aminophosphine 4, while the best yield of complex 3 was obtained
in a one-hour reaction. A drawback of using KPPh2 in reactions with
CPC 1 is that overall product yields are lower than those obtained by
us in the best experiments with LiPPh2 [7]. The proposed mechanism
for obtaining 4 from CPC 1 in THF is supported by 31P{1H} NMR data
and includes the consecutive formation of the corresponding
complexes with one and two bridging PPh2 groups.
Currently, we are studying stoichiometric and catalytic reactions
of mono- and dinuclear m-Cl and m-OAc CPCs with different metal
phosphides in order to obtain a set of structurally different P*- and
C*,P*-chiral aminophosphines, potent catalysts in asymmetric
transformations.
4. Experimental
4.1. General methods and materials
All reactions with KPPh2 were carried out under a positive
pressure of argon using Schlenk techniques. Purifications by column
chromatography were carried out using Natland silica gel 60 (230
mesh). Preparative thin-layer chromatography (TLC) was carried out
using 200 ꢂ 250 mm glass plates with an unfixed layer of Natland or
Merck silica gel 60 (230 mesh). Analytical TLC was performed on
It is noteworthy that for aminophosphine 4 (L), its Pd(0) complex
PdL3 has been reported [46]. The air-sensitive complex was instable
at rt in solutions. The 31P NMR spectrum of this compound exhibited
two signals, at
d
17.3 and ꢀ16.7 ppm (d8-PhMe, ꢀ40 ꢁC, 85% H3PO4).
The latter singlet was due to the presence of the free amino-
phosphine in the sample. It was also determined that in this
complex, the aminophosphine was coordinated to the metal
primarily or exclusively through the P atom. For comparison, Pd(II)
complexes LPdMe2, LPdMeX (X ¼ I, Br, Cl or OTf) and LPdCl2 of
aminophosphine 4 exhibited bidentate P,N chelate coordination
[46e49]. Compounds LPdMeCl and LPdMe(OTf) both provided a 31P
Whatman silica gel 60 (F254) 250 mm precoated plates. Compounds
were visualized on TLC plates using UV light (254 nm) and/or iodine
stain. Routine 1H (500 MHz), 13C{1H} (126 MHz) and 31P{1H}
(202 MHz) as well as DEPT, COSY and HMQC spectra were recorded
on a Bruker AVANCE 500 NMR spectrometer. Chemical shifts are
reported in ppm with SiMe4 as an internal standard (1H and 13C) or
P(OEt)3 as an external standard (31P). Spinespin coupling constants,
J, are given in Hz. Spectra of the products obtained in reactions of the
KPPh2 reagents with CPCs were recorded in CDCl3 unless otherwise
stated. Toluene was dried by refluxing over K/benzophenone,
distilled under Ar, and stored over potassium. THF was distilled over
K/benzophenone under Ar immediately before use. Acetone was
purified by distillation over KMnO4. Other solvents were distilled
over CaH2. Potassium metal was obtained from Acros. The 0.5 M
NMR signal at
d 37.7 ppm (CDCl3, rt) [48], while the spectrum of
LPdCl2 had a singlet at 22.9 ppm (CDCl3, rt, 85% H3PO4) [49]. In our
study, the 31P{1H} NMR spectra of 1:4.5 reaction mixtures of dimer 1
and KPPh2 exhibited a few low-intensity singlets in the 5e15 ppm
region but no signals between 15 and 50 ppm (see, for example,
Fig. 3b, d8-THF, P(OEt)3 as standard). These singlets may belong to Pd
(0) or Pd(II) complexes with a dative PePd bond; however, their
structures are impossible to determine.

V.A. Stepanova et al. / Journal of Organometallic Chemistry 696 (2011) 871e878
877
KPPh2 solutions in THF were either purchased from SigmaeAldrich
Appendix. Supplementary material
Co. or synthesized in our lab by using a known general method21 as
described below. Chlorodiphenylphosphine was distilled under
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2010.10.020.
vacuum prior to use. Complex di-(m-chloro)bis-{[2-(N,N-dimethy-
lamino)methyl]phenyl-C,N}dipalladium(II) (1) was obtained from
the corresponding benzylamine in 95% yield using a reported
procedure [55]. Other chemicals were used as purchased without
further purification.
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Acknowledgment
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