Multifaceted palladium catalysts towards the tandem diboration-arylation reactions of alkenes

Article abstract of DOI:10.1002/chem.200800931

Novel Pd²⁶⁺ compounds have been synthesized in high yield. These compounds and their Pd₂⁴⁺ counterparts as synthetic precursors mediate the diboration of vinylarenes and aliphatic 1-alkenes, and under mild and basic reaction conditions they produce a variety of 1,2-diboronate esters with excellent conversions and chemo-selectivities. The presence of bis(catecholato)diboron (B₂cat²) favours the reduction of Pd^III to Pd^II, while the catalytic precursor of Pd^II is transformedinto Pd⁰-nanoparticles. An "in situ" catalytic tandem reaction has been designed to transform the diboronate intermediates into the monoarylated product, which after oxidative workup, provides the carbohydroxylated adduct. Eventually, the same catalyst performs both sequences with total conversion from the alkene.

Full text of DOI:10.1002/chem.200800931

DOI: 10.1002/chem.200800931
Multifaceted Palladium Catalysts Towards the Tandem Diboration–Arylation
Reactions of Alkenes
Dirk Penno,[b] Vanesa Lillo,[a] Igor O. Koshevoy,[b] Mercedes Sanaffl,[b]
M. Angeles Ubeda,*[b] Pascual Lahuerta,*[b] and Elena Fernµndez*[a]
6+
Abstract: Novel Pd2
compounds
selectivities. The presence of bis(cate-
cholato)diboron (B2cat2) favours the
reduction of PdIII to PdII, while the cat-
alytic precursor of PdII is transformed
into Pd0-nanoparticles. An “in situ”
catalytic tandem reaction has been de-
signed to transform the diboronate in-
termediates into the monoarylated
product, which after oxidative workup,
provides the carbohydroxylated adduct.
Eventually, the same catalyst performs
both sequences with total conversion
from the alkene.
have been synthesized in high yield.
These compounds and their Pd2
4+
counterparts as synthetic precursors
mediate the diboration of vinylarenes
and aliphatic 1-alkenes, and under mild
and basic reaction conditions they pro-
duce a variety of 1,2-diboronate esters
with excellent conversions and chemo-
Keywords: catalysis · chemoselec-
tivity · cross-coupling · diboration ·
palladium · tandem reactions
Introduction
examples, the catalytic systems have to be switched from the
Pt- or Rh-mediated diboration of alkenes or alkynes to Pd-
À
Because of their stability, relatively low toxicity and easy ac-
cessibility, organoboron derivatives are considered very
useful organometallic compounds for transmetalation with
transition-metal complexes.[1] Catalytic access to organobor-
onate esters, by boron addition to unsaturated molecules,[2]
has enhanced their use for tandem purposes.[3] In particular,
the catalytic diboration of alkenes and alkynes afforded
alkyl- and alkenyldiboronates, which are susceptible to
being functionalized “in situ” by means of a cross-coupling
reaction towards targeted molecules.[4] However, in all these
mediated Suzuki–Miyaura C C bond formation. We became
interested in finding a unique catalytic system that could
perform both sequences, and we focused on Pd complexes.
While palladium catalytic systems were shown to be effi-
cient in the diboration of allenes[5] and the borylative cycli-
sation of 1,6-enynes,[6] the diboration of alkenes was first
achieved by our group when the base NaOAc was involved
in the reaction.[7] It has been postulated that the base plays
a role in the cleavage of diboron to afford metal–boryl inter-
mediates in the catalytic cycle, instead of the unfavourable
palladium–diboryl species expected by the oxidative addi-
[8]
À
tion of B B to Pd.
We have reported the synthesis and characterization of
[a] V. Lillo, Dr. E. Fernꢀndez
Departament de Quꢁmica Fꢁsica i Inorgꢂnica
Universitat Rovira i Virgili
the paddlewheel bis-cyclometalated palladium(II) com-
4+
(O2CR)2] (1),[9] with a Pd2
pounds, cis-[Pd2ACHTUNGTRNENUG(C6H4PPh2)2CAHTUNGTRENNGUN
C/Marcel.lꢁ Domingo s/n, 43007
Tarragona (Spain)
Fax : (+34)977-559563
core and no metal–metal bond. Cyclic voltammetry shows
that some of these compounds have two reversible oxidation
peaks, so they are potential starting products for the synthe-
sis of dinuclear palladium compounds in higher oxidation
states. Most transition elements are known to have paddle-
wheel compounds but very little is known about those with
E-mail: mariaelena.fernandez@urv.cat
[b] D. Penno, Dr. I. O. Koshevoy, Dr. M. Sanaffl, Dr. M. A. Ubeda,
Prof. P. Lahuerta
Departament de Quꢁmica Inorgꢂnica
Universitat de Valencia
Dr. Moliner 50, 46100 Burjassot
Valencia (Spain)
E-mail: angeles.ubeda@uv.es
pascual.lahuerta@uv.es
6+
an M2 core in Group 10. Only one compound has been
6+
spectroscopically characterized with a Ni2 core[10] and an-
6+
other one with a Pd2 core.[11] We recently reported the
preliminary results of the high yield synthesis of very pure
À
paddlewheel compounds that had a Pd Pd single bond with
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800931.
6+
a Pd2 core.[12] In this paper, we describe the synthesis and
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FULL PAPER
erature procedure.[9,13,14] [Pd
obtained through 3 as the intermediate compound, while
cis-[Pd2A(C6H4PPh2)2A(O2CR)2X2] compounds (R=CH3, X=
ACHTUNGNERTN(UNG C6H4PPh2)X]4 (X=I (4c)) was
C
CHTUNGTRENNUNG
Cl (2a); R=CMe3, X=Cl (2b); R=CF3, X=Cl (2c); R=
CH3, X=Br (2d)) were obtained by oxidation of the corre-
sponding cis-[Pd2ACHTUNGRTNEN(GU C6H4PPh2)2HCAUTNGTNER(NUGN O2CR)2] (1) with PhI·Cl2 or
Br2 (Scheme 1).
characterisation of compounds 2a–d with the general formu-
la cis-[Pd2A(C6H4PPh2)2A(O2CR)2X2] (X=Cl, Br) (2) obtained
C
CHTUNGTRENNUNG
by the oxidation of compounds 1a–c.
Both families of compounds, 1 and 2, contain labile car-
boxylate ligands. To get further insight into the role of the
OAc fragment during the Pd-mediated diboration of al-
kenes, we decided to explore these new compounds in the
catalytic diboration of alkenes. As well as reporting the first
application of these complexes in catalysis, we also studied
whether the electronic influence of the alkyl group in the
carboxylate ligand, (R=CH3, CMe3, CF3 and C6F5), can
affect the reaction outcome. The cationic complex cis- [Pd2-
Scheme 1. Synthesis of cis-[Pd2ACHTUNGTRNENG(U C6H4PPh2)2CAHTUNTGRNE(NUGN O2CR)2X2] compounds.
The ORTEP view for complex 2d, is shown in Figure 1.
The corresponding figures for compounds 2a–c are very
similar and can be found in the Supporting Information (see
Figures S1–3). Table 1 includes some representative bond
lengths and angles for compounds of type 1 and 2 that have
been structurally characterised. The structures of all the
(BF4)2 (3)[13] and the tretranuclear
ACHTUNGTRENNUNG(C6H4PPh2)2ACHTUNGTRENNUNG(NCCH3)4]ACHTGUNTRENNUGN
palladium complexes [PdACHTNUTGRNEUNG
(C6H4PPh2)X]4 (X=Cl (4a),[14b] Br
(4b),[14a] I (4c)) without bridging carboxylate ligands have
also been tested in catalysis.
4+
6+
compounds show a Pd2 (1) or Pd2 (2) unit, bridged by
two carboxylate groups and two ortho-metalated phosphine
Results and Discussion
Palladium catalysts: cis-[Pd
pounds, (R=CH3 (1a), CMe3 (1b), CF3 (1c), C6F5 (1d), cis-
[Pd2A(C6H4PPh2)2A(NCCH3)4](BF4)2 (3) and [Pd(C6H4PPh2)X]4
(X=Cl (4a), Br (4b)) were synthesized according to the lit-
2CAHTUNGTRENNUG(C6H4PPh2)2ACHUTNGTRNE(NUNG O2CR)2] com-
G
N
N
ACHTUNGTRENNUNG
Figure 1. ORTEP view of compound 2d with ellipsoids representing 30%
probability. H atoms are omitted for clarity.
Table 1. Selected bond lengths [Š] and angles [8] for different dinuclear palladium compounds (X=Cl, Br).
Compound
1b[a]
1c[a]
1d[a]
2a[b]
2b[b]
2c[b]
2d[b]
À
Pd1 Pd2
2.6779(15)
2.226(3)
1.974(10)
–
2.7229(8)
2.2273(12)
1.982(4)
–
2.7078(6)
2.2303(17)
1.992(6)
–
2.5294(17)
2.255(4)
2.028(15)
2.462(4)
168.22(12)
88.57(11)
100.6
2.5241(9)
2.2621(19)
2.028(7)
2.4262(18)
167.87(5)
88.17(5)
101.7
2.5434(4)
2.2511(11)
2.021(4)
2.4144(11)
167.45(3)
88.40(3)
2.5411(9)
2.258(2)
2.035(8)
2.5967(11)
167.67(4)
88.45(6)
101.8
À
Pd1 P1
À
Pd1 C42
À
Pd1 X1
X1-Pd1-Pd2
P1-Pd1-Pd2
–
–
–
87.83(6)
96.3(1)
87.50(3)
94.3(1)
86.85(4)
96.8(1)
P1-Pd1-Pd2-P2
99.41(4)
[a] See reference [9]. [b] See this paper.
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10649

E. Fernꢀndez, M. A. Ubeda, P. Lahuerta et al.
ligands, in a head-to-tail ar-
rangement. In the case of the
6+
Pd2
compounds, the struc-
ture is completed with two
chlorides for compounds 2a–c
or two bromides for compound
Scheme 2.
2d, in axial positions. The Pd–
6+
Pd distance in the Pd2
compounds is in the range of
Table 2. Palladium(II) and (III) complexes mediate catalytic diboration/
oxidation of styrene.[a]
2.5241(9)–2.5434(4) Š, 0.012–0.020 Š shorter than in the
Pd2 compounds (2.6779(15)-2.7229(8) Š). This observation
4+
Entry Catalytic
system
NaOAc
(equiv)
B2cat2
(equiv)
Conv
[%][b]
Diol
À
[%][b]
is consistent with the formation of a Pd Pd single bond in
the oxidized species. Additionally, this Pd–Pd distance short-
ening gives rise to larger P-Pd-Pd bond angles and P-Pd-Pd-
P torsion angles to accommodate the bridging metalated li-
gands. The same observation applies to the carboxylate li-
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1b
1c
1d
2a
2b
2c
2d
3
–
0.5
1
1
1
1
1
1
1
1
1
1
–
3
3
1
2
3
3
3
3
3
3
3
3
3
3
54
80
38
79
81
87
99
95
86
97
98
99
84
94
32
51
60
91
99
81
99
99
99
99
99
99
44
99
À
À
gands. Pd P and Pd C bond lengths are slightly longer for
the oxidized species.
When comparing the oxidised compounds 2a and 2d,
which only differ in the nature of the axial halide, no big dif-
9
6+
10
11
12
13
14
ferences in the bond lengths and angles around the Pd2
core can be observed, excluding logically the Pd–halide
bond lengths, 2.462(4) and 2.5967(11) Š, respectively.
When comparing the three compounds with axial chloride
3
1
À
ligands, compounds 2a and 2b have similar Pd Pd bond
lengths, 2.5294(17) and 2.5241(9) Š, respectively, while com-
[a] Standard conditions: substrate/Pd 1:0.05, solvent=THF, T=258C, t=
4 h. [b] Determined by H NMR spectroscopy.
1
pound 2c is slightly longer, 2.5434(4) Š, probably because
À
of the presence of CF3 groups
in the bridging carboxylate li-
À
gands. The Pd Cl bond lengths
roughly show the opposite
trend, 2a: 2.462(4), 2b:
2.4262(18)
and
2c:
2.4144(11) Š.
Diboration–arylation reactions
of alkenes: The catalytic dibo-
ration of alkenes was first per-
formed on styrene as a model
substrate, in the presence of
bis(catecholato)diboron
Scheme 3.
(B2cat2) (Scheme 2).
The catalyst precursor 1a proved to be inactive until an
excess of diboron was added to the reaction mixture
(Table 2, entry 1). Chemoselectivity to the diborated product
was enhanced by the presence of the base NaOAc, up to
quantitative values when combined with an excess of
(B2cat2), (Table 2, entries 2–5). This is of particular interest
because the catalytic diboration can compete with the cata-
lytic dehydrogenative borylation,[2] giving rise to alkenylbor-
onates and hydroborated byproducts, (Scheme 3).
Under these optimised reaction conditions, complexes
1b–d also proved to be very active and selective in the 1-
phenyl 1,2-ethanediol formation within 4 h at room temper-
ature (Table 2, entries 6–8). Surprisingly, complexes 2a–d,
which are based on PdIII, were also very active and chemose-
lective for the diborated product (Table 1, entries 9–12). To
clarify the role of the carboxylate ligand present in the cata-
lyst precursor, we made a comparative catalytic study with
the solvated cationic complex [Pd
2CAHTUGTNREN(NUG C6H4PPh2)2ACHTUNTGERN(NUGN NCCH3)4]-
AHCTUNGTRENNUNG
ylate groups.[13] Under reaction conditions similar to those in
Table 2 entry 1, moderate diol formation was observed, al-
though the addition of one equivalent of NaOAc favoured
the reaction quantitatively (Table 2, entries 13–14). The
presence of the carboxylate anion in the catalyst precursor
does not seem to be critical for the cleavage of the diboron,
À
as counterion BF4 also seems to contribute to this end, in
view of entry 13 (Table 2) and the previously reported dibo-
ration reaction catalysed by Cu–NHC complexes.[15]
To check whether 1a and 2a could also be applied to
other substrates, we studied the diboration of other terminal
and internal vinylarenes. Although both catalyst precursors
performed with total chemoselectivity towards the diol
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Multifaceted Palladium Catalysts
FULL PAPER
Table 3. Palladium(II) and (III) complexes mediate catalytic diboration/
oxidation of vinylarenes.[a]
Table 4. Complex 4 mediates the catalytic diboration/oxidation of alke-
nes.[a]
Entry
Substrate
Catalytic system
Conv [%][b]
Diol [%][b]
Entry
Substrate
Catalytic system
Conv [%][b]
99
Diol [%][b]
99
1
4a
1
1a
79
99
2
3
4b
4c
99
99
99
99
2
1a
92
99
3
4
1a
1a
40
17
99
99
4
4b
99
99
5
6
4b
4b
99
95
99
99
5
6
2a
2a
99
96
99
99
7
8
9
4b
4b
4b
87
99
99
99
99
99
7
8
2a
2a
99
94
99
99
[a] Standard conditions: substrate/Pd 1:0.05, B2cat2 (3 equiv), NaOAc
(1 equiv), solvent=THF, T=258C, t=4 h. [b] Determined by 1H NMR
spectroscopy.
[a] Standard conditions: substrate/Pd 1:0.05, B2cat2 (3 equiv), NaOAc
(1 equiv), solvent=THF, T=258C, t=4 h. [b] Determined by 1H NMR
spectroscopy.
product (Table 3), only complex 2a gave quantitative con-
version even in the diboration of internal vinylarenes, such
as indene and trans-b-methylstyrene.
cies formed from 4a and 2a in the presence of B2cat2 differs
from that of 1a with B2cat2.
To gain further insight into the role of the catalyst, we
monitored the metal catalyst under different reaction condi-
tions by using NMR spectroscopy. Complex 2a (0.01 mmol)
was dissolved in a sealable NMR tube with 1 mL of
[D8]THF, and the colour immediately changed from red to
yellow when B2cat2 (0.03 mmol) and NaOAc (0.01 mmol)
were added. The 31P NMR spectra showed a significant shift,
from d=À13.9 ppm typical of PdIII in 2a to d=+26.8 ppm
assigned to a new PdII species, generated in situ. Analogous-
ly, the 11B NMR spectra also showed how the original signal
at d=30.1 ppm due to B2cat2 decreased in favour of a new
signal at d=23 ppm, which is characteristic of B2cat3, and
the signal at d=13.6 ppm associated with the interaction of
B2cat2–NaOAc.[7] The diboron seems to reduce PdIII to PdII,
just as B2X4 has been reported to reduce PdII to Pd0.[16] The
new palladium species formed in situ is assigned to complex
4a, in comparison with the NMR data of the previously iso-
lated complex.[14]
Following the synthetic methodology described in the lit-
erature,[14] pure complexes 4a–b were prepared and used in
the catalytic diboration reaction of vinylarenes so that their
catalytic activities could be checked and compared with
those observed for complexes 2a–b. Interestingly, catalyst
precursors 4a–c proved to be as active and as chemoselec-
tive in the diboration/oxidation of styrene as complexes 2a–
b (Table 4). The nature of X (X=Cl, Br, I) does not change
the reaction outcome. Further catalytic studies with complex
4b provided quantitative conversions of the diol product for
both terminal and internal vinylarenes. The fact that the cat-
alytic behaviour was similar suggests that the catalytic spe-
To get a more complete picture of this hypothesis, styrene
consumption and diboronate ester formation were moni-
tored at short intervals during the first 3 h of the catalytic
diboration with 1a, 2a and 4a in the presence of B2cat2 and
NaOAc (Figure 2). A typical catalytic experiment was car-
ried out in an NMR tube. Styrene consumption and diboro-
nate product formation were followed by monitoring the
1
doublets at d=5.7 and 3.3 ppm, respectively, in the H NMR
spectra.
To gain further insight into the role of the catalyst, we
monitored the metal catalyst under reaction conditions
using NMR spectroscopy. When complex 1a (0.01 mmol)
was dissolved in a sealable NMR tube with 1 mL of
[D8]THF, the colour changed to brownish when B2cat2
(0.03 mmol) and NaOAc (0.01 mmol) were added. The
31P NMR spectra showed how the original signal corre-
sponding to 1a, at d=+21 ppm, progressively disappeared.
The 11B NMR spectra also showed the disappearance of the
signal corresponding to B2cat2, at d=30.1 ppm, and the ap-
pearance of signals at d=23 (B2cat3) and 13.6 ppm (B2cat2–
NaOAc).[7] Interestingly, a black solid was recovered and
characterized as “in situ” phosphine-stabilized palladium
nanoparticles, the core size and size distribution of which
were examined by TEM. The picture showed disperse nano-
particles (6.9Æ3.0) nm in diameter (Figure 3). Thus, the re-
duction of PdII to Pd0 by B2cat2 in 1a parallels a previous
study that reported that PPh3-stabilized gold nanoparticles
can be obtained by reducing [AuClACTHNUTRGNEUNG(PPh3)] with diboron
[16a]
B2X4
and [Au2Cl2ACTHUGNETRNNU(G binap)] (binap=2,2’-bis(diphenylphos-
phino)-1,1’-binaphthyl) with diboron B2cat2.[16b]
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E. Fernꢀndez, M. A. Ubeda, P. Lahuerta et al.
Figure 2. Evolution of the catalytic diboration of styrene with catalyst
Figure 3. “In situ” Pd nanoparticles: a) core size distribution, b) TEM mi-
crograph (”500000).
*
*
precursor a) 1a, b) 2a and c) 4a. : styrene consumption, : diboronate
ester formation.
To learn about the nature of the active catalytic species in
these systems, we performed three new diboration of styrene
experiments, with 1a, 2a and 4a as catalyst precursors, and
with an excess of Hg.[17] While complex 1a became substan-
tially inactive, catalyst precursors 2a and 4a gave quantita-
tive conversion. Therefore, we decided to conduct new cata-
lytic reactions with the enantiomerically pure complex 4b in
an attempt to observe the induction of stereoselectivity.[13]
However, when 3,3-dimethyl-1-butene was diborated with
4b and B2cat2 in the presence of the base NaOAc, no enan-
tioselectivity was observed even though total conversion and
selectivity were achieved within 4 h of reaction at room tem-
perature. Considering that all these catalyst precursors have
backbone chirality this observation suggests that, in the di-
boration process, the metal intermediates lose their chirality
because of the cleavage of the complex structure.
Finally, we focused on checking whether complexes 2 and
4 could perform the tandem single-pot diboration of al-
kenes/Suzuki–Miyaura cross coupling, by using only 5 mol%
of palladium throughout the tandem reaction (Scheme 4).
We were very pleased to observe that catalyst precursor 4b
performs this tandem reaction highly satisfactorily. Thus,
once the diboration of 3,3-dimethyl-1-butene with B2cat2 has
been properly completed, (after 4 h at room temperature in
the presence of 4b and the base NaOAc), the addition of
Scheme 4.
aryl halides and aryl triflates provided total monoarylation
of the primary alkylboronic ester. Further oxidative workup
led to total conversion of the alkene into the carbohydroxy-
lated adduct (Table 5, entry 1). Similar results were observed
when complex 2a was used instead (Table 5, entry 2). Aryl
halides performed better than aryl triflates as reactive
agents (Table 5, entries 3–5). The catalytic conditions for the
cross-coupling reaction were optimised, and 10% H2O was
required to complete the monoarylation process. The ab-
sence of H2O only provided 50% of desired product. The ef-
ficiency of the base Cs2CO3 guaranteed the sequence of ary-
lation, despite the excess of NaOAc present in the reaction.
Like 3,3-dimethyl-1-butene, the substrate vinylcyclohexane
performed the tandem reaction with identical success,
(Table 5, entry 6).
Morken et al.,[4a] have been able to prepare chiral non-
symmetric 1,2-diboron adducts by Rh-mediated diboration
and further Pd-mediated cross coupling favoured a new C
À
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Multifaceted Palladium Catalysts
FULL PAPER
Table 5. Palladium complexes mediate catalytic diboration/monoarylation/oxidation of alkenes.[a]
Entry
Cat.
Substrate
ArX
Product
Conv [%][b]
1
4b
99
2
2a
90
3
4
5
4b
4b
4b
99
99
30
6
4b
99
[a] Standard conditions: substrate/Pd 1:0.05, B2cat2 (3 equiv), NaOAc (1 equiv), THF, 258C, 4 h, then Cs2CO3 (3 equiv), ArX (2 equiv), THF/H2O 10%,
1
758C, 15 h. [b] Determined by GC analysis and H NMR spectroscopy.
Experimental Section
À
C bond formation from the less-hindered C B bond, while
À
[18]
the remaining C B was then oxidised. However, there are
some differences between the work of Morken et al. and
General: PhI·Cl2
and the palladium complexes 1a–d[9] and 4a–b[14]
were prepared by following reported methods. Commercially available
reagents were purchased from Sigma-Aldrich. Solvents were used with-
out further purification to synthesize the palladium complexes. All cata-
lytic reactions were carried out under an atmosphere of dry nitrogen, and
the necessary organic solvents were dried, distilled and degassed before
use. NMR spectra were recorded on Varian Gemini 300, Varian Mercury
400, Bruker Avance 300 and Bruker Avance 400 spectrometers. Chemical
ours: while they used [RhACTHNUTRGENNU(G acac)ACHTUNGTERN(NUGN ndb)]/(S)-Quinap (5 mol%;
acac=acetylacetone, ndb=(2,3,5,6-h)-norbornadiene, (S)-
Quinap=(S)-1-(2-diphenylphosphino-1-naphthyl)isoquino-
line) for the efficient asymmetric diboration sequence and
[PdCl2ACHTUNGTRENNUNG(dppf)] (dppf=1,1’-bis(diphenylphosphino)ferrocene)
1
shifts were reported relative to tetramethylsilane for H, 85% H3PO4 for
complexes (10 mol%) for the cross-coupling sequence, we
only required 5 mol% of Pd complex for both sequences,
but with three equivalents of diboron. The percentages of
product formed have also been significantly improved from
60–77% with the mixed catalytic system Rh/Pd to 100%
with our Pd system.
31P and BF3·OEt2 for 11B. Electronic spectra were measured on a Shimad-
zu UV-2501PC spectrometer in CH2Cl2 solution. Elemental analyses
were performed by Robertson Microlit Laboratories, Madison, New
Jersey (US), and the analytical laboratory of the University of Joensuu
(Finland).
General synthetic procedure for palladium complexes 2a–c: A sample of
the corresponding palladium complex 1a–c (0.042 mmol) was suspended
in diethyl ether (6 mL) and then cooled to À108C. A slight excess of
PhI·Cl2 (13 mg, 0.055 mmol) in acetonitrile (1 mL) was added dropwise
to the yellow suspension while the mixture was stirred. The colour of the
suspension changed from yellow to red. The reaction mixture was stirred
for 15 min at about À58C. The pale-red/orange solution was decanted off
and the red precipitate was washed with a 5:1 v/v mixture of diethyl
ether/acetonitrile (3”3 mL) and then dried under vacuum to give the
products in high yield (2a: 90, 2b: 96, 2c: 78%). Analytically pure sam-
ples were obtained by dissolving a sample in dichloromethane and then
filtering the solution and evaporating the solvent under vacuum. Finally
the solid was washed with acetonitrile and dried under vacuum.
Compound 2a: 31P{1H} NMR (CDCl3, 208C): d=À13.8 ppm (s);
1H NMR (CDCl3, 208C): d=8.22 (m, 2H), 7.74 (m, 1H), 7.42 (m, 4H),
7.20 (m, 4H), 6.99 (m, 1H), 6.80 (m, 1H), 6.74 (m, 1H), 1.14 ppm (s,
3H); UV/Vis (CH2Cl2): lmax (e)=499 (8.7”103), 434 (1.8”103), 379 nm
(1.2”104 mÀ1 molÀ1); elemental analysis calcd (%) for C40H34Cl2P2Pd2O4:
C 51.95, H 3.71; found: C 51.51, H 3.63.
Conclusion
6+
Novel singly bonded Pd2 compounds were synthesized in
high yield. These compounds and a family of structurally re-
lated di- and tetranuclear Pd2 compounds show high selec-
4+
tivity in the catalytic diboration of alkenes. The presence of
bis(catecholato)diboron (B2cat2) favoured the reduction of
PdIII to PdII, while the catalytic precursor of PdII was trans-
formed into Pd0-nanoparticles. Halides seem to stabilise the
Pd complexes with respect to reduction to Pd0 nanoparticles,
even in the presence of excess acetate. More interestingly
some of these compounds perform catalytic tandem dibora-
tion–arylation reactions of alkenes by using a single catalyst.
This process is the first example of the multifaceted proper-
ties of a palladium complex participating in different catalyt-
ic cycles with identical success.
X-ray crystal structure data for compound 2a: C40H34Cl2O4P2Pd2; ortho-
¯
rhombic; space group=P421/c; a=22.299(3), b=22.299(3), c=
17.250(3) Š; a=b=g=908; V=8577(2) Š3; Z=8; MoKa radiation;
Chem. Eur. J. 2008, 14, 10648 – 10655
ꢃ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10653

E. Fernꢀndez, M. A. Ubeda, P. Lahuerta et al.
273(2) K; 11321 reflections, 6135 independent (m=1.073 mmÀ1); refine-
ment (on F2) with SHELXTL (Version 6.1), 454 parameters, 0 restraints;
(3 mL) was added. A fine orange precipitate immediately formed, which
was collected by filtration and washed with acetone and diethyl ether
(51 mg, 93%). 31P{1H} NMR (CDCl3, 208C): d=24.0 ppm (s); 1H NMR
(CDCl3, 208C): d=7.52–7.41 (m, 12H), 7.28–7.10 (m, 32H), 6.51–
6.41 ppm (m, 12H); elemental analysis calcd (%) for C72H56I4P4Pd4: C
43.08, H 2.98; found: C 43.71, H 2.85.
R1 =0.0690 (I>2s), wR2 (all data)=0.2020; GOF=1.129; max/min resid-
À3
ual electron density=2.152/À1.291 eŠ
.
Compound 2b: 31P{1H} NMR (CD2Cl2, 208C): d=À14.0 ppm (s);
1H NMR (CD2Cl2, 208C): d=8.33 (m, 2H), 7.76 (m, 1H), 7.50 (m, 4H),
7.22 (m, 4H), 7.05 (m, 1H), 6.83 (m, 2H), 0.61 ppm (s, 9H); UV/Vis
(CH2Cl2): lmax (e)=500 (1.4”104), 428 (3.9”103), 381 nm (2.0”
104 mÀ1 molÀ1); elemental analysis calcd (%) for C46H46Cl2P2Pd2O4: C
53.99, H 4.12; found: C 53.49, H 3.63.
Typical catalytic diboration and subsequent oxidation: Bis(catecholato)-
diboron (0.6 mmol) was added to a solution of the catalyst (5 mol%,
0.01 mmol Pd) and sodium acetate (0.2 mmol) in THFACTHUNRTGNEUNG(dry) (2 mL) under
nitrogen. The solution was stirred for 5 min and the substrate (0.2 mmol)
was then added. The mixture was stirred for 4 h at room temperature.
NaOH (aq, 3m, 1 mL) and H2O2 (30%, 1 mL) were added carefully and
stirring at room temperature was continued for 2 h. The oxidation was
quenched by adding a saturated aqueous solution of sodium thiosulfate
(1 mL) and NaOH (aq, 1m, 10 mL). Then the reaction mixture was ex-
tracted with ethyl acetate (3”20 mL) and the united organic phases were
washed with brine (20 mL), dried over magnesium sulfate and carefully
dried in vacuo so that the styrene was not removed. The products ob-
tained were analysed by 1H NMR spectroscopy to determine the degree
of conversion and the nature of the reaction products.
X-ray crystal structure data for compound 2b: C46H46Cl2O4P2Pd2; ortho-
ACHTUNGTRENNUNGrombic; space group=Pbna; a=11.7110(3), b=17.1950(4), c=
21.1460(7) Š; a=b=g=908; V=4258.2(2) Š3; Z=4; MoKa radiation;
293(2) K; 45790 reflections, 4528 independent (m=1.088 mmÀ1); refine-
ment (on F2) with SHELXTL (Version 6.1), 257 parameters, 0 restraints;
R1 =0.0487 (I>2s), wR2 (all data)=0.1260; GOF=1.031; max/min resid-
À3
ual electron density=0.605/À0.784 eŠ
.
Compound 2c: 31P{1H} NMR (CDCl3, 208C): d=À8.8 ppm (q, JF-P
=
5
1.2 Hz); 19F NMR (CDCl3, 208C): d=À74.2 ppm (d, JP-F 1.2 Hz);
1H NMR (CDCl3, 208C): d=8.25 (m, 1H), 8.21 (m, 1H), 8.04 (m, 1H),
7.59 (m, 2H), 7.49 (m, 2H), 7.45–7.26 (m, 4H), 7.14 (m, 1H), 6.97 (m,
1H), 6.84 ppm (m, 1H).
5
Typical catalytic diboration, subsequent Suzuki reaction and oxidation:
Bis(catecholato)diboron (0.6 mmol) was added to a solution of the cata-
lyst (5 mol%, 0.01 mmol Pd) and NaOAc (0.2 mmol) in THFACHTUNGTRENNUNG(dry)
X-ray
crystal
structure
data
for
compound
2c·2CH2Cl2:
(2 mL) under nitrogen. The solution was stirred for 5 min, the substrate
(0.2 mmol) was added and stirring was continued for 4 h at room temper-
ature. After heating to reflux, cesium carbonate (0.6 mmol), substrate
(0.4 mmol) and water (degassed, 0.2 mL) were added and the reaction
mixture was stirred for 15 h. After cooling to room temperature, NaOH
(aq, 3m, 1 mL) and H2O2 (30%, 1 mL) were added carefully and stirring
was continued for 2 h. The oxidation was quenched by adding a saturated
aqueous solution of sodium thiosulfate (1 mL) and NaOH (aq, 1m,
10 mL). Then the reaction mixture was extracted with ethyl acetate (3”
20 mL) and the united organic phases were washed with brine (20 mL),
dried over magnesium sulfate and dried in vacuo. The products obtained
were analysed by 1H NMR spectroscopy[4a] to determine the degree of
conversion and the nature of the reaction products.
(Æ)-1-Cyclohexyl-2-phenylethanol: 1H NMR (400 MHz, CDCl3): d=7.23
(1H, m), 6.78 (3H, m), 3.80 (3H, 1 s; OMe), 3.58 (1H, ddd, J=9.6, 6.0,
3.6 Hz), 2.85 (1H, dd, J=13.6, 3.6 Hz), 2.56 (1H, dd, J=13.6, 9.5 Hz),
2.04–1.67 (5H, m), 1.44–1.41 (1H, m), 1.29–1.07 ppm (5H, m); 13C NMR
(75 MHz, CDCl3): d=159.9, 141.0, 129.7, 122.8, 115.9, 11.9, 77.0, 55.3,
43.4, 41.0, 29.5, 28.2, 26.7, 26.4 ppm.
¯
C42H32Cl6F6O4P2Pd2; triclinic; space group P1; a=11.4873(9), b=
12.2811(10), c=17.4297(14 Š; a=96.8940(10), b=91.5210(10), g=
111.8540(10)8; V=2258.9(3) Š3; Z=2; MoKa radiation; 173(2) K; 20358
reflections, 10494 independent (m=1.288 mmÀ1); refinement (on F2) with
SHELXTL (Version 6.1), 559 parameters, 0 restraints; R1 =0.0541 (I>
2s), wR2 (all data)=0.1033; GOF=1.061; max/min residual electron den-
À3
sity=1.542/À1.161 eŠ
.
Synthesis of 2d: A suspension of 1a (50 mg, 0.042 mmol) in diethyl ether
slight excess of bromine (5 mL,
(6 mL) was cooled to À108C.
A
0.097 mmol) in ether (1 mL) was added dropwise while the mixture was
stirred. The colour changed from yellow to dark red. The reaction mix-
ture was stirred for 15 min at about À58C. The pale-red solution was de-
ACHTUNGTRENNUNGcanted off and the red precipitate was washed with diethyl ether (3”
3 mL) and then dried under vacuum to give the product in high yield
(59 mg, 98%). Single crystals suitable for X-ray diffraction were obtained
by diffusion of diethyl ether into a dichloromethane solution of the com-
pound at À158C. 31P{1H} NMR (CDCl3, 208C): d=À15.0 ppm (s);
1H NMR (CDCl3, 208C): d=8.41–8.35 (m, 4H), 8.08–8.04 (m, 2H), 7.05–
7.55 (m, 16H), 7.00–6.95 (m, 2H), 6.89–6.84 (m, 2H), 6.81–6.74 (m, 2H),
1.23 ppm (s, 6H); elemental analysis calcd (%) for C40H34Br2P2Pd2O4: C
47.41, H 3.38; found: C 46.09, H 3.38.
Acknowledgements
X-ray crystal structure data for compound 2d: C40H34Br2O4P2Pd2; ortho-
¯
rhombic; space group=P421/c; a=22.565(3), b=22.565(3), c=
The authors are grateful for financial support from the MEC of Spain
(CTQ2007–60442/BQU, CTQ2005–08351 and a grant to I.O.K), Consolid-
er Ingenio 2010 (CSD2006–0003) and UV (VSegles grant to D.P.).
17.279(4) Š; a=b=g=908; V=8798(3) Š3; Z=8; MoKa radiation,
293(2) K; 16930 reflections, 9894 independent (m=2.742 mmÀ1); refine-
ment (on F2) with SHELXTL (Version 6.1); 453 parameters, 0 restraints;
R1 =0.0608 (I>2s), wR2 (all data)=0.1791, GOF=1.049; max/min resid-
À3
ual electron density: 1.658/À1.531 eŠ
.
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10654
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Received: May 15, 2008
Published online: October 17, 2008
Chem. Eur. J. 2008, 14, 10648 – 10655
ꢃ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10655