PCN- and PCS-pincer palladium complexes as tandem catalysts in homoallylation reactions

Article abstract of DOI:10.1002/adsc.201000204

Novel PCN- and PCS-pincer palladium complexes 2-(dimethylamino)methyl-5- methoxy-6-(diphenylphosphinoxy)phenylpalladium(II) bromide (1), 2-(phenylimino)-5-methoxy-6-(diphenylphosphinoxy)phenylpalladium(II) bromide (2), 2-(phenylthiomethyl)-5-methoxy-6-(di

Full text of DOI:10.1002/adsc.201000204

FULL PAPERS
DOI: 10.1002/adsc.201000204
PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
in Homoallylation Reactions
Jie Li,a Maxime Siegler,b Martin Lutz,b Anthony L. Spek,b
Robertus J. M. Klein Gebbink,a,* and Gerard van Kotena,*
a
Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Faculty of Science, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands
Fax : (+31)-30-252-3615; e-mail: r.j.m.kleingebbink@uu.nl or g.vankoten@uu.nl
Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University,
b
Padualaan 8, 3584 CH Utrecht, The Netherlands
Received: March 16, 2010; Revised: August 10, 2010; Published online: October 12, 2010
Dedicated to Prof. Dr. Carmen Najera on the occasion of an important birthday in friendship and with great
admiration for her excellent contribution to synthetic organic chemistry.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000204.
Abstract: Novel PCN- and PCS-pincer palladium in the tandem reaction of aldehydes or sulfonimines
complexes
(diphenylphosphinoxy)phenylpalladium(II) bromide investigated. It was found that the catalytic activities
(1), 2-(phenylimino)-5-methoxy-6-(diphenylphos- are very dependent on the combination of the E-
phinoxy)phenylpalladium(II) bromide (2), 2-(phenyl- donor moieties in the pincer ligand. Generally, PCS-
thiomethyl)-5-methoxy-6-(diphenylphosphinoxy)phe- pincer complex 3 and its cationic complex 3a outper-
nylpalladium(II) bromide (3), 2-(phenylthio)methyl- form PCN-pincer complexes 1 and 2 as well as their
5-methoxy-6-(diphenylphosphinoxy)phenylpalladi- cationic complexes 1a and 2a in both the homoallyla-
um(II) chloride (4) have been synthesized (55–95% tion and tandem reaction. PCS-pincer palladium
2-(dimethylamino)methyl-5-methoxy-6- with allyl chlorides and hexamethyldistannane were
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
yield) by using a flexible and straightforward syn- complexes 3 and 3a seem to benefit in a positive
thetic route starting from isovanillin as the common sense from the combination and cooperativity of a p-
precursor. The structures of complexes 1, 2 and, 4 in accepting phosphorus donor and a s-donating sulfur
the solid state were determined using X-ray diffrac- donor.
tion and showed a typical pincer-type geometry. The
catalytic activities of 1–3 in the homoallylation reac-
tions of aldehydes and allyl
ACHTUNGTNER(NUNG tributyl)stannane as well Keywords: homoallylation; palladium; pincer com-
as of their corresponding cationic complexes 1a–3a plexes; tandem catalysis
Introduction
genation, Michael addition, aldol condensation, ho-
moallylation, and a number of other reactions.[1]
The greater part of the known ECE-pincer metal
In the past decades, ECE-pincer metal complexes
have attracted much attention and a variety of struc- complexes have identical ortho-substituents along the
[2]
tures with various donor atom containing groups, e.g., Cipso metal bond, i.e., these are of the NCN, PCP,[3]
ꢀ
E=NR2, SR, PR2, SeR, OPR2, OP(OR)2 have been or SCS[4] types. Recently, ECE’-type (E¼E’) pincer
synthesized.[1a,b,c] A common characteristic of some of complexes, i.e., pincer ligands in which the ortho-sub-
these complexes is their robustness, i.e., thermal sta- stituents are different, have opened a new aspect of
bility and stability of the s-M C bond against oxida- pincer chemistry.[5–7] As elaborated in Figure 1, some
ꢀ
tion and hydrolysis. In addition, a range of ECE- examples of PCN- and PCS-pincer palladium com-
ꢀ
pincer metal complexes show remarkable performan- plexes and their applications in C C cross coupling
ces as catalysts in C C cross coupling, transfer hydro- reactions,[5] aldol condensations and tandem cataly-
ꢀ
sis[6] have been independently reported by different
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PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
Figure 2. Strategy to access PCN- and PCS-pincer palladium
complexes.
lytic performances in the stannylation/homoallylation
tandem reaction.
The use of isovanillin as the key building block, as
pioneered by Nishiyama and co-workers (see complex
D in Figure 1),[11] has allowed the regioselective intro-
duction of oxazolinyl nitrogen donor and diphenyl-
phosphinite phosphorus donor functionalities by che-
moselective conversion of the two independent func-
tional groups, i.e., the aldehyde and hydroxy groups.
Here, we report on the development of a synthetic
Figure 1. Representative examples of PCN- and PCS-pincer
palladium complexes.
groups. The PCN-pincer Pd complexes A and B are methodology to synthesize ECE’-pincer arene and
able to catalyze the Suzuki–Miyaura arylation aryl bromide ligands along with their corresponding
of benzyl halides and other non-activated coupling palladium complexes starting from isovanillin as the
partners,[5b,c]
[Pd{C6H3A(CH2PPh2)-2-(CH2SPh)-6}
while
the
PCS-pincer
(MeCN)]
complex general building block (Figure 2).
G
G
G
(BF4)
C
is an excellent catalyst in the stannylation/homoallyla-
tion tandem reaction of allyl chloride, hexamethyldi- Results
tin and aldehydes or aryl sulfonimines (Scheme 1).[6]
It is worthy of note that several pincer palladium Synthesis of PCN-Pincer Palladium Complex 1
complexes have been reported as active catalysts for
only one of the two reactions that are part of this Different from the literature procedure,[5a,12] which in-
tandem reaction.[6,8–10] For example, the corresponding volved subsequent reduction, bromination and amina-
PCP-pincer palladium complex [Pd
ACHTUGNTRENGUN{C6H3CAHTUNTGREN(NUGN CH2PPh2)2- tion reaction steps, in the present synthetic route to 6
2,6}(MeCN)](BF4) is an active catalyst in the homoal- the aldehyde group of isovanillin 5 was directly con-
A
ACHTUNGTRENNUNG
lylation of aldehydes or sulfonimines with allylstanna- verted into a (dimethylamino)methyl moiety in mod-
ne,[8a] but is not able to catalyze the stannylation reac- erate yield through a one-pot reductive amination re-
tion. Vice versa, NCN-pincer palladium complexes action (Scheme 2).[13] The resulting 6 was then reacted
catalyze the stannylation reaction, but not the homo- with diphenylphosphine chloride in the presence of
allylation.[10] On the other hand, SCS-pincer palladi- triethylamine to yield the corresponding ECE’-pincer
um complexes, like complex C, are able to catalyze arene ligand 7 in good yield and acceptable purity.
each of the two reactions of the tandem reaction.[6]
Compound 7 was used in the next metallation step
These observations prompted us to study the syn- without further purification. A stoichiometric amount
theses of ECE’-pincer palladium complexes bearing of PdCl2 was added to a solution of 7 in toluene and
other combinations of s-donating (i.e., N or S) and p- the resulting reaction mixture was refluxed at 1108C
accepting (i.e., P) donors in order to study their cata- under N2 for 18 h. After purification, the desired
Scheme 1. Cationic PCS-pincer Pd complex C-catalyzed stannylation/homoallylation tandem reaction.
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PCN-pincer coordination mode. The palladium atom
is located at the central position of a distorted square-
planar structure, with the chloride group occupying
the remaining trans-position with respect to Cipso. The
nitrogen donor and phosphorus donor are mutually
ꢀ
ꢀ
trans-positioned, with
a
P1 Pd1 N1 angle of
159.53(5)8. The phenyl group C11/C16 is disordered.
The value of the occupancy factor given for the major
component of the disordered phenyl group refined to
0.646(14).
Synthesis of PCN-Pincer Palladium Complex 2
As depicted in Scheme 3, the novel PCN pincer palla-
dium complex 2 has been derived from isovanillin
bromide 8.[14] Reaction of 8 with aniline in refluxing
chloroform gave imine 9 in good yield. A subsequent
reaction with diphenylphosphine chloride in the pres-
ence of triethylamine afforded the new PCN-pincer
aryl bromide ligand 10 in quantitative yield. After re-
Scheme 2. Synthesis of PCN-pincer palladium complex 1.
Reagents and conditions: i) NMe2H·HCl, NaOAc,
NaBH3CN, ethanol, 24 h, 53%; ii) Ph2PCl, Et3N, 0 8C to
room temperature, 16 h, quantitative yield; iii) PdCl2, tolu-
ene, 1108C, 18 h, 55%.
PCN-pincer palladium chloride complex 1 was ob- moving NEt3·HCl by simple filtration and without fur-
tained in 55% yield. ther purification, a solution of 10 in toluene was
The molecular geometry of 1 in the solid state was added to a solution of a zerovalent Pd precursor {i.e.,
determined by single crystal X-ray diffraction analysis [Pd2A(dba)3·CHCl3]} in toluene. The reaction mixture
CTHUNGTRENNUNG
at 150(2) K. Suitable (colourless) single crystals of 1 was warmed to 808C for 5 h to yield PCN-pincer pal-
were obtained by slow diffusion of hexanes into a di- ladium chloride complex 2 in 84% yield.
chloromethane solution containing the complex at
room temperature. As shown in Figure 3, the mono- of hexanes into a dichloromethane solution of 2) was
ACHTUNGTRENNUNGanionic pincer aryl ligand adopts the typical mer- determined by single crystal X-ray diffraction analysis
The structure of 2 in the solid state (slow diffusion
at 150(2) K. As shown in Figure 4, 2 adopts a typical
mer-pincer coordination mode. Different to complex
1, complex 2 possesses an almost perfect square-
planar structure, as the double bond of the imine can
become conjugated with the aromatic backbone. The
palladium atom is located at the central position of
this square-planar structure, while the bromine atom
Figure 3. Displacement ellipsoid plot (50% probability
level) of the asymmetric unit of 1 at 150(2) K. H atoms and
the minor components of the disordered phenyl ring C11/
C16 are omitted for the sake of clarity. Selected bond
ꢀ
ꢀ
lengths [ꢁ] and angles [8]: Pd1 C1 1.957(2), Pd1 P1
Scheme 3. Synthesis of PCN-pincer complex 2. Reagents and
conditions: i) PhNH2, CHCl3, MgSO4, reflux, 16 h, 85%; ii)
Ph2PCl, Et3N, 0 8C to room temperature, 16 h, quantitative
ꢀ
ꢀ
ꢀ
2.1858(6), Pd1 N1 2.1592(17), Pd1 Cl1 2.3695(6), N1 C7
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1.488(3), P1 O1 1.6380(16), P1 Pd N1, 159.53(5), C1 Pd1
Cl1 177.26(7).
yield; iii) Pd2ACHTNUGTRNEG(UN dba)3·CHCl3, toluene, 808C, 5 h, 84%.
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PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
occupies a position trans to Cipso. The nitrogen and
phosphorus donors are mutually located in trans-posi-
ꢀ
ꢀ
tions with a P1 Pd1 N1 angle of 158.08(4)8, which is
very similar to that of 1 (vide supra).
Synthesis of PCS-Pincer Palladium Complexes 3 and
4
Besides PCN-pincer Pd complexes 1 and 2, the novel
PCS-pincer complexes 3 and 4 were synthesized as a
proof of principle. As depicted in Scheme 4, the hy-
droxy group of isovanillin bromide 8 was first protect-
ed by an acetyl group to prevent side product forma-
tion during the bromination step. Subsequently, alde-
hyde 11 was converted into alcohol 12 by reduction in
the presence of NaBH4 in ethanol. Benzylic bromine
13 was prepared by reacting alcohol 12 with a small
excess of PBr3 under mild conditions. These first
three steps produced the respective products in suffi-
cient purity to enable their direct use in the next step.
Thioether formation to afford compound 14 was ac-
complished by reacting 13 with NaSPh in dry THF at
room temperature. Without purification, the resulting
acetyl ester product was saponified and the resulting
crude mixture was subjected to flash chromatography
on silica gel to isolate compound 14 in a yield of
57%. The phosphorus donor was introduced via a
Figure 4. Displacement ellipsoid plot (50% probability
level) of the asymmetric unit of 2 at 150(2) K. H atoms are
omitted for the sake of clarity. Selected bond lengths [ꢁ]
ꢀ
ꢀ
ꢀ
and angles [8]: Pd1 C1 1.9658(16), Pd1 P1 2.2020(5), Pd1
ꢀ
ꢀ
ꢀ
N1 2.1777(14), Pd1 Br1 2.5037(2), N1 C20 1.297(2), P1 O1
ꢀ
ꢀ
ꢀ
ꢀ
1.6513(12), P1 Pd N1, 158.08(4), C1 Pd1 Br1 177.68(5).
Scheme 4. Synthesis of PCS-pincer palladium complexes 3 and 4. Reagents and conditions: i) Ac2O, pyridine, 808C, 40 min,
90%; ii) NaBH4, EtOH, room temperature, 30 min, 95%; iii) 1.3 equiv. PBr3, Et2O, 0 8C, 2 h, 87%; iv) a) NaSPh, THF, room
temperature, 16 h; b) 3 mL 2.5N NaOH water solution, EtOH, 30 min, 57%; v) ClPPh2, Et3N, THF, room temperature, 16 h,
quantitative yield; vi) Pd2ACHTNUGTRNE(UGN dba)3·CHCl3, toluene, 808C, 5 h, 78%; vii) AgBF4, wet acetone, aqueous NaCl solution, 1 h, 95%.
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Jie Li et al.
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PCN- and PCS-Pincer Palladium Complex-Catalyzed
Homoallylation
The catalytic performances of the new PCN- and
PCS-pincer complexes were initially evaluated in the
homoallylation of aryl aldehydes with allyl-
AHCTUNGTRENNUNG
(16), PCS-pincer complex 3 affords the desired homo-
allylic alcohol 16a in good yield (Table 1, entry 3),
whereas complexes 1 and 2 merely gave 68% and
21% yields, respectively (Table 1, entries 1 and 2). Im-
proved product yields were obtained for all three
pincer complexes when para-NO2 functionalized elec-
trophile 17, which is an activated substrate (i.e., Ham-
mett constant of p-NO2 group is sp =0.78[18]), was
used (Table 1, entries 4–6 vs. entries 1–3). According
to these initial tests, complex 3 showed the best cata-
lytic activities amongst the three new pincer com-
plexes. Remarkably, in the case of complexes 1 and 2,
some Pd black was found to be present in the reaction
mixture after 5 h, whereas no Pd black was observed
for complex 3. This observation strongly indicated
that PCN complexes 1 and 2 decomposed during the
catalytic processes at 608C in DMF. Another sub-
strate, p-methylbenzaldehyde 18 (i.e., Hammett con-
stant of p-methyl group is sp =ꢀ0.17[18]) was evaluat-
ed as a slightly less electrophilic substrate using com-
plex 3 as catalyst. The yield of its homoallylic alcohol
Figure 5. Displacement ellipsoid plot (50% probability
level) of the asymmetric unit of 4 at 295(2) K. H atoms and
the minor components of the three phenyl groups are omit-
ted for the sake of clarity. Selected bond lengths [ꢁ] and
ꢀ
ꢀ
ꢀ
angles [8]: Pd1 C1 1.982(3), Pd1 P1 2.2011(9), Pd1 S1
ꢀ
ꢀ
ꢀ
2.3774(10), Pd1 Cl1 2.3718(9), S1 C7 1.807(4), P1 O2
ꢀ
ꢀ
ꢀ
ꢀ
1.632(2), P1 Pd S1, 165.02(3), C1 Pd1 Cl1 176.35(8).
ꢀ
P O coupling by reacting 14 with diphenylphosphine product 18a dramatically decreased to 33% (Table 1,
chloride in the presence of triethylamine. Finally, pal- entry 7). Finally, 3 catalyzes the conversion of p-bro-
ladation was smoothly carried out by oxidatively in- mobenzaldehyde 19 (i.e., Hammett constant of p-Br
serting
a
zerovalent
Pd
species
{i.e.,
ꢀ
A
CHTUNGTRENNUNG
gave the novel PCS-pincer palladium complex 3 in
good yields after purification.
Table 1. Homoallylation reactions of aryl aldehydes and
allyl(tributyl)stannane catalyzed by Pd complexes 1–3.
AHCTUNGTRENNUNG
As we could not obtain suitable single crystals for
3, complex 4 bearing a chloride anion was pre-
pared.[15] Colourless crystals of this complex were ob-
tained by slow diffusion of hexanes into a dichlorome-
thane solution of 4 at room temperature. The struc-
ture of 4 was determined by single X-ray diffraction
analysis at room temperature. As shown in Figure 5,
the palladium atom is located at the central position
of a distorted square-planar structure, in which the
chloride group occupies a trans position with respect
to Cipso. The sulfur and phosphorus donors are mutu-
ally located in trans positions, with a P1 Pd1 S1
angle of 165.02(3)8, which is larger than those of PCN
pincer complexes 1 [159.53(5)8] and 2 [158.08(4)8].
The values of the occupancy factors given for the
major components of the disordered phenyl groups
C8/C13, C15/C20 and C21/C26 refined to 0.743(6),
0.504(10) and 0.52(2), respectively.
Entry
Electrophile
Catalyst
Product
Yield [%][a]
1
2
3
4
5
6
7
8
16
16
16
17
17
17
18
19
1
2
3
1
2
3
3
3
16a
16a
16a
17a
17a
17a
18a
19a
68 (75)
21 (30)
84 (90)
90 (>95)
85 (>95)
93 (>95)
33 (40)
ꢀ
ꢀ
71 (75)
[a]
1
Isolated yields; product formation estimated by H NMR
spectroscopy with mesitylene as internal standard pre-
sented in parentheses.
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PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
group is sp =0.2318) into homoallylic alcohol 19a in a complexes 1a–3a. These cationic complexes, compris-
yield of 71% (Table 1, entry 8).
ing one equivalent of H2O as co-ligand, were easily
prepared by reacting the corresponding pincer halide
complexes 1–3 with AgBF4 in CH2Cl2 in the presence
of water (Figure 6). The catalytic activities of these
complexes in the tandem reaction are summarized in
Table 2, Table 3, and Table 4.
PCN- and PCS-Pincer Palladium Complex-Catalyzed
in Tandem Catalysis
As cationic PCS-pincer palladium complex C was
In order to compare their catalytic activities, the
found to be an active catalyst for the stannylation/ho- cationic PCN- and PCS-pincer complexes, 1a–3a were
moallylation tandem reaction,[6] we were interested in tested in the tandem reaction of p-NO2-benzaldehyde
investigating the catalytic performance of the corre- 17 and allyl chloride in the presence of hexamethyl-
sponding cationic PCN- and PCS-pincer palladium distannane. Complexes 1a and 3a afforded compara-
ble isolated yields as earlier reported (63%) for the
PCS-pincer complex C (see Figure 1),[6] while the
PCN-pincer complex 2a showed a slightly lower cata-
lytic activity. Complex 3a outperformed PCN-pincer
complexes 1a and 2a as well as PCS-pincer complex C
according to the conversions of electrophile 17 and
the isolated yields of homoallyl product 17a (Table 2,
entry 3 vs. entries 1 and 2).
A reaction condition optimization was then carried
out with complex 3a and electrophile 17. It was found
that the conversions of 17 depended highly on the po-
larity of the solvent in which the reaction was carried
out. Polar solvents like THF and DMF afforded good
conversions and isolated yields (Table 2, entries 3 and
4), whereas non-polar solvents like benzene and di-
chloromethane gave moderate conversions of 30–50%
(Table 2, entries 5 and 6). Accordingly, DMF was
Figure 6. Cationic PCN- and PCS-pincer complexes 1a–3a.
kept as the optimal solvent in further catalytic tests.
Table 2. Tandem reactions of aldehydes catalyzed by pincer complexes 1a–3a.[a]
Entry
Electrophile
Cat.
Conditions (solvent/T [8C]/time)
Product
Conversion [%][b]
Yield [%][c]
1
2
3
4
5
6
17
17
17
17
17
17
17
17
20
16
1a
2a
3a
3a
3a
3a
3a
3a
3a
3a
DMF/r.t./18 h
DMF/r.t./18 h
DMF/r.t./18 h
THF/r.t./18 h
DCM/r.t./18 h
Benzene/r.t./18 h
DMF/40/18 h
DMF/40/18 h
DMF/r.t./16 h
DMF/r.t./16 h
17a
17a
17a
17a
17a
17a
17a
17a
20a
16a
70
60
85
80
50
30
80
59
43
70
65
–
–
7
68
71
68[e]
18[e]
8[d]
9
80
71[e]
19[e]
10
[a]
Reagents and conditions: 0.1 mmol of aldehyde, 0.12 mmol of allyl chloride, 0.12 mmol of Me3SnSnMe3, 0.005 mmol of Pd
complex, appointed solvent, reaction temperature and time.
Conversions of electrophiles were monitored by H NMR with mesitylene as internal standard.
Isolated yields after column chromatography.
2.0 equivalents of allyl chloride were used.
Detected by GC with mesitylene as internal standard.
[b]
[c]
[d]
[e]
1
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The reaction temperature was then slightly elevated novel class of homoallylic compounds that allow
to 408C using 2.0 equivalents of allyl chloride, which facile deprotection to yield primary allylamines.[16]
did not show an improvement of the conversions
In addition, cinnamyl chloride was examined as al-
(Table 2, entry 3 vs. entries 7 and 8). The catalytic ac- lylic reagent in order to reveal the diastereoselectivity
tivities of 3a in reactions with two other electrophiles of 3a in the tandem catalysis. Entries 1 and 2 in
were then investigated. The trifluoromethyl-function- Table 4 show that both the isolated yield and diaste-
alized electrophile 20 (i.e., Hammett constant of p- reoselectivity of this reaction are dependent on the
CF3 group is sp =0.54[18]) gave a slightly decreased nature of the solvent in which the reaction was car-
conversion and isolated yield compared to those of ried out. For p-NO2-benzaldehyde 17 both the yield
benchmark electrophile 17 (Table 2, entry 9). In con- and diastereomeric ratio (i.e., anti/syn) were remarka-
trast, the conversion and isolated yield dramatically bly improved by using DMF instead of THF as the
decreased to less than 20% for benzaldehyde 16 (R= solvent (Table 4, entry 1 vs. entry 2). In contrast to
H) (Table 2, entry 10).
the strong solvent effect, an increase of the tempera-
Next to aryl aldehyde substrates, a series of sulfon- ture did not significantly affect the yield and diaste-
ACHTUNGTRENNUNG
conversion and 65% isolated yield, which is consider-
ably different than the conversion and isolated yield
observed in the tandem reaction for benzaldehyde Discussion
(Table 3, entry 3 vs. Table 2, entry 10). Even the N,N-
dimethylsulfamoyl-protected sulfonimines 23 and 24 In general, the title ECE’-pincer palladium complexes
were converted providing reasonable yields of the are able to catalyze the homoallylation and stannyla-
corresponding allylic products (Table 3, entries 4 and tion/homoallylation tandem reactions with aldehydes
5). These two homoallylsulfonamides belong to a and sulfonimines as electrophiles. Probably due to the
Table 3. Cationic PCS-pincer palladium complex 3a-catalyzed tandem allylation of sulfonimines.[a]
Entry
Electrophile
Product
Conversion [%][b]
Yield [%][c]
1[d]
2
3
4
5
21
21
22
23
24
21a
21a
22a
23a
24a
80
90
80
75
60
63
77
65
60
45
[a]
Reagents and conditions: 0.1 mmol of sulfonimine, 0.12 mmol of allyl chloride, 0.12 mmol of Me3SnSnMe3, 0.005 mmol of
Pd complex, 1 mL of DMF, 408C unless otherwise stated.
Conversions of electrophiles were monitored by H NMR with mesitylene as internal standard.
Isolated yields.
The reaction was performed at room temperature.
[b]
[c]
[d]
1
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PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
Table 4. PCS-pincer complex 3a catalyzed tandem allylation reactions with cinnamyl chloride.[a]
Entry
Electrophile
Product
Conditions (solvent/T [8C])
Conversion [%][b]
Yield [%][c]
dr[d]
1
2
3
4
17
17
17
21
17b
17b
17b
21b
THF/r.t.
DMF/r.t.
DMF/40
DMF/40
50
85
85
80
30
65
63
48e
3:2
5:1
6:1
1:5
[a]
Reagents and conditions: 0.1 mmol of aldehyde or sulfonimine, 0.12 mmol of cinnamyl chloride, 0.12 mmol of
Me3SnSnMe3, 0.005 mmol of Pd complex 3a, appointed solvent, temperature and reaction time.
Conversions of electrophiles were detected by H NMR with mesitylene as internal standard.
Isolated yields.
Diastereomeric ratio: anti/syn.
Only the pure syn product was isolated.
[b]
[c]
[d]
[e]
1
combination and cooperation of two different donor lytic activities without the concomitant formation of
moieties, these pincer complexes display quite dissim- Pd black.
ilar catalytic activities not only to each other, but also
The catalytic use of complex 3 in the homoallyla-
to the previously reported symmetrical ECE-pincer tion is limited to relatively strong electrophiles bear-
complexes. For instance, PCN-pincer complexes 1 and ing electron-withdrawing groups and shows a rather
2 showed low catalytic activities towards the bench- low catalytic activity towards weak electrophilic sub-
mark electrophile benzaldehyde in the homoallylation strates in comparison with earlier reported phosphite
reactions, and Pd black formed during the course of or phosphonite PCP-pincer palladium complexes
the reactions. Presumably, PCN-pincer ligands are (Table 1, entries 7 and 8).[8a,b,e,f,9] Importantly, it was
hemi-labile under the reaction conditions (608C, found that the increasing order of yields agreed with
DMF), and the Pd atoms might dissociate from the the increasing order of Hammett constants of the
complexes during catalysis (see Scheme 5). Different- para-functional groups in the aldehyde substrates.
ly, PCS-pincer complex 3 could take advantage of the This observation implies that the electrophilic charac-
combination of S and P donors, and conserves its con- ter of the aldehydes can affect the rate-determining
figuration during catalysis. Thus, it shows better cata- step in the homoallylation of aldehydes. This observa-
Scheme 5. Reaction pathway of PCE-pincer Pd complexes-catalyzed stannylation/homoallylation tandem reaction.
Adv. Synth. Catal. 2010, 352, 2474 – 2488
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Jie Li et al.
FULL PAPERS
tion strongly suggests that the catalytic activity of
According to the Hammett constant for a meta-
ꢀ
ECE’-pincer complexes in homoallylation reactions OMe group with respect to the C Pd s-bond, i.e.,
might not be further improved by simply combining sm =0.12,18 the meta-OMe groups in the ECE’-pincer
one electron-enriched donor and one electron-defi- complexes 1–3 and 1a–3a withdraw electron density
ꢀ
ꢀ
cient donor.
from the Cipso Pd bonds and thus polarize the Cipso
The stannylation/homoallylation tandem reaction Pd bonds in the direction of Cipso, consequently, it
involves three independent steps (Scheme 5): (i) stan- makes the Pd metal centers more positively charged.
nylation of allyl chloride, which is favoured by an Therefore, these pincer complexes can be considered
electron-enriched Pd metal center;[10] (ii) transmetala- as relatively electron-deficient and they are expected
tion of allylstannane and pincer palladium complex to to accelerate the transmetallation step in the allyla-
generate an h1-allyl Pd intermediate, which requires tion process but to slow down both the stannylation
an electron-deficient Pd metal center;[8a–c] and (iii) an of allyl chlorides and homoallylation of Pd h1-allyl
aryl aldehyde or aryl sulfonimine substrate electro- moieties according to previous reports.[8a–c] Efforts are
philically attacks the g position of the h1-allyl moiety, currently ongoing to corroborate this line of reasoning
which is enhanced by an electron-enriched Pd metal with the analogous complex that does not have a
center.[8a–c] Clearly, these reactions proceed under OMe group.
very complicated electronic requirements, and thus
the rational design of pincer ligands and the fine-
tuning of the electronic properties of their donor moi- Conclusions
eties are crucial for the further development of these
catalyzed reactions. Electronically tuneable ECE’- In this paper, we have developed a series of unsym-
pincer metal complexes seem to be an ideal choice in metrical PCN- and PCS-pincer palladium complexes
this respect, particularly when the complexes bear starting from isovanillin as the common building
both s-donating (i.e., N or S) and p-accepting (i.e., P) block, by using tuneable and straightforward synthetic
donors.
routes. Their syntheses enjoy good yields, mild reac-
In the tandem reactions of aldehydes, allyl chloride tion conditions, and excellent flexibility. Additionally,
and hexamethyldistannane, cationic aqua PCS-pincer the detailed structures of the PCN- (1 and 2) and
complex 3a also outperformed PCN-pincer complexes PCS- (3a) pincer palladium complexes in the solid
1a and 2a, albeit that its catalytic activity towards less state have been determined by single crystal X-ray
electrophilic substrates is still not ideal, which is con- analyses. The catalytic performances of these com-
sonant with the results observed for the correspond- plexes have been examined in homoallylation reac-
ing, neutral PCS-pincer palladium chloride 3 in homo- tions of aldehydes with allylACTHNUTRGNEU(GN tributyl)stannane as well
allylation reactions (vide supra). Interestingly, the cat- as the tandem reactions of allyl chlorides, aldehydes
alytic activity of complex 3a was not significantly af- or sulfonimines, and hexamethyldistannane in order
fected by using less electrophilic sulfonimine sub- to understand the influence of the combination of dif-
strates, which indicates that the electrophilic ferent E-donors on the course of these reactions. In
character of sulfonimines cannot predominate the the homoallylation reactions, PCS-pincer complex 3
rate-determining step in their homoallylation. Impor- outperforms PCN-pincer complexes 1 and 2 to afford
tantly, under mild ambient conditions Pd-black was yields warranting further investigations for strong
not formed in any of the tandem catalysis reactions. electrophiles, although its catalytic activity is rather
Electron-deficient phosphinite PCS-pincer complex low towards less electrophilic substrates. In the
3a overall showed comparable catalytic activities and tandem reaction, it was also found that cationic PCS-
stereoselectivities to the relatively electron-enriched pincer complex 3a is superior to cationic PCN-pincer
phosphine
[Pd{C6H3A(CH2PPh2)-2-(CH2SPh)-6}
regardless of the combination of electrophile [i.e., p- markably, aryl sulfonimines generally present higher
cationic
PCS-pincer
complex
C
complexes 1a and 2a in terms of catalytic activities to-
A
N
ACHTUNGTRENNUNG
NO2-benzaldehyde
and
N-(4- reactivities in comparison with aryl aldehydes when
nitrobenzylidene)benzenesulfonAHCTUNGTREGaNNUN mide] and allyl chlo- PCS-pincer complex 3a is used as the catalyst. These
ride (i.e., allyl chloride and cinnamyl chloride). Yet, investigations have shown the remarkable catalytic
3a is outperformed by SCS-pincer complex [Pd{C6H3 properties of PCS-pincer palladium complexes in the
(CH2SPh)2-2,6}(MeCN)] (BF4) bearing an electron- stannylation/homoallylation tandem reaction, but also
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
donating SCS-pincer ligand in both catalytic activity showed that these are inferior to the activity of sym-
and diastereoselectivity.[6] These observations imply metrical SCS-pincer palladium complexes. Due to the
that the better catalytic activity in the tandem reac- flexible synthetic strategy, the catalytic performances
tion could require pincer complexes bearing relatively of PCS-pincer complexes could be further improved
stronger electron-donating pincer ligands and less by varying either the thioether or the phosphinite
electrophilic palladium(II) metal centers.
functionalities with different substituents. Current ef-
2482
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Adv. Synth. Catal. 2010, 352, 2474 – 2488

PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
56.2, 45.2; anal. calcd. for C 66.27, H 8.34, N 7.73; found: C
66.26, H 8.40, N 7.69.
forts in our laboratory aim at both the development
of more active PCS-pincer complexes and of their
chiral analogues in asymmetric tandem catalysis, as
well as at the understanding of the subtle electronic
demand of the catalytic stannylation/homoallylation
reaction.
1-(Dimethylamino)methyl-3-diphenylphosphinooxy-4-
methoxybenzene (7)
Compound 6 (0.182 g, 1 mmol) was in dissolved dry toluene
(20 mL) and pre-cooled to 08C on an ice bath. To this solu-
tion was consecutively added Et3N (280 mL, 2 mmol) and
Ph2PCl (189 mL, 1 mmol) via an airtight syringe. After the
addition, the reaction mixture was allowed to warm to room
temperature and was vigorously stirred for 16 h. The result-
ing white suspension was filtered through a short pad of
Celite, after which the filtrate was concentrated under re-
duced pressure to give crude compound 7, which was ob-
tained in quantitative yield and used in the next step with-
out further purification. Yield:0. 037 g (quantitative);
1H NMR (CDCl3, 400 MHz): d=7.66–7.74 (m, 4H, PPhH),
7.35–7.48 (m, 6H, PPhH), 6.71 (d, 3J=8.0 Hz, 1H, ArH),
Experimental Section
General Remarks
All reactions were performed under a dry N2 atmosphere
using standard Schlenk techniques unless stated otherwise.
Glassware was oven dried or flame dried prior to use. Di-
ethyl ether and THF were freshly distilled over metallic
sodium prior to use. Toluene, Et3N, and DMF were distilled
over CaH2 and stored under N2 at ꢀ308C. Diphenylphos-
phine chloride was freshly distilled under reduced pressure
prior to use. 2-Bromo-3-hydroxy-4-methoxybenzaldehyde 8
was prepared according to a reported procedure.[14] All
other reagents were purchased from ACROS Organics and
Sigma–Aldrich Chemical Co. Inc, and used as received.
1H NMR (400.0 MHz), 13C NMR (100.6 MHz), and
31P NMR (161.9 MHz) spectra were recorded at room tem-
perature in CDCl3 on a Varian 400 MHz spectrometer.
Chemical shift values are reported in ppm (d) relative to
(CH3)4Si (1H and 13C NMR) or a capillary containing 85%
H3PO4 in D2O (31P{1H} NMR). Flash chromatography was
performed using ACROS silica gel, 0.06–0.200 mm, pore di-
ameter ca. 6 nm. MS measurements were carried out on an
Applied Biosystems Voyager DE-STR MALDI-TOF MS.
Elemental microanalyses were performed by Dornis und
Kolbe, Mikroanalytisches Laboratorium, Mꢃlheim a/d Ruhr,
Germany.
3
6.42 (d, J=8.0 Hz, 1H, ArH), 6.38 (s, 1H, ArH), 3.98 (s,
3H, OCH3), 3.41 (s, 2H, CH2NMe2), 2.34 (s, 6H, NMe2);
13C NMR (CDCl3, 100.6 MHz): d=168.6, 153.9 (d, J=
3.0 Hz), 148.5, 144.7, 142.5 (d, J=18 Hz), 131.2 (d, J=
7.6 Hz), 131.0 (d, J=7.6 Hz), 129.8 (d, J=4 Hz), 128.3 (d,
J=7.5 Hz), 126.1, 123.4, 110.9, 53.6, 50.7; 31P{1H} NMR
(CDCl3, 161.9 Hz): d=126.4 (s).
2-(Dimethylamino)methyl-5-methoxy-6-
(diphenylphosphinoxy)phenyl palladium(II) Bromide
(1)
To a solution of compound 7 (0.19 g, 0.5 mmol) in dry tolu-
ene (15 mL) was added PdCl2 (0.09 g, 0.5 mmol) and this
mixture was refluxed at 1108C under N2 for 18 h. After
cooling, the reaction mixture was filtered through a short
pad of Celite and the filtrate was concentrated under re-
duced pressure to yield a yellowish solid. The solid was sub-
jected to flash chromatography with EtOAc/hexanes (9:1,
v/v) as eluent. Analytically pure product was obtained as a
slightly yellow and air stable solid. Yield: 0.139 g
1-(Dimethylamino)methyl-3-hydroxy-4-methoxy-
benzene (6)
1
Isovanillin (2 g, 13.15 mmol) was reacted with dimethyla-
mine hydrochloride (2.32 g, 28.45 mmol) and sodium cyano-
borohydride (1.44 g, 22.85 mmol) in methanol (100 mL) in
the presence of sodium acetate (1.82 g, 22.20 mmol). The
pH value of the solution was maintained in the range of 7–8
throughout the reaction, if necessary by the addition of con-
centrated HC1. The solution was stirred at room tempera-
ture during 24 h. Acetone (## mL) was then added, followed
by 6N HCl (until pH 2–3 was reached). The solvent was re-
moved under vacuum, and the residue was dissolved in
water (50 mL) and extracted with EtOAc (4ꢄ15 mL). The
combined organic layers upon evaporation of the solvent
yielded a mixture of starting aldehyde and the correspond-
ing alcohol. The remaining aqueous phase was made basic
(pH 8–9) and extracted with EtOAc (5ꢄ25 mL). The organ-
(0.275 mmol, 55%); H NMR (CDCl3, 400 MHz): d=7.97–
3
8.03 (m, 4H, PPhH), 7.47–7.49 (m, 6H, PPhH), 6.70 (d, J=
3
8.0 Hz, 1H, ArH), 6.64 (d, J=8.0 Hz, 1H, ArH), 4.05 (s,
2H, CH2NMe2), 3.86 (s, 3H, OCH3), 2.89 (s, 6H, NMe2);
13C NMR (CDCl3, 100.6 MHz): d=50.5 (d, J=11 Hz), 146.8,
144.5 (d, J=20 Hz), 140.3, 133.5 (d, J=54 Hz), 131.8–132.2
(m), 128.8 (d, J=20 Hz), 117.6, 110.9, 56.6, 50.5; 31P{1H}
NMR (CDCl3, 161.9 Hz): d=155.4 (s). MS (MALDI-TOF):
m/z=470.81, calcd. for [MꢀCl]+: 470.82; anal. calcd. for
C22H23NClO2PPd: C 52.19, H 4.58, N 2.77, P 6.12, Pd 21.02;
found: C 52.22, H 4.61, N 2.71, P 6.09, Pd 21.01;
1-Phenylimino-2-bromo-3-hydroxy-4-methoxybenzene
(9)
ic extracts were dried over MgSO4, and the solvent was re-
moved under vacuum, thus yielding an amber oil residue.
An analytical sample was recrystallized from a saturated hot
hexanes solution of the crude product. Yield: 1.27 g (53%),
Compound
8
(0.4620 g, 2 mmol),[13] aniline (184 mL,
2 mmol), and MgSO4 (1 g) were suspended in CHCl3
(20 mL) and refluxed at 758C under N2 for overnight. After
cooling, the reaction mixture was filtered through a short
pad of Celite and the filter cake was washed with CH2Cl2
(3ꢄ5 mL). The combined filtrate was concentrated to dry-
ness in vacuo to yield a yellow-brown solid that was used
for the next step without further purification. An analytical
1
white solid; H NMR (CDCl3, 400 MHz): d=6.83–6.91 (m,
3H, ArH), 6.05 (br s, 1H, OH), 3.91 (s, 3H, OCH3), 3.39 (s,
2H, CH2NMe2), 2.28 (s, 6H, NMe2); 13C NMR (CDCl3,
100.6 MHz): d=146.2, 145.8, 131.5, 121.0, 116.0, 110.7, 63.9,
Adv. Synth. Catal. 2010, 352, 2474 – 2488
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asc.wiley-vch.de
2483

Jie Li et al.
FULL PAPERS
sample was recrystallized from a saturated hot hexanes solu-
tion of the crude product. Yield: 0.52 g (85%); 1H NMR
(CDCl3, 400 MHz): d=8.81 (s, 1H, PhN=CHAr), 7.91 (d,
3J=8.4 Hz, 1H, ArH), 7.40 (m, 2H, NPhH), 7.27 (m, 3H,
room temperature under N2 for 1 h. Subsequently, pyridine
was removed from the mixture under reduced pressure on a
rotary evaporator. The resulting oily product was redis-
solved in demineralized water (20 mL) and the pH value of
the mixture was then adjusted to 7 by adding saturated
aqueous NaHCO3 solution. The neutralized aqueous solu-
tion was extracted with EtOAc (3ꢄ15 mL) and the com-
bined organic layers were dried over MgSO4. After filtra-
tion, filtrate solution was evaporated to dryness to afford a
white solid. The product was pure enough to be used in the
next step without further purification. Yield: 1.22 g (90%);
1H NMR (CDCl3, 400 MHz): d=10.14 (s, 1H, ArC=OH),
7.76 (d, 3J=8.8 Hz, 1H, ArH), 6.94 (d, 3J=8.8 Hz, 1H,
ArH), 3.83 (s, 3H, OCH3), 2.33 (s, 3H, OAc); 13C NMR
(CDCl3, 100.6 MHz): d=190.3, 167.8, 157.4, 138.2, 128.9,
127.2, 122.6, 111.2, 56.8, 20.5; anal. calcd.: C 43.98, H 3.32,
Br 29.26; found: C 43.91, H 3.28, Br 29.31.
3
NPhH), 6.93 (d, J=8.4 Hz, 1H, ArH), 6.06 (br s, 1H, OH),
3.98 (s, 3H, OCH3); 13C NMR (CDCl3, 100.6 MHz): d=
159.4, 151.9, 149.7, 143.3, 129.4, 128.0, 126.4, 121.0, 112.7,
109.9, 56.7; anal. calcd.: C 54.92, H 3.95, N 4.58; found: C
54.96, H 3.91, N 4.53.
1-Phenylimino-2-bromo-3-diphenylphosphinoxy-4-
methoxybenzene (10)
Compound 9 (0.3062 g, 1 mmol) was azeotropically dried on
dry toluene (3ꢄ5 mL), and afterwards dissolved in dry tolu-
ene (20 mL) and precooled to 08C on an ice bath. To this
solution was consecutively added Et3N (280 mL, 2 mmol)
and Ph2PCl (189 mL, 1 mmol) via an airtight syringe. After
addition, the reaction mixture was allowed to warm to room
temperature and was vigorously stirred for 16 h. The result-
ing white suspension was filtered through a short pad of
Celite and the filter cake was washed with EtOAc (3ꢄ
5 mL). The filtrate was concentrated under reduced pressure
and was fully dried in vacuum to give crude compound 10,
which was used in the next step without further purification.
Yield:0.496 g (quantitative); 1H NMR (CDCl3, 400 MHz):
2-Bromo-3-hydroxymethyl-6-methoxyphenyl Acetate
(12)
To a solution of 11 (1.22 g, 4.5 mmol) in absolute ethanol
(20 mL) was added NaBH4 (0.1 g, 2.5 mmol) in one portion.
The reaction mixture was stirred at room temperature for
1 h. Ethanol was removed on a rotary evaporator and the
resulting mixture was diluted with water (20 mL) and ex-
tracted with EtOAc (3ꢄ20 mL). The combined organic
layers were dried over MgSO4, followed by filtration. A
white solid product was obtained after removing the solvent
under vacuum. The product was enough pure to be used in
the next step without further purification. An analytical
sample was recrystallized from a saturated hot hexanes solu-
tion of the crude product. Yield: 1.18 g (95%); 1H NMR
3
d=8.84 (s, 1H, CH=NPh), 7.80 (t, J=4.4 Hz, 4H, PPhH),
7.43–7.47 (m, 6H, PPhH), 7.24–7.41 (m, 6H, ArH+NPhH),
6.94 (d, 3J=8.8 Hz, 1H, ArH), 3.64 (s, 3H, OCH3);
13C NMR (CDCl3, 100.6 MHz): d=159.6, 151.1, 142.1 (d, J=
18 Hz), 132.0 (d, J=11 Hz), 131.3 (d, J=23 Hz), 129.4 (d,
J=12 Hz), 128.6, 128.5, 128.4, 126.3, 124.5, 122.4, 121.3,
111.6, 56.2; 31P{1H} NMR (CDCl3, 161.9 Hz): d=126.8 (s).
3
(CDCl3, 400 MHz): d=7.23 (d, J=8.4 Hz, 1H, ArH), 6.86
2-(Phenylimino)-5-methoxy-6-(diphenylphosphinoxy)-
phenylpalladium(II) Bromide (2)
3
(d, J=8.4 Hz, 1H, ArH), 4.59 (s, 2H, ArCH2OH), 3.78 (s,
3H, OCH3), 2.93 (s very br., 1H, CH2OH), 2.34 (s, 3H,
OAc); 13C NMR (CDCl3, 100.6 MHz): d=168.4, 153.2,
151.8, 137.9, 130.5, 126.3, 117.6, 109.7, 64.6, 56.5, 21.2; anal.
calcd.: C 43.66, H 4.03, Br 29.05; found: C 43.70, H 3.98, Br
29.09.
To a solution of compound 10 (0.2450 g, 0.5 mmol) in dry
toluene (15 mL) was added [Pd2ACHTNUTRGEN(UNG dba)3CCHCl3] (0.258 g,
0.25 mmol) and this mixture was warmed to 808C under N2
for 5 h. After cooling, the reaction mixture was filtered
through a short pad of Celite and the filtrate was concen-
trated under reduced pressure to yield a yellowish solid mix-
ture. The solid was subsequently subjected to flash chroma-
tography with EtOAc/hexanes (3:7 v/v) as eluent. The prod-
uct was obtained as a light yellow and air stable solid. Yield:
2-Bromo-3-bromomethyl-6-methoxyphenyl Acetate
(13)
1
To a solution of 12 (1.18 g, 4.3 mmol) in dry THF (20 mL)
pre-cooled at 08C on an ice bath was added freshly distilled
PBr3 (0.50 mL, 5 mmol) via syringe. After the addition, the
ice bath was removed and the reaction was quenched by
carefully adding saturated aqueous NaHCO3 (10 mL) after
being stirred at room temperature for 1.5 h. Afterwards, the
mixture was extracted with EtOAc (3ꢄ20 mL) and the com-
bined organic layers were dried over MgSO4. Removal of all
volatiles under vacuum yielded a yellowish oily product.
The product was pure enough to be used in the next step
without further purification. An analytical sample was ob-
tained by high vacuum flash distillation (0.5 mmHg/1858C).
Yield: 1.26 g (3.74 mmol, 87%); 1H NMR (CDCl3,
0.2387 g (0.4 mmol,80%); H NMR (CDCl3, 400 MHz): d=
3
8.07 (q, J=6.4 Hz, 4H, PPhH), 7.46–7.54 (m, 6H, PPhH),
3
7.40 (t, J=7.2 Hz, 2H, NPhH), 7.26–7.31 (m, 3H, NPhH),
8.24 (d, 4JP,H =4.8 Hz, 1H, CH=NPh), 7.22 (d, 3J=8.0 Hz,
1H, ArH), 6.70 (d, 3J=8.0 Hz, 1H, ArH), 3.94 (s, 3H,
OCH3); 13C NMR (CDCl3, 100.6 MHz): d=173.0, 156.5,
150.3 (d, J=10.4 Hz), 148.3 (d, J=20.7 Hz), 148.6, 138.1,
132.2–132.9 (m), 128.8–129.2 (m), 127.7, 125.6, 123.7, 109.5,
56.4; 31P{1H} NMR (CDCl3, 161.9 Hz): d=159.2 (s). MS
ꢀ
(MALDI-TOF): m/z=516.86, calcd. for C26H21NO2PPd [M
Br]+: 516.84; Anal. calcd.: C 52.33, H 3.55, N 2.35, P 5.19,
Pd 17.83; found: C 52.28, H, 3.65, N 2.38, P 5.14, Pd 17.81;
3
3
400 MHz): d=7.26 (d, J=8.8 Hz, 1H, ArH), 6.84 (d, J=
8.8 Hz, 1H, ArH), 4.56 (s, 2H, ArCH2Br), 3.77 (s, 3H,
OCH3), 2.32 (s, 3H, OAc); 13C NMR (CDCl3, 100.6 MHz):
d=164.8, 138.3, 129.4, 129.1, 122.0, 114.3, 56.2, 26.7, 20.3;
2-Bromo-3-formyl-6-methoxyphenyl Acetate (11)
Compound 8 (1.16 g, 5 mmol) and Ac2O (0.47 mL, 5 mmol)
were dissolved in pyridine (15 mL) and vigorously stirred at
2484
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Adv. Synth. Catal. 2010, 352, 2474 – 2488

PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
anal. calcd.: C 35.54, H 2.98, Br 47.28; found: C 35.44, H
3.05, Br 47.23.
0.25 mmol) and the mixture was warmed to 808C under N2
for 5 h. After cooling, the reaction mixture was filtered
through a short pad of Celite and the filtrate was concen-
trated under reduced pressure to yield a yellowish solid mix-
ture. The solid was subjected to flash chromatography with
EtOAc/hexanes [3:7 (v/v)] as eluent. An analytic sample
was obtained as a light yellow and air stable solid. Yield:
0.2400 g (0.39 mmol, 78%); 1H NMR (CDCl3, 400 MHz):
d=8.03 (q, 3J=8.0 Hz, 4H, PPhH), 7.79–7.82 (m, 2H,
SPhH), 7.45–7.52 (m, 6H, PPhH), 7.34–7.36 (m, 3H,
CH2SPhH), 6.85 (d, 3J=8.0 Hz, 1H, ArH), 6.70 (d, 3J=
8.0 Hz, 1H, ArH), 4.62 (s, 2H, ArCH2SPh), 3.86 (s, 3H,
OCH3); 13C NMR (CDCl3, 100.6 MHz): d=174.4, 157.8,
152.0 (d, J=14 Hz), 148.0, 142.5 (d, J=2 Hz), 142.0, 133.3,
133.0, 132.5 (d, J=3 Hz), 132.0 (d, J=14 Hz), 130.0, 129.6
(d, J=14 Hz), 118,9, 111.4, 57.1, 50.5; 31P{1H} NMR (CDCl3,
161.9 Hz): d=153.0 (s). MS (MALDI-TOF): m/z=535.90,
calcd. for C26H21NO2PPd [MꢀBr]+: 535.91; anal. calcd.: C
50.71, H 3.60, P 5.03, Pd 17.28, S 5.21; found: C 50.68, H
3.51, P 5.04, Pd 17.27, S 5.25;
1-(Phenylthio)methyl-2-bromo-3-hydroxy-4-methoxy-
benzene (14)
Compound 13 (0.25 g , 0.76 mmol) and NaSPh (0.1 g,
0.76 mmol) were stirred in dry THF (20 mL) at room tem-
perature for 16 h. After quenching the reaction with demin-
eralized water (20 mL), the crude product was extracted
with EtOAc (3ꢄ30 mL) and the combined organic layers
were dried over MgSO4. A yellow oily mixture was obtained
after removing of all volatiles. Without further purification,
it was directly treated with 2.5 N NaOH (3 mL) and ethanol
(10 mL) at room temperature for 30 min. The reaction mix-
ture was then diluted with demineralized water (50 mL) and
neutralized with acetic acid. The mixture was extracted with
EtOAc (3ꢄ30 mL) and the combined organic layers were
dried over MgSO4 and concentrated on a rotary evaporator
after filtration. The resulting beige oily product was subject-
ed to flash chromatography with EtOAc/hexanes [1:1 (v/v)]
as eluent, Rf =0.85. The desired product was obtained as a
1
beige viscous oil. Yield: 0.14 g (0.43 mmol, 57%); H NMR
(CDCl3, 400 MHz): d=7.33 (apparent d, 3J=7.6 Hz, 2H,
2-(Phenylthio)methyl-5-methoxy-6-(diphenylphos-
phinoxy)phenylpalladium(II) Chloride (4)
3
3
SPhH), 7.24 (t, J=7.6 Hz, 2H, SPhH), 7.19 (apparent t, J=
3
7.2 Hz, 1H, SPhH), 6.78 (d, J=8.4 Hz, 1H, ArH), 6.68, (d,
3J=8.4 Hz, 1H, ArH), 6.04, (s br, 1H, ArOH), 4.19 (s, 2H,
ArCH2SPh), 3.85 (s, 3H, OCH3); 13C NMR (CDCl3,
100.6 MHz): d=158.0, 141.9, 136.4, 129.9, 129.0, 126.8, 125.2,
123.6, 116.0, 114.7, 58.2, 37.8; anal. calcd.: C 51.70, H 4.03,
Br 24.57, S 9.86; found: C 51.61, H 3.98, Br 24.59, S 9.79;
To a solution of 3 (0.2400 g, 0.39 mmol) in acetone (20 mL),
AgBF4 (0.082 g, 0.42 mmol) was added and the reaction mix-
ture was stirred at room temperature for 1 h. The resulting
suspension was filtered over Celite and the residue was sub-
sequently washed with CH2Cl2 (15 mL) and demineralized
water (10 mL). A saturated aqeuous NaCl solution was
added and the biphasic mixture was stirred at room temper-
ature for 1 h. Afterwards, the two layers were separated and
the aqueous layer was further washed with CH2Cl2 (3ꢄ
10 mL). The combined organic layers were dried over
MgSO4 and evaporated to dryness. The residue was redis-
solved in a minimal amount of CH2Cl2, from which the
product precipitated upon addition of hexanes. After centri-
fugation, 4 was isolated as a slightly yellow powder. Yield:
0.2120 g (0.37 mmol, 95%); 1H NMR (CDCl3, 400 MHz):
d=8.03 (q, 3J=8.0 Hz, 4H, PPhH), 7.79–7.82 (m, 2H,
SPhH), 7.45–7.52 (m, 6H, PPhH), 7.34–7.36 (m, 3H,
CH2SPhH), 6.82 (d, 3J=8.0 Hz, 1H, ArH), 6.69 (d, 3J=
8.0 Hz, 1H, ArH), 4.59 (s, 2H, ArCH2SPh), 3.86 (s, 3H,
OCH3); 13C NMR (CDCl3, 100.6 MHz): d=171.4, 158.6,
151.8 (d, J=14 Hz), 147.9, 142.5 (d, J=2 Hz), 141.7, 133.3,
132.8, 132.5 (d, J=3 Hz), 131.5(d, J=14 Hz), 129.8, 129.5 (d,
J=14 Hz), 118,9, 110.8, 56.5, 50.0; 31P{1H} NMR (CDCl3,
161.9 Hz): d=150.8 (s). MS (MALDI-TOF): m/z=535.90,
calcd. for C26H21NO2PPd [MꢀCl]+: 535.91; anal. calcd.: C
54.65, H 3.88, P 5.42, Pd 18.63, S 5.61; found: C 54.70, H
3.91, P 5.34, Pd 18.27, S 5.52.
1-(Phenylthio)methyl-2-bromo-3-diphenylphosphin-
oxy-4-methoxybenzene (15)
Compound 14 (0.1630 g, 0.5 mmol) was azeotropically dried
in dry toluene 3 times, dissolved in dry toluene (20 mL) and
pre-cooled to 08C on an ice bath. To this solution was con-
secutively added Et3N (140 mL, 1 mmol) and Ph2PCl
(94.5 mL, 0.5 mmol) via an airtight syringe. After addition,
the reaction mixture was allowed to warm to room tempera-
ture and was vigorously stirred for 16 h. The resulting white
suspension was filtered through a short pad of Celite, and
the filtrate was concentrated under reduced pressure and
thoroughly dried under vacuum to give crude compound 15,
which was obtained in quantitative yield and used in the
next step without further purification. Yield: 0.260 g (quanti-
1
3
tative); H NMR (CDCl3, 400 MHz): d=7.83 (q, J=8.0 Hz,
4H, PPhH), 7.58–7.75 (m, 2H, SPhH), 7.37–7.48 (m, 6H,
3
PPhH), 7.24–7.31 (m, 3H, CH2SPhH), 6.71 (d, J=7.6 Hz,
1H, ArH), 6.43 (d, 3J=7.6 Hz, 1H, ArH), 4.34 (s, 2H,
ArCH2SPh), 3.95 (s, 3H, OCH3); 13C NMR (CDCl3,
100.6 MHz): d=163.4, 153.9 (d, J=3.6 Hz), 148.5, 144.9,
142.1 (d, J=18 Hz), 131.7 (d, J=7.5 Hz), 131.4 (d, J=
7.5 Hz), 129.8 (d, J=4 Hz), 128.3 (d, J=7.8 Hz), 129.7,
128.6, 117.5, 110.5, 57.1, 23.4, 14.9; 31P{1H} NMR (CDCl3,
161.9 Hz): d=126.9 (s).
General Procedure for Synthesizing Cationic Pincer
Palladium Complexes 1a–3a
PCN- or PCS-pincer palladium complexes 1–3 (0.1 mmol)
and AgBF4 (0.020 g, 0.1 mmol) were suspended in dichloro-
methane (5 mL). A few drops of demineralized water were
added to the reaction mixture and the resultant mixture was
stirred at room temperature for 1 h. To the mixture was
2-(Phenylthiomethyl)-5-methoxy-6-(diphenylphos-
phinoxy)phenylpalladium(II) Bromide (3)
To a solution of compound 15 (0.2550 g, 0.5 mmol) in dry
toluene (15 mL) was added Pd2ACHTNUTRGNEUNG(dba)3·CHCl3 (0.258 g,
Adv. Synth. Catal. 2010, 352, 2474 – 2488
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2485

Jie Li et al.
FULL PAPERS
added MgSO4 (0.5 g) and the white suspension was further
stirred for some time. Afterwards, the suspension was fil-
tered over a short pad of Celite and the filter cake was
washed with freshly distilled dichloromethane (3ꢄ5 mL).
The combined solutions were concentrated under vacuum
and the product was precipitated from the solution by the
addition of hexanes or ether. A white solid was then collect-
ed by centrifuge and was completely dried under vacuum.
Data for complex 1a: Yield: 0.050 g (0.087 mmol, 87%),
saturated aqueous KF solution (1 mL) and the reaction mix-
ture was stirred at room temperature for another 12 h. Af-
terwards, the reaction mixture was extracted with EtOAc
(3ꢄ2 mL) and the combined organic layers were dried over
MgSO4. The dry solution was concentrated on a rotary evap-
orator and was subjected to flash chromatography with
EtOAc/hexanes (3:7 v/v) as eluent. Chemical conversions
1
were estimated by H NMR with mesitylene as internal stan-
1
dard. The H and 13C NMR spectral data of the resulting al-
1
white solid. H NMR (CDCl3, 400 MHz): d=7.96–8.02 (m,
lylic alcohols agreed with those of reported compounds.[7c]
3
4H, PPhH), 7.47–7.50 (m, 6H, PPhH), 6.71 (d, J=8.0 Hz,
1H, ArH), 6.65 (d, 3J=8.0 Hz, 1H, ArH), 4.06 (s, 2H,
CH2NMe2), 3.84 (s, 3H, OCH3), 2.89 (s, 6H, NMe2);
13C NMR (CDCl3, 100.6 MHz): d=171.9, 155.8, 150.5 (d, J=
10 Hz), 146.8, 144.5 (d, J=18.8 Hz), 140.3, 133.5 (d, J=
50 Hz), 131.7–132.1 (m), 128.9 (d, J=18.8 Hz), 118.6, 110.9,
56.7, 50.5; 31P{1H} NMR (CDCl3, 161.9 Hz): d=152.3 (br s);
19F{1H} NMR (CDCl3, 376.3 Hz): d=ꢀ153.1 (s). MS
(MALDI-TOF): m/z= 488.86, calcd. for C22H25NO3PPd
[MꢀBF4]+: 488.83, m/z=470.80, calcd. for C22H23NO2PPd
[MꢀBF4ꢀH2O]+: 470.82; anal. calcd.: C 45.90, H 4.38, N
2.43, P 5.38, Pd 18.49; found: C 45.85, H 4.44, N 2.40, P
5.35, Pd 18.51.
Data for complex 2a: Yield: 0.049 g (0.08 mmol, 80%).
1H NMR (CDCl3, 400 MHz): d=8.23 (d, 4JP, H =5 Hz, 1H,
CH=NPh), 8.10 (q, 3J=6.8 Hz, 4H, PPhH), 7.46–7.54 (m,
6H, PPhH), 7.38 (t, 3J=7 Hz, 2H, NPhH), 7.26–7.33 (m,
3H, NPhH), 7.23 (d, 3J=8.0 Hz, 1H, ArH), 6.71 (d, 3J=
8.0 Hz, 1H, ArH), 3.93 (s, 3H, OCH3); 13C NMR (CDCl3,
100.6 MHz): d=172.8, 156.5, 150.1 (d, J=10 Hz), 148.6,
148.4 (d, J=21 Hz), 138.5, 132.3–132.7 (m), 129.1 (m), 128.7,
127.9, 125.6, 123.8, 109.5, 56.3; 31P{1H} NMR (CDCl3,
161.9 Hz): d=156.2 (br s); 19F{1H} NMR (CDCl3, 376.3 Hz):
d=ꢀ153.3 (s); MS (MALDI-TOF): m/z=534.80, calcd. for
C26H23NO3PPd [MꢀBF4]+: 534.86, m/z=516.88, calcd. for
C26H21NO2PPd [MꢀBF4ꢀH2O]+: 516.84; anal. calcd.: C
50.23, H 3.73, N 2.25, P 4.98, Pd 17.12; found: C 50.30, H
3.78, N 2.20, P 4.91, Pd 17.14.
Data for complex 3a: Yield: 0.058 g (0.09 mmol, 90%);
1H NMR (CDCl3, 400 MHz): d=8.02 (q, 3J=8.0 Hz, 4H,
PPhH), 7.80–7.83 (m, 2H, SPhH), 7.46–7.53 (m, 6H, PPhH),
7.32–7.35 (m, 3H, CH2SPhH), 6.88 (d, 3J=8.4 Hz, 1H,
ArH), 6.72 (d, 3J=8.4 Hz, 1H, ArH), 4.63 (s, 2H,
ArCH2SPh), 3.87 (s, 3H, OCH3); 13C NMR (CDCl3,
100.6 MHz): d=174.2, 152.1 (d, J=14 Hz), 148.0, 142.5 (d,
J=2 Hz), 142.1, 133.3, 133.0, 132.7 (d, J=3 Hz), 132.5 (d,
J=14 Hz), 130.3, 129.9 (d, J=14 Hz), 118,7, 111.5, 57.1,
50.6; 31P{1H} NMR (CDCl3, 161.9 Hz): d=151.7 (br s);
19F{1H} NMR (CDCl3, 376.3 Hz): d=ꢀ153.2 (s); MS
(MALDI-TOF): m/z=553.85, calcd. for C26H24O3PPdS
[MꢀBF4]+: 553.93, m/z=535.85, calcd. for C26H22O2PPdS
[MꢀBF4ꢀH2O]+: 535.91; anal. calcd.: C 48.74, H 3.78, P
4.83, Pd 16.61; found: C 48.65, H 3.80, P 4.81, Pd 16.58.
General Procedure for Catalytic Tandem Reactions
In a flame-dried Schlenk flask, aryl aldehyde or sulfonimine
(0.1 mmol), hexamethyldistannane (25 mL, 0.12 mmol),
pincer complex (0.005 mmol), solvent (1 mL), and a stirring
bar were loaded and the resulting mixture was vigorously
stirred at the indicated temperature for 16–18 h. The reac-
tion was quenched by addition saturated aqueous KF solu-
tion (1 mL) and the reaction mixture was stirred at room
temperature for another 12 h. Afterwards, the reaction mix-
ture was extracted with EtOAc (3ꢄ2 mL) and the combined
organic layers were dried over MgSO4. The dry solution was
concentrated on a rotary evaporator and was subjected to
flash chromatography with EtOAc/hexanes (3:7 v/v) as
1
eluent. Chemical conversions were estimated by H NMR or
GC with mesitylene as internal standard. The 1H and
13C NMR spectral data of the resulting allylic alcohols
agreed with those of reported compounds.[6,8c]
X-Ray Diffraction Analysis
All reflection intensities were measured using a Nonius
KappaCCD diffractometer equipped either with a rotating
anode (1 and 2) or a sealed tube (4) with graphite-mono-
chromated Mo Ka radiation (l=0.71073 ꢁ) under the pro-
gram COLLECT.[19] The programs PEAKREF[20] or
HKL2000[21] were used to refine the cell dimensions. Data
reduction was done using the programs EVALCCD[22] or
HKL2000.[21] The structures were solved either with the pro-
grams DIRDIF0823] or SHELXS-97[24] and were refined on
F2 with SHELXL-97.[24] Multi-scan semi-empirical absorp-
tion corrections based on symmetry-related measurements
were applied to all sets of data with the program
SADABS.[25] The temperature of data collection was con-
trolled using the system OXFORD CRYOSTREAM 600
(manufactured by OXFORD CRYOSYSTEMS). The H-
atoms (except for the H atom attached to C20 in 2) were
placed at calculated positions (AFIX 23 or AFIX 43 or
AFIX 137) with isotropic displacement parameters having
values 1.2 or 1.5 times Ueq of the attached C atom. The co-
ordinates and the isotropic displacement parameter of the H
atom attached to C20 (2) were refined freely.
For 1, data were collected at 150 K after the crystal had
been flash cooled from room temperature. One phenyl ring
of complex 1 is disordered. Restraints were applied so that
the minor and major components of the disordered phenyl
group have similar geometries (FLAT, SADI and SAME in-
structions). The SIMU instruction (i.e., a restraint for which
neighbouring atoms have similar Uij) had to be applied to
all disordered atoms in order to get a satisfactory refine-
ment.
General Procedure for Catalytic Homoallylation
Reactions
In a flame-dried Schlenk flask, aryl aldehyde (0.1 mmol), al-
lylstannane
(37 mL,
0.12 mmol),
pincer
complex
(0.005 mmol), dry DMF (1 mL) and a stirring bar were
loaded and the resulting mixture was vigorously stirred at
608C for 16 h. The reaction was quenched by the addition
2486
asc.wiley-vch.de
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 2474 – 2488

PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts
For 2, data were collected at 150 K after the crystal had
been flash cooled from room temperature. The structure of
complex 2 is ordered.
Am. Chem. Soc. 2005, 127, 12273–12281; d) J. S.
Fossey, M. L. Russell, A. K. Malik, C. J. Richards, J.
Organomet. Chem. 2007, 692, 4843–4848; e) M. Q.
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Klein Gebbink, G. van Koten, Coord. Chem. Rev. 2004,
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For 4, crystals shattered if the temperature was set at
200 K or below. This behaviour may result from the exis-
tence of a destructive phase transition. Subsequently, no
data could be collected at 150 K. Near 230 K, a few frames
were collected but the diffraction pattern looked rather
complicated (i.e., extra spots were not integrated correctly
during the process of cell determination). Data were then
collected near 295 K, at which the diffraction corresponds to
that of a single crystal. It seems to be likely that a solid-
solid phase transition occurs somewhere between room tem-
perature and 230 K, but its study is beyond the scope of this
paper. The structure of 4 is disordered: the phenyl groups
attached to the P and S atoms are disordered. The aniso-
tropic displacement parameters for the disordered Cphenyl
atoms attached directly to P and S atoms (e.g., C8 and C8’)
were constrained to be the same using the EADP instruc-
tion. The ISOR and SIMU instructions were applied to all
disordered atoms in order to get a satisfactory refinement.
Restraints were applied so that the minor and major compo-
nents of the disordered rings have similar geometries using
the SADI and SAME instructions.
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CCDCC 741774, CCDC 71775, and CCDC 71776 contain
the supplementary crystallographic data for compounds 1, 2,
and 4, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
We gratefully thank Utrecht University for financial support.
This work was also supported in part (A. L. S., M. L.) by the
Council for the Chemical Sciences of The Netherlands Or-
ganization for Scientific Research (CW-NWO).
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