Diethylzinc-mediated allylation of carbonyl compounds catalyzed by [(NHC)(PR3)PdX2] and [(NHC)Pd(η3-allyl)Cl] complexes

Article abstract of DOI:10.1002/ejic.200900891

[(NHC)(PR₃)PdX₂] complexes (NHC = N-heterocyclic carbene) are active precatalysts in the palladium-catalyzed allylation of carbonyl compounds with allylic acetates and diethylzinc. A comparative study examining the catalytic activity of a series of six of these complexes was carried out with allyl and cinnamyl acetates. [(IMeSMe)(PPh₃)PdI ₂] was found to be the most versatile precatalyst (IMesMe = 1-mesityl-3-methylimidazol-2-ylidene) and the scope of the reaction was investigated with this complex. [(IMeSMe)(PPh₃)-PdI₂] catalyzes the allylation of aromatic (except 4-nitrobenz-aldehyde) and aliphatic aldehydes (including enolizable aldehydes) with cinnamyl acetate to give the corresponding homoallylic alcohols in 57-98 % yields and dlastereoselectivities ranging from 70:30 to 92:8. The allylation of acetone also takes place under the same conditions, leading to the expected adduct in 63 % yield. The reaction with cyclohexenyl acetate proceeds at room temperature to afford the homoallylic alcohols in 40-78% yields with excellent diastereoselec-tivities (>98:2), but is limited to aromatic aldehydes. An experimental study concerning the mechanism, of the transformation was also carried out. We first demonstrated that the phosphane ligand was not essential for the reaction to take place. [(NHC)Pd(allyl)Cl] complexes are active precatalysts and lead to similar yields in the presence or in the absence of PPh₃. Transmetalation of [(NHC)Pd(allyl)Cl] complexes with diethyl- or dimethylzinc, which is a determining step for the mechanism, was studied by ¹H NMR spectroscopy. The reaction of [(IPr)Pd(allyl)Cl] with dimethylzinc affords rapidly [(IPr)Pd(η³-allyl)(Me)] but no detectable trace of allylzinc species [IPr = 1, 3-bis(2, 6-diisopropylphenyl)imidazol-2-ylidene]. [(IPr)Pd(η³-allyl)(Me)] was found to be a nucleophilic species able to react smoothly at room temperature with an aldehyde in the absence of zinc to form the corresponding homoallylic alcohol.

Full text of DOI:10.1002/ejic.200900891

FULL PAPER
DOI: 10.1002/ejic.200900891
Diethylzinc-Mediated Allylation of Carbonyl Compounds Catalyzed by
[(NHC)(PR3)PdX2] and [(NHC)Pd(η3-allyl)Cl] Complexes
Alexandre Flahaut,[a] Krimo Toutah,[a] Pierre Mangeney,[a] and Sylvain Roland*[a]
Keywords: Allylation / Carbene ligands / Palladium / Reaction mechanisms
[(NHC)(PR3)PdX2] complexes (NHC = N-heterocyclic car-
bene) are active precatalysts in the palladium-catalyzed al-
lylation of carbonyl compounds with allylic acetates and di-
ethylzinc. A comparative study examining the catalytic ac-
tivity of a series of six of these complexes was carried out
with allyl and cinnamyl acetates. [(IMesMe)(PPh3)PdI2] was
found to be the most versatile precatalyst (IMesMe = 1-mes-
ityl-3-methylimidazol-2-ylidene) and the scope of the reac-
tion was investigated with this complex. [(IMesMe)(PPh3)-
PdI2] catalyzes the allylation of aromatic (except 4-nitrobenz-
aldehyde) and aliphatic aldehydes (including enolizable al-
dehydes) with cinnamyl acetate to give the corresponding
homoallylic alcohols in 57–98% yields and diastereoselectivi-
ties ranging from 70:30 to 92:8. The allylation of acetone also
takes place under the same conditions, leading to the ex-
pected adduct in 63% yield. The reaction with cyclohexenyl
acetate proceeds at room temperature to afford the homoal-
tivities (Ͼ98:2), but is limited to aromatic aldehydes. An ex-
perimental study concerning the mechanism of the transfor-
mation was also carried out. We first demonstrated that the
phosphane ligand was not essential for the reaction to take
place. [(NHC)Pd(allyl)Cl] complexes are active precatalysts
and lead to similar yields in the presence or in the absence of
PPh3. Transmetalation of [(NHC)Pd(allyl)Cl] complexes with
diethyl- or dimethylzinc, which is a determining step for the
mechanism, was studied by 1H NMR spectroscopy. The reac-
tion of [(IPr)Pd(allyl)Cl] with dimethylzinc affords rapidly
[(IPr)Pd(η3-allyl)(Me)] but no detectable trace of allylzinc
species [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylid-
ene]. [(IPr)Pd(η3-allyl)(Me)] was found to be a nucleophilic
species able to react smoothly at room temperature with an
aldehyde in the absence of zinc to form the corresponding
homoallylic alcohol.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
lylic alcohols in 40–78% yields with excellent diastereoselec- Germany, 2009)
allylmetal species. Therefore, in contrast to the Tsuji–Trost
Introduction
reaction, the direct participation of an allylpalladium spe-
cies in the key step (reaction with the aldehyde) is not sys-
tematic. Moreover, in reactions where pathway A is consid-
ered, the nucleophilic behavior of the allyl moiety could not
always be easily proved experimentally.
Palladium-catalyzed transformations involving allylpalla-
dium species represent an important area of homogeneous
catalysis.[1] Among these, allylation of nucleophiles, com-
monly called the Tsuji–Trost reaction, has been widely
studied.[1,2] In this reaction, it is clearly established that the
allyl fragment of the transient cationic π-allyl palladium
species behaves as an electrophile, allowing the formation
of allylated products after attack of nucleophiles. Allylation
reactions of electrophiles, catalyzed by PdII or Pd0, have
also been reported. Various efficient catalytic systems have
been developed for the allylation of aldehydes, imines,
ketones, or Michael acceptors.[3] Two main mechanisms,
fundamentally different, have been proposed for these reac-
tions (Scheme 1). In pathway A, (η1-allyl)PdII I species, hav-
ing a nucleophilic allyl moiety, are directly involved in the
allylation step.[4] In pathway B, intermediate allylpalladium
species II does not react directly with the electrophile but
leads after transmetalation to the formation of nucleophilic
Scheme 1. Main pathways proposed for the allylation of aldehydes
with allylpalladium complexes.
In the allylation reactions described by Yamamoto using
catalytic bis(allyl)palladium (1) and allylstannanes (Fig-
ure 1),[5] the homoallylic alcohol resulting from the reaction
of 1 with benzaldehyde was observed by NMR spec-
troscopy and isolated.[5a,7b] The nucleophilic properties of
one of the allyl ligands was also confirmed by DFT calcula-
[a] Institut Parisien de Chimie Moléculaire UMR CNRS 7201,
Université Pierre et Marie Curie, Paris 6
4, place Jussieu, bât. F pièce 655, BC 43, 75252 Paris Cedex 05,
France
Fax: +33-1-44273056
E-mail: sylvain.roland@upmc.fr
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Diethylzinc-Mediated Allylation of Carbonyl Compounds
tions.[6] Szabó reported allylation reactions of carbonyl plexes 6 were efficient precatalysts in the allylation reaction
compounds and imines by using catalytic PdII “pincer” of aldehydes.[21] In the presence of allylstannanes,[21a] the
complexes with tridentate P–C–P ligands.[7] This methodol- transient formation of (η1-allyl)Pd species was shown by
ogy was based on the in situ generation of nucleophilic (η1- NMR spectroscopy, and a mechanism similar to that pro-
allyl)PdII species 2, generally from allylstannanes.[8] In this posed by Szabó was considered (pathway A, Scheme 1).
case, the formation of 2 was confirmed by NMR spec-
troscopy[7a,7b,7d,9] and its reactivity studied by DFT calcula-
tions.[10] A low activation barrier was found for the reaction
of the η1-allyl complex with aldehydes.[7b,7e,11]
Figure 1. Nucleophilic allylpalladium complexes.
Figure 2. NHC–Pd complexes used in allylation reactions of car-
bonyl compounds.
The palladium-catalyzed allylation reaction of carbonyl
compounds or imines by using allylic acetates and dieth-
ylzinc was initially developed by Tamaru and many applica-
tions have been reported.[12] It was proposed for this trans-
Herein we wish to present our study concerning the use
of [(NHC)(PR3)PdX2] complexes 7 as precatalysts in the
diethylzinc-mediated palladium-catalyzed allylation of car-
bonyl compounds as well as an experimental study of the
mechanism, carried out with [(NHC)Pd(allyl)Cl] complexes,
demonstrating that nucleophilic [(NHC)Pd(allyl)(Alk)] spe-
cies 8 are likely involved in the key step of the reaction.
formation
a mechanism corresponding to pathway B
(Scheme 1) in which the intermediate cationic π-allyl palla-
dium complex would react with Et2Zn to form a nucleo-
philic allylzinc species.[12a] Phosphorous ligands are gen-
erally necessary in this reaction[3] and asymmetric versions
have been reported with chiral monodentate
P li-
gands.[12d,12g,12h,12j] In this case, the mechanism initially
considered cannot easily account for the enantioselectivities
obtained. In particular, when allyl or cinnamyl acetates are
used, the reaction should proceed through achiral allylzinc
Results and Discussion
A series of six [(NHC)(PR3)PdX2] complexes 7a–f with
species unable to generate any enantioselectivity.[3e] There- different phosphorus, NHC, and halides ligands was pre-
fore, Minnaard and Feringa proposed that nucleophilic pared (Figure 3). We initially selected this family of com-
[(L2)(allyl)Pd(Et)] species 3 could be generated in situ after plexes for several reasons. We assumed that (i) the π-ac-
transmetalation by Et2Zn and react directly with the alde- cepting properties of the phosphorus ligand could favor the
hyde.[12h,13]
reduction of PdII species into Pd0; (ii) the oxidative addition
Recently, it has been considered that the strong σ-donor of allylic acetates that generates [(NHC)Pd(allyl)(OAc)]
effect associated with the N-heterocyclic carbenes complexes could be favored by the σ-donor effects of both
(NHCs)[14] could be exploited to confer nucleophilic prop- the NHC and the P-ligand; (iii) the phosphorus ligand
erties on the allyl palladium complexes and promote the could stabilize underligated (NHC)Pd0 complexes but could
allylation of electrophiles such as aldehydes. Numerous ap- also dissociate to free a coordination site; (iv) a single
plications of NHC–Pd complexes in the Tsuji–Trost reac- [(NHC)(PR3)PdX2] precatalyst may allow the formation of
tion have been reported.[15] It has been generally observed various transient [(NHC)Pd(η3-R-allyl)OAc] complexes
that the allyl moiety of the intermediate [(NHC)Pd- and therefore of a wide range of homoallylic alcohols; (v)
(allyl)(L)] cationic complexes was poorly electrophilic.[16] In these complexes must be more appropriate than [(NHC)-
contrast, Sigman showed in 2003 that the allyl fragment Pd(allyl)Cl] precatalysts, as contamination by unexpected
of [(NHC)Pd(allyl)Cl] complexes reacted with HCl to form homoallylic alcohols arising from the transfer of the allyl
propene.[17] Szabó described in 2004 the first application of fragment initially present on the precatalyst cannot take
NHC–Pd complexes in the allylation reaction of carbonyl place. We decided to compare the influence of two NHC
compounds (Figure 2).[7b] However, a low catalytic activity ligands, IPr and IMesMe (Figure 3), which had given signif-
was observed with the bis(NHC) “pincer” complex 4 icantly different results in the Tsuji–Trost reaction.[15l] In
tested.[18] In 2007, Jarvo showed that allylpalladium com- allylic alkylation reactions catalyzed by [(IMesMe)Pd(allyl)-
plexes of type 5, bearing bidentate NHC ligands, behaved Cl], high reaction rates and yields were observed with vari-
as nucleophiles and were able to react directly with alde- ous allylic acetates, whereas the efficiency of [(IPr)Pd(allyl)-
hydes.[19] Complex 5 catalyzes allylations of aldehydes with Cl] was limited to allyl acetate.
allylstannanes[19] and conjugate allylation reactions of α,β-
Complexes 7a–e, bearing IPr as ligand, were obtained in
unsaturated N-acylpyrroles using allylboronic esters.[20] 68–94% yields by reaction of the phosphorus ligand [PPh3,
Later, Shi and co-workers reported that bis(NHC)PdII com- P(nBu)3 or P(OPh)3] with the appropriate [{(IPr)PdX2}2]
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A. Flahaut, K. Toutah, P. Mangeney, S. Roland
FULL PAPER
Figure 3. [(NHC)(PR3)PdX2] complexes and NHC ligands. IPr =
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IMesMe
mesityl-3-methylimidazol-2-ylidene.
= 1-
dimeric complex 9 or 10 (Scheme 2).[22] [{(IPr)PdI2}2] (9)
was synthesized in 74% yield from imidazolium salt 11
(IPr·HCl) by using a reported procedure.[23] Complex 7f
was obtained in 51% yield in two steps from imidazolium
salt 12 (IMesMe·HI) by the same procedure as that used
for 7d and 7e. In this case, the intermediate dimeric complex
[{(IMesMe)PdI2}2] is not stable enough to be isolated ana-
lytically pure in reasonable yields and fully characterized.
It was directly engaged in the second step after rapid purifi-
cation by filtration through silica gel. Slow evaporation of
a diluted solution of 7f in Et2O afforded small red crystals
suitable for X-ray diffraction analysis. As shown in Fig-
ure 4, 7f is a trans complex with a distorted square-planar
coordination around the palladium center.
Figure 4. Molecular structure of 7f. Selected angles [°] C1–Pd1–P1
174.91(16), I1–Pd1–I2 157.49(2), I1–Pd1–P1 90.84(4), I1–Pd1–C1
88.44(14), I2–Pd1–P1 93.31(4), I2–Pd1–C1 89.27(14). Selected
bond lengths [Å]: Pd1–C1 2.023(5), Pd1–P1 2.3459(14), Pd1–I1
2.6326(5), Pd1–I2 2.6352(5).
Table 1, Entry 3). In a second set of experiments with 4-
bromobenzaldehyde and complexes 7a–f, higher yields
ranging from 45 to 75% were obtained (Table 1, Entries 4–
9) and [(IPr)(PPh3)PdCl2] (7a) was found to be the most
efficient complex, affording homoallylic alcohol 14b in 75%
yield (Table 1, Entry 4). A close but slightly lower yield of
70% was obtained with [(IMesMe)(PPh3)PdI2] (7f) (Table 1,
Entry 9). In reactions catalyzed by 7a–e, bearing the same
NHC ligand (IPr), no straightforward influence of the elec-
tronic properties of the phosphorus ligand and of the na-
ture of the halide ligand on the yields could be evidenced
(Table 1, Entries 1–8). However, PPh3 appeared to be the
most appropriate phosphorus ligand: in reactions using
benzaldehyde as the electrophile, triphenylphosphanyl com-
plexes 7a and 7d led to the formation of 1-phenyl-3-buten-
1-ol (14a) in 54 and 60% yields (Table 1, Entries 1 and 3),
Scheme 2. Synthesis of complexes 7a–f.
whereas
a lower yield of 23% was obtained with
The efficiency of [(NHC)(PR3)PdX2] complexes 7a–f was [(IPr){P(OPh)3}PdCl2] (7b) (Table 1, Entry 2); When 4-bro-
first investigated in the allylation reaction of benzaldehyde mobenzaldehyde was used as the electrophile and
and 4-bromobenzaldehyde with allylic acetate (13) under dichlorido complexes 7a–c as the precatalysts (Table 1, En-
standard reaction conditions usually reported for this trans- tries 4–6), the best yield was also achieved with 7a bearing
formation[12] (Scheme 3). All reactions were stirred for 16 h PPh3 as the phosphorus ligand (75%; Table 1, Entry 4) and
at 20 °C in the presence of 7a–f (5 mol-%) and 3.5 equiv. of the replacement of PPh3 by P(OPh)3 (complex 7b) or
diethylzinc. The allylation of benzaldehyde with 13 has been P(nBu)3 (complex 7c) led to a significant decrease in the
described previously using Pd(PPh3)4[12a] or palladium com- yields (Table 1, Entries 5 and 6). In contrast to these ob-
plexes generated from Pd(PhCN)2Cl2 and phosphoramid- servations, the effect of the phosphorus ligand [PPh3 or
ites ligands as catalysts.[12h] Homoallylic alcohol 14a was P(nBu)3] was found to be negligible in reactions catalyzed
obtained in 77 and 73% yield, respectively. Allylation reac- by the diiodido complexes 7d and 7e (Table 1, Entries 7 and
tions using 13 have not been reported with NHC–Pd com- 8). Finally, the influence of the NHC ligand was compared
plexes. As shown in Table 1, complexes 7a–f proved to be under the same conditions using the diiodido complexes
suitable precatalysts for this Et2Zn-mediated reaction. In [(IPr)(PPh3)PdI2] (7d) and [(IMesMe)(PPh3)PdI2] (7f).
preliminary experiments performed with benzaldehyde and Homoallylic alcohol 14b was obtained in 70% yield with 7f
complexes 7a, 7b, or 7d, homoallylic alcohol 14a was ob- (Table 1, Entry 9) but in a lower yield of 55% with 7d
tained in 23–60% yields (Table 1, Entries 1–3). The best (Table 1, Entry 7), thus suggesting that IMesMe should be
yield was achieved with [(IPr)(PPh3)PdI2] (7d) (60%; more appropriate.
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Diethylzinc-Mediated Allylation of Carbonyl Compounds
Table 2. Comparison of complexes 7a–f in the allylation of 4-bro-
mobenzaldehyde with cinnamyl acetate.
Entry
Complex
Yield [%][a]
anti/syn[b]
1
2
3
4
5
6
7a
7b
7c
7d
7e
7f
12
10
22
32
34
76
n.d.[c]
n.d.[c]
36:64
62:28
91:9
Scheme 3. Diethylzinc-mediated allylation of aldehydes with allylic
acetate (13) catalyzed by 7a–f (diethylzinc 1  in hexanes).
92:8
Table 1. Allylation of aromatic aldehydes catalyzed by 7a–f by
using allylic acetate (13).
1
[a] Yield determined by analysis of the H NMR spectrum of the
crude reaction mixture by comparison with the internal standard.
[b] Ratio determined by analysis of the H NMR spectrum of the
Entry
Complex
Ar
Product
Yield [%][a]
1
1
2
3
4
5
6
7
8
9
7a
7b
7d
7a
7b
7c
7d
7e
7f
Ph
Ph
Ph
14a
14a
14a
14b
14b
14b
14b
14b
14b
54
23
60
75
50
45
55
52
70
crude reaction mixture. [c] Not determined.
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
monophosphane ligands.[12d] Aromatic aldehydes led to the
formation of the homoallylic alcohols in 70–80% yield but
a lower yield of 55% was reported with 2,2-dimethylpro-
panal. The anti isomer was obtained as the major product
with excellent diastereoselectivities (98:2–99:1). The reac-
tion was not described with enolizable aldehydes or ketones.
In 2009, the allylation of benzaldehyde with 15, catalyzed
by chiral bis(NHC)–PdII complexes 6, was reported. The
corresponding homoallylic alcohol 17c was obtained in
72% yield with a low anti/syn ratio of 55:45.[21b] As shown
in Table 3, our conditions proved to be ineffective with 4-
nitrobenzaldehyde (16b) but led in the other cases to the
expected adducts 17c–k in 57–98% yield and diastereoselec-
tivites ranging from 59:41 to 88:12. The results obtained
with aromatic aldehydes suggested that the electronic prop-
erties of the substituent on the aryl group could have a sig-
nificant effect on the yield. The allylation product was not
detected by using 4-nitrobenzaldehyde (16b), whereas 4-
methoxybenzaldehyde (16f) led to the corresponding adduct
17f in 95% yield (Table 3, Entries 1 and 4). Similar results
were reported by Onomura.[12j] However, the influence of
the substitution was found to be less marked with aromatic
aldehydes 16c, 16d, and 16g, which led to the corresponding
homoallylic alcohols in yields ranging from 57 to 75%
(Table 3, Entries 4–6). Similar diastereomeric ratios (85:15–
92:8) were observed with aldehydes 16c, 16f, and 16g de-
rived from benzaldehyde, whereas 1-naphthylaldehyde (16d)
gave a lower dr of 70:30. Nonaromatic aldehydes, including
crotonaldehyde (16h) and aliphatic aldehydes 16i–k, fur-
nished the branched products in 60–67% yields with dr val-
ues varying from 59:41 to 85:15 (Table 3, Entries 6–9).
Interestingly, enolizable aldehydes 16i and 16j are compati-
ble with the catalytic system (65–67% yield; Table 3, En-
tries 7 and 8). The more bulky aldehydes 16j and 16k gave
the best diastereoselectivities with anti/syn ratios of 85:15
1
[a] Yields determined by analysis of the H NMR spectrum of the
crude reaction mixture by comparison with the internal standard
(di-tert-butyl-4,4Ј-biphenyl).
Next, we examined the catalytic activity of 7a–f in the
allylation of 4-bromobenzaldehyde with cinnamyl acetate
(15; Scheme 4). The results are presented in Table 2. In this
study, a clear and dramatic effect of the NHC ligand was
observed. Yields ranging from 10 to 34% were obtained
with complexes 7a–e bearing IPr as the NHC ligand,
whereas 7f afforded homoallylic alcohol 17a in a signifi-
cantly higher yield of 76%. A similar influence of the NHC
ligand was previously observed by our group in allylic alky-
lation reactions with cinnamyl acetate and dimethyl malon-
ate.[15l] A diastereomeric ratio (dr) of 92:8 (anti/syn) was ob-
tained for 17a in the reaction catalyzed by complex 7f, and
the linear product was not detected in the crude reaction
mixture.
Scheme 4. Allylation of aldehydes with cinnamyl acetate (15) and
complex 7f.
The scope of the reaction was then examined with cinn- and 83:17, respectively. 2,2-Dimethylpropanal (16k) gave
amyl acetate (15), complex 7f, and various aromatic or ali- the highest yield (98%) but a significant amount of linear
phatic aldehydes 16b–l (Scheme 5, Figure 5, and Table 3). product 18k (38%) was produced during the reaction
The Et2Zn-mediated allylation of benzaldehyde was initially (Table 3, Entry 9). The lowest anti/syn ratios were obtained
described with the benzoate analogue of 15 and Pd(PPh3)4. with crotonaldehyde (16h) and butyraldehyde (16i; Table 3,
Under these conditions, the homoallylic alcohol was ob- Entries 6 and 7). The Lewis basicity of the carbonyl com-
tained in 83% yield and an anti/syn ratio of 91:9.[3a] To the pound seems to have a significant effect on the reaction, as
best of our knowledge, cinnamyl acetate (15) was mainly the best conversions were achieved by using the more elec-
used in the asymmetric version of the reaction with chiral tron-rich aldehydes 16f and 16k.
Eur. J. Inorg. Chem. 2009, 5422–5432
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A. Flahaut, K. Toutah, P. Mangeney, S. Roland
FULL PAPER
The allylation of 3-phenylpropanal catalyzed by palladium
complexes bearing [2,2]paracyclophane monophosphane li-
gands was reported. The expected homoallylic alcohol was
obtained in 38% yield (dr = 91:9).[12k] Phosphoramidite li-
gands proved to be ineffective for the allylation of aliphatic
aldehydes.[12h] Interestingly, bis(NHC)–PdII complexes 6
were recently shown to catalyzed the allylation of aromatic
and aliphatic aldehydes in THF at 50 °C with acetate 19.
By using 10 mol-% of catalyst, aromatic aldehydes fur-
nished the homoallylic alcohols in 74–96% yield with dia-
stereomeric ratios varying from 90:10 to Ͼ99:1, whereas ali-
phatic aldehydes produced the corresponding adducts in
lower yields (58–61%) and syn/anti ratios ranging from
84:16 to 90:10.[21b] As shown in Table 4, complex 7f (5 mol-
%) catalyzes the allylation of aromatic aldehydes with cyclo-
hexenyl acetate (19) at room temperature and leads to the
formation of the homoallylic alcohols in 40–78% yields
(Table 4, Entries 1–7). High diastereoselectivities were ob-
tained in all cases with syn/anti ratios superior to 98:2. The
best result was reached with 4-bromobenzaldehyde (16a),
which led to 20a in 78% yield after 16 h (Table 4, Entry 1).
Prolonged reaction times in this case did not improve signif-
icantly the yield. The allylation reaction is slower with benz-
aldehyde (16c), which led to homoallylic alcohol 20c in 32%
yield after 16 h and 76% after 45 h (Table 4, Entries 2 and
3). Surprisingly, the more electron-rich aldehydes 16e and
16f gave the lowest results, maximum yields of 59 and 40%,
respectively, being reached after 45 h (Table 4, Entries 5 and
7). No important evolution of the yields was observed be-
tween 16 and 45 h (Table 4, Entries 4–7). Our conditions
proved to be ineffective with aliphatic aldehydes 16i–k
(Table 4, Entries 8–10) and the maximum yield (12%) was
obtained with n-butanal (16i) (Table 4, Entry 8). Attempts
to optimize the conditions with 16j were unsuccessful. Pro-
longed reaction times (20 °C, 120 h) or higher temperatures
(50 °C, 48 h) did not improve the yield.
Scheme 5. Allylation of aldehydes with cinnamyl acetate (15) and
complex 7f.
Figure 5. Carbonyl compounds 16 used in this study.
Table 3. Allylation of aldehydes with cinnamyl acetate catalyzed by
7f.
Entry
RCHO
Product
Yield [%][a] anti/syn[b]
1
2
3
4
5
6
7
8
9
16b
16c
16d
16f
16g
16h
16i
17b
17c
17d
17f
17g
17h
17i
Ͻ5
75
57
95
75
63
67
n.d.[b]
85:15
70:30
87:13
88:12
59:41
65:35
85:15
83:17
16j
17j
65
16k
17k
60 (98)[c]
1
[a] Determined by analysis of the H NMR spectrum of the crude
reaction mixture by comparison with the internal standard. [b] Not
determined. [c] Yield including linear product 18k.
Interestingly, the reaction is also effective with acetone
(16l) as the electrophile. The corresponding homoallylic
alcohol 17l was obtained in 63% yield (Scheme 6).
Scheme 7. Allylation of aldehydes with cyclohexenyl acetate and
complex 7f.
Having investigated the scope and limitations of the reac-
tion, we decided to focus our attention on the study of its
mechanism. As presented in the introduction, two funda-
mentally different pathways have been proposed for this re-
Scheme 6. Allylation of acetone with cinnamyl acetate (15) and
complex 7f.
The scope of the reaction with 7f was further studied action. Transmetalation of the π-allylpalladium intermedi-
with cyclohexenyl acetate (19) (Scheme 7 and Table 4). Pal- ates by Et2Zn is one of the key steps of the transformation.
ladium-catalyzed allylation reactions of aldehydes with 19 The outcome of this reaction must determine if allylzinc or
in the presence of Et2Zn have been reported with chiral (allyl)Pd(Et) species are formed, and consequently, if the
monophosphane or phosphoramidite ligands.[12d,12h,12k] reaction proceeds through pathway A or pathway B
Aromatic aldehydes are generally the most appropriate elec- (Scheme 1). In reactions catalyzed by 7f, the π-allyl interme-
trophiles, leading to the homoallylic alcohols in good to diates can be either neutral [(NHC)Pd(η3-allyl)(OAc)] or
excellent yields and diastereoselectivities. The reaction with cationic [(NHC)(PPh3)Pd(η3-allyl)][OAc] complexes gener-
aliphatic aldehydes was generally found to be more difficult. ated, as in the Tsuji–Trost reaction, by oxidative addition
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Diethylzinc-Mediated Allylation of Carbonyl Compounds
1
Table 4. Allylation of aldehydes with cyclohexenyl acetate (19) and formed at 20 °C in [D8]THF and monitored by H NMR
complex 7f.
spectroscopy (Scheme 9). Unfortunately, a rapid degrada-
Entry RCHO
Product
t [h]
Yield [%][a] syn/anti[a]
tion of the complex was observed, with precipitation of Pd
black. We decided then to study the reaction with [(IPr)-
Pd(η3-allyl)(Cl)] (22), bearing the same NHC ligand as
complexes 7a–e, and to use Me2Zn instead of Et2Zn to
avoid β-H elimination reactions. In this case, we observed
the rapid formation of [(IPr)Pd(η3-allyl)(Me)] (23), and
complete conversion was achieved within 10 min
(Scheme 9). Complex 23 is a stable complex whose synthesis
and characterization was reported by Pörschke in 2005.[27]
1
2
3
4
5
6
7
8
9
16a
16c
16c
16e
16e
16f
16f
16i
20a
20c
20c
20e
20e
20f
20f
20i
16
16
45
16
45
16
45
16
16
16
78
32
76
53
59
33
40
12
7
Ͼ98:2
Ͼ98:2
Ͼ98:2
Ͼ98:2
Ͼ98:2
Ͼ98:2
Ͼ98:2
n.d.[b]
n.d.[b]
n.d.[b]
16j
20j
20k
1
10
16k
Ͻ2
In the H NMR spectrum, we observed the characteristic
1
[a] Determined by analysis of the H NMR spectrum of the crude
reaction mixture by comparison with the internal standard. [b] Not
determined.
singlet of the Pd–Me at δ = –0.34 ppm ([D8]THF). This
spectrum also showed the presence of ZnMe2 (δ =
–0.86 ppm)[28] and MeZnCl (δ = –0.82 ppm)[29] although
their respective amounts could not be exactly determined.
More importantly, allylzinc species that could be formed
during the reaction were not detectable. A single set of sig-
nals corresponding to a single allyl fragment, which is that
of 23, was visible. It has been suggested that zinc–palladium
aggregates could be involved in the R2Zn-mediated al-
lylation reaction.[12h,30] However, we were unable to detect
such species in the reaction medium by 1H NMR spec-
troscopy.
of the allylic acetate to (NHC)Pd0 or (NHC)(PPh3)Pd0 spe-
cies. We assumed that either of these complexes (neutral or
cationic) must be involved in the transmetalation step. This
prompted us to study the mechanism with [(NHC)Pd(η3-
allyl)Cl] complexes, which are structurally similar to the
neutral complex cited above and should be directly involved
in the catalytic cycle. Firstly, it was necessary to control
their catalytic activity and to determine the importance of
the phosphane ligand. The allylation of 4-bromobenzalde-
hyde (16a) with allylic acetate (13) was then tested by using
[(IMesMe)Pd(η3-allyl)Cl] (21)[24] as the catalyst (Scheme 8,
conditions A). The reaction led to expected homoallylic
alcohol 14b in 68% yield. This demonstrated that 21 could
catalyze the reaction and that PPh3 was not essential for
this one to take place. Performed in the presence of PPh3
(conditions B), the same reaction afforded 14b in a slightly
better yield of 76%. However, comparison of these results
with that obtained with [(IMesMe)(PPh3)PdI2] (7f) (70%
yield; Table 1, Entry 9) suggested that this difference is not
significant.[25]
Scheme 9. Transmetalation reactions with Et2Zn or Me2Zn (1  in
hexanes). Solution of Pd complex (0.08  in [D8]THF).
The reactivity of 23 in the absence of zinc was then inves-
tigated. Complex 23 was synthesized and isolated according
to a reported procedure by reaction of 22 with methyllith-
ium.[27] The reaction of 23 with 4-bromobenzaldehyde (16a)
1
was monitored by H NMR spectroscopy in [D8]THF at
20 °C (Scheme 10). As shown in Figure 6, 23 reacts
smoothly with the aldehyde to form homoallylic adduct 24.
Conversions of 45 and 73% were measured after 45 min
and 6.5 h, respectively, the maximum (77%) being reached
after 24 h. The exact nature of intermediate 24 was not de-
Scheme 8. Allylation of 4-bromobenzaldehyde catalyzed by com-
plex 21. Influence of the phosphane.
1
termined. However, its H NMR spectrum is very close to
We hypothesized that the participation of monoligated
NHC–Pd complexes rather than (NHC)(PPh3)Pd com-
plexes in the rate-determining step could account for the
negligible influence of the phosphane. Consequently, in or-
der to simplify our study, the transmetalation step was in-
vestigated with [(NHC)Pd(η3-allyl)Cl] complexes in the ab-
sence of phosphane. The transmetalation of PdII complexes
with dialkylzinc has been scarcely studied although it is of
importance in catalytic reactions, particularly in Negishi
couplings. The first experimental observations on this reac-
tion was reported in 2007.[26] We first attempted to deter-
that of 14b except for the hydrogen on the carbon bearing
the oxygen atom. The formation of 14b was confirmed by
adding D2O to the NMR sample at the end of the reaction.
Scheme 10. Reaction of 23 with aldehyde 16a.
These experiments support the mechanism depicted in
mine the species generated by addition of a stoichiometric Figure 7, in which nucleophilic [(NHC)Pd(allyl)(R)] species
amount of Et2Zn to complex 21. The reaction was per- (R = Et or Me), would be involved. This catalytic cycle is
Eur. J. Inorg. Chem. 2009, 5422–5432
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A. Flahaut, K. Toutah, P. Mangeney, S. Roland
FULL PAPER
amount of aldehyde 16a, in the presence of PPh3
(Scheme 11). However, whatever the conditions used (THF,
20 °C or reflux), no formation of 14b was detected after
16 h (Table 5, Entries 1 and 2) and a degradation of the
complex was observed. In contrast, addition of Et2Zn to
the same mixture at 20 °C led to the rapid formation of 14b,
a conversion of 59% being achieved within 30 min (Table 5,
Entry 3). As shown previously in Scheme 9, the direct ad-
dition of Et2Zn to complex 21 in the absence of aldehyde
leads instantaneously to the degradation of the complex.
Comparison of these two reactions shows, as observed by
Tamaru,[12a] that the application of Barbier-type procedure
is essential for the allylation.
Figure 6. 1H NMR monitoring of the reaction of 23 (0.17  in [D8]-
THF) with 16a at 20 °C. Yield of homoallylic adduct 24 determined
by integration of the central allyl proton and comparison with the
internal reference (TMS).
basically identical to that considered by Minnaard and Fer-
inga with phosphoramidite ligands,[12h] except for the L/Pd
ratio and the coordination mode of the allyl moiety in the
nucleophilic allylpalladium intermediate. In our study, the
transmetalation reaction was found to be faster than the
allylation reaction. In addition, we previously observed
high reaction rates in the allylic alkylation by using di-
methyl malonate and allylic acetate catalyzed by similar
complexes,[15l] an indication that the oxidative addition
must also be fast. Although the concentrations in the cata-
lytic reaction are different, this suggests that the rate-de-
termining step could be the allylation step.
Scheme 11. Reaction of 21 with aldehyde 16a. Reagents: 21
(0.02 mmol), 16a (0.02 mmol), PPh3 (0.02 mmol) in THF (0.5 mL).
Table 5. Stoichiometric reactions of complex 21 with aldehyde 16a.
Entry
Conditions
Conv. [%][a]
1[a]
2[a]
3
16 h, 20 °C
16 h, reflux
0[b]
0[b]
Et2Zn[c], 30 min, 20 °C
59[d]
[a] [D8]THF was used as the solvent. [b] Compound 14b was not
detectable by 1H NMR spectroscopy. [c] Et2Zn (1  in hexanes,
0.02 mmol). [d] Conversion determined by analysis of the 1H NMR
spectrum of the crude reaction mixture by comparison with the
amount of residual aldehyde. The reaction was quenched with
NH4Cl, extracted with Et2O, dried (MgSO4), and concentrated.
Conclusions
In conclusion, we demonstrated that [(NHC)(PR3)PdX2]
and [(NHC)Pd(η3-allyl)Cl] complexes are suitable catalysts
for the allylation reaction of carbonyl compounds by using
allylic acetates and diethylzinc. The investigation of the re-
action scope with complex 7f revealed interesting results but
also showed limitations. This complex catalyzes the al-
lylation of various aldehydes, including enolizable aliphatic
aldehydes, as well as acetone, by using cinnamyl acetate.
The reaction with cyclohexenyl acetate was found to be lim-
ited to aromatic aldehydes but led to excellent diastereo-
selectivities. We also demonstrated experimentally for the
first time that transmetalation of π-allylpalladium interme-
diates with dialkylzinc led to the formation of nucleophilic
Figure 7. Proposed catalytic cycle of the diethylzinc-mediated al-
lylation reaction with [(NHC)Pd(allyl)X] complexes (X = Cl or
AcO; R = Me or Et).
Finally, we decided to control the reactivity of [(NHC)- (allyl)(alkyl)PdII species able to react directly with alde-
Pd(η3-allyl)Cl] complexes toward aldehydes without Et2Zn. hydes in the absence of zinc. The rate-determining step of
Indeed, as presented in the introduction, it was reported the catalytic reaction is very likely the allylation step, which,
that cationic allylpalladium complexes 5, bearing bidentate interestingly, is also the stereodetermining step. This may
NHC–P ligands, were able to react directly with stoichio- allow the development of asymmetric versions by using eas-
metric amounts of aldehydes in various solvents at 60– ily accessible chiral monodentate NHC ligands. Investi-
70 °C to form the corresponding homoallylic alcohols.[19] gations concerning this topic are currently underway as are
We tested the reaction of complex 21 with a stoichiometric DFT calculations on the mechanism.
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Eur. J. Inorg. Chem. 2009, 5422–5432

Diethylzinc-Mediated Allylation of Carbonyl Compounds
0.088 mmol). The resulting mixture was stirred at 20 °C and moni-
tored by TLC. After 3.5 h, a second portion of P(nBu)3 (22 µL,
0.088 mmol) was added. After 30 min, the mixture was concen-
trated, and the solid residue was purified by flash chromatography
on silica gel (cyclohexane/EtOAc, 8:2) to give a yellow solid (56 mg,
Experimental Section
General: All experiments were performed under an atmosphere of
argon by using standard Schlenk techniques unless stated other-
wise. THF was dried with sodium benzophenone ketyl under an
atmosphere of argon and distilled prior to use. CH2Cl2 (REC-
TAPUR, stabilized with 0.1% of EtOH) and THF were degassed
by vacuum/argon cycles. Reagents were purchased from Acros, Al-
drich, or Strem and used as received. [{(IPr)PdCl2}2] (9),[31] 1,3-
(2,6-diisopropylphenyl)imidazolium chloride (11),[32] 1-mesityl-3-
methylimidazolium iodide (12),[33] and [(IPr)Pd(allyl)Cl] (22)[34]
were prepared according to literature procedures. 1H NMR and
13C NMR spectra were recorded with a Bruker ARX-250 or Av-
ance-400 spectrometer. Proton chemical shifts (δ) are reported rela-
tive to TMS. Carbon chemical shifts are reported relative to the
NMR solvent (CDCl3, 77.23 ppm). Melting points are uncorrected
and were measured with a Stuart Scientific apparatus SMP3. Ele-
mental analyses were performed at the ICSN (microanalytical ser-
vice). Compounds 14a,[35,36] 14b,[36] 17a,[7f] 17b,[7f,37] 17c,[7f,12g,38]
17d,[12g] 17f,[12g] 17h,[38] 17i,[39] 17j,[40] 17k,[12f] 17l,[41] 18k,[12f] 20c,[42]
20e,[43] 20f,[37] 20i,[43,44] 20j,[42,35b] and 20k[35b] have previously been
described in the literature.
1
82%). M.p.149–154 °C. H NMR (400 MHz, CDCl3): δ = 7.45 (t,
J = 7.6 Hz, 2 H), 7.30 (d, J = 7.6 Hz, 4 H), 7.08 (d, J = 1.5 Hz, 2
H, N-CH=CH-N), 3.13 (m, 4 H, CH iPr), 1.51–1.42 (m, 6 H, P-
CH2 nBu), 1.39 (d, J = 6.6 Hz, 12 H, CH3 iPr), 1.26–1.12 (m, 12
H, CH2 nBu), 1.08 (d, J = 7.1 Hz, 12 H, CH3 iPr), 0.77 (t, J =
6.9 Hz, 9 H, CH3 nBu) ppm. 13C NMR (100 MHz, CDCl3): δ =
2
175.6 (d, JC,P = 193 Hz, C carbene), 146.7 (C Ar), 135.5 (C Ar),
129.6 (CH Ar), 124.0 (d, 4JC,P = 5.1 Hz, N-CH=CH-N), 123.5 (CH
Ar), 28.6 (iPr), 26.3 (iPr), 25.8 (CH2 nBu), 24.1 (d, JC,P = 13.7 Hz;
CH2 nBu), 22.9 (iPr), 20.4 (d, JC,P = 26.6 Hz; CH2 nBu), 13.7 (CH3
nBu) ppm. 31P NMR (75 MHz, CDCl3):
δ = 8.9 ppm.
C39H63Cl2N2PPd (768.23): calcd. C 60.97, H 8.27, N 3.65; found
C 60.74, H 8.05, N 3.49.
[{(IPr)PdI2}2] (10): To a solution of 1,3-(2,6-diisopropylphenyl)-
imidazolium chloride (11; 219 mg, 0.5 mmol) in THF (40 mL) was
added Pd(OAc)2 (113 mg, 0.5 mmol) and sodium iodide (300 mg,
2 mmol). Potassium tert-butoxide (67 mg, 0.6 mmol) was then
added in portions, and the mixture was stirred for 16 h at 20 °C.
The mixture was directly poured in a short column of silica gel,
and the orange-brown fraction was collected by elution with THF.
The solvent was evaporated, and the brown solid obtained was dis-
solved in hot cyclohexane with a few drops of EtOAc and purified
by flash chromatography on silica gel (pentane/Et2O, 9:1) to give
an orange-brown solid (246 mg, 74%). M.p. Ͼ280 °C. 1H NMR
(250 MHz, CDCl3): δ = 7.52 (t, J = 7.7 Hz, 4 H, CH Ar), 7.34 (d,
J = 7.7 Hz, 8 H, CH Ar), 7.09 (s, 4 H, N-CH=CH-N), 3.27 (m, 4
H, CH iPr), 2.74 (m, 4 H, CH iPr), 1.47 (d, J = 6.6 Hz, 12 H, CH3
iPr), 1.25 (d, J = 6.3 Hz, 12 H, CH3 iPr), 1.08 (d, J = 6.9 Hz, 12
H, CH3 iPr), 0.93 (d, J = 6.6 Hz, 12 H, CH3 iPr) ppm. 13C NMR
(50 MHz, CDCl3): δ = 166.0 (C carbene), 146.8, 146.4, 135.8, 130.7,
125.8, 125.1, 124.7, 29.5 (iPr), 26.9 (iPr), 24.4 (iPr) ppm.
C54H72I4N4Pd2 (1497.63): calcd. C 43.31, H 4.85, N 3.74; found C
43.51, H 4.74, N 3.57.
[(IPr)(PPh3)PdCl2] (7a): To a solution of [{(IPr)PdCl2}2] (9;
113 mg, 0.1 mmol) in THF (2 mL) was added PPh3 in one portion
(52.5 mg, 0.2 mmol). The resulting mixture was stirred at 20 °C for
20 min. The color of the solution changed from orange to yellow.
The mixture was concentrated, and the solid residue was purified
by flash chromatography on silica gel (pentane/Et2O, 9:1) to give a
pale-yellow solid (159 mg, 96%). M.p. 222–224 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.47 (t, J = 7.8 Hz, 2 H), 7.28 (d, J =
7.8 Hz, 4 H), 7.23–7.15 (m, 9 H), 7.11–7.05 (m, 8 H), 3.12 (m, 4
H, CH iPr), 1.23 (d, J = 6.5 Hz, 12 H, CH3 iPr), 1.00 (d, J =
6.8 Hz, 12 H, CH3 iPr) ppm. 13C NMR (100 MHz, CDCl3): δ =
170.9 (d, 2JC,P = 198 Hz, C carbene), 146.8 (iPr-C Ar), 135.5 (N-C
1
Ar), 134.9 (d, JC,P = 10.3 Hz, CH Ph), 130.3 (d, JC,P = 44.5 Hz,
C Ph), 129.8 (CH Ar), 129.7 (d, JC,P = 1.7 Hz, CH Ph), 127.5 (d,
4
4
JC,P = 10.3 Hz, CH Ph), 124.2 (d, JC,P = 6 Hz, N-CH=CH-N),
123.8 (CH Ar), 28.6 (iPr), 26.3 (iPr), 22.9 (iPr) ppm. 31P NMR
[(IPr)(PPh3)PdI2] (7d): To a solution of {[(IPr)PdI2]2} (10; 322 mg,
0.215 mmol) in THF (15 mL) was added PPh3 (113 mg, 0.43 mmol)
in one portion. The resulting mixture was stirred at 20 °C for 3 h.
The color of the solution changed from brown-red to orange-red.
The mixture was concentrated, and the solid residue was purified
by flash chromatography on silica gel (pentane/Et2O, 9:1) to give
an orange solid (345 mg, 68%). M.p. 193–197 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.46 (t, J = 7.6 Hz, 2 H), 7.28–7.08 (m, 21
H), 3.35 (m, 4 H, CH iPr), 1.27 (d, J = 6.6 Hz, 12 H, CH3 iPr),
1.00 (d, J = 6.8 Hz, 12 H, CH3 iPr) ppm. 13C NMR (100 MHz,
(75 MHz, CDCl3):
δ = 21.6 ppm. HRMS (ESI) calcd. for
C45H51Cl2N2PPdNa [M+ + Na] 847.20995; found 847.21019.
C45H51Cl2N2PPd (828.2): calcd. C 65.26, H 6.21, N 3.38; found C
65.37, H 6.35, N 3.44.
[(IPr){P(OPh)3}PdCl2] (7b): To a solution of [{(IPr)PdCl2}2] (9;
113 mg, 0.1 mmol) in THF (2 mL) was added P(OPh)3 (53 µL,
0.2 mmol). The color of the solution changed from orange to yel-
low. The resulting mixture was stirred at 20 °C for 20 min. The
mixture was concentrated, and the solid residue was purified by
flash chromatography on silica gel (pentane/Et2O, 9:1) to give a
pale-yellow solid (156 mg, 89%). M.p. 74–76 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.54 (t, J = 7.6 Hz, 2 H), 7.28 (d, J =
7.6 Hz, 4 H), 7.10–6.95 (m, 17 H), 3.01 (m, 4 H, CH iPr), 1.22 (d,
J = 6.6 Hz, 12 H, CH3 iPr), 1.04 (d, J = 6.8 Hz, 12 H, CH3 iPr)
2
CDCl3): δ = 168.3 (d, JC,P = 198 Hz, C carbene), 146.6 (C Ar),
1
136.5 (C Ar), 135.3 (d, JC,P = 10.3 Hz, CH Ph), 133.9 (d, JC,P
=
4
48 Hz, C Ph), 129.9 (CH Ar), 129.6 (d, JC,P = 1.7 Hz, CH Ph),
127.1 (d, JC,P = 10.3 Hz, CH Ph), 125.0 (d, JC,P = 6.0 Hz, N-
4
CH=CH-N), 124.1 (CH Ar), 29.3 (iPr), 26.4 (iPr), 23.6 (iPr) ppm.
31P NMR (75 MHz, CDCl3): δ = 16.2 ppm. C45H51I2N2PPd
(1011.1): calcd. C 53.45, H 5.08, N 2.77; found C 53.98, H 5.08, N
2.62.
ppm. 13C NMR (100 MHz, CDCl3): δ = 168.1 (d, JC,P = 300 Hz,
2
C carbene), 150.8 (d, 2JC,P = 5.14 Hz, P-O-C Ph), 146.7 (iPr-C Ar),
4
134.9 (N-C Ar), 130.0 (CH Ar), 129.3 (CH Ar), 124.6 (d, JC,P
=
8.6 Hz, N-CH=CH-N), 124.3 (CH Ar), 123.9 (CH Ar), 120.4 (d,
3JC,P = 5.1 Hz, CH Ph), 28.5 (iPr), 26.3 (iPr), 22.8 (iPr) ppm. 31P
[(IPr){P(nBu)3}PdI2] (7e): To a solution of [{(IPr)PdI2}2] (10;
35 mg, 0.023 mmol) in THF (1.6 mL) was added P(nBu)3 (12 µL,
0.047 mmol). The resulting mixture was stirred at 20 °C and moni-
tored by TLC. After 15 h, a second portion of P(nBu)3 (12 µL,
0.047 mmol) was added. After 30 min, the mixture was concen-
NMR (75 MHz, CDCl3):
(876.2): calcd. C 61.68, H 5.87, N 3.20; found C 61.36, H 5.88, N
3.07.
δ = 92.3 ppm. C45H51Cl2N2O3PPd
[(IPr){P(nBu)3}PdCl2] (7c): To a solution of [{(IPr)PdCl2}2] (9;
50 mg, 0.044 mmol) in THF (2.5 mL) was added P(nBu)3 (22 µL, trated, and the solid residue was purified by flash chromatography
Eur. J. Inorg. Chem. 2009, 5422–5432
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A. Flahaut, K. Toutah, P. Mangeney, S. Roland
FULL PAPER
on silica gel (cHex/EtOAc, 98:2) to give a yellow solid (42 mg,
94%). M.p. 129–131 °C. H NMR (400 MHz, CDCl3): δ = 7.44 (t,
mol), allylic acetate (0.24 mmol), and aldehyde or ketone
(0.2 mmol) in dry THF (1.5 mL) was added dropwise at 20 °C a
1
J = 7.8 Hz, 2 H), 7.29 (d, J = 7.8 Hz, 4 H), 7.20 (d, J = 1.3 Hz, 2 solution of Et2Zn (1  in hexanes, 0.7 mL, 0.7 mmol, 3.5 equiv.).
H, N-CH=CH-N), 3.41 (m, 4 H, CH iPr), 1.95 (m, 6 H, P-CH2 The mixture was stirred for 16 h (or 45 h) at 20 °C before quench-
nBu), 1.44 (d, J = 6.8 Hz, 12 H, CH3 iPr), 1.30–1.05 (m, 12 H, CH2
ing with aqueous saturated NH4Cl (5 mL). The mixture was stirred
nBu), 1.08 (d, J = 6.5 Hz, 12 H, CH3 iPr), 0.80 (t, J = 6.9 Hz, 9 vigorously for 30 min. Et2O (5 mL) was added, and the organic
H, CH3 nBu) ppm. 13C NMR (100 MHz, CDCl3): δ = 170.5 (d,
layer was separated. The aqueous layer was extracted with Et2O
(2ϫ5 mL). The combined organic layer was dried (Na2SO4), fil-
2JC,P = 189 Hz, C carbene), 145.4 (C Ar), 135.3 (C Ar), 128.7 (CH
4
Ar), 123.8 (d, JC,P = 5.1 Hz, N-CH=CH-N), 122.8 (CH Ar), 28.1 tered through Celite, and concentrated under reduced pressure
(iPr), 25.9 (d, JC,P = 29.1 Hz, CH2 nBu), 25.6 (CH2 nBu), 25.5 (iPr),
23.1 (d, JC,P = 13.7 Hz, CH2 nBu), 22.5 (iPr), 12.7 (CH3 nBu) ppm.
(15 Torr, 20 °C). The yield was determined by 1H NMR spec-
troscopy of the crude reaction mixture by comparison with the in-
31P NMR (75 MHz, CDCl3): δ = –1.03 ppm. C39H63I2N2PPd ternal standard.
(951.13): calcd. C 49.25, H 6.68, N 2.95; found C 49.31, H 6.54, N
2.81.
1-(3,4,5-Trimethoxyphenyl)-2-phenyl-3-buten-1-ol (17g): The crude
was purified by flash chromatography on silica gel (pentane/Et2O,
1:1) to give a colorless oil. Data for the anti isomer: 1H NMR
(400 MHz, CDCl3): δ = 7.18–7.05 (m, 3 H, CH Ph), 6.96 (d, J =
8.3 Hz, 2 H, CH Ph), 6.18 [s, 2 H, (MeO)3C6H2], 6.19 (ddd, J =
8.8, 10.3, 19.3 Hz, 1 H, CH=CH2), 5.24–5.17 (m, 2 H, CH=CH2),
4.67 (d, J = 8.1 Hz, 1 H, CH-OH), 3.70 (s, 3 H, Me), 3.61 (s, 6 H,
Me), 3.38 (t, J = 8.3 Hz, 1 H, CH-Ph), 2.45 (br. s, 1 H, OH) ppm.
13C NMR (100 MHz, CDCl3): δ = 152.6 (C Ar), 140.6 (C Ar),
137.9 (CH), 137.3 (C Ar), 128.4 (CH Ph), 128.3 (CH Ph), 126.6
(CH), 118.5 (CH2), 103.6 (CH), 77.3 (CH-OH), 60.8 (CH3 OMe),
59.5 (CH-Ph), 55.9 (CH3 OMe) ppm. Data for the syn (visible and
representative signals): 1H NMR (400 MHz, CDCl3): δ = 5.87 (ddd,
J = 18.0, 10.5, 7.7 Hz, 1 H, CH=CH2), 4.98 (dt, J = 10.3, 1.3 Hz,
1 H, CH=CH2), 4.86 (dt, J = 17.1, 1.3 Hz, 1 H, CH=CH2), 4.78
(d, J = 7.7 Hz, 1 H, CH–OH) ppm. HRMS (ESI): calcd. for
C19H22O4Li [M+ + Li] 321.16731; found 321.16625. C19H22O
(266.38): calcd. C 72.59, H 7.05; found C 72.68, H 7.11.
[(IMesMe)(PPh3)PdI2] (7f): To a solution of 1-methyl-3-mesityl-
imidazolium iodide (12; 164 mg, 0.5 mmol) in THF (40 mL) was
added, under a nitrogen atmosphere, Pd(OAc)2 (113 mg, 0.5 mmol)
and sodium iodide (300 mg, 2 mmol). Potassium tert-butoxide
(67 mg, 0.6 mmol) was then added in portions, and the mixture was
stirred for 6 h at 20 °C with exclusion of light. The mixture was
directly poured onto a short column of silica gel, and the orange
fraction was collected by elution with THF. Evaporation of the
solvent and drying afforded an orange-red solid (225 mg). The solid
was dissolved in CH2Cl2 (20 mL) and PPh3 (94 mg, 0.36 mmol) was
added in one portion. The mixture was stirred for 15 min at 20 °C.
The color of the solution changed from dark-red to orange. Evapo-
ration of the solvent and drying afforded an orange solid (310 mg).
1H NMR analysis of this solid in CDCl3 showed the presence of
96% of the expected complex 7f, 4% of the bis(carbene) complex
[(IMesMe)2PdI2], and trace amounts of PPh3. The orange solid was
dissolved in ethyl acetate (20 mL) and stirred for 15 min. The pale-
yellow precipitate that appeared {[(IMesMe)2PdI2]} was separated
by filtration, and the solution was concentrated to dryness. The
residue was dissolved in CH2Cl2 (3 mL) and pentane (20 mL) was
added slowly. The red-orange precipitate formed was collected by
filtration, washed with pentane, and dried to afford an orange solid
(208 mg, 51%). The sample contained less than 2% of the bis(car-
bene) complex [(IMesMe)2PdI2]. Small red crystals suitable for X-
ray analysis were obtained by slow evaporation of a diluted Et2O
solution of the complex containing a small amount of CH2Cl2.
syn-1-Cyclohex-2-enyl(4-bromophenyl)methanol (20a): The crude
was purified by flash chromatography on silica gel (pentane/Et2O,
1
8:2) to give a colorless oil. H NMR (400 MHz, CDCl3): δ = 7.46
(d, J = 8.3 Hz, 2 H, CH Ph), 7.21 (d, J = 8.3 Hz, 2 H, CH Ph),
5.84 (ddd, J = 10.2, 6.0, 2.6 Hz, 1 H, CH2-CH=CH), 5.38 (dd, J =
10.2, 2.0 Hz, 1 H, CH=CH-CH), 4.57 (d, J = 6.1 Hz, 1 H, CH-
OH), 2.45 (m, 1 H, CH-CH-OH), 1.98 (m, 2 H, CH2-CH=CH),
1.80–1.70 (m, 1 H, CH2), 1.69–1.58 (m, 1 H, CH2), 1.55–1.45 (m,
2 H, CH2) ppm. 13C NMR (100 MHz, CDCl3): δ = 142.2 (C Ar),
131.7 (CH Ar), 131.4 (CH2-CH=CH), 128.6 (CH Ar), 127.9
(CH=CH-CH), 121.5 (C Ar), 77.0 (CH-OH), 43.5 (CH-CH-OH),
25.6 (CH2), 23.9 (CH2), 21.4 (CH2) ppm. HRMS (ESI): calcd. for
C13H15BrONa [M+ + Na] 289.01985; found 289.02251. C13H15BrO
(267.16): calcd. C 58.44, H 5.66; found C 58.08, H 5.97.
1
M.p. 203–206 °C. H NMR (250 MHz, CDCl3): δ = 7.55–7.25 (m,
15 H), 7.08 (t, J = 1.7 Hz, 1 H, N-CH=CH-N), 6.96 (s, 2 H, CH
Ar), 6.95 (t, J = 1.7 Hz, 1 H, N-CH=CH-N), 4.00 (s, 3 H, N-CH3),
2.44 (s, 3 H, CH3 Mes), 2.31 (s, 6 H, CH3 Mes) ppm. 13C NMR
2
(62.5 MHz, CD2Cl2): δ = 160.9 (d, JC,P = 195.0 Hz, C carbene),
CCDC-723321 (for 7f) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
139.9 (C Mes), 137.1 (C Mes), 135.9 (d, JC,P = 10.8 Hz, CH Ph),
4
134.1 (d, 1JC,P = 45.5 Hz, C Ph), 130.7 (d, JC,P = 2.2 Hz, CH Ph),
4
129.9 (CH Mes), 128.3 (d, JC,P = 9.8 Hz, CH Ph), 125.6 (d, JC,P
= 5.5 Hz, N-CH=CH-N), 123.5 (d, 4JC,P = 6.5 Hz, N-CH=CH-N),
40.5 (N-CH3), 22.2 (CH3 Mes), 21.8 (CH3 Mes) ppm. 31P NMR
(75 MHz, CD2Cl2): δ = 16.2 ppm. HRMS (ESI): calcd. for
C31H31I2N2P104PdNa [M+ + Na] 842.92468; found 842.92472.
C31H31I2N2PPd (822.79): calcd. C 45.25, H 3.80, N 3.40; found C
45.10, H 3.80, N 3.52. Data for the minor complex [(IMesMe)
2PdI2]: Pale-yellow solid. 1H NMR (250 MHz, CDCl3): δ = 6.96
(d, J = 1.9 Hz, 2 H), 6.79 (s, 4 H), 6.72 (d, J = 1.9 Hz, 2 H), 4.09
(s, 6 H, N-CH3), 2.44 (s, 6 H, CH3 Mes), 2.00 (s, 12 H, CH3 Mes)
ppm. HRMS (ESI): calcd. for C26H32I2N4PdNa [M+ + Na]
782.96434; found 782.96489. The complex is little soluble in most
organic solvents.
Acknowledgments
Financial support from Centre National de la Recherche Sci-
entifique (CNRS), Université Pierre et Marie Curie, and Clariant,
France is gratefully acknowledged. We thank P. Le Floch and N.
Mézailles for helpful discussion and Dr P. Herson for X-ray analy-
ses.
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5431

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Received: September 8, 2009
Published Online: October 23, 2009
5432
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Eur. J. Inorg. Chem. 2009, 5422–5432