Ferrocenyl phosphine-oxazaphospholidine oxide ligands for the Suzuki-Miyaura coupling of hindered aryl bromides and chlorides

Article abstract of DOI:10.1139/V08-113

A series of ferrocenyl oxazaphospholidine phosphines that differ electronically and sterically were investigated as ligands for the Suzuki-Miyaura cross-coupling reactions. One of these compounds, 1, was shown to be highly effective in the coupling reactions of bulky aryl bromides with boronic acids when combined with Pd(OAc)₂, while another, 2, was capable of coupling aryl chlorides with boronic acids. However, these ligands were less effective in asymmetric induction.

Full text of DOI:10.1139/V08-113

1
71
Ferrocenyl phosphine–oxazaphospholidine oxide
ligands for the Suzuki–Miyaura coupling of
1
hindered aryl bromides and chlorides
Daniele Vinci, Nelson Martins, Ourida Saidi, John Bacsa, Amadeu Brigas, and
Jianliang Xiao
Abstract: A series of ferrocenyl oxazaphospholidine phosphines that differ electronically and sterically were investi-
gated as ligands for the Suzuki–Miyaura cross-coupling reactions. One of these compounds, 1, was shown to be highly
effective in the coupling reactions of bulky aryl bromides with boronic acids when combined with Pd(OAc) , while an-
2
other, 2, was capable of coupling aryl chlorides with boronic acids. However, these ligands were less effective in asym-
metric induction.
Key words: Suzuki–Miyaura coupling, ferrocenyl phosphines, aryl bromides, aryl chlorides, palladium.
Résumé : Une série de ferrocényl oxazaphospholidine phosphines qui présentent des différences tant électroniques que
stériques a été étudiée comme ligands pour les réactions de couplage croisé de Suzuki–Miyaura. On a montré qu’un de
ces composés, 1, lorsqu’il est combiné au Pd(OAc) , est très efficacle dans les réactions de couplage de bromures
2
d’aryles avec des acides boroniques alors que le composé 2 permet d’effectuer le couplage de chlorures d’aryles avec
les acides boroniques. Toutefois, ces ligands sont moins efficaces dans l’induction asymétrique.
Mots-clés : couplage de Suzuki–Miyaura, ferrocényl phosphines, bromures d’aryles, chlorures d’aryles, palladium.
[
Traduit par la Rédaction] Vinci et al. 175
Introduction
scribed (20–28). In particular, biaryl monophosphine ligands
have proven to be highly effective, partially meeting the re-
quirement of increased catalytic performance (29). However,
there remain some key issues that need to be addressed;
these include problems such as hydrolysis of Pd–Ar com-
plexes, slow transmetalation or slow trans to cis isomeri-
zation of intermediate palladium species, leading to the
formation of unwanted byproducts (18, 29–34).
Ferrocene-based ligands have been employed successfully
in many catalytic transformations (35) due to their ease of
functionalization and the stability of most of its derivatives.
Its proven record in homogeneous catalysis has been demon-
strated. The Suzuki–Miyaura reaction is no exception to this
trend (36, 37). Recently, we reported a new series of
ferrocenyl-phosphine ligands that were synthesized by ortho-
lithiation of (2R,4S,5R)-3,4-dimethyl-2-ferrocenyl-5-phenyl-
[1,3,2]oxazaphospholidine 2-oxide (Scheme 1) (38). Herein,
we present our findings on the application of the ligands 1–4
in the Suzuki–Miyaura reactions.
The palladium-catalyzed cross-coupling reactions of
organoboron compounds and organic halides or triflates have
emerged as a powerful and general methodology for the for-
mation of carbon–carbon bonds (1–7). Advances in recent
years have made the reactions scalable and cost effective (8).
However, methods that are generally applicable to the syn-
thesis of bulky and axially chiral biaryls, common structural
motifs in natural products and chiral ligands, are still rare
(
9–17). Although reasonably good results in the enantio-
selective Suzuki coupling have been achieved using P^N lig-
ands based on biaryl and ferrocene backbones, the reaction
times, scope of substrates, and catalyst loadings remain to be
improved (18, 19).
Suzuki–Miyaura coupling of hindered aryl bromides and
chlorides is a particularly challenging area of research. Re-
cently, new and improved catalytic systems for the coupling
of these demanding substrates have been developed and de-
Received 27 March 2008. Accepted 30 May 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on
September 2008.
3
Dedicated to Professor Dick Puddephatt for his outstanding contributions to chemistry and mentoring.
2
D. Vinci, N. Martins, O. Saidi, J. Bacsa, and J. Xiao. Department of Chemistry, University of Liverpool, Liverpool L69 7ZD,
U.K.
N. Martins and A. Brigas. C CIQA, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 8005-139 Faro, Portugal.
1
This article is part of a Special Issue dedicated to Professor R. Puddephatt.
Corresponding author (e-mail: j.xiao@liv.ac.uk).
2
Can. J. Chem. 87: 171–175 (2009)
doi:10.1139/V08-113
© 2008 NRC Canada

1
72
Can. J. Chem. Vol. 87, 2009
Scheme 1. Ferrocenyl phosphino ligands synthesized by ortho-
Table 1. Suzuki coupling with Pd(OAc) and ligands 1–4.
2
lithiation and examined in this study.
Entry
Ligand
L/Pd ratio
Conv. (%)
Yield (%)a
1
2
3
4
5
6
7
8
9
0
1
12
3
1
1
2
2
3
3
4
4
1
2
3
4
2
1
2
1
2
1
2
1
2
1
2
2
2
>99
78
55
>99
91
78
97
90
61
27
39
53
22
98
63
50
94
90
76
91
79
54
21
22
39
8
Experimental3
Procedures for Suzuki–Miyaura coupling reactions
The coupling of 1-bromo-2-methoxynaphthalene with 1-
naphthalene boronic acid to give 2-methoxy-1,1′-binaphthyl,
b
5
, is described as a typical example. An oven-dried carousel
b
1
1
reaction tube was charged with 1-naphthalene boronic acid
b
b
(
(
0.75 mmol), CsF (1.5 mmol), 1 (10 µmol), and Pd(OAc)
2
5 µmmol). The aryl bromide (0.5 mmol) was introduced at
1
FcPPh2
this stage (or after degassing by injection through a rubber
septum in case of a liquid). The reaction vessel was evacu-
Note: Reaction conditions: 1.0 equiv. of aryl bromide, 1.5 equiv. of
boronic acid, 3.0 equiv. of CsF, 1 mol% Pd(OAc) , 105 °C, 4 h in
2
ated and backfilled with N . This process was repeated five
dioxane.
2
a
times, after which dioxane (3 mL) was introduced. The reac-
tion mixture was then heated at 100 °C for 4 h. After cool-
ing down to ambient temperature, the conversion was
Isolated yields; hydrolysis products were observed in the case of low
yields.
b
Reaction time: 1 h.
1
determined from the crude H NMR, and isolated yield was
obtained after a standard work-up and purification by flash
chromatography (100% hexane). The product 2-methoxy-
monochromated Mo Kα radiation (λ = 0.710 73 Å). The
crystal was cooled to T = 110(2) K. A total of 19537 reflec-
1
,1′-binaphthyl, 5, was obtained in 98% yield. Analytic de-
tions was collected with 2θ
= 55.1°. Of these reflections,
13146 were unique (R = 0.0406). The reflection data were
max
1
tails: mp 106 to 107 °C. H NMR (CDCl , 400 MHz, ppm)
3
int
δ: 3.75 (s, 3H), 7.14 (d, 1H, J = 8.8 Hz), 7.19–7.28 (m, 2H),
integrated using SAINT v6.45a (39). The data were cor-
rected for absorption, decay, and other errors using
SADABS V2007-2 (40). The structure was solved by direct
methods and refined using SHELX (41). X-SEED (42), a
graphical interface to SHELX, was used to generate repre-
sentations of the crystal structure.
7
7
7
.30–7.33 (m, 2H), 7.42–7.47 (m, 3H), 7.61 (t, 1H, J =
.7 Hz), 7.86 (d, 1H, J = 8.0 Hz), 7.93 (d, 1H, J = 8.0 Hz),
1
3
.94 (d, 1H, J = 8.0 Hz), 7.97 (d, 1H, J = 8.8 Hz). C NMR
(
1
1
CDCl , 100 MHz, ppm) δ: 56.8, 113.8, 123.2, 123.5, 125.5,
3
25.5, 125.7, 125.8, 126.2, 126.4, 127.7, 127.8, 128.2,
28.4, 129.0, 129.4, 132.9, 133.7, 134.2, 134.5, 154.6. MS
Crystal data: 2(C H Cl FeNO P Pd), C H N, CHCl ,
3
2
31
2
2
2
7
5
3
+
m/z: 284 (M ). The products 6–19 were obtained in a similar
M = 1701.38, orange block, 0.20 mm × 0.20 mm ×
0.10 mm, monoclinic, space group P21 (No. 4), a =
manner.
9
.4590(19), b = 19.219(4), c = 19.677(4) Å, β = 96.99(3)°,
3
3
X-ray structural determination of Pd-1
V = 3550.5(12) Å , Z = 2, D
= 1.591 g/cm , F = 1720.
calcd. 000
The complex Pd-1 was synthesized by heating PdCl2-
Final GoF = 1.011, R1 = 0.0517, wR2 = 0.1042, R indices
2
(
PhCN) with 1 in dry dichloromethane with a 91% yield.
H NMR (CDCl , 400 MHz, ppm) δ: 0.78 (s, br, 3H), 2.00
based on 10244 reflections with I >2σ(I) (refinement on F ),
2
1
–1
842 parameters, 1 restraint, µ = 1.266 mm . Absolute struc-
3
(
d, 3H, J = 11.2 Hz), 3.69 (m, 1H), 4.15 (br, 1H), 4.75 (s,
ture parameter = –0.00(2).
5
H), 4.80 (br, 1H), 4.83 (br, 1H), 5.70 (br, 1H), 7.30–7.90
3
1
(
4
m, 15H). P NMR (CDCl , 162 MHz, ppm) δ: 16.4(s),
3
Results and discussion
37
54
6.6(s). HR-MS (ES+) m/z: calcd. for C H Cl FeNO P Pd:
32 31 2 2
3
5
56
+
7
19.9920, C H Cl FeNO P Pd: 719.9903 [M – Cl] ;
Ligand screening
3
2
31
2 2
found: 719.9918.
Single crystal X-ray data were collected on a Bruker D8
diffractometer with an APEX detector using graphite
The ligands 1–4 are hemilabile, with P^O chelation to Pd
being the most likely. Sterically and electronically, however,
they show different behaviour, and it was of interest to see
3
Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of Un-
published Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 3800. For more in-
formation on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/cms/unpub_e.shtml. CCDC 691402 contains the crystallographic data for
this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
©
2008 NRC Canada

Vinci et al.
173
Table 2. Suzuki coupling of sterically hindered partners with 1.
Fig. 1. The X-ray structure of Pd-1. Selected bond distances (Å)
and angles (°): Pd(1)–O(2) 2.071(4), Pd(1)–P(1) 2.229(2), Pd(1)–
Cl(2) 2.262(2), Pd(1)–Cl(1) 2.364(2); O(2)-Pd(1)-P(1) 88.93(14),
O(2)-Pd(1)-Cl(2) 172.8(1), P(1)-Pd(1)-Cl(2) 89.94(7), O(2)-Pd(1)-
Cl(1) 88.4(1), Cl(2)-Pd(1)-Cl(1) 92.77(7).
how this difference would impact the palladium-catalyzed
Suzuki coupling. At the outset, we examined the coupling
between the sterically hindered 1-bromo-2-methoxynaph-
thalene and 1-naphthalene boronic acid and compared the re-
sults with a similar but less bulky phosphine. The results are
reported in Table 1.
As can be seen, ligand 1 showed excellent results when
the ligand/catalyst ratio was set to 2. The reaction was com-
plete after 4 h and the coupling product was recovered in
Note: Reaction conditions: 1.0 equiv. of aryl bromide, 1.5 equiv. of
boronic acid, 3 equiv. of CsF, 100 °C in dioxane, 1 mol% Pd(OAc) , 2
2
mol% 1. Yield refers to isolated yields.
a
Hydrolysis product was observed.
9
8% yield after purification by column chromatography (Ta-
ble 1, entry 1). At a lower concentration of ligand, the reac-
tion slowed down, lowering the yield to 63% (Table 1, entry
b
NEt (3 equiv.) and toluene as solvent were used.
3
2
). In contrast, the reaction with ligand 2 was found to go to
completion when a 1/1 ligand/palladium (L/Pd) ratio was
used (Table 1, entry 4). At higher concentrations, the rate
dropped dramatically (Table 1, entry 3). Ligands 3 and 4
show similar behaviour and their results are somewhat simi-
lar to those obtained with the ligand 1. To better appreciate
the differences among the ligands, the coupling time was re-
duced. As is seen from the entries 9–12 in Table 1, the
ligand 1 gave the best yields, and it appears that an electron
rich phosphine does not benefit the coupling. The coupling
was considerably less efficient with the less bulky ligand
FePPh2 (Fc = ferrocenyl) (Table 1). Thus, the oxaza-
phospholidine moiety of 1–4, in combination with the phos-
phine group, is able to stabilize the active palladium species
and accelerate the reaction rate. The differences between 1
and 2 could partially be explained by assuming a stable, ac-
tive Pd species requires two equivalents of 1 but only one
equivalent of the bulkier 2 (43).
ture parameter is further evidence that the crystals are in-
deed chiral. The molecular structure of one of the two mole-
cules is shown in Fig. 1. The coordination geometry about
the Pd(II) atom is square planar, with P, O, and the two Cl
atoms cis to each other. The two Pd–Cl bonds are noticeably
different [2.262(2) and 2.364(2) Å]. The longer Pd–Cl bond,
which is trans to P(1), is likely to be due to the stronger
trans effect of a P atom than an O atom. The three chiral
centres of P(2), C(4), and C(5) have R, S, and R configura-
tions, respectively. The complex is also planar chiral with a
R configuration.
During the Suzuki coupling, the phosphoryl oxygen could
dissociate from the palladium to facilitate the trans-
metalation and recoordinate to induce a faster reductive
elimination. This may well be the case with the bulkier 2. In
the case of 1, 3, and 4, however, the need for 2 equiv. of
ligand for a more efficient coupling appears to suggest that
the phosphoryl oxygen is replaced by the phosphine group
of a second ligand.
The X-ray crystal structure of the complex, Pd-1, formed
between 1 and PdCl (PhCN) , confirms that 1 behaves as a
2
2
chelating ligand, with both the phosphorus atom and the
phosphoryl oxygen atom of the oxazaphospholidine ring co-
ordinating to the Pd(II) centre (Fig. 1). The compound crys-
Aryl bromides as substrates
tallizes in the chiral space group P2 with 2 Pd-1 molecules,
One of the biggest challenges in the use of a Suzuki–
Miyaura reaction is the coupling of hindered substrates.
Since the ligand 1 afforded the best results in the coupling of
1-bromo-2-methoxynaphthalene and 1-naphthalene boronic
acid, its catalytic activity towards other hindered substrates
was studied. The results of the coupling of various sterically
1
a benzonitrile, and a chloroform molecule in the asymmetric
unit, but the two organometallic molecules are chemically
and nearly structurally identical. Examination of the orienta-
tion of the two molecules and the crystal packing reveals no
missing symmetry elements, and the refined absolute struc-
©
2008 NRC Canada

1
74
Can. J. Chem. Vol. 87, 2009
Table 4. Asymmetric Suzuki coupling.
Table 3. Suzuki coupling of aryl chlorides using 2.
T
(°C)
Time
(h)
Conv.
(%)
Ee
(%)
a
Entry
Ligand
L/Pd
1
2
3
4
5
6
7
1
1
2
2
2
3
4
2
1
1
1
2
2
2
70
50
50
70
70
70
70
12
12
12
12
7
85
78
55
>99
80
84
26 (R)
18 (R)
7 (S)
9 (S)
27 (R)
14 (R)
3 (R)
12
12
92
Note: Reaction conditions: 1.0 equiv. of aryl bromide, 1.5 equiv. of
boronic acid, 3.0 equiv. of CsF, dioxane, 5 mol% Pd(OAc) .
2
a
Determined by HPLC using a DAICEL OJ column (90/10 hex-
ane/iPrOH).
sponding bromides. The results of the coupling reactions
obtained with 2 are summarized in Table 3.
The reaction between 4-chloroacetophenone and
phenylboronic acid at 105 °C using 1 mol% of Pd and 1
equiv. of 2 was complete in just 1 h (Table 3, entry 1). Re-
ducing the amount of palladium to 0.1 mol% resulted in a
predictable decrease of the reaction rate; complete conver-
sion was reached after 18 h (Table 3, entry 2). Increasing the
steric bulk on the boronic acid also resulted in a decrease in
the reaction rate (Table 3, entry 3). Although the coupling of
the 4-chloroacetophenone with 2-tolylboronic acid also oc-
curred at a 0.1 mol% Pd loading, the reaction took more
than one day to reach completion. The rest of the reactions
were therefore performed using 1 mol% Pd.
Note: Reaction conditions: 1.0 equiv. of aryl chloride, 1.5 equiv. of
boronic acid, 3.0 equiv. of CsF, 105 °C in dioxane, 1 mol% Pd(OAc) , 1
2
mol% ligand 2. Yield refers to isolated yields.
a
Pd(OAc) (0.1 mol%).
2
As can be seen from Table 3, going from an activated
chloride to a deactivated one like 2-chlorotoluene, the yield
was not affected under the conditions employed. The cou-
pling of sterically hindered boronic acids, such as 1-
naphthylboronic acid and 2-tolylboronic acid (Table 3, en-
tries 6 and 7), ran smoothly to completion after 8 h. How-
ever, a further increase in the steric bulk of the chlorides did
eventually affect the reaction rate. Thus, with 2-chloro-meta-
xylene as substrate, the coupling reactions went to comple-
tion only after 24 h (entries 8–10). The results obtained with
hindered aryl bromides and sterically hindered aryl boronic
acids employing ligand 1 are reported in Table 2.
The couplings involving the formation of the trisubstituted
biaryls proceeded smoothly to completion. The reactions
were complete after 4 h and no hydrolysis byproducts were
observed in the crude mixtures. However, the reaction lead-
ing to tetra-substituted biaryl compounds resulted in low
yields (Table 2, entries 7 and 8), and a significant amount of
hydrolysis product, 2-methoxynaphthalene.
The important factor that impedes the coupling of partners
in entry 7 of Table 2 is likely to be the increased steric bulk
of the boronic acid, which makes the transmetallation diffi-
cult. This may in turn give preference to protonolysis of the
Pd–Ar intermediate, leading to the hydrolysis product. Ac-
2
are better than or comparable with those obtained using re-
lated electron-rich ferrocenyl phosphines (44, 45).
Asymmetric coupling
So far, few catalysts have been developed for asymmetric
Suzuki cross-coupling reactions (18, 19). In the course of
this study, we examined the ligands 1–4 for the asymmetric
coupling of 1-naphthylboronic acid and 1-bromo-2-meth-
oxynaphthylboronic acid. The reaction conditions are
slightly different from the ones reported in Table 1. The con-
versions and ee values obtained are reported in Table 4.
Ligand 1 yielded a 85% conversion and 26% ee as its best
results (Table 4, entry 1). Decreasing the temperature or
the L/Pd ratio did not bring about any amelioration. Al-
though the ligand 2 was previously found to give a higher
activity using a lower L/Pd ratio (Table 1, entry 4), it gave a
higher enantioselectivity (27% ee) when 2 equiv. of the
ligand were used (Table 4, entry 5). Most notably, the con-
figuration of the product was reversed in the case of the lat-
ter, suggesting that the active catalyst varies with the L/Pd
ratio. The higher ee for the L/Pd ratio of 2 is probably due to
cordingly, a combination of CsF and NEt as base and a
3
nonprotic solvent like toluene afforded some improvement in
the conversion. The yield increased from 12% to 43% (Ta-
ble 2, entry 8), although 2-methoxynaphthalene was still ob-
served.
Aryl chlorides as substrates
To find the best ligand for these substrates, we screened
the ligands 1–4 for the coupling of the activated 4-
chloroacetophenone with phenylboronic. In sharp contrast to
the coupling of the bromides, only the ligand 2 was found to
give satisfactory results; hence, the other ligands were not
studied further. Apparently, a more electron-rich ligand is
needed for the aryl chlorides, which are expected to be less
reactive in the oxidative addition at Pd(0) than the corre-
©
2008 NRC Canada

Vinci et al.
175
the formation of a Pd-diphosphine species, which increases
the steric bulk around the palladium. Ligand 4, the electron-
poor analogue of 1, afforded the poorest enantioselectivity,
which may partly stem from its ease of dissociation from the
metal centre. However, it is difficult to draw conclusions re-
lating the electronic and steric properties of ligand with the
asymmetric induction observed, not least because there are
probably more than one active catalytic species operating in
the reaction media.
14. K. Kamikawa, A. Tachibana, S. Sugimoto, and M. Uemura.
Org. Lett. 3, 2033 (2001).
1
5. G. Bringmann, M. Breuning, and S. Tasler. Synthesis, 4, 525
1999).
(
1
1
6. G. Bringmann and D. Menche. Acc. Chem. Res. 34, 615 (2001).
7. K.C. Nicolaou, H. Li, C.N.C. Boddy, J.M. Ramanjulu, T.Y.
Yue, S. Natarajan, X.J. Chu, S. Brase, and F. Rubsam. Chem.
Eur. J. 5, 2584 (1999).
1
1
2
2
2
2
2
8. J. Yin and S.L. Buchwald. J. Am. Chem. Soc. 122, 12051
(
2000).
9. M. Genov, A. Almorin, and P. Espinet. Chem. Eur. J. 12, 9346
2006).
Conclusion
(
0. S.D. Walker, T.E. Barder, J.R. Martinelli, and S.L. Buchwald.
This paper presents results on the application of a series
of electronically and sterically varied ferrocenyl ligands to
the Suzuki–Miyaura cross-coupling reactions of aryl bro-
mides and chlorides. Ligands 1 and 2 were proven to gener-
ate very effective catalysts for the coupling of sterically
hindered partners. However, ligand 1 was ineffective in the
coupling of aryl chlorides. For these substrates, including
deactivated ones, the bulkier, electron-rich ligand 2 was nec-
essary. A strong dependence of the catalytic activity on
the L/Pd ratio was observed and this varied with the nature
of the ligand. In contrast with 1, an active catalyst was
formed when 1 equiv. of 2 relative to the palladium was
used, a scenario reminiscent of those observed with other
bulky, electron-rich phosphines (43). The satisfying perfor-
mance of the ligands in terms of reaction rates and substrate
scope could partly be ascribed to the presence of the
oxazaphospholidine moiety, which renders the ligand to be
potentially hemilabile during the catalysis, thereby stabiliz-
ing the active palladium species. These ligands were also
shown to induce enantioselection in the Suzuki coupling re-
action; however, their performance remains to be improved
in this respect.
Angew. Chem. Int. Ed. 43, 1871 (2004).
1. J. Yin, M.P. Rainka, X.X. Zhang, and S.L. Buchwald. J. Am.
Chem. Soc. 124, 1162 (2002).
2. J.P. Wolfe and S.L. Buchwald. Angew. Chem. Int. Ed. 38,
2
413 (1999).
3. J.P. Wolfe, R.A. Sinjer, B.H. Yang, and S.L. Buchwald. J. Am.
Chem. Soc. 121, 9550 (1999).
4. D.W. Old, J.P. Wolfe, and S.L. Buchwald. J. Am. Chem. Soc.
1
20, 9722 (1998).
25. N. Gurbuz, I. Ozdemir, S. Demir, and B. Centinkaya. J. Mol.
Catal. 209, 23 (2004).
26. S.D. Cho, H.K. Kim, H.S. Yim, M.R. Kim, J.K. Lee, J.J. Kim,
and Y.J. Yoond. Tetrahedron, 63, 1345 (2007).
27. T.E. Barder, S.D. Walker, J.R. Martinelli, and S.L. Buchwald.
J. Am. Chem. Soc. 127, 4685 (2005).
2
2
3
8. N. Marion, O. Navarro, J. Mei, E.D. Steven, N.M. Scott, and
S.P. Nolan. J. Am. Chem. Soc. 128, 4101 (2006).
9. K. Billingsley and S.L. Buchwald. J. Am. Chem. Soc. 129,
3
358 (2007).
0. A.N. Cammidge and K.V.L. Crépy. Chem. Commun. 1723,
2000).
(
3
3
1. A.N. Cammidge and K.V.L. Crépy. Tetrahedron, 60, 4377 (2004).
2. A.M. Herrbach, O. Baudoin, D. Guénard and F. Guéritte. J.
Org. Chem. 68, 4897 (2003).
Acknowledgements
3
3
3
3. A.S. Castanet, F. Colobert, P.E Broutin, and M. Obringer. Tet-
rahedron: Asymmetry, 13, 659 (2002).
4. N. Hadei, E.A.B. Kantchev, C.J. O’Brien, and M.G. Organ.
Org. Lett. 7, 1991 (2005).
We gratefully acknowledge Johnson Matthey and the
Fundação para a Ciência e a Tecnologia (SFRH/BD/17262/
2
004 NM) for financial support.
5. R.G. Arrayas, J. Adrio, and J.C. Carretero. Angew. Chem. Int.
Ed. 45, 7674 (2006).
36. F.Y. Kwong, W.H. Lam, C.H. Yeung, K.S. Chan, and A.S.C.
Chan. Chem. Commun. 1922 (2004).
References
1
2
3
. A. Suzuki. Acc. Chem. Res. 15, 178 (1982).
. A. Suzuki. Pure Appl. Chem. 57, 1749 (1985).
. N. Miyaura and A. Suzuki. J. Synth. Org. Chem. Jpn. 46, 848
3
3
3
4
4
7. T.E. Pickett, F.X. Roca, and C.J. Richards. J. Org. Chem. 68,
592 (2003).
2
8. D. Vinci, N. Mateus, X. Wu, F. Hancock, A. Steiner, and J.
Xiao. Org. Lett. 2, 215 (2006).
(
1988).
4
5
. A. Suzuki. Pure Appl. Chem. 63, 419 (1991).
. N. Miyaura and A. Suzuki. J. Synth. Org. Chem. Jpn. 51, 1043
9. Bruker AXS, Inc. SAINT-PLUS. Version 6.45a [computer pro-
gram]. Bruker AXS Inc., Madison, Wisconsin. 2005.
0. Bruker AXS, Inc. SADABS. Version 2007-2 [computer pro-
gram]. Bruker AXS Inc., Madison, Wisconsin. 2007.
1. G.M. Sheldrick. SHELXL-97 [computer program]. University
of Göttingen, Germany. 1997.
2. L.J. Barbour. J. Supramol. Chem. 1, 189 (2001).
3. I.D. Hills, M.R. Netherton, and G.C. Fu. Angew. Chem. Int.
Ed. 42, 5749 (2003).
(
1993).
6
7
8
9
. A. Suzuki. Pure Appl. Chem. 66, 213 (1994).
. N. Miyaura and A. Suzuki. Chem. Rev. 95, 2457 (1995).
. S. Liu and J. Xiao. J. Mol. Catal. A: Chem. 270, 1 (2007).
. G. Bringmann, R. Walter, and R. Weirich. Angew. Chem. Int.
Ed. 29, 977 (1990).
4
4
1
1
0. T.G. Gant and A.I. Meyers. Tetrahedron, 50, 2297 (1994).
1. A.I. Meyers, J.R. Flisak, and R.A. Aitken. J. Am. Chem. Soc.
4
4
4. J.F. Jensen and M. Johannsen. Org. Lett. 5, 3025 (2003).
5. C. Baillie, L. Zhang, and J. Xiao. J. Org. Chem. 69, 7779
1
09, 5446 (1987).
1
1
2. K. Kamikawa and M. Uemera. Synlett, 938 (2000).
3. M. Uemura, A. Daimon, and Y. Hayashi. J. Chem. Soc. Chem.
Commun. 1943 (1995).
(
2004).
©
2008 NRC Canada