Application of an air stable Pd oxazoline complex for Heck, Suzuki, Sonogashira and related C-C bond-forming reactions

Article abstract of DOI:10.1016/j.tetlet.2004.08.082

The novel complex trans-[PdCl₂(η¹-N-2-ethyl-2- oxazoline)₂] is shown to be an active and oxidatively robust catalyst for C-C bond-forming reactions (Heck, Sonogashira, Ullman, Suzuki), which can be carried out in air witho

Full text of DOI:10.1016/j.tetlet.2004.08.082

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7689–7691
Application of an air stable Pd oxazoline complex for Heckq,
Suzuki, Sonogashira and related C–C bond-forming reactions
a,
Robert A. Gossage, Hilary A. Jenkins and Paras N. Yadav
b,
a
*
a
The Chester Woodleigh Small Laboratory of Inorganic Chemistry, 6 University Avenue, Elliott Hall,
Department of Chemistry, Acadia University, Wolfville, Nova Scotia, Canada B4P 2R6
b
Department of Chemistry, St. Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3
Received 4 August 2004;revised 13 August 2004;accepted 16 August 2004
This communication is dedicated to the memory of a mentor and friend, Dr. John Malito, who passed away October 21, 2003
1
2 2
Abstract—The novel complex trans-[PdCl (g -N-2-ethyl-2-oxazoline) ] is shown to be an active and oxidatively robust catalyst for
C–C bond-forming reactions (Heck, Sonogashira, Ullman, Suzuki), which can be carried out in air without rigorous solvent/sub-
strate purification and in the absence of additional free ligand.
ꢀ
2004 Elsevier Ltd. All rights reserved.
1.Introduction
2.Results and discussion
Several early and late transition metal (TM) complexes
are known to mediate C–C bond formation, but in recent
years Pd compounds (formed either prior to reactivity or
in situ) have demonstrated significant versatility in this
Our research interests include fundamental and applied
5
studies of oxazoline (ox) ligands and the TM complexes
6
derived from them. The general area of coordination
chemistry of monodentate ox ligands, specifically with
1
–3
5
area. A wide variety organic transformations are facil-
itated by complexes that incorporate a number of diverse
ligand classes (for example, P, N, O, S, etc. as donor
Pd, is an under-explored area of research; thus justify-
ing our present examination. The treatment of a solu-
tion of 2-ethyl-2-oxazoline, a cheap and commercially
4
7
atoms) combined with Pd. One of the few drawbacks
available polymer precursor, with methanolic Li PdCl
2
4
in the application of most Pd derivatives is the air-sensi-
tive nature of the active species/intermediates (presuma-
bly Pd(0) complexes) and/or oxidation potential of the
ligands during catalysis. Hence, inert atmosphere condi-
tions are often necessary for efficient catalysis;this aspect
remains a serious impediment to large-scale industrial use
2
,3
of Pd-mediated technology. In addition, quantities of
ÔfreeÕ ligand are also often necessary to stabilise Pd during
catalysis, a situation that may make product purification
more difficult. Therefore, there is still a need for new and
oxidatively robust Pd-based systems for applications in
synthetic organic chemistry.
Keywords: C–C bond formation;Palladium;Heck;Sonogashira;
Suzuki;Catalysis;Oxazoline.
q
Oxazoline Chemistry Part VI. For Part V, see: Ref. 6e.
Corresponding author. Tel.: +1 902 5851109;fax: +1 902 5851114;
e-mail: rgossage@acadiau.ca
*
Correspondence author concerning the crystallographic portion of
this manuscript: e-mail: hilary.jenkins@smu.ca
Figure 1.
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.082

7690
R. A. Gossage et al. / Tetrahedron Letters 45 (2004) 7689–7691
Table 1. Catalysis results using complex 1 as mediator of C–C bond formation
a
Entry
Aryl–X
Substrate
Product
Time (h)
% Yield
TON
b
2
3
3
2
3
3
3
3
3
1
2
1
PhI
PhI
PhI
PhI
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
PhB(OH)
PhB(OH)
PhI
t-Stilbene
t-Stilbene
t-Stibene
t-Stilbene
48
24
18
18
24
24
24
3
87
84
8.6 · 10
1.7 · 10
2.0 · 10
1.9 · 10
>2.0 · 10
1.5 · 10
1.3 · 10
>1.3 · 10
>1.3 · 10
1.1 · 10
1.9 · 10
b
2
b
3
77
71
b,c
4
b
5
p-NO
PhBr
2
PhI
t-b-NO
2
-styrene
100
77
b
6
t-Stilbene
t-Stilbene
Biphenyl
p-Ph-anisole
Biphenyl
PhCCPh
b
7
p-Br-anisole
PhBr
65
d
8
2
2
100
100
45
d
9
p-Br-anisole
PhI
PhI
3
e
1
1
0
1
3
f
PhC„CH
3
32
a
1
All yields refer to yields of pure ( H NMR, mp, IR) products isolated by extraction and flash column chromatography;TON = turn over number
per hour.
b
Conditions: 0.00053mmol 1, 25mmol aryl-halide, 30mmol styrene, 25mL DMF, 30mmol sodium acetate, 140–150 ꢁC; t-stilbene = trans-stilbene;
t-b-NO
2
-styrene = trans-b-(4-nitrophenyl)styrene.
.0053mmol of 1 used.
Conditions: 0.00053mmol 1, 10 mL toluene, 2mmol aryl-halide, 3mmol PhB(OH)
Conditions: 0.014mmol 1, 5mL DMF, 2mmol PhI, 1mmol 1,4-dihydroquinone, 2mmol K
Conditions: 0.0011mmol 1, 2mL pyrollidine, 2mmol PhI, 1mmol CuI, 2.4mmol Ph–C„CH, 90ꢁC.
c
d
e
f
0
2
, 4mmol K
CO
2
CO
3
, 110ꢁC.
2
3
, 110ꢁC.
gives an air stable complex analysing for [PdCl (2-ethyl-
2
2-ox) ] (1) in good yield. Proton NMR spectroscopy
2
indicates a single ox ligand environment. Single crystal
In conclusion, a new Pd complex has been synthesised
and fully characterised and found to be an effective cat-
alyst, in air, for a number of C–C bond-forming reac-
tions. We are currently expanding this chemistry to
include enantioselective substrate activation;the investi-
gation of the mechanism of reactions mediated by 1 and
its (chiral) analogues will be disclosed in a future
publication.
8
X-ray diffraction of a recrystallised sample of 1 revealed
9
(
trans stereochemistry of ox ligands bound via the
Fig. 1) a mononuclear (formally) Pd(II) complex with
1
5,10
N-atom in an g fashion, as expected.
characterised analogues of 1 are rare but include
Structurally
1
11
trans-[PdCl (g -N-2-phenyl-2-ox) ] (2) and a related
2
2
naphthalene complex described by van Koten et al.,
0
1
Acknowledgements
trans-[PdCl (g -N-4,4-dimethyl-2-(2 -napthyl)-2-ox) ]
2
2
1
2
(
cis/trans isomers in solution, in contrast to our NMR
3). The latter species appears to exist as a mixture of
This work is supported by NSERC (Canada). Johnson–
Matthey Ltd is thanked for a generous loan of Pd salts.
T. D. Hamilton is acknowledged for her assistance in the
characterisation of 1.
1
1
observations with 1 (and earlier studies of 2). The
1,12
1
bond lengths and angles
both 2 and 3 are similar in most respects to that of 1.
of the solid-state forms of
9
We have further investigated (Table 1) the use of solu-
tions of 1, in open air, as a mediator of C–C bond for-
mation under typical Heck (entries 1–7), Suzuki (entries
References and notes
1. Crabtree, R. H. The Organometallic Chemistry of the
Transition Metals, 3rd ed.;John-Wiley: Toronto, 2001,
Section 9.6;Spessard, G. O.;Miessler, G. L. Organome-
tallic Chemistry;Prentice-Hall: Toronto, 1997, Chapter 9–
8
–9), Ullmann (entry 10) and Sonogashira (entry 11)
reaction conditions without addition of free ligand nor
1
3
rigorous exclusion of moisture or air. Complex 1 is an
effective catalyst for all of these classes of C–C bond-
forming reactions. Yields range from 32% to 100%
under standard conditions and TONÕs are moderate
11;Omae, I. Applications of Organometallic Compounds;
John Wiley: Toronto, 1998, Chapter 20.
2
3
. Negishi, E. J. Organomet. Chem. 2002, 653, 34;Li, J. J.;
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Uozumi, Y. Org. Lett. 2004, 6, 1833;van der Boom, M.;
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1
–4,13,14
(
used were of reagent grade and not purified further.
Table 1);
all solvents and reagents that were
1
5
There are few fully characterised (pre-formed) Pd-based
systems that have been shown to be effective for a pleth-
ora of different C–C bond-forming processes and even
fewer can operate in air and without additional lig-
2
–4,14
and.
pects with the further advantage of using a simple, very
This system effectively combines these two as-
7
inexpensive (or readily synthesised on a large scale ) lig-
and (cf. bulky phosphines, carbenes), which is stable to
oxidative decomposition and low in molecular weight.
The robust nature of 1, presumably combined with the
stability of the Pd intermediates during catalysis in open
air, makes this a very attractive system for C–C coupling
reactions.
O.;Kaur, H.;Mahjoor, P.;Nolan, S. P.
2004, 69, 3173;Hillier, A. C.;Grasa, G. A.;Viciu, M. S.;
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R.;Rataboul, F.;Reirmeier, T.;Monsees, A.;Fuhrmann,
002, 653, 69;Basic phosphines: Su, W.;Urgaonkar, S.; 9. X-ray data of complex 1: FW = 375.56;formula:
˚
C
10
H
18
N
2
O
tal system: monoclinic. Space group: P2(1)/n. Unit cell
2
Cl
2
Pd;temp.: 291(2)K; k = 0.71073A. Crys-
˚
˚
C.;Shaikh, N.;Dingerdissen, U.;Beller, M.
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245;Imines: Grasa, G. A.;Hillier, A. C.;Nolan, S. P.
Chem.
dimensions: a = 8.8593(10)A; a = 90ꢁ; b = 8.0909(9)A;
˚
c = 10.0689(11)A;
b = 92.857(2)ꢁ;
c = 90ꢁ.
720.84(14)A ; Z = 2;density (calcd): 1.730Mg/m
Vol. =
˚
3
3
3
.
Absorption coefficient: 1.650mm ; F(000): 376. Crystal
ꢀ
1
Org. Lett. 2001, 3, 1077;Palladacycles: Bedford, R. B.
Chem. Commun. 2003, 1787, and references cited therein;
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hedron Lett. 2004, 45, 4113;Nitriles: Ma, S.;Wei, Q.;Ren,
H. Tetrahedron Lett. 2004, 45, 3517;Polymer systems:
Mastrorilli, P.;Nobile, C. F. Coord. Chem. Rev. 2004, 248,
3
size: 0.25 · 0.25 · 0.3mm . Theta range for data collec-
tion: 2.99ꢁ to 24.99ꢁ. Index ranges: ꢀ10 6 h 6 10,
ꢀ9 6 k 6 6, ꢀ10 6 l 6 11. Reflections collected: 3707;
independent reflections: 1272 [R(int) = 0.0157]. Complete-
ness to theta = 24.99ꢁ: 99.9%. Absorption correction:
none. Refinement method: Full-matrix least-squares on
3
77;Thioureas: Yang, D.;Chen, Y.-C.;Zhu, N.-Y. Org.
2
Lett. 2004, 6, 1577;Ionic liquids: Park, S. B.;Alper, H.
Chem. Commun. 2004, 1306.
. G o´ mez, M.;Muller, G.;Rocamora, M. Coord. Chem. Rev.
F . Data/restraints/parameters: 1272/0/80;Goodness-of-fit
on F : 1.084. Final R indices [I > 2sigma(I)]: R1 = 0.0161,
2
5
wR2 = 0.0442;
R
wR2 = 0.0446. Extinction coefficient: 0.0478(17);largest
indices (all data): R1 = 0.0173,
1
999, 193–195, 769.
˚
ꢀ3
6
. (a) Barclay, T. M.;del Rı´o, I.;Gossage, R. A.;Jackson, S.
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.
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3
2, 363;(c) Button, K. M.;Gossage, R. A. J. Heterocycl.
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8
. Bassiri, T. G.;Levy, A.;Litt, M. Polym. Lett. 1967, 5,
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71.
. Synthesis of complex 1: To a MeOH solution of Li
0.28M;2.0mL: 0.56mmol) was added 7mL of MeOH
and 2-ethyl-2-oxazoline (1.0mL;9.9mmol) at room tem-
2
PdCl
4
13. Heck, R. F. Palladium Reagents in Organic Syntheses;
Academic: Toronto, 1990.
(
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Chem.
perature in open air. The dark brown mixture was stirred
for 1h during which time the solution lightened in colour
and an orange precipitate formed. This solid was then
isolated by filtration and washed with MeOH (3 · 7mL),
Org. Chem. 2001, 66, 8677;Li, G. Y. J. Org. Chem. 2002,
67, 3643;Li, G. Y. J. Organomet. Chem. 2002, 653, 63;
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4330.
pet. ether (5mL) and Et
air. Complex 1 was thus obtained as an orange solid (yield
.14g: 67%). An analytical sample, also found to be
2
O (2 · 5mL) and then dried in
0
suitable for single crystal X-ray diffraction, was obtained
by layering a dichloromethane solution (5mL) of 1 with an
equal volume of MeOH and subsequent storage at ꢀ20ꢁC
15. Aryl-chlorides were ineffective substrates in air under the
standard Heck conditions described herein (product
yields: <5%). Stille coupling mediated by 1 using PhI or
1
for several days. Mp 178–179ꢁC (decomp.). H NMR
3 3
p-bromoacetophenone with Bu SnPh or Bu Sn(allyl)
(
300MHz;CDCl
.00 (t, 2H, CH
3
;294K): d = 4.54 (t, 2H, J = 9.8, CH
2
O),
CH ),
.28 (t, 3H, CH ). Calcd (found) for C H N O Cl Pd: C,
under standard conditions (toluene, 100ꢁC, 8h) gave low
yields of coupled product in open air. Aspects of these
results in relation to the data presented herein will be
disclosed separately.
4
1
3
2
N), 3.00 (q, 2H, J = 7.3Hz, CH
2
3
3
10 18
2
2
2
1.98 (31.73), H, 4.83 (4.63), N, 7.46 (7.41)%.