The ionic liquid ethyltri-n-butylphosphonium tosylate as solvent for the acid-catalysed hetero-Michael reaction

Article abstract of DOI:10.1016/j.tet.2006.09.052

A new and convenient method for the acid-catalysed Michael addition reactions of alcohols, thiols and amines to methyl vinyl ketone has been developed using the ionic liquid ethyltri-n-butylphosphonium tosylate. The reaction conditions are mild and obviat

Full text of DOI:10.1016/j.tet.2006.09.052

Tetrahedron 62 (2006) 11039–11043
The ionic liquid ethyltri-n-butylphosphonium tosylate as
solvent for the acid-catalysed hetero-Michael reaction
*
Nazira Karodia, Xihan Liu, Petra Ludley, Dimitrios Pletsas and Grace Stevenson
School of Life Sciences, University of Bradford, Bradford BD7 1DP, UK
Received 13 July 2006; revised 4 September 2006; accepted 15 September 2006
Available online 10 October 2006
Abstract—A new and convenient method for the acid-catalysed Michael addition reactions of alcohols, thiols and amines to methyl vinyl
ketone has been developed using the ionic liquid ethyltri-n-butylphosphonium tosylate. The reaction conditions are mild and obviate the
need for toxic and expensive Lewis acid catalysts, offering advantages over more commonly used systems.
Ó 2006 Elsevier Ltd. All rights reserved.
8
a–d
1
. Introduction
times. In contrast, the aza-Michael addition route usually
requires acid- or base-catalysis to activate one of the sub-
1
2,9
The Michael addition is a powerful reaction for the forma-
tion of carbon–carbon and carbon–hetero atom bonds. Both
strates. High costs coupled with environmental issues
due to the large amount of acidic effluents being generated
are drawbacks associated with Lewis acid-catalysed aza-
Michael reactions. Thiols are typically difficult to use in
the presence of Lewis acidic metals since they are known
to poison these catalysts. In the thia-Michael reaction, the
2
Michael- and hetero-Michael additions are widely used in
inter- and intra-molecular reactions to give products, which
include important building blocks for many biologically
active molecules. Hetero-Michael addition of amines to
a,b-unsaturated carbonyl compounds gives b-amino ke-
tones, which are attractive for their use as synthetic inter-
1
0
1
1
thiols are generally activated by deprotonation.
3
mediates of anticancer agents, antibiotics and other drugs.
The past five years have seen concerted efforts to develop
effective, economical and environmentally friendly method-
ologies for the hetero-Michael addition. Some of these excit-
ing developments include the use of an azaphosphatrane
Michael addition of a thiol to an a,b-unsaturated ketone
results in the formation of a sulfur–carbon bond; this is
a key reaction in the synthesis of biologically active com-
pounds such as the calcium antagonist diltiazem. Similarly,
b-oxy ketones are also important in organic synthesis.
4
12
13
Ò
14
nitrate salt,
CeCl $7H O/NaI,
3
Nafion SAC-13,
2
5
15
16
17
18
Bi(NO ) , Bi(OTf) , InBr3, Cu(BF ) , and strong
3 3 3 4 2
1
9
Moreover the b-amino, b-thio and b-oxy ketone functional-
ities occur in many natural products.
Brønsted acids. There are also examples of the use of
ammonium ionic liquids as catalysts and/or solvents for the
hetero-Michael addition of thiols, and aliphatic amines.
6
2
0
21
The uncatalysed addition reaction of alcohols to a,b-unsatu-
rated ketones proceeds very slowly and gives only moderate
yields of the b-oxy ketones. Other methods most commonly
employed use red mercury oxide and boron trifluoride ether-
With a view to establishing more practical reaction con-
ditions, as well as facilitating the recovery of the products,
we have investigated the application of the phospho-
nium ionic liquid, ethyltri-n-butylphosphonium tosylate
7
a,b
ate as catalysts, which are both toxic. These methods also
require working under an inert atmosphere. Other methods
widely used are the acid- or base-catalysed reaction of the
neat reagents but in this case the reaction mixture needs to
be neutralised very carefully, otherwise the ethers decom-
pose on distillation. A significant drawback to this method
is that the reaction is difficult to control and can be very
violent thus requiring cooling or the use of an inert solvent.
The Mannich reaction is a classic method for the preparation
(n-Bu PEtOTs) in the acid-catalysed hetero-Michael addi-
3
tion of a series of alcohols, thiols and amines to methyl
vinyl ketone (MVK) as the prototypical Michael acceptor
(Scheme 1). This method obviates the need for expensive
metal catalysts and strong bases, and instead uses cheap
and readily available reagents.
O
O
TsOH, n-Bu3PEtOTs
8
a
of b-amino ketones but this reaction has serious disadvan-
tages, including drastic reaction conditions and long reaction
Nu
NuH = ROH, RNH2, R2NH, RSH
R = aliphatic, aromatic
*
Corresponding author. Tel.: +44 1274233790; fax: +44 1274235600;
e-mail: n.karodia@bradford.ac.uk
3
Scheme 1. Acid-catalysed hetero-Michael addition in n-Bu PetOTs.
0
040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.09.052

11040
N. Karodia et al. / Tetrahedron 62 (2006) 11039–11043
Phosphonium tosylates have proven to be effective solvents
for organic synthesis, having the advantages of high thermal
stability, tolerance towards air and moisture and low vapour
pressures. Phosphonium tosylates are cheap, non-corrosive
and some are solid at room temperature, making them easy
ethyl tosylate) at low cost. Initially the hetero-Michael reac-
tion was attempted in the ionic liquid without any catalyst,
however, no detectable yield of product was observed. p-Tol-
uenesulfonic acid monohydrate (TsOH$H O) was chosen as
2
the catalyst because it is the conjugate acid of the solvent an-
ion. The optimum quantity for the reaction was established
as 2% by systematically varying the quantity of the catalyst
and monitoring the yield of the product obtained.
2
2,23
to handle and to separate from the reaction products.
2
. Results and discussion
Selected results are summarised in Table 1. The reaction
with the aliphatic alcohols, which are weak nucleophiles,
(Entries 1–8) gave yields ranging from 12–75%, which com-
Ethyltri-n-butylphosphonium tosylate (n-Bu PEtOTs) was
3
chosen because its melting point permits reactions at rela-
tively low temperatures. It also crystallises readily, making
it easy to separate from the product and recycle. In addition,
it is economically and readily synthesised from commer-
cially available starting materials (tri-n-butylphosphine and
7
,14
pare well with other methods.
Varying the ratio of donor
and acceptor did not affect the yields significantly (Entries
1–3). In our adaptation, the best isolated yield (59%) for
the reaction with ethanol (Entry 3) compares well with the
Table 1. Hetero-Michael addition to methyl vinyl ketone (MVK)
a
Yield (%) (isolated)
Entry
Michael donor
MVK/Michael donor
Time [h]
Product
Reference
O
O
O
O
O
2
4
1
2
3
4
5
EtOH
1:1
3
3
3
6
3
62
67
27
27
27
27
28
Et
Et
Et
Et
Bu
O
O
O
O
O
EtOH
1.5:1
1:1.5
1:1
EtOH
66 (59)
64
EtOH
n-BuOH
1:1
76 (63)
O
6
7
8
9
i-PrOH
t-BuOH
1:1
1:1
1:1
1:1
3
3
3
3
52 (45)
22 (12)
80 (75)
29
30
19
26
i-Pr
O
O
O
O
O
t-Bu
O
6 5 2
C H CH OH
Ph
O
C
6
H
5
OH
100
Ph
Bu
O
O
O
10
11
12
13
14
15
Bu
2
NH
1:1
1:1
1:1
1:1
1:1
2.1:1
1
1
1
3
3
3
82
83
80
90
95
31
32
31
12
33
34
N
Bu
H
N
N
NH
N
O
O
Me
N
H
N
Me
O
H
N
NH2
NH2
b
O
N
2
100 (98)
O
(
continued)

N. Karodia et al. / Tetrahedron 62 (2006) 11039–11043
11041
Table 1. (continued)
Entry Michael donor
a
Yield (%) (isolated)
MVK/Michael donor
2.1:1
Time [h]
3
Product
Reference
34
N
NH2
2
16
100 (98)
O
O
NH2
N
2
1
1
7
8
2.1:1
1:1
3
3
100 (98)
95 (90)
33
O2N
O2N
O
SH
SH
20a
S
O
1
2
9
1:1
1:1
3
3
100 (96)
100
14
12
S
S
SH
0
O
ꢀ
Reactions were conducted at 40 C on a 5.0 mmol scale.
Yield determined by H NMR.
A small amount (2%) of the bis-addition product was also obtained.
a
1
b
7
a
yields for the BF $OEt -catalysed reaction (52–77%) and
3
4-piperidin-1-yl-butan-2-one and 4-morpholinobutan-2-one
in 83% and 80% yields, respectively, which compares fa-
vourably with the reaction catalysed by the azaphosphatrane
2
is only slightly lower than the reaction catalysed with
sodium ethanolate (71%). The only method giving signif-
icantly higher yields (95%) is the acid-catalysed reaction of
MVK in an excess of ethanol. The reactions with n-butanol
Entry 5) and benzyl alcohol (Entry 8) were similar to those
of the other primary alcohols and good yields of 63% and
5% were obtained. The reaction of benzyl alcohol com-
pares well with the Brønsted acid-catalysed reaction where
2
4
nitrate where yields of 88% and 90% were obtained after
20 h reaction time. The Cu(acac) -catalysed reaction in
2
5
12
2
(
the ionic liquid [bmim]BF , appears to be the most efficient
4
for the reactions of aliphatic amines giving high yields in
10–60 min. Excellent yields were also obtained with
aromatic amines (Entries 12–14) and these are superior to
2
1b
7
1
9
19
72% yield was obtained after 48 h. In comparison to the
primary alcohols, the yield for isopropanol (Entry 6) was
the yields obtained with strong Brønsted acids.
marginally lower (w 45%) but still superior to the known
We found that both aliphatic and aromatic thiols reacted
equally well with methyl vinyl ketone in the Bu PEtOTs/
2
4
base-catalysed (12–20%) and the BF $OEt -catalysed re-
3
2
3
7
actions (34%). Unsurprisingly, tert-butanol (Entry 7) gave
lower yield (22%), as expected for a hindered nucleophile.
A yield was not quoted for the equivalent BF $OEt /HgO-
TsOH system and the Michael adducts were obtained in
excellent yields (95–100%) (Entries 18, 20). This is a favour-
able contrast with the Brønsted acid catalyst, Tf NH, which
3
2
2
7
19
method. In comparison to the pyridine-catalysed reaction
of phenol (Entry 9) with MVK at 100 C, the Bu PEtOTs/
was used in acetonitrile as the solvent. Thiophenol could
not be used as a reactant in that system and a yield of only
ꢀ
TsOH system was more efficient giving 100% conversion
3
1
9
66% was obtained after 1 h for benzyl mercaptan. The
yields were similar to those obtained with bismuth triflate.
2
6
16
in only 3 h. These oxo-Michael reactions in ethyltri-n-
butylphosphonium tosylate cannot be compared with the
corresponding reactions in imidazolium ionic liquids since
there are no reports of oxo-Michael reactions in this class
of ionic liquids.
While the reaction of thiophenol in the ammonium ionic
liquid, 1-pentyl-3-methylimidazolium bromide was rapid
and the Michael adduct was isolated in 75% yield after
2
0b
only 45 min,
the reaction catalysed by an azaphospha-
12
trane nitrate salt resulted in 95% yield after 40 h. There
appears to be an advantage in using ionic liquids as solvents/
catalysts for this reaction.
A broad range of aliphatic and aromatic amines were also
effective nucleophiles, generating 4-aminobutanone deriva-
tives (Entries 10–17). The reactions were fast and excellent
yields were obtained (82–98%) with both primary and
secondary amines. These results compare very well with
the recently reported improved method for the reaction of
di-n-butylamine with MVK, involving Lewis acid catalysis
by a CeCl $7H O/NaI system supported in SiO , which
3. Conclusion
We have developed a new method for the hetero-Michael
addition of alcohols, thiols and amines to methyl vinyl
ketone, which uses very small amounts of a non-aqueous,
non-volatile and inexpensive acid as catalyst. The yields
are good, even when equimolar amounts of the reagents
are used. In addition, we are avoiding some of the problems
associated with the other methods recently developed. The
reaction conditions are mild and there is no need to work
under an inert atmosphere.
3
2
2
1
3
gave the addition products in 84% yield after 5 h.
Whilst the yield is comparable with that obtained in the
Bu PEtOTs/TsOH system (82%) the reaction time was
3
greatly extended. Furthermore, the workup involved extrac-
tion, washing and flash chromatography, which are unneces-
sary with the reactions in the phosphonium ionic liquid.
Similarly the reactions of piperidine and morpholine gave

11042
N. Karodia et al. / Tetrahedron 62 (2006) 11039–11043
The role of the ionic liquid in the hetero-Michael reaction is
not yet understood and further investigations are necessary.
These results show that the phosphonium ionic liquid acts
differently to the ammonium-based ionic liquids in that
both aliphatic and aromatic amines reacted equally well in
the phosphonium ionic liquid while only aliphatic amines
have been successful Michael donors in ammonium ionic
4.3. Representative procedure
In a typical reaction methyl vinyl ketone (350 mg, 5 mmol),
ethanol (230 mg, 5 mmol), p-toluenesulfonic acid (20 mg,
0.1 mmol) and ionic solvent n-Bu PEtOTs (2 g) were placed
3
in a 25 mL round-bottomed flask fitted with a condenser and
a magnetic stirrer bar. The mixture results in a homogeneous
2
1
ꢀ
liquids. The oxo-Michael reaction has not been reported
in ammonium ionic liquids and therefore comparisons can-
not be made, however, the thia-Michael reaction has been
well studied and excellent yields were reported, which are
solution on heating at 40 C and the reaction mixture was
stirred at this temperature for 3 h. The product was isolated
by Kugelrohr distillation or extraction with diethyl ether
(3ꢂ10 mL) followed by filtration through a silica plug to
2
0
comparable to the results obtained in this study.
give 4-ethoxybutan-2-one; dH (CDCl ) 1.18 (3H, t, J 7,
3
CH CH –O), 2.19 (3H, s, CH CO), 2.69 (2H, t, J 6,
3
2
3
O–CH CH CO), 3.49 (2H, q, J 7, CH CH –O), 3.68 (2H, t,
2
2
3
2
2
7
J 6, O–CH CH CO); d (CDCl ) 15.0 (CH CH –O), 30.3
2
2
2
C
3
3
4. Experimental
(
6
CH C]O), 43.7 (O–CH CH CO), 65.4 (O–CH CH CO),
3 2 2 2 2
6.3 (CH CH –O), 207.2 (C]O).
3 2
2
7
4
.1. General
Similarly, all the products of the hetero-Michael reactions
were analysed by H NMR spectroscopy and the data
compared with the literature (Table 1).
Chemicals were obtained from Aldrich or Lancaster. Methyl
vinyl ketone and aniline were distilled prior to use, all other
1
1
13
materials were used as received. H and C NMR spectra
were recorded at 270 and 68 MHz, respectively, on a Jeol
GX270 spectrometer. All spectra were measured in CDCl3
as the solvent and the chemical shifts were referenced to tet-
ramethylsilane (TMS) as an internal standard (0 ppm). The
References and notes
3
1
1. Michael, A. J. Prakt. Chem. 1889, 349.
P NMR spectrum was recorded at 121 MHz on a Bruker
AM-300 spectrometer at 121 MHz in CDCl as the solvent
2
. Perlmutter, P. Conjugate Addition Reactions in Organic
Synthesis; Tetrahedron Organic Chemistry Series; Pergamon:
Oxford, UK, 1992; Vol. 9.
3
and phosphoric acid as the external standard. The IR spec-
trum was recorded using a KBr disc on a Nicolet 140
ꢁ
1
3. (a) Banik, B. K.; Becker, F. F.; Banik, I. Bioorg. Med. Chem.
FTIR spectrometer in the range 4000–400 cm . The accu-
rate mass measurement was carried out at the EPSRC
National Mass Spectrometry Service Centre, Chemistry
Department, University of Wales, Swansea. The melting
point was recorded on a Reichert hot-stage melting point
apparatus and is uncorrected.
2
1
004, 12, 2523; (b) Graul, A.; Castaner, J. Drugs Future
997, 22, 956.
4
5
. Budriesi, R.; Carosati, E.; Chiarini, A.; Cosimelli, B.; Cruciani,
G.; Ioan, P.; Spinelli, D.; Spisani, R. J. Med. Chem. 2005, 48,
2
445.
. (a) Calter, M. A.; Liao, W. J. Am. Chem. Soc. 2002, 124, 13127;
b) Nicolaou, K. C.; Ritz ꢀe n;, A.; Namoto, K. Chem. Commun.
2001, 1523.
(
4.2. Preparation of ethyltri-n-butylphosphonium
tosylate
6. Misra, M.; Luthra, R.; Singh, K. L.; Sushil, K. Comprehensive
Natural Products Chemistry; Barton, D. H. R., Nakanishi, K.,
Meth-Cohn, O., Eds.; Pergamon: Oxford, UK, 1999; Vol. 4.
7. (a) Milas, N. A.; Sakal, E.; Plati, J. T.; Rivers, J. T.; Gladding,
J. K.; Grossi, F. X.; Weiss, Z.; Campell, M. A.; Wright, H. F.
J. Am. Chem. Soc. 1948, 70, 1597; (b) Jarolim, V.; Sorm, F.
Collect. Czech. Chem. Commun. 1975, 40, 1059.
8. (a) Arend, M.; Westermann, B.; Risch, N. Angew. Chem., Int.
Ed. 1998, 37, 1044; (b) Jung, M. E. Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford,
UK, 1991; Vol. 4; (c) Kobayashi, S.; Ishitani, H. Chem. Rev.
1999, 99, 1069; (d) Ishitani, H.; Ueno, M.; Kobayashi, S.
J. Am. Chem. Soc. 2000, 122, 8180.
9. (a) Bull, S. D.; Davies, S. G.; Delgado-Ballester, S.; Fenton, G.;
Kelly, P. M.; Smith, A. D. Synlett 2000, 1257; (b) Davies, S. G.;
McCarthy, T. D. Synlett 1995, 700; (c) Rosenthal, D.;
Brandrup, G.; Davis, K. H.; Wall, M. E. J. Org. Chem. 1965,
30, 3689; (d) Perez, M.; Pleixates, R. Tetrahedron 1995, 51,
8355; (e) Asao, N.; Shimada, T.; Sudo, T.; Tsukada, N.;
Yazawa, K.; Gyoung, Y. S.; Yamamoto, Y. J. Org. Chem.
1997, 62, 6274.
10. (a) Anastas, P. T.; Warner, J. C. Green Chemistry, Theory and
Practice; Oxford University Press: Oxford, UK, 1998; (b)
Clark, J.; Macquarrie, D. Handbook of Green Chemistry and
Technology; Blackwell: Malden, MA, 2002.
Ethyltri-n-butylphosphonium tosylate was prepared by heat-
ing a solution of tri-n-butylphosphine (11 g, 54 mmol) and
ethyl tosylate (11 g, 54 mmol) in dry toluene (40 mL) at
ꢀ
1
00 C under a nitrogen atmosphere for 16 h. The solvent
was evaporated to give a white solid, which was suspended
in dry ether, filtered and washed with dry ether to furnish
ꢀ
the product (96%) as a white solid, mp 73–76 C (lit.:
ꢀ
050w (CH), 3021w (CH), 2950s (CH), 2929s (CH),
22a
ꢁ1
7
3
2
8
3
1
4.5–75.2 C),
IR nmax/cm
(KBr) 3086w (CH),
865s (CH), 1465m (P–C), 1129vs (S]O), 1043s, 1014s,
15s, 710s, 681s, 579s; dH (CDCl ) 0.91 (9H, t, J 7,
3
ꢂCH (CH ) P), 1.22 (3H, dt, J 18 and 8, CH CH P),
3
2 3
3
2
.42–1.56 (12H, m, 3ꢂCH (CH ) CH P), 2.17–2.39 (6H,
3
2 2
2
m, 3ꢂCH (CH ) CH P), 2.32 (3H, s, CH C H ), 2.33 (2H,
3
2 2
2
3 6 5
0
tolyl), 7.16 (2H, d, J 8, H-2 and 2 of tolyl); d (CDCl )
dt, J 13 and 8, CH CH P), 7.11 (2H, d, J 8, H-3 and 3 of
3
2
0
C
3
6
d, J 7, CH (CH ) P), 18.3 (3C, d, J 47, CH CH CH CH P),
.1 (d, J 5, CH CH P), 12.6 (d, J 50, CH CH P), 13.5 (3C,
3 2 3 2
3
2 3
3
2
2
2
2
J 13, CH CH CH P), 126.1 (C-3 and 3 of tolyl), 128.4
1.3 (CH C H ), 23.7 (3C, d, J 4, CH CH P), 23.9 (3C, d,
3 6 5 2 2
0
C-2 and 2 of tolyl), 138.9 (C-4 of tolyl), 144.5 (C-1 of
2
2
2
0
(
+
tolyl); d +35.2; m/z (EI) 231.2236 (M , C H P requires
P
14 32
ꢁ
31.2236), 171.0109 (M , C H SO requires 171.0121).
7 7 3
2

N. Karodia et al. / Tetrahedron 62 (2006) 11039–11043
11043
1
1
1. Kondo, T.; Mitsudo, T. Chem. Rev. 2000, 100, 3205.
2. Fetterly, B. M.; Jana, N. K.; Verkade, J. G. Tetrahedron 2006,
Karodia, N. Tetrahedron Lett. 2001, 42, 2011; (d) Ludley, P.;
Karodia, N. ARKIVOC 2002, 172.
6
2, 440.
23. Del Sesto, R. E.; Corley, C.; Robertson, A.; Wilkes, J. S.
J. Organomet. Chem. 2005, 2536.
24. Ferry, N.; McQuillin, F. J. J. Chem. Soc. 1962, 103.
25. Halbig, P.; Treibs, A. Deutsches Reich Patent DE 734724,
1943; Chem. Abstr. 1944, 38, 1248.
1
3. Bartoli, G.; Bosco, M.; Marcantoni, E.; Petrini, M.; Sambri, L.;
Torregiani, E. J. Org. Chem. 2002, 66, 9052.
4. Wabnitz, T. C.; Yu, J.; Spenser, J. Synlett 2003, 1070.
5. Srivastava, N.; Banik, B. K. J. Org. Chem. 2003, 68, 2109.
6. Alam, M. M.; Varala, R.; Adapa, S. R. Tetrahedron Lett. 2003,
1
1
1
26. Cheralathan, K. K.; Kumar, I. S.; Palanichamy, M.; Murugesan,
V. Appl. Catal. A: Gen. 2003, 241, 247.
4
4, 5115.
1
7. Bandini, M.; Cozzi, P. G.; Giacomini, M.; Melchiorre, P.;
Selva, S.; Umani-Ronchi, A. J. Org. Chem. 2002, 67, 3700.
8. Garg, S. K.; Kumar, R.; Chakraborti, A. K. Tetrahedron Lett.
27. (a) Caselli, A. S.; Collins, D. J.; Stone, G. M. Aust. J. Chem.
1982, 35, 799; (b) Kaptein, B.; Monaco, V.; Broxterman,
Q. B.; Schoenmaker, H. E.; Kamphies, J. Recl. Trav. Chim.
Pays-Bas 1995, 114, 231.
28. Yoneda, N.; Kiuchi, T.; Fukuhara, T.; Suzuki, A.; Olah, G. A.
Chem. Lett. 1984, 1617.
1
2
9. Wabnitz, T. C.; Spenser, J. Org. Lett. 2003, 5, 2141.
005, 46, 1721.
1
2
0. (a) Ranu, B. C.; Dey, S. S.; Hajra, A. Tetrahedron 2003, 59,
2
417; (b) Ranu, B. C.; Dey, S. S. Tetrahedron 2004, 60,
183; (c) Yadav, J. S.; Reddy, B. V. S.; Baishya, G. J. Org.
29. Terasawa, T.; Okada, T. Tetrahedron 1977, 33, 595.
30. de Kimper, N.; Sulmon, P.; Moeens, L.; Schamp, N.; Declerq,
J.-P.; van Meerssche, M. J. Org. Chem. 1986, 51, 3839.
31. Cabral, J.; Laszlo, P.; Mah ꢀe , L.; Montaufier, M.-T.;
Randriamahefa, S. L. Tetrahedron Lett. 1989, 30, 3969.
32. Chen, J. J.; Lu, C. V.; Brockman, R. N. Tetrahedron Lett. 2003,
44, 3459.
4
Chem. 2003, 68, 7098.
2
2
1. (a) Xu, L.-W.; Li, J.-W.; Zhou, S.-L.; Xia, C.-G. New J. Chem.
2
004, 28, 188; (b) Kantam, M. L.; Neeraja, V.; Kavita, B.;
Neelima, B.; Chaudhuri, M. K.; Hussain, S. Adv. Synth.
Catal. 2005, 347, 763.
2. (a) Karodia, N.; Guise, S.; Newlands, C.; Andersen, J. Chem.
Commun. 1998, 2341; (b) Comyns, C.; Karodia, N.; Zeler,
S.; Andersen, J. Catal. Lett. 2000, 67, 113; (c) Ludley, P.;
33. Miller, K. J.; Kitagawa, T. T.; Abu Omar, M. M. Organometal-
lics 2001, 20, 4403.
34. Kawasaki, A.; Ogata, Y. Bull. Chem. Soc. Jpn. 1970, 43, 276.