NbCl5: A novel Lewis acid in allylation reactions

Article abstract of DOI:10.1055/s-2002-28504

Allylation reactions promoted by niobium pentachloride are described. Allylstannanes were added to aromatic and aliphatic aldehydes and aromatic imines in good yields. Excellent syn diastereoselectivities were obtained in the addition of (E)-cinnamylstannane to benzaldehyde (49:1) and in the addition of crotylstannane to N-benzylideneaniline (46:1).

Full text of DOI:10.1055/s-2002-28504

928
SPECIAL TOPIC
NbCl5: A Novel Lewis Acid in Allylation Reactions
N
bC
l
:
A
N
ove
l
L
a
ewis
A
cid
r
in Allyl
l
ation R
o
eactions s Kleber Z. Andrade,* Neucírio R. Azevedo, Guilherme R. Oliveira
5
Instituto de Química, Universidade de Brasília, C.P. 4478, 70910-970, Brasília, DF, Brazil
Fax +55(61)2734149; E-mail: ckleber@unb.br
Received 3 March 2002
Abstract: Allylation reactions promoted by niobium pentachloride
are described. Allylstannanes were added to aromatic and aliphatic
aldehydes and aromatic imines in good yields. Excellent syn dia-
stereoselectivities were obtained in the addition of (E)-cinnamyl-
stannane to benzaldehyde (49:1) and in the addition of
Scheme 2
crotylstannane to N-benzylideneaniline (46:1).
Key words: allylation, homoallylic compounds, stereoselectivity,
niobium pentachloride, Lewis acids
Motivated by this work and other recent works that dem-
onstrated the Lewis acidity of niobium2 and tantalum2c,d,f
compounds in organic reactions, we decided to investigate
the use of NbCl5 on the allylation of aldehydes and imines
with allylstannanes, which are far more nucleophilic than
allylsilanes.3
Introduction
Whereas the Lewis-acid-mediated addition of allylstan-
nanes to carbonylic compounds has been extensively
studied,1 the allylation of aldimines still needs further de-
velopment. The homoallylic alcohols and amines so ob-
tained are important and versatile intermediates in the
synthesis of more complex products since the double-
bond moiety can be further functionalized (Scheme 1).
This represents an advantage over the synthetically analo-
gous aldol addition reaction.
NbCl5 has a great oxophilicity and is readily available in
our country since Brazil accounts for about 60% of total
niobium production.4 Furthermore, its use as a Lewis acid
is restricted to a few reactions2 and no work has been pub-
lished regarding its use on allylation reactions.
This paper describes our results on the use of NbCl5 as a
Lewis acid with emphasis on the scope and stereoselectiv-
ity of these reactions as well as some mechanistic consid-
erations.5
Allylation of Aldehydes
In order to investigate the feasibility of the allylation reac-
tion, various aldehydes were reacted with allyltri-n-butyl-
stannane, in the presence of NbCl5 (Scheme 3).
Scheme 1
A variety of Lewis acids have been employed successfully
in these reactions, including TiCl4, BF3 Et2O, SnCl4,
InCl3, AlCl3, and MgBr2. The search for new reagents ca-
pable of mediating these reactions is still a matter of much
concern. Suzuki et al.2b reported the double addition and
exclusive formation of a cyclopropane derivative when al-
lylsilanes (2 equiv) were reacted with aldehydes in the
presence of NbCl5 (Scheme 2). This constitutes a com-
pletely different outcome from the normal allylation prod-
ucts, which in turn could be obtained under shorter
reaction periods (5 min, 60% yield).
Scheme 3
In our search for the best experimental conditions, the fol-
lowing parameters were evaluated: solvent, temperature,
stoichiometric relationship and the order in which the re-
agents were added. The results are summarized in
Table 1.
The best results were obtained when 2 equivalents of the
allylstannane were used, at –15 °C, in diethyl ether as the
solvent (entries 6, 10 12, 14, and 16). NbCl5 promoted the
ring-opening of the THF, leading to polymeric products
(entry 7). When the more coordinating solvents DME and
Synthesis 2002, No. 7, 21 05 2002. Article Identifier:
1437-210X,E;2002,0,07,0928,0936,ftx,en;C00902SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

SPECIAL TOPIC
NbCl5: A Novel Lewis Acid in Allylation Reactions
929
CH3CN were used, the yields lowered (entries 9 and 13)
or the reaction failed completely (entries 8, 11 and 15).
When isobutyraldehyde was used yields were low (ca.
20–30%), probably due to the volatility of the product. In
contrast, a much better yield was obtained when 2-phenyl-
propionaldehyde was employed (Table 2).
Scheme 4
Table 2 Results of the Reaction Between Allylstannane and ( )-2-
Phenylpropionaldehyde in Diethyl Ether
Table 1 Experimental Conditions and Results of the Reactions Be-
tween Aldehydes and Allyltri-n-buylstannanea
Entry
Temp (°C)
–78
Syn:anti
3:1
Yield (%)a
Entry
R
Solvent
Temp
(°C)
Time
(h)
Yieldc
(%)
1
2
63
75
1
2
Phb
CH2Cl2
CH2Cl2
CH2Cl2
Et2O
–78
–78
–15
–78
–15
–15
–78
–43
–15
–15
–15
–15
–15
–15
–15
–15
2.5
1.5
1
41
38
45
48
47
100
–
–15
1.6:1
Ph
a Isolated yields of the chromatographically pure products (syn:anti
mixture).
3
Ph
4
Phb
2
5
Phb
Et2O
2
Next we examined the stereo- and regioselectivity of these
NbCl5 promoted allylation reactions. In this way, crotyl-
stannane (3-methylallylstannane, 1:1 mixture of isomers)
and benzaldehyde were reacted in the presence of NbCl5,
in a variety of conditions (Table 3). The reaction proved
to be regiospecific in that no linear product was formed
under any conditions (Scheme 5).
6
Ph
Et2O
0.5
2.5
2
7
Phb
THF
8
Phb
CH3CN
DME
Et2O
–
9
Ph
2
70
50
–
10
11
12
13
14
15
16
p-OMePh
p-OMePh
p-NO2Ph
p-NO2Ph
p-ClPh
p-ClPh
2-furyl
1.5
2
DME
Et2O
1.5
2
70
50
71
–
Scheme 5
DME
Et2O
Table 3 Results of the Reaction Between Crotylstannanea and Ben-
zaldehyde in Diethyl Ether
0.5
2
DME
Et2O
Entry
Temp
(°C)
Mode of Syn:antic Time
Yieldd
(%)
0.5
50
addition
normal
normal
normal
inverse
(h)
1.5
0.3
0.3
0.3
1
–15
–15
–15
–15
1.9:1
2.6:1
2.5:1
2.2:1
63
91
83
91
a Unless otherwise noted 2 equiv of allylstannane were employed.
b Reaction with 1 equiv of allylstannane in relation to the aldehyde.
c Isolated yields of the chromatographically pure products.
2
3b
4
The cyclopropane compound obtained by Suzuki
(Scheme 2) was never observed in the reaction with allyl-
stannane. These interesting results show the versatility of
NbCl5 as a Lewis acid.
a In all cases 2 equiv of crotylstannane were used.
b 2 equiv of NbCl5 were used.
c Ratio determined by GC and 1H NMR integration.
After establishing the best experimental conditions, we
turned our attention to the stereoselectivity of these reac-
tions. The Cram/anti-Cram selectivity was addressed in
the reaction between ( )-2-phenylpropionaldehyde and
allylstannane in the presence of NbCl5 (Scheme 4).
d Isolated yields of the chromatographically pure products (syn:anti
mixture).
It is reported that the mode of addition of the reagents in
SnCl4- and TiCl4-mediated allylation reactions alters the
diastereomer ratio of the homoallylic alcohol as well as
the regiochemistry of the reaction, due to transmetalla-
tion.8 The reactions performed with normal addition (ad-
dition of crotylstannane to a suspension of benzaldehyde
and NbCl5 in Et2O) afforded a mixture of diastereomers in
good yields (Table 3, entries 1 to 3). The diastereomeric
ratio was determined by GC analysis of the mixture of
The diastereofacial selectivity was low (3:1), even at –78
°C. These results are similar to those obtained with other
Lewis acids such as BF3 Et2 and TiCl4.6 The diastere-
omers were identified by the 1H NMR chemical shifts of
the methyl groups (syn 1.35, d, J = 6.9 Hz; anti 1.30,
d, J = 7.1 Hz) by comparison with those in the literature7
and quantified by GC.
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York

930
C. K. Z. Andrade et al.
SPECIAL TOPIC
1
products and by H NMR integration of the carbinolic imine activation by chlorotrimethylsilane,18 catalytic lan-
protons (syn: 4.60, d, J = 5.3 Hz and anti: 4.35, d, thanide triflates19 and catalytic Pd(II) or Pt(II) complex-
J = 7.2 Hz). These data are in accordance with those re- es.20
ported.9
We initiated our study by searching for the best solvent
The best results were obtained at –15°C with shorter reac- system. Ethyl ether, acetonitrile and dichloromethane
tion periods. When the reactions were performed at lower were investigated in the allylation of N-benzylidene-
temperatures (–78 °C) or when 2 equiv of NbCl5 were aniline (9) at different temperatures (Scheme 7). The re-
used (entry 3), no significant improvements were sults are shown in Table 4.
achieved either in yield or in selectivity.
Either CH3CN or CH2Cl2 gave satisfactory results at
In the reactions with inverse addition (addition of benzal- –15 °C. The yields were the same at lower temperatures
dehyde to a suspension of crotylstannane and NbCl5 in using CH2Cl2 (entry 1) and decreased in all cases when the
Et2O), the yields and diastereoselectivities were similar to reaction was allowed to reach room temperature. In con-
those obtained with normal addition (entry 4). Further- trast to the allylation of aldehydes, Et2O was not a good
more, linear products were not detected under any condi- solvent (entry 2). A control experiment indicated that no
tions, as already mentioned. Although no spectroscopic reaction takes place in the absence of NbCl5.
studies were performed,10 these results suggest that the
transmetallation process is not occurring.
In order to get a better insight into the diastereoselectivity
and to confirm the influence of the stannane geometry on
the diastereomeric ratios obtained in the NbCl5-mediated
additions to aldehydes, the reaction between E-
Scheme 7
cinnamylstannane11 (3-phenylallylstannane) and benzal-
dehyde was studied (Scheme 6).
Table 4 Experimental Conditions and Yields of the Reactions be-
tween N-Benzylideneaniline (9) and Allyltri-n-butylstannanea
Yields (%)b
Entry
Solvent
CH2Cl2
Et2O
0 °C r.t. –15 °C
–78 °C
1
2
3
27
18
62
82
41
80
83
–
Scheme 6
CH3CN
–
The reaction proceeded smoothly, but in this case the syn
diastereomer was obtained almost exclusively, in good
yield. The diastereomeric ratio (49:1) was determined by
GC.
a 1 equiv of NbCl5 and 2 equiv of allylstannane were employed.
b Isolated yields of the chromatographically pure products.
This result is in sharp contrast to those obtained with
ZnCl2 in DMF12 and SnCl2 in CH3CN,13 which gave pre-
dominantly the anti isomer.14 In both cases, a cyclic tran-
sition state was proposed to explain the anti
stereoselectivity. Although it is reported that BF3 Et2 is
also syn selective,12 the reaction is run at –78 °C. The ad-
vantage here is that the reaction can be warmed to 0 °C
without lowering the selectivity.
Next, the stoichiometry of the reaction was assessed
(Table 5). We found that an excess of stannane improves
the yields but is not critical to the success of the reaction,
except when Et2O was used (entry 2). The lower yields
obtained with 1 equiv of allylstannane may be due to its
slight decomposition in the presence of NbCl5.
To confirm these results, an enriched (5:1) mixture of the
anti isomer was prepared following a procedure of Wilson
et al.15 The spectroscopic data for the major isomer
matched those reported for the anti isomer16 whereas the
data for the minor isomer are the same we obtained in the
NbCl5-mediated reaction.17
Table 5 Influence of an Excess of Allylstannane on the Allylation
of N-Benzylideneaniline (9), at –15 °C
Yields (%)a
Entry
Solvent
CH2Cl2
Et2O
–15 °Cb
72
–15 °Cc
82
1
2
3
17
41
Allylation of Imines
CH3CN
60
80
As already mentioned, there is much less work in the lit-
erature dealing with allylation of imines as compared to
aldehydes. Recent improvements in this reaction deal with
a Isolated yields of the chromatographically pure products.
b Imine:NbCl5:stannane = 1:1:1
c Imine:NbCl5:stannane = 1:1:2
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York

SPECIAL TOPIC
NbCl5: A Novel Lewis Acid in Allylation Reactions
931
In this way, we defined the ideal conditions for the NbCl5-
mediated allylation of imines as being CH2Cl2 as the sol-
vent,21 at –15 °C and using an excess of allylstannane. Un-
der these conditions a variety of aromatics aldimines were
allylated in reasonable to good yields (Scheme 8,
Table 6).
Scheme 9
Scheme 8
Table 6 Results of the Reactions Between Aldimines and Allyltri-
n-butylstannane
Entry
Amine
10
R
R1
Yielda (%)
Scheme 10
1
2
3
4
5
6
Ph
Ph
82
80
59
51
35
85
Table 7 Results of the Reaction Between Crotylstannanea and N-
Benzylideneaniline (9) in CH2Cl2
11
p-NO2Ph
2-Furyl
p-ClPh
p-ClPh
p-OMePh
Ph
Entry
Temp (°C) Mode of
addition
Syn:anti
Yieldb (%)
12
Ph
13
Ph
1
2
–15
–78
normal
normal
9:1
60
62
14
n-Bu
Ph
46:1
15
a 2 equiv of crotylstannane were used.
a Isolated yields of the chromatographically pure products.
b Isolated yields of chromatographically pure products (syn:anti mix-
ture).
In some cases, bis-allylated22 by-products were also ob-
tained (ca 10–15%) when 2 equiv of allylstannane were
used (Figure 1). This can be circumvented using only a
slight excess of this reagent.
Similar to the allylation of aldehydes, the allylation of
imines proved to be regiospecific and no linear products
were formed under any conditions. The allylated products
were obtained in good yields and excellent diastereoselec-
tivities (entries 1 and 2). The diastereomeric ratio was de-
termined by GC analysis of the mixture of products and by
1H NMR integration of the proton on the carbon attached
to the nitrogen (syn: 4.33, d, J = 4.5 Hz and anti: 4.07,
d, J = 7.2 Hz) and methyl groups (syn: 1.01, d, J = 6.9
Hz and anti: 0.99, d, J = 6.9 Hz). These data are in ac-
cordance with those reported.24
Figure 1 Bis-allylated by-products
At –78 °C, the syn isomer is obtained almost exclusively,
regardless of the stereochemistry of the stannane double
bond. These results are even superior to those obtained
with other Lewis acids such as BF3 Et2 and TiCl4.24,25
Also, this is in sharp contrast to the results obtained on the
allylation of aldehydes (Scheme 5, Table 3). This led us to
propose different transition states for these reactions as
discussed below.
The activation of the imines probably occurs via the for-
mation of an iminium salt with NbCl5 (Scheme 9). As an
example, a dark-orange precipitate forms when imine 9 is
added to a suspension of NbCl5 in CH2Cl2. Similarly, a
white precipitate attributed to the iminium salt formed
when TMSCl was used as additive has been observed.23
The stereo- and regioselectivity of these NbCl5-promoted
allylation reactions were assessed in the reaction between
crotylstannane (3-methylallylstannane, 1:1 mixture of
isomers) and imine 9 in the presence of NbCl5
(Scheme 10, Table 7).
Mechanistic considerations
The mechanism of these reactions is still a matter of much
controversy. Yamamoto et al.6 proposed an acyclic anti-
periplanar transition state for the BF3 Et2O-mediated re-
action between crotylstannane and benzaldehyde, in
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York

932
C. K. Z. Andrade et al.
SPECIAL TOPIC
which the syn isomer is obtained almost exclusively, re- Based on this model, we can postulate that the slight over-
gardless of the double-bond geometry in the crotylstan- all preference for the syn homoallylic alcohol in these
nane.26
NbCl5 mediated reactions (1:1 mixture of stannanes) is
due to a relatively large preference for transition state
ET1 over ET4 (for the E substrate) while there is no clear
preference between ET2 and ET3 (for the Z substrate)
which have steric interactions similar in energy.
Based on the results obtained here on the allylation of al-
dehydes, it seems that the geometry of the crotylstannane
double bond plays an important role in the reaction selec-
tivity.
In the E-cinnamylstannane allylation of aldehydes, a syn-
synclinal transition state (ET5) can also be invoked to ex-
plain the selective formation of the syn isomer although
the antiperiplanar transition state (ET6) should not be
ruled out28 (Figure 4).
In this way, the syn-synclinal transition state proposed by
Keck et al.27 seems to furnish a more reasonable explana-
tion to account for the low selectivity obtained. In this
conformation, the stannane methylene group resides in a
syn orientation in relation to the carbonyl oxygen, allow-
ing an interaction between the HOMO of the olefin and
the LUMO of the carbonyl group (Figure 2).
Figure 4 Syn-synclinal and antiperiplanar transition states for
E-cinnamylstannane
Figure 2 Frontier orbital interaction between the allylstannane and
the aldehyde
Our results on the allylation of imines indicated that the
selectivity is not affected by the double-bond geometry
and the syn isomer is obtained almost exclusively. This
could be better explained through an acyclic antiperipla-
nar transition state (Figure 5). Among the possible geom-
etries, including the synclinals, the antiperiplanar (ET7)
seems to be preferred since it has less steric crowding as
compared to the others.
Figure 3 represents the syn-synclinal transition states for
the E- and Z-crotylstannane (ET1 to ET4) proposed by
Keck and adapted for the NbCl5-mediated reactions. Tran-
sition state ET1, according to Keck, is the lowest in ener-
gy since there is only one steric interaction between the
aldehyde phenyl group and the stannane methyl group.
ET2 and ET4 show interactions between the stannane
methylene group and the Lewis acid and between the phe-
nyl group and the methyl group. ET3 shows an interaction
between the methyl group and the bulky Lewis acid.
Figure 5 Antiperiplanar transition states proposed for the allylation
of imines
Current work
Our current work on allylation reactions deals with the
generation of N-acyliminium ions by NbCl5 and subse-
quent addition of allylsilane (Scheme 11). Some applica-
tions of this methodology in the synthesis of alkaloids are
in progress. So far, our preliminary results are promising,
as can be seen by the yields of the allylated products 19
and 20, and will be published in more details soon.
Figure 3 Syn-synclinal transition states for E- and Z-crotyl-
stannanes
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York

SPECIAL TOPIC
NbCl5: A Novel Lewis Acid in Allylation Reactions
933
pressure to furnish the crude product which was purified by silica
gel chromatography (hexanes–EtOAc).
1-Phenyl-3-buten-1-ol (1)
IR (film): 3390, 3076, 1641, 1453, 757, 700 cm–1.
1H NMR (200 MHz, CDCl3): = 7.26–7.40 (m, 5 H, Ph), 5.71–5.94
(m, 1 H, CH), 5.10–5.24 (m, 2 H, CH2), 4.75 (dd, 1 H, J = 7.3, 3.0
Hz, CH), 2.48–2.58 (m, 2 H, CH2), 2.12 (br, 1 H, OH).
13C NMR (50 MHz, CDCl3): = 43.7 (CH2), 73.2 (CH), 118.3
(CH2), 125.8 (CH), 127.7 (CH), 128.4 (CH), 134.5 (CH), 143.8
(C0).
Scheme 11
MS: m/z (%) = 51 (20), 63 (3), 79 (88), 91 (3), 107 (100), 115 (4),
130 (4).
1-(4-Methoxyphenyl)-3-buten-1-ol (2)
Conclusion
IR (film): 3403, 3075, 1640, 1513, 1463 cm–1.
1H NMR (200 MHz, CDCl3): = 7.28 (d, 2 H, J = 8.3 Hz, Ph), 6.88
(d, 2 H, J = 8.3, Ph), 5.69–5.90 (m, 1 H, CH), 5.07–5.20 (m, 2H,
CH2), 4.67 (t, 1 H, J = 6.5 Hz, CH), 3.80 (s, 3 H, CH3), 2.4 (tt, 2 H,
J = 6.8, 1.2 Hz, CH2), 2.28 (br, 1 H, OH).
13C NMR (50 MHz, CDCl3): = 43.8 (CH2), 55.2 (CH3), 73.1 (CH),
113.7 (CH), 118.2 (CH2), 127.0 (CH), 134.7 (CH), 136.1 (C0),
159.0 (C0).
In summary, the scope and limitations of the NbCl5-medi-
ated allylation of aldehydes and imines have been studied.
The methodology allows the obtention of homoallylic al-
cohols and amines in good yields with excellent regio-
and stereoselectivities. NbCl5 proved to be an effective
Lewis acid and a promising tool in the control of diastere-
oselectivity in these reactions. The exceptional selectivity
obtained with cinnamylstannane (aldehydes) and with
crotylstannane (imines) makes NbCl5 an attractive choice
when the syn isomer is required.
MS: m/z (%) = 31 (5), 44 (5), 51 (20), 61 (28), 76 (28), 113 (78), 128
(41), 142 (63), 157 (100), 160 (59), 174 (14).
1-(4-Nitrophenyl)-3-buten-1-ol (3)
IR (film): 3412, 3078, 1641, 753, 700 cm–1.
All reactions involving NbCl5 were performed under argon in a
flame-dried flask. Ethereal solvents were distilled from sodium ben-
zophenone ketyl. CH2Cl2 and CH3CN were distilled from CaH2 pri-
or to use. NbCl5 was supplied by Companhia Brasileira de
Mineração e Metalurgia (CBMM) and sublimed before use. Alde-
hydes were distilled immediately before use except p-nitrobenzal-
dehyde, which is solid. Imines were prepared from the
corresponding aldehydes and amines in CH2Cl2, in the presence of
Na2SO4. Crotyl and cinnamylstannane were prepared according to
a literature procedure.11
1H NMR (200 MHz, CDCl3): = 8.16 (d, 2 H, J = 9.2 Hz, Ph), 7.50
(d, 2 H, J = 9.2 Hz, Ph), 5.66–5.87 (m, 1 H, CH), 5.08–5.20 (m, 2
H, CH2), 4.86 (dd, 1 H, J = 7.3, 5.1 Hz, CH), 2.64 (br, 1 H, OH),
2.6–2.61 (m, 2 H, CH2)
13C NMR (50 MHz, CDCl3): = 43.7 (CH2), 72.0 (CH), 119.4
(CH2), 123.5 (CH), 126.7 (CH), 133.3 (CH), 147.1 (C0), 151.2 (C0).
MS: m/z (%) = 46 (4), 50 (24), 63 (1), 77 (41), 93 (1), 104 (28), 127
(72), 151 (100), 174 (9).
1-(4-Chlorophenyl)-3-buten-1-ol (4)
IR spectra were recorded on a BOMEM Hartman & Braun-Michel-
son MB series 100 LASER FT-IR. 1H NMR spectra were recorded
on Varian EM390, Bruker ARX 200 and DRX 400 spectrometers.
Chemical shifts are reported in ppm from tetramethylsilane as inter-
nal reference. Data are reported as follows: chemical shift, multi-
plicity (s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet
of doublets, m = multiplet, etc), integration, coupling constant and
assignment. 13C NMR spectra were recorded on Bruker ARX 200
(50 MHz) and DRX 400 (100 MHz) spectrometers. Chemical shifts
are reported in ppm from tetramethylsilane with solvent as internal
standard (CDCl3: = 77.0). Data are reported as follows: chemical
shift and assignment. Mass spectra were obtained on a Perkin-Elm-
er Q-Mass 910 mass spectrometer.
IR (film): 3372, 3078, 1641, 1492, 745, 720 cm–1.
1H NMR (90 MHz, CCl4): = 7.4 (s, 4 H, Ph), 5.7–6.2 (m, 1 H, CH),
5.1–5.4 (m, 2 H, CH2), 4.8 (t, 1 H, J = 7.2 Hz, CH), 2.5 (t, 2 H,
J = 7.2 Hz, CH2), 2.0 (s, 1 H, OH).
MS: m/z (%) = 51 (2), 63 (4), 77 (89), 113 (27), 129 (7), 141 (100),
164 (2).
1-(2-Furyl)-3-buten-1-ol (5)
IR (film): 3393, 3078, 1642, 1433, 737 cm–1.
1H NMR (200 MHz, CDCl3): = 7.27 (s, 1 H, Ar), 6.20-6.40 (m, 2
H, Ar), 5.70-6.10 (m, 1 H, CH), 5.00-5.40 (m, 2 H, CH2), 4.77 (t, 1
H, J = 7.2 Hz, CH), 2.64 (t, 2 H, J = 7.2 Hz, CH2), 2.0 (br, 1 H, OH).
GC analyses were performed on Shimadzu GC-14B and Varian Star
3400 spectrometers using DB-WAX column. Column chromatog-
raphy was performed on silica gel (70-230 mesh).
13C NMR (50 MHz, CDCl3): = 40.0 (CH2), 66.9 (CH), 106.1 (CH),
109.8 (CH), 118.5 (CH2), 133.5 (CH), 141.6 (CH), 146.5 (C0).
MS: m/z (%) = 28 (4), 39 (23), 49 (17), 63 (23), 77 (9), 89 (38), 91
(100), 96 (34), 119 (55).
Allylation of Aldehydes; General Procedure
To a fresh sublimed NbCl5 (1 mmol) suspension in Et2O (8 mL) at
–15 °C, under an Ar atmosphere was added the aldehyde (1 mmol),
followed by the addition of allylstannane (2 mmol). After the time
specified in Table 1, the reaction was quenched with sat. NH4Cl (10
mL), extracted with Et2O (3 15 mL) and stirred for 1 h with 10%
KF in H2O (10 mL). The reaction mixture was then washed several
times with brine, dried with Na2SO4 and concentrated at reduced
2-Phenyl-5-hexen-3-ol (6)
IR (film): 3422, 3084, 1641, 1453, 762, 700 cm–1.
1H NMR (200 MHz, CDCl3): anti isomer: = 7.10–7.40 (m, 5 H,
Ph), 5.70–6.00 (m, 1 H, CH), 5.00–5.30 (m, 2 H, CH2), 3.60–3.90
(m, 1 H, CH), 2.70–2.90 (m, 1 H, CH), 1.90–2.20 (m, 2 H, CH2),
1.56 (s, 1H, OH), 1.30 (d, 3 H, J = 7.1 Hz, CH3). syn isomer:
=
7.10–7.40 (m, 5 H, Ph), 5.70–6.00 (m, 1 H, CH), 5.00–5.30 (m, 2 H,
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SPECIAL TOPIC
CH2), 3.60–3.90 (m, 1 H, CH), 2.70–2.90 (m, 1 H, CH), 1.90–2.20
(m, 2 H, CH2), 1.56 (s, 1 H, OH), 1.35 (d, 3 H, J = 6.9 Hz, CH3).
13C NMR (50 MHz, CDCl3): 16.4 (CH3, syn), 17.7 (CH3, anti),
38.9 (CH, anti), 39.5 (CH, syn), 45.3 (CH2), 74.9 (CH), 117.7 (CH,
anti), 118.1 (CH, syn), 126.4, 126.6, 127.7, 128.1, 128.4, 135.0,
143.2, 144.3.
13C NMR (50 MHz, CDCl3): = 43.3 (CH2), 57.1 (CH), 117.3 (CH),
118.3 (CH2), 125.5 (CH), 126.3 (CH), 127.0 (CH), 129.0 (CH),
134.6 (CH), 143.6 (C0), 147.4 (C0).
N-[1-(4-Nitrophenyl)-3-butenyl)]phenylamine (11)
IR (film): 3410, 3072, 2922, 2853, 1640, 1603, 1519, 1345, 993,
922, 854, 751, 693 cm–1.
1H NMR (200 MHz, CDCl3): = 8.18 (d, 2 H, J = 8.8 Hz, Ar), 7.54
(d, 2 H, J = 8.5 Hz, Ar), 7.03–7.13 (m, 2 H, Ar), 6.64–6.72 (m, 1 H,
Ar), 6.42 (dd, 2 H, J = 6.5, 2.0 Hz, Ar), 5.63–5.84 (m, 1 H, CH),
5.15–5.26 (m, 2 H, CH2), 4.50 (dd, 1 H, J = 8.0, 5.2 Hz, CH), 4.23
(br, 1 H, NH), 2.41–2.70 (m, 2 H, CH2).
MS: m/z (%) = 51 (12), 57 (11), 77 (34), 91 (73), 105 (100), 128
(41), 143 (26), 157 (15).
2-Methyl-1-phenyl-3-buten-1-ol (7)
IR (film): 3411, 1638, 1453, 761, 701 cm–1.
1H NMR (200 MHz, CDCl3): anti isomer: = 7.20–7.40 (m, 5 H,
Ph), 5.64–5.90 (m, 1 H, CH), 4.90–5.30 (m, 2 H, CH2), 4.35 (d, 1 H,
J = 7.8 Hz, CH), 2.50 (m, 1 H, CH), 1.60 (s, 1 H, OH), 0.86 (d, 3 H,
J = 6.8 Hz, CH3). syn isomer: = 7.20–7.40 (m, 5 H, Ph), 5.64-5.90
(m, 1 H, CH), 4.90–5.30 (m, 2 H, CH2), 4.60 (d, 1 H, J = 5.4 Hz,
CH), 2.50 (m, 1 H, CH), 2.00 (s, 1 H, OH), 1.00 (d, 3 H, J = 6.8 Hz,
CH3).
13C NMR (50 MHz, CDCl3): = 42.8 (CH2), 56.9 (CH), 113.5 (CH),
118.1 (CH2), 119.3 (CH2), 123.0 (CH), 127.3 (CH), 129.3 (CH),
133.4 (CH), 146.5 (C0), 147.1 (C0), 151.5 (C0).
MS: m/z (%) = 77 (1), 181 (39), 227 (100), 268 (0.2).
N-[1-(2-Furyl)-3-butenyl]phenylamine (12)
IR (film): 3411, 3077, 3052, 2918, 1640, 1603, 994, 921, 748, 691
13C NMR (50 MHz, CDCl3): = 14.0 (CH3, syn), 16.5 (CH3, anti),
44.6 (CH, syn), 46.1 (CH, anti), 77.2 (CH), 115.4 (CH2, syn), 116.7
(CH2, anti), 126.0, 126.8, 127.3, 127.6, 128.0, 128.2, 140.3 (CH,
syn), 142.6 (CH, anti).
cm–1.
1H NMR (200 MHz, CDCl3): = 7.33 (dd, 1 H, J = 1.8, 0.9 Hz, Ar),
7.08–7.22 (m, 2 H, Ar), 6.56–6.73 (m, 3 H, Ar), 6.27 (dd, 1 H,
J = 3.2, 1.8 Hz, Ar), 6.15 (dt, 1 H, J = 3.2, 0.8 Hz, Ar), 5.64–5.85
(m, 1 H, CH), 5.08–5.21 (m, 2 H, CH2), 4.54 (t, 1 H, J = 6.2 Hz,
CH), 3.97 (br, 1 H, NH), 2.61–2.68 (m, 2 H, CH2).
MS: m/z (%) = 51 (9), 65 (4), 77 (32), 79 (55), 107 (100), 129 (40),
144 (15).
1,2-Diphenyl-3-buten-1-ol (8)
Syn Isomer
IR (film): 3382, 3080, 1637, 1453, 750, 690 cm–1.
13C NMR (50 MHz, CDCl3): = 39.2 (CH2), 51.3 (CH), 106.0 (CH),
110.1 (CH), 113.5 (CH), 117.8 (CH), 118.4 (CH2), 129.1 (CH),
133.9 (CH), 141.5 (CH), 146.5 (C0), 155.3 (C0).
1H NMR (400 MHz, CDCl3): = 7.01–7.38 (m, 10 H, Ph), 5.89
(ddd, 1 H, J = 18.1, 10.3, 7.9 Hz, CH), 4.78–5.04 (m, 3 H, CH and
CH2), 3.63 (t, 1 H, J = 8.1 Hz, CH), 1.94 (s, 1 H, OH).
N-[1-(4-Chlorophenyl)-3-butenyl]phenylamine (13)
IR (film): 3413, 3077, 2925, 2852, 1639, 1601, 993, 918, 823, 749,
692 cm–1.
13C NMR (100 MHz, CDCl3): 58.5 (CH), 77.5 (CH), 117.2 (CH),
127.0, 127.8, 128.1, 128.7, 128.8, 137.0, 140.2, 141.9.
1H NMR (200 MHz, CDCl3): = 7.28 (s, 4 H, Ar), 7.02–7.12 (m, 2
H, Ar), 6.61–6.69 (m, 1 H, Ar), 6.41–6.48 (m, 2 H, Ar), 5.63–5.83
(m, 1 H, CH), 5.11–5.23 (m, 2 H, CH2), 4.35 (dd, 1 H, J = 7.9, 5.2
Hz, CH), 4.10 (br, 1 H, NH), 2.36–2.65 (m, 2 H, CH2).
MS: m/z (%) = 51 (52), 65 (20), 79 (88), 91 (51), 107 (95), 128
(100), 165 (4), 206 (7).
13C NMR (50 MHz, CDCl3): = 43.2 (CH2), 56.6 (CH), 113.5 (CH),
117.7 (CH2), 118.7 (CH2), 127.7 (CH), 128.8 (CH), 129.1 (CH),
132.6 (C0), 134.2 (CH), 142.2 (C0), 147.1 (C0).
Anti Isomer
1H NMR (400 MHz, CDCl3): = 7.00–7.38 (m, 10 H, Ph), 6.25
(ddd, 1 H, J = 19.1, 10.2, 8.9 Hz, CH), 5.10–5.30 (m, 3 H, CH and
CH2), 3.54 (t, 1 H, J = 8.3 Hz, CH), 2.34 (d, 1 H, J = 2.1 Hz, OH).
13C NMR (100 MHz, CDCl3): = 59.2 (CH), 77.2 (CH), 118.4
(CH), 126.5, 126.6, 127.4, 127.9, 128.3, 128.7, 137.8, 140.5, 141.7.
N-Butyl-1-(4-chlorophenyl)-3-butenyl-1-amine (14)
IR (film): 3331, 3077, 2928, 1640, 1639, 1595, 1090, 993, 917, 828,
741, 681 cm–1.
1H NMR (200 MHz, CDCl3): = 7.26 (d, 4 H, J = 1.7 Hz, Ar), 5.58–
5.80 (m, 1 H, CH), 5.02–5.12 (m, 2 H, CH2), 3.61 (t, 1 H, J = 6.8
Hz, CH), 2.30–2.45 (m, 4 H, 2 CH2), 1.53 (br, 1 H, NH), 1.18–1.49
(m, 4 H, 2 CH2), 0.86 (t, 3 H, J = 6.9 Hz, CH3).
13C NMR (50 MHz, CDCl3): = 14.0 (CH3), 20.4 (CH2), 32.4
(CH2), 43.2 (CH2), 47.4 (CH2), 61.9 (CH), 118.0 (CH), 128.4 (CH),
128.5 (CH), 132.4 (C0), 135.2 (CH), 142.8 (C0).
Allylation of Imines; General Procedure
To a fresh sublimed NbCl5 (1 mmol) suspension in CH2Cl2 (1.5 mL)
at –15 °C, under an argon atmosphere, was added a solution of the
imine (1 mmol) in CH2Cl2 (1.5 mL), followed by the addition of al-
lylstannane (2 mmol), after 10 min. The reaction was monitored by
TLC and after the consumption of the imine (1–2 h) it was quenched
with sat. NH4Cl (10 mL), extracted with Et2O (3 15 mL) and
stirred for 2 h with 10% KF in H2O (10 mL). The reaction mixture
was then washed several times with brine, dried with Na2SO4 and
concentrated at reduced pressure to furnish the crude product,
which was purified by silica gel chromatography (hexanes–EtOAc).
N-[1-(4-Methoxyphenyl)-3-butenyl]phenylamine (15)
IR (film): 3410, 3004, 2922, 2834, 1638, 1603, 1246, 993, 918, 829,
749, 692 cm–1.
1H NMR (200 MHz, CDCl3): = 7.28–7.35 (m, 2 H, Ar), 7.02–7.17
(m, 2 H, Ar), 6.86–6.93 (m, 2 H, Ar), 6.64–6.72 (m, 1 H, Ar), 6.51–
6.57 (m, 2 H, Ar), 5.70–5.90 (m, 1 H, CH), 5.14–5.27 (m, 2 H, CH2),
4.38 (dd, 1 H, J = 7.7, 5.4 Hz, CH), 4.18 (br, 1 H, NH), 3.82 (s, 3 H,
OCH3), 2.43–2.70 (m, 2 H, CH2).
13C NMR (50 MHz, CDCl3): = 43.2 (CH2), 55.2 (CH), 56.5 (CH3),
113.4 (CH), 113.9 (CH), 117.9 (CH), 118.2 (CH2), 127.3 (CH),
129.1 (CH), 134.8 (CH), 135.5 (C0), 147.4 (C0), 158.5 (C0).
N-(1-Phenyl-3-butenyl)phenylamine (10)
IR (film): 3411, 3078, 2853, 1640, 1601, 1452, 993, 917, 748, 692
cm–1.
1H NMR (200 MHz, CDCl3): = 7.17–7.39 (m, 5 H, Ar), 7.03–7.10
(m, 2 H, Ar), 6.59–6.67 (m, 1 H, Ar), 6.46–6.50 (m, 2 H, Ar), 5.65–
5.86 (m, 1 H, CH), 5.10–5.23 (m, 2 H, CH2), 4.37 (dd, 1 H, J = 7.9
and 5.2 Hz, CH), 4.15 (br, 1 H, NH), 2.39–2.68 (m, 2 H, CH2).
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York

SPECIAL TOPIC
NbCl5: A Novel Lewis Acid in Allylation Reactions
935
N-Allyl-N-[1-(4-nitrophenyl)-3-butenyl]phenylamine (16)
1H NMR (200 MHz, CDCl3): = 8.16 (d, 2 H, J = 8.8 Hz, Ar), 7.45
(d, 2 H, J = 9.0 Hz, Ar), 7.16–7.27 (m, 3 H, Ar), 6.60–6.85 (m, 2 H,
Ar), 5.63–6.05 (m, 2 H, 2 CH), 5.02–5.19 (m, 5 H, 1 CH, 2 CH2),
3.75–3.80 (m, 2 H, CH2), 2.77–2.87 (m, 2 H, CH2).
13C NMR (50 MHz, CDCl3): = 36.3 (CH2), 49.4 (CH2), 61.9 (CH),
115.2 (CH), 116.6 (CH2), 117.8 (CH2), 118.3 (CH), 123.6 (CH),
128.5 (CH), 129.2 (CH), 134.8 (CH), 135.3 (CH).
(5) Preliminary communications of this work: (a) Andrade, C.
K. Z.; Azevedo, N. R. Tetrahedron Lett. 2001, 42, 6473.
(b) Andrade, C. K. Z.; Oliveira, G. R. Tetrahedron Lett.
2002, 43, 1935.
(6) Yamamoto, Y.; Yatagai, H.; Ishihara, Y.; Maeda, N.;
Maruyama, K. Tetrahedron 1984, 40, 2239.
(7) Heathcock, C. H.; Kiyooka, S.; Blumenkopf, T. A. J. Org.
Chem. 1984, 49, 4214.
(8) Keck, G. E.; Abbott, D. E.; Boden, E. P.; Enholm, E. J.
Tetrahedron Lett. 1984, 25, 3927.
MS: m/z (%) = 77 (17), 178 (18), 268 (100), 308 (17).
(9) Coxon, J. M.; van Eyk, S. J.; Steel, P. J. Tetrahedron 1989,
45, 1029.
N-Allyl-N-[1-(4-chlorophenyl)-3-butenyl]phenylamine (17)
1H NMR (200 MHz, CDCl3): = 7.15–7.30 (m, 6 H, Ar), 6.70–6.80
(m, 3 H, Ar), 5.61–5.93 (m, 2 H, 2 CH), 4.96–5.16 (m, 5 H, 1 CH,
2 CH2), 3.73 (d, 2 H, J = 4.9 Hz, CH2), 2.75 (t, 2 H, J = 7.0 Hz,
CH2).
13C NMR (50 MHz, CDCl3): = 36.0 (CH2), 48.7 (CH2), 61.5 (CH),
128.5 (CH), 129.0 (CH), 129.1 (CH), 132.8 (C0), 135.4 (CH), 136.0
(CH), 139.4 (C0), 148.7 (C0).
(10) For spectroscopic studies on the transmetallation involving
Sn and B, see: (a) Denmark, S. E.; Wilson, T.; Willson, T.
M. J. Am. Chem. Soc. 1988, 110, 984. (b) Denmark, S. E.;
Weber, E. J.; Wilson, T. M.; Willson, T. M. Tetrahedron
1989, 45, 1053. (c) Naruta, Y.; Nishigaichi, Y.; Maruyama,
K. Tetrahedron 1989, 45, 1067. (d) Keck, G. E.; Andrus, M.
B.; Castellino, S. J. Am. Chem. Soc. 1989, 111, 8136.
(11) For its synthesis, see: Carofiglio, T.; Marton, D.; Tagliavini,
G. Organometallics 1992, 11, 2961.
MS: m/z (%) = 77 (17), 178 (18), 268 (100), 308 (17).
(12) Koreeda, M.; Tanaka, Y. Chem. Lett. 1982, 1299.
(13) In CH2Cl2 the reaction is syn-selective (82:18): Yasuda, M.;
Sugawa, Y.; Yamamoto, A.; Shibata, I.; Baba, A.
Tetrahedron Lett. 1996, 37, 5951.
(14) The anti isomer is also obtained exclusively in the reaction
of cinnamyl chloride with aldehydes mediated by tin and
aluminum. See ref.8 Also: Uneyama, K.; Nanbu, H.; Torii, S.
Tetrahedron Lett. 1986, 27, 2395.
N-(2-Methyl-1-phenyl-3-butenyl)phenylamine (18)
IR (film): 3414, 1638, 1602, 1090, 994, 918, 748, 692 cm–1.
1H NMR (200 MHz, CDCl3): = 7.18–7.37 (m, 5 H, Ar), 6.99–7.10
(m, 2 H, Ar), 6.56–6.64 (m, 1 H, Ar), 6.47 (d, 2 H, J = 7.6 Hz, Ar),
5.65–5.84 (m, 1 H, CH), 5.09–5.24 (m, 2 H, CH2), 4.33 (d, 1 H,
J = 4.5 Hz, CH), 4.20 (br, 1 H, NH), 2.43–2.76 (m, 1 H, CH), 1.01
(d, 3 H, J = 6.9 Hz, CH3).
(15) Reaction between cinnamyl bromide and benzaldehyde,
mediated by zinc in aqueous media: Wilson, R. S.;
Guazzaroni, M. E. J. Org. Chem. 1989, 54, 3087.
(16) 1H NMR (CDCl3) anti: = 1.54 (br, 1 H), 3.54 (t, 1 H, J = 8.3
Hz), 5.10–5.30 (m, 3 H), 6.25 (ddd, 1 H, J = 19.1, 10.2, 8.9
Hz), 7.00–7.38 (m, 10 H). See: Coxon, J. M.; Simpson, G.
W.; Steel, P. J.; Trenerry, V. C. Aust. J. Chem. 1984, 37, 65.
(17) 1H NMR (CDCl3) syn: = 1.94 (br, 1 H), 3.63 (t, 1 H, J = 8.0
Hz), 4.78–5.04 (m, 3 H), 5.89 (ddd, 1 H, J = 7.8, 10.3, 18.1
Hz), 7.01–7.38 (m, 10 H). For comparison, see ref.27
(18) Wang, D.-K.; Dai, L.-X.; Hou, X.-L. Tetrahedron Lett.
1995, 36, 8649.
(19) (a) Bellucci, C.; Cozzi, P. G.; Umani-Ronchi, A.
Tetrahedron Lett. 1995, 36, 7289. (b) Kobayashi, S.;
Iwamoto, S.; Nagayama, S. Synlett 1997, 1099.
(c) Kobayashi, S.; Bujima, T.; Nagayama, S. Chem.
Commun. 1998, 19. (d) Kobayashi, S.; Ishitani, H. J. Syn.
Org. Chem. Jpn. 1998, 56, 357.
(20) (a) Nakamura, H.; Iwama, H.; Yamamoto, Y. Chem.
Commun. 1996, 1459. (b) Nakamura, H.; Iwama, H.;
Yamamoto, Y. J. Am. Chem. Soc. 1996, 118, 6641.
(c) Nakamura, H.; Nakamura, K.; Yamamoto, Y. J. Am.
Chem. Soc. 1998, 120, 4242. (d) Bao, M.; Nakamura, H.;
Yamamoto, Y. Tetrahedron Lett. 2000, 41, 131. (e) Bao,
M.; Nakamura, H.; Yamamoto, Y. Angew. Chem. Int. Ed.
2001, 40, 3208.
(21) The choice for CH2Cl2 was based on the fact that the reaction
in this solvent could be run at lower temperature (–78 °C) as
compared to CH3CN.
(22) An example of palladium-catalyzed double-allylation
reaction has recently been reported: Nakamura, H.; Aoyagi,
K.; Shim, J.-G.; Yamamoto, Y. J. Am. Chem. Soc. 2001, 123,
372.
(23) Wang, D.-K.; Dai, L.-X.; Hou, X.-L. Tetrahedron Lett.
1995, 36, 8649.
(24) Yamamoto, Y.; Komatsu, T.; Maruyama, K. J. Org. Chem.
1985, 50, 3115.
13C NMR (50 MHz, CDCl3): = 15.3 (CH3), 43.6 (CH), 61.3 (CH),
113.3 (CH), 115.8 (CH2), 117.1 (CH), 126.9 (CH), 127.4 (CH),
128.1 (CH), 129.0 (CH), 140.2 (CH), 142.6 (C0), 147.1 (C0).
MS: m/z (%) = 77 (22), 104 (15), 182 (100), 237 (1).
Acknowledgement
The authors thank the Instituto de Química, Universidade de
Brasília, for its financial support during the early stages of this
work. We also thank CBMM for NbCl5 samples, CAPES for fel-
lowship, Prof. Antônio Gilberto Ferreira (UFSCar), Leyla Tavares
(UFSCar) and Wender Alves (UnB) for high field NMR spectra.
References
(1) Recent reviews on allylation reactions of aldehydes and
imines: (a) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93,
2207. (b) Nishigaichi, Y.; Takuwa, A.; Naruta, Y.;
Maruyama, K. Tetrahedron 1993, 49, 7395. (c) Marshall, J.
A. Chem. Rev. 1996, 96, 31. (d) Bloch, R. Chem. Rev. 1998,
98, 1407.
(2) (a) Suzuki, K.; Hashimoto, T.; Maeta, H.; Matsumoto, T.
Synlett 1992, 125. (b) Maeta, H.; Nagasawa, T.; Handa, Y.;
Takei, T.; Osamura, Y.; Suzuki, K. Tetrahedron Lett. 1995,
36, 899. (c) Howarth, J.; Gillespie, K. Tetrahedron Lett.
1996, 37, 6011. (d) Yamamoto, M.; Nakazawa, M.;
Kishikawa, K.; Kohmoto, S. Chem. Commun. 1996, 20,
2353. (e) Chandrasekhar, S.; Takhi, M.; Reddy, Y. R.;
Mohapatra, S.; Rao, C. R.; Reddy, K. V. Tetrahedron 1997,
53, 14997. (f) Kobayashi, S.; Busujima, T.; Nagayama, S.
Chem.–Eur. J. 2000, 6, 3491.
(3) Hagen, G.; Mayr, H. J. Am. Chem. Soc. 1991, 113, 4954.
(4) For a review on niobium compounds, see: Nowak, I.; Ziolek,
M. Chem. Rev. 1999, 99, 3603.
(25) Keck, G. E.; Enholm, E. J. J. Org. Chem. 1984, 50, 146.
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York

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C. K. Z. Andrade et al.
SPECIAL TOPIC
(26) For a complete discussion of the possible transition states
geometries, see ref.24
(27) (a) Keck, G. E.; Savin, K. A.; Cressman, E. N. K.; Abbott,
D. E. J. Org. Chem. 1994, 59, 7889. (b) Keck, G. E.;
Dougherty, S. M.; Savin, K. A. J. Am. Chem. Soc. 1995, 117,
6210.
(28) An eight-membered cyclic transition state with synclinal
arrangement of the reacting C=C and C=O double bonds has
been suggested for the reaction between aldehydes and
allylsilane promoted by BF3 Et2 , based on computational
evidence Bottoni, A.; Costa, A. L.; Tommaso, D. D.; Rossi,
I.; Tagliavini, E. J. Am. Chem. Soc. 1997, 119, 12131.
Synthesis 2002, No. 7, 928–936 ISSN 0039-7881 © Thieme Stuttgart · New York