The role of hydrogen bonding in the intramolecular cyclization of carbenes

Article abstract of DOI:10.1071/CH03139

The cyclization products from the photolysis of 3-arylamino-4- ethoxycarbonyl-2-heteroarylisoxazol-5(2H)-ones are interpreted as arising from intramolecular cyclization of carbenes. The mode of cyclization appears to be controlled by hydrogen bonding of t

Full text of DOI:10.1071/CH03139

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2004, 57, 491–496
The Role of Hydrogen Bonding in the Intramolecular
Cyclization of Carbenes
David W. Jeffery,A Troy Lister,A and Rolf H. PragerA,B
A School of Chemistry, Physics and Earth Sciences, Flinders University,
Adelaide SA 5001, Australia.
B Author to whom correspondence should be addressed (e-mail: rolf.prager@flinders.edu.au).
The cyclization products from the photolysis of 3-arylamino-4-ethoxycarbonyl-2-heteroarylisoxazol-5(2H)-ones
are interpreted as arising from intramolecular cyclization of carbenes. The mode of cyclization appears to be
controlled by hydrogen bonding of the 3-NH group with either the 2-heteroaryl group or the 4-ethoxycarbonyl group.
Manuscript received: 23 May 2003.
Final version: 5 September 2003.
R
H
R
R
Introduction
R
R
N
N
ꢀ or hn
ꢁCO2
N
O
N
N
O
O
O
ꢂ
O
OH
O
We have reported a considerable number of photo-
lysis and pyrolysis reactions of isoxazol-5(2H)-ones 1
(Scheme 1),[1–3] in which the common intermediate is gener-
ally the imidoylcarbene 2. When a C(3) hydoxyl group[4,5] or
amino group[6] was present, however, the pathway was often
complicated by the presence of several tautomers, each of
which followed its own decomposition pathway, particularly
on photolysis.
Thus the presence of a 3-hydroxyl group led to separate
photolysis pathways from the three tautomers 3a–3c,[4,5] and
the 3-phenylamino analogue 4a gave a single photolysis prod-
uct via tautomer 4b (Scheme 2),[6] which, however, does not
involve a carbene intermediate.
O
O
O
HO
Ph
Ph
Ph
1
2
3a
3b
3c
Scheme 1.
H
N
N
O
O
NHCONHPh
CO2Et
NHPh
NHPh
OEt
hn, MeOH
O
O
H
MeO2C
CO2Et
O
4a
4b
The photolysis of N-heteroaryl derivatives of 4a was
intriguing in two ways. Firstly, photolysis now occurred
exclusively through the lactone form, resulting in carbene-
derived products. Secondly, the carbene intermediate was
captured either by the nitrogen atom of the 2-heteroaryl
group, to form an imidazopyridine,[7,8] or by the 3-arylamino
group, leading to an indole. However, the less nucleophilic
3-nitropyridyl derivative 5 gave exclusively the imidazo-
pyridine 6,[7] whereas the more nucleophilic isoquinolinyl
derivative 7 gave both imidazoquinoline 8 and indole 9 in
a 2 : 1 ratio (Scheme 3).[6] Since this distribution of prod-
ucts appeared counter-intuitive, we have investigated the
photolysis of this system in further detail.
Scheme 2.
methyl group at N(2). Methylation of 4a with base and
methyl iodide gave the same product as from the reaction
of N-methylhydroxylamine with the thioamide 10, namely
the 2-methylated material 11 (Scheme 4). This product gave
a single compound on photolysis, but the spectroscopic data
did not allow unequivocal assignment of structure. X-ray
analysis∗ showed the structure to be 12, which, like the prod-
uct from 4, is consistent with its origin from the 5-hydroxy
tautomer. As we were interested in the synthetic applications
of possible polycyclic heterocycles from this system, we have
now concentrated on the 2-aryl compounds.
While the isoxazolone 5 gave only the imidazopyridine 6
on photolysis (Scheme 5), we hoped that the presence of a
more electron-rich N-aryl group at C(3) would promote for-
mation of the indole. However, photolysis of isoxazolone 13
once again gave only the corresponding imidazopyridine 14.
Results and Discussion
Since isoxazolone 4a underwent photolysis as its hydroxy-
isoxazole tautomer 4b, and its 2-aryl derivative 5 as the
lactone tautomer, we first examined the effect of having a
∗ We thank Dr M. R. Taylor for determining the single crystal X-ray structure, which is unequivocal, but the quality of the crystal precludes publication
of the data.
© CSIRO 2004
10.1071/CH03139
0004-9425/04/050491

492
D. W. Jeffery, T. Lister and R. H. Prager
NO2
NO2
NO2
N
N
hn
N
CO2Et
ꢁCO2
N
N
O
N
CO2Et
NHPh
CO2Et
NHPh
NHPh
O
5
6
N
N
hn
ꢁCO2
NHPh
N
N
CO2Et
H
N
H
N
ꢂ
N
N
HN
O
N
NHPh
O
EtO2C
EtO2C
CO2Et
7
8
9
2 : 1
Scheme 3.
Me
N
Me
S
MeN
HO2C
N
MeN
HO2C
NHPh
CO2Et
O
O
NPh
NPh
CO2Et
NHPh
NPh
CO2Et
EtO2C
HO
O
CO2Et
CO2Et
10
11
O
NPh
CO2Et
O
NHMe
MeN
HO2C
O
MeN
O
MeHN
NPh
NPh
NPh
CO2Et
C
O
O
H
CO2Et
EtO2C
12
Scheme 4.
NO2
NO2
NO2
OMe
OMe
OMe
N
N
N
N
hn
H
N
N
N
OMe
O
O
ꢁCO2
N
N
OMe
H
H
O
O
O
O
CO2Et
EtO
EtO
OMe
13a
13b
OMe
OMe
N
H
N
OMe
N
O2N
N
N
CO2Et
OMe
H
O
N
14
O2N
OEt
Scheme 5.

Role of Hydrogen Bonding in Cyclization of Carbenes
493
O
O
Since electronic factors in the 3-arylamino group now
appeared to be only secondary, we considered the possibility
that hydrogen-bonding factors were paramount. If hydrogen
bonding in the reactants could be maintained throughout
the formation of the carbene intermediate, the formation
of the imidazole from the weakly hydrogen-bonding nitro-
pyridine group could be understood by the preference for the
conformation 13b.
H
H
H
N
N
N
Me
N
O
Me
O
Me
O
Me
CO2Et
CO2Et
O
CO2Et
CO2Et
15
(Z )-16
(E )-16
Scheme 6.
Since there is evidence to suggest that the carbene forma-
tion from an isoxazolone is a stepwise process initiated by
homolysis of the N O bond,[9] the initially formed carbene
is likely to be a triplet. While triplet carbonylcarbenes are
planar, the corresponding singlet carbenes are nonplanar,[10]
with nearly orthogonal RCCO dihedral angles. The latter
orientation allows overlap of the in-plane oxygen lone pair of
the carbonyl group with the empty p-orbital of the singlet car-
bene, as well as the π-bond of the carbonyl group to conjugate
with the filled carbene orbital.[10] Wang and co-workers have
shown that the singlet–triplet gap in aryl(methoxycarbonyl)
carbenes depends on the substituent in the aryl group, the
triplet being of lower energy with electron-withdrawing
groups and the singlet with electron-donating groups.[11] In
addition, the bond angle about the triplet carbene carbon is
larger than that of the singlet.[12] Polar solvents also favour
the singlet state.
We also have carried out severalAM1-level geometry opti-
mization calculations on the decomposition of the system 15
to (Z)-16 and to (E)-16,† representing a typical system in
which the carbene is captured by a nucleophile attached to
the nitrogen. While AM1 energies are unlikely to be very
accurate, they are adequate for our purpose here in rational-
izing the preference of different conformers.The calculations
are collected in Table 1, and the general situation is summa-
rized in Scheme 6.The initially formed triplet carbene (Z)-16
is lower in energy by 110 kJ mol−1 than its singlet state. On
the other hand, the energy of the triplet state of the carbene
(E)-16 is almost identical with that of triplet (Z)-16, and
hence there will be a minimal energy barrier to their intercon-
version. With the carbene (E)-16, however, the singlet state
is of considerably lower energy (by 177 kJ mol−1), and this
leads to essentially spontaneous cyclization of a nucleophile
with the carbene, Scheme 7. Consistent with the previously
cited calculations,[10–12] the triplet states of (Z)-16 and (E)-
16 have larger angles than the corresponding singlet states.
The ester carbonyl group in the initially formed triplet (Z)-
16 is not coplanar with the carbene, unlike its (E)-isomer,
although the ester and carbene groups are orthogonal in
singlet (Z)-16 as expected.[10] On the other hand, the confor-
mation of the singlet (E)-16 is essentially planar, due to the
overriding interaction of the formyl π-bond with the empty
carbene π-orbital.
Table 1. AM1-calculated parameters for carbene 16
(Z)-16 (E)-16
Triplet
Singlet
Triplet Singlet
Carbene angle [◦]
155.2
43.15
0
133.6 149.9
131.0
14.4
Dihedral angle 3-4-5-6 [◦]A
91.0
+110
9.6
+6
Energy [kJ mol−1 B
]
−171
A Atom 6 is the ester carbonyl group.
Relative values.
B
2
N
O
slow
N
N
N
3
ꢁCO2
4
O
fast
N
fast
N
N
Scheme 7.
O
O
H
H
H
N
N
N
NH2
O
O
NH2
NH2
3
4
CO2Et
O
6
O
5
CO2Et
EtO
17
(Z)-18
(E )-18
Scheme 8.
Table 2. AM1-calculated parameters for carbene 18
(Z)-18
(E)-18A
Triplet
Singlet Triplet Singlet
Carbene angle [◦]
150.1
43.5
Very large
124.6
84.2
3.76
146.1
176.5
2.39
134.6
177.3
Dihedral angle 3-4-5-6 [◦]
NH–ester distance [Å]
2.31
(C=O)
(OEt)
+7
(OEt)
−188
Energy [kJ mol−1 B
]
0
+105
A Lowest energy (E) conformation.
B Relative values.
We have also carried out similar calculations on the
carbenes derived from 3-aminoisoxazolones. In the simplest
case, that of the carbene 18 derived from the isoxazolone 17
(Scheme 8), the calculations are compiled in Table 2. The
amino group clearly does not influence the conformation of
the carbene (Z)-18 formed from it, and hydrogen bonding of
the NH with the ester group is precluded by the large car-
bene bond angles. However, after isomerization to (E)-18,
† The terms (Z) and (E) refer to the geometry about the C=N bond. While the designations to carbene 16 are not correct, we wish to enable subsequent
comparison with the carbenes 18, 19, 20, and 24.

494
D. W. Jeffery, T. Lister and R. H. Prager
NO2
NO2
NO2
N
NO2
NO2
N
N
N
N
N
N
N
H
hn
N
NHMe
N
N
3
5
O
O
N
N
ꢁCO2
CO2Et
4
MeHN
6
O
H
H
O
O
O
CO2Et
O
EtO
EtO
EtO
(Z)-19
(E)-19
5b
5a
Scheme 9.
N
Table 3. AM1-calculated parameters for carbene 19
(Z)-19 (E)-19
N
imidazopyridine
H
N
O
6
O2N
OEt
Triplet Singlet Triplet Singlet
Carbene angle [◦]
147.9
6.63
2.3
0
122.1 153.9
132.6
135.5
Scheme 10.
Dihedral angle 3-4-5-6 [◦]
NH–N(pyr) distance [Å]
100.5
2.3
28.9
4.75
+21
4.25A
Energy [kJ mol−1 B
]
+101
−146.1
5.8 ppm and the doublet of doublets due to H(5)ꢀ at 6.0 ppm,
considerably upfield from their usual positions and indicating
thatthequinolineanddimethoxybenzeneringswerenowcon-
strained to stacking. The isoxazolone 21[8] was photolyzed in
acetone at 300 nm to give a single product 22. No signifi-
cant quantity of 23 was isolated. This is of synthetic interest,
as it complements the base-catalyzed reaction of 21, which
leads only to 23.[8] We suspect that hydrogen bonding and
π-stacking in 21 might promote carbene formation from this
conformer, leading to (Z)-24 and (E)-24a and then rapidly to
pyrroloquinoline 22 (Scheme 12). Ab initio geometry calcu-
lations on the singlet carbene (E)-24 showed that regardless
of the extent of rotation about the C(3) NH bond, the carbene
would spontaneously collapse to the tautomer of 22, namely
22a, and not to the indole 23. This suggests that the carbene
cascade leading to the carbene (E)-24 and thus to 22 is faster
than the tautomerization of (E)-24 to (E)-25, which could
give the indole 23.
To further probe the importance of hydrogen bond-
ing in the choice of products, several attempts have been
made to synthesize other N-arylated derivatives, 26, of 4
(Scheme 13). The thioamide 10 and its S-methylated deriva-
tive were treated with phenylhydroxylamine under varying
conditions but failed to react. In an alternative approach,
phenylhydroxylamine was first treated with phenyliso-
thiocyanate to form the N-hydroxythiourea 27,[14] but all
attempts to react this with ethyl malonyl chloride to give
28 were unsuccessful. Other unsuccessful approaches to
obtain 26 included the treatment of 4 with 5-chloro-
1,3-dimethoxybenzene in the presence of lithium diiso-
propyl amide, with the diazonium salts from aniline or
3-methoxyaniline, or with 2,4-dinitrofluorobenzene. In addi-
tion, several attempts have been made to trap the intermedi-
ate carbene 24 with the highly reactive bicyclopropylidene
29,[15–17] but the (presumably) bimolecular reaction was
unable to compete with the intramolecular reactions of the
carbene, and the product composition was unchanged.
A N(pyr)–carbene 2.6 Å.
Relative values.
B
such hydrogen bonding is possible although unlikely to be
significant.
For the model carbene 19 (Scheme 9), the corresponding
values for the lowest-energy conformer are shown in Table 3.
In the carbene (Z)-19, hydrogen bonding of the amino NH
group and the pyridine N atom is made possible by the large
carbene bond angle; hydrogen bonding with the ester group
is precluded.
Ab initio geometry optimization calculations at the AM1
level (Gaussian98W) show that the hydrogen-bonded con-
formation of the ester for isoxazolone 5a is 35.9 kJ mol−1
lower in energy than the hydrogen-bonded conformation of
the pyridyl for 5b, Scheme 10. However, since this is not
maintained in the carbene, and pyridine N NH bonding
will be weak, the facile isomerization of the (Z)- to the
(E)-carbene occurs and leads to exclusive formation of the
imidazopyridine 6.
With 7, on the other hand, while the energy difference
between the quinolinyl hydrogen-bonded system in the
isoxazolone 7b and the ester hydrogen-bonded conformer
of the isoxazolone 7a is essentially the same as above
(36.1 kJ mol−1), the singlet carbene (Z)-20 is stabilized by
hydrogen bonding between the NH and the isoquinoline N
atom, and hence the singlet–triplet barrier for the carbene 20
is now significantly decreased, with respect to the isomeriza-
tion of the carbene (Z)-20 to (E)-20, allowing the formation
of some indole (Scheme 11).‡
Similar calculations for the N-(4-quinolinyl)isoxazolone
21 confirm that hydrogen bonding in the isoxazolone and
aromatic π-interactions[13] result in the conformation shown
(21) being preferred by 28.2 kJ mol−1. In support of this,
the H NMR spectrum of the isoxazolone 21[8] showed the
1
presence of a one-proton doublet of doublets due to H(6)ꢀ at
‡ At this stage we cannot rule out the possibility that the indole is the result of insertion of the carbene 20 into a C–H bond of the phenyl ring, but this would
appear to give no role to the N(2) aryl group.

Role of Hydrogen Bonding in Cyclization of Carbenes
495
N
N
hn
NH
N
imidazopyridine
O
N
ꢁCO2
N
O
H
O
O
OEt
EtO
7a
(E )-20
N
H
N
H
hn
indole
N
O
N
N
ꢁCO2
N
O
EtO2C
CO2Et
7b
(Z )-20
Scheme 11.
OMe
OMe
OMe
Me
N
Me
N
5؅
6؅
5؅
6؅
H
N
OMe
N
NH
N
NHAr
hn
NHAr
N
N
O
OMe
OMe
N
N
N
N
ꢁCO2
H
O
H
N
H
CO2Et
CO2Et
Me
O
Me
O
CO2Et
OEt
Me
EtO
21
(Z)-24
(E )-24
22a
22
OMe
OMe
H
N
H
N
N
HN
OMe
N
OMe
EtO2C
N
CO2Et
Me
Me
(E )-25
23
Scheme 12.
after formation of the carbene. But the 3-NH group does
continue to hydrogen bond with the 2-heteroaryl N-atom.
Combined with the effects of π–π stacking, this can direct the
conformationofthecarbeneanddriveitsmodeofcyclization.
R
Ph
N
Ph
N
NHPh
CO2Et
C
NHPh
N
NHPh
O
O
S
HO
O
O
S
CO2Et
Experimental
26
27
Scheme 13.
28
29
Melting points were determined on a Reichert hot stage microscope
and are uncorrected. 1H and 13C (300 and 75.5 MHz) NMR mea-
surements were recorded on a Gemini Varian 300 spectrometer in
deuteriochloroform with tetramethylsilane as internal standard, unless
otherwisestated. InfraredspectrawererecordedonaPerkin–Elmer1600
FT-IR spectrophotometer using fused sodium chloride cells, measured
as Nujol mulls or films. Electrospray ionization (ESI) mass spec-
tra were recorded at Monash University, Melbourne. The protonated
molecular ion (MH+) mass to charge ratio (m/z) is reported. Micro-
analyses were performed at the University of Otago, New Zealand.
Gas chromatography–mass spectrometry data was recorded on a Varian
Conclusions
2-Aryl-3-arylamino-4-ethoxycarbonylisoxazol-5(2H)-ones
undergo photolytic extrusion of CO2 to give carbenes which
insertintoeitherthe2-arylor3-arylaminogroups. Suchinser-
tions are not dictated by purely electronic factors. Hydrogen
bonding of 3-NHAr and the CO2Et groups is not maintained

496
D. W. Jeffery, T. Lister and R. H. Prager
Saturn 4D GC/MS/MS fitted with a Zebron 30 m × 0.25 mm ID 5%
phenyl polysiloxane column. X-ray crystal data were recorded by the
University of Canterbury, New Zealand, and the crystal structures
solved by Dr Max Taylor. AM1 calculations were carried out using
MacSpartan (15 and 16) or Gaussian 98W. Details are available in the
Accessory Materials.
7.95 (1H, d, J 7.2), 7.62 (1H, d, J 8.1), 7.38–7.27 (3H, m), 6.62 (1H, d,
J 2.4), 6.57 (1H, dd, J 8.4, 2.4), 4.46 (2H, q, J 7.1), 3.87 (3H, s), 3.85
(3H, s), 3.06 (3H, s), 1.49 (3H, t, J 7.1). δC (CDCl3/TFA) 165.0, 160.0,
154.7, 154.0, 147.9, 136.0, 133.0, 129.4, 126.9, 126.4, 121.8, 119.1,
118.5, 116.9, 113.0, 104.7, 100.0, 86.8, 60.6, 55.7, 55.5, 21.8, 14.4.
Attempts to Trap Photolysis Intermediates from 21
Ethyl 2-Methyl-5-oxo-3-phenylamino-2,5-dihydroisoxazole-
4-carboxylate 11
Amixtureofisoxazolone21(50 mg, 0.13 mmol)andbicyclopropylidene
(103 mg, 1.28 mmol) was photolyzed in anhydrous acetone (20 mL)
by sunlight through pyrex for 5 d. The solvent was removed to yield
a red solid. 1H NMR and TLC analysis indicated the presence of
21 and the pyrroloquinoline 22, and polymerization products of the
bicyclopropylidene.
To a stirred solution of isoxazolone 4a[6] (0.300 g, 1.21 mmol) in ace-
tone (10 mL) under an atmosphere of nitrogen was added triethylamine
(200 µL, 1.45 mmol). The solution was stirred for 10 min at room tem-
peraturebeforemethyliodide(300 µL, 4.83 mmol)wasaddeddropwise,
and the resulting solution was stirred for 67 h. The solvent was removed
in vacuo, yielding a yellow solid which was extracted into EtOAc
and washed with 1 M HCl (5 mL) and brine (3 × 50 mL). The organic
extracts were dried (MgSO4), the solvent removed, and the yellow solid
was recrystallized from ethanol to give the isoxazolone 11 as colour-
less crystals (0.201 g, 63%). mp 142–143◦C. (Found: C 59.2, H 5.4,
N 10.6%. C13H14N2O4 requires C 59.5, H 5.4, N 10.7%). vmax/cm−1
3416, 1756, 1673, 1582. δH 9.40 (1H, bs), 7.47–7.36 (2H, m), 7.32–7.20
(3H, m), 4.35 (2H, q, J 7.2), 3.02 (3H, s), 1.37 (3H, t, J 7.2). δC 167.9,
165.4, 164.9, 136.2, 129.9, 126.7, 122.7, 77.8, 60.6, 39.9, 14.4.
Alternative Preparation: Methylhydroxylamine hydrochloride
(570 mg, 6.8 mmol) in ethanol (40 mL) was neutralized with excess of
potassium carbonate, and the solution filtered. Diethyl phenylthiocar-
bamoylmalonate (1.0 g, 3.3 mmol)[18] was added to the filtrate, and the
solution refluxed for 20 h. The solvent was evaporated, water (20 mL)
and 3 M HCl (10 mL) were added, and the mixture was extracted with
ethylacetate.Thedriedextractwasevaporatedtoapaleyellowoil(65%),
which slowly crystallized. The crystals had identical spectral properties
to the sample above.
Accessory Materials
AM1-level ab initio calculations for 5a, (Z)-5a, 5b, (E)-5b,
7a, (Z)-7a, 7b, 13a, 13b, 21a, and 21b are available from
the author or, until May 2009, from the Australian Journal of
Chemistry.
Acknowledgments
The authors are grateful for financial support from the
Australian Research Council. We also acknowledge the
very generous gift of a sample of bicyclopropylidene from
Professor Armin De Meijere (Göttingen).
References
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Ethyl 1-Methyl-2,5-dioxo-3-phenyl-2,3,4,5-tetrahydroimidazole-
4-carboxylate 12
Isoxazolone 11 (0.070 g, 0.267 mmol) was dissolved in anhydrous ace-
tone (150 mL) and photolyzed at 300 nm for 2.5 h. The solvent was
removed in vacuo, yielding a yellow oil which was triturated with and
recrystallized from ethanol to give the title compound 12 as colour-
less crystals (0.013 g, 22%). mp 110–112◦C. (Found: C 58.9, H 5.4,
N 10.6%. C13H14N2O4 requires C 59.5, H 5.4, N 10.7%). vmax/cm−1
3400, 1774, 1742, 1716, 1599. δH 7.51–7.33 (4H, m), 7.24–7.14 (1H,
m), 5.13 (1H, s), 4.35–4.13 (2H, m), 3.14 (3H, s), 1.22 (3H, t, J 7.0). δC
164.9, 164.0, 153.0, 136.4, 129.4, 125.4, 119.9, 63.8, 63.2, 25.5, 13.9.
[10] Y. Wang, C. M. Hadad, T. P. Toscano, J. Am. Chem. Soc. 2002,
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Chem. 2000, 207, 89.
Photolysis of Isoxazolone 13
Isoxazolone 13[8] (100 mg) in acetonitrile (150 mL) was photolyzed at
300 nm under nitrogen for 1 h through pyrex. The solvent was removed
to yield a red solid, which was recrystallized from ethanol to give
ethyl 6-nitro-2-(2,4-dimethoxyphenyl)aminoimidazo[1,2-a]pyridine-3-
carboxylate 14 as red needles (61 mg, 68%). mp 220◦C (lit.[8] 218–
221◦C), identical with an authentic sample.[8]
Ethyl 4-Methyl-2-(2,4-dimethoxyphenyl)aminopyrrolo[2,3-c]-
quinoline-3-carboxylate 22
N-aryl isoxazolone 21[8] (0.100 g, 0.222 mmol) was photolyzed at
300 nm under a nitrogen atmosphere in acetone (150 mL) for 4 h. The
solvent was removed in vacuo to yield a brown solid, which was recrys-
tallized from ethanol to give the title compound as brown crystals
(0.061 g, 68%). mp 114–117◦C. (Found: [M + H]+ (ESI) 406.1768.
C23H24N3O+ requires 406.1766). vmax/cm−1 1643, 1601, 1558, 1419,
1349, 1234, 41204, 1075. δH (CDCl3/TFA) 11.44 (1H, s), 9.05 (1H, bs),
[16] A. de Meijere, S. I. Kozhushkov, Eur. J. Org. Chem. 2000,
3809. doi:10.1002/1099-0690(200012)2000:23<3809::AID-
EJOC3809>3.0.CO;2-X
[17] A. de Meijere, S. Kozhushkov, T. Spath, M. von Seebach,
S. Lohr, H. Nuske, T. Pohlmann, M. Es-Sayed, et al., Pure Appl.
Chem. 2000, 72, 1745.
[18] D. E. Worrall, J. Am. Chem. Soc. 1918, 40, 415.