Cationic carbenoid rearrangement of 2-phenyl substituted gem-dihalogenospiropentanes

Article abstract of DOI:10.1002/ejoc.201000427

The series of 2-phenyl- and 2,2-diphenyl gem-dihalogenospiropentanes were employed as model compounds to study the carbenoid rearrangement with the use of methyllithium. The scope and limitations of this skeletal rearrangement are outlined and its mechani

Full text of DOI:10.1002/ejoc.201000427

FULL PAPER
DOI: 10.1002/ejoc.201000427
Cationic Carbenoid Rearrangement of 2-Phenyl Substituted
gem-Dihalogenospiropentanes
Kseniya N. Sedenkova,[a] Elena B. Averina,*[a] Yuri K. Grishin,[a] Victor B. Rybakov,[a]
Tamara S. Kuznetzova,[a] and Nikolay S. Zefirov[a]
Keywords: Carbenoids / Carbocations / Rearrangement / Spiro compounds
The series of 2-phenyl- and 2,2-diphenyl gem-dihalogeno-
spiropentanes were employed as model compounds to study
the carbenoid rearrangement with the use of methyllithium.
The scope and limitations of this skeletal rearrangement are
outlined and its mechanism is discussed.
carbenoid intermediate as the only possible pathway of the
rearrangement of I into III–V. Aiming to find out the mech-
anistic reasons laying beyond this fact and taking into ac-
count the ability of phenyl substituents to stabilize the
neighboring carbocationic center, we synthesized a series
of 2-phenyl-substituted gem-dihalogenospiropentanes and
studied their reactions with methyllithium.
Introduction
Reaction of gem-dihalogenocyclopropanes with alkyllith-
ium reagents is known as a traditional way to obtain substi-
tuted allenes (Doering–Skattebol–Moore reaction).[1] How-
ever, we have recently reported that some gem-dihalogeno-
spiropentanes react with methyllithium in an unusual way:
lowering the reaction temperature prevents the formation
of allenes and leads to carbenoid skeletal rearrangement.[2]
The final steps of this reaction are quite variable and may
include halogenophilic processes,[3] insertion into a C–H
bond of the solvent,[4] and substitution of the halogen atom
for a methyl group.[5] The preferable reaction path is deter-
mined by the substituents in the spiropentane moiety as
well as by the nature of the halogen atoms (Scheme 1).
Results and Discussion
Model gem-dihalogenospiropentanes 1–9 were obtained
by cycloaddition of dihalogenocarbenes to alkylididenecy-
clopropanes, which were synthesized by a Wittig reaction[6]
(Table 1). We chose Makosza conditions[7] for the cyclo-
addition of mixed dihalogenocarbenes and Doering–
Hoffmann conditions[8] for the cycloaddition of dibromo-
carbene. In the synthesis of mixed bromochloro-substituted
Table 1. Synthesis of model gem-dihalogenospiropentanes.
Entry
gem-Dihalogenospiropentane
Isolated yields [%]
1
2
3
4
5
6
7
8
9
R1 = Ph, Hal = F
R1 = Ph, Hal = Cl
R1 = Ph, Hal = Br
R1 = Me, Hal = F
R1 = Me, Hal = Cl
R1 = Me, Hal = Br
R1 = H, Hal = F
R1 = H, Hal = Cl
R1 = H, Hal = Br
35[a]
61[b]
58[c]
39[a]
60[b]
53[c]
51[a]
53[b]
55[c]
Scheme 1. Reaction of gem-dihalogenospiropentanes with meth-
yllithium.
While studying a series of model gem-dihalogeno-substi-
tuted spiropentanes in the reaction with methyllithium, we
had to accept the carbocationic transformation of lithium
[a] Conditions: CHBr2F, dibenzo-18-crown-6, NaOH 50%,
CH2Cl2. [b] Conditions: CHBr2Cl, TEBAC (triethylbenzylammo-
nium chloride), NaOH 50%, CH2Cl2. [c] Conditions: CHBr3,
tBuOK, petroleum ether.
[a] Department of Chemistry, Moscow State University,
Leninskie Gory, 119992 Moscow, Russian Federation
Fax: +7-095-9390290
E-mail: elaver@org.chem.msu.ru
Eur. J. Org. Chem. 2010, 4145–4150
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. B. Averina et al.
FULL PAPER
spiropentanes, we used dibenzo-18-crown-6 as a phase-
transfer catalyst[9] to avoid a mixture of products.
gem-Dihalogenospiropentanes 1–9 were studied in the re-
action with methyllithium under appropriate conditions de-
pending on the halogen atoms. Previously, we showed that
the most favorable conditions for the rearrangement of
gem-dibromospiropentanes were –55 °C,[2,3] whereas bro-
mofluorospiropentanes reacted with methyllithium at 0 °C
or higher temperatures.[5] Bromochloro-substituted spiro-
pentanes 2, 5, and 8 were treated with methyllithium at
–65 °C.
We found that the reaction of 2,2-diphenylspiropentanes
1–3 with methyllithium led to monomeric (10) or dimeric
(11) products resulting from isomerization of the spiropen-
tane moiety into a four-membered ring followed by SE sub-
stitution and fusion of the cycles (Scheme 2). Their ratio
was found to depend dramatically on the nature of the halo-
gen atoms in the starting spiropentane. Treatment of bro-
mofluoride 1 with methyllithium afforded only monomeric
product 10, whereas the reaction of bromochloride 2 led to
dimeric product 11 exclusively, and dibromospiropentane 3
gave a mixture of both products in high total yield.
Scheme 3. Formation of tricyclic product 10.
Scheme 4. Formation of fused hexocyclic system 11.
Scheme 2. Reaction of 2,2-diphenyl gem-dihalogeno-substituted
spiropentanes with methyllithium.
Obviously, the formation of products 10 and 11 is a result
of intra- or intermolecular carbocationic cyclization with
the participation of the phenyl ring. The presence of two
phenyl rings at the 2-position to the carbenoid center is
essential for the formation of polycycles 10 and 11. We can
suppose that this cyclization proceeds by ortho-electrophilic
substitution in the phenyl ring, where the carbocationic cen-
ter of intermediate IX acts as the intramolecular electro-
phile. Double methylation of intermediate XI with meth-
yllithium (acting as nucleophilic methyl source) and methyl
bromide (as electrophilic methyl source) leads to compound
10 (Scheme 3).
The structure of compound 10 was determined by using
NMR spectroscopy (for NMR spectroscopic data see the
Experimental Section). In particular, NOE experiments
indicate the relative positions of the H atom and the methyl
groups in the internal five-membered cycle. NOE enhance-
ments for the H atom were observed after irradiation of
both methyl groups.
Figure 1. ORTEP-3[12] view of the molecular structure of 11 with
the atom numbering scheme. Displacement ellipsoids are drawn at
30% probability level. The hydrogen atoms are presented as small
circles of arbitrary radius.
Interaction of intermediate XI with carbocation IX re-
The NMR spectra also confirm the structure of com-
sults in the second act of electrophilic substitution, leading pound 11. The protons of the four-membered cycles give
to a seven-membered ring (Scheme 4). The structure of hex- two superimposed ABCDE spin systems. In accordance
ocyclic compound 11 was proven by single crystal X-ray with known data,[13] cyclobutane and methylenecyclobut-
analysis[10,11] (Figure 1).
ane fragments have characteristic differences in the values
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Eur. J. Org. Chem. 2010, 4145–4150

Carbenoid Rearrangement of Phenyl-Substituted gem-Dihalogenospiropentanes
2
4
of the J and J coupling constants (see the Experimental
In the case of monophenylspiropentanes 7–9 (the sub-
Section). The sign of long-range coupling constants was de- strates containing a phenyl group as the only 2-substituent),
termined by using the tickling double resonance technique. the sole isolated product was allene 13, that is, the product
As a whole, the scheme of the reactions of 2,2-diphenyl- of the Doering–Skattebol–Moore reaction[1] (Scheme 7).
gem-dihalogenospiropentanes described above involves a Though compound 13 was the main product of the reaction
number of cationic intermediates, some of them participat- of bromofluoride 7 with methyllithium, the yield was quite
ing in electrophilic ortho-substitution processes leading to low, which was probably caused by the instability of the
unique polycyclic structures.
resulting allene, which smoothly decomposes even when
The replacement of one of the phenyl substituents for a stored at low temperature. For dihalogenides 8 and 9 we
methyl group significantly changes the reaction pathway. observed NMR signals of some rearrangement products
Thus, 2-methyl-2-phenyl-substituted spiropentanes 4–6 re- and the mixture of unidentified compounds, which could
acted with methyllithium in quite a different way. For all be a result of their further transformation. We did not iso-
the halogenides, no electrophilic substitution pathway was late any rearrangement products and only allene 13 was ob-
observed, and the main product of the reaction was cyclo- tained in low yields in these reactions. The lower stability
butene 12, a usual product of dihalogenospiropentane–cy- of the rearrangement products may be related to the pos-
clobutene rearrangement (compare III, Scheme 1), with the sibility of side reactions of the intermediates, containing a
exception of two methyl groups replacing halogen atoms proton in the 2-position, in the presence of methyllithium.
(Scheme 5). In the case of dibromide 6, product 12 was ob-
tained as a complex mixture of unidentified compounds
that made the product difficult to isolate. For dihalogenides
4 and 5, the yields of product 12 were high enough (over
70% in the case of dibromide 4) before purification, but we
observed partial decomposition of 12 during purification
by column chromatography.
Scheme 7. Reaction of 2-phenyl-substituted gem-dihalogenospiro-
pentanes with methyllithium.
Conclusions
Thus, significant progress was made in our studying of
Scheme 5. Reaction of 2-methyl-2-phenyl-substituted gem-dihalo-
the unusual dihalogenospiropentane rearrangement. We
genospiropentanes with methyllithium. Yields of 12 refer to iso-
have shown that the incorporation of two phenyl rings in
lated product.
the 2-position of the starting gem-dihalogenospiropentanes
The presence of methyl groups in cyclobutene 12 should
be the result of two subsequent acts of substitution, the
first one being the electrophilic substitution at the saturated
carbon atom in methyllithium (intermediate XVII as the
electrophilic reagent, Scheme 6) and the second step being
nucleophilic substitution reaction in methyl bromide (inter-
mediate XIX as nucleophilic reagent).
led to the formation of polycyclic fused compounds in the
reactions with methyllithium. The products obtained by this
route unambiguously confirmed the electrophilic nature of
the spiropentyllithium carbenoids and a carbocationic-like
mechanism of the studied rearrangement.
Experimental Section
General Remarks: NMR spectra were recorded with a Bruker
Avance-400 spectrometer; chemical shifts δ were measured with ref-
erence to the solvent (1H: CDCl3, δ = 7.24 ppm; 13C: NMR CDCl3,
δ = 77.13 ppm). Mass spectra were recorded with a Finnigan MAT
95 XL spectrometer (70 eV) by using electron impact ionization
(EI) and GC–MS coupling. MALDI-TOF spectra were recorded
in positive mode by using a Bruker Ultraflex mass spectrometer
with dithranol as a matrix. Accurate mass measurements (HRMS)
were carried out by using a Jeol GCMate II mass spectrometer
(70 eV). Microanalyses were performed with a Carlo Erba 1106
instrument. Infrared spectra were recorded with a Thermo Nicolet
FTIR-200 spectrometer. Analytical thin-layer chromatography was
carried out with Silufol silica gel plates (supported on aluminum);
detection was done by UV lamp (254 and 365 nm) and chemical
staining (iodine vapor). Column chromatography was performed
on silica gel 60 (230–400 mesh, Merck). Starting compounds 1,1Ј-
(cyclopropylidenemethylene)dibenzene, (1-cyclopropylideneethyl)-
Scheme 6. Formation of product 12.
The ability of intermediates IX and XVII to act as elec-
trophiles can be explained by their comparatively long life-
times resulting from the additional stabilization of the cat-
ionic center by the conjugated phenyl groups.
Eur. J. Org. Chem. 2010, 4145–4150
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4147

E. B. Averina et al.
FULL PAPER
benzene, and (cyclopropylidenemethyl)benzene were synthesized by
a known procedure.[6] All other starting materials were commer-
cially available. All reagents except commercial products of satisfac-
tory quality were purified by literature procedures prior to use.
8.98 (1JC,H = 163 Hz, CH2, B), 11.35 (1JC,H = 164 Hz, CH2, A),
26.78 (2JC,F = 8.6 Hz, Cspiro), 26.95 (2JC,F = 8.7 Hz, Cspiro), 34.94
2
(1JC,H = 161 Hz, JC,F = 11.7 Hz, CH, B), 37.71 (1JC,H = 163 Hz,
2JC,F = 11.0 Hz, CH, A), 86.29 (1JC,F = 311 Hz, CBrF, A), 92.43
(1JC,F = 310 Hz, CBrF, B), 127.16 (1JC,H = 160 Hz, CH, Ph, A),
127.30 (1JC,H = 160 Hz, CH, Ph, B), 128.22 (1JC,H = 160 Hz, 2 CH,
Ph), 128.43 (1JC,H = 160 Hz, 2 CH, Ph), 128.59 (1JC,H = 159 Hz, 2
CH, Ph), 128.78 (1JC,H = 159 Hz, 2 CH, Ph), 134.35 (C, Ph, A),
General Procedure for the Addition of Bromofluorocarbene to Alk-
enes: A 50% aqueous solution of NaOH (22 mL) was added drop-
wise to a stirred mixture of the corresponding alkene (11.0 mmol),
CHBr2F (2.53 g, 13.2 mmol), and TEBAC (0.1 g, 0.3 mmol) in
dichloromethane (22 mL) at 0 °C under an atmosphere of argon.
The reaction mixture was warmed up to room temperature and
stirred for 24 h. Then, it was treated with ice (20 g). The organic
phase was separated, and the water phase was extracted with
dichloromethane (3ϫ10 mL). The combined organic fractions
were washed with water (50 mL) and dried with MgSO4. The sol-
vent was evaporated in vacuo; the residue was purified by prepara-
tive column chromatography.
135.79 (C, Ph, B) ppm. IR (film): ν = 3095, 3072, 3041, 3013, 1609,
˜
1528, 1499, 1459, 1348, 1188, 1136, 1100, 935, 870, 704, 648, 598,
526 cm–1. C11H10BrF (241.10): calcd. C 54.80, H 4.18; found C
54.71, H 4.10.
General Procedure for the Addition of Bromochlorocarbene to Alk-
enes: A 50% aqueous solution of NaOH (22 mL) was added drop-
wise to a stirred mixture of the corresponding alkene (11.0 mmol),
CHBr2Cl (2.74 g, 13.1 mmol), and dibenzo-18-crown-6 (0.1 g,
0.3 mmol) in dichloromethane (22 mL) at 0 °C under an atmo-
sphere of argon. The reaction mixture was warmed up to room
temperature and stirred for 24 h. Then it was treated with ice (20 g).
The organic phase was separated, and the water phase was ex-
tracted with dichloromethane (3ϫ10 mL). The combined organic
fractions were washed with water (50 mL) and dried with MgSO4.
The solvent was evaporated in vacuo; the residue was purified by
preparative column chromatography.
1-Bromo-1-fluoro-2,2-diphenylspiro[2.2]pentane (1):[5] Yield: 35%
(1.22 g), colorless solid, m.p. 106–108 °C, Rf = 0.2 (petroleum
1
ether). H NMR (400.1 MHz, CDCl3, 25 °C): δ = 1.30 (ddd, 2JH,H
3
3
= 5.1 Hz, JH,H = 6.7 Hz, JH,H = 9.5 Hz, 1 H, CH2), 1.45 (ddd,
2JH,H = 5.0 Hz, JH,H = 6.4 Hz, JH,H = 9.5 Hz, 1 H, CH2), 1.52
(dddd, JH,H = 5.0 Hz, JH,H = 6.7 Hz, JH,H = 9.1 Hz, JH,F
3
3
2
3
3
3
=
2
3
0.9 Hz, 1 H, CH2), 1.72 (dddd, JH,H = 5.1 Hz, JH,H = 6.4 Hz,
3JH,H = 9.1 Hz, JH,F = 9.0 Hz, 1 H, CH2), 7.24–7.38 (m, 10 H,
3
Ph) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C): δ = 8.3 (3JC,F
4 Hz, CH2), 11.3 (CH2), 32.0 (2JC,F = 9 Hz, Cspiro), 43.0 (2JC,F
=
=
1-Bromo-1-chloro-2,2-diphenylspiro[2.2]pentane (2): Yield: 61%
(2.23 g), colorless solid, m.p. 95–96 °C, Rf = 0.2 (petroleum ether).
10 Hz, 1 C), 93.3 (1JC,F = 312 Hz, CBrF), 127.2 (CH, Ph), 127.2
(CH, Ph), 128.2 (2 CH, Ph), 128.5 (2 CH, Ph), 129.3 (2 CH, Ph),
129.5 (2 CH, Ph), 139.0 (C, Ph), 141.0 (3JC,F = 4 Hz, 1 C, Ph) ppm.
1H NMR (400.1 MHz, CDCl3, 25 °C): δ = 1.35 (ddd, JH,H
=
2
4.6 Hz, 3JH,H = 6.1 Hz, 3JH,H = 9.2 Hz, 1 H, CH2), 1.45 (ddd, 2JH,H
3
3
= 4.8 Hz, JH,H = 6.1 Hz, JH,H = 9.2 Hz, 1 H, CH2), 1.74 (ddd,
3
3
IR (nujol): ν = 3068, 3040, 2970, 2940, 2860, 1600, 1500, 1470,
2JH,H = 4.8 Hz, JH,H = 6.1 Hz, JH,H = 9.2 Hz, 1 H, CH2), 1.80
˜
1390, 1180, 1060, 715, 650, 605 cm–1. MS (EI, 70 eV): m/z (%) = (ddd, JH,H = 4.6 Hz, JH,H = 6.1 Hz, JH,H = 9.2 Hz, 1 H, CH2),
2
3
3
318 (12) [M]+, 316 (12) [M]+, 237 (100) [M – Br]+, 222 (29), 203
(35), 159 (32), 115 (19), 109 (21), 91 (43). C17H14BrF (317.20): 7.50 (4 H, 4 CH, Ph) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C):
7.26–7.31 (m, 2 H, 2 CH, Ph), 7.34–7.39 (m, 4 H, 4 CH, Ph), 7.45–
calcd. C 64.37, H 4.45; found C 64.52, H 4.60.
δ = 10.67 (CH2), 12.41 (CH2), 36.15 (Cspiro), 44.62 (CPh2), 60.0
(CBrCl), 127.09 (CH, Ph), 127.15 (CH, Ph), 128.16 (2 CH, Ph),
128.24 (2 CH, Ph), 129.34 (2 CH, Ph), 129.46 (2 CH, Ph), 104.44
1-Bromo-1-fluoro-2-methyl-2-phenylspiro[2.2]pentane (4):[5] Yield:
39% (1.09 g), colorless liquid, Rf = 0.4 (petroleum ether). Two iso-
(C, Ph), 141.87 (C, Ph) ppm. IR (nujol): ν = 3092, 3061, 3029, 2975,
˜
1
mers, A/B = 1:1. H NMR (400.1 MHz, CDCl3, 25 °C): δ = 1.10
2947, 2870, 1598, 1582, 1495, 1451, 1382, 1152, 1089, 1040, 818,
720, 622, 546 cm–1. C17H14BrCl (333.65): calcd. C 61.20, H 4.23;
found C 61.27, H 4.23.
2
3
3
(ddd, JH,H = 4.6 Hz, JH,H = 6.2 Hz, JH,H = 9.3 Hz, 1 H, CH2),
1.21–1.44 (m, 4 H, CH2), 1.54 (ddd, 2JH,H = 5.1 Hz, 3JH,H = 6.2 Hz,
3JH,H = 9.3 Hz, 1 H, CH2), 1.59–1.69 (m, 2 H, CH2), 1.70 (d, 4JH,F
= 1.9 Hz, 3 H, CH3), 1.72 (d, 4JH,F = 2.4 Hz, 3 H, CH3), 7.35–7.48 1-Bromo-1-chloro-2-methyl-2-phenylspiro[2.2]pentane (5): Yield:
(m, 10 H, CH, Ph) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C): δ 60% (1.79 g), colorless liquid, Rf = 0.5 (petroleum ether). Two iso-
= 6.21 (3JC,F = 3.8 Hz, CH2), 8.91 (CH3), 9.01 (3JC,F = 3.5 Hz, mers, A/B = 1:1. H NMR (400.1 MHz, CDCl3, 25 °C): δ = 1.10
1
CH2), 10.88 (CH3), 20.41 (3JC,F = 5.2 Hz, CH2), 24.90 (3JC,F
=
(ddd, JH,H = 4.7 Hz, JH,H = 6.2 Hz, JH,H = 9.5 Hz, 1 H, CH2),
2
3
3
3.6 Hz, CH2), 30.90 (2JC,F = 8.1 Hz, 1 C), 31.04 (2JC,F = 9.6 Hz, 1
C), 34.54 (2JC,F = 9.9 Hz, 1 C), 35.43 (2JC,F = 10.9 Hz, 1 C), 93.98
1.26–1.34 (m, 3 H, CH2), 1.34 (ddd, 2JH,H = 5.0 Hz, 3JH,H = 6.2 Hz,
3JH,H = 9.7 Hz, 1 H, CH2), 1.45 (ddd, JH,H = 5.2 Hz, JH,H
=
2
3
(1JC,F = 314 Hz, CBrF), 94.86 (1JC,F = 308 Hz, CBrF), 127.07 (2 6.5 Hz, JH,H = 9.7 Hz, 1 H, CH2), 1.74 (s, 3 H, CH3), 1.77 (s, 3
3
CH, Ph), 128.25 (2 CH, Ph), 128.32 (2 CH, Ph), 128.36 (2 CH, H, CH3), 1.74–1.81 (m, 2 H, CH2), 7.32–7.45 (m, 10 H, CH,
Ph), 128.42 (2 CH, Ph), 138.89 (C, Ph), 141.90 (C, Ph) ppm. IR
Ph) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C): δ = 8.64 (1JC,H
=
(film): ν = 3070, 3030, 3000, 2930, 2880, 1600, 1500, 1450, 1380,
164 Hz, CH2), 10.19 (1JC,H = 162 Hz, CH2), 11.23 (1JC,H = 164 Hz,
CH2), 12.62 (1JC,H = 164 Hz, CH2), 23.93 (1JC,H = 129 Hz, CH3),
26.58 (1JC,H = 129 Hz, CH3), 35.14 (C), 35.30 (C), 36.75 (C), 36.82
(C), 60.97 (CBrCl), 61.46 (CBrCl), 127.09 (1JC,H = 158 Hz, CH,
Ph), 127.12 (1JC,H = 158 Hz, CH, Ph), 128.19 (1JC,H = 156 Hz, 2
˜
1190, 1100, 1005, 980, 875, 780, 710, 640 cm–1. C12H12BrF (255.13):
calcd. C 56.49, H 4.74; found C 56.70, H 4.73.
1-Bromo-1-fluoro-2-phenylspiro[2.2]pentane (7): Yield: 51% (1.35 g),
colorless liquid, Rf = 0.5 (petroleum ether). Two isomers, A/B 1:0.9.
1H NMR (400.1 MHz, CDCl3, 25 °C): δ = 1.19–1.30 (m, 3 H, CH2,
CH, Ph), 128.23 (1JC,H = 156 Hz, 2 CH, Ph), 128.41 (1JC,H
=
158 Hz, 2 CH, Ph), 128.50 (1JC,H = 158 Hz, 2 CH, Ph), 140.86 (C,
2
A, B), 1.31–1.42 (m, 3 H, CH2, A, B), 1.48 (dddd, JH,H = 3.6 Hz,
Ph), 142.61 (C, Ph) ppm. IR (film): ν = 3065, 3036, 3000, 2971,
˜
3JH,H = 5.6 Hz, 3JH,H = 9.4 Hz, 4JH,F = 0.6 Hz, 1 H, CH2, B), 1.55
2934, 2878, 1605, 1498, 1449, 1381, 1143, 1086, 1034, 1008, 945,
824, 765, 703, 602 cm–1. C12H12BrCl (271.58): calcd. C 53.07, H
4.45; found C 53.24, H 4.58.
2
3
3
4
(dddd, JH,H = 4.8 Hz, JH,H = 6.3 Hz, JH,H = 9.2 Hz, JH,F
=
=
3
0.8 Hz, 1 H, CH2, A), 3.02 (s, 1 H, CH, A), 3.10 (d, JH,F
13.6 Hz, 1 H, CH, B), 7.26–7.38 (m, 10 H, 5 CH, Ph, A, 5 CH,
Ph, B) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C): δ = 7.79 (1JC,H
1-Bromo-1-chloro-2-phenylspiro[2.2]pentane (8): Yield: 53%
3
= 165 Hz, CH2, A), 8.45 (1JC,H = 163, JC,F = 3.5 Hz, CH2, B), (1.50 g), colorless liquid, Rf = 0.5 (petroleum ether). Two isomers,
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Carbenoid Rearrangement of Phenyl-Substituted gem-Dihalogenospiropentanes
1
A/B = 1:0.8. H NMR (400.1 MHz, CDCl3, 25 °C): δ = 1.30–1.44
(m, 2 H, CH2, A, 2 H, CH2, B), 1.46–1.59 (m, 2 H, CH2, A, 2 H,
CH2, B), 3.15 (s, 1 H, CH, B), 3.30 (s, 1 H, CH, A), 7.35–7.45 (m,
5 H, 5 CH, Ph, A, 5 H, 5 CH, Ph, B) ppm. 13C NMR (100.6 MHz,
General Procedure for the Reaction of gem-Dihalogenospiropentanes
with Methyllithium: A solution of 1.5  methyllithium (6.0 mmol,
4.0 mL for dibromospiropentanes and bromochlorospiropentanes;
8.0 mol, 5.3 mL for bromofluorospiropentanes) in ether was added
dropwise to a solution of dihalogenospiropentane (4.0 mmol) in
absolute ether (10 mL) under an atmosphere of argon at low tem-
perature (–65 °C for bromochlorospiropentanes, –55 °C for dibro-
CDCl3, 25 °C): δ = 9.91 (1JC,H = 165 Hz, CH2, A), 10.51 (1JC,H
165 Hz, CH2, B), 10.94 (1JC,H = 165 Hz, CH2, B), 12.20 (1JC,H
166 Hz, CH2, A), 31.06 (Cspiro, A), 31.19 (Cspiro, B), 39.30 (1JC,H
=
=
=
164 Hz, CH, B), 39.94 (1JC,H = 164 Hz, CH, A), 52.97 (CBrCl), mospiropentanes, 0 °C for bromofluorospiropentanes). The reac-
56.13 (CBrCl), 127.41 (1JC,H = 160 Hz, CH, Ph, A), 127.46 (1JC,H tion mixture was stirred (1.5 h for dibromospiropentanes, 10 h for
= 160 Hz, CH, Ph, B), 128.06 (1JC,H = 160 Hz, 2 CH, Ph, B), bromochlorospiropentanes, 5 h for bromofluorospiropentanes).
128.12 (1JC,H = 160 Hz, 2 CH, Ph, A), 129.05 (1JC,H = 161 Hz, 2
Then it was warmed up to 0 °C and quenched with an equal
CH, Ph, B), 129.13 (1JC,H = 165 Hz, 2 CH, Ph, A), 135.09 (C, Ph, amount of cold water. The organic phase was separated, and the
A), 136.09 (C, Ph, B) ppm. IR (film): ν = 3070, 3039, 3008, 2935,
water phase was extracted with diethyl ether (3ϫ3 mL). The com-
˜
2867, 1604, 1499, 1459, 1158, 1069, 1040, 960, 891, 830, 781, 749,
bined organic fractions were washed with water (5 mL) and dried
700, 615, 567, 556 cm–1. C11H10BrCl (257.55): calcd. C 51.30, H with MgSO4. The solvent was evaporated; the products were iso-
3.91; found C 51.41, H 4.16.
lated by preparative column chromatography.
General Procedure for the Addition of Dibromocarbene to Alkenes:
A solution of bromoform (3.34 g, 1.16 mL, 13.2 mmol) in petro-
leum ether (3 mL) was added dropwise to a stirred mixture of the
corresponding alkene (11.0 mmol) and tBuOK (1.95 g, 26.4 mmol)
in petroleum ether (10 mL) at 0 °C under an atmosphere of argon.
After 20 min the resulting mixture was slowly warmed up to room
temperature and then stirred for 48 h. Then it was quenched with
cold water (10 mL). The organic phase was separated, and the
water phase was extracted with diethyl ether (3ϫ10 mL). The com-
bined organic layers were dried with anhydrous MgSO4. The sol-
vent was evaporated in vacuo; the residue was purified by prepara-
tive column chromatography.
2a,7-Dimethyl-7-phenyl-2,2a,7,7a-tetrahydro-1H-cyclobuta[a]-
indene (10):[5] Yield: 51% (0.50 g) from bromofluorospiropentane
4, 32% (0.31 g) from dibromospiropentane 6. Colorless oil, Rf =
1
0.2 (petroleum ether). H NMR (400.1 MHz, C6D6, 25 °C), cyclo-
3
butane protons (ABCDE system): δ = 1.02 (dddd, JA,B = 2.2 Hz,
3
4
2JA,C = –11.9 Hz, JA,D = 9.0 Hz, JA,E = 3.5 Hz, 1 H, HA), 1.35
1,1-Dibromo-2,2-diphenylspiro[2.2]pentane (3): Yield: 58% (2.41 g),
colorless solid, m.p. 126–127 °C (petroleum ether). 1H NMR
(400.1 MHz, CDCl3, 25 °C): δ = 1.35–1.39 (m, 2 H, CH2), 1.83–
1.86 (m, 2 H, CH2), 7.23–7.28 (m, 2 H, 2 CH, Ph), 7.30–7.36 (m,
4 H, 4 CH, Ph), 7.43–7.47 (m, 4 H, 4 CH, Ph) ppm. 13C NMR
(100.6 MHz, CDCl3, 25 °C): δ = 13.02 (2 CH2), 37.00 (Cspiro), 44.51
(CBr2), 46.29 (CPh2), 127.17 (2 CH, Ph), 128.24 (4 CH, Ph), 129.39
3
3
(s, 3 H, CH3), 1.41 (s, 3 H, CH3), 1.70 (dddd, JA,B = 2.2 Hz, JB,C
= 8.9 Hz, JB,D = –11.1 Hz, JB,E = 1.6 Hz, 1 H, HB), 2.15 (m,
2
3
3
3
4
2JA,C = –11.2 Hz, JB,C = 8.9 Hz, JC,D = 10.2 Hz, JC,E = 0.5 Hz,
1 H, HC), 2.32 (m, JA,D = 9.0 Hz, JBD = –11.05 Hz, JC,D
=
2
3
10.2 Hz, JD,E = 8.0 Hz, 1 H, HD), 3.24 (m, JA,E = 3.5 Hz, JB,E
3
4
3
= 1.6 Hz, JC,E = 0.5 Hz, JD,E = 8.0 Hz, 1 H, HE) ppm; aromatic
protons: δ = 7.18–7.24 (m, 1 H, Ph), 7.25–7.38 (m, 6 H, Ph), 7.42–
7.48 (m, 2 H, Ph) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C): δ
= 18.6 (1JC,H = 125 Hz, CH3), 23.0 (1JC,H = 138 Hz, CH2), 26.2
4
3
(4 CH, Ph), 141.78 (2 C, Ph) ppm. IR (nujol): ν = 3070, 3045, 3014,
˜
2936, 2869, 1601, 1498, 1450, 1382, 1149, 1067, 1039, 978, 845,
780, 716, 626, 611, 542 cm–1. C17H14Br2 (378.10): calcd. C 54.00,
H 3.73; found C 54.11, H 3.65.
(1JC,H = 128 Hz, CH3), 28.9 (1JC,H = 138 Hz, CH2), 48.4 (1JC,H
=
138 Hz, CH), 52.3 (C), 57.9 (C), 125.5 (CH, Ph), 125.8 (CH, Ph),
125.9 (CH, Ph), 126.5 (CH, Ph), 127.0 (CH, Ph), 127.6 (2 CH, Ph),
127.7 (2 CH, Ph), 143.3 (C, Ph), 146.7 (C, Ph), 151.5 (C, Ph) ppm.
1,1-Dibromo-2-methyl-2-phenylspiro[2.2]pentane (6): Yield: 53%
(1.84 g), colorless solid, m.p. 97 °C, b.p. 63–65 °C/2 Torr. 1H NMR
2
3
(400.1 MHz, CDCl3, 25 °C): δ = 1.16 (ddd, JH,H = 4.6 Hz, JH,H
IR (film): ν = 3020, 2985, 2950, 2925, 2870, 1600, 1525, 1500, 1470,
˜
3
2
1430, 1380, 1125, 1080, 940, 772, 750, 730, 675 cm–1. MS (EI,
70 eV): m/z (%) = 248 (26) [M]+, 233 (15), 220 (54), 205 (100), 192
(20), 178 (15), 143 (26), 128 (13), 115 (9), 91 (14), 77 (4). C19H20
(248.36): calcd. C 91.88, H 8.12; found C 91.63, H 8.34.
= 6.3 Hz, JH,H = 9.4 Hz, 1 H, CH2), 1.32 (ddd, JH,H = 4.6 Hz,
3JH,H = 6.7 Hz, JH,H = 9.1 Hz, 1 H, CH2), 1.40 (ddd, JH,H
=
3
2
3
3
5.0 Hz, JH,H = 6.7 Hz, JH,H = 9.4 Hz, 1 H, CH2), 1.72 (s, 3 H,
CH3), 1.82 (ddd, 2JH,H = 5.0 Hz, 3JH,H = 6.3 Hz, 3JH,H = 9.1 Hz, 1
H, CH2), 7.27–7.40 (m, 5 H, CH, Ph) ppm. 13C NMR (100.6 MHz,
CDCl3, 25 °C): δ = 10.75 (CH2), 13.18 (CH2), 26.35 (CH3), 36.05
(C), 36.58 (C), 47.98 (C), 127.05 (CH, Ph), 128.11 (2 CH, Ph),
128.33 (2 CH, Ph), 142.34 (C, Ph) ppm. C12H12Br2 (316.03): calcd.
C 45.61, H 3.83; found C 45.52, H 3.90.
5,13b-Diphenyl-7,7a,8,9,9a,13b-hexahydro-6H-dibenzo[a,h]dicyclo-
buta[c,e]azulene (11): Yield: 69% (0.60 g) from bromochlorospiro-
pentane 5, 65% (0.57 g) from dibromospiropentane 6, colorless
crystals (prism), m.p. 201 °C (MeOH/CHCl3). 1H NMR
(400.1 MHz, C6D6, 25 °C, cyclobutane protons, ABCDE system):
δ = 1.65 (m, 2JA,B = –10.8 Hz, 3JA,C = 1.6 Hz, 3JA,D = 8.2 Hz, 3JA,E
1,1-Dibromo-2-phenylspiro[2.2]pentane (9): Yield: 55% (1.82 g), col-
orless liquid, Rf = 0.3 (petroleum ether). 1H NMR (400.1 MHz,
CDCl3, 25 °C): δ = 1.36–1.41 (m, 2 H, CH2), 1.43–1.49 (m, 1 H,
CH2), 1.56–1.62 (m, 1 H, CH2), 3.25 (s, 1 H, CH), 7.33–7.41 (m, 5
2
3
= 1.2 Hz, 1 H, A = H21A), 2.14 (m, JA,B = –10.8 Hz, JB,C
=
8.1 Hz, JB,D = 10.9 Hz, JB,E = 7.7 Hz, 1 H, B = H21B), 2.45 (m,
3
3
3JA,C = 1.6 Hz, JB,C = 8.1 Hz, JC,D = –11.8 Hz, JC,E = 4.2 Hz,
3
2
4
H, 5 CH, Ph) ppm. 13C NMR (100.6 MHz, CDCl3, 25 °C): δ = 1 H, C = H22A), 2.80 (m, JA,D = 8.2 Hz, JB,D = 10.9 Hz, JC,D
=
3
3
2
11.86 (CH2), 12.80 (CH2), 31.97 (Cspiro), 39.92 (CH), 39.96 (CBr2), –11.8 Hz, 4JD,E = 0.2 Hz, 1 H, D = H22B), 3.41 (m, JA,E = 1.2 Hz,
3
127.54 (CH, Ph), 128.16 (2 CH, Ph), 129.15 (2 CH, Ph), 136.27 (C,
3JB,E = 7.7 Hz, 4JC,E = 4.2 Hz, 4JD,E = 0.2 Hz, 1 H, E = H20) ppm;
Ph) ppm. IR (film): ν = 3068, 3037, 3005, 2968, 2935, 2863, 1604, methylenecyclobutane protons (ABCDE system) δ = 1.65 (m, 2JA,B
˜
3
3
3
1496, 1457, 1381, 1117, 1061, 1030, 953, 822, 770, 746, 731, 699,
675, 620, 609, 550 cm–1. C11H10Br2 (302.01): calcd. C 43.75, H 3.34;
found C 43.52, H 3.45.
= –11.8 Hz, JA,C = 9.7 Hz, JA,D = 3.9 Hz, JA,E = 4.0 Hz, 1 H,
A = H3A), 2.14 (m, JA,B = –11.8 Hz, JB,C = 9.1 Hz, JB,D
=
2
3
3
9.4 Hz, JB,E = 9.0 Hz, 1 H, B = H3B), 2.45 (m, JA,C = 9.7 Hz,
3
3
Eur. J. Org. Chem. 2010, 4145–4150
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4149

E. B. Averina et al.
FULL PAPER
3JB,C = 9.1 Hz, JC,D = –17.0 Hz, JC,E = –4.5 Hz, 1 H, C = H4A), 103 (5), 102 (22), 91 (3), 89 (4), 77 (3), 76 (3), 63 (6), 51 (4). HRMS
2
4
3
3
2
2.80 (m, JA,D = 3.9 Hz, JB,D = 9.4 Hz, JC,D = –17.0 Hz, 4JD,E
=
(EI): calcd. for C11H10 142.0783; found 142.0758.
1.6 Hz, 1 H, D = H4B), 3.41 (m, JA,E = 4.0 Hz, JB,E = 9.0 Hz,
4JC,E = –4.5 Hz, 4JD,E = 1.6 Hz, 1 H, E = H2) ppm; phenyl groups:
δ = 6.51–6.56 (m, 2 H), 7.04–7.07 (m, 3 H), 6.96 (tt, 3JH,H = 7.3 Hz,
4JH,H = 1.2 Hz, 1H, p-H), 7.08–7.13 (m, 2 H, m-H), 7.19–7.23 (2
3
3
Acknowledgments
We thank the Russian Foundation for Basic Research (Project 09-
03-00717-a) for financial support of this work.
3
4
H, o-H); benzyl groups: 6.49 (dd, JH,H = 7.8 Hz, JH,H = 1.4 Hz,
3
4
1 H), 6.77 (dd, JH,H = 7.5 Hz, JH,H = 1.4 Hz, 1 H), 6.91 (ddd,
3
4
3JH,H = 7.5 Hz, JH,H = 7.8 Hz, JH,H = 1.5 Hz, 1 H), 7.04 (ddd,
3
4
3JH,H = 7.5 Hz, JH,H = 7.8 Hz, JH,H = 1.4 Hz, 1 H), 7.28–7.30
(m, 1 H), 7.32–7.35 (m, 2 H), 7.42 (br. d, 3JH,H = 7.6 Hz, 1 H) ppm.
13C NMR (100.6 MHz, CDCl3, 25 °C): δ = 20.96 (1JC,H = 136 Hz,
[1] a) W. v. E. Doering, P. M. LaFlamme, Tetrahedron 1958, 2, 75–
79; b) W. R. Moore, H. R. Ward, J. Org. Chem. 1960, 25, 2073;
c) L. Skattebøl, Acta Chem. Scand. 1963, 17, 1683–1693; d) H.
Hopf in The Chemistry of Ketenes, Allenes and Related Com-
pounds (Ed.: S. Patai), Wiley, New York, 1980, part 2, pp. 779–
901; e) J. Backes, U. H. Brinker in Houben-Weyl (Ed.: M. Re-
gitz), Thieme, Stuttgart, 1989, vol. E19b, pp. 391–541; f) R. R.
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in Organic Chemistry (Ed.: B. Halton), JAI Press, Greenwich,
1991, vol. 1, pp. 19–54; h) E. Lee-Ruff in Houben-Weyl (Ed.:
A. de Meijere), Thieme, Stuttgart, 1997, vol. E17c, pp. 2388–
2418; i) L. K. Sydnes, Chem. Rev. 2003, 103, 1133–1150.
[2] a) K. A. Lukin, N. S. Zefirov, D. S. Yufit, Y. T. Struchkov, Tet-
rahedron 1992, 48, 9977–9984; b) K. A. Lukin, N. S. Zefirov in
Chemistry of Cyclopropyl Group (Ed.: Z. Rappoport), Wiley,
Chichester, 1995, pp. 861–885.
[3] a) E. B. Averina, T. S. Kuznetsova, A. N. Zefirov, A. E. Kopo-
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101–102; b) E. B. Averina, T. S. Kuznetsova, A. E. Lysov, K. A.
Potekhin, N. S. Zefirov, Dokl. Akad. Nauk 2000, 375, 481–483;
Dokl. Chem. 2000, 375, 257–259 (Engl. Trans.); c) E. B. Aver-
ina, R. R. Karimov, K. N. Sedenkova, Yu. K. Grishin, T. S.
Kuznetzova, N. S. Zefirov, Tetrahedron 2006, 62, 8814–8821.
[4] a) E. B. Averina, K. N. Sedenkova, Yu. K. Grishin, T. S. Kuz-
netsova, N. S. Zefirov, ARKIVOC 2008, 4, 71–79; b) K. N. Sed-
enkova, E. B. Averina, Yu. K. Grishin, T. S. Kuznetsova, N. S.
Zefirov, Zh. Org. Khim. 2008, 44, 962–969; Russ. J. Org. Chem.
2008, 44, 950–957 (Engl. Trans.).
CH2, C3), 20.96 (1JC,H = 136 Hz, 1 C, CH2, C3), 25.82 (1JC,H
=
135 Hz, 1 C, CH2, C21), 28.97 (1JC,H = 136 Hz, 1 C, CH2, C4),
29.75 (1JC,H = 135 Hz, 1 C, CH2, C22), 48.0 (1JC,H = 138 Hz, 1 C,
CH, C2), 52.49 (1JC,H = 140 Hz, 1 C, CH2 C20), 67.64 (1 C, C1),
67.79 (1 C, C13), 124.90 (2 C, CH=), 125.18 (1 C, CH=), 125.85 (1
C, CH=), 126.07 (1 C, CH=), 126.61 (1 C, CH=), 126.72 (1 C,
CH=), 126.85 (2 C, CH=), 127.45 (2 C, CH=), 127.60 (1 C, CH=),
128.21 (2 C, CH=), 128.73 (1 C, CH=), 129.38 (1 C, CH=), 129.55
(2 C, CH=), 133.95 (1 C, C=), 138.63 (1 C, C=), 141.55 (1 C, C=),
143.85 (1 C, C=), 146.74 (1 C, C=), 147.41 (1 C, C=), 148.11 (1 C,
C=), 148.84 (1 C, C=) ppm. IR (nujol): ν = 3060, 2963, 2946, 2865,
˜
1600, 1497, 1468, 1379, 1116, 762, 745, 717 cm–1. MS (EI, 70 eV):
m/z (%) = 438 (7), 437 (37), 436 (100) [M]+, 409 (11), 408 (41), 407
(29), 391 (10), 380 (16), 379 (11), 359 (10), 332 (22), 331 (86), 330
(49), 329 (27), 317 (17), 316 (15), 315 (31), 304 (14), 303 (22), 302
(25), 291 (20), 289 (15), 253 (21), 252 (15), 228 (11), 217 (15), 215
(25), 204 (14), 203 (14), 202 (16). MS (MALDI-TOF): m/z (%) =
436 (23). C34H28 (436.59): calcd. C 93.54, H 6.46; found C 93.40,
H 6.55.
[1-Methyl-1-(2-methylcyclobut-1-en-1-yl)ethyl]benzene (12):[5] Yield:
45% (0.33 g) from bromofluorospiropentane 1, 36% (0.26 g) from
bromochlorospiropentane 2, 6% (0.04 g) from dibromospiropen-
tane 3. Colorless solid, Rf = 0.3 (petroleum ether). 1H NMR
(400.1 MHz, CDCl3, 25 °C): δ = 1.48 (s, 6 H, 2 CH3), 1.57 (s, 3 H,
CH3), 2.20–2.28 (m, 4 H, 2 CH2), 7.18–7.24 (m, 1 H, CH, Ph),
7.32–7.43 (m, 4 H, 4 CH, Ph) ppm. 13C NMR (100.6 MHz, CDCl3,
25 °C): δ = 15.4 (1JC,H = 126 Hz, CH3), 25.8 (1JC,H = 137 Hz, CH2),
28.1 (1JC,H = 127 Hz, 2 CH3), 28.9 (1JC,H = 137 Hz, CH2), 29.8
(C), 125.6 (CH, Ph), 126.3 (2 CH, Ph), 128.0 (2 CH, Ph), 135.1 (C,
[5] E. B. Averina, K. N. Sedenkova, I. S. Borisov, Yu. K. Grishin,
T. S. Kuznetzova, N. S. Zefirov, Tetrahedron 2009, 65, 5693–
5701.
[6] K. Utimoto, M. Tamura, K. Sisido, Tetrahedron 1973, 29,
1169–1171.
[7] a) M. Makosza, M. Wawrzyniewicz, Tetrahedron Lett. 1969,
˛
10, 4659–4662; b) K. Muëller, F. Stier, P. Weyerstahl, Chem.
Ber. 1977, 110, 124–137; c) L. Anke, D. Reinhard, P. Weyer-
stahl, Liebigs Ann. Chem. 1981, 591–602.
Ph), 146.6 (C=), 148.7 (C=) ppm. IR (nujol): ν = 3030, 3000, 2970,
˜
2875, 1665, 1600, 1475, 1430, 1372, 1250, 1170, 772, 715, 685 cm–1.
MS (EI, 70 eV): m/z (%) = 186 (12) [M]+, 171 (100), 156 (12), 143
(48), 129 (21), 115 (11), 91 (23), 77 (9), 41 (10), 39 (5). C14H18
(186.29): calcd. C 90.26, H 9.74; found C 90.32, H 9.71.
[8] W. E. Doering, A. K. Hoffmann, J. Am. Chem. Soc. 1954, 76,
6162–6165.
[9] M. Fedoryn´ski, Synthesis 1977, 783–784.
[10] a) Enraf–Nonius, CAD4Software, version 5.0, Enraf–Nonius,
Delft, The Netherlands, 1994; b) G. M. Sheldrick, Acta Crys-
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tallogr., Sect. B 2002, 58, 380–388.
[11] CCDC-739011 (for 11) contains the supplementary crystallo-
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www.ccdc.cam.ac.uk/data_request/cif.
[12] L. J. Farrugia, J. Appl. Crystallogr. 1997, 30, 565–566.
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Chem. 1976, 54, 657–664.
[14] V. Usieli, S. Sarel, Tetrahedron Lett. 1973, 14, 1349–1350.
Received: March 30, 2010
(2-Cyclopropylidenevinyl)benzene (13):[14] Yield: 24% (0.14 g) from
bromofluorospiropentane 7, 18% (0.10 g) from bromochlorospiro-
pentane 8, 14% (0.08 g) from dibromospiropentane 9. Colorless oil,
1
Rf = 0.5 (petroleum ether). H NMR (400.1 MHz, CDCl3, 25 °C):
δ = 1.68–1.74 (m, 2 H, CH2), 1.74–1.79 (m, 2 H, CH2), 6.33 (dt,
5
5JH,H = 3.6 Hz, JH,H = 7.1 Hz, 1 H, CH), 7.17–7.22 (m, 1 H, CH,
Ph), 7.31–7.35 (m, 4 H, 4 CH, Ph) ppm. 13C NMR (100.6 MHz,
CDCl3, 25 °C): δ = 8.70 (2 CH2), 79.99 (C=), 96.62 (CH), 126.49
(CH, Ph), 126.55 (2 CH, Ph), 128.52 (2 CH, Ph), 131.62 (C, Ph),
189.98 (=C=) ppm. IR (film): ν = 3092, 3060, 3035, 2938, 2880,
˜
2863, 2026, 1960, 1602, 1498, 1455, 1265, 1168, 802, 755, 710 cm–1.
MS (EI, 70 eV): m/z (%) = 143 (8), 142 (85) [M]+, 141 (100), 140
(3), 139 (10), 116 (5), 115 (40), 139 (10), 116 (5), 115 (40), 114 (2),
Published Online: May 31, 2010
4150
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 4145–4150