A cyclopropyl-homoallyl rearrangement accompanying the borane-mediated reduction of tosylhydrazones

Article abstract of DOI:10.1002/ejoc.200300134

The Wolff-Kishner reduction of strained cyclopropyl ketones 9-oxo-2,8-didehydronoradamantane (1) and 5-oxo-2,4-didehydrobrendane (5) proceeds without rearrangement, while the borane-mediated reduction of the corresponding tosylhydrazones 1a and 5a affords the rearranged products tricyclo[4.2.1.0^3.8]non-4-ene (3) and 4-brendene (7), respectively. Rearrangement also occurs during the reduction of the less-strained tosylhydrazones 8a and 10a derived from cyclopropyl methyl ketone (8) and bicyclo[4.1.0]heptan-2-one (10). The results obtained in experiments with deuterated reagents support a concerted mechanism of diazene decomposition/cyclopropyl-homoallyl rearrangement. This rearrangement offers a great possibility for the preparation of new polycyclic molecules and, moreover, is a convenient method for regiospecific isotopic labeling using deuterated reagents. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200300134

FULL PAPER
A Cyclopropyl-Homoallyl Rearrangement Accompanying the Borane-Mediated
Reduction of Tosylhydrazones
[a]
[a]
[a]
[a]
´
´
Goran Kragol, Iva Benko, Jasmina Muharemspahic, and Kata Mlinaric-Majerski*
´
Dedicated to Professor Nenad Trinajstic on the occasion of his 65th birthday
Keywords: Small-ring systems / Hydrazones / Reduction / Rearrangements
The Wolff−Kishner reduction of strained cyclopropyl ketones
9-oxo-2,8-didehydronoradamantane (1) and 5-oxo-2,4-dide-
hydrobrendane (5) proceeds without rearrangement, while
the borane-mediated reduction of the corresponding tosylhy-
drazones 1a and 5a affords the rearranged products tricy-
The results obtained in experiments with deuterated re-
agents support a concerted mechanism of diazene decom-
position/cyclopropyl-homoallyl rearrangement. This re-
arrangement offers a great possibility for the preparation of
new polycyclic molecules and, moreover, is a convenient
clo[4.2.1.03,8]non-4-ene (3) and 4-brendene (7), respectively. method for regiospecific isotopic labeling using deuterated
Rearrangement also occurs during the reduction of the less-
strained tosylhydrazones 8a and 10a derived from cyclopro-
pyl methyl ketone (8) and bicyclo[4.1.0]heptan-2-one (10).
reagents.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
The enormous importance of the cyclopropyl group in
the various fields of chemistry lies in its versatile reactivity.
The propensity of cyclopropane-containing molecules to
undergo a variety of ring-opening reactions is well docu-
mented.[1] Although the bonding characteristics of the
cyclopropane ring would suggest that they should stabilize
adjacent carbanions, experiments have shown that cyclo-
propanes themselves are very poor acceptors.[2] Neverthe-
less, the cyclopropyl moiety has been used extensively in
synthetic chemistry.[3Ϫ7]
Scheme 1
During the course of our studies on the chemistry of
strained polycyclic molecules that contain the cyclopropyl
moiety,[4Ϫ6] we found that the cyclopropane ring-opening
reactions of cyclopropyl ketone 1 are very dependent upon
the reaction conditions used.[7] The WolffϪKishner re-
duction of 1 gives the nonrearranged product 2,8-dide-
hydronoradamantane (2), while the borane-mediated re-
duction of the tosylhydrazone 1a and electron-transfer re-
duction of 1 both give a ring-opening reaction with forma-
tion of the tricyclo[4.2.1.03,8]nonane compounds 3 and 4,
respectively (Scheme 1).
To the best of our knowledge this is the first example of
a cyclopropyl-homoallyl rearrangement accompanying the
reduction of cyclopropyl ketone tosylhydrazone derivatives.
Nevertheless, because of the similarity between cyclopropyl
ketone and enone systems, this reaction could be regarded
as an extension of borane-driven transformations of enone
hydrazones into transposed alkenes.[8]
To gain more knowledge regarding the ring opening of
cyclopropylcarbonyl systems with a negative charge built
up on the carbon atom adjacent to the ring, we have also
investigated reduction reactions, as these may involve cyclo-
propylmethyl anions or closely related structures.
-
[a]
Since it is known that the reduction of tosylhydrazones
with boron hydrides[9] offers a mild and convenient alterna-
tive to the WolffϪKishner and Clemmensen reduction, in
this work we studied the borane-mediated reduction of
´
Ruder Boskovic Institute, Department of Organic Chemistry
and Biochemistry,
P. O. Box 180, 10002 Zagreb, Croatia
Fax: (internat.) ϩ 385-1/4680195
E-mail: majerski@rudjer.irb.hr
2622
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200300134
Eur. J. Org. Chem. 2003, 2622Ϫ2625

Cyclopropyl-Homoallyl Rearrangement
FULL PAPER
cyclopropyl ketone tosylhydrazones in which the cyclopro-
pyl ring is either incorporated into strained polycyclic mol-
ecules (such as 1a and 5a) or into less-strained compounds
(such as 8a and 10a).
Results and Discussion
5-Oxo-2,4-didehydrobrendane (5) was prepared accord-
ing to a literature procedure.[10] The corresponding tosylhy-
drazone 5a was readily prepared by our usual procedure[11]
requiring no acid or base catalysis.[12] Upon the reduction
of tosylhydrazone 5a with an excess of borane in THF and
the decomposition of the resulting borane derivative under
basic conditions, the cleavage of the cyclopropane ring oc-
curred and 4-brendene (7) was obtained as the only prod-
uct.[13] However, WolffϪKishner reduction of ketone 5 gave
the unrearranged product 2,4-didehydrobrendane (6;
Scheme 2).
Scheme 3
It has been suggested that an alkyl diimide intermediate
is formed in both the WolffϪKishner reduction of ke-
tones[15] and in the reduction of the corresponding tosylhy-
drazones with boron hydrides.[16] However, the mechanism
of the subsequent base-promoted decomposition of diazene
derivatives and the possible existence of carbanionic species
is still questionable.[17] Our results indicate that the last
steps in the borane-mediated reduction of tosylhydrazones
and the WolffϪKishner reduction are rather different. To
obtain more insight into the mechanism of the cyclopropyl
rearrangement during the borane-mediated reduction of
cyclopropyl ketone tosylhydrazones, we performed the
experiments with tosylhydrazone 5a using common and/or
deuterated reagents (BD3·THF and NaOD/D2O). The com-
bination of common BH3·THF and deuterated NaOD/D2O
gave only exo-2-deuterio-4-brendene (7a), while the combi-
nation of deuterated BD3·THF and NaOD/D2O yielded
exo-2,5-dideuterio-4-brendene (7b).[18,19] The formation of
only the exo-2-deuterio derivative 7a favors the concerted
mechanism of the diazene decomposition/cyclopropyl-
homoallyl rearrangement during the borane-mediated re-
duction of cyclopropyl ketone tosylhydrazones without the
formation of free carbanion (Scheme 4).
Scheme 2
To prove the generality of this rearrangement we also
studied the borane-mediated reduction of nonstrained
cyclopropyl ketone tosylhydrazones 8a and 10a derived
from cyclopropyl methyl ketone (8) and bicyclo[4.1.0]hep-
tan-2-one (10), respectively. Cyclopropyl methyl ketone is
commercially available, while bicyclo[4.1.0]heptan-2-one
was prepared by a SimmonsϪSmith cyclopropanation of
cyclohex-2-en-1-one. The best yield of the cyclopropanation
(58%) was obtained when the ratio of cyclohex-2-en-1-one,
Zn/CuCl2 and CH2I2 was 1:8:4. The borane-mediated re-
duction of cyclopropyl ketone tosylhydrazones 8a and 10a
also proceeded with cyclopropane ring opening, and the
corresponding cis- and trans-2-pentene (9) and 3-methyl-
cyclohexene (11) were obtained, respectively, as the sole ole-
finic products (Scheme 3). The WolffϪKishner reduction of
cyclopropyl methyl ketone afforded an unrearranged prod-
uct.[14]
Scheme 4
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 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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´
´
G. Kragol, I. Benko, J. Muharemspahic, K. Mlinaric-Majerski
FULL PAPER
Conclusion
5.3, 6.0, 10.5, 13.5, 17.5, 19.1, 21.3, 128.0, 129.3, 135.3, 143.8,
160.1 ppm.
In summary, we have found that a cyclopropyl-homoallyl
rearrangement occurs during the borane-mediated re-
duction of various cyclopropyl ketone tosylhydrazones,
while the WolffϪKishner reduction of the corresponding
cyclopropyl ketones gives unrearranged products. The re-
sults obtained with deuterated reagents imply that the bor-
ane-mediated reduction proceeds through a concerted
mechanism of diazene decomposition/cyclopropyl-homoal-
lyl rearrangement. In addition, this cyclopropyl-homoallyl
rearrangement offers a great possibility for the preparation
of new polycyclic molecules and, moreover, is a convenient
method for regiospecific isotopic labeling employing deuter-
ated reagents.
Preparation of Tosylhydrazone 10a: According to the General Pro-
cedure, ketone 10 (4.47 g, 40.0 mmol) was mixed with p-TsHNNH2
(7.93 g, 40.0 mmol) in dry ethanol (70 mL). Dilution with water
(200 mL), extraction with diethyl ether (4 ϫ 70 mL), drying and
solvent evaporation gave a yellowish product. Recrystallization
from ethanol gave tosylhydrazone 10a (2.19 g, 20%); m.p. 154Ϫ156
°C. IR (KBr): ν˜ ϭ 3230 (s), 3020 (w), 2940 (m), 2860 (w), 1600
(m), 1395 (m), 1330 (s), 1165 (s) cmϪ1 1H NMR (CDCl3): δ ϭ
.
0.26Ϫ0.34 (m), 0.75Ϫ0.81 (m), 0.97Ϫ2.05 (m), 2.15Ϫ2.25 (m), 2.42
(s, CH3), 2.44 (s, CH3), 7.30 (d, J ϭ 8.0 Hz), 7.84 (d, J ϭ 8.0 Hz)
ppm. 13C NMR (CDCl3): δ ϭ 7.9, 8.7, 10.8, 11.6, 13.3, 15.4, 17.7,
18.5, 20.6, 21.3, 22.0, 23.8, 31.1, 127.8, 129.4, 135.6, 143.7,
160.1 ppm. HRMS: calcd. for C14H18N2O2S 278.10835; found
278.10871.
General Procedure for the Reduction of Tosylhydrazones with Bor-
ane: A 1  solution of BH3 in THF was added under nitrogen to
an ice-cold solution of the corresponding tosylhydrazone in dry
THF. After stirring for 15 min, the ice/water bath was removed and
the reaction mixture was stirred for a further 1 h. The excess of
BH3·THF was destroyed by careful addition of water until no reac-
tion was observed. Then a 5  aqueous NaOH solution was added
and stirring was continued at room temperature for 1 h. The reac-
tion mixture was diluted with water and the product was extracted
with pentane and dried with anhydrous MgSO4. After filtration,
the solution was concentrated under reduced pressure and the resi-
due was filtered through a short column of alumina (activity I),
with pentane as eluent, to give the olefinic products as highly vol-
atile substances.
Experimental Section
General Remarks: The purity of all compounds was determined by
1
GLC and/or H and 13C NMR spectroscopy. GLC analyses were
carried out with a Varian 3300 gas chromatograph fitted with a
DB-210 capillary column. 1H and 13C NMR spectra were recorded
with a Varian Gemini 300 spectrometer. IR spectra were recorded
with a PerkinϪElmer M-297 spectrophotometer and high-resolu-
tion mass spectra were recorded with an Extrel FTMS 2001 spec-
trometer. Melting points were determined with a Kofler apparatus.
General Procedure for the Preparation of Tosylhydrazones: A mix-
ture of the corresponding cyclopropyl ketone and p-TsHNNH2 (1:1
ratio) in dry ethanol was stirred at 50 °C for 1 h, cooled to room
temperature and left in the refrigerator overnight. Water was then
added to the resulting suspension and the product was extracted
with diethyl ether and dried with anhydrous MgSO4. The solvent
was evaporated to give a mixture of two isomers of the correspond-
ing tosylhydrazone as a white solid.
Reduction of Tosylhydrazone 5a with BH3·THF: According to the
General Procedure, tosylhydrazone 5a (0.453 g, 1.5 mmol) was re-
duced with a 1  solution of BH3·THF (2 mL, approx. 2 mmol) in
dry THF (8 mL). After addition of a 5  NaOH aq. solution
(2 mL) and stirring for 1 h, the mixture was diluted with water
(20 mL). Extraction with pentane (4 ϫ 15 mL), followed by drying,
concentrating, and purification gave 4-brendene (7)[13,19] (0.040 g,
Preparation of Tosylhydrazone 5a: According to the General Pro-
cedure, ketone 5 (0.38 g, 2.84 mmol) was mixed with p-TsHNNH2
(0.526 g, 2.84 mmol) in dry ethanol (3.5 mL). Dilution with water
(30 mL), extraction with diethyl ether (4 ϫ 15 mL), drying and sol-
vent evaporation gave tosylhydrazone 5a (0.77 g, 90%); m.p.
142Ϫ144 °C (ref.[12a] 183Ϫ188 °C). IR (KBr): ν˜ ϭ 3220 (s), 3060
(w), 2980 (s), 2920 (m), 2880 (m), 1670 (m), 1600 (m), 1170 (vs)
1
22%). H NMR (CDCl3): δ ϭ 1.16 (d, J ϭ 11.7 Hz, 2 H, 2-Hendo
and 9-Hendo), 1.37Ϫ1.47 (m, 2 H, 2-Hexo and 9-Hexo), 1.59 (s, 2 H,
8-H), 2.32Ϫ2.38 (m, 3 H, 3-H, 6-H and 1-H), 2.91Ϫ2.96 (m, 1 H,
7-H), 6.01 (br. s, 2 H, 4-H and 5-H) ppm. 13C NMR (CDCl3): δ ϭ
36.1 (t, 2 C, C-2 and C-9), 39.3 (t, 1 C, C-8), 42.1 (d, 1 C, C-1),
42.9 (d, 2 C, C-3 and C-6), 54.0 (d, 1 C, C-7), 139.4 (d, 2 C, C-4
and C-5).
cmϪ1 1H NMR (CD3OD): δ ϭ 1.23Ϫ1.34 (m), 1.58Ϫ1.72 (m),
.
1.88Ϫ2.20 (m), 2.42Ϫ2.60 (m with distinguishable s at δ ϭ
2.54 ppm, CH3), 2.74 (br. s), 3.00Ϫ3.08 (m), 7.46 (d, J ϭ 8 Hz),
7.88 (d, J ϭ 8 Hz) ppm. 13C NMR (CD3OD): δ ϭ 21.7, 31.8, 32.0,
33.7, 36.2, 36.6, 37.2, 37.4, 38.2, 42.1, 42.6, 43.0, 43.6, 43.7, 51.2,
51.3, 129.4, 130.9, 138.0, 145.5, 178.5, 178.7 ppm. C16H18N2O2S
(302.398): calcd. C 63.55, H 6.00 N, 9.26; found C 63.55, H 6.22,
N 9.46.
Reduction of Tosylhydrazone 8a with BH3·THF: According to the
General Procedure, tosylhydrazone 8a (3.12 g, 12.0 mmol) was re-
duced with a 1  solution of BH3·THF (17 mL, approx. 17 mmol)
in dry THF (65 mL). After addition of a 5  NaOH aq. solution
(16 mL) and stirring for 1 h, the product was distilled off (36 °C)
at atmospheric pressure through a Vigreux column with a dry ice/
acetone cooling condenser to yield a mixture (0.126 g, 15%) of cis-
and trans-2-pentene[20] (1:2 ratio).
Preparation of Tosylhydrazone 8a: According to the General Pro-
cedure, ketone
8
(2.94 mL, 30.0 mmol) was mixed with p-
Reduction of Tosylhydrazone 10a with BH3·THF: According to the
General Procedure, tosylhydrazone 10a (1.0 g, 3.6 mmol) was re-
duced with a 1  solution of BH3·THF (5 mL, ca. 5 mmol) in dry
TsHNNH2 (5.81 g, 30.0 mmol) in dry ethanol (50 mL). Dilution
with water (200 mL), extraction with diethyl ether (4 ϫ 75 mL),
drying and solvent evaporation gave tosylhydrazone 8a (7.09 g, THF (20 mL). After addition of a 5  NaOH aq. solution (5 mL)
94%); m.p. 127Ϫ129 °C (ref.[20] 123 °C). IR (KBr): ν˜ ϭ 3210 (s),
and stirring for 1 h, the mixture was diluted with water (30 mL)
3020 (w), 2990 (w), 2925 (w), 1625 (m), 1325 (s), 1165 (s) cmϪ1. 1H and extracted with pentane (3 ϫ 30 mL). Most of the solvent was
NMR (CDCl3): δ ϭ 0.63Ϫ0.69 (m), 0.84Ϫ0.90 (m), 1.50Ϫ1.65 (m distilled off at atmospheric pressure and then the residue was trans-
with 2 distinguishable s at δ ϭ 1.64 and 1.65 ppm, CH3), 2.41 (s),
ferred in vacuo to afford 3-methylcyclohexene (11), traces of pen-
7.29 (d, J ϭ 8.2 Hz), 7.83 (d, J ϭ 8.2 Hz). 13C NMR (CDCl3): δ ϭ tane and THF. The yield of the reaction was calculated to be 12.5%
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Cyclopropyl-Homoallyl Rearrangement
FULL PAPER
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[7]
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ˇ
´
´
M. Sindler-Kulyk, Z. Majerski, D. Pavlovic, K. Mlinaric-Ma-
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ˇ
´
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´
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´
Wolff؊Kishner Reduction of 5-Oxo-2,4-didehydrobrendane (5): A
solution of ketone 5 (0.27 g, 2.0 mmol), diethylene glycol (10 mL),
98Ϫ100% hydrazine hydrate (0.5 mL, 10.0 mmol) and KOH
(0.37 g, 6.6 mmol) was heated at 100 °C for 2 h and then at 210 °C
for 5 h, during which time the product sublimed. The sublimate
was washed with pentane (20 mL) and dried with anhydrous
MgSO4. The pentane was evaporated to give 0.115 g (48%) of 2,4-
didehydrobrendane (6). IR (KBr): ν˜ ϭ 3040 (w), 2960 (s), 2930 (s),
5331Ϫ5334.
´
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.
9.6 Hz, 1 H, 5-Hendo), 1.15 (dd, J ϭ 13.1, 5.8 Hz, 1 H, 2-H), 1.27
(dd, J ϭ 9.6, 9.3 Hz, 1 H, 5-Hexo), 1.51Ϫ1.56 (m, 2 H, 4-H and 9-
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H, 6-H), 2.33 (br. s, 1 H, 1-H), 2.55 (br. s, 1 H, 7-H) ppm. 13C
NMR (CDCl3): δ ϭ 24.0 (d, C-3), 30.4 (d, C-4), 30.6 (d, C-2), 35.0
(d, C-6), 37.6 (d, C-1), 39.5 (t, C-9), 39.8 (t, C-5), 44.2 (d, C-7),
51.7 (t, C-8) ppm. HRMS: calcd. for C9H12 120.09390; found
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[13]
The 1H and 13C NMR spectra of 7 are identical with literature
Preparation of Ketone 10: Cyclohex-2-en-1-one (5 mL, 0.05 mol)
and CH2I2 (16.63 mL, 0.2 mol) were added under nitrogen to a
freshly prepared suspension of Zn/Cu dust, obtained by the ultra-
sound-promoted reaction of zinc (26.15 g, 0.4 mol) and CuCl
(39.60 g, 0.4 mol) in dry diethyl ether (125 mL). The reaction mix-
ture was stirred at 40Ϫ50 °C for 48 h and the excess of Zn/Cu
dust was then destroyed by addition of a saturated aqueous NH4Cl
solution until no reaction was observed. The suspension was di-
luted with water (200 mL), filtered, and the filtrate was extracted
with diethyl ether (4 ϫ 60 mL). The extracts were combined, dried
with anhydrous MgSO4 and the solvents evaporated under reduced
pressure to afford a heavy yellow oil. The pure ketone 10 (3.34 g,
59%) was obtained by distillation under reduced pressure as a clear
yellowish oil; b.p. (10 Torr) 85 °C. The NMR spectra are in agree-
ment with the literature data.[22]
[13a]
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[20]
The incorporation of deuterium in a specific position of the
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Acknowledgments
We thank the Ministry of Science and Technology of the Republic
of Croatia for financial support of this study (Project 0098052).
1
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Received March 4, 2003
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