Incorporation of an allene unit into α-pinene via β- elimination

Article abstract of DOI:10.1002/hlca.200690145

The two double-bond isomers 3-iodo-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (6b) and 3-iodo-4,6,6-trimethylbicyclo[3.1.1]hept-2-ene (11) were synthesized by reacting 2,6,6-trimethylbicyclo[3.1.1]heptan-3-one (9) with hydrazine, followed by treatment with I₂ in the presence of Et₃N. Treatment of 11 with t-BuOK as base in diglyme at 220 resulted in the formation of 9 and 6,6-dimethyl-4-methylidenebicyclo[3.1.1]hept-2-ene (12). For the formation of 9, the cyclic allene 7 is proposed as an intermediate. Treatment of the second isomer, 6b, with t-BuOK at 170 gave rise to the diene 12 and the dimerization product 17. The underlying mechanism of this transformation is discussed. On the basis of density-functional-theory (DFT) calculations on the allene 7 and the alkyne 15, the formation of the latter as the intermediate was excluded. Verlag Helvetica Chimica Acta AG.

Full text of DOI:10.1002/hlca.200690145

Helvetica Chimica Acta – Vol. 89 (2006)
1449
Incorporation of an Allene Unit into a-Pinene via b-Elimination
a
b
a
by Benan Kilbas ), Akin Azizoglu ), and Metin Balci* )
a) Department of Chemistry, Middle East Technical University, TR-06531 Ankara
b
)
Department of Chemistry, Balikesir University,TR-10100 Balikesir
e-mail: mbalci@metu.edu.tr)
(
The two double-bond isomers 3-iodo-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (6b) and 3-iodo-4,6,6-
trimethylbicyclo[3.1.1]hept-2-ene (11) were synthesized by reacting 2,6,6-trimethylbicyclo[3.1.1]hep-
tan-3-one (9) with hydrazine, followed by treatment with I in the presence of Et N. Treatment of 11
A
H
R
U
G
2
3
with t-BuOK as base in diglyme at 2208 resulted in the formation of 9 and 6,6-dimethyl-4-methylidene-
bicyclo[3.1.1]hept-2-ene (12). For the formation of 9, the cyclic allene 7 is proposed as an intermediate.
Treatment of the second isomer, 6b, with t-BuOK at 1708 gave rise to the diene 12 and the dimerization
product 17. The underlying mechanism of this transformation is discussed. On the basis of density-func-
tional-theory (DFT) calculations on the allene 7 and the alkyne 15, the formation of the latter as the inter-
mediate was excluded.
Introduction. – The synthesis of cyclic allenes with eight or less skeletal C-atoms,
known as highly strained organic compounds, has for the past decades attracted
increasing interest [1]. Besides synthetic considerations, theoretical chemists have
been keen on investigating these compounds to obtain insight into their structural
and unusual physico-chemical properties [2]. Strained cyclic allenes are nonplanar
and chiral rather than planar zwitterionic or carbene-like species, even in the case of
the highly strained cyclohexa-1,2-diene and cyclohepta-1,2-diene [3].
Among the numerous synthetic approaches [1] to cyclic allenes, the Doering–
Moore–Skattebol method and the b-elimination method have been most widely studied
in the literature. The first one, discovered by Moore [4] and co-workers, and by Skatte-
bol [5], involves the conversion of 1,1-dihalocyclopropanes [6] to the corresponding
cyclic allenes upon treatment with alkyllithium reagents [7][8]. The latter – first
attempted by Favorskii [9] to prepare cyclopenta-1,2-diene by treatment of vinyl bro-
mide with t-BuOK – is the reaction of the corresponding vinyl halides with bases
(
for recent examples, see [10]).
More recently, we have reported both experimental and theoretical studies related
to the Doering–Moore–Skattebol reaction to generate a cyclic allene incorporated into
a-pinene (1) [8]. As shown in Scheme 1, four products were isolated, the major being
the carbene-insertion product 4; the others were derived from allene dimerization.
Although the Doering–Moore–Skattebol method was successful in obtaining the
desired allene, it gave the ring-enlarged product 5 where the allene bonds are located
in a seven-membered ring. Hence, incorporation of an allene unit into the a-pinene (1)
skeleton without ring enlargement would generate the six-membered cyclic allene 7
(Scheme 2), which would cause considerable deviation from linear geometry. In this
©
2006 Verlag Helvetica Chimica Acta AG, Zürich

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Helvetica Chimica Acta – Vol. 89 (2006)
Scheme 1
work, we present a method of accessing the highly reactive intermediate 7 by applica-
tion of a b-elimination route, as outlined in Scheme 2.
Scheme 2
Results and Discussion. – First, we attempted to synthesize the vinyl bromide 6a by
bromination of a-pinene (1), followed by HBr elimination. However, none of our
efforts produced the desired bromine-addition products. Since a-pinene has a strong
tendency for Wagner–Meerwein rearrangement, we isolated in all cases the rearranged
dibromides instead of the desired regular addition products.
Therefore, we turned our attention to the synthesis of the corresponding vinyl io-
dide 6b as the key intermediate, having an efficient leaving group. As shown in
Scheme 3, hydroboration of 1 [11] followed oxidation with pyridinium chlorochromate
(
(
PCC) gave the ketone 9 [12], which was converted to the hydrazone 10 as a mixture of
E)/(Z)-isomers by treatment with hydrazine hydrate at 1108. Treatment of isomeric 10
with I [13] in the presence of E N in THF resulted in the formation of 6b and 11 in a
A
H
R
U
G
2
3
ratio of 2 :3. After column-chromatographic separation (0.4% AgNO on silica gel), the
3
compounds were obtained in pure forms and could be characterized.
The thermal stabilities of 6b and 11 were checked separately. We found that no
interconversion under C=C bond isomerization takes place at temperatures of up to
2508. Furthermore, the vinyl iodide 11 was stable in the presence of base at 2008.
After the successful synthesis of the key compound 11, it was submitted to the base-
induced elimination of HI in diglyme at 2208 in a sealed tube using t-BuOK as the base.
This, indeed, led to dehydroiodination, and three products, 12, 9, and 1, were found in
54, 32, and 6% yield, respectively (Scheme 4).
The formation of the ketone 9 may be rationalized according to Scheme 4. Nucleo-
philic addition of t-BuOK to the central allene C-atom of the formed allenic intermedi-

Helvetica Chimica Acta – Vol. 89 (2006)
1451
Scheme 3
Scheme 4
ate 7 gives rise to the enol ether 13. The hydrolysis of the latter (on silica gel) produces
the ketone 9. Furthermore, deprotonation of 7 by the strong base might also generate
+
the allylic anion 14, which can take up H to form the conjugated diene 12.
Recently, Christl et al. [14] reacted the allene precursor 3-bromo-1,2-dihydronaph-
thalene (and derivatives thereof) with t-BuOK and obtained a mixture of naphthalene
and enol ether as the major products. The formation of the major product was also
rationalized by the formation of the desired allene intermediate. Furthermore, we
did not have any evidence for an alternative b-elimination, which would lead to the for-
mation of a cyclic alkyne 15. Bottini et al. [15] reported that the change of halide from
bromide or iodide to chloride, the use of DMSO in place of Et2
A
H
R
U
G
vated temperatures all favor allene formation on the cost of alkyne formation. There-

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Helvetica Chimica Acta – Vol. 89 (2006)
fore, we also run the elimination reaction of 11 in DMSO to see whether the product
distribution would be affected or not. However, careful analysis of the reaction mixture
did not reveal any remarkable change in product distribution. Actually, the formation
of 9 can also be explained by formation of 15, followed by addition of t-BuOH. How-
ever, an asymmetric alkyne can be attacked at two different C-atoms. The observation
that the ketone 16 was not detected at all thus excludes the formation of 15.
1
To further support allene formation, we carried out DFT calculations ) on the cyclic
allene and the alkyne. The cyclic allene 7 is ca. 10.6 kcal/mol more stable than the cyclic
alkyne 15 (Figure).
Figure. Optimized structures of the allene 7 and the alkyne 15. Calculated relative energies (B3LYP/
LanL2DZ level) and selected bond distances (in Å) are shown.
Finally, the isomer 6b was submitted to base-induced HI elimination (Scheme 5).
Dehydroiodination occurred at lower temperature (sealed tube, diglyme, 1708), and
two compounds, the diene 12 and the dimerization product 17 [16], were formed in
54 and 32% yield, respectively. Meticulous examination of the reaction mixture did
not reveal the formation of any other products. Ketone 9, possibly formed by elimina-
tion of 11, was not detected. Based on this observation, we assume that the allenic inter-
mediate 7 was not formed during the elimination reaction of 6b. The underlying reac-
tion mechanism is not entirely clear at this stage. However, we tentatively propose the
pathway outlined in Scheme 6.
In a first step, the base deprotonates the Me group (instead of the adjacent CH2
group) under formation of the carbanion 18. The latter then may displace the I-atom
to form the corresponding carbene 19, which, in turn, undergoes CÀH insertion result-
1)
For details, see the Exper. Part.

Helvetica Chimica Acta – Vol. 89 (2006)
1453
Scheme 5
Scheme 6
ing in the formation of the conjugated diene 12 as the major product. Furthermore, the
+
allylic anion 18 can take up H to produce the kinetically controlled product 20.
DFT Calculations (at the RB3LYP/LanL2DZ level of theory) showed that the
energy difference between 6b and 20 is ca. 3.4 kcal/mol, the isomer 6b being thermody-
namically more stable. At a reaction temperature of 1708, the CÀI bond can easily
undergo homolytic cleavage yielding the allylic radical 21, whose dimerization gives
rise to 17.
To trap the cyclic allene 7 directly with a diene, base-supported elimination of 11
was conducted in the presence of furan in a sealed tube at elevated temperature
(Scheme 7). However, careful GC/MS studies of the resulting reaction mixture did
not reveal any evidence for the formation of the trapping product 22. The reaction mix-
ture mainly consisted of the dimerization product 17, beside a complex product mix-
ture.
Conclusion. – We have described a route to the highly strained cyclic allene 7, which
can be generated from 3-iodo-4,6,6-trimethylbicyclo[3.1.1]hept-2-ene (11) by b-elimi-

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Helvetica Chimica Acta – Vol. 89 (2006)
Scheme 7
nation of HI with t-BuOK as base. Alkyne formation was excluded on the basis of the
formed products, and according to theoretical calculations. Interestingly, base-sup-
ported elimination of the isomer 6b follows a different route giving rise to the insertion
and dimerization products 12 and 17, respectively.
The authors are indebted to the Scientific and Technical Research Council of Turkey (Grant TUBI-
TAK-MISAG-216) and the Turkish Academy of Sciences for financial support.
Experimental Part
General. TLC: 0.2-mm silica gel 60 F254 aluminum plates (Merck). Column chromatography (CC):
À1
1
13
silica gel (60 mesh; Merck). IR Spectra: soln. in 0.1-mm cells or KBr pellets: in cm . H- and C-
1
13
NMR: at 400 ( H) and 100 MHz ( C); apparent multiplicities are given in all cases; d in ppm, J in Hz.
MS: in m/z (rel. %).
(1R,2R,3R,5S)-2,6,6-Trimethylbicyclo[3.1.1]heptan-3-ol (8) [11]. To a soln. of 1 (68.0 g, 0.50 mol) and
NaBH (19 g, 0.50 mol) in THF (160 ml) was added dropwise precooled BF ·OEt (71.0 g, 0.50 mol) at 08
4
3
2
under N atmosphere. The mixture was kept 3 h at this temp. Then, 3M aq. NaOH soln. (167 ml) and 30%
2
H O soln. (250 ml) were added at À108. After stirring for 2 h, the reaction was complete, and the solvent
2
2
was evaporated. After addition of H O, the mixture was extracted with CH Cl , the org. phase was
2
2
2
washed with sat. aq. NaHCO soln. and H O, and dried (Na SO ). Evaporation of the solvent gave crys-
3
2
2
4
1
talline 8 (71 g, 92%). M.p. 53–558. H-NMR (400 MHz, CDCl ): 4.21 (dt, J=9.2, 4.9, 1 H); 2.70–2.63 (m,
3
1
1
H); 2.55–2.51 (m, 1 H); 2.12–2.07 (m, 2 H); 1.96 (dt, J=6.3, 1.7, 1 H); 1.87 (ddd, J=13.9, 4.5, 2.6, 1 H);
.62 (br. s, 1 H); 1.39 (s, 3 H); 1.30 (d, J=7.4, 3 H); 1.21 (d, J=9.8, 1 H); 1.11 (s, 3 H). C-NMR (100
13
MHz, CDCl ): 71.9; 48.2; 48.1; 42.2; 39.5; 38.6; 34.8; 28.1; 24.1; 21.2.
3
(1R,2R,5S)-2,6,6-Trimethylbicyclo[3.1.1]heptan-3-one (9) [12]. A soln. of pyridinium chlorochro-
mate (PCC; 119.8 g, 1.40 mol) in CH Cl (800 ml) was added to a soln. of 8 (71.0 g, 0.46 mol) in
2
2
CH Cl (250 ml) at 08. When the addition was complete, the mixture was stirred at r.t. for 3 h. The solvent
2
2
was evaporated, and the residue was worked up by extraction with H O/CH Cl . The org. phase was
2
2
2
washed with sat. aq. NaHCO soln. and H O, and dried (Na SO ). After solvent removal, the residue
3
2
2
4
was passed over silica gel (70 g), eluting with CH
Torr to give 9 (63.0 g, 90%). H-NMR (400 MHz, CDCl ): 2.69–2.64 (m, 2 H); 2.55–2.45 (m, 3 H);
2
Cl
2
, and then further purified by distillation at 558/5
1
3
13
2
.19–2.14 (m, 1 H); 2.1 (dt, J=6.4, 1.5, 1 H); 1.36 (s, 3 H); 1.24 (d, J=7.5, 3 H); 0.93 (s, 3 H). C-
NMR (100 MHz, CDCl ): 214.0, 51.6, 45.4, 45.0, 39.5, 39.3, 34.8, 27.5, 22.3, 17.1.
3
(1R,2R,3E,5S)- and (1R,2R,3Z,5S)-2,6,6-trimethylbicyclo[3.1.1]heptan-3-one Hydrazone (10). Com-
pound 9 (63.0 g, 0.412 mol) was reacted with neat hydrazine hydrate (21.6 g, 0.45 mol) at 1108 for 18 h.
The residue was extracted with H O and CHCl . The org. phase was washed with sat. aq. NaHCO soln.
2
3
3
and H O, and dried (K CO ). After removal of the solvent, 10 (64.0 g, 93%) was obtained as a (Z)/(E) 1:3
2
2
3
1
mixture. IR (KBr): 3465w, 3400m, 3209w, 2974s, 2939s, 2865s, 1635m, 1469m, 1365m. H-NMR (400 MHz,
CDCl ): 4.92 (br. s, 2 H); 2.81–2.26 (m); 2.48–2.26 (m); 2.03 (m); 1.82–1.79 (dt, J=5.7, 2.0); 1.26 (s); 1.19
3
13
(
3
3
d, J=6.3); 1.12 (d, J=7.0), 0.88 (s), 0.83 (s). C-NMR (100 MHz, CDCl ; (E)-isomer): 153.9; 46.2; 38.8;
8.3; 30.9; 28.6; 27.1; 20.1; 18.6. C-NMR (100 MHz, CDCl ; (Z)-isomer): 154.4; 46.4; 43.4; 38.7; 38.3;
3.7; 27.1; 22.2; 20.0. EI-MS (70 eV): 167 (100, [M+H] ), 151 (62), 134 (14), 83 (19). Anal. calc. for
3
13
3
+
C H N : C 72.24, H 10.91; found: C 72.06, H 10.82.
10
18
2

Helvetica Chimica Acta – Vol. 89 (2006)
1455
(1S,5S)-3-Iodo-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (6b) and (1S,4R,5R)-3-Iodo-4,6,6-trimethylbi-
cyclo[3.1.1]hept-2-ene (11). A soln. of I (97.0 g, 0.38 mol) in anh. THF (160 ml) was added to a mechan-
2
ically stirred soln. of 10 (64.0 g, 0.383 mol) and Et3N (40.5 g, 0.4 mol) in THF (300 ml) over a period of 15
A
H
R
U
G
min. After the addition, the mixture was stirred for 1 h at r.t. When the reaction was complete, the solvent
was evaporated, H O (300 ml) was added, and the mixture was extracted with hexane. The org. phase was
2
washed with sat. aq. NaCl soln. and H O, dried (Na SO ), and concentrated. The products were distilled
2
2
4
under vacuum to afford 6b and 11 in a ratio of 2 :3 (total yield: 25 g, 24.7%). The two isomers 6b and 11
(
500 mg) were separated by CC (100 g SiO with 0.4 g AgNO ; hexane).
2
3
Data of 11 (first fraction; anal. pure). Colorless liquid. IR (KBr): 3037w, 2963s, 1711m, 1382m,
1
1
365m, 957m. H-NMR (400 MHz, CDCl ): 6.72 (d, J=6.8, 1 H); 2.57 (tq, J=7.0, 2.3, 1 H); 2.27 (dt,
3
A-part of AB system, J=9.1, 5.5, 1 H); 2.1 (q-like, J=6.8, 1 H); 1.98 (dt, J=6.3, 2.3, 1 H); 1.35 (d, B-
13
part of AB-system, J=9.1, 1 H); 1.29 (s, 3 H); 1.11 (d, J=7.0, 3 H); 0.95 (s, 3 H). C-NMR (100 MHz,
+
CDCl ): 146.8; 117.3; 49.4; 46.7; 46; 42.4; 26.3; 25.7; 22; 20.1. EI-MS (70 eV): 263 (100, [M+H] ), 220
3
(
97), 135 (28), 93 (38), 92 (67). Anal. calc. for C H I: C 45.82, H 5.77; found: C 45.68, H. 5.62.
10 15
1
Data of 6b (second fraction; 90% pure). IR (KBr): 2861s, 1641w, 1465m, 1381m, 1367m, 898m. H-
NMR (400 MHz, CDCl ): 2.75 (dt, A-part of AB system, J=17.0, 2.1, 1 H); 2.65 (dt, B-part of AB system,
3
J=17.0, 2.3, 1 H); 2.4 (dt, A-part of AB system, J=8.9, 5.5, 1 H); 2.25 (dd, J=5.5, 3.8, 1 H); 1.93 (m, 1 H);
1
3
1
.82 (t, J=2.1, 3 H); 1.35 (d, B-part of AB system, J=8.9, 1 H); 1.27 (s, 3 H); 0.86 (s, 3 H). C-NMR (100
+
MHz, CDCl ): 148.4; 90.4; 49.6; 46.4; 45.6; 43.9; 31.6; 27.6; 25.8; 21.4. EI-MS (70 eV): 263 (55, [M+H] );
3
2
20 (100), 135 (54), 93 (58), 91 (67). Anal. calc. for C H I: C 45.82, H 5.77; found: C 45.98, H 5.92.
10 15
Reaction of 11 with t-BuOK. A soln. of t-BuOK (2.24 g, 0.02 mol) and 11 (2.0 g, 0.007 mol) in diglyme
25 ml) was placed in a glass tube. The tube was sealed and heated to 1708 for 8 h. Then, H O was added,
(
2
and the mixture was extracted with Et
2
A
H
R
U
G
soln. and H O, and dried (Na SO ). After removal of the solvent, the residue was purified by CC (40 g
2
2
4
SiO ; hexane) to afford the diene 12 (0.55 g, 54%), followed by a-pinene (1; 0.063 g, 6%). Further elution
2
2 2
with CH Cl provided the ketone 9 (0.37 g, 32%).
1
Data of 6,6-Dimethyl-4-methylidenebicyclo[3.1.1]hept-2-ene (12). Colorless oil. H-NMR (400 MHz,
CDCl ): 6.29 (t, J=7.5, 1 H); 6.01 (d, J=8.5, 1 H); 4.66 (s, 1 H); 4.63 (s, 1 H); 2.63 (br. t, J=5.6, 1 H); 2.55
3
13
(
ddd, J=8.5, 5.4, 3.0, 1 H); 2.28 (br. q, J=6.0, 1 H); 1.5 (d, J=8.5, 1 H); 1.35 (s, 3 H); 0.85 (s, 3 H). C-
NMR (100 MHz, CDCl ): 150.2; 138.3; 126.6; 107.3; 51.9; 43.7; 43; 36.1; 26.3; 22.4.
3
Reaction of 6b with t-BuOK. A soln. of t-BuOK (3.36 g, 0.03 mol) and 6b (2.5 g, 0.0085 mol) in
diglyme (15 ml) was heated in a sealed glass tube at 1708 for 8 h. Then, H O was added, and the residue
2
was extracted with Et2
A
H
R
U
G
dried (Na SO ). After removal of the solvent, the residue was purified by CC (25 g SiO ; pentane). The
2
4
2
first fraction gave 12 (0.41 g, 32%), and the second fraction afforded the dimer 17 (1.45 g, 56%).
1
Data of 2,2’-Ethane-1,2-diylbis(6,6-dimethylbicyclo[3.1.1]hept-2-ene) (17). Colorless oil. H-NMR
(
400 MHz, CDCl ): 5.18 (br. s, 1 H); 2.35 (dt, A-part of AB system, J=8.5, 5.6, 1 H); 2.25–2.15 (br.
3
AB system, J=17.3, 2 H); 2.05 (m, 1 H); 1.9 (t, J=5.3, 1 H); 1.87 (s, 2 H); 1.26 (s, 3 H); 1.16 (d, B-
1
3
part of AB system, J=8.5, 1 H); 0.84 (s, 3 H). C-NMR (100 MHz, CDCl ): 148.6; 116.1; 46.3; 41.3;
3
9
3
+
8.4; 35.2; 32; 31.7; 26.8; 21.7. EI-MS (70 eV): 271 (9, [M+H] ), 228 (20), 202 (6), 171 (10), 135 (50),
3 (100).
Computational Methods. Density-functional-theory (DFT) calculations were carried out with
Becke’s three-hybrid method [17] and the Lee–Yang–Parr exchange functional (B3LYP) [18], as imple-
mented in the GAUSSIAN 03W software [19]. All calculations were performed with a LANL2DZ basis
set, which includes the effective core potential (ECP) proposed by Hay and Wadt plus double basis-x for
Cl- and I-atoms [20]. Stationary points were characterized as minima or saddle points by analytically
evaluating harmonic vibrational frequencies. All energies reported in the discussion were calculated at
the B3LYP/LANL2DZ level, and include unscaled zero-point vibrational energies.
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Received May 8, 2006