The preparation of the last remaining acyclic isomers of benzene

Article abstract of DOI:10.1002/ejoc.200400869

The three acyclic isomers of benzene - 2,3-hexadien-5-yne, 3-methyl-1,2-pentadien-4-yne, and 1,2,3,4-hexatetraene have been prepared by simple routes from readily available starting materials. The hydrocarbons as well as several side products formed during their synthesis were characterized by their spectroscopic data and typical cycloaddition and isomerization reactions.

Full text of DOI:10.1002/ejoc.200400869

FULL PAPER
The Preparation of the Last Remaining Acyclic Isomers of Benzene[‡]
Helmut Maurer[a][‡‡] and Henning Hopf*[a]
Keywords: Benzene isomers / Alkynes / Allenes / Cuprates / Cycloadditions
The three acyclic isomers of benzene – 2,3-hexadien-5-yne,
-methyl-1,2-pentadien-4-yne, and 1,2,3,4-hexatetraene –
by their spectroscopic data and typical cycloaddition and
isomerization reactions.
3
have been prepared by simple routes from readily available
starting materials. The hydrocarbons as well as several side
products formed during their synthesis were characterized
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
cross-conjugation is displayed by benzene isomers such as
pentafulvene and 2-ethynyl-1,3-butadiene (see below),
whereas linear conjugation is demonstrated by hydrocarbons
such as 1,3-hexadien-5-yne. Although some electronic fea-
Structural and functional complexity of chemical com-
pounds depend on the number and type of their constituent
atoms. And yet, as shown by the homologous and the vi-
nylogous principle, once a certain degree of complexity has
been reached the addition of further atoms often does not
lead to principally new properties (functions) any more.
Rather, incremental changes are observed when the growth
process is continued. In fact, it can be stated that for many
classes of compounds from a certain number of atoms on
no fundamentally new properties emerge. These considera-
tions may be illustrated for the unsaturated and/or cyclic
tures such as antiaromaticity, show up first in the C Hn
n
series with n = 4 already (1,3-cyclobutadiene), they are re-
peated here (e. g. in 1-vinyl-1,3-cyclobutadiene). Of course,
the number of combinations grows further rapidly if one
progresses to higher values of n (C H allows 7437 combi-
8
8
[2]
nations, C H 369 067 ), but no additional bonding fea-
10
10
tures of fundamental importance arise. In other words: six
carbon and six hydrogen atoms are just enough to generate
the various π-systems that are of greatest theoretical and
practical value; progressing to higher analogs does not give
rise to principally new π-electron patterns.
hydrocarbons with the sum formula C H . Obviously, the
n
n
number of combinations that can be generated from the
respective carbon and hydrogen atoms increases rapidly
with growing n. Counting only hydrocarbons with covalent
bonds and maintaining the usual valencies of carbon and
hydrogen, that is excluding radicals, carbenes etc., there is
no covalent structure for n = 1, one for n = 2 (acetylene),
none for n = 3, and 11 for n = 4. Now a steep increase sets
in and for C H not only more than 200 combinations can
Returning to the C H hydrocarbons, these can be classi-
6
6
fied into five groups: a) Acyclic (15 isomers), monocyclic
61 isomers), bicyclic (82 isomers), tricyclic (45 isomers),
(
and tetracyclic (15 isomers). Out of these groups only one –
[3]
the “acyclic isomers of benzene” – consist of compounds
that should all be preparatively obtainable under normal
laboratory conditions. In all other groups structural fea-
tures are present that will make the isolation of the respec-
tive isomers either impossible or only under very special
conditions such as matrix isolation or within the mass spec-
trometer. These features are inter alia triple bonds in three-
to six-membered rings, allene groups in these rings, bridge-
head double bonds (violation of Bredt’s rule) etc.
6
6
be constructed – the exact number is 218 – but these dem-
onstrate principles (functions) of fundamental importance
for organic chemistry for the first time in full width. Clearly
benzene, the prototypical aromatic hydrocarbon is of great-
est importance. But there are other isomers showing other
important structural and reactivity concepts: [3]Radialene
is the smallest conceivable hydrocarbon consisting of semi-
cyclic double bonds only (radialenes) and with [3]prismane
the polycyclic prismane series begins. The phenomenon of
The acyclic C H isomers are summarized in Scheme 1.
6
6
Whereas most of these hydrocarbons have been extensively
[7–19]
studied – see references
leading to the pertinent litera-
ture – there are three isomers, which have either been men-
tioned only cursorily in the chemical literature, viz. 2,3-
hexadien-5-yne (8) or not at all: 3-Methyl-1,2-pentadien-
4-yne (15) and 1,2,3,4-hexatetraene (12). Although we syn-
thesized these three cumulenes nearly 30 years ago, we
never published their preparation, the main reason being
that we were concentrating our efforts in this area on the
1]
[
[
‡] Alkynes and Cumulenes, XXV. Part XXIV: Ref.[
a] Institut für Organische Chemie, Technische Universität
Braunschweig,
[4]
Hagenring 30, 38106 Braunschweig, Germany
Fax: +49-531-391 5388
E-mail: H.Hopf@tu-bs.de
[
‡‡] Taken in part from the Ph. D. thesis of H. Maurer, Karlsruhe,
1977.
2702
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400869
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The Preparation of the Last Remaining Acyclic Isomers of Benzene
FULL PAPER
chemical and structural properties of other acyclic benzene
isomers such as 7, 10, and 13. However, a recent computa-
tional study by Sastry and co-workers on the C H poten-
6
6
tial energy surface aimed at determining the relative sta-
bility of the 218 C H isomers and their synthetic feasibil-
6
6
ity[ prompts us to report our results from 1977. With
5]
[6]
this publication the preparation of the first class of C H6
6
isomers is hence complete.
Scheme 2. Preparation of 2,3-hexadien-5-yne (8).
The first hydrocarbon isolated by preparative gas
chromatography was 1,2-pentadien-4-yne (17), a known hy-
drocarbon, which was identified by comparison of its spec-
troscopic data with those reported in the literature.[ Next
eluted the vinylallene 18 followed by the desired C H iso-
22]
6
6
mer 2,3-hexadien-5-yne (8) as the main product, and readily
identified by the spectroscopic and analytical data given in
the experimental section. As the most highly alkylated pro-
duct the vinylallene derivative 19 was finally obtained. To
have additional chemical proof for the new hydrocarbons,
Scheme 1. The acyclic isomers of benzene.
18 and 19 were treated in benzene for 6 h at 50 °C with a
slight excess of tetracyanoethene (TCNE). After work-up
1
Preparation of 2,3-Hexadien-5-yne (8) and 3-Methyl-1,2-
pentadien-4-yne (15)
two colorless solids were obtained, which according to H
NMR analysis (experimental section) have the structures 20
and 21. When the dienyne 8 was isomerized with potassium
For the preparation of 8 and its branched isomer 15, we tert-butoxide in tert-butyl alcohol at 60 °C 2,4-hexadiyne
used the alkylation of appropriate propargylacetates with (4) was obtained as the sole rearrangement product.[ The
dimethyllithium cuprate first reported by Crabbé and co- base-catalyzed isomerization of allenes and allenynes to al-
10]
[
20]
workers.
Thus 3-acetoxy-1,4-pentadiyne (16) was pre- kynes is a well-known phenomenon, which has been ob-
[21]
[23]
pared from the corresponding alcohol by treatment with served many times.
We assume that in the present case
acetyl chloride in pyridine at 0 °C (70% yield). The ester is the allenic proton at C-2 is removed first initiating an isom-
a colorless liquid which even at –78 °C slowly turns brown erization of 8 into 9. Repetition of this process could then
and then black. It can, however, be stored in solution at lead to 4.
–30 °C indefinitely. For alkylation, 16 was slowly added to
For the preparation of 3-methyl-1,2-pentadien-4-yne (15)
a solution of dimethyllithium cuprate in ether at –15 °C. the linear acetate 22 was prepared from 2,4-pentadiyn-1-
[
24]
After warming the reaction mixture to room temperature ol
with acetyl chloride in pyridine at 0 °C (79% yield).
and work-up, the ethereal solution was concentrated and The colorless liquid is much less stable than its branched
after vacuum transfer (0.01 Torr) separated by preparative isomer 16 and decomposes rapidly at room temperature
gas chromatography. The very complex product mixture with formation of a black polymeric material. The behavior
showed at least 12 peaks, eight of which, amounting to ap- of 22 during distillation is unpredictable and we have expe-
proximately 10%, were discarded. The structures and yields rienced vigorous decompositions with this diacetylene.
(isolated yields after gas chromatography) of the four main Methylation of 22 was carried out as described above for
products are shown in Scheme 2.
16, and again a complex mixture of hydrocarbon products
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H. Maurer, H. Hopf
FULL PAPER
Scheme 3. Preparation of 3-methyl-1,2-pentadien-4-yne (15).
was obtained (Scheme 3). The four main products, formed
in 1.9:0.7:2.9:1 ratio (GC analysis; yields were not deter-
mined quantitatively) could be separated from at least 8
trace components by preparative gas chromatography.
Again, the desired compound 15 was the main compo-
nent and ethynylallene (17) eluting as the second hydro-
carbon, was also produced. As the compound with the
shortest retention time the vinylallene 23, a known hydro-
[
25]
carbon,
was isolated. Surprisingly, toluene (24) was ob-
tained in all experiments, so its formation is evidently not
an artefact. The structures of all these compounds follow
unambiguously from their spectral and analytical data
given in the experimental section. Furthermore, 23 when
reacted with TCNE furnished the Diels–Alder adduct 25,
also identified by its spectra.
Although we did not carry out mechanistic experiments
the following sequence of steps, based on analogies from
[
20]
the literature, appears reasonable (Scheme 4).
In the first step of the methylation the cuprate reacts with
the acetates 16 and 22, respectively, under propargyl re-
III
arrangement to the Cu intermediates 26 and 27, while the
acetate group is displaced. Methyl transfer and elimination
of methyl copper subsequently provides the desired methyl
allenes 8 and 15. On the other hand 26 and 27 are evidently
long-lived enough under the reaction conditions to allow
hydrolysis to the parent allenyne 17 on work-up. The origin
of the other isolated products, the vinyl allenes 18, 19, and
23, and especially toluene (24) remains obscure at present.
For these higher alkylation products to be generated,
methyl groups have to be added to the triple bond of a
suitable precursor hydrocarbon. Although a control experi-
ment showed that lithium dimethyl cuprate does not react
with 2-penten-5-yne, another experiment with the acetate
Scheme 4. Mechanisms of formation of 2,3-hexadien-5-yne (8) and
16 was revealing. Treating this substrate with the cuprate
3
-methyl-1,2-pentadien-4-yne (15) and their side products.
prepared from one equivalent of copper(i) iodide and five
equivalents of methyllithium led to a product mixture in
which the primary product 8 and the hydrolysis product 17
were not present anymore. Instead 18, 19 and 24 had been
formed in 0.44:4:1 ratio, indicating that allenynes can be
alkylated after all. No further experiments were undertaken
to shed light on these observations. The formation of aro-
matic compounds (benzene, toluene) has often been ob-
[26]
clic acetylenic and allenic C H and C H precursors.
6
6
7
8
Since the aromatization process was of no interest here, no
effort was undertaken to investigate its mechanism.
Preparation of 1,2,3,4-Hexatetraene (12)
In 1973 Corey and Ruden reported on the stereoselective
served in base-catalyzed and thermal isomerizations of acy- preparation of E- and Z-enynes by coupling propargyl tri-
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The Preparation of the Last Remaining Acyclic Isomers of Benzene
FULL PAPER
phenylphosphonium bromide (28) with a carbonyl compo- Experimental Section
nent, n-butyllithium functioning as the base.[ In the case
27]
General: UV: Cary-14. IR: Perkin–Elmer 221 and Beckman IR-8.
MS (70 eV): Varian MAT CH-5. GC: Varian-Aerograph 1740–1.
NMR: Varian A-60 in CDCl , if not given otherwise; TMS as in-
3
ternal standard. Since the compounds prepared in this study are
all air-sensitive the experiments were carried out under nitrogen.
of cyclohexane carbaldehyde rather than the expected enyne
they isolated cyclohexyl-1,2,3-butatriene and reasoned that
the initially generated propargylidenetriphenylphosphorane
had undergone a base-catalyzed isomerization to the cu-
mulenic phosphorane 30 prior to the Wittig condensation
possibly involving 29 (Scheme 5).
3
ol
-Acetoxy-1,4-pentadiyne (16): To a solution of 1,4-pentadiyn-3-
[21]
(19 g, 0.125 mol) in anhydrous pyridine (60 mL) was added
slowly at 0 °C freshly distilled acetyl chloride (11.7 g, 0.15 mol).
When the addition was complete the temperature was increased to
room temp. and the brown reaction mixture was extracted with
diethyl ether (2 × 200 mL). The pyridine was removed from the
combined ethereal extracts by careful washing with water (5 ×
150 mL) and 0.1 m hydrochloric acid. After neutralization with hy-
drogencarbonate solution and drying with sodium sulfate, the sol-
vent was removed by rotary evaporation. The remaining yellow oil
was vacuum-transferred under high vacuum at 40 °C: 10.8 g (71%)
1
of 16 as colorless oil (b.p./16 Torr: 61 °C). H NMR: δ = 2.14 (s, 3
H, acetate), 2.61 (d, J = 2.5 Hz, 2 H, –CϵC–H), 6.00 (t, J = 2.5 Hz,
–
1
1
H, –CH–) ppm. IR (film): ν˜ = 3310 cm (s) and 2150 (m, –CϵC–
H), 1750 (s), 1230 (s), 1050 (s, acetate). MS (EI): m/z = 122 (3)
+
[
M ], 107 (5), 80 (20), 79 (25), 63 (72), 62 (71), 43 (100). C
7 6 2
H O
(
122.12): calcd. C 68.84, H 4.95; found C 70.13, H 5.19 (the un-
stable ester turns yellow quickly during weighing, probably a
reason for the faulty elemental analysis).
2,3-Hexadien-5-yne (8): To a suspension of lithium (1.8 g, 0.26 mol)
in 80 mL of anhydrous ether was added a solution of 15.6 g (0.11
mol) of methyl iodide in 50 mL of ether at such a rate as to keep
the reaction mixture gently refluxing. After completion of the ad-
dition the mixture was refluxed for 1 h and the prepared methyllith-
ium solution transferred under nitrogen to a dropping funnel of
a second three-necked reaction flask containing a suspension of
copper(i) iodide (9.7 g, 0.05 mol) in 150 mL of anhydrous ether at
Scheme 5: Generation of 1,2,3,4-hexatetraene (12).
If this process could be performed with acrolein it should
offer a route to 1,2,3,5-hexatetraene (11), a C H isomer
still unknown at the time when we carried out the experi-
ment, and since then obtained by an alternate protocol.
In fact, the Wittig reaction between acrolein and 30 did not
lead to 11 but provided its isomer 12 and benzene in trace
amounts. The highly unstable 12, it polymerizes rapidly
6
6
–15 °C. When the methyllithium was added (keeping the reaction
[
17]
temperature at all times below –10 °C) the lemon yellow methyl
copper precipitated but soon dissolved again to provide a grey
cuprate solution. This solution was cooled to –15 °C and 5.3 g (0.04
mol) of 16 in 100 mL of ether was added slowly. When the addition
even in solution at –10 °C, was characterized by its NMR was complete the temperature was raised to room temperature and
and vibrational spectra. In the proton spectrum the methyl the mixture stirred for 4 h. For work-up 150 mL of aqueous sat.
group appears as a doublet of triplets (J = 7.5, J = 1.5 Hz) ammonium chloride solution was added at –10 °C, the ether phase
1
2
was separated and the aqueous phase extracted several times with
ether. The combined organic phases were neutralized with hydro-
gencarbonate solution, washed with water, and dried with sodium
sulfate. When the solvent was subsequently removed by distillation
some 1,2-pentadien-4-yne (17) was lost as shown by GC-analysis
at δ = 1.96 and the remaining protons resonate at 5.17 (un-
resolved ps-d, J = 7.5 Hz, 2 H, terminal cumulenic H) and
5.71 (m, 1 H, nonterminal cumulenic H). The appearance
of the spectrum strikingly resembles that of 1,2-butadiene
5
(
(
methylallene) although the J coupling constant is higher
(
Carbowax, 55 °C). The remaining oily residue was separated from
7
J = 3.0 Hz) than the value for the J coupling of 12. The
IR spectrum (in CCl ) with bands at 2070 cm (m), 1675
polymeric components by vacuum transfer (0.001 Torr) and sub-
jected to preparative gas chromatography (Carbowax, 55 °C). Four
–
1
4
(s), 1610 (s) and 840 (m) support the structure proposal, main fractions were isolated.
the aliphatic C,H-valence vibrations appearing at 2960 (s-
m), 2920 (m), and 2870 (s). Unfortunately, the high reactiv-
ity of the cumulene, which is also displayed by the parent
Fraction 1. 1,2-Pentadien-4-yne (17): The volatile, polymerization-
prone colorless liquid (0.3 g, 12%) had spectroscopic data identical
[21,22]
with those reported in the literature.
[
28]
hydrocarbon
prevented further study of this interesting
Fraction 2. 4-Methyl-1,2,4-hexatriene (18): 0.12 g, 3.5%; unstable
cumulene. As far as the mode of formation of 12 is con-
cerned it could be produced by base-catalyzed isomeriza-
tion of the not isolated vinylbutatriene (11). For the origin
colorless liquid that quickly turns brown, then black at room.
1
temp.). H NMR: δ = 1.65 (ps-s, 3 H, CH
3
), 1.75 (ps-s, 3 H, CH
3
),
4
5
.92 (br. d, J = 7.0 Hz, 2 H, C=C=CH
.80 (t, J = 7.0 Hz, 1 H, –CH=C=). IR (film): ν˜ = 1940 cm (s,
2
), 5.40 (m, 1 H, –CH=C),
of benzene many routes can be proposed; although its for-
–1
mation is not surprising,[
23,26]
with the scant experimental
C=C=C), 2970 (s), 2918 (s), 2860 (s), 1620 (w, C=C), 1435 (s), 1375
evidence a decision between these alternatives is presently (s), 1105 (m), 875 (s), 840 (s, C=C=CH ). To the NMR solution of
2
hardly possible.
this fraction a small excess of TCNE was added and the mixture
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H. Maurer, H. Hopf
FULL PAPER
heated at 50 °C until all allene signals had vanished (ca. 6 h):
ether. The combined organic phases were treated successively with
0.1 m hydrochloric acid, satd. hydrogencarbonate solution and
water before they were dried with sodium sulfate. The major part
of the solvent was removed by distillation and the remaining
higher-boiling fraction was purified by vacuum-transfer. Separa-
tion by preparative gas chromatography (Carbowax, 60 °C) yielded
four fractions (ratio: 1.9:0.7:2.9:1, no quantitative separation was
performed).
1
4,4,5,5-Tetracyano-1,6-dimethyl-3-methylidenecyclohexene (20):
H
NMR: δ = 1.62 (d, J = 7.0 Hz, 3 H, =CCH
3
), 1.97 (s, 3 H, CH
.29 (m, 1 H, ring CH), 5.70 and 5.94 (2×ps-s, 1 H each, =CH
3
),
),
3
6
2
.27 (br. ps-s, 1 H, endocyclic =CH) ppm. Since only very small
amounts of this unstable material was available no further spectra
were recorded.
Fraction 3. 2,3-Hexadien-5-yne (8): 1.14 g, 28%; colorless liquid.
1
1 2 3
= 6.8, J = 3.8 Hz, 3 H, CH
), 2.84 Fraction 1. 2,3-Dimethyl-1,3,4-pentatriene (23):[25] 1H NMR: δ =
H NMR: δ = 1.73 (dd, J
= 1.6, J = 2.2 Hz, 1 H, –CϵCH), 5.17–5.64 (br. m, 1.87 (m, 6 H, 2×CH ), 4.91 (m, 4 H, 2×=CH ) ppm. IR (CCl ):
HC=C=CH–) ppm. IR (CCl
(dd, J
1
2
3
2
4
): ν˜ = 3321 cm–1 (s) and 2110 (w, ν˜ = 3097 cm (w), 1940 (s, C=C=C), 1620 (m, C=C), 850 (s,
–1
–
–
1
4
CϵCH), 1968 (s, –HC=C=CH–), 2959 (w), 2931 (w), 1442 (m),
=CH ). UV (ethanol): λ
m/z = 94 (100) [M ], 79 (60), 77 (70), 53 (45), 51 (55), 39 (76). A
max
(log ε) = 214.5 nm (4.30). MS (70 eV):
2
+
+
373 (m). MS (70 eV): m/z = 78 (100) [M ], 63 (25), 52 (50), 51
(55), 50 (50), 39 (27). UV (ethanol): λmax (log ε) = 216 nm (3.94). sample of 23 in benzene was treated at 50 °C with a slight excess
6 6
C H
(78.11): calcd. C 92.26, H 7.74; found C 91.84, H 7.94. For of TCNE to provide 4,4,5,5-Tetracyano-1,2-dimethyl-3-methyli-
1
the base-catalyzed isomerization 8 (100 mg; 1.28 mmol) was treated
in a NMR tube with 1 mL of a 0.113 n solution of potassium tert-
butoxide in tert-butyl alcohol for 5 h at 60 °C under nitrogen. The
reaction mixture was taken up in 10 mL of ether, the organic phase
was dried with sodium sulfate and subjected to GC separation
denecyclohexene (25): H NMR: δ = 1.97 (br. s, 6 H, 2×CH ), 3.17
3
(m, 2 H, –CH –), 5.85 (m) and 6.02 (m, 2 H, =CH ) ppm.
2
2
Fraction 2. 1,2-Pentadien-4-yne (17): Identified by spectral compari-
son (see above).
Fraction 3. 3-Methyl-1,2-pentadien-4-yne (15): Colorless oil. 1
NMR: δ = 1.83 (t, J = 3.2 Hz, 3 H, CH ), 2.92 (ps-t, J = 1.4 Hz,
= 1.4 Hz, 2 H, =C=CH ) ppm.
H
(
Carbowax, 55 °C). As the sole isomerization product 2,4-hexadi-
yne (4) was isolated in quantitative yield (95 mg, 95%) as shown
3
by its spectroscopic and analytical data.[
10]
1 H, CϵCH), 4.91 (dq, J
1
1
= 3.2, J
2
2
–
IR (CCl
4
): ν˜ = 3310 cm (s) and 2095 (m, CϵCH), 1943 (s) and
Fraction 4. 3-Methyl-2,4,5-heptatriene (19): 0.11 g, 2.5%; colorless,
8
55 (s, C=C=CH
2
). UV (ethanol): λmax (log ε) = 213 nm (4.17). MS
1
unstable liquid. H NMR: δ = 1.59 (m, 3 H, CH
3
), 1.67 (s, 3 H,
), 5.35 (m, 2 H, CH=CMe and
C=C=CHMe), 5.77 (m, 1 H, =CMe–CH=C=) ppm. IR (CCl ): ν˜
3050 cm–1 (w), 3000 (w), 2980 (m-s), 2920 (s), 2857 (m), 1940 (m,
C=C=C), 878 (s), 818 (s), 720 (s). UV (ethanol): λmax (log ε) =
+
(
(
70 eV): m/z = 78 (100) [M ], 77 (60), 63 (39), 52 (79), 51 (95), 50
64), 39 (35). The hydrocarbon is volatile and unstable preventing
3 3
CH ), 1.82 (m, 3 H, CH
4
an exact elemental analysis.
=
Fraction 4. Toluene (24): Identified by comparison with an authen-
tic sample.
+
2
8
21 nm (4.50). MS (70 eV): m/z = 108 (65) [M ], 93 (10), 91 (91),
0 (24), 78 (80), 65 (18), 53 (17), 39 (23). To the NMR solution of
this fraction a small excess of TCNE was added and the mixture
heated at 50 °C until all allene signals had vanished (ca. 6 h):
1
,2,3,4-Hexatetraene (12): Within 20 min a solution of n-butyllith-
ium (10.3 g, 0.075 mol) in 30 mL of anhydrous tetrahydrofuran was
added at –10 °C to a suspension of propargyl triphenylphospho-
4
,4,5,5-Tetracyano-3-ethylidene-1,6-dimethylcyclohexene (21): Off-
nium bromide (28)[
29]
(27.4 g, 0.072 mol) in 150 mL of tetra-
1
white needles (ethanol), m.p. 121–122 °C. H NMR: δ = 1.62 (d, J
7.0 Hz, 3 H, –CHCH –), 1.97 (m, 6 H, 2×CH ), 3.30 [m, 1 H, –
C(Me)H–], 6.35 (m, 2 H, olefinic H) ppm. IR (KBr): ν˜ = 2255 cm
hydrofuran. The reaction mixture turned yellow during the ad-
dition, and when it was complete, the precipitate had dissolved and
an intense yellow solution been formed. After additional stirring
=
3
3
–
1
(
(
9
w, CN), 1620 (m, C=C), 820 (s), 807 (s). MS (70 eV): m/z = 236
(
30 min) at –10 °C the reaction temperature was lowered to –40 °C
+
100) [M ], 221 (58), 194 (71), 182 (44), 171 (37), 144 (32), 108 (84),
and acrolein (3.92 g, 0.07 mol) in 30 mL of tetrahydrofuran was
added during 20 min. The now orange-red solution was stirred at
0
3 (92), 91 (35), 77 (40), 51 (35), 41 (39), 39 (58). UV (ethanol):
max (log ε) = 242 nm (3.21).
λ
°C for 2 h during which time an amorphous precipitate formed.
1
-Acetoxy-2,4-pentadiyne (22): To a solution of 22.2 g (0.28 mol)
To the reaction mixture was added 200 mL of pentane followed by
150 mL of a saturated ammonium chloride solution. The organic
phase was separated and thoroughly washed with ice water to re-
move the main part of the THF, a process accompanied by con-
tinuous production of polymeric material. The pentane was re-
moved in vacuo (100–200 Torr, 10 °C) and the remaining turbid
solution directly subjected to GC separation (Carbowax, 70 °C, ra-
tio 12:benzene ca. 0.55:1). The spectroscopic data of the isolated
[
24]
of 2,4-pentadiyn-1-ol
in 500 mL of anhydrous pyridine 25.0 g
(0.28 mol) of freshly distilled acetyl chloride was added at 0 °C at
such a rate as to keep the temperature of the reaction mixture be-
low 5 °C. After stirring for 2 h the reaction was worked-up as de-
scribed above. Vacuum-transfer furnished 27 g (79%) of 22, color-
1
less oil (b.p./13 Torr: 74–75 °C). H NMR: δ = 2.09 (s, 3 H, CH
3
),
2
–
.28 (t, J = 1.0 Hz, 1 H, –CϵCH), 4.73 (d, J = 1.0 Hz, 2 H,
): ν˜ = 3310 cm–1 (s) and 2060 (w, diyne droplet of 12 are discussed in the main section.
CH –) ppm. IR (CDCl
3
2
unit), 1750 (s), 1215 (s) and 1027 (m, acetate). UV (ethanol): λmax
(
(
5
log ε) = 209 nm (2.41), 218 (2.56), 228 (2.64), 241 (2.67), 254
2.46). C (122.1): calcd. C 68.84, H 4.95; found C 68.23, H
.50.
7
6 2
H O
[
[
1] A. Krüger, H. Hopf, Chem. Eur. J. 2001, 7, 4378–4385.
2] We thank Prof. Dr. L. Ernst (Technical University
Braunschweig) for the calculation of these values by using the
MOLGEN program (Chr. Benecke, R. Grund, R. Hohberger,
A. Kerber, R. Laue, Th. Wieland, University of Bayreuth, Ver-
sion 3, 1995).
3
-Methyl-1,2-pentadien-4-yne (15): To a solution of lithium di-
methyl cuprate prepared as described above from 4.0 g (0.57 mol)
of lithium, 41.2 g (0.29 mol) of methyl iodide and 21.7 g (0.114
mol) of copper(i) iodide in 450 mL of diethyl ether 7.0 g (0.057
mol) of 22 in 100 mL of ether was added at –10 °C. After stirring
at room temp. for 3 h the reaction mixture was hydrolyzed with
satd. ammonium chloride solution. The insoluble material was re-
moved by filtration, and the filtrate was extracted thoroughly with
[
[
3] H. Hopf, Chem. Ber. 1971, 104, 1499–1506.
4] Ashe has described the formation of 8 as a side product in the
CuCl-catalyzed reaction of ethynylmagnesium bromide with 3-
1
bromobutyne; however, with the exception of the H NMR
spectroscopic data no other spectroscopic data of 8 were re-
2706
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2005, 2702–2707

The Preparation of the Last Remaining Acyclic Isomers of Benzene
FULL PAPER
ported: A. J. Ashe, III, W.-T. Chan, Tetrahedron Lett. 1975, 16,
Soc. 1975, 97, 5467–5472; c) Preparative uses: H. Hopf, Tetra-
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Houk, J. Am. Chem. Soc. 1998, 120, 5622–5627.
14] This paper.
2749–2752.
[
5] T. C. Dinadayalane, U. D. Priyakumar, G. N. Sastry, J. Phys.
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[
[
15] 1,2-Hexadien-4-yne (9): H. Hopf, Chem. Ber. 1971, 104, 3087–
[
[
3095.
7] 1,5-Hexadiyne (bipropargyl, 1): a) Synthesis: L. Brandsma,
Synthesis of Acetylenes, Allenes and Cumulenes – Methods and
Techniques, Elsevier, Amsterdam, 2004, p. 56; b) Structure:
D. L. Powell, P. Klaeboe, A. Phongsatha, B. N. Cyvin, S. J. Cy-
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Acetylenes, Allenes and Cumulenes – Methods and Techniques,
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Ashe, W.-T. Chan, J. Org. Chem. 1979, 44, 1409–1413.
[
16] 1,2,4,5-Hexatetraene (biallenyl, 10): a) Synthesis: H. Hopf, An-
gew. Chem. 1970, 82, 703; Angew. Chem. Int. Ed. Engl. 1970,
9
6
, 732; H. Hopf, J. Kleinschroth, I. Böhm, Org. Synth. 1981,
0, 41–48; b) Structure: D. H. Christensen, H. Hopf, P. Kla-
eboe, D. L. Powell, Spectr. Chim. Acta 1973, 29A, 7–16; M.
Traetteberg, G. Paulen, H. Hopf, Acta Chem. Scand. 1973, 27,
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1
1
972, 11, 419–420; H. Hopf, G. Schön, Liebigs Ann. Chem.
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[
[17] 1,2,3,5-Hexatetraene (vinylbutatriene, 11): H. Maurer, H.
Hopf, Angew. Chem. 1976, 88, 687 –688; Angew. Chem. Int. Ed.
Engl. 1976, 15, 628–629.
[
9] 1,3-Hexadiyne (3): a) Synthesis: L. Brandsma, Synthesis of
Acetylenes, Allenes and Cumulenes – Methods and Techniques,
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Hebd. Seances Acad. Sci. 1963, 257, 121.
[18] 3-Methylen-1-penten-4-yne (2-ethynyl-1,3-butadiene, 13): a)
Synthesis: H. Hopf, Chem. Ber. 1971, 104, 1499–1506; b) Struc-
ture: H. Priebe, C. J. Nielsen, P. Klaeboe, H. Hopf, H. Jäger, J.
Mol. Struct. 1987, 158, 249–257; c) preparative uses: H. Hopf,
H. Bader, H. Jäger, Chem. Ber. 1989, 122, 1193–1198.
[19] 3-Methyl-1,4-pentadiyne (14): a) Synthesis: A. J. Ashe, W.-T.
Chang, Tetrahedron Lett. 1975, 16, 2749–2752; b) Structure:
W. R. Roth, V. Staemmler, M. Neumann, C. Schmuck, Liebigs
Ann. 1995, 1061–1118.
[
10] 2,4-Hexadiyne (4): a) Synthesis: L. Brandsma, Synthesis of
Acetylenes, Allenes and Cumulenes – Methods and Techniques,
Elsevier, Amsterdam, 2004, p. 93; b) Structure: J. W. White, J.
Mol. Struct. 1982, 79, 188–214; c) Preparative uses: R. West,
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of Acetylenes, Allenes and Cumulenes – Methods and Tech-
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[
[
[
[21] E. R. H. Jones, H. H. Lee, M. C. Whiting, J. Chem. Soc. 1960,
3483–3489.
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[23] H. Hopf, Chem. Ber. 1971, 104, 3087–3095.
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[26] H. Hopf, Tetrahedron Lett. 1970, 11, 1107–1109 and refs. cited
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1
105–1109 and refs. cited therein; b) Structure: E. Toernung,
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981, 71, 71–89; c) Preparative uses: A. L. Klebanski, M. Jelen-
1
jewski, V. Tschuganov, Zh. Obshch. Khim. (J. Gen. Chem. U.
S. S. R.) 1947, 17, 1436–1450 (Chem. Abstr. 1948, 42, 2920).
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[27] E. J. Corey, R. A. Ruden, Tetrahedron Lett. 1973, 14, 1495–
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1970, 82, 703; Angew. Chem. Int. Ed. Engl. 1970, 9, 732; b) [28] J. L. Ripoll, J. Chem. Soc. Chem. Commun. 1976, 235–236; J. L.
Structure: P. Klaeboe, A. Phongsatha, B. N. Cyvin, S. J. Cyvin,
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Traetteberg, H. Hopf, Acta Chem. Scand., Ser. A 1981, 35, 365–
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371; P. Bischof, R. Gleiter, H. Hopf, F. T. Lenich, J. Am. Chem.
Received: December 09, 2004
Eur. J. Org. Chem. 2005, 2702–2707
www.eurjoc.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2707