Polar reactions of acyclic conjugated bisallenes

Article abstract of DOI:10.3762/bjoc.9.5

The chemical behaviour of various alkyl-substituted, acyclic conjugated bisallenes in reactions involving polar intermediates and/or transition states has been investigated on a broad scale for the first time. The reactions studied include lithiation, reaction of the thus formed organolithium salts with various electrophiles (among others, allyl bromide, DMF and acetone), oxidation to cyclopentenones and epoxides, hydrohalogenation (HCl, HBr addition), halogenation (Br2 and I2 addition), and [2 + 2] cycloaddition with chlorosulfonyl isocyanate. The resulting adducts were fully characterized by spectroscopic and analytical methods; they constitute interesting substrates for further organic transformations.

Full text of DOI:10.3762/bjoc.9.5

Polar reactions of acyclic conjugated bisallenes
Reiner Stamm and Henning Hopf*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 36–48.
Institut für Organische Chemie, Technische Universität Braunschweig,
Hagenring 30, D-38106 Braunschweig (Germany), Fax: +49 (0)531 /
391 5388
doi:10.3762/bjoc.9.5
Received: 22 August 2012
Accepted: 07 December 2012
Published: 08 January 2013
Email:
Henning Hopf* - H.Hopf@tu-bs.de
Alkynes and cumulenes part XXVIII, for part XXVII see [1].
Associate Editor: M. P. Sibi
* Corresponding author
Keywords:
β-lactams; conjugated bisallenes; cyclopentenones; epoxidation;
halogen addition; hydrohalogenation; ionic additions; metalation
© 2013 Stamm and Hopf; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The chemical behaviour of various alkyl-substituted, acyclic conjugated bisallenes in reactions involving polar intermediates and/or
transition states has been investigated on a broad scale for the first time. The reactions studied include lithiation, reaction of the thus
formed organolithium salts with various electrophiles (among others, allyl bromide, DMF and acetone), oxidation to cyclopen-
tenones and epoxides, hydrohalogenation (HCl, HBr addition), halogenation (Br2 and I2 addition), and [2 + 2] cycloaddition with
chlorosulfonyl isocyanate. The resulting adducts were fully characterized by spectroscopic and analytical methods; they constitute
interesting substrates for further organic transformations.
Introduction
Whereas the use of hexa-1,2,4,5-tetraene (1) and its derivatives butyl derivative 3 (7,7-dimethylocta-1,2,4,5-tetraene) as an
in pericyclic reactions is well documented [2-6], relatively little asymmetric starting material for our polar reactions (Scheme 1).
is known about the behavior of noncyclic, conjugated bis-
allenes in ionic or polar reactions, whether these involve meta-
lation processes, the addition of halogens and hydrogen halides,
or oxidation reactions, to name but a few. To fill this gap we
initiated a research program, hoping also that polar reactions of
the bisallenes would lead to functionalized derivatives that
could be useful for other preparative investigations, just as in
Scheme 1: The alkylated conjugated bisallenes 1– 3 as model
systems for polar reactions.
the pericyclic case [2-6]. The results of these initial studies are
presented here.
Since the parent hydrocarbon 1 (biallenyl) is rather unstable [7], Hydrocarbon 2, prepared by Skattebøl in the early 1960s
we decided to use the tetramethyl derivative 2 (2,7-dimethyl- [3,8,9], is a crystalline solid at room temperature and stable for
octa-2,3,5,6-tetraene) as a symmetric substrate and the tert- very long times. Bisallene 3, first prepared by Ruitenberg,
36

Beilstein J. Org. Chem. 2013, 9, 36–48.
Kleijn, Westmijze, Meijer and Vermeer [10] is decidedly more methylocta-2,3,5,6-tetraene (5, pentamethylbiallenyl) in good
stable than the parent hydrocarbon 1: in solution the colorless yield (77%) (Scheme 2). The hydrocarbon, an oily substance at
oil can be kept for weeks at room temperature before discol- room temperature, which solidifies in the deep freezer, is
oration sets in, and in neat form it withstands polymerization for perfectly stable and can be handled easily. It is characterized by
about a week in the deep freezer. The protocols for making its spectroscopic data (see Supporting Information File 1), in
these two derivatives from readily available starting materials particular by a septet at δ 5.58 ppm with 5J = 2.8 Hz for the
are straightforward and high-yielding.
remaining allenic proton in the 1H NMR spectrum and by two
singlets at δ 202.5 and 202.8 ppm for the central allene carbon
atoms in the 13C NMR spectrum. When the methylation is
carried out after 3 equiv of n-butyllithium have been added,
small amounts of a dimethyl product, i.e., the fully alkylated
biallenyl 7 can be isolated, possibly formed via the dilithiated
Results and Discussion
Reactions of 2/4 with electrophiles
Metalation of 2 and quenching of 4 with simple
halides
Treating 2 in THF at −30 °C with a slight excess of n-butyl- intermediate 6 (up to 15% depending on the exact alkylation
lithium (1.2 equiv) in the presence of tetramethylethylendi- conditions). However, since the purification of this product
amine (TMEDA) and quenching the formed monoanion 4 with required gas-chromatographic separation, a stepwise approach
methyl iodide in THF results in the formation of 2,4,7-tri- was adopted to prepare it: the monomethylation product 5 was
Scheme 2: Alkylation and silylation of 2.
37

Beilstein J. Org. Chem. 2013, 9, 36–48.
subjected to the metalation/methylation sequence again and hydrocarbons: two monoallylated products, 14 and 17, and two
yielded pure 7 in very good yield (86%). This hydrocarbon, bisallylated ones, 19 and 20, with the relative yields determined
symmetric and hence relatively high-melting again (mp by gas chromatography given in Scheme 3; ca. 10% of the reac-
72–75 °C), is a known compound [11]; however, in our hands tion mixture was unreacted starting material 2.
its spectroscopic properties differed slightly from the published
ones (see Supporting Information File 1).
Whereas the structure assignments of 14 and 20 are based on
the usual spectroscopic data (see Experimental section;
Changing to trimethylsilyl chloride and tert-butyldimethylsilyl Supporting Information File 1) and are unequivocal, for 17 and
chloride as quenching reagents resulted in the expected forma- 19 only GC–MS data were available, and the given structures
tion of 8 and 9, which are both produced in very good yields are hence speculative and rest only on their mass spectra.
and are oils again. Their structures follow from the spectro- Hydrocarbon 14 was obtained in 80% purity by preparative gas
scopic data listed in Supporting Information File 1. Since 5, 8, chromatography; its complete separation from 17 failed. The
and 9 still have a “free” allenic hydrogen atom they can be alky- bisallyl derivative 20 was isolated in analytically pure form by
lated/silylated again, yielding 10, 11, and 12, all in excellent preparative gas chromatography. All hydrocarbons are color-
yield. The highly symmetric derivatives 11 and 12 are again less oils that, even at −15 °C (deep freeze), are stable for a
solids, whereas 10 is an oil at room temperature. We have limited time only.
reported on the X-ray analysis of 11 in an earlier study [12].
A simple rationalization of the product formation, in which 14
Metalation of 2 and quenching of 4 with allyl bro-
mide (13)
and 17 are the primary products, is given in Scheme 3. Hydro-
carbon 14 is the direct allylation product of 4, whereas for the
In the next series of experiments 13 was introduced as the generation of 17 we have to assume that its precursor
quenching reagent for the monolithiated bisallene 4 in the hope monoanion has the propargyl structure 16. In all these struc-
of introducing additional unsaturation into the reaction prod- tures the lithium atom is only meant to mark the position of the
ucts. Although this goal was accomplished, the overall process carbon atom at which the electrophile attacks; no proposition
is considerably more complex (Scheme 3). Treatment of 2 or 4 about the nature of the carbon–lithium bond or the structure of
in THF at −35 °C with 13 resulted in the formation of four the anion is intended. Further allylation of 14 under the reac-
Scheme 3: Allylation of the monoanion 4.
38

Beilstein J. Org. Chem. 2013, 9, 36–48.
Scheme 4: Metalation/silylation of hydrocarbon 3.
tion conditions and formation of the symmetric bisallyl deriva- by preparative gas chromatography; even the analytical GC
tive of 2, hydrocarbon 15, was not observed.
(capillary column) did not show full resolution. Assuming that
the hydrogen atom at the substituted end of 3 is not replaced by
On the other hand, metalation of 17 to the monoanion 18 and its the TMS-group for steric and electronic reasons, four monosily-
reaction with allyl bromide (13) could furnish the bisallyl lated products are possible, i.e., silylation at the unsubstituted
derivatives 19 and 20. Note that these reaction pathways are not end of 3 leading to two diastereomers. General structure 21
the only ones conceivable. For example, 14 could also provide (Scheme 4) accounts for our experimental observations.
20 if the propargyl anion of the former reacts with 13.
Regardless of the exact structures of the silylation products 21,
Metalation/trimethylsilylation of 3
we did not observe any selectivity in this transformation.
In an attempt to learn about the regioselectivity of the lithiation/
quenching process, the asymmetric bisallene 3 was subjected to Quenching of 4 with N,N-dimethylformamide (DMF)
the above conditions, employing a three-fold excess of the and acetone
metalation reagent. Compared to the tetramethylderivative 2, Metalated allenes are known to be converted into allenic alde-
this hydrocarbon possesses five different allenic C–H bonds, hydes on DMF treatment [13-15].
which could in principle be replaced by a trimethylsilyl
substituent.
The bisallene 2 reacts analogously: after quenching of the
monoanion 4 with DMF and acidic work-up, the bisallenic alde-
As it turned out, 3 is considerably less reactive than 2: after hyde 22 is obtained in 75% yield and 90% purity (GC analysis
comparable reactions times (see Supporting Information File 1) of the raw product). The compound is apparently not very
only 37% of the substrate had been converted into products. As stable, though: after distillation, 22 is isolated with analytical
shown by GC–MS analysis the reaction mixture contained four purity (>99%) as a slightly yellow oil, but with a reduced yield
mono silylated products (m/z = 206, [M]+) in 1:2.7:3.0:5.3 ratio, of 48%, the remainder being nonvolatile oligomeric com-
whose mass spectra were virtually identical. We therefore pounds. The structure of the monoaldehyde (see Scheme 5)
assume that the structures of these products are also very unambiguously follows from its spectroscopic data, in particu-
similar. Unfortunately, these compounds could not be separated lar the absorption of the formyl proton at δ 9.49 and the allenic
Scheme 5: Quenching of 4 with DMF and acetone.
39

Beilstein J. Org. Chem. 2013, 9, 36–48.
septet at 5.59 ppm with the typical coupling constant 5J of anionic intermediates with a propargyl structure. The “half-
2.8 Hz in the 1H NMR spectrum. In the 13C NMR spectrum the rearranged” diol 24 is a slightly yellow oil at room temperature.
central allenic carbon atoms are revealed by signals at 204.6 Structure-defining are the missing allene proton and three
and 216.0 ppm; the carbonyl group absorbs at 190.9 ppm.
signals for carbon atoms not connected to a hydrogen atom: the
central allene carbon atom at 210.0 ppm and the two acetylene
Replacing the DMF by acetone should lead to a tertiary bisal- carbon atoms at δ 72.0 and 75.0 ppm.
lenic alcohol, and this is indeed the case. However, over all the
situation becomes more complex. As illustrated in Scheme 5 The symmetric diol 25 crystallizes from ethanol with one
three alcohols are produced; their yields are only moderate and equivalent of included water. Stoichiometric host/guest asso-
become even poorer on separation due to the instability of these ciates have often been observed for this class of compounds
products. Again, the conversion is incomplete leading to the [16]. The NMR spectra of 25 show only a small number of
recovery of 2 in 25% yield.
signals due to the symmetry of the molecule. In the 13C NMR
spectrum the acetylenic (δ 74.2 and 83.8 ppm) and tertiary
The three alcohols produced have the structures 23–25 as carbon atoms (δ 41.4 and 66.7 ppm) are clearly visible as
shown in Scheme 5. According to GC analysis of the raw pro- singlets. All other spectroscopic details agree with the proposed
duct mixture they are produced in 13, 9, and 39% yield, respect- structure (Supporting Information File 1).
ively. However, only 10, 4, and 8% survive the elaborate and
time-consuming separation by distillation, chromatography, and Miscellaneous reactions of 2 and 4 with elec-
recrystallization.
trophiles
The carboxylation of simple metalated allenes has often been
The analytically pure (>98%) monoalcohol 23, a colorless oil at described in the chemical literature [17]. And the process is
room temperature, is the product expected from the reaction of apparently successful for 2/4 also: the 1H NMR spectrum of the
4 with acetone. The spectroscopic data leave no doubt about its raw product, obtained in ca. 70% yield, shows the expected
structure. The 1H NMR spectrum displays the typical septet signals at δ 5.59 ppm (5J = 2.9 Hz) for the remaining allene
(5J = 3.2 Hz) of the “remaining” allenic hydrogen atom at δ proton and at 9.5 ppm (broad signal) for the carboxyl group.
5.52 ppm. The OH-group absorbs at δ 2.30 ppm, a signal that However, all attempts to isolate the new bisallene derivative 26
vanishes after D2O-exchange. In the 13C NMR spectrum two resulted in resin formation and destruction of the original
different central allenic carbon atoms are recognizable again (δ signals (Scheme 6).
199.6 and 201.9 ppm), and the saturated tertiary carbon atom
absorbs as a singlet at δ 71.5 ppm. All other spectroscopic data In another series of experiments we tried to use 4 as a building
(see Supporting Information File 1) also agree with the struc- block for different tri- and tetraallenes. As indicated in
ture proposal.
Scheme 6 all of these experiments failed. Neither could we
prepare the “coupling product” 27 by treatment of 4 with the
The other two alcohols, 24 and 25, both of them diols, are obvi- biselectrophile dimethylysilyl dichloride nor the dimer 28 by
ously bisalkylation products and involve the generation of the direct action of iodine on the organolithium compound 4. In
Scheme 6: Further reactions of 2/4 with various electrophiles.
40

Beilstein J. Org. Chem. 2013, 9, 36–48.
this latter case we noted after work-up and GC–MS analysis of propanones (the so-called “allenoxide-cyclopropanone
the raw product mixture that addition products of 2 had been rearrangement”) or are oxidized a second time to provide dioxa-
produced besides mostly polymeric material. We will return to spiro[2.2]pentane derivatives [18-20].
the addition of iodine to 2 later. The coupling of various
propargyl halides (29, R = H, CH3; X = Cl, Br) in the As far as functionalized allenes are concerned, the contribu-
presence of a Pd-catalyst did not lead to the hoped for hydro- tions of Bertrand, Grimaldi, and co-workers [21-24] are particu-
carbon 30.
lar noteworthy in the present context. The French authors
demonstrated that vinylallenes, hydrocarbons rather similar to
the systems studied here, provide cyclopentenone derivatives in
Oxidation of conjugated bisallenes
The oxidation of allenes has already been studied previously. In a process mimicking the Nazarov cyclization [25].
seminal papers Crandall and his students described the epoxida-
tion of differently substituted monoallenes and showed that The first conjugated bisallenes, including the tetramethyl
methylene oxiranes are the initial oxidation products. These, derivative 2, were epoxidized by Pasto et al. [26]. These authors
however, are often not isolated but react further either to cyclo- showed that this hydrocarbon provides the products 35 and 36
on treatment with m-chloroperbenzoic acid (MCPBA) in
dichloromethane (product ratio: 71:29). With excess dimethyl-
dioxirane (DMDO) the epoxide 37 was produced solely,
evidently generated by further oxidation of the monooxidation
product 36. The still remaining double bond in 37 is not
oxidized under these conditions (Scheme 7).
To rationalize their findings these authors propose monooxida-
tion of 2 with MCPBA to the alleneoxide 31 initially, which is
in equilibrium with its conformational isomer 32. Ring-opening
of these strained intermediates then provides the “stretched” and
the “closed” zwitterions 33 and 34, respectively. Whereas the
open form 33 is intercepted by the anion derived from the epox-
idation reagent, m-chlorobenzoic acid (MCBA-anion), to
provide 35, the curved intermediate 34 can easily cycloiso-
merize to the cyclopentenone 36. That the nature of the oxi-
dation reagent plays a crucial role in these reactions was
demonstrated by oxidation of 8,8-dimethylnona-2,3,5,6-
tetraene, which reacts to a complex mixture of cyclopentenone
derivatives of type 36 simply on exposure to air (derivative 2
does not react under these conditions) [26].
Since some of the above oxidation products were not fully puri-
fied and characterized we decided to investigate the epoxida-
tion of several bisallenes prepared in this study more carefully.
The oxidation of the tetramethylbisallene 2 was carried out with
magnesium monoperoxy phthalate (MMPP) since we had also
observed the formation of addition products of 2 (m-chloroben-
zoates) when we used MCPBA for epoxidation. Indeed, ketone
36 could be obtained in 80% yield with MMPP (GC analysis),
with no other oxidation products being observed (Scheme 8).
Compound 36 is a highly volatile ketone, and this may explain
the pronouncedly reduced yield (32%) when it was isolated
from the product mixture by column chromatography followed
Scheme 7: Oxidation of conjugated bisallenes with different oxidizing
agents according to [26].
by rotary evaporation of the solvent. The spectroscopic data of
36 are identical with those given in the literature [26]. As far as
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Beilstein J. Org. Chem. 2013, 9, 36–48.
File 1). Again, we believe that the Pasto mechanism explains
the outcome of the process well.
Lifting the symmetry of the starting material would allow
insight into the regioselectivity of the epoxidation process. We
therefore took bisallene 5 and subjected it to MMPP treatment
(fivefold excess). The reaction took place in acceptable yield
(65%) and provided a mixture of two bis oxidation products, 40
and 41, in 5:1 ratio. Although the two regioisomers were sep-
arated on the analytical GC, the attempted preparative GC sep-
aration failed because the products did not survive the required
column temperature (100 °C). Although the structural differ-
ence between the two isomers is rather small, they can be distin-
guished by the absorption of the remaining olefinic proton in
the 1H NMR spectrum. In the main isomer 40 this proton (2-H)
is right next to the carbonyl group and absorbs at δ 6.18 ppm,
whereas in the side product this proton (3-H) appears at δ 6.86
ppm. Both protons couple with the neighboring methyl
substituents (q, 4J = 1.4 and 1.2 Hz, respectively) [27].
Scheme 8: Oxidation of 2, 7 and 5 with MMPP.
the oxidation mechanism is concerned it agrees with the sugges- Applying the same mechanism as in Scheme 7 leads to the
tion of Pasto (see above).
conclusion that it is the more highly substituted allene moiety
that is attacked preferentially by the epoxidation reagent.
When the epoxidation reagent was employed in excess (five-
fold) the epoxide 37 was produced in 66% yield (with refer- Another asymmetric bisallene is the tert-butyl derivative 3. It
ence to 2) as a colorless oil. Since the analytical and spectro- has already been observed by Pasto and co-workers that tert-
scopic data are incomplete in the literature [26], these data are butylated bisallenes can be oxidized by air (see above) [26]. We
given in full in Supporting Information File 1.
therefore kept a solution of 3 in dichloromethane for 4 days
under an atmosphere of pure oxygen. The only oxidation pro-
The next bisallene to be oxidized was the fully substituted hexa- duct is 5-tert-butyl-4-methenyliden-2-cyclopenten-1-one (44,
methyl derivative 7, prepared as described in Scheme 2. Reac- Scheme 9), produced in 20% yield. An increase in oxidation
tion with 0.5 equiv of MMPP resulted in the formation of 68% time does not result in a yield increase, since the already
of 38 and a trace amount of the epoxide 39 (8%), while 24% of produced 44, which is evidently not very stable, starts to form
the substrate was recovered (GC analysis). Since the complete intractable polymers. Typical features of the 1H NMR spectrum
separation of the two products was difficult, the experiment was of this cyclic ketone are the intense singlet at δ 1.00 ppm for the
repeated with excess MMPP (1.5 equiv); in this case the yield tert-butyl substituent and the multiplet of the tertiary ring
of 39 was 62% and the ketoepoxide could be separated and hydrogen atom at δ 2.50 ppm, which couples with the semi-
characterized spectroscopically (see Supporting Information cyclic methylene protons. In the 13C NMR spectrum this
Scheme 9: Oxidation of the asymmetric bisallene 3 by air.
42

Beilstein J. Org. Chem. 2013, 9, 36–48.
sp3-hybridized carbon atom absorbs at δ 57.7 ppm (doublet, an epoxide analogous to 46 had taken place. We assume that the
low-field shift because of the neighboring carbonyl group, double bond in 47 is so highly shielded by the neighboring tert-
which absorbs at δ 208.6 ppm).
butyldimethylsilyl substituent that it is not attacked by MMPP
anymore. To obtain analytically pure material we attempted to
To rationalize our observations we adopt the radical mecha- purify the raw product by preparative gas chromatography.
nism proposed previously by Pasto [26]. In the first step, To realize a sufficiently short retention time, the column
oxygen attacks the substrate under formation of the diradical 42. temperature had to be increased to 240 °C. Astonishingly the
This, in turn, closes from its resonance structure 43 to yield the resulting oil, obtained in poor yield (11%) showed a singlet at
isolated product 44. Had the oxidation taken place at the unsub- δ 6.45 ppm, obviously caused by an olefinic proton. This means
stituted allene moiety of 3, the tert-butyl substituent would have that during the separation process (which employed hydrogen as
ended up at the semicyclic double bond according to this mech- the carrier gas) the initial product 47 had lost one of its volumi-
anism. Since a second tert-butyl signal could not be detected in nous substituents. The spectroscopic data (see Supporting Infor-
the NMR spectrum of the product mixture, the reaction mation File 1) suggest that it is 48 that has been produced under
evidently takes place with very high regioselectivity.
the purification conditions.
Hydrohalogenation and halogenation of
In a final set of epoxidation experiments the highly hindered
silyl derivatives 11 and 12 were oxidized with MMPP acyclic conjugated bisallenes
(Scheme 10).
Hydrohalogenation of 2 and 5
The addition of protic acids HX (with X usually being Cl and
Br) to allenes has been studied quite carefully. The process
usually takes place according to the Markovnikoff rule and
often cannot be stopped at the monoaddition stage [28]. For
simple allenes, including the parent hydrocarbon, another side
reaction leads to cyclodimerisation products (1,3-dihalo-
cyclobutanes) [29]. With hydrochloric and hydrobromic acid,
vinylallene (penta-1,2,4-triene) provides (E)- and (Z)-2-halo-
1,3-pentadiene [21,30].
For the reaction of 2 with hydrogen chloride, a solution of the
hydrocarbon in diethyl ether was cooled to −70 °C under
nitrogen and an excess of HCl gas in ether was added. The
temperature was increased to −30 °C, and the solution was then
kept at this temperature for 5 h; afterwards it was slowly raised
to room temperature. After work-up an oily mixture was
Scheme 10: Epoxidation of the disilylbisallenes 11 and 12.
Treatment of 11 with MMPP (0.5 equiv) in methanol for 2 days obtained, which by preparative gas chromatography could be
yielded a mixture of the ketone 45 (54%) and the secondary separated into two components. One component consisted of
product 46 (24%), with the rest of the product mixture being the (E)- and (Z)-3-chloro-2,7-dimethl-octa-2,4,6-triene (55 and 56,
unchanged starting material. Since the two products could not together 45%), the other was m-cymene (57, 20%, Scheme 11).
be separated by preparative gas chromatography, the oxidation
was repeated with 0.7 equiv of MMPP and extended reaction Whereas 57 could be readily purified by gas chromatography
time (10 d, rt). Now all starting material was oxidized and 46 and identified by spectral comparison with the authentic hydro-
could be isolated by preparative GC in analytically pure form. carbon, the two diastereomers 55 and 56 could not be separated
The spectroscopic data are unremarkable and allow an unam- and were analyzed as a mixture. All spectroscopic and analyt-
biguous structure assignment (see Supporting Information ical data (see Supporting Information File 1) of this mixture
File 1).
point to the structure(s) given in Scheme 11.
The oxidation of the sterically more shielded 12 was even To rationalize these findings we propose the following addition
slower than that of 11: after 3 days at room temperature only mechanism (Scheme 11). The bisallene 2 possesses three
47% of the substrate had reacted. According to spectroscopic different positions to which a proton can be added: the two
analysis (see Supporting Information File 1) the ketone 47 had terminal carbon atoms of the allene moiety and its central
been formed in the transformation, i.e., no further oxidation to carbon atom. Although the two terminal atoms are differently
43

Beilstein J. Org. Chem. 2013, 9, 36–48.
Scheme 11: The addition of HCl and HBr to the bisallenes 2 and 5.
substituted the resulting cations are both of the vinyl type and (X = Br) now being 20:1, and their combined yield amounting
cannot be stabilized by mesomeric or hyperconjugative effects. to 53% with 27% of m-cymene 57 generated as the third,
This, however, is not the case for the central sp-hybridized aromatic product. The increase in diastereoselectivity is
carbon atom and the protonation will hence take place there. expected, considering the larger ionic radius of bromide
The resulting cation 49 is tertiary and can be stabilized addition- compared to chloride.
ally by interaction with the neighboring (allyl) double bond as
well as the remote allene group; a corresponding resonance The case of the higher, asymmetrically substituted bisallene 5
structure is shown in 52. Intermediate 49 can adopt two extreme allowed the investigation of the discrimination between two
conformations: the stretched (transoid) structure 49 and the different allene subunits. Under similar conditions as those
coiled (cisoid) structure 50. For the latter form we can again above (5 h at −45 °C, then slow temperature increase to room
formulate a resonance structure, 53. In principle, both the 49/50 temperature) HCl addition resulted in the formation of four
and the 52/53 pair can equilibrate, but for both pairs we would products (GC–MS-analysis of the raw product mixture): two,
expect the transoid structure to be more stable (less internal produced in 9 and 7% yield, were isomers of the starting ma-
steric interaction). When 52 is intercepted by the nucleophile terial. We assume that they are hydrocarbons comparable to 57
(Cl− in this case) the (E)-diastereomer 55 results; likewise 53 (R = CH3); however, their separation was unsuccessful. The
furnishes the (Z)-isomer 56. From these considerations we other two products, produced in 70 and 8% yield, are mono-
would expect 55 to be the dominant product. If 50 loses a chlorides. They could be separated (under considerable ma-
proton the butadienylallene 51 results, which by reprotonation terial loss) by preparative gas chromatography, and according to
at its allene carbon atom, followed by electrocyclization is their spectral data (see Supporting Information File 1), we
converted into 54. Proton loss of this σ-complex yields the assign structures 55 and 56 to them (Scheme 11).
isolated aromatic product 57 in the last step. Thermal cycliza-
tions of tetraenes such as 51 to aromatic compounds have been As in the case of the HCl addition to 2, the UV spectra of the
reported in the literature [31].
products were particularly valuable for structure assignment;
they clearly show that the products obtained possess a conju-
For the interception of the cations in Scheme 11 by bromide gated triene chromophore. The formation of the monochlorides
very similar results are obtained, the ratio of 55 (X = Br) and 56 very likely proceeds via the carbocationic intermediates 49/50
44

Beilstein J. Org. Chem. 2013, 9, 36–48.
and 52/53 (R = CH3). For the generation of the aromatic mixture, but all attempts to separate at least the major adducts
isomers of the substrate we postulate a butadienylallene inter- by chromatography and/or recrystallization failed.
mediate, 51, again.
More is known already about the addition of iodine to conju-
Halogenation of 2, 3 and 11
gated bisallenes. For example, Neumann and Schriewer added
Among the addition of halogens the addition of bromine and iodine to the meso-derivative 61 and obtained a diiodide to
iodine to various allenes has been particularly well studied [28]. which they assigned structure 62 [34]. Although the binding
Whereas bromine is added to the more highly substituted sites of the two halogen atoms are apparently correct, the given
double bond of simple monoallenes, iodine prefers the addition stereochemistry is disputable, since no unambiguous stereo-
at the less substituted end. Whether the same or a similar mech- structure assignment was performed. It is surprising that the d,l-
anistic path is followed in these reactions is unknown. As com- diastereomer of 61 furnishes the same iodine addition product
pounds more related to the bisallenes studied here, several as the meso-compound, as claimed by the authors [33].
vinyallenes have been treated with one equivalent of bromine to
yield adducts in which the most highly substituted double bond Turning to our more symmetric 2, we prepared the analogous
has been attacked, leaving the conjugated system unaffected diiodide 63 in very good yield under similar conditions. Note
[32,33]. Applied to the conjugated bisallene 2, this mode of ad- that we were also unable to determine the stereochemistry of the
dition should provide either the monoadduct 58 or the bisadduct central double bond from our spectroscopic data given in
59 as the primary product (Scheme 12).
Supporting Information File 1 because of symmetry properties
of the product.
The addition of excess bromine (2 equiv) to 2 was carried out in
trichloromethane solution at −40 °C. The GC–MS analysis of For the monosubstituted bisallene derivative 3 the situation is
the raw product mixture showed that at least three dibromides stereochemically more complex. Treatment of this chiral com-
(monoadducts) and a tetrabromide as well as 57 (R = H) had pound with iodine in tetrachloromethane at −20 °C caused the
been formed. However, the actual isolation of any of these formation of two products. To make the formation of bis (or
products by preparative gas chromatography was unsuccessful. higher) adducts unlikely, we not only treated the hydrocarbon
Since two of the bromine substituents in the putative tetrabro- with just one equivalent of the halogen, but always kept its
mide 59 are in allylic positions, we reasoned that we could concentration during the addition process very low to reduce the
hydrolyze this adduct to the corresponding diol. We therefore chance of a subsequent halogenation. The two products
stirred a slurry of 59 in water in the presence of neutral (symbolized by the general structure 64 in Scheme 13) were
aluminum oxide and indeed could isolate, after work-up, the obtained with a total yield of 64% and in ca. 15:1 ratio (GC
diol 60 in the form of colorless cubes. The IR spectrum of the analysis). Although the two products could not be separated,
compound is dominated by an intense OH absorption band at spectroscopic data could be derived for the main component in
3220 cm−1; in the 1H NMR spectrum both the olefinic the product mixture. The protons of the central double bond
(δ 6.99 ppm) and the methyl protons (δ 1.54 ppm) appear as couple with a coupling constant of 13.5 Hz; this double bond is,
sharp singlets; the UV spectrum reveals the presence of a diene hence, very likely trans-configured. Since this value is about
chromophore (see Supporting Information File 1).
half way between the (E) and (Z) coupling constants of other
oligoenes [35] an unambiguous decision is difficult on this basis
Repeating the bromine addition experiment with the fully alone. We prefer the (E)-configuration, though, since the alter-
substituted bisallene derivative 11 also leads to bromine addi- native diastereomer and the path leading to it would be much
tion as demonstrated by GC–MS analysis of the raw product more sterically hindered. Unfortunately, all other olefinic
Scheme 12: The addition of bromine to the bisallene 2.
45

Beilstein J. Org. Chem. 2013, 9, 36–48.
protons absorbed as (pseudo) singlets (at δ 6.04, 6.44 and
6.59 ppm). We therefore could not assign the orientation
between the iodine and the tert-butyl group at one of the
terminal double bonds.
Also open at the present time is the question of whether the
iodine addition takes place by a radical or an ionic mechanism.
Addition of chlorosulfonyl isocyanate to 2
The addition of heterocumulenic systems to several allenes has
been studied previously. For instance, Moriconi and Kelly
added chlorosulfonyl isocyanate (CSI, 66) to monoallenes, such
as tetramethyallene (65, 2,4-dimethylpenta-2,3-diene), at ice-
bath temperature and obtained, after aqueous work-up, a mix-
ture of the β-lactam 68 and the cross-conjugated amide 69;
other alkylated allenes reacted similarly (Scheme 14) [36,37].
To rationalize their findings they proposed the initial genera-
tion of a zwitterionic intermediate 67, which subsequently
either ring-closes to 68 or is converted into 69 by proton loss
and hydrolysis of the chlorosulfonyl group.
Scheme 13: The addition of iodine to the conjugated bisallenes 61, 2
and 3.
Scheme 14: Addition of chlorosulfonyl isocyanate (CSI, 66) to allenes.
46

Beilstein J. Org. Chem. 2013, 9, 36–48.
When the bisallene 2 was treated with 66 (1 equiv) at 0 °C the the formation of cyclopentenone derivatives in a Nazarov-type
cycloaddition was complete after ca. 4 h as shown by moni- cyclization. The addition of hydrohalides leads to halo-1,3,5-
toring the isocyanate band at 2260 cm−1 in the IR spectrum. For trienes whereas bromine and iodine addition furnish conjugated
work-up the reaction mixture was hydrolyzed with ice water dihalodienes and trienes. Finally, the reaction of 2 with chloro-
and the organic phase separated. On standing, a small amount of sulfonyl isocyanate provides a β-lactam derivative formed in a
a solid crystallized from the aqueous phase. To this we assign formal [2 + 2] cycloaddition via a zwitterionic intermediate.
structure 75 according to its spectral data (Supporting Informa-
tion File 1). Of particular diagnostic importance are its amide Taken together, these studies show that conjugated bisallenes
bands (3515 and 1679 cm−1) in the IR spectrum and the absorp- [39], which are readily available by high-yielding synthetic
tion maximum at 284 nm in the electronic spectrum (triene transformations from simple substrates, are useful starting ma-
chromophore). The central double bond of the triene system is terials for the preparation of a plethora of novel organic com-
trans-configured (3J = 15.2 Hz) [35].
pounds.
The main product is the β-lactam derivate 71. Since this prima-
ry addition product displayed some erratic properties in our
hands (sometimes it polymerized, sometimes it survived the
work-up), we decided to remove the reactive ClSO2 group by
hydrolyzing it with sodium sulfite solution under slightly basic
conditions (KOH, pH = 8). To the resulting colorless solid we
assign structure 72. The IR spectra of the substrate 71 and the
free lactam 72 are similar with the exception of the absorption
band for the “free” N–H band at 3430 cm−1. The carbonyl
group of the lactam is shifted to lower wavenumbers
(1728 cm−1) as compared to 71 (1760 cm−1); all other spectro-
scopic data (Supporting Information File 1) also agree with the
structural proposal 72.
Supporting Information
Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-5-S1.pdf]
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