Halogenoborane mediated allene cyclooligomerization

Article abstract of DOI:10.1039/C8SC04790A

The halogenoboranes XB(C₆F₅)₂ (X: Cl, Br) react with allene to give a mixture of the cyclotrimer 1,3,5-trimethylenecyclohexane (1) and the related halogenoborylated cyclotetramerization products 3a and 3b. Alkyl-substituted allenes were catalytically cyclotrimerized metal-free by XB(C₆F₅)₂ (X: Cl, Br) to give cis,trans-2,4,6-trialkyl-1,3,5-trimethylenecyclohexanes under mild conditions.

Full text of DOI:10.1039/C8SC04790A

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Halogenoborane mediated allene
cyclooligomerization†
Cite this: DOI: 10.1039/c8sc04790a
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of Chemistry
Xin Tao, Christian Wolke, Constantin G. Daniliuc,
Gerald Kehr
¨
*
and Gerhard Erker
Received 26th October 2018
Accepted 21st December 2018
The halogenoboranes XB(C6F5)2 (X: Cl, Br) react with allene to give a mixture of the cyclotrimer 1,3,5-
trimethylenecyclohexane (1) and the related halogenoborylated cyclotetramerization products 3a and 3b.
Alkyl-substituted allenes were catalytically cyclotrimerized metal-free by XB(C6F5)2 (X: Cl, Br) to give
cis,trans-2,4,6-trialkyl-1,3,5-trimethylenecyclohexanes under mild conditions.
DOI: 10.1039/c8sc04790a
rsc.li/chemical-science
Allenes serve as important organic building blocks.1 The parent 4.84 (9-CH2]) and d 4.63, 4.48 (4,6-CH2]) as well as the AX-
allene, propadiene, has been cyclotrimerized at a variety of spin system of the diastereotopic 3,7-CH2 pairs (d 2.24, 2.00,
transition metal catalysts, but this usually gave predominately 2JHH ¼ 14.0 Hz) and the AB pattern of the 5-CH2 at d 2.58, 2.54.
1
1,2,4-trimethylenecyclohexane and only minor amounts of the The 1-CH2 group at boron and the 8-CH2 unit give rise to H
1,3,5-trimethylenecyclohexane isomer 1.2,3 Compound 1 had NMR signals at d 2.18 and d 2.26, respectively (atom numbering
been synthesized in stoichiometric reaction sequences.4 We scheme analogous to Fig. 1). Compound 3a shows a broad 11B
recently developed the metal-free, HB(C6F5)2 catalysed forma- NMR resonance at d 71.5, which is typical for a planar tri-
tion of isomerically pure 1 by allene cyclotrimerization under coordinate Lewis acidic borane in this situation (D19Fm,p
mild conditions.5 We also had recently shown that a small series 13.2 ppm, for details see the ESI†).
¼
of 1-alkynes could be converted to linear oligomers by treatment
The reaction of allene with BrB(C6F5)2 (2b) was carried out
with either of the halogenoboranes XB(C6F5)2 2a (X: Cl) or 2b (X: analogously. The reaction was directly followed by NMR spec-
Br). The resulting products contained a halide substituent at troscopy. It resulted in the formation of the products 1 and 3b in
one end of the oligoacetylene chain and the B(C6F5)2 functional a ca. 1 : 2 ratio (4 h, r.t.). Compound 3b was characterized from
group at the other.6 This prompted us to investigate which type the mixture by NMR spectroscopy. It shows similar spectra as its
of a reaction allene would undergo with either of the reagents 2a
and b. We have performed these reactions and here report
about their surprising outcome.
We rst carried out the reaction of excess allene with
ClB(C6F5)2 (2a) in d8-toluene in a Young NMR tube at room
temperature. Aer 7 h reaction time we monitored the NMR
features of a mixture that contained the products 1,3,5-trime-
thylenecyclohexane (1) and the chloroborylated allene tetramer
3a in a ca. 1 : 1 ratio (both present in ca. 8 mol% in the mixture)
plus unreacted 2a (ca. 10 mol%) and allene (ca. 75 mol%). Aer
24 h the amount of the pair of allene cyclooligomerization
products had almost doubled and there remained only ca.
3 mol% of the chloroborane 2a. That was almost completely
consumed aer 48 h reaction time at room temperature. The
products 1 and 3a were identied spectroscopically from the
mixture [1: d 4.53, 2.70 (1H)]. Compound 3a shows the typical
1
olenic exo-methylene pairs of H NMR resonances at d 5.08,
¨
¨
¨
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster,
¨
Corrensstraße 40, 48149 Munster, Germany. E-mail: erker@uni-muenster.de
Fig. 1 A view of the molecular structure of compound 4b. Selected
bond lengths (A˚) and angles (ꢀ): B1–C1 1.634(2) B1–N21 1.661(2) C4–
C11 1.323 (2) C6–C12 1.327(2) C9–C10 1.351(9) C9–Br1 1.910(5) B1–
C1–C2 124.4(1) C1–C2–C8 108.5(1) C2–C8–C9 116.3(3) C8–C9–C10
124.5(6) C8–C9–Br1 118.4(4).
† Electronic supplementary information (ESI) available: Additional experimental
details, further spectral and crystallographic data. CCDC 1862533 to 1862535
and 1881527. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8sc04790a
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chloro-substituted analogue 3a (see the ESI† for details and the
depicted spectra).
We prepared compound 3a on a preparative scale. Aer
direct treatment with pyridine we isolated the Lewis adduct 4a
as a white solid in 53% yield (Scheme 1). It was characterized by
C, H, N elemental analysis, by spectroscopy and by X-ray
diffraction (see the ESI† for details). It showed very similar
parameters as the analogous bromine containing compound 4b
(see below).
Treatment of in situ prepared 3b with a slight excess of
pyridine gave the adduct 4b, which we isolated in 72% yield as
a white solid on a 100 mg scale. The X-ray crystal structure
analysis (Fig. 1) showed the central six-membered ring that was
constructed by connecting three allene units. It features the exo-
methylene C]C double bonds annulated at carbon atoms C4
and C6 and it has the CH2–CBr]CH2 unit, derived from the
fourth connected allene unit, attached at the ring carbon atom
C2. This carbon atom also bears the CH2–B(C6F5)2 substituent,
which has the pyridine donor added to its boron atom. The
central six-membered carbocycle of compound 4b features
a distorted chair-like ring conformation.
In solution (CD2Cl2) compound 4b shows the 1H NMR ]CH2
signals of the symmetry equivalent pair of exo-methylene
groups at the ring carbons C4 and C6 and the pair of signals
of the C9]CH2 moiety. The C3/C7 CH2 hydrogen atoms are
pairwise diastereotopic and the C8 and C1 CH2 groups both
Scheme
cyclooligomerization.
2
Reaction pathway for the XB(C6F5)2 mediated allene
1
show H NMR singlets. Compound 4b shows a 11B NMR reso-
3, represents a competing stoichiometric reaction branch that
eventually removes the XB(C6F5)2 reagent from the system
(Scheme 2) and thus terminates the catalytic sequence of the
formation of 1.
nance in the typical tetra-coordinated borane range at d ꢁ1.4
19
(D
F
m,p
¼ 5.6 ppm).
It is probably reasonable to assume that our reaction
sequence starts with a halogenoboration reaction7 of allene
(Scheme 2). This would generate the allylborane 5 that probably
undergoes an allylboration8 reaction with a second equivalent
of allene to form the intermediate 6. Since this contains an
allylborane subunit it can undergo another allylboration reac-
tion to give 7. This sequence could in principle be propagated to
form a respective series of linear borylated allene oligomers 7, 8
etc., were there not the attractive possibility of these systems to
undergo alternative intramolecular allylboration.9 In the case of
7 this would lead to the cyclization product 9. Subsequent
XB(C6F5)2 elimination provides an attractive pathway to the
observed product 1,3,5-trimethylenecyclohexane (1). This would
in principle constitute a cyclotrimerization of allene catalysed
by the XB(C6F5)2 reagents. However, the intramolecular allyl-
boration of 8, giving the other experimentally observed products
Compounds 3a,b are reactive boron Lewis acids. With the
t
bulky Bu3P Lewis base (10) they react by ring closure10,11 to
eventually give the zwitterionic product 12 (Scheme 3). As
a typical example, the reaction of the in situ generated chloride
containing derivative 3a was carried out with a ca. two molar
t
equivalents of Bu3P in toluene (24 h, 60 ꢀC). Workup with
pentane and dichloromethane in this case gave a ca. 79 : 21
mixture of compound 12 (characterization see below) and the
[tBu3PH+]Clꢁ phosphonium salt 11a (31P NMR: d 47.9, JPH
1
ꢂ463 Hz, some of the stoichiometric by-product 11a was
probably lost during the workup procedure).
The reaction between the more reactive borane 3b and tBu3P
was carried out similarly (toluene, 24 h, r.t.). Workup in this
case gave the pure compound 12 as a white solid, which we
isolated in 65% yield. It was characterized by C, H, N elemental
analysis, by spectroscopy and by X-ray diffraction (single crys-
tals of compound 12 were obtained from dichloromethane/
Scheme 1 Reaction of allene with the halogenoboranes 2a and 2b.
Scheme 3 Reaction of the products 3 with two molar equiv. of tBu3P.
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pentane by the diffusion method). The X-ray crystal structure Compound 14c was isolated in 44% yield as a colorless oil. The
analysis shows the newly formed borataspiro[5,5]undecene NMR spectra showed the symmetry features of the cis,trans-
framework (Fig. 2) which was selectively formed by boron 2,4,6-trialkyl-substituted 1,3,5-trimethylenecyclohexane diaste-
induced allene tetramerization followed by phosphane induced reomer. It features the 1H NMR triplets of the 2,6-CH and the 4-
ring closure reaction. The zwitterionic compound has the bulky CH ring hydrogens in a 2 : 1 intensity ratio. The 1-H2C] unit
tBu3P-substituent attached at the C(9)]C(10) carbon–carbon shows one 1H NMR resonance, whereas the 3,5-H2C] moieties
double bond. The adjacent six-membered ring shows carbon showed two due to their stereochemically unsymmetrical envi-
atoms C4 and C6 which serve both as the ring sp2-carbons of the ronment (see the ESI† for further details). The BrB(C6F5)2
pair of exo-methylene C]CH2 groups.
borane is an equally efficient metal-free cyclotrimerization
In solution (CD2Cl2) compound 12 shows the typical 1H NMR catalyst for the n-octylallene 13c. The catalytic reaction was
doublet of the tBu3P-substituent. It features the ]CH signals of carried out under analogous conditions and gave the product
the newly formed internal olenic moiety at d 8.21 [1H: d, 3JPH
¼
14c in 46% yield aer workup (some minor byproduct was
27.8 Hz; 13C: d 177.0 (broad)]. The BCH2 group shows up as a 1H isolated (ca. 20%) but not positively identied as yet). We per-
NMR signal at d 1.20 and the C(8)H2 methylene group as formed the XB(C6F5)2 catalysed alkylallene cyclotrimerization
a doublet at d 2.06 (3JPH ¼ 4.5 Hz). The pair of olenic exo- reaction for a second example: the reaction of n-dodecylallene
ꢀ
methylene groups of the adjacent carbocyclic six-membered (13d) with either of the Cl/BrB(C6F5)2 borane (10 mol%, 60 C,
1
ring shows typical H NMR signals at d 4.61/4.41 and the reso- 48 h in d8-toluene) gave the tri-substituted cyclotrimer 14d with
nances of the pairwise diastereotopic methylene hydrogen cis,trans-attachment of the long chained alkyl groups as the
atoms at the C3/C7 pair and at C5 (see the ESI† for details).
major product. We isolated it as a colorless oil in 40% yield from
We also reacted a pair of alkyl-substituted allenes with the the ClB(C6F5)2 catalysed reaction [54% with BrB(C6F5)2] (see the
halogenoboranes 2 and found a marked dependence of the ESI† for the characterization of 14d).
overall reaction pathway on the substitution pattern. In the two
We had previously shown that HB(C6F5)2 serves as an effi-
cases investigated the competition between the two previously cient metal-free catalyst for the cyclotrimerization of allene to
observed pathways, namely catalytic cyclotrimerization and 1,3,5-trimethylenecyclohexane and of cyclohexylallene to cis,-
stoichiometric formation of a tetramer derivative, was shied to trans-2,4,6-tricyclohexyl-1,3,5-trimethylenecyclohexane.5
Our
present study shows that the strongly electrophilic halogen-
the catalytic side.
We treated n-octylallene (13c), which we prepared according oboranes XB(C6F5)2 (X: Cl, Br) in principle can induce the same
to a procedure reported by Ma et al.12 with 10 mol% of catalytic reaction. Both these strongly electrophilic halogeno-
ClB(C6F5)2 (2a) in d8-toluene solution at 60 ꢀC. Workup aer boranes serve as catalysts for the cyclotrimerization of long-
48 h reaction time involving purication by chromatography chain n-alkylallenes 13c,d to give the respective cyclotrimers
gave the cyclotrimer 14c as the major product (Scheme 4). 14c,d as the major products. Ring-closure and elimination is
sufficiently effective to close the catalytic cycle with liberation of
the respective XB(C6F5)2 (X: Cl, Br) catalyst (Scheme 2). However,
the reaction seems to be sensitive to sterics in the competition
between ring-closure and further allene incorporation. In the
case of the unsubstituted parent allene substrate we found
a substantially competing further oligomerization step which
eventually leads to the formation of the stoichiometric products
3a and 3b, thereby consuming the borane catalyst of the allene
cyclotrimerization reaction and, thus, leading to termination of
the catalytic reaction in this specic case. Nevertheless, our
study has shown that allene oligomerization beyond trimeri-
zation is possible with such borane systems. This makes us
hopeful that it might be possible to open novel ways of utili-
zation of allenes by a further development of metal-free reac-
tions along the characteristic lines that have shown up in this
study.
Fig. 2 A view of the molecular structure of the zwitterionic FLP
product 12. Selected bond lengths (A˚) and angles (ꢀ): B1–C1 1.630(4)
B1–C10 1.624(4) C4–C11 1.328(4) C6–C12 1.323(5) C9–C10 1.343(4)
C9–P1 1.834(3) B1–C1–C2 113.5(2) C1–C2–C8 108.5(2) C1–B1–C10
107.0(2) C2–C2–C7 108.0(2) C2–C8–C9 114.0(2) C8–C9–C10 Scheme 4 XB(C6F5)2 (X: Cl, Br) catalyzed cyclotrimerization of alkyl
121.4(2) C9–C10–B1 125.7(3).
substituted allenes.
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We have started to use the allene cyclotrimerization products
1 and 14 as the starting materials for the conversion to the
respective arene isomers. It is known that 1,3,5-trimethylene-
cyclohexane is resistant to thermal isomerization, but it was
reported that it could be isomerized by treatment with acid.2a,13
We repeated this reaction: treatment of 1 with 5 mol% of
p-toluene sulfonic acid in toluene solution at r.t. for 1.5 h
cleanly converted 1 to mesitylene (Scheme 5). We also treated
the n-octyl- and cyclohexylallene cyclotrimers 14c14 and 14e5
with p-toluene sulfonic acid. Compound 14c was fully converted
on a preparative scale with 10 mol% of the acid catalyst during
48 h at r.t. The hexa-substituted arene 15c was isolated as an oil
1
in 78% aer workup. It was characterized by H and 13C NMR
spectroscopy (see the ESI† for details). Compound 14e needed
slightly more vigorous conditions. It required treatment with
10 mol% of the p-toluene sulfonic acid catalyst at 80 ꢀC for 5 h to
become converted. This reaction is not overly selective, but we
isolated the aromatic isomer 15e in 37% yield from the reaction
mixture. The compound was characterized by spectroscopy and
by an X-ray crystal structure analysis (see the ESI† for details). In
principle, these reactions have shown that the metal-free X–
B(C6F5)2 catalyzed allene cyclotrimerization opens attractive
pathways to the synthesis of interesting highly substituted
arene products.
Conflicts of interest
There are no conicts to declare.
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