Borane-Induced Dimerization of Arylallenes

Article abstract of DOI:10.1002/anie.201808436

A series of arylallenes react with HB(C₆F₅)₂ in a 2:1 molar ratio to give the tail-to-tail 1,6-diaryl-2-boryl-hexa-1,5-diene coupling products. The reaction of the phenylallene substrate with HB(C₆F₅)

Full text of DOI:10.1002/anie.201808436

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Accepted Article
Title: Borane-Induced Dimerization of Arylallenes
Authors: Gerhard Erker, Xin Tao, Constantin G Daniliuc, Dustin
Dittrich, and Gerald Kehr
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Borane-Induced Dimerization of Arylallenes
Xin Tao, Constantin G. Daniliuc, Dustin Dittrich, Gerald Kehr, Gerhard Erker*[a]
Abstract: A series of arylallenes react with HB(C6F5)2 in a 2:1 molar
ratio to give the tail-to-tail 1,6-diaryl-2-boryl-hexa-1,5-diene coupling
products. The reaction of the phenylallene substrate with HB(C6F5)2
was shown to initially give the 2-boryl-3,4-diphenyl-1,5-hexadiene
head-to-head coupling product which then was shown to rearrange by
a thermally induced Cope rearrangement to the final product.
Allenes serve as important synthetic building blocks. Due to their
unique composition they undergo a great variety of chemical
transformations, among them various carbon-carbon coupling
reactions.[1] We had recently addressed questions about the
selective cyclotrimerization reaction of the parent allene (1).
Allene can be cyclotrimerized metal catalyzed, but this invariably
leads to 1,2,4-trimethylenecyclohexane as the major product with
only minor amounts of the 1,3,5-trimethylenecyclohexane isomer
2.[2,3] Stoichiometric syntheses of 2 had been reported.[4]
Scheme 2. HB(C6F5)2 induced dimerization of arylallenes.
Figure 1. A view of the molecular structure of compound 7a (thermal ellipsoids
are shown at 15% probability). Selected bond lengths (Å) and angles (°) C1C2
1.342(4), C2C3 1.522(4), C2B1 1.554(3), C3C4 1.531(4), C5C6 1.319(4),
C1C2C3 117.9(2), C4C5C6 125.6(3), C2C3C4C5 67.5(3),  BCCC
360.0.
Scheme 1. HB(C6F5)2 catalyzed cyclotrimerization of alkylallenes.
We reported[5] that the selective cyclotrimerization of allene (1) to
give 2 is catalyzed by the borane HB(C6F5)2 (3).[6] The analogous
reaction of cyclohexylallene 4a led selectively to the trans,cis-
tricyclohexyl substituted 1,3,5-trimethylenecyclohexane 5a.[5] We
have now found a second closely related example, namely the
HB(C6F5)2-catalyzed cyclotrimerization of n-octylallene (4b) to
give the analogous product 5b (Scheme 1) (see the Supporting
Information for details).
We reacted two molar equivalents of phenylallene 6a with
HB(C6F5)2 in toluene r.t, 3 days). Workup gave the 2-boryl-1,6-
diphenyl-substituted 1,5-hexadiene product 7a as a pale yellow
crystalline material which we isolated in 44% yield. It was
characterized by C,H elemental analysis, by NMR spectroscopy
and by X-ray diffraction. (Figure 1) Allene-allene coupling had
apparently taken place between the terminal =CH2 groups, and
the components of one equivalent of the HB(C6F5)2 borane had
been added to carbon atoms C2 and C5. Both the C1-C2 and C5-
C6 carbon-carbon double bonds are found E-configurated. In the
crystal the C6-chain features a gauche conformation at the central
C2C3C4C5 moiety. We note that there is a short contact
between carbon atoms C18 and C31, possibly indicating a -
stacking interaction[8] between the respective aryl groups in
compound 7a.
We have now investigated the reaction of a small series of
substituted arylallenes 6a-d[7] with HB(C6F5)2 and found that they
showed a markedly different chemical behavior: instead of
catalytic cyclotrimerization the substrates
6 underwent a
stoichiometric reaction with HB(C6F5)2 to give unique boryl-
substituted linear dimeric carbon-carbon coupling products.
The reactions of the p-X-aryl substituted allenes 6b (X = CH3), 6c
(X = F) and 6d (X = Ph) with HB(C6F5)2 were carried out in similar
ways (2:1 stoichiometry, toluene, r.t., 3 days) giving the analogous
2-boryl substituted E,E-1,6-diaryl-1,5-hexadiene derivatives [7b
(39%), 7c (39%), 7d (54%), see Scheme 2]. They showed similar
NMR features in solution as 7a. The characteristics of the X-ray
crystal structures of compounds 7b and 7c are analogous to those
[a]
Dr. X. Tao, Dr. C. G. Daniliuc, D. Dittrich, Dr. G. Kehr,
Prof. Dr. G. Erker
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster, Germany
E-mail: erker@uni-muenster.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201808436
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[14b]
of 7a, whereas the structure of compound 7d deviates slightly. In
contrast to the gauche type framework of the former it shows an
antiperiplanar arrangement of the C3-C2 and C4-C5 vectors[9] at
the newly formed C3(sp3)-C4(sp3) carbon-carbon single bond. We
assumed that this reaction sequence probably starts with a
HB(C6F5)2 hydroboration step[10] of the allene substrate,[11]
followed by allylboration[12,13] of a second arylallene equivalent. In
order to investigate this assumption, we carried out a number of
experiments. Their outcome was directly monitored by NMR
spectroscopy without any workup.
Therefore, we assume an essential participation of the
B(C6F5)2 substituent in the Cope rearrangement of 11a. It may
possibly proceed through the zwitterionic borata-alkene stage 12a
by making use of the high tendency of the B(C6F5)2 group for
carbanion stabilization.[15] This pathway would conceptually be
related to the reaction course typically taken in catalyzed Cope
rearrangement processes.[16]
We characterized the intermediate 11a of this reaction sequence
structurally by derivatization. For that purpose, we generated it in
a separate experiment and trapped this Lewis acidic borane with
pyridine. We isolated the pyridine adduct 13a as a white solid in
42% yield. Single crystals suited for the X-ray single crystal
structure analysis were obtained from dichloromethane/pentane
by the diffusion method. It shows that the pair of allenes had
indeed become head-to-head coupled.
Figure 2. A view of the molecular structure of compound 13a, formed from the
CC coupling product 11a by trapping with pyridine (thermal ellipsoids are
shown at 30% probability). Selected bond lengths (Å) and angles (°) C1C2
1.336(3), C2C3 1.542(3), C2B1 1.634(3), C3C4 1.566(3), C5C6 1.326(4),
C1C2C3 119.5(2), C4C5C6 124.2(4), C2C3C4C5 56.0(3).
Scheme 3. Mechanistic investigation of HB(C6F5)2-induced dimerization of
phenylallene {product composition given in mol%}.
It features the pair of phenyl substituents at the newly formed
C3(sp3)C4(sp3) single bond. This unit is found in the typical
staggered conformation. The central carbon atoms (C3, C4) have
the C5=C6 vinyl group and the C2[B]=C1 alkenyl substituent
attached and oriented in an overall twist-boat like conformational
arrangement (Figure 2). We note that in compound 13a the pair
of phenyl groups at carbon atoms C3 and C4 are arranged facing
each other with an angle between the aryl planes of 47.3 °. The
distance between the ipso-carbon atoms (C11, C21) of these
arene rings amounts to 2.891 Å. The boron atom in compound
13a shows a slightly distorted tetrahedral coordination geometry
with bonds to the alkenyl carbon atom C2, the pair of C6F5
substituents (BCCC = 336.5°) and the coordinated pyridine ligand
(for the NMR characterization of compound 13a see the
Supporting Information).
We also reacted the 2-borylhexadiene derivative 11a with PtBu3.
This resulted in a typical frustrated Lewis pair (FLP) reaction,[17,18]
namely trans-1,2-phosphane/borane addition to the pendant
C5C6 carbon-carbon double bond to yield 14a. Triphenyl-
phosphane underwent an similar FLP addition reaction with 11a
to give 15a, only in this case did the phosphane nucleophile add
to the =CH- group of the vinyl functionality to give a six-membered
ring product (Scheme 3).[19]
We first reacted HB(C6F5)2 with one molar equiv. of phenylallene.
We found that a mixture was formed containing the allylborane 8a
(62 mol%) accompanied by the isomeric alkenylboranes 9 (15
mol%) and 10 (16 mol%) as well as a small amount of the follow-
up product 11a (7 mol%, see Scheme 3). Addition of a second
equivalent of phenylallene (6a) to the mixture resulted in rapid
disappearance of the allylborane 8a with selective formation of
the 3,4-diphenyl-2-boryl substituted 1,5-hexadiene product 11a.
The pair of the alkenylboranes (9, 10) was practically not touched
by this reaction.
Compound 11a shows typical NMR features of the vinyl group [1H:
 5.34, 5.17, 5.70 (ABX)] as well as the C[B(C6F5)2]=CH2 moiety
[1H:  5.84, 5.76; 13C:  159.8, 137.3; 11B:  58.3; 19Fm,p = 12.8
ppm]. The central PhCH-CHPh-unit shows a pair of 1H NMR
resonances at  4.34, 3.83 with 3JHH = 11.5 Hz.
Keeping compound 11a for 24 h at r.t. in the CD2Cl2 solution
eventually resulted in the formation of the phenylallene
coupling/borylation product 7a presumably by means of a
thermally induced Cope-rearrangement reaction. Again, the
unreactive phenylallene hydroboration isomers
remained practically unaltered in the solution.
9 and 10
Many Cope rearrangements proceed at markedly higher
temperatures than the 11a to 7a conversion.[14] The closely related
rac-3,4-diphenyl-1,5-hexadiene to trans,trans-1,6-diphenyl-1,5-
hexadiene rearrangement was reported to take place at ca. 100°C.
In situ generation of the (phenylallyl)borane intermediate 8a
allowed us to carry out a coupling reaction between differently
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substituted allenes. In situ generated 8a was reacted with p-
fluorophenylallene (6c, 3 days, r.t.) to yield 7e as a pale yellow
crystalline material isolated in 38% yield (Scheme 4).
double bond potential -stacking interaction (as observed in
compound 13a, Figure 2) along the reaction path (e.g. in [8+6]≠)
might help to explain the specific regioselectivity in these
dimerizing arylallene/HB(C6F5)2 coupling reactions. The formation
of the final products 7 can then be explained by a subsequent
Cope-rearrangement (see above).[20]
Scheme 4. Synthesis of compound 7e.
The cyclotrimerization pathway of the alkylallenes 4 appears to
start with HB(C6F5)2 hydroboration of the C=CH2 group to
generate the allylboranes 16b (Scheme 5). The catalytic
cyclotrimerization reaction then requires to proceed by a series of
two consecutive intermolecular allylboration reactions at the less
substituted allene C=CH2 double bond, followed by an
intramolecular allylboration cyclization step to generate 19b.
Subsequent retro-hydroboration then closes the catalytic cycle to
form 5 with liberation of the HB(C6F5)2 catalyst. Repulsive
substituent interaction seems to favor the regiochemical outcome
of this reaction type along the chosen pathway. A stoichiometric
experiment reacting HB(C6F5)2 with 4b gave indication of the
potential sequential formation of 16b, 17b and 19b, respectively,
as major components of the reaction mixture (see the Supporting
information for details).
Scheme 6. Sequence of allylboration and Cope rearrangement in the HB(C6F5)2
mediated dimerization of phenylallene.
Alkenylboranes are important building blocks in organic synthesis.
They undergo a variety of useful reactions, most notably the many
varieties of metal complex catalyzed cross coupling
reactions.[21,22] The B(C6F5)2 substituted alkenes have also been
used in this way.[23] We generated compound 7a in situ (see
above) and used it as a component in palladium-catalyzed
Suzuki-type coupling. Treatment of a solution of 7a in THF with 4-
iodotoluene in the presence of 10 mol% of Pd(PPh3)4 (19 h, 65
oC) eventually gave the triarylhexadiene 21, which we isolated in
68% yield[24] as a colorless oil after column chromatography
(Scheme 7)
Scheme 5. HB(C6F5)2 catalyzed cyclotrimerization of alkylallenes.
Scheme 7. Utilization of compound 7a.
The productive HB(C6F5)2 plus arylallene reaction also starts with
hydroboration at the unsubstituted allene double bond, although
this reaction is less regioselective (Scheme 3). In the arylallene
reactions the allylboration reaction apparently goes to the aryl-
substituted styryl-type allene carbon-carbon double bond to
selectively give the primary head-to-head coupling product 11
(Scheme 3 and Figure 2). It maybe that in addition to electronically
preferred allylborane attack on the styryl-type phenylallene C=C
Hydroboration products are often oxidatively converted to
alcohols.[25] Oxidative cleavage of alkenylboranes leads to the
respective carbonyl products.[26] Therefore, we treated the
hexadienylborane 7a under typical conditions with H2O2 under
aqueous basic conditions (NaOH). Workup then gave the ketone
20 as a colorless oil, which we isolated in 42% yield [13C NMR: 
207.0 (C=O)].
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In conclusion, in this study the borane HB(C6F5)2 has served as a
stoichiometric reagent for the borylating tail-to-tail coupling
reaction between two arylallenes. The situation of the reaction of
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Financial support from the European Research Council is
gratefully acknowledged.
Keywords: allene • carbon-carbon coupling • metal-free • Cope-
rearrangement • allene dimers and trimers
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Mono-alkylallenes are catalytically
cyclotrimerized by HB(C6F5)2 , whereas
arylallenes react with HB(C6F5)2 to
eventually give the stoichiometric 1,6-
diaryl-2-boryl-hexa-1,5-diene tail-to-tail
coupling products.
Xin Tao, Constantin G. Daniliuc, Dustin
Dittrich, Gerald Kehr, Gerhard Erker*
Page No. – Page No.
Borane-Induced Dimerization of
Arylallenes
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