Borane-induced ring closure reaction of oligomethylene-linked bis-allenes

Article abstract of DOI:10.1039/c9sc03870a

The trimethylene-linked bis-allene 3a reacts with Piers' borane [HB(C₆F₅)₂] by a hydroboration/allylboration sequence to generate the cyclization product 5a. Its pyridine adduct was isolated and characterized by X-ray diffraction. Compound 5a undergoes a typical frustrated Lewis pair 1,2-P/B alkene addition reaction with PPh₃ to give the heterobicyclic bridged olefinic zwitterionic product 9a. The tetramethylene-linked bis-allene 3b and its phenylene annulated analogue 3c react with HB(C₆F₅)₂ to give the analogous seven-membered ring products 5b,c under mild conditions. The cyclization product 5a undergoes a series of sequential allylboration reactions with two equivalents of allene followed by ring-closure to give the four-component coupling product 12a. It undergoes FLP addition to an exo-methylene group upon treatment with PPh₃. Compound 12a is oxidatively converted to the boron-free alcohol.

Full text of DOI:10.1039/c9sc03870a

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Received 00th January 20xx,
Accepted 00th January 20xx
Xin Tao,a Karel Škoch,a Constantin G. Daniliuc,a Gerald Kehr,a Gerhard Erker*a
DOI: 10.1039/x0xx00000x
The trimethylene-linked bis-allene 3a reacts with Piers’ borane [HB(C6F5)2] by a hydroboration/allylboration sequence to
generate the cyclization product 5a. Its pyridine adduct was isolated and characterized by X-ray diffraction. Compound 5a
undergoes a typical frustrated Lewis pair 1,2-P/B alkene addition reaction with PPh3 to give the heterobicyclic bridged
olefinic zwitterionic product 9a. The tetramethylene-linked bis-allene 3b and its phenylene annulated analogue 3c react with
HB(C6F5)2 to give the analogous seven-membered ring products 5b,c under mild conditions. The cyclization product 5a
undergoes a series of sequential allylboration reactions with two equivalents of allene followed by ring-closure to give the
four-component coupling product 12a. It undergoes FLP addition to an exo-methylene group upon treatment with PPh3.
Compound 12a is oxidatively converted to the boron-free alcohol.
frameworks.6f,g It should be noted that the vast majority of
these reaction is metal catalysed.
Introduction
Allenes are important building blocks in organic synthesis.1 They
show interesting and useful stereochemical properties.2 We
had recently shown that the borane [HB(C6F5)2]3 catalyses the
cyclotrimerization of allene as well as a small series of mono-
alkyl-substituted allenes
1,3,5-trimethylene cyclohexanes
metal-free conditions (Scheme 1).1,4,5
1
to selectively give the respective
2
as single isomers under
Chart 1. Examples of cyclization reactions of bis-allenes.
This posed the question what the favoured reaction pathway
would be if we treated e.g. oligomethylene-linked bis-allenes
with HB(C6F5)2,7 i.e. under metal-free conditions. We have now
performed these reactions starting from two examples of that
bis-allene family. It turned out that a different cyclization type
prevailed under these conditions. The outcome of these
reactions will be presented and discussed below.
Scheme 1. HB(C6F5)2-catalyzed cyclotrimerization of alkyl-substituted allenes.
There is a rich cyclization chemistry of bis-allenes reported in
the literature (see Chart 1). Systems I (mostly with X = NTs, less
frequently CR2 or O) were reported to rearrange to II thermally
induced.6a They added R3Si-SnBu3/ or -GeR3 reagents Pd(0)
catalysed or radical induced to give the products III 6b,c
.
With
H2NR nucleophiles cyclization to medium-sized rings (e.q. IV
was reported.6d The products
and VI of internal [2+2]
)
V
cycloaddition were formed under the influence of Au(I)6e or
Pd(0)6a catalysis, respectively. In some cases coupling between
two bis-allenes occurred to give hetero-steroidal
Results and discussion
Borane-induced ring-closure reaction of the bis-allenes
The allenes 3a and 3b (Scheme 2) were prepared by a variant of
the Crabbé reaction as described by Ma et al.8 Copper (I)
induced treatment of 1,6-heptadiyne with para-formaldehyde
as C1-building block and dicyclohexylamine gave the
trimethylene-linked bis-allene 3a6h (40% isolated). The
analogous reaction starting from 1,7-octadiyne gave the
a.Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149 Münster, Germany. Email: erker@uni-muenster.de
Electronic Supplementary Information (ESI) available: Additional experimental
details, further spectral and crystallographic data. CCDC deposition numbers are
1922906-1922913 and 1957134. See DOI: 10.1039/x0xx00000x
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tetramethylene-linked bis-allene 3b6i (63% isolated, see the ESI We reacted the tetramethylene-linked bis-allene 4b with
View Article Online
DOI: 10.1039/C9SC03870A
for details). The phenylene containing bis-allene 3c was HB(C6F5)2 and found that the seven-membered cyclization
synthesised analogously.
product 5b was formed analogously. Treatment of the in situ
generated borane 5b with pyridine gave the respective pyridine
adduct 6b, which we isolated crystalline in 39% yield.
Compounds 5b and 6b were characterized by spectroscopy (see
the ESI for details); product 6b was characterized by C,H,N
elemental analysis and by an X-ray crystal structure analysis
(Figure 2). It shows the newly formed seven-membered ring in
a
−
typical cycloheptene boat-like conformation.9 The
CH2B(C6F5)2 moiety is attached at the sp2-carbon atom C4 (the
boron atom bears a pair of C6F5 groups and the coordinated
pyridine ligand in a pseudo-tetrahedral geometry). The vinyl
substituent bonded to carbon atom C3.
Scheme 2. HB(C6F5)2-induced ring closure of the bis-allenes 3.
We reacted compound 3a with one molar equivalent of Piers’
borane [HB(C6F5)2]. The rapid reaction (r.t., minutes, in CD2Cl2)
gave the cyclized product 5a. It was not isolated but
Figure 1. A view of the molecular structure of the borane pyridine adduct 6a
Selected bond lengths (Å) and angles (°): B1 N31 1.613(3), B1 C5 1.651(3), C1 C2
1.322(4), C4 C6 1.337(3), B1 C5 C4 117.7(2).
.
−
−
−
−
−
−
characterized by spectroscopy [1H NMR:
(C6-H, see Figure 1 for the unsystematic atom numbering
 5.55 (olefinic =CH−)
scheme),

5.52, 5.03,
−  69.3;
CH=CH2 substituent); 11B NMR:
19

F
m,p
= 13.1 ppm]. Compound 5a was then generated in situ
on a preparative scale and trapped by subsequent addition of
pyridine. We isolated the pyridine adduct 6a as a white
crystalline solid in 73% yield. The X-ray crystal structure analysis
of compound 6a showed the newly formed cyclohexene core
that has a vinyl group attached in the allylic position and it bears
the
−CH2B(C6F5)2(pyridine) substituent adjacent to it at the
olefinic ring carbon atom C4 (Figure 1). In solution (CD2Cl2) we
monitored the typical 1H/13C NMR features of the vinyl group at
carbon atom C3 and the central doubly substituted cyclohexene
1
core. The H NMR features of the coordinated pyridine moiety
show up at 
8.67, 8.11 and 7.65 (11B:  −0.6). Due to the chiral
Figure 2. Molecular structure of compound 6b. Selected bond lengths (Å) and
angles (°): B1 N31 1.627(5), B1 C5 1.645(6), C1 C2 1.285(8), C4 C6 1.340(6),
B1 C5 C4 119.9(3).
−
−
−
−
centre (ring carbon C3) the C6F5 groups at boron are
diastereotopic and give rise to a 1:1 set of the respective o,p,m-
C6F5 19F NMR signals.
−
−
The reaction of the bis-allene 3a responded to the
We assume that the reaction started with hydroboration of the stoichiometry of the HB(C6F5)2 reagent since the primary ring-
terminal =CH2 unit of one allenyl group by the HB(C6F5)2 closure product 5a contained a reactive pendant vinyl group.
reagent. This resulted in the formation of the functionalized Therefore, the reaction of 3a with two molar equivalents of
borane
4
which contained a reactive allylborane unit opposite Piers’ borane gave the bis-borane product 7a. Its 11B NMR
73.3 and 69.4) that are
to a reactive allene unit. This situation was set up for an internal spectrum showed two signals (


allylboration reaction of the allene, which directly opened a attributed to the pair of Lewis acidic planar-tricoordinate boron
pathway to the observed cyclization product 5a. Addition of the centres. Consequently, we observed two sets of 19F NMR
pyridine Lewis base to the strongly Lewis acidic B(C6F5)2 group resonances of the pairs of the C6F5 substituents at the two
gave 6a (Scheme 2).
B(C6F5)2 groups. The in situ generated bis-borane 7a was then
treated with two molar equivalents of pyridine to give the
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respective double pyridine adduct 8a (isolated as a white solid NMR [P]
−
CH signal is found at
−

3.64 and the single olefinic
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DOI: 10.1039/C9SC03870A
in 72% yield). It was characterized by C, H, N elemental analysis, =CH
by X-ray diffraction (the structure is shown in the ESI) and by
spectroscopy. Due to the chiral centre (C3) the pair of C6F5
groups at each B(C6F5)2(pyridine) moiety are diastereotopic and,
consequently, we observed four sets of o,m,p-C6F5 19F NMR
signals of compound 8a. We observed two sets of pyridine
−
signal at  5.17.
1
1H/13C NMR resonances and located the single olefinic H NMR
signal (C6-H) at  4.35 (t, JHH = 3.3 Hz, 1H).
We performed the reaction of phenylene bridged bis-allene
system 3c in a similar way. The reaction of 3c with one molar
equiv. of HB(C6F5)2 was carried out in CD2Cl2 at r.t. and the
almost instantaneously in situ generated product was
characterized by NMR spectroscopy [1H: δ 5.48/5.13/5.08 (vinyl
substituent), 5.72 (ring-CH=), 11B: δ 71.7, 19F: Δδ19Fm,p= 13.2].
The latter heteroatom NMR signals are typical for the presence
of strongly Lewis acidic tricoordinate boron with this
substituent pattern. Compound 5c was treated with pyridine
and the respective pyridine/borane Lewis adduct 6c was
isolated in 37% yield after workup involving pentane extraction.
The NMR spectra now show a 11B NMR resonance in the typical
Scheme 3. FLP reaction of the boranes
5 with PPh3.
range of tetracoordinate boron (δ
−0.5) and the C6F5
substituents at boron are diastereotopic (see the ESI for further
details).
Subsequent FLP ring-closure reactions
The compounds
5 each contain a sterically encumbered,
strongly Lewis acidic borane functionality and in its vicinity an
accessible reactive vinyl group. We used this for carrying out a
typical frustrated Lewis pair10 reaction, namely
borane/phosphane addition to the C=C double bond.11
a 1,2-
Figure 3. Molecular structure of the P/B FLP addition product 9a. Selected bond
lengths (Å) and angles (°): B1
1.549(2), C4 C6 1.333(2), C1
112.4(1), C2 C3 C4 109.2(1), C3
109.3(1), P1 C2 C3 111.9(1), P1
−
C1 1.647(2), B1
B1 C5 105.7(1), B1
C4 C5 114.9(1), C4
C2 C3 C4 174.9(1) , P1
−
C5 1.640(2), P1
C1 C2 114.6(1), C1
C5 B1 108.9(1), P1
C2 C1
−
C2 1.856(2), C1
C2
C2
−
−
−
C2
C3
C1
−
−
−
−
−
−
−
−
−
−
−
−
−
We reacted the in situ generated cyclization product 5a with
triphenylphosphane at room temperature. The reaction was
practically instantaneous under these typical conditions and we
isolated the P/B addition product 9a to the internal vinyl group
as a white powder in 76% yield (Scheme 3). Single crystals of
compound 9a that were suited for the X-ray crystal structure
analysis were obtained at room temperature from a solution in
dichloromethane that was layered with pentane. It showed that
1,2-phosphane/borane addition to the vinyl substituent had
occurred. The internal -B(C6F5)2 Lewis acid had been added to
the =CH2 terminus of the alkene and the external PPh3
−
−
−
−
−
−
−
−
B1 175.9(1).
The seven-membered ring compounds 5b and 5c react
analogously with PPh3. Compound 5b was in situ generated and
triphenylphosphane was added. We isolated the product 9b in
61% yield by crystallization. It was characterized by C, H-
elemental analysis, by spectroscopy and by X-ray diffraction.
The molecular structure is similar to that of 9a. It also contains
a trans-relationship of the C3-H and C2-H hydrogen atoms of the
heterobicyclic ring system. The hetero-NMR signals occur at

−
13.3 (11B) and 30.5 (31P). Compound 9c was prepared

nucleophile to the
heterobicyclo[4,4,0]decene type system features a bridgehead
C=C double bond (C4 C6) and a borate system inside the
−CH= carbon atom. The resulting zwitterionic
analogously from 5c (see Scheme 3). It was isolated as a white
solid in 77% yield after workup and characterized by NMR
spectroscopy and by an X-ray crystal structure analysis. For
further details of the characterization of the compounds 9b and
9c including their depicted molecular structures see the ESI.
−
heterocyclic six-membered ring that was formed in the FLP
+
addition reaction. Consequently, the -PPh3 -phosphonium
substituent is found attached at the same ring at carbon atom
C2. We have isolated a single diastereoisomer of 9a from this
reaction; it features the hydrogen atoms at carbon atoms C2
and C3 oriented trans to each other at the heterobicyclic
framework (Figure 3). In solution compound 9a shows the NMR
Subsequent cyclooligomerization reaction with allene
Internal allene allylboration12 represents the important step of
the ring-closure reaction sequence starting from
3 to form the
heteronuclear resonances at  −12.3 (11B) and

27.9 (31P). A
products . The compounds themselves each contain an
5
5
diastereotopic pair of C6F5 substituents is bonded at boron,
allylborane functionality which might show the respective
reactivity towards added allene reagents. Therefore, we
exposed the cyclization product 5a to an excess of the parent
1
giving rise to two separate sets of 19F NMR resonances. The H
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allene H2C=C=CH2. An NMR experiment revealed a close to consecutive C-C coupling between 5a and VietwwAorticlemOonllianer
complete conversion to the new product 12a within 24 h at equivalents of allene. The ring carbonDOaIt:o1m0.10C339/Cb9eSaCr0s38t7h0eA
room temperature. We carried out this reaction on a remaining vinyl substituent; carbon atoms C10 and C12 are both
preparative scale under analogous conditions. Workup part of the pair of exo-methylene groups 1,3-positioned in the
o
involving crystallization from pentane at
−
35 C (3 d) gave the second ring. The
−
CH2B(C6F5)2 group is attached at the
crystalline product 12a, which we isolated in 37% yield. bridgehead carbon (C4 in Figure 4).
Compound 12a was characterized by C, H elemental analysis, by In solution (CD2Cl2) compound 12a shows the typical NMR
spectroscopy and by an X-ray crystal structure analysis. This features of the vinyl substituent. The pair of =CH2 exo-
1
showed (Figure 4) that the endocyclic allylborane moiety of the methylene groups shows a total of four H NMR resonances (

starting material 5a had reacted with two molar equivalents of 4.85/4.61 and
H2C=C=CH2 to give the functionalized decalin derivative 12a
1H NMR signals of a pair of diastereotopic hydrogen atoms (AB
Apparently, compound 5a had undergone an allylboration system at
2.30/2.15; 13C: 36.7). The corresponding 11B NMR
76.4, i.e. in a typical range of a strongly Lewis
 4.68/4.42). The −CH2[B] substituent shows the
.


reaction with allene to generate the intermediate 10a. This feature is at

itself represents an “elongated” allylborane system, that acidic tri-coordinated boron atom in this substituent situation.13
subsequently took up another allene equivalent to give 11a. The Consequently, we observed three 19F NMR signals of the pair of
19
intermediate 11a could in principle have reacted with further the C6F5 substituents at boron with a large 
allene, but instead its allylborane “found” the remaining exo- chemical shift separation.
methylene group at the six-membered core with which it
F
m,p
= 11.5 ppm
Some typical reactions of the borane 12a
underwent a favoured intramolecular allylboration reaction4 to
directly give the observed product 12a (Scheme 4).
Compound 12a is a reactive borane and it contains C=C double
bond functionalities. Therefore, it should be suitable to undergo
typical FLP addition to one of the olefinic units in the presence
of an external phosphane nucleophile.11 We, consequently,
reacted the in situ generated borane 12a with
triphenylphosphane in dichloromethane. The reaction with
PPh3 was instantaneous. The volatiles were removed and the
residue was washed with pentane to give the P/B addition
product 13a, which we isolated as a white solid in 56% yield (see
Scheme 5). Compound 13a was characterized by C, H-elemental
analysis, by spectroscopy and by X-ray diffraction (Figure 5). It
showed that a P/B FLP addition had taken place at the proximal
C=CH2 moiety (C12=C15 in compound 12a, see Figure 4) by
using the adjacent pendent internal borane and the external
phosphane. Compound 13a is the isomer that was formed by
borane addition to the C=CH2 terminus and, consequently,
phosphane addition to the sp2-ring carbon atom. This resulted
Scheme 4. Formation of compound 12a and allene by sequential allylboration in the formation of a heterocyclic six-membered ring-system
reactions.
that had become 1,3-attached at the “lower” six-membered
decalin ring of compound 12a. The phosphonium PPh3+ moiety
is found attached at the new bridgehead atom C12 (Figure 5).
Figure 4. Molecular structure of compound 12a. Selected bond lengths (Å) and
angles (°): B1−C5 1.554(3), C1−C2 1.313(3), C4−C5 1.557(3), C4−C6 1.566(3),
C10−C14 1.324(3), C12−C15 1.324(3), B1−C5−C4 123.2(2), C5−C4−C6 109.0(2).
Figure 5. A view of the P/B FLP alkene addition product 13a. Selected bond lengths (Å)
and angles (°): B1−C5 1.640(6), B1−C15 1.667(6), P1−C12 1.858(4), C1−C2 1.302(6),
C4−C6 1.565(5), C10−C14 1.328(6), C12−C15 1.555(5), C5−B1−C15 110.3(3),
C12−C15−B1 114.3(3), P1−C12−C15−B1 -129.8(3).
The X-ray crystal structure analysis of compound 12a shows the
newly formed trans-decalin framework that was formed by the
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Compound 13a shows a typical borate 11B NMR resonance at 12a, similar as we had seen it in the formation of itVsieiwsoAmrticelerO1n3linae
,
 −15.1 in solution (CD2Cl2, 273 K) and a phosphonium 31P NMR only that in this case PPh3 addition had taken place at the =CH2
signal at
DOI: 10.1039/C9SC03870A

31.1. It shows the 19F NMR features of a pair of terminus of the exo-methylene group concurrent with borane
diastereotopic C6F5 groups at the boron atom (for further addition to its adjacent doubly substituted sp2-carbon atom.
details of the NMR characterization of compound 13a see the The structure of the resulting P/B zwitterion 14a is depicted in
ESI).
Figure 6.
1
Compound 13a is the P/B FLP addition product that is formed The H NMR spectrum of compound 14a (in CD2Cl2, at 299 K)
under kinetic control. When we stored the CD2Cl2 solution of shows the P-coupled system of the exocyclic
compound 13a for 7 days at room temperature the resulting at 4.18/2.70 and the resonances of the endocyclic
NMR spectra showed the formation of an equilibrium mixture group at 1.32/0.35. The heteroatom NMR signals occur at
of 13a (ca. 20 mol%), the starting material 12a (plus PPh3, ca. 8
6.8 (11B) and 19.9 (31P), respectively and we observed two sets
−
CH2−
[P] moiety

−
CH2 [B]
−


−
mol%) and the new compound 14a (65 mol%) (plus some minor of 19F NMR signals of the pair of diastereotopic C6F5 groups at
contaminants). The major product 14a, apparently formed boron (for further details see the ESI).
under thermodynamic control, was prepared similarly on a We eventually converted the borane-induced multi-component
preparative scale (24 h, r.t., CH2Cl2 layered with pentane) and cyclization product 12a to a boron-free derivative.14 This was
crystallized from the mixture. Crystalline compound 14a was carried out in the usual way of oxidative deborylation as it is
isolated in 60% yield and the product was characterized by C, H- done in the conventional hydroboration chemistry.15 Treatment
elemental analysis, by spectroscopy and by X-ray diffraction.
of the strongly electrophilic -B(C6F5)2 borane 12a with
NaOH/H2O2 gave the alcohol 15a that we isolated as a white
solid in 42% yield after workup (see the ESI for its
characterization by NMR spectroscopy).
We briefly investigated the reaction of the bis-allenic ether
166c,h,j with HB(C6F5)2. The reaction was carried out in CD2Cl2
solution at r.t. The products of the reaction were not isolated
but directly identified in situ generated from the solution. We
subsequently added a total of three molar equivalents of
HB(C6F5) to eventually achieve a complete conversion of
compound 16 with a clean product formation. The NMR analysis
(for details see the ESI) revealed that the by far predominant
reaction was ether cleavage. This gave the (C6F5)2B-O-CH2-
CH=C=CH2 cleavage product 17 (see Scheme 6) and butadiene
(18) as primary products. The latter was then subsequently
converted by added HB(C6F5)2 to the bis-hydroboration product
19. The boryl ether 17 also was not stable under the reaction
conditions, probably due to subsequent ether cleavage with
additional HB(C6F5)2 (see the ESI for details). We also
investigated briefly the reaction of the respective bis-allenic N-
tosyl amine with HB(C6F5)2, but that gave a complicated mixture
of as yet unidentified products.
Scheme 5. Reaction of compound 12a with PPh3 and oxidative replacement of the
boryl group.
Figure 6. A projection of the molecular structure of compound 14a. Selected bond
lengths (Å) and angles (°): B1
−
C5 1.657(4), B1
−
C12 1.695(3), P1
−
C15 1.823(2),
C1−
C2 1.301(13), C4
−
C6 1.556(3), C10
−
C14 1.325(4), C12
−C15 1.540(3),
C5−
B1−C12 99.9(2), C12
−
C15 B1 112.1(2), B1
−
−
C12 C15 P1 179.1(2).
−
−
The X-ray crystal structure analysis of 14a shows the presence
of five-membered boratacycle that is 1,3-annulated to the
“lower” six-membered ring of the trans-decalin framework. It
was apparently formed by a 1,2-P/B FLP addition to the
C12=C15 carbon-carbon double bond of the starting material
Scheme 6. Reaction of the bis-allenic ether 16 with HB(C6F5)2.
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(a) X. Jiang, X. Cheng and S. Ma, Angew. Chem. Int. Ed., 2006,
Conclusions
View Article Online
45, 8009; (b) S.-K. Kang, T.-G. Baik, A. ND. OKuI:l1a0k.1,0Y3.-9H/C.9HSaC,0Y3.8L7i0mA
and J. Park, J. Am. Chem. Soc., 2000, 122, 11529; (c) Y.-T. Hong,
S.-K. Yoon, S.-K. Kang and C.-M. Yu, Eur. J. Org. Chem., 2004,
With this study we have found a new variant of our borane
induced carbon-carbon coupling reactions between allene
building blocks. In this case the reaction starts as it is commonly
observed in our systems by 1,2-[B]
allene =CH2 group by the strongly electrophilic HB(C6F5)2
hydroboration reagent to probably generate reactive
allylborane intermediate in situ, which is set for undergoing
rapid intramolecular ring-closure with the pendant second
allenyl moiety to generate the products 5a to 5c, respectively.
These are then obviously protected by their special geometry
from undergoing further intermolecular allylborane coupling
under the applied reaction conditions, so that the reaction
stopped at the functionalized six-or seven-membered ring
4628; (d) J. Cheng, X. Jiang and S, Ma, Org. Lett., 2011, 13
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−
H addition16 to a terminal
a
7
8
9
For examples of thermally induced reactions of bis-allenes,
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87, 4506; (b) W. R. Roth, M. Heiber and G. Erker, Angew.
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products. The compounds
5 are, however, in principle still
active allylboration reagents. This we could show by the rapid
reaction of the example 5a with the parent allene H2C=C=CH2.
Two equivalents of allene were consumed in a sequence of
consecutive intramolecular allylboration reactions, followed by
a final intramolecular allylboration ring-closure reaction to give
the four-component coupling product 12a. This in turn was
E. L. Eliel, S. H. Wilen and M. P. Doyle (Eds), Basic Organic
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oxidatively converted to the boron-free product 15a
. These
metal-free reactions are markedly different from the common
metal catalysed bis-allenic cyclization reactions reported in the
literature (see Chart 1 and the respective references). We will
see how the products of our metal free cyclization reactions and
their follow-up products (and related systems) might become
easily available useful reagents for further external C-C coupling
reactions using either of the newly generated functionalities.
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Conflicts of interest
There are no conflicts to declare.
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Notes and references
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1
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13 For a comparison, the 11B NMR data of B(C6F5)3 is

59.5. (a)
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14 We had previously shown that alkenyl-B(C6F5)2 derivatives
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coupling with iodobenzene: (a) C. Chen, G. Kehr, R. Fröhlich
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2
3
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16 For an example of a rare alternative 1,1-hydroboration
reaction, see: A. Ueno, J. Yu, X. Tao, C. G. Daniliuc, G. Kehr
and G. Erker, Organometallics, 2018, 37, 2665.
4
5
(a) X. Tao, G. Kehr, C. G. Daniliuc and G. Erker, Angew. Chem.
Int. Ed., 2017, 56, 1376; (b) see also: X. Tao, C. Wölke, C. G.
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,
13922.
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DOI: 10.1039/C9SC03870A
TOC
Text for TOC: The borane induced cyclization of the bis-allene
followed by reaction with two molar equivalents of allene
gives the functionalized decalin product.
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