Borylative Radical Cyclizations of Benzo[3,4]cyclodec-3-ene-1,5-diynes and N-Heterocyclic Carbene-Boranes

Article abstract of DOI:10.1002/chem.201700689

Borylative radical cyclization of benzo[3,4]cyclodec-3-ene-1,5-diynes to provide 5-borylated 6,7,8,9-tetrahydrobenzo[a]azulenes has been developed. The experimental results suggest that the reaction proceeds by a radical chain mechanism, in which di-tert-butyl hyponitrite (TBHN) works as a good radical initiator to form boryl radicals from N-heterocyclic carbene–boranes (NHC-boranes). The present reaction is a rare model that illustrates addition of boryl radicals to alkynes.

Full text of DOI:10.1002/chem.201700689

A Journal of
Accepted Article
Title: Borylative Radical Cyclizations of Benzo[3,4]cyclodec-3-ene-1,5-
diynes and N-Heterocyclic Carbene-Boranes
Authors: Takashi Watanabe, Daisuke Hirose, Dennis P Curran, and
Tsuyoshi Taniguchi
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To be cited as: Chem. Eur. J. 10.1002/chem.201700689
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Borylative Radical Cyclizations of Benzo[3,4]cyclodec-3-ene-1,5-
diynes and N-Heterocyclic CarbeneBoranes
Takashi Watanabe,[a] Daisuke Hirose,[b] Dennis P. Curran,*[c] and Tsuyoshi Taniguchi*[a]
Abstract: Borylative radical cyclization of benzo[3,4]cyclodec-3-ene-
1,5-diynes to provide 5-borylated 6,7,8,9-
Recently, Ogawa and co-workers reported methods for
diboronation of alkynes putatively involving addition of
pinacolboryl radicals to alkynes.[11]
tetrahydrobenzo[a]azulenes has been developed. The experimental
results suggest that the reaction proceeds by a radical chain
mechanism, and di-tert-butyl hyponitrite (TBHN) works as a good
radical initiator to form boryl radicals from N-heterocyclic carbene-
boranes (NHC-boranes). The present reaction is a rare model to
illustrate addition of boryl radicals to alkynes.
Programmed cascade reactions initiated by intermolecular
addition of various carbon- and heteroatom-centered radicals to
triple bonds are useful methods to construct functionalized cyclic
frameworks.[12] With the intention to develop a new radical
reaction involving addition of boryl radicals to alkynes followed
by radical cyclization, we tried radical reactions of two model
substrates, enyne 1a[13] and alkynal 1b[14]
,
with 1,3-
The chemistry of boron-centered radicals has been recently an
attractive and challenging subject in the main group chemistry
because boron compounds have been historically used mainly
for ionic and metal-catalyzed reactions. Interesting structures
and reactivity of several boron-centered radicals have been
identified thus far.[1] However, the application of boron-centered
radicals to synthetic methods has less explored compared with
those of other radical species.[2]
dimethylimidazol-2-ylidene borane (2a). To promote formation of
a boryl radical from 2a, we added di-tert-butyl hyponitrite
(TBHN)[15]
, which forms highly reactive tert-butoxy radical
(tBuO•) with release of nitrogen gas under heating conditions.[7b]
However, cyclized products 3a and 3b were not detected in
either case (Figure 1B). Since 5-exo-trig radical cyclization onto
double bonds is generally fast,[16] the absence of cyclized
products suggests that addition of the NHC-boryl radical to a
simple terminal alkyne is unfavorable.
Borane complexes and borohydride compounds typically
work as a hydride source. Since homolytic bond dissociation
energies (BDEs) of B−H bonds are typically large,[3] these do not
apparently work as a hydrogen donor to radicals. However, well-
designed radical reactions with borohydride compounds have
been reported.[4] N-Heterocyclic carbene-boranes (NHC-
boranes) are good precursors of relatively stable boryl radicals
because their B−H BDEs are lower than those of typical borane
complexes.[5] Therefore, they can work as replacements of
typical radical mediators such as tributyltin hydride (Bu3SnH) in
Barton-McCombie reactions[6], radical dehalogenation of
organohalides[7]
and
decyanations
of
malononitriles.[8]
Nevertheless, there are only a few examples of addition of NHC-
boryl radicals to carboncarbon multiple bonds. NHC-boryl
radicals are nucleophilic and induce polymerization of acrylates
by the addition reaction.[9] On the other hand, the addition of
boron-centered radicals including NHC-boryl radicals to alkynes
has been much less explored (Figure 1A). Lalevée and co-
workers reported that addition of triethylamineboryl radical
(Et3NBH2•) to cyclohexylacetylene was relatively fast in
electron paramagnetic resonance (EPR) experiments.[10]
Figure 1. Background of this study.
[a]
T. Watanabe, Dr. T. Taniguchi
School of Pharmaceutical Sciences, Institute of Medical,
Pharmaceutical and Health Sciences
Kanazawa University
During another preliminary study, we conducted Masamune-
Bergman reactions[17] of benzo[3,4]cyclodec-3-ene-1,5-diyne
(4)[18] with NHC-borane 2a under heating conditions. In many
experiments, we isolated small amounts of 5-borylated 6,7,8,9-
tetrahydrobenzo[a]azulene 5a (Figure 1C). The structure was
supported by NMR spectroscopy, high resolution mass
spectrometry and UV-vis spectroscopy. The 11B NMR spectrum
showed a triplet at −25.0 ppm in CDCl3, indicating production of
a B-monosubstituted NHC-borane compound.
Kakuma-machi, Kanazawa 920-1192 (Japan)
E-mail: tsuyoshi@p.kanazawa-u.ac.jp
Dr. D. Hirose
Graduate School of Natural Science and Technology,
Kanazawa University
Kakuma-machi, Kanazawa 920-1192 (Japan)
Prof. Dr. D. P. Curran
Department of Chemistry
[b]
[c]
University of Pittsburgh
Pittsburgh, PA 15260 (USA)
E-mail: curran@pitt.edu
Production of borylated carbocycle 5a suggests that addition
of a boryl radical to an alkyne occurred. Product 5a could be
Supporting information for this article is given via a link at the end of
the document.
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formed by a radical cascade reaction induced by intermolecular
addition of NHC-boryl radical 6 formed by the reaction of 2a with
p-benzyne, which is a biradical intermediate, to the alkyne of
diyne 4.[19] This was supported by detection of 1,2,3,4-
tetrahydroanthracene.[18] Alkenyl radical 7 formed by the addition
reaction of NHC-boryl radical 6 could cyclize onto the other
alkyne to form cyclopentyl radical 8, while 8 could abstract a
hydrogen atom from NHC-borane 2a to give product 5a. Since
the NHC-boryl radical 6 is generated again in the final step, the
overall process is a chain reaction. Both stabilization of the
resultant alkenyl radical 7 by the aromatic ring and the strain of
alkynes might promote addition of the NHC-boryl radical to the
alkynes in this reaction. For the cyclization step, there is a
theoretical study of 5-endo-dig cyclization of 1,2-
diethynylbenzenes, and it suggested that 5-endo-dig cyclization
after addition of radicals to the alkyne does not have a high
barrier.[20] The radical cyclization of diyne 4 would be more
favorable due to its strained structure.[21]
cyclized product 5a (Entries 8 and 9). However, the use of
decreased amount of NHC-borane 2a (2 equiv) significantly
diminished the yield of product 5a (Entry 10). On the other hand,
a larger amount of 2a (10 equiv) slightly improved the result
(Entry 11). In the present reaction, most of the unreacted NHC-
borane could be recovered by silica gel chromatography. Thus,
we
succeeded
in
obtaining
5-borylated
6,7,8,9-
tetrahydrobenzo[a]azulene 5a in acceptable yield by optimizing
reaction conditions. The product based on Masamune-Bergman
reaction, such as 1,2,3,4-tetrahydroanthracene, was hardly
detected in these cases.
Table 1. Optimization of the reactions between 4 and 2a.[a]
In this communication, we describe borylative radical
cyclization of benzo[3,4]cyclodec-3-ene-1,5-diynes with NHC-
Entry
TBHN
(equiv)
Solvent
Temp
[ºC]
Time
[h]
5a
4
boranes
to
produce
5-borylated
6,7,8,9-
[%][b]
[%][b]
tetrahydrobenzo[a]azulenes, which illustrates that boryl radical
addition to alkynes has been realized.[22,23]
1
0.2
0.2
0.2
0.2
0.3
0.2
0.2
0.35
0.4
0.4
0.4
PhH
60
4
22
35
30
33
39
40
46
55
59
31
65
60
33
40
8
2
PhH
80
2
3[c]
4[d]
5
PhH
80
0.5
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
PhH
80
PhH
80
18
22
28
19
trace
23
0
6[e]
7
PhH
100
100
100
100
100
100
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
8
9
Scheme 1. Proposed mechanism of the reaction of 4 with boryl radical 6 to
produce 5a.
10[f]
11[g]
We looked for a suitable radical initiator that would efficiently
induce the radical chain reaction to 5-borylated 6,7,8,9-
tetrahydrobenzo[a]azulene 5a. Azobisisobutyronitrile and
Et3B/O2, which are typical radical initiators, did not form 5a at 60
[a] Conditions: 4 (0.15 mmol), 2a (0.75 mmol), TBHN (0.03−0.06 mmol) in
solvent (1.5 mL) at 60−100 ˚C. [b] Yield of isolated compounds. [c] 7.5 mL
(0.02 M) of solvent was used. [d] 0.3 mL (0.5 M) of solvent was used. [e] The
reaction was performed in a sealed tube. [f] 2 equiv (0.3 mmol) of 2a was
used. [g] 10 equiv (1.5 mmol) of 2a was used.
ºC and room temperature.[8,9] However, the use of
a
substoichiometric amount (0.2 equiv) of TBHN induced the
reaction of diyne 4 with NHC-borane 2a (5 equiv) in heating
benzene (0.1 M) to provide cyclized product 5a in 22% yield
(Table 1, Entry 1). A significant amount of the starting material
was not consumed in this reaction. Elevating the temperature
and increasing the amount of TBHN improved the results, but
there was still recovery of the starting material (Entries 2, 5 and
6). There was no impact in the diluted conditions (0.02 M) (Entry
3), but the reaction in a high concentration (0.5 M) complicated
the reaction with low recovery of the starting material (Entry 4).
Replacing the solvent with benzotrifluoride at 100 ˚C slightly
improved the conversion ratio of product 5a (Entry 7).[24]
Increasing the amount of TBHN further improved yield of
We next surveyed preparative reactions of several NHC-
boranes with diyne 4 under the conditions of Entry 9 in Table 1
(Figure 2). The reaction of diyne
4
with 1,3-
dimethylbenzimidazol-2-ylidene borane (2b) gave a complex
mixture, and cyclized product 5b could not be isolated in a pure
form. However, when 1,3,4,5-tetramethylimidazol-2-ylidene
borane (2c) and 1,3-diisopropylimidazol-2-ylidene borane (2f)
were reacted with diyne 4, the corresponding cyclized products
5c and 5f were obtained in 59% and 51% yield. 1,4-Dimethyl-
1,2,4-triazolium-5-ylidene
borane
(2d)
provided
the
corresponding cyclized product 5d in 40% yield, but the reaction
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with 4,5-dichloro-1,3-dimethylimidazol-2-ylidene borane (2e) was
complex like the reaction with 5b, and the corresponding
cyclized product 5e was isolated in 19% yield. Small amounts of
side products tentatively identified as 1,2-hydroborated
compounds of the alkyne were detected along with 5d and 5e
(see the Supporting Information). The reaction with dipp-
imidazolylidene borane 2g (dipp is 2,6-diisopropylphenyl) did not
produce cyclized compound 5g at all probably due to a steric
factor, and starting 4 was recovered in 77% yield. These results
imply that the borylative radical reaction is sensitive to electronic
and steric factors of NHCs.
Reactions of some diyne analogues were next tested with
NHC-borane 2a, and results are shown in Figure 2. Reactions of
diynes having a 3,4-disubstituted arene provided 5-borylated
6,7,8,9-tetrahydrobenzo[a]azulene derivatives 9−12 in 36−57%
yield under the standard conditions. Reactions of diynes having
electron-donating groups such as methyl or methoxy groups
provided cyclized products 11 and 12 but do not fully completed
under standard conditions. The use of increased amounts of
reagents (TBHN: 0.5 equiv, 2a: 10 equiv) increased
consumption of the starting diynes. These results suggest that
addition of the NHC-boryl radical to electron-rich alkynes is less
favorable compared to the case of diyne 4. This is consistent
with nucleophilicity of NHC-boryl radicals.[6b,9] These results also
support that the rate-limiting step might be addition of the NHC-
boryl radical to the alkyne. Thus, these examples illustrated that
the reaction provided an access to the novel borylated ring-
fused system.
The effect of ortho-substituted groups of diynes was tested
using representative diynes 13, 16, and 19. The reaction of
diyne 13 having an ortho-methyl group provided product 14 as a
major product, indicating that the boryl radical reacted with a
less hindered alkyne. A small amount of another isomer 15 was
also isolated in this reaction. Two reactions using different
amounts of reagents gave almost the same results, and the ratio
of products 14 and 15 was approximately 80:20. The reaction of
16 having an ortho-methoxy group was more complex than that
of 13, and cyclized product 17 (20% yield) and a complex
mixture including many side products were isolated. The starting
diyne 16 was fully consumed in this reaction. We next performed
the reaction at a lower temperature (80 ˚C) with 10 equiv of 2a
and 0.4 equiv of TBHN to suppress excessive side reactions.
Product 17 was obtained in slightly improved yield, and main
side products could be tentatively identified as 1,2-
hydroboration compounds of alkynes (ca. 18% yield) because
some unidentified products disappeared in this case (see the
Supporting Information). We could not find another cyclized
isomer 18 from isolated products in this reaction. Analogue 19
having methoxy groups at two ortho-positions of alkynes did not
give a cyclized product 20. In this reaction, no borylated side
products were detected by 11B NMR analysis of the reaction
mixture, and significant amounts of the starting material did not
seem to be consumed. Thus, ortho-substituent groups seemed
to sterically affect the reaction.
Figure 2. Results of TBHN-initiated radical reactions between several diynes
and NHC-boranes. Reaction conditions: diyne (0.15 mmol), NHC-borane (0.75
mmol), TBHN (0.06 mmol) in PhCF3 (1.5 mL) for 0.5 h at 100 ˚C. [a] Yield in
parenthesis is based on a recovered starting material. [b] 10 equiv (1.5 mmol)
of 2a and 0.5 equiv (0.075 mmol) of TBHN were used. [c] 10 equiv (1.5 mmol)
of 2a and 0.4 equiv (0.06 mmol) of TBHN were used at 80 ˚C.
We briefly investigated the isotope effect in the radical
reaction of diyne 4 using a mixture of NHC-borane 2a and NHC-
borane 2a-d having deuterium atoms (90% D), of which the total
H/D ratio was approximately equal (Scheme 2). Cyclized
compound 5a-d having deuterium atoms was detected in
isolated products. The ratio of H to D on the cyclopentene
moiety was estimated at 70:30 by 1H NMR analysis. The
observed value is not large but is consistent with typical
Bu3SnH-mediated radical reactions (kH/kD = ca. 2.3),[25] indicating
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that the step of irreversible B−H bond cleavage determined the
produced from the major isomer (C10 isomer) under basic
conditions. Prolonging the reaction time slightly increased the
amount of the minor isomer (C9 isomer).
distribution of the product.[26]
Scheme 2. Isotope effect in the reaction of 4 with the NHC-borane.
We tried to transform the borylated compound 5a into other
derivatives by Suzuki-Miyaura reaction. It is known that NHC-
arylboranes (NHC-BH2Ar) can be transformed into the
corresponding NHC-difluoro(aryl)boranes (NHC-BF2Ar), which
work as precursors of Suzuki-Miyaura reaction, with
Selectfluor.[27] However, treatment of 5a with Selectfluor did not
provide the corresponding difluoroborane derivative at all. In this
reaction, a mixture of unidentified products having no boron
component was isolated, suggesting that alkenes of 5a reacted
with Selectfluor. Next, we tried to transform B−H bonds to B−Cl
bonds with N-chlorosuccinimide (NCS). Since NHC-BCl2R could
be isolated by silica gel chromatography, we tested a one-pot
Suzuki-Miyaura reaction by forming a borate intermediate in situ
under basic conditions. After treatment of 5a with NCS in CH2Cl2
Scheme 3. Transformation of 5a by Suzuki-Miyaura reaction.
In conclusion, we developed cascade radical reactions of
benzo[3,4]cyclodec-3-ene-1,5-diynes
carbene−boranes in the presence of di-tert-butyl hyponitrite to
provide 5-borylated 6,7,8,9-tetrahydrobenzo[a]azulene
and
N-heterocyclic
and stirring for 15 min at
0
˚C, 4-iodoanisole, 1,1'-
bis(diphenylphosphino)ferrocene]palladium(II) dichloride with
dichloromethane [PdCl2(dppf)•CH2Cl2], sodium carbonate, and
THF were directly added to the reaction mixture, and the
resultant mixture was heated for 2.5 hours. In this reaction, a
desired coupling product 21 was not obtained at all, and a
deboronated product 22 was exclusively obtained in 87% yield
(Scheme 3, Eq. 1, Method A). This result indicated that olefins of
5a were intact by NCS. A deboronated product 22 was hardly
detected in a mixture of 5a and NCS in CDCl3 by 1H NMR
analysis. When water was subsequently added to this mixture, a
deboronated product 22 was detected along with a compound
tentatively identified as the corresponding boronic acid by 1H
NMR analysis.[28] Obviously, deboronation occurred in the step
of hydrolysis of NHC-BCl2R. In the absence of water, sodium
carbonate would exclusively cause deboronation to give 22. In
light of these NMR experiments, Suzuki-Miyaura reaction was
performed in THF and water (10:1), but a coupling compound 21
was not produced. When the temperature was elevated to 100
ºC in toluene and water (10:1) in Suzuki-Miyaura reaction, a
small amount of the desired product 21 was detected. Eventually,
we obtained coupling product 21 in 51% yield by a one-pot
strategy consisting of treatment of 5a with NCS, subsequent
addition of THF/water (10:1) and stirring at room temperature for
1 hour, and Suzuki-Miyaura reaction at 100 ºC in toluene
(Scheme 3, Eq. 1, Method B). When 4-iodonitrobenzene was
used as a coupling partner, the corresponding coupling product
was isolated as a mixture 23 including two olefinic isomers
(Scheme 3 Eq. 2). The minor isomer (C9 isomer) would be
derivatives. Examples of the synthesis of such carbocyclic
systems are limited.[29] The reaction was assumed to be a
radical chain process involving intermolecular boryl radical
addition to the alkyne followed by cyclization. It should be noted
that this approach would be difficult due to release of free
borane or large B−H BDEs if typical borane complexes or
borohydrides are used. However, this study illustrated that
boron-centered radicals are applicable to not only intermolecular
radical addition to alkynes but also cascade reactions by
properly designing the substrates and reaction conditions. We
anticipate that this study will serve as a motif to develop various
cascade reactions with boron-centered radicals in the future.
Acknowledgements
D.H. and T.T. are thankful to Profs. Shigeyoshi Kanoh, Katsuhiro
Maeda and Tomoyuki Ikai (Kanazawa University) for their kind
support. This work was supported by MEXT/JSPS KAKENHI
Grant-in-Aid for Scientific Research (C) (Grant No. 16K08159)
and Grant-in-Aid for JSPS Fellows (Grant No. 14J02441). D.P.C
thanks the US National Science Foundation for support.
Keywords: alkynes • boron • main group chemistry • radical
reactions • strained molecules
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[1]
Recent examples: a) C.-W. Chiu, F. P. Gabbaï, Angew. Chem. Int. Ed.
2007, 46, 1723–1725; Angew. Chem. 2007, 119, 1753–1755; b) T.
Matsumoto, F. P. Gabbaï, Organometallics 2009, 28, 4252–4253; c) Y.
Aramaki, H. Omiya, M. Yamashita, K. Nakabayashi, S. Ohkoshi, K.
Nozaki, J. Am. Chem. Soc. 2012, 134, 19989–19992; d) P. Bissinger, H.
Braunschweig, A.; Damme, I. Krummenacher, A. K. Phukan, K.
Radacki, S. Sugawara, Angew. Chem. Int. Ed. 2014, 53, 7360–7363;
Angew. Chem. 2014, 126, 7488–7491; e) D. Lu, C. Wu, P. Li, Chem.
Eur. J. 2014, 20, 1630–1637; f) A. J. Rosenthal, M. Devillard, K. Miqueu,
G. Bouhadir, D. Bourissou, Angew. Chem. Int. Ed. 2015, 54, 9198–
9202; Angew. Chem. 2015, 127, 9330–9334; g) M. F. Silva Valverde, P.
Schweyen, D. Gisinger, T. Bannenberg, M. Freytag, C. Kleeberg, M.
Tamm, Angew. Chem. Int. Ed. 2017, 56, 1135–1140; Angew. Chem.
2017, 129, 1155–1160.
Saeki, S. Kawaguchi, L.-B. Han, A. Nomoto, A. Ogawa, Tetrahedron
2016, 72, 7832–7838.
[12] U. Wille, Chem. Rev. 2013, 113, 813–853.
[13] K.; Miura, H. Satio, N. Fujisawa, A. Hosomi, J. Org. Chem. 2000, 65,
8119–8122.
[14] C. González-Rodoríguez, L. Escalante, J. A. Varela, L. Castedo, C.
Saá, Org. Lett. 2009, 11, 1531–1533.
[15] D. G. Mendenhall, Tetrahedron Lett. 1983, 24, 451–452.
[16] M. Newcomb, In Radicals in Organic Synthesis, 1st ed.; P. Renaud, M.
P. Sibi, Eds.; Wiley-VCH: Weinheim, Germany, 2001. Vol. 1. pp 317–
336.
[17] a) N. Darby, C. U. Kim, J. A. Salaün, K. W. Shelton, S. Takada, S.
Masamune, J. Chem. Soc., Chem. Commun. 1971, 23, 1516–1517; b)
R. R. Jones, R. G. Bergman, J. Am. Chem. Soc. 1972, 94, 660–661; c)
R. G. Bergman, Acc. Chem. Res. 1973, 6, 25–31; d) T. P. Lockhart, P.
B. Comita, R. G. Bergman, J. Am. Chem. Soc. 1981, 103, 4082–4090.
[18] M. F. Semmelhack, T. Neu, F. Foubelo, J. Org. Chem. 1994, 59, 5038–
5047.
[2]
[3]
A. Baralle, A. Baroudi, M. Daniel, L. Fensterbank, J.-P. Goddard, E.
Lacôte, M.-H. Larraufie, G. Maestri, M. Malacria, C. Ollivier, In
Encyclopedia of Radicals in Chemistry, Biology and Materials.1st ed.; C.
Chatgilialoglu, A. Studer, Eds.; John Wiley & Sons Ltd.: Chichester, UK,
2012; Vol. 2, pp 767–816.
[19] An example of radical polymerization intiated by
a p-benzyne
J. Hioe, A. Karton, J. M. L. Martin, H. Zipse, Chem. Eur. J. 2010, 16,
6861–6865.
intermediate: J. D. Rule, S. R. Wilson, J. S. Moore, J. Am. Chem. Soc.
2003, 125, 12992–12993.
[4]
[5]
T. Kawamoto, I. Ryu, Org. Biomol. Chem. 2014, 12, 9733–9742.
D. P. Curran, A. Solovyev, M. Makhlouf Brahmi, L. Fensterbank, M.
Malacria, E. Lacôte, Angew. Chem. Int. Ed. 2011, 50, 10294–10317;
Angew. Chem. 2011, 123, 10476–10500.
[20] I. V. Alabugin, M. Manoharan, J. Am. Chem. Soc. 2005, 127, 9534–
9545.
[21] In a preliminary experiment, a less strained 11-membered cyclic diyne
(benzo[3,4]cycloundec-3-ene-1,5-diyne) did not cause similar borylative
radical cyclization. Further studies are currently in progress.
[22] A nice review on borylative cyclization has been recently published: E.
Buñuel, D. J. Cárdenas, Eur. J. Org. Chem. 2016, 5446–5464.
[23] An example of synthesis of a 4-borylated 9-phenyl-2,3-dihydro-1H-
fluorene by borylative cyclization of a diyne with boron trichloride:
Warner, A. J.; Lawson, J. R.; Fasano, V.; Ingleson, M. J. Angew. Chem.
Int. Ed. 2015, 54, 11245–11249; Angew. Chem. 2015, 127, 11397–
11401.
[6]
a) S.-H. Ueng, M. Makhlouf Brahmi, ꢀ. Derat, L. Fensterbank, E.
Lacꢁte, M. Malacria, D. P. Curran, J. Am. Chem. Soc. 2008, 130,
10082–10083; b) S.-H. Ueng, A. Solovyev, X. Yuan, S. J. Geib, L.
Fensterbank, E. Lacꢁte, M. Malacria, M. Newcomb, J. C. Walton, D. P.
Curran, J. Am. Chem. Soc. 2009, 131, 11256–11262; c) S.-H. Ueng, L.
Fensterbank, E. Lacôte, M. Malacria, D. P. Curran, Org. Lett. 2010, 12,
3002–3005.
[7]
a) S.-H. Ueng, L. Fensterbank, E. Lacꢁte, M. Malacria, D. P. Curran,
Org. Biomol. Chem. 2011, 9, 3415–3420; b) X. Pan, E. Lacôte, J.
Lalevée, D. P. Curran, J. Am. Chem. Soc. 2012, 134, 5669–5674.
T. Kawamoto, S. J. Geib, D. P. Curran, J. Am. Chem. Soc. 2015, 137,
8617–8622.
[24] E. Hasegawa, Y. Ogawa, K. Kakinuma, H. Tsuchida, E. Tosaka, S.
Takizawa, H. Muraoka, T. Saikawa, Tetrahedron 2008, 64, 7724–7728.
[25] a) D. J. Carlsson, K. U. Ingold, J. Am. Chem. Soc. 1968, 90, 7047–
7055; b) M. S. Oderinde, M. G. Organ, Chem. Eur. J. 2013, 19, 2615–
2618.
[8]
[9]
a) M.-A. Tehfe, M. Makhlouf Brahmi, J.-P. Fouassier, D. P. Curran, M.
Malacria, L. Fensterbank, E. Lacꢁte, J. Lalevꢂe, Macromolecules 2010,
43, 2261–2267; b) M.-A. Tehfe, J. Monot, M. Makhlouf Brahmi, H.
Bonin-Dubarle, D. P. Curran, M. Malacria, L. Fensterbank, E. Lacꢁte, J.
Lalevꢂe, J.-P. Fouassier, Polym. Chem. 2011, 2, 625–631; c) J. C.
Walton, M. Makhlouf Brahmi, J. Monot, L. Fensterbank, E. Lacôte, M.
Malacria, Q. Chu, S.-H. Ueng, A. Solovyev, D. P. Curran, J. Am. Chem.
Soc. 2011, 133, 10312–10321.
[26] E. M. Simmons, J. F. Hartwig, Angew. Chem. Int. Ed. 2012, 51, 3066–
3072; Angew. Chem. 2012, 124, 3120–3126.
[27] S. Nerkar, D. P. Curran, Org. Lett. 2015, 17, 3394–3397.
[28] Examples of protodeboronation of boronic acids: a) H. G. Kuivila, K. V.
Nahabedian, J. Am. Chem. Soc. 1961, 83, 2159–2153; b) H. G. Kuivila,
J. F. Reuwer, Jr., J. A. Mangravite, Can. J. Chem. 1963, 41, 3081–
3090.
[10] J. Lalevꢂe, N. Blanchard, A.-C. Chany, M.-A. Tehfe, X. Allonas, J.-P.
Fouassier, J. Phys. Org. Chem. 2009, 22, 986–993.
[29] G. Wang. C. Chen, J. Peng, Chem. Commun. 2016, 52, 10277–10280.
[11] a) A. Yoshimura, Y. Takamachi, L.-B. Han, A. Ogawa, Chem. Eur. J.
2015, 21, 13930–13933; b) A. Yoshimura, Y. Takamachi, K. Mihara, T.
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10.1002/chem.201700689
Chemistry - A European Journal
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T. Watanabe, D. Hirose, D. P. Curran,*
T. Taniguchi*
Page No. – Page No.
Borylative Radical Cyclizations of
Benzo[3,4]cyclodec-3-ene-1,5-diynes
and N-Heterocyclic CarbeneBoranes
Addition of boryl radicals to alkynes occurs when a mixture of strained
benzo[3,4]cyclodec-3-ene-1,5-diynes and N-heterocyclic carbeneboranes are
exposed to radical conditions with di-tert-butyl hyponitrite. The addition reaction
induces subsequent radical cyclization to provide 5-borylated 6,7,8,9-
tetrahydrobenzo[a]azulenes. Further transformation of the borylated compound has
been achieved by using a modified procedure for Suzuki-Miyaura reaction.
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