BN-Phenanthrenes: Synthesis, Reactivity, and Optical Properties

Article abstract of DOI:10.1021/acs.orglett.9b00530

Two series of BN-phenanthrenes, including the parental compounds, have been synthesized in an efficient manner from commercially available materials. Both parental BN-phenanthrenes are strongly fluorescent in solution. Their absorption and emission spectr

Full text of DOI:10.1021/acs.orglett.9b00530

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
BN-Phenanthrenes: Synthesis, Reactivity, and Optical Properties
Chen Zhang,† Lei Zhang,‡ Chao Sun,† Wenfang Sun,†,∥ and Xuguang Liu*,†,§
†Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering,
Tianjin University of Technology, Tianjin 300384, People’s Republic of China
‡School of Science, Tianjin Chengjian University, Tianjin 300384, People’s Republic of China
§Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
∥Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States
S
* Supporting Information
ABSTRACT: Two series of BN-phenanthrenes, including the parental com-
pounds, have been synthesized in an efficient manner from commercially available
materials. Both parental BN-phenanthrenes are strongly fluorescent in solution.
Their absorption and emission spectra are greatly dependent on the BN position
and orientation. Moreover, the reactivities of the two series of BN-phenanthrenes
toward electrophiles are completely different, and these species can be further
functionalized by cross-coupling reactions.
oping with heteroatoms is an effective way to tune the
D
properties of polycyclic aromatic hydrocarbons (PAHs).1
In particular, BN-codoped PAHs, in which pairs of C atoms
have been replaced by BN units, have attracted great interest,
because of their potential to create new optoelectronic
materials.2 In fact, several BN-PAHs have been reported to
be highly effective and sometimes superior to their carbona-
ceous analogues in fields such as organic field-effect transistors
(OFETs).3 While many BN-perturbed analogues of PAHs now
exist,2,3 the fundamental understanding of the effect of BN/CC
replacement on the structure and properties of the PAHs must
be improved. Thus, the basic knowledge obtained from simple
BN-PAHs is highly valuable.
Phenanthrene, one of the simplest PAHs, is of great interest
to organic and material chemists because of its high stability
and electroluminescent properties.4a−c Moreover, phenan-
threnes are also found as core structure in natural products.4d
Only four BN-phenanthrene derivatives have been synthesized
and isolated to date (Figure 1). Dewar first reported BN-
phenanthrene 1 in 19585a and 2 in 1962.5b Vaquero and
Figure 1. Reported BN-phenanthrenes and this work.
highly desirable, because their electronic structures are not
influenced by substituent effects.9 Herein, we describe the
synthesis and properties of the two missing isomers of BN-
phenanthrene, 5 and 6, including the parental molecules.10
Undaunted by this challenge, the key step in the synthesis was
identifying a suitable starting material that can be easily
transformed to the target molecule by organic transformations.
The syntheses of parental BN-phenanthrenes 5a and 6a are
shown in Scheme 1. The key step involves the borylative
cyclization of the corresponding amino vinyl naphthalene
amines. The synthesis of BN-phenanthrene 5a commenced
with the conversion of the hydroxyl group of commercially
available 2-nitronaphthalen-1-ol to trifluoromethanesulfonate
(OTf) by treatment with trifluoromethanesulfonic anhydride
́
́
Garcıa-Garcıa provided an improved synthetic route to 2 in
2017 and studied its properties and reactivity.6 BN-
phenanthrene 3, with an internal BN unit, was synthesized
by the Piers group in 2007.7 In 2013, Wang’s group reported
the synthesis of BN-phenanthrene 4 by a photoelimination
reaction.8 The presence of BN units in phenanthrenes has a
substantial influence on their properties.5−8 For instance, most
BN-phenanthrenes (1, 3, and 4) are strongly emissive (2 is
not). As shown in Figure 1, the BN units in the reported BN-
phenanthrene derivatives are mainly located on the middle B
ring of the phenanthrene. To the best of our knowledge, BN/
CC replacements in the terminal A/A′ ring have not been
disclosed. Moreover, unsubstituted BN-PAHs (“parental”) are
Received: February 11, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00530
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Scheme 1. Synthetic Route to Parental BN-Phenanthrenes
5a and 6a
Figure 2. NICS (1) values of phenanthrene and parental BN-
phenanthrenes 5a and 6a. Determined at the B3LYP/6-311+G(2d,p)
level of theory.
the middle ring of 5a (−9.17) and 6a (−9.29) are more
negative than those of the corresponding ring in carbonaceous
phenanthrene (−8.00),13 which indicates the middle rings of
BN-phenanthrene 5a and 6a have stronger aromaticity. We
also investigated the reactivity of the parental BN-phenan-
threnes 5a and 6a. The bromination of these isomers could not
be achieved using either bromine (Br2) or N-bromosuccini-
mide (NBS) as the brominating reagent, presumably because
of the effect of the B−H bond.14
(Tf2O). Suzuki cross coupling of intermediate 8 with 4,4,5,5-
tetramethyl-2-vinyl-1,3,2-dioxaborolane furnished 2-nitro-1-
vinylnaphthalene (9). The reduction of 9 using iron powder
gave 1-vinylnaphthalen-2-amine (10). Treatment of 10 with
boron trichloride followed by lithium aluminum hydride
(LiAlH4) afforded parental BN-phenanthrene 5a. 5a is
relatively stable in the air for several hours and can be purified
by chromatography on silica gel. We found that 5a is not stable
in air for long periods and should be stored under an inert
atmosphere.
We then focused on preparing BN-phenanthrenes bearing a
phenyl group on the boron atom, which should improve their
stability (Scheme 2), facilitating further manipulations.
Scheme 2. Synthetic Route to BN-Phenanthrenes 5b and 6b
The initial synthesis of another isomer of BN-phenanthrene,
6a, was staring from the 2-bromonaphthalen-1-amine. After a
substantial effort, we found that the cross coupling of 2-
bromonaphthalen-1-amine with either vinylboronic acid
pinacol ester, vinylmagnesium bromide or tributyl(vinyl)tin
failed in our hand. Ultimately, the successful synthesis of 6a
started with 1-nitro-2-naphthaldehyde (11), which can be
easily prepared by a previously reported procedure (see
Scheme S10 in the Supporting Information).11 The Wittig
reaction between 11 and a triphenyl phosphonium ylide
(generated in situ from Ph3PMeI) afforded 1-nitro-2-vinyl-
naphthalene (12), which can be reduced to 2-vinylnaphthalen-
1-amine (13) by iron powder. Similarly, the borylative
cyclization/reduction sequences using BCl3/LiAlH4 furnished
parental BN-phenanthrene 6a in 70% yield over two steps. We
found that 6a is less stable than 5a and decomposed gradually
during purification. Pure 6a could be obtained by preparative
thin-layer chromatography and should be stored under an inert
atmosphere immediately after purification.
Gratifyingly, the electrophilic borylation of 1-vinylnaphtha-
len-2-amine (10) or 2-vinylnaphthalen-1-amine (13) with
dichlorophenylborane (PhBCl2) produced BN-phenanthrenes
5b (99%) and 6b (94%) in excellent yields. Both 5b and 6b are
quite stable toward chromatography on silica gel. Compounds
5b and 6b were smoothly brominated using bromine (Br2) as
the brominating reagent, which is remarkably different from
the inertness of 5b and 6b toward NBS as a brominating
reagent; using NBS as brominating reagent resulted in either
poor reactivity (with 5b) or no reaction at all (with 6b).
Interestingly, we found that the regioselectivities of the
halogenation reactions of these two isomers are completely
different. The bromination of BN-phenanthrene 5b occurs
exclusively at the α-carbon adjacent to boron, even when using
more than 2 equiv of Br2. On the other hand, under the same
conditions, BN-phenanthrene 6b yielded a mixture of the
mono- (6c) and dibrominated products (6c′), which can be
separated by careful chromatography. When the amount of Br2
was increased to two equivalents, dibrominated product 6c′
became the only product. Notably, when using 3 equiv of Br2,
6c′ was not further brominated. The different behaviors of
isomers 5b and 6b highlight the tremendous influence of the
location of the BN unit on the reactivity of the BN-
phenanthrene. The difference between 5b and 6b is that 6b
has a C−H in para position of the NH group on the central
The 1H NMR chemical shifts of the N−H protons in
parental BN-phenanthrenes 5a and 6a are in the aromatic
region (5a, 8.48 ppm; 6a, 9.30 ppm), which suggests these
species are aromatic. The chemical shifts of B−H in 5a (5.18
ppm) and 6a (5.23 ppm) are ∼5.20 ppm, which is the shift
typical of BN aromatics containing B−H bonds.9 Unfortu-
nately, we were not able to obtain X-ray crystal structures of 5a
and 6a. To understand the effects of the location and
orientation of the BN unit on the aromaticity of BN-
phenanthrene, we performed nucleus-independent chemical
shift (NICS) calculations (Figure 2).12 The NICS (1) values of
the BN rings in 5a and 6a are comparable (5a, −6.58; 6a,
−6.52), indicating the BN-containing rings in the two isomers
have similar aromaticities (Figure 2). The NICS (1) values of
B
DOI: 10.1021/acs.orglett.9b00530
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Letter
ring, which can be brominated, but for 5b this position is fused
to another benzene ring and therefore not amenable to
bromination.
To provide a qualitative explanation of the regioselectivity of
the bromination reactions, we performed theoretical calcu-
lation on the relative stability of arenium ions (after attack of
Br+) on BN-phenanthrenes 5b and 6b (see Figure 3, as well as
shorter than the typical B−C single bond covalent radius (1.58
Å).16a,b Shorter N−C bonds were also observed in the inner
ring (5c, 1.376 Å; 6c′, 1.393 Å). Interestingly, the C7−C8
bond connecting the two terminal rings is longer in 5c (1.361
Å) than in 6c′ (1.309 Å), which is also consistent with the
calculated NICS (1) values, in which the middle ring of 6a
showed greater aromaticity than that of 5a. Both BN-
phenanthrenes have a planar framework, and the dihedral
angles between the C4BN ring and the other terminal benzene
ring are quite small (5c, 5.6°; 6c′, 1.3°). The phenyl groups
connected to the B atom are out of the main plane with
dihedral angles of ∼32° for 5c and 54° for 6c′, which indicate
their diminished conjugation with the planar BN-phenanthrene
core (see Tables S3−S8 in the Supporting Information).
The normalized UV-vis absorption and emission spectra of
BN-phenanthrenes 5a, 5b, 6a, and 6b and carbonaceous
phenanthrene in cyclohexane are shown in Figure 5, and the
Figure 3. Calculated stability of arenium ions, corresponding to the
attack of Br+ on different positions of BN-phenanthrenes 5b and 6b,
determined at the B3LYP/6-311+G(2d,p) level of theory.
Figure S6 in the Supporting Information). In the case of 5b,
the arenium ion intermediate generated by E+ attacking at the
carbon (C-12) adjacent to boron was treated as the zero-
energy reference, which is also the typical position where
bromination of 1,2-dihydro-1,2-azaborine occurred.15 The
relative energies of the arenium ions in all of the remaining
positions in 5b are >10 kcal/mol, which is consistent with the
experimental results, in which electrophilic aromatic sub-
stitution occurs exclusively at the C-12 position of 5b. In the
case of 6b, the second lowest-lying arenium ions was observed
at C-7 (2.23 kcal/mol), which is consistent with the
dibromination results observed experimentally.
Thanks to the high stability of the B-phenyl BN-
phenanthrene derivative, we were able to obtain the crystal
structures of 5c and 6c′, which allowed us to determine the
solid-state structures of both scaffolds, although the quality of
the crystal structure of 6c′ is poor. [The structures of 5c and
6c′ are depicted in Figure 4.] The results confirmed the
regioselectivity of the bromination reactions. The BN bond
lengths of 5c (1.426 Å) and 6c′ (1.408 Å) are close to the
localized BN double bonds (1.403(2) Å),16a,b and they are
much shorter than typical BN single bonds (1.56 Å). The B−C
bonds in the inner rings (5c, 1.528 Å; 6c′, 1.509 Å) are also
Figure 5. Normalized absorption (left) and emission spectra (right)
of phenanthrene, parental BN-phenanthrenes 5a and 6a (top), and
phenyl substituted BN-phenanthrenes 5b and 6b (bottom) in
cyclohexane at a concentration of 10−5 M.
maxima of their absorption and emission bands are presented
in Table 1 (see Table S9 in the Supporting Information). The
low energy bands of parental BN-phenanthrenes 5a and 6a are
substantially red-shifted (5a, λabs = 346 nm; 6a, λabs = 356 nm)
compared to the carbonaceous phenanthrene (294 nm). The
overall absorption spectrum of 5a is blue-shifted, compared to
6a. The trend is consistent with the calculated results (see
Table S2 in the Supporting Information). The introduction of
a phenyl ring on the boron red-shifted the absorption (5b, λabs
= 352 nm; 6b, λabs = 360 nm), because of conjugation between
the B-Ph moiety and the main scaffold.
Phenanthrene is a well-known fluorophore with a relatively
weak emission at 348 nm (Φ = 0.09).7 We also measured the
emission properties of the BN-phenanthrenes, and all four BN-
phenanthrenes showed strong emission in solution (Figure 5,
Table 1). The emission maxima of 5a are comparable to those
of carbonaceous phenanthrene, and the emission maxima of 6a
were red-shifted. Again, B-Ph 5b (λem = 360 nm) and 6b (λem
= 365 nm) displayed red-shifted emission bands, compared to
their corresponding B−H analogues 5a (λem = 348 nm) and 6a
(λem = 354 nm). This trend is consistent with that observed in
their absorption spectra. The solution-state fluorescence
quantum yields of BN-phenanthrenes (5a, 0.79; 5b, 0.92; 6a,
Figure 4. Solid-state structures of BN-phenanthrenes 5c and 6c′ with
views parallel and perpendicular to the polycyclic planes. Thermal
ellipsoids are set at the 50% probability level. H atoms have been
omitted for the sake of clarity.
C
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Table 1. Photophysical Properties of 5a, 5b, 6a, and 6b
a
b
c
e
ε (M−1 cm−1
)
λonset (nm)
λem (λex) (nm)
Φpl
EG (eV)
opt
compound
λabs (nm)
d
Phen
5a
5b
6a
6b
294
345
356
352
360
11926
12016
14877
6811
301
358
364
363
369
348 (294)
348 (301)
360 (300)
354 (300)
365 (283)
0.09
0.79
0.92
0.56
0.85
4.12
3.46
3.41
3.42
3.36
9908
a
b
c
Refers to the lowest-energy peak maxima values. Refers to the highest-energy peak maxima values. Relative to 9,10-diphenylanthracene in
cyclohexane (λex = 336 nm, Φpl = 0.93 0.03).17 Adopted from ref 7. Optical band gap EGopt = 1240/λonset
d
e
0.56; 6b, 0.85) are higher than that of all-carbon phenanthrene
(0.09). Notably, the quantum yield of BN-phenanthrene 5 is
higher than that of BN-phenanthrene 6.
Transition-metal-catalyzed cross-coupling reactions have
been widely used to construct the C−C bonds of aromatic
compounds.18 In particular, the cross coupling of halogenated
BN-PAHs has been well-documented.6,9f,19 The reactivities of
brominated BN-phenanthrenes 5c, 6c, and 6c′ were evaluated
(see Scheme 3). To our delight, brominated BN-phenan-
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00530.
■
S
Full details on the synthesis of all BN-phenanthrenes 5
and 6; NMR spectra, UV-vis and photoluminescence
data, electrochemical data, X-ray crystallographic data,
and theoretical calculations; experimental and computa-
tional details (PDF)
a
Scheme 3. Further Functionalization of 5c, 6c, and 6c′
Accession Codes
CCDC 1896392 and 1896393 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xuguangliu@tjut.edu.cn.
ORCID
Lei Zhang: 0000-0003-0100-8357
Wenfang Sun: 0000-0003-3608-611X
Xuguang Liu: 0000-0002-7859-3139
Notes
a
The reaction conditions were not optimized.
threnes 5c could participate in Suzuki-type (5e, 72%), Heck-
type (5d, 78%) and Sonogashira-type reactions (5f, 39%) and
afforded the corresponding products in good yields. Bromi-
nated BN-phenanthrenes 6c and 6c′ could also undergo
Suzuki-type cross-coupling reactions to produce the corre-
sponding products in decent yields (6d, 43%; 6e, 66%). The
photophysical properties of functionalized products 5c−5f and
6c−6e can be found in the Supporting Information (Figures
S8−S10 and Table S9).
In summary, we have developed an efficient synthesis of two
types of BN-phenanthrenes, including the parental species, by
selecting the appropriate starting material. All the BN-
phenanthrenes were fully characterized, and two of them
were unambiguously confirmed by single-crystal X-ray analysis.
The aromaticities of the parental BN-phenanthrenes were
quantified by experimental and computational studies. The BN
position and orientation substantially impact the optical
properties of the compound. Both types of BN-phenanthrenes
are strongly emissive in solution. Notably, the position of the
BN unit has a significant effect on the reactivity of the
compound toward bromination, which was further explained
by DFT calculations. The exciting photophysical properties
and successful metal-catalyzed coupling reactions make these
BN-phenanthrenes broadly applicable in future works.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
Natural Science Foundation of Tianjin (No.
17JCZDJC37700), National Natural Science Foundation of
China (No. 21502140), Tianjin University of Technology, and
the Shanghai Institute of Organic Chemistry (No. K2015-10).
We also thank the Training Project of Innovation Team of
Colleges and Universities in Tianjin (No. TD13-5020) and the
support from the University Student Innovation Program.
REFERENCES
■
̇
́
́
(1) (a) Stępien, M.; Gonka, E.; Zyła, M.; Sprutta, N. Chem. Rev.
2017, 117, 3479−3716. (b) Narita, A.; Wang, X.; Feng, X.; Mu
̈
llen, K.
Chem. Soc. Rev. 2015, 44, 6616−6643.
(2) (a) Liu, Z.; Marder, T. B. Angew. Chem. 2008, 120, 248−250
[Angew. Chem., Int. Ed. 2008, 47, 242−244] . (b) Bosdet, M. J. D.;
Piers, W. E. Can. J. Chem. 2009, 87, 8−29. (c) Campbell, P. G.;
Marwitz, A. J. V.; Liu, S.-Y. Angew. Chem. 2012, 124, 6178−6197
[Angew. Chem., Int. Ed. 2012, 51, 6074−6092] . (d) Abbey, E. R.; Liu,
S.-Y. Org. Biomol. Chem. 2013, 11, 2060−2069. (e) Ashe, A. J., III
Organometallics 2009, 28, 4236−4248. (f) Wang, X.; Wang, J.; Pei, J.
Chem. - Eur. J. 2015, 21, 3528−3539. (g) Morgan, M. M.; Piers, W. E.
D
DOI: 10.1021/acs.orglett.9b00530
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Dalton. Trans. 2016, 45, 5920−5924. (h) Helten, H. Chem. - Eur. J.
2016, 22, 12972−12982. (i) Wang, J. Y.; Pei, J. Chin. Chem. Lett.
X. Org. Lett. 2014, 16, 5024−5027. (d) Baggett, A. W.; Vasiliu, M.; Li,
B.; Dixon, D. A.; Liu, S.-Y. J. Am. Chem. Soc. 2015, 137, 5536−5541.
́
2016, 27, 1139−1146. (j) Belanger-Chabot, G.; Braunschweig, H.;
Roy, D. K. Eur. J. Inorg. Chem. 2017, 2017, 4353−4368. (k) Giustra,
Z. X.; Liu, S.-Y. J. Am. Chem. Soc. 2018, 140, 1184−1194. (l) Wang, J.
Univ. Chem. 2017, 32, 1−6. (m) Huang, J.; Li, Y. Front. Chem. 2018,
6, 1−22.
(3) (a) Hatakeyama, T.; Hashimoto, S.; Seki, S.; Nakamura, M. J.
Am. Chem. Soc. 2011, 133, 18614−18617. (b) Wang, X.; Lin, H.; Lei,
T.; Yang, D.; Zhuang, F.; Wang, J.; Yuan, S.; Pei, J. Angew. Chem., Int.
Ed. 2013, 52, 3117−3120. (c) Wang, X.; Zhuang, F.; Wang, R.; Wang,
X.; Cao, X.; Wang, J.; Pei, J. J. Am. Chem. Soc. 2014, 136, 3764−3767.
(4) (a) Boden, B. N.; Jardine, K. J.; Leung, A. C. W.; MacLachlan,
M. J. Org. Lett. 2006, 8, 1855−1858. (b) Li, J.; Chang, H.; Feng, C.;
Wu, Y. Org. Lett. 2016, 18, 6444−6447. (c) Kaleta, J.; Mazal, C. Org.
Lett. 2011, 13, 1326−1329. (d) Bera, K.; Sarkar, S.; Jalal, S.; Jana, U. J.
́
Org. Chem. 2012, 77, 8780−8786. (e) Kovacs, A.; Vasas, A.;
Hohmann, J. Phytochemistry 2008, 69, 1084−1110.
(5) (a) Dewar, M. J. S.; Kubba, V. P.; Pettit, R. J. Chem. Soc. 1958,
3073−3076. (b) Dewar, M. J. S.; Kaneko, C.; Bhattacharjee, M. K. J.
Am. Chem. Soc. 1962, 84, 4884−4887.
́
(6) Abengozar, A.; García-García, P.; Sucunza, D.; Frutos, L. M.;
́
Castano, O.; Sampedro, D.; Perez-Redondo, A.; Vaquero, J. J. Org.
̃
Lett. 2017, 19, 3458−3461.
(7) Bosdet, M. J. D.; Jaska, C. A.; Piers, W. E.; Sorensen, T. S.;
Parvez, M. Org. Lett. 2007, 9, 1395−1398.
(8) Lu, J.; Ko, S.; Walters, N. R.; Kang, Y.; Sauriol, F.; Wang, S.
Angew. Chem., Int. Ed. 2013, 52, 4544−4548.
(9) (a) Liu, Z.; Ishibashi, J. S. A.; Darrigan, C.; Dargelos, A.;
Chrostowska, A.; Li, B.; Vasiliu, M.; Dixon, D. A.; Liu, S.-Y. J. Am.
Chem. Soc. 2017, 139, 6082−6085. (b) Ishibashi, J. S. A.; Marshall, J.
̀
L.; Maziere, A.; Lovinger, G. J.; Li, B.; Zakharov, L. N.; Dargelos, A.;
Graciaa, A.; Chrostowska, A.; Liu, S.-Y. J. Am. Chem. Soc. 2014, 136,
15414−15421. (c) Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.;
Dixon, D. A.; Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 973−977.
(d) Abbey, E. R.; Lamm, A. N.; Baggett, A. W.; Zakharov, L. N.; Liu,
S.-Y. J. Am. Chem. Soc. 2013, 135, 12908−12913. (e) Abbey, E. R.;
Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2011, 133, 11508−
11511. (f) Brown, A. N.; Li, B.; Liu, S.-Y. J. Am. Chem. Soc. 2015, 137,
8932−8935.
(10) The synthesis of 5 via a different method was independendently
reported by Vaquero and co-workers while we revised our manuscript.
́
See: Abengozar, A.; García-García, P.; Sucunza, D.; Sampedro, D.;
́
Perez-Redondo, A.; Vaquero, J. J. Org. Lett. 2019, 21, 2550−2554.
(11) Riesgo, E. C.; Jin, X.; Thummel, R. P. J. Org. Chem. 1996, 61,
3017−3022.
(12) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van
Eikema Hommes, N. J. R. J. Am. Chem. Soc. 1996, 118, 6317−6318.
(13) NICS (1) values of phenanthrene have been previously
reported: see refs 7 and 12. Here, we report with our own equipment,
for the sake of consistency.
(14) Both of the BN-phenanthrenes 5a and 6a decomposed under
the bromination reaction conditions.
(15) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Org. Lett. 2007, 9, 679−
681.
(16) (a) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.;
Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1−S17.
(b) Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008,
130, 7250−7252.
(17) (a) Brouwer, A. M. Pure Appl. Chem. 2011, 83, 2213−2228.
(b) Meech, S.; Phillips, D. J. Photochem. 1983, 23, 193−217.
(18) (a) Corbet, J.; Mignani, G. Chem. Rev. 2006, 106, 2651−2710.
(b) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111,
1417−1492.
́
́
(19) (a) Abengozar, A.; García-García, P.; Sucunza, D.; Perez-
Redondo, A.; Vaquero, J. J. Chem. Commun. 2018, 54, 2467−2470.
(b) Molander, G. A.; Wisniewski, S. R.; Traister, K. M. Org. Lett.
2014, 16, 3692−3695. (c) Sun, F.; Lv, L.; Huang, M.; Zhou, Z.; Fang,
E
DOI: 10.1021/acs.orglett.9b00530
Org. Lett. XXXX, XXX, XXX−XXX