Synthesis, Characterization, and Functionalization of 1-Boraphenalenes

Article abstract of DOI:10.1002/anie.201803180

1-Boraphenalenes have been synthesized by reaction of BBr₃ with 1-(aryl-ethynyl)naphthalenes, 1-ethynylnaphthalene, and 1-(pent-1-yn-1-yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench-stable products. A

Full text of DOI:10.1002/anie.201803180

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Accepted Article
Title: Synthesis, Characterization and Functionalization of 1-Bora-
phenalenes
Authors: Michael Ingleson, Rachel Kahan, Daniel Crossley, Jessica
Cid, and James Radcliffe
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803180
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10.1002/anie.201803180
Angewandte Chemie International Edition
COMMUNICATION
Synthesis, Characterization and Functionalization of 1-Bora-
phenalenes
R. J. Kahan, D. L. Crossley, J. Cid, J. E. Radcliffe, M. J. Ingleson*[a]
Abstract: 1-Bora-phenalenes have been synthesized by reaction of
BBr3 with 1-(aryl–ethynyl)naphthalenes, 1-ethynylnaphthalene and
1-(pent-1-yn-1-yl)naphthalene and can be selectively functionalised
at boron or carbon to form bench stable products. All these 1-bora-
phenalenes have LUMOs localized on the planar C12B core that are
closely comparable in character to isoelectronic phenalenyl cations.
In contrast to the comparable LUMOs, the aromatic stabilization of
the C5B ring in 1-bora-phenalenes is dramatically lower than the C6
rings in phenalenyl cations. This is due to the occupied orbitals of π
symmetry being less delocalised in the 1-bora-phenalenes.
electronic structures and thus are not directly comparable to the
phenalenyls. Even 3 which has a tricyclic core isoelectronic to 1+
has some LUMO character located on the exocyclic aromatic
groups and is not completely planar (see Fig. S8), therefore is
distinct to 1+. To generate a boron doped PAH more comparable
to the phenalenyl cation the analogue should be planar, be
isoelectronic to 1+, and have a LUMO that is closely comparable
in character to 1+. Notably our calculations indicate this is the
case for the 1-bora-phenalenes (e.g. Fig. 1, bottom right).
Recently, a number of routes have been developed to
synthesize B-doped PAHs.6, 13 In this area the combination of
alkyne borylative cyclisation14 and intramolecular boron-Friedel
Crafts enabled the formation of boracycles (Scheme 1, top).15
Compound B and derivatives (scheme 1, top right) do contain a
1-bora-phenalene (C12B) subunit. However, the additional fused
rings present in B leads to non-planarity within the C12B subunit
and LUMOs that are delocalised beyond the tricyclic subunit.
Herein we report the serendipitous synthesis of a planar 1-bora-
phenalene containing no additional annulation. This enabled the
subsequent development of a simple route to 1-bora-phenalenes,
which can be readily functionalized at varying positions.
Calculations revealed comparable LUMOs for these 1-
boraphenalenes and isoelectronic 1+, however, the occupied π
orbitals of the 1-bora-phenalenes are distinct in character to 1+
leading to lower aromatic stabilization of the C5B ring.
Phenalenyl, 1 is an open shell poly-aromatic hydrocarbon (PAH)
containing 13 carbon atoms and 13 π electrons.1 Since
Haddon’s seminal report in 1975,2 1, and derivatives, have been
of considerable interest for studying fundamental bonding
phenomena (multi-centre bonding / σ Vs. π dimerisation),2,3 and
for a range of applications (organic semiconductors, spin
memory, electrode materials).4 The non-bonding SOMO of
phenalenyl is key to its unique properties (Fig. 1), and
phenalenyls display amphoteric redox behaviour, with oxidation
furnishing a 12 π electron cation and reduction a 14 π electron
anion.1 The key properties of
1 can be modulated by
functionalisation of the periphery or by incorporation of
6
heteroatoms.5, While the incorporation of N, O and S into
phenalenyls is well documented,1, 5, 7 there are only two reports
incorporating boron to the best of our knowledge, and in both
boron is “co-doped” with nitrogen (e.g. 2 and 3, Fig. 1).8-10
However, computational studies on bora-phenalenes have
indicated potentially interesting molecular and bulk properties.11
Scheme 1. Top, the combined borylative cyclisation/intramolecular SEAr
reaction. Bottom, this work to form 1,2- and 1,3-dibromo-1-bora-phenalenes.
Previously, the cyclisation of 2-(phenylethynyl)-1,1'-
biphenyl led to the 9-borylated phenanthrene, compound A
(Scheme 1, top left).14 However, on attempting the borylative
cyclisation of 4a the expected compound, C (Scheme 2, left),
was not observed. Instead the addition of BBr3 in ortho-
dichlorobenzene (o-DCB) and heating to 140 °C led on workup
to the 1-hydroxy-1-bora-phenalene, 6a (Scheme 2, right). In
addition to 6a, the formation of products from HBr addition to the
triple bond of 4a were observed (HBr is the by-product from the
SEAr reaction). Repeating the reaction in the presence of 2,4,6-
tri-tert-butylpyridine (TBP) and using excess BBr3 (as [H-
TBP][BBr4] is now the ultimate by-product from SEAr) prevents
this side reaction and leads to good yields of 6a. Analogous
reactivity is observed when terphenyl is replaced by p-tolyl and
6c also was isolated as a bench stable solid. In contrast, the
Figure 1. Top, phenalenyl, 1, and reported B,N-phenalenes / extended B-E
phenalenes (refs: 8, 9 and 12). Bottom, bora-phenalene isomers. The LUMOs
are calculated at the B3LYP/6-311G(d,p) level (0.05 isovalue).
While notable work on di- and tetra-benzophenalenes
containing boron (with and without co-doping with N/O/S) has
been reported,12 these more extended compounds have distinct
[a]
Dr. R. A. Kahan, Dr. D. L. Crossley, Dr. J. Cid, Dr. J. E. Radcliffe,
Dr. M. J. Ingleson*
School of Chemistry, University of Manchester, Manchester, M13
9PL, United Kingdom,
michael.ingleson@manchester .ac.uk:
Supporting information for this article is given via a link at the end
of the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201803180
Angewandte Chemie International Edition
COMMUNICATION
formation of 5b was complicated by the 6-endo-dig cyclisation to
form the 9-borylated phenanthrene (analogous to C) which is
competitive in this case, with a 1:1 ratio of products formed (see
SI). For 4a, the 6-endo-dig cyclisation presumably is disfavoured
because of the greater steric bulk around the alkyne giving rise
exclusively to 5a (while the 6-endo-dig cyclisation is not possible
with 4c). The 1H NMR spectrum of 6a has a characteristic broad
singlet at δ = 5.75 ppm for the B-OH, which is at 5.97 ppm for 6b
and 6.11 ppm for 6c. The 11B NMR resonances for 6a - 6c are
typical for boracyclic borinic acids (δ11B = 37-38 ppm).
bromide. Lastly, an intramolecular SEAr of the proximal
naphthalene moiety can occur (step E) to form the six
membered boracycle of the 1,2-dibromo-1-bora-phenalenes 5a-
c. A related trapping of a vinyl cation by a proximal naphthalene
during
the
borylative
cyclisation
of
1,2-bis(1-
naphthylalkynyl)benzene with B(C6F5)3 has been reported.16
Scheme 3 Proposed mechanism for the formation of 6a-c.
Scheme 2. 1-Bora-phenalene formation. [a] heated to 140 °C for 4a; 120 °C
for 4b; and 70 °C for 4c. [b] Isolated yield. [c] In-situ conversion.
The proposed mechanism has an aryl group to stabilise
the vinyl cation formed post step A. Replacement of this aryl with
an alkyl or hydrogen would disfavour the formation of this vinyl
cation. Therefore we anticipated that reaction of the terminal
alkyne, 1-ethynylnaphthalene, 7a, with > 1 eq. BBr3 would
instead result in trans-haloboration17 to form 8 (Scheme 4) which
positions a vinylBBr2 group for intramolecular SEAr (akin to step
E, Scheme 3) to form 1,3-dibromo-1-bora-phenalene (9). Thus
7a and excess BBr3 were combined and NMR spectroscopy
indicated the quantitative formation of the haloborated product 8
within minutes of BBr3 addition (δ11B = 49.9 ppm). In solution, 8
slowly transforms to 9 over 48 h at 20oC. Compound 9 forms
quantitatively (by in-situ NMR spectroscopy), and crystallises
from the o-DCB solvent during the reaction. The solid state
structure of 9 (scheme 4, bottom right) has positional disorder of
B1 and C2, and a mirror plane along the C1-C4-C5 axis,
precluding detailed discussion of any metrics.
Notably, on exposure to wet solvent the borinic acid
derived from 9 is not observed, instead protodeboronation
occurs. This can be used to transform 9 into 10a by addition of
Hünigs base/pinacol (Scheme 4, top right), with 10a forming via
protodeboronation and then an E2 elimination from the
haloalkene. Comparable reactivity was observed for 1-(pent-1-
yn-1-yl)naphthalene (7b) to furnish 10b. The identification of 10a
was confirmed by X-ray diffraction studies which revealed
distorted C1-C2-C3 angles (174.1(3)°), C2-C3-CtA (where CtA =
centroid of ring A, 175.60(17)°) and B1-C11-CtB (168.33(16)°).
10a/b can be synthesised directly from 7a/b with no isolation of
intermediates, and are the first reported 8-borylated-1-alkynyl
naphthalenes to the best of our knowledge. To confirm the
formation of 9 proceeds via 8, a solution of pinacol was added
after 20 minutes to the mixture derived from 7a/BBr3 to form 8-
BPin. This led to quantitative conversion to 8-BPin but it was
Single crystal X-ray diffraction studies on 6a and 6c
(Figure 2) revealed a trigonal planar geometry around boron,
effectively orthogonal exocyclic aryls and planar 1-bora-
phenalene units (max. deviation from the C12B mean plane 0.08
Å). Key metrics include short C11-C12 distances (1.350(4) Å for
6a and 1.348-1.356 Å for 6c, there are three molecules in the
asymmetric unit (asu) for 6c) and much longer B-C1 bonds
(1.540(4) Å for 6a and 1.533 -1.550 Å for 6c) and C8-C12 single
bonds (1.478(4) and 1.481-1.485 Å). The short B-O distances
(6a = 1.362(4) and 6c = 1.351-1.369 Å) indicate π donation from
the hydroxyl to boron. These distances suggest minimal
endocyclic π delocalisation in the boracycle (ring C), in contrast
the isoelectronic phenalenyl cations have a much smaller C-C
t
bond distance range (1.392 - 1.416 Å for the Bu3 substituted
phenalenyl cation),3a indicating significant
throughout all rings in the all carbon analogues.
π
delocalisation
Figure 2 Molecular structures of 6a and 6c with ellipsoids at 50% probability,
hydrogen atoms (except B-OH) have been omitted for clarity.
The differing reactivity observed for 4a-c compared with 2-
(phenylethynyl)-1,1'-biphenyl (which forms A) presumably arises
because the naphthyl moiety can intercept the vinyl carbocation
in a 5-endo-dig cyclisation (step B, Scheme 3). A plausible
mechanism involving tautomerisation and B-C bond cleavage
can be proposed (step C) followed by a 1,2- migration of
isolated in only 30
% yield by crystallisation (8-BPin
decomposes under basic conditions via protodeboronation/E2
elimination to furnish 7a and was unstable on silica).
1-Bora-phenalene derivatives that are bench stable and
contain exocyclic boron-substituents that do not π donate to
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10.1002/anie.201803180
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COMMUNICATION
1) BBr3,[a] oDCB, 48 h
B
O
O
2) Pinacol[b]
X-Ray
10a
R
DCM, NEtiPr2, 1 h
R=H=10a (60%)[c]
R=nPr=10b (40%)[c]
,
+ Pinacol/
NEtiPr2
B
A
- HBr
+ BBr3
C
BBr2
B
Br
Br
Br
9 (61%)
R = H = 7a
R = nPr = 7b
H
8
R
+ Pinacol
X-Ray 8-Bpin
BPin
H
Br
8-BPin
X-Ray 9
Scheme 4: The formation of compounds 8, 9 and 10x. [a] 1.2 equivalents
BBr3 for 8a. [b] 1.4 equivalents for 8a. [c] Isolated yield. Inset molecular
structures of 8-BPin, 9 and 10a with ellipsoids depicted at 50% probability.
Hydrogen atoms have been omitted for clarity for 8-BPin and 10a.
Scheme 5. Functionalisation of 5c and 6c, top right solid state structure of 12
(hydrogens omitted for clarity) and middle right the LUMO for 14 (isovalue =
0.05, the LUMO for 11, 12 and 13 are closely comparable). Bottom, the
formation of 15 from 9 and the LUMO of 9 and 15, (isovalue = 0.05).
boron were next targeted. 6c reacts with MesMgBr to form two
species in a ca 9:1 ratio (Scheme 5, top). The major product (11)
is functionalised at carbon, leaving the borinic acid moiety intact,
as indicated by a resonance at 6.04 ppm in the 1H NMR
spectrum for the B-OH. The minor product (12) is functionalised
at boron, leaving the vinyl bromide group intact. In contrast,
functionalisation of the bromo congener 5c with MesMgBr
results predominantly in the formation of 12 along with minor
unidentified species, with the formation of 11 not observed. 12 is
bench stable and can be isolated in 55% yield with a δ11B of 58.0
ppm. Increasing the ratio of MesMgBr : 5c/6c to more than 10 :1
did not result in any significant double arylation of either
compound at 20oC or at raised temperatures. Furthermore, the
reaction of 12 in a sealed tube with excess MesMgBr at 100 °C
resulted in the formation of the di-arylated compound 13 as only
the minor product with 14 the major product (Scheme 5, middle).
The formation of 14 presumably occurs by Grignard metathesis
generating the Grignard reagent derived from 12 which upon
aqueous work up is hydrolysed to 14. As observed for 5c, 9 can
be readily functionalised at boron, but in this case superior yields
were obtained used ZnMes2, which afforded bench stable 15 in
88% yield. Despite repeated attempts, only compound 12 was
amenable to crystallisation in our hands. The solid state
structure of 12 revealed a planar 1-bora-phenalene unit and
effectively orthogonal aryl groups. The bond metrics in the
boracycle were closely comparable to those found in 6a and 6c,
including a short C11-C12 distance of 1.360(4) Å.
orthogonal in each compound. Most notably, the LUMO for each
compound is effectively identical (see Figure S7), being located
on the C12B core and being predominantly non-bonding in nature,
with zero orbital coefficients on exocyclic groups in contrast the
LUMO of isoelectronic 3. Thus the replacement of {C-H}+ in 1+
for {B-R} has minimal effect on the nature of this frontier orbital.
This is notable as the non-bonding character of this frontier
orbital is crucial for the unique redox-properties of phenalenyls.1
In contrast to the LUMO, the occupied π orbitals are distinct for
the 1-bora-phenalenes compared to 1+. For 5c and 9 the HOMO
is principally located on rings A and C (scheme 5 and Figure S7)
with some contribution from the exocyclic bromines. For the B-
Mes substituted compounds the HOMO and HOMO-1 are both
located on the mesityl group, (see Fig. S7), but the occupied π
orbitals on the C12B core are also more localized than in in 1+
where the highest energy occupied π orbitals are delocalized
throughout the phenalenyl C13 core (see Figure S6).
Nucleus-independent chemical shifts (NICS) were
determined for the reported compounds, the perprotio 1-bora-
phenalene (C12BH9) and the isoelectronic analogue 1+ (see
Table S3). For all the 1-bora-phenalenes the boracycle (ring C)
is effectively non-aromatic (NICS(1) values between -0.3 and -
1.6) while the naphthyl unit has significant aromaticity (NICS(1)
values for rings A and B are between -10.4 and -9.6). This is
distinct to the more symmetric aromatic structure of D3h 1+
(NICS(1) = -7.8) and to 3 (which is dominated by a single highly
aromatic Clar sextet). Therefore while the LUMO of the 1-bora-
phenalenes and 1+ are closely comparable in character, the
overall π electronic structures are different due to the lower
symmetry on incorporating boron and the higher energy of the B
pπ orbital relative to the C pπ orbitals (see Fig. S6-7). To estimate
Compounds 5c, 9 and 12-15 were calculated at the
B3LYP/6-311G(d,p) level. All six calculated structures have C=C
distances for the C1-C2 unit (numbering as per Scheme 4,
bottom right) between 1.35 - 1.37 Å comparable to those found
in the solid state structures of 6a, 6c and 12. The C12B cores are
planar in each case with the Mes moieties oriented effectively
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10.1002/anie.201803180
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COMMUNICATION
the effect of this on the aromatic stabilization of the C5B ring in
1-bora-phenalenes relative to isoelectronic carbocations the
isomerization method was used (calculations at the B3LYP/6-
311G(d,p) level, Scheme 6, eq. 1 and 2).19 This revealed that
this 1-bora-phenalene has a much lower aromatic stabilization
energy than the isoelectronic carbocation congener. The lower
aromatic stabilization energy for the 1-boraphenalenes was
supported by an isodesmic reaction (eq. 3),20 which confirmed
the greater aromatic stability of phenalenyl cations (> 12 kcal
mol-1). While it has been demonstrated numerous times that the
LUMOs of B doped PAHs and carbocation analogues are often
similar in nature,6 to fully understand the properties of these
isoelectronic pairs consideration of the occupied π orbitals is
also essential.
lower aromatic stabilization of the C5B ring than observed in
each ring in the D3h phenalenyl cations due to the less
delocalized nature of the occupied orbitals of π symmetry in the
1-bora-phenalenes. For the B-Mes containing bora-phenalenes
a reversible reduction wave is observed well separated from the
second reduction process, indicating that the 13 π electron
radical anion, analogous to the phenalenyl radical is accessible.
Further
studies
into
generating
1-bora-phenalenenes,
particularly examples enabling access to isolable 13 π electron
radical anions, are currently ongoing.
Acknowledgements
The research leading to these results has received funding from
the European Research Council under framework 7 (Grant
number 305868) and the Horizon 2020 Research and Innovation
Program (Grant no. 769599), the Leverhulme Trust (RPG-2014-
340), the University of Manchester, and the Royal Society. We
also acknowledge the EPSRC (grant number EP/K039547/1) for
financial support. Additional research data supporting this
publication are available as supplementary information
accompanying this publication.
Scheme 6. Electronic energies (kcal mol-1) for a range of isodesmic reactions.
Conflict of Interest
With an understanding of the electronic structure of 12-15
in hand the propensity of these to undergo redox was
investigated. The first reduction wave is reversible (Table 1 and
Figures S2-S5) and its potential mirrors the trend observed
computationally with 12 and 15 containing one inductively
The authors declare no conflict of interest.
Keywords: boron, borylation, aromaticity, phenalenyl, polycyclic
aromatic hydrocarbons
withdrawing bromine substituent having
a less negative
reduction potential than 13 and 14. For 12-14 the second
reduction event is significantly more negative than the first. This
separation and the reversible nature of the first reduction wave
indicates that 13 π electron radical anions should be accessible.
However, attempts to date to chemically reduce 13 and 14 have
led to either complex diamagnetic mixtures (with 14) or NMR
silent product(s) that have frustrated isolation (with 13).
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First Reduction
E1/2 (V)
-1.75
Second
Reduction (V)a
-2.71
Epeak (V)
-1.84
LUMO (eV)
-3.11
12
13
14
15
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-1.89
-2.97
-2.72
-1.95
-1.89
-2.98
-2.68
-1.85
-1.74
-3.13
-2.11
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possessing good bench stability. Notably, the nature of the
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in the extensively studied all carbon phenalenyl cation
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Angewandte Chemie International Edition
COMMUNICATION
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10.1002/anie.201803180
Angewandte Chemie International Edition
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Entry for the Table of Contents
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R. J. Kahan, D. L. Crossley, J. Cid, J. E.
Radcliffe, M. J. Ingleson*
Page No. – Page No.
Synthesis, Characterization and
Functionalization of 1-Bora-
phenalenes.
Naphthyl-alkynes react with BBr3 to form the first boron only doped phenalenes. All
the studied 1-bora-phenalenes have LUMOs localized on the C12B core that are
comparable to the isoelectronic phenalenyl cations.
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