Sequential C?H Borylation and N-Demethylation of 1,1′-Biphenylamines: Alternative Route to Polycyclic BN-Heteroarenes

Article abstract of DOI:10.1002/anie.201902499

Described herein is an unprecedented access to BN-polyaromatic compounds from 1,1′-biphenylamines by sequential borane-mediated C(sp²)?H borylation and intramolecular N-demethylation. The conveniently in situ generated Piers’ borane from a borinic acid reacts with a series of N,N-dimethyl-1,1′-biphenyl-2-amines in the presence of PhSiH₃ to afford six-membered amine-borane adducts bearing a C(sp²)?B bond at the C2′-position. These species undergo an intramolecular N-demethylation with a B(C₆F₅)₃ catalyst to provide BN-isosteres of polyaromatics. According to computational studies, a stepwise ionic pathway is suggested. Photophysical characters of the resultant BN-heteroarenes shown them to be distinctive from those of all-carbon analogues.

Full text of DOI:10.1002/anie.201902499

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Sequential C−H Borylation and N-Demethylation of 1,1’-
Biphenylamines: An Alternative Route to Polycyclic BN-
Heteroarenes
Authors: Sukbok Chang, Jianbo Zhang, Hoimin Jung, Dong-Wook Kim,
and Sehoon Park
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902499
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10.1002/anie.201902499
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Sequential CH Borylation and N-Demethylation of 1,1’-
Biphenylamines: An Alternative Route to Polycyclic BN-
Heteroarenes
Jianbo Zhang, Hoimin Jung, Dong-Wook Kim, Sehoon Park,* and Sukbok Chang*
Dedicated to Professor Chul-Ho Jun for his honorable retirement
Abstract: Described herein is an unprecedented access to BN-
polyaromatic compounds from 1,1’-biphenylamines via borane-
mediated sequential C(sp2)H borylation and intramolecular N-
demethylation. The conveniently in situ generated Piers’ borane from
a borinic acid reacts with a series of N,N-dimethyl-1,1'-biphenyl-2-
amines in the presence of PhSiH3 to afford six-membered amine-
borane adducts bearing a sp2 CB bond at the C2’-position. These
species undergo an intramolecular N-demethylation by B(C6F5)3
catalyst to provide BN-isosteres of polyaromatics. According to
computational study,
a stepwise ionic pathway is suggested.
Photophysical characters of the resultant BN-heteroarenes were
shown to be distinctive from those of all-carbon analogues.
Polyaromatic hydrocarbons (PAHs) have drawn great interests in
materials chemistry as
a
key component of organic
semiconductors for applications in electronic devices thanks to
their unique photophysical and electronic properties being
susceptible to molecular structure.[1] In this regard, considerable
efforts to substitute the carbon atoms with heteroatoms in the
polyaromatic compounds have been made for the invention of
new organic materials as isosteres of the parent PAHs with
distinctive chemical and electronic properties.[2] Among various
isosteres, substitution of a C=C unit with an isoelectronic BN
moiety is one of the most widely studied approaches.[3] Since the
Scheme 1. (a) Examples of polycyclic BN-heteroarenes and precedent
synthetic routes. (b) Frustrated Lewis pair-mediated sp2 CH borylation. (c)
Unconventional route to polycyclic BN-heteroarenes (this work).
Dewar’s pioneering work on BN-heteroarene chemistry,[4a]
a
number of research groups including Piers, Ashe, Liu, and
Perepichka have expanded the scope by developing new
synthetic methods with their applications [Scheme 1a(i)].[4b-p]
However, numerous BN-polyaromatics are still synthesized with
the conventional multistep procedures. One general strategy
leading to the sp2 CB bond in these methods is the Friedel-
Crafts-type cyclization although it often requires the use of
corrosive and/or toxic stoichiometric reagents (e.g. BCl3, RLi, or
RSnBu3; R = alkyl) [Scheme 1a(ii)].[3d,4a-b] From a synthetic point
of view, developing a benign approach for the construction of BN-
aromatic frameworks is highly desirable, but remains still
challenging.
On the other hand, it has been demonstrated that N,B-based
frustrated Lewis pairs (FLPs) can cooperatively mediate CH
bond activation via a transition state to selectively form a sp2 CB
bond in an intermolecular fashion (Scheme 1b).[5] In this
concerted pathway, N atom serves as a Lewis base for
deprotonation of a CH bond of substrates, while the Lewis acidic
boron is selectively connected to the deprotonated carbon center
to give a zwitterion intermediate which is eventually converted to
the corresponding borylated amines upon the release of H2. While
an alternative SEAr pathway can also be considered, it requires
relatively higher activation energy according to the literature.[5f]
Inspired by these precedence, we conceived a two-step
strategy to six-membered BN unit-containing polyarenes: (i)
C(sp2)H borylation of 1,1’-biphenylamines with Piers’ borane,[6]
followed by dearylation with silane[7] to furnish a biphenyl amine-
boron adduct, and (ii) Lewis acid-promoted intramolecular N-
dealkylation leading to BN-heteroarenes (Scheme 1c).[4,8]
Described herein is an unprecedented synthetic route to a range
of polycyclic BN-heteroarenes from biaryl amines. The first step
of CH borylation shows broad scope and good scalability, while
the subsequent N-demethylation is enabled via the catalysis of a
Lewis acidic borane.
[*]
Dr. J. Zhang, H. Jung, Dr. D.-W. Kim, Dr. S. Park, Prof. Dr. S. Chang
Center for Catalytic Hydrocarbon Functionalizations
Institute for Basic Science (IBS)
and
Department of Chemistry
Korea Advanced Institute of Science & Technology (KAIST)
Daejeon 34141, Republic of Korea
E-mail: sehoonp@kaist.ac.kr; sbchang@kaist.ac.kr
Supporting information for this article is given via a link at the end of
the document.
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Table 1: Scope of amino groups of 1,1’-biphenyl substrates.[a]
present CH borylation were subsequently explored (Table 2).
1,1’-Biphenylamine and its derivatives bearing substituents at the
C4’-position (1a1i) readily reacted with Piers’ borane to provide
a series of 2’-mono-boryl-1,1’-biphenylamines in 63~97% yields
(2a2i). Noteworthy is that the present procedure is also operative
in a (semi)-gram scale in good to high yields (2a and 2e). In
addition, phenoxy (2f) and halide groups (2g2i) were compatible
with the present conditions. Interestingly, electron-donating
Table 2: Substrate scope of C(sp2)H borylation with Piers’ borane.[a]
[a] Reaction conditions: 1,1’-biphenyl substrates (0.2 mmol), (C6F5)2BOH (1.2
equiv), and PhSiH3 (4.8 equiv) in CH2Cl2 (0.8 mL) at 60 oC for 24 h. Isolated
yields. [b] ORTEP with thermal ellipsoids set at 50% probability level. The
selected bond lengths of BN are 1.634 Å (entry 2), 1.650 Å (entry 4, 2a), and
the BP is 1.972 Å (entry 7). [c] Piers' borane was pre-generated in situ prior to
addition of 1a. [r.s.m.] = recovery of starting materials.
We commenced the current study by performing
stoichiometric reactions of (C6F5)2BOH with 1,1’-biphenylamines
bearing variable N-moieties in the presence of phenylsilane
(Table 1). The reaction of 1,1’-biphenyl substrates possessing a
primary (1-NH2) or secondary amine group (1-NHMe) with in situ
generated (C6F5)2BH gave the corresponding adducts in 48% and
30%, respectively (entries 12), while a bulkier secondary amine
moiety was not effective (entry 3). Solid structure of the latter
adduct was obtained to show the bond length of BN being 1.634
Å.[5e-f] Pleasingly, the desired CH borylation occurred efficiently
when N,N-dimethyl-1,1’-biphenylamine (1a) was subjected to the
above conditions to give 2a in 77% (entry 4). A boryl moiety in 2a
is presumed to form by facile metathesis between the
presupposed ArB(C6F5)2 (Ar = 1,1’-biphenyl) and PhSiH3 with
liberation of C6F5[Si] (see the S.I.).[7] A reaction of 1a with pre-
generated (C6F5)2BH indeed afforded 2a in 64% (entry 5).
However, replacement of the methyl group by ethyl (1-NEt2) gave
only trace amounts of borylated product (entry 6). Moreover, 1-
PPh2 bearing a diphenylphosphino moiety was converted to a
phosphonium borohydride zwitterion in 71% (entry 7). The
observation that the CH borylation was highly sensitive to the N-
substituents of substrates was also disclosed by Fontaine, where
the efficiency was found to be dependent on the identity of amino
groups of the employed reagents.[5d]
[a] Reaction conditions: 1,1’-biphenylamines (1) (0.2 mmol), (C6F5)2BOH (1.2
equiv), and PhSiH3 (4.8 equiv) in CH2Cl2 (0.8 mL) at 60 oC for 24 h. Yields and
regioisomeric ratio of isolated products 2. [b] For 48 h.
With the insights obtained from the design of an effective
molecular skeleton (Table 1, entry 4), scope and scalability of the
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substituents at the C4’ position led to higher yields when
compared to the electron-withdrawing groups (e.g. 2f vs. 2h).
Reactions of C3’-substituted biphenyls brought out steric-
shows a covalent BN bond with a distance of ca. 1.40 Å, which
is in the range of bond lengths in analogous BN-heteroarenes.[4d-
e,16]
Delightfully, a range of amine-borane adducts 2 were
dependent regioselectivities: while
a
reaction of 3’-fluoro
converted to the corresponding BN-heteroaromatics in 34~88%
isolated yields (3a, 3e, 3i, and 3q3t). The N-demethylation
process was quite sensitive to the conformational variation in
substrates, often requiring increased catalyst loadings to obtain
satisfactory product yields (e.g. 3i, 3s3t). Solid structures of 3a,
3q, and 3s revealed the BN bond distances ranging from 1.402
to 1.408 Å along with the presence of dihedral angels (8.4o, 5.1o,
and 16.8o, respectively) between two aryl rings neighboring the
BN-aromatic unit. It is noteworthy that these BN bond lengths
are shorter than that (1.464 Å) of a previously reported B2N2-
triphenylene,[4d] and such shorter BN bonds in 3 can be attributed
to the presence of an electron-withdrawing C6F5 group on B atom.
substrate gave low regioselectivity (1:3), bulkier substituents
induced the formation of CB bond highly selectively at the
sterically more accessible carbon center (2k2l).[5a-b] The present
procedure displayed high fidelity with variation at the C5-position
of biphenylamine substrates (2o2p). Moreover, 3- or 1-
phenylnaphthalen-2-amines and 2-naphthylaniline were all found
to be facile substrates to give the corresponding CH borylated
products in 64~79% on 1~2 mmol scale (2q2t). Some
compounds were not fruitful for the current CH borylation mainly
due to the faster decomposition of Piers’ borane or
dearomatization of substrates (see the S.I.).
Next, we turned our attention to an unprecedented
intramolecular N-demethylation of 2. To estimate its feasibility,
DFT calculations were conducted on the basis of two mechanistic
scenarios (thermal[9] vs. ionic paths[10]) using 2a as a model
substrate (Scheme 2). Our computational study suggested that a
thermal N-demethylation requires an extremely demanding
Table 3: Substrate scope of the B(C6F5)3-catalyzed N-demethylation.[a]
‡
energy barrier (GA ~60 kcal/mol), while a stepwise ionic path
involving a B(C6F5)3-mediated BH bond activation followed by
‡
hydride transfer proceeds with ~38 kcal/mol (GC ).[11] It is
interesting to note that the activation of BH bond of diarylborane
was unfolded by the cooperative action of Lewis acidic B(C6F5)3
and an amino group to eventually form an ammonium borate
‡
intermediate (Int-1) via TSB (GB = ~28 kcal/mol).[12] Under this
ionic pathway, the nucleophilic attack at the N-methyl group of Int-
1 by borohydride is predicted to enable the sp3 CN bond
cleavage via TSC. Although a considerable energy barrier (~38
kcal/mol) was estimated by DFT calculations, we envisaged that
the desired conversion will occur with catalytic amount of B(C6F5)3
at elevated temperatures.
[a] Reaction conditions: B(C6F5)3 (5 mol%) and 2 (0.5 mmol) in 1,2-C6H4Cl2 at
180 oC for 24 h. Isolated yields. [b] 20 Mol% of B(C6F5)3 employed.
Scheme 2. Plausible reaction pathways for the N-demethylation based on DFT
calculations.
The current two-step procedure also proved to be applicable
for the synthesis of bis-BN-polyaromatics: N-methylated
[1,1':4',1''-terphenyl]-2,2''-diamine (1u) underwent a double CH
borylation to form white precipitate 2u (not shown) in CH2Cl2
solvent, which was then converted to the corresponding bis-BN-
heteroarene 3u via boron catalysis in 25% overall yield. The solid
Indeed, the B(C6F5)3-catalyzed N-demethylation of 2a took
place at 180 oC to produce the desired BN-heteroarene 3a in 88%
yield in 24 h (Table 3).[13] To our best knowledge, this reaction
represents the first example of borane-catalyzed N-
dealkylation.[14,15] An X-ray crystallographic structure of 3a clearly
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Chem. Commun. 2015, 51, 6257; c) M. Stępień, E. Gońka, M. Żyła, N.
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structure of 3u was unambiguously confirmed by X-ray
crystallography, which shows a trans orientation with dihedral
angels of 6.2o and 8.7o between aryl rings adjacent to BN-
aromatic units. Interstingly, intermolecular - interactions
(closest contact ca. 3.64 Å) were also shown in the solid sturcture
of 3u (see the S.I.).[17]
[3]
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Photophysical properties of obtained BN-heteroarenes were
briefly examined. Aminoborane product 3a displayed absorbed
wavelength at abs = 331 nm which is somewhat red-shifted
relative to that of phenanthrene (abs = 293 nm). An absorption
band of 3a was almost same as those (abs = 326~329 nm) of its
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BN-PAHs (3q, 3s, and 3u) exhibited lower-energy absorption
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at 338 nm, which is similar to that of phenanthrene (em = 347
nm).[4e] The fluorescence emissions of tetracyclic BN-
heteroarenes 3q and 3s (em = 372 and 383 nm, respectively)
were bathochromic shifted (25~45 nm) relative to those of tricyclic
phenanthrene and 3a. Interestingly, a small Stokes shift was
observed for 3q (2 nm), whereas its isomer 3s had a relatively
larger Stokes shift (16 nm).[4e,18] The emission of a pentacyclic bis-
BN-product 3u (em = 386 nm) was red-shifted by 14 nm relative
to that of 3q, but Stokes shift was small (7 nm).
In summary, we have developed the metal-free, borane-
mediated sequential CH borylation of biaryl amines and N-
demethylation of borylated adduct intermediates to provide a
diverse range of new B,N-heteroaromatic compounds. While the
present two-step procedure liberates only gaseous byproducts
(H2 and CH4), the second step of N-demethylation is postulated to
proceed via a B(C6F5)3-mediated outer-sphere ionic pathway. The
observed photophysical properties of the resultant B,N-
polyaromatics are distinctive from those of all-carbon analogues.
This work offers a new synthetic avenue to access BN-
polyaromatic hydrocarbons.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: Piers' borane • C–H Borylation • N-Dealkylation •
BN-heteroarenes • Isosterism
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[18] F. W. D. Rost, In Fluorescence Microscopy; Cambridge University Press,
Cambridge and New York, 1995.
[13] The N-demethylation reaction of 2a at 130~150 oC gave rise to inferior
yields of 3a (2~12%) under otherwise similar conditions (see the S.I.).
[14] Being consistent with DFT-based insights, no reaction occurred either in
the absence of B(C6F5)3 or in the presence of less acidic BPh3 as a
catalyst (see the S.I.).
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10.1002/anie.201902499
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
Jianbo Zhang, Hoimin Jung, Dong-Wook
Kim, Sehoon Park,* and Sukbok Chang*
Page No. – Page No.
Sequential CH Borylation and N-
Demethylation of 1,1’-
Biphenylamines: An Alternative
Route to Polycyclic BN-Heteroarenes
A
two-step process of borane-mediated sequential CH borylation and N-demethylation of
biphenylamines has been developed to provide a diverse range of new B,N-heteroarenes. The
unprecedented intramolecular N-demethylation of borane-amine adduct intermediates was enabled by
B(C6F5)3 catalyst albeit at high temperature. The present procedure is featured by transtion metal-free
conditions to liberate gaseous byproducts (H2 and CH4).
This article is protected by copyright. All rights reserved.