BCl3-Induced Annulative Oxo- and Thioboration for the Formation of C3-Borylated Benzofurans and Benzothiophenes

Article abstract of DOI:10.1002/anie.201610014

BCl₃-induced borylative cyclization of aryl-alkynes possessing ortho-EMe (E=S, O) groups represents a simple, metal-free method for the formation of C3-borylated benzothiophenes and benzofurans. The dichloro(heteroaryl)borane primary products can be protected to form synthetically ubiquitous pinacol boronate esters or used in situ in Suzuki–Miyaura cross couplings to generate 2,3-disubstituted heteroarenes from simple alkyne precursors in one pot. In a number of cases alkyne trans-haloboration occurs alongside, or instead of, borylative cyclization and the factors controlling the reaction outcome are determined.

Full text of DOI:10.1002/anie.201610014

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International Edition: DOI: 10.1002/anie.201610014
German Edition: DOI: 10.1002/ange.201610014
Synthetic Methods
BCl3-Induced Annulative Oxo- and Thioboration for the Formation of
C3-Borylated Benzofurans and Benzothiophenes
Andrew J. Warner, Anna Churn, John S. McGough, and Michael J. Ingleson*
Abstract: BCl3-induced borylative cyclization of aryl-alkynes
possessing ortho-EMe (E = S, O) groups represents a simple,
metal-free method for the formation of C3-borylated benzo-
thiophenes and benzofurans. The dichloro(heteroaryl)borane
primary products can be protected to form synthetically
ubiquitous pinacol boronate esters or used in situ in Suzuki–
Miyaura cross couplings to generate 2,3-disubstituted hetero-
arenes from simple alkyne precursors in one pot. In a number
of cases alkyne trans-haloboration occurs alongside, or instead
of, borylative cyclization and the factors controlling the
reaction outcome are determined.
Whilst B(C6F5)3 was crucial in developing metal-free
alkyne borylative cyclization it leads to zwitterionic products
such as A (Scheme 1). The use of these species in subsequent
functional group transformations is not established, currently
limiting their synthetic utility.[10] Using alternative boron
Lewis acids such as BCl3 to effect borylative cyclization
enables the formation of organo-boronic acid derivatives on
work-up,[11] and consequentially access to the myriad of
already proven transformations. However, this is an under-
developed approach with demonstrated, modular protocols
scarce. Two notable exceptions are 1) the BCl3-induced
alkyne borylative cyclization where
a (hetero)aromatic
B
enzofurans and benzothiophenes are important structures
moiety is the nucleophile attacking the BCl3-activated
alkyne (Scheme 2, top left),[12] and 2) the use of B-chloro-
catecholborane to produce borylated lactones via cyclitive
alkyne oxoboration (Scheme 2, bottom left).[13] Both proto-
cols generate desirable boronic acid derivatives on (trans)es-
terification, and are complementary to electrophilic iodina-
tive cyclization (which generates organic electrophiles).[14]
found in pharmaceutical targets (e.g., desketoraloxifene) and
organic materials.[1,2] The boronic acid derivatives of these
heteroaromatics are desirable as they are bench-stable, have
low toxicity and are effective in many functional group
transformations, including the ubiquitous Suzuki–Miyaura
cross coupling reaction.[3] Typically, the formation of these
ꢀ
ꢀ
borylated compounds is achieved via the C H or C X
borylation of the pre-formed heteroaromatic.[3b,4] An alter-
native more efficient approach is to form the heteroaromatic
ꢀ
scaffold and the C B bond in one pot via the borylative
cyclization of alkynes. This can be mediated by transition
metal catalysts[5] or in the absence of a metal catalyst by using
strong boron electrophiles.[6] The latter approach was pio-
neered using B(C6F5)3 which on addition to appropriately
substituted alkynes led to a range of borylated heterocycles,
including products derived from aminoboration[7] and oxobo-
ration (Scheme 1).[8] Other catalyst-free cyclitive elemento-
borations have been reported, albeit to a lesser extent,[5,6] with
reports of cyclitive thioboration particularly rare.[9]
Scheme 2. Previous relevant borylative cyclization reactions and this
work.
From these studies key requirements enabling borylative
cyclization without metal catalysts can be identified, including
that the boron electrophile must: a) bind reversibly to the
heteroatomic moiety, and b) induce borylative cyclization
preferentially to dealkylation reactions (e.g., cyclization
Scheme 1. Borylative cyclization of substituted alkynes with B(C6F5)3.
ꢀ
occurs prior to O R cleavage). Guided by these herein we
report our studies into the reaction of BCl3 with 2-alkynyl-
anilines, anisoles and thioanisoles, which led to the develop-
ment of a simple new route to important boronic acid
derivatives of benzothiophenes and benzofurans. This route
(Scheme 1, bottom right) is catalyst-free and thus distinct to
a recent cyclitive alkyne oxo-boration report which required
Au catalysts (Scheme 1, top right).[15]
[*] A. J. Warner, A. Churn, J. S. McGough, Dr. M. J. Ingleson
School of Chemistry, University of Manchester
Oxford Road, Manchester, M13 9PL (UK)
E-mail: Michael.ingleson@manchester.ac.uk
Our studies commenced by combining equimolar BCl3
and N,N-dimethyl-2-(phenylethynyl)aniline (1) for compar-
ison with B(C6F5)3 which formed zwitterion A.[7] In contrast to
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610014.
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Figure 1. Trans-haloboration of 1 with BCl3. Top right, solid state
structure of 2, thermal ellipsoids at 50% probability and hydrogens
omitted for clarity. Bottom, a previously reported alkyne trans-halobora-
tion.
Scheme 3. BCl3-induced borylative cyclization of 2-alkynyl-anisoles.
Bottom right, solid state structure of 4g, thermal ellipsoids at 50%
probability and hydrogens omitted for clarity. [a] 12 h. [b] 6 mmol scale
to produce 1.16 g of 4g. [c] Using non-purified solvents under ambient
atmosphere.
B(C6F5)3 addition of BCl3 did not lead to a borylated indole
with X-ray diffraction studies revealing it had instead formed
2 (Figure 1), the product from alkyne trans-haloboration. The
reactivity disparity between BCl3 and B(C6F5)3 is attributed to
stronger N!B coordination with BCl3 due to the lower steric
crowding around boron. Notably, 2 is not the expected
product from the direct haloboration of an alkyne with BCl3,
(< 5 min at 208C), as indicated by the consumption of 3a
along with the generation of CH3Cl (d1H 3.02 ppm) and a new
major resonance centered at 51 ppm in the 11B NMR spec-
trum, consistent with a heteroaryl-BCl2 species. A minor
broad resonance at 14.2 ppm in the 11B NMR spectrum was
also observed. Esterification with pinacol/NEt3 enabled the
isolation of 4a in 56% yield without column chromatography.
No intermediates are observed so detailed discussion of the
mechanism is not warranted, although alkyne activation by
BCl3 and cyclization presumably occurs prior to demethyla-
tion based on the slow ether cleavage observed on combining
anisole and BCl3. It is noteworthy that a non-linked analogue
of B, 1,2-bis(2-methoxyphenyl)ethyne, undergoes rapid trans-
haloboration and demethylation with both BCl3 and BBr3,
thus the reactivity disparity between 3a and B is not due to
the use of different boron trihalides.
Exploration of the substrate scope revealed that electron-
donating and -withdrawing groups on the anisole ring are
compatible in certain positions (4b–e). Furthermore, boryla-
tive cyclization is not limited to diarylalkynes with benzyl-
and methyl-substituted alkynes converted to the benzofurans
4 f and 4g in good yield, with the structure of 4g confirmed by
X-ray crystallography. 4g was also accessible on a gram scale
and using non-purified solvents under ambient conditions in
good yield. Whilst a phenyl group substituted with an
electron-withdrawing group para to the alkyne led to the
borylated benzofuran in moderate isolated yield (4h), when
ester and nitro groups were incorporated into the anisole ring
para to the alkyne this led to low conversions to the
benzofuran-BCl2 species (the d11B 51 ppm is the minor
component). Instead a d11B 15 ppm resonance was the major
product with 3i (Scheme 4), whilst for 3j (Scheme 4), where
a naphthyl group has been incorporated resulting in an
increase in the steric environment around the alkyne, the
major d11B resonance is centered at 14 ppm. With both these
substrates after the addition of BCl3 the 1H NMR spectra
which would proceed by syn-addition of Cl2B Cl,[16] suggest-
ꢀ
ing 2 is formed by a different mechanism. Precedence for
alkyne trans-haloboration is extremely limited, with com-
pound C (Figure 1), the trans-haloboration/demethylation
product from the addition of BBr3 to o-alkynyl-anisole B
a notable exception.[17] With direct haloboration precluded it
is possible that the reaction proceeds from the (N,N-Me2-
aniline)–BCl3 adduct by chloride transfer from boron to
carbon, related to that calculated for intramolecular alkyne
trans-hydroboration.[18]
With the formation of C3-borylated indoles disfavored
under these conditions due to trans-haloboration the propen-
sity of o-alkynyl anisoles to undergo borylative cyclization
was explored. The rapid formation of C from B clearly
indicates that trans-haloboration also is viable with o-alkynyl-
anisoles, however, this reaction was proposed to proceed via
initial ether demethylation then haloboration.[17] While ether
cleavage of anisoles with BBr3 is well documented, detailed
studies into the mechanism are rare,[19] but one recent report
ꢀ
calculated that PhO Me cleavage is a bimolecular process
[19a]
ꢀ
involving two Me(Ph)O BBr3 moieties.
Thus B may be
prearranged to undergo rapid ether cleavage and other o-
alkynyl anisoles may be less prone to ether cleavage,
particularly using BCl3 instead of BBr3. Consistent with this
the combination of equimolar anisole and BCl3 in DCM at
208C resulted in the formation of a single 11B resonance at
32 ppm with minimal ether cleavage observed even after 30 h
at 208C (only ca. 2.5% CH3Cl was formed by 1H NMR
spectroscopy). The 32 ppm 11B chemical shift is consistent
with an equilibrium between the Lewis adduct and free BCl3
and anisole. Thus anisole binding to BCl3 is reversible and
ether cleavage is not significant at 208C, suggesting that
alkyne borylative cyclization using BCl3 is viable.
1-Methoxy-2-(phenylethynyl)benzene (3a) was cyclized
in DCM using BCl3 (Scheme 3). The reaction was rapid
2
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Scheme 5. Double BCl3-induced borylative cyclization.
Scheme 4. Trans-haloboration with strong electron-withdrawing/bulky
groups.
using BCl3 (Scheme 5). 7 represents a versatile precursor to
2,3,6,7-tetraarylbenzo[1,2-b:4,5-b’]difurans which are of inter-
est as hole transport materials.[2] To the best of our knowledge
3,7-diborylated benzodifurans have not been previously
reported.
While the purified borylated benzofurans reported herein
are effective in Suzuki–Miyaura cross couplings (e.g., 4g with
4-bromo-toluene) to enhance the utility of this methodology
a one-pot borylative cyclization/Suzuki–Miyaura cross cou-
pling procedure was developed (Scheme 6). This does not
require isolation of the borylated benzofuran, instead the
benzofuran-BCl2 product is hydrolyzed in situ to the boronic
acid and then subjected to conventional Suzuki–Miyaura
cross coupling conditions. This one-pot procedure is a simple
and rapid way to generate 2,3-disubstituted benzofurans from
simple alkynyl precursors in good yield (72% isolated yield of
8).
revealed that minimal CH3Cl had formed (consistent with d11B
51 ppm being a minor resonance). Instead a singlet was
observed at 4.56 and 4.49 ppm, respectively from 3i and 3j,
more consistent with an intact ArylOMe unit coordinated to
a Lewis acid. Attempts to isolate the product derived from 3i
after esterification with Et3N/pinacol led to isolation of the
starting alkyne, presumably due to E2 elimination. The
naphthyl derivative 5 was formed as the major product post
1
esterification, with H, 13C{1H}, 11B NMR spectroscopy fully
consistent with haloboration, a formulation supported by
mass spectroscopy. Therefore to form borylated benzofurans
in acceptable isolated yields by BCl3-induced borylative
cyclization significant bulk around the alkyne and strong
EWG in the para position (to the alkyne) of the anisole
moiety have to be avoided.
With the substituent effects probed the functional group
tolerance of BCl3-induced borylative cyclization was further
explored using the “robustness screen” methodology;[20]
specifically, monitoring the cyclization of 3b in the presence
of various additives. This revealed that borylative cyclization
was not affected by additives containing nitro, vinyl or CF3
groups (in each case > 80% of the borylated benzofuran was
formed with the additive not consumed). However, benz-
aldehyde and acetone were not compatible, with the addition
of BCl3 to separate reactions containing these additives and
3b leading to additive consumption and significantly reduced
benzofuran formation. Other Lewis basic groups were
compatible with borylative cyclization provided that
> 2 equivalents of BCl3 was used, with the first equivalent
of BCl3 coordinating to the Lewis basic group (in each case
> 70% conversion to the borylated benzofuran was observed
in the presence of a tertiary amine, a tertiary amide, a pyridine
and a nitrile). Established routes to 3-borylated-2-organo-
benzofurans generally proceed from 3-halo-2-organo-benzo-
furans by metallation/quenching with B(OR)3, or by Pd-
catalyzed Miyaura borylation.[3b] Notably these routes are not
compatible with some of the functional groups tolerated by
BCl3-induced borylative cyclization (e.g., amide/nitrile groups
are generally incompatible with metallation). Furthermore,
Scheme 6. One pot borylative cyclization and Suzuki–Miyaura cross
coupling.
o-Alkynyl-thioanisoles and BCl3 were explored next to
assess if BCl3 induced borylative cyclization was possible via
alkyne thio-boration. Firstly, equimolar thioanisole and BCl3
were combined which led to a species with d11B 7.9 ppm,
ꢀ
indicating significant adduct formation, but importantly no S
Me cleavage was observed. Furthermore, previous work has
shown that thioanisole-(BHxCl3ꢀx) (x = 1 or 2) compounds are
effective hydroborating agents at 208C indicating that an
electrophilic borane is accessible from these Lewis adducts.[21]
Therefore BCl3 was added to methyl(2-(phenylethynyl)-
phenyl)sulfane (9a) in DCM with in situ 11B NMR spectros-
copy revealing one major product had formed with a broad
resonance centered at 4 ppm, which does not correspond to
ꢀ
this methodology is complementary to iridium-catalyzed C
a
3-BCl2-benzothiophene species (expected d11B ca.
H borylation which provides C2- or C7-borylated benzofur-
ans.[4] Finally, it worth emphasizing that 4a–h are formed at
ambient temperature without a catalyst using inexpensive
BCl3, in contrast the previous borylative cyclization route to
C3-borylated benzofurans required pre-installation of the
borane (using NaH/CatBCl), Au catalysis, raised temper-
atures and ꢁ 20 h.[15]
52 ppm).[22] This is consistent with no chloromethane being
observed in the 1H NMR spectrum. Methylsulfonium cations
are significantly weaker methylating agents (less prone to
Me+ transfer to nucleophiles) than methyloxonium cations,[23]
therefore we surmised that the major compound is the
zwitterion 10a analogous to A (Scheme 7). In our hands
crystalline material of 10 could not be isolated therefore
support for this assignment was provided by combining 9a
with BCl3 (to form 10a) and then adding Et3N as a stronger
nucleophile to induce demethylation. This led to formation of
Multiple borylative cyclizations also proceed with appro-
priately substituted diynes, with 6 converted to 7, a dibory-
lated diaryl-benzo[1,2-b:4,5-b’]difuran, in excellent yield
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groups on the anisole moiety para to the alkyne, or significant
steric bulk are absent. This methodology is a simple, scalable
and metal-free route to useful benzofuran and benzothio-
phene boronic acid derivatives, many of which would be
challenging to access by other established borylation method-
ologies.
Scheme 7. Borylative cyclization followed by demethylation/dehaloge-
nation to form benzothienyl-BCl2 and then subsequent esterification.
Acknowledgements
We acknowledge the ERC under framework 7 (Grant number
305868), and the Royal Society (M.J.I.) for financial support.
We also acknowledge the EPSRC (grant number EP/
K039547/1) for financial support. Additional research data
supporting this publication are available as Supporting
Information accompanying this publication.
[Et3NMe]+ (by 1H NMR spectroscopy) and a new major
broad 11B resonance at 6.3 ppm attributed to the product from
demethylation of 10a by Et3N. On addition of one equivalent
of AlCl3 this compound was then converted to a new major
species displaying a broad 11B resonance at 52.9 ppm fully
consistent with a benzothiophene-BCl2 compound.[22] The
same boron species is formed by initial addition of AlCl3 to
10a followed by Et3N. Esterification of the d11B 52.9 ppm
species with excess pinacol/Et3N led to the desired product
11a in good isolated yield (68%), unequivocally confirming
that borylative cyclization has taken place. This reaction is
notable as a rare example of cyclitive alkyne thioboration.[9c]
It should be noted that attempts to directly esterify the
zwitterion 10a led to significantly lower isolated yields of 11a
(38%). This is attributed to 10a having a greater propensity to
undergo protodeboronation due to the more nucleophilic
anionic benzothienyl-BCl3 moiety (relative to benzothienyl-
BCl2).
With the functional group tolerance already assessed in
benzofuran formation other thioanisole substrates were
selected to assess if alkyne haloboration was a competitive
pathway. As there was no evidence (in situ or post work-up)
for haloboration with 9a bulkier substituents, naphthyl and
mesityl, 9b and 9c, respectively, were incorporated into the
alkyne. Addition of BCl3 to these alkynes resulted in similar
outcomes to that observed with 9a with no evidence for
haloboration in either case, suggesting it is not a competitive
reaction with thioanisoles. Again, the isolated yield of the
benzothiophene pinacol boronate ester is higher on addition
of Et3N/AlCl3 prior to esterification (e.g., for producing 11b
yield = 48% direct from the zwitterion 10b whereas it is 73%
on esterification after addition of Et3N/AlCl3). To demon-
strate further that this methodology allows access to other-
wise challenging to synthesize boronic acid derivatives 11d
was produced in 55% isolated yield. Compound 11d is not
readily accessible by established borylation routes commenc-
ing from 2-(thiophen-3-yl)benzo[b]thiophene (e.g., Ir-cata-
lyzed borylation and halogenation/lithiation approaches
would all proceed at the thienyl alpha position).[3b,4]
Conflict of interest
The authors declare no conflict of interest.
Keywords: annulation · borylation · cross coupling ·
electrophilic cyclization · organoboranes
[1] 2,3-substituted benzofurans and benzothiophenes are privileged
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[6] R. L. Melen, Chem. Commun. 2014, 50, 1161.
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[8] For
a key initial publication see: a) R. L. Melen, M. M.
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b) C. Eller, G. Kehr, C. G. Daniliuc, R. Frçhlich, G. Erker,
Organometallics 2013, 32, 384; During the reviewing of this
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was reported: c) D. J. Faizi, A. J. Davis, F. B. Meany, S. A. Blum,
Angew. Chem. Int. Ed. 2016, 55, 14286.
In conclusion, two distinct reaction pathways operate on
ꢀ
addition of BCl3 to arylalkynes possessing ortho E Me (E =
NMe, O or S) moieties, specifically borylative cyclization and
trans-haloboration. The latter occurs with N,N-dimethyl-2-
(phenylethynyl)aniline whilst all the o-alkynyl-thioanisoles
studied react selectively by borylative cyclization. For o-
alkynyl-anisoles both pathways are observed, with borylative
cyclization dominating provided strong electron-withdrawing
[10] To our knowledge [ArylB(C6F5)3]ꢀ species have not been utilised
in Suzuki – Miyaura cross coupling, although ArylB(C6F5)2
species have been, for a recent example see: C. Eller, G. Kehr,
4
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C. G. Daniliuc, D. W. Stephan, G. Erker, Chem. Commun. 2015,
[18] K. Yuan, N. Suzuki, S. K. Mellerup, X. Wang, S. Yamaguchi, S.
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G.-Q. Liu, Y.-M. Li, Angew. Chem. Int. Ed. 2015, 54, 12636;
Angew. Chem. 2015, 127, 12827.
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Chem. Int. Ed. 2015, 54, 11245; Angew. Chem. 2015, 127, 11397.
[13] D. J. Faizi, A. Issaian, A. J. Davis, S. A. Blum, J. Am. Chem. Soc.
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[14] For cyclization using Group 16 and 17 electrophiles see (and
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Chem. 2009, 74, 1141.
[20] K. D. Collins, F. Glorius, Nat. Chem. 2013, 5, 597.
[21] M. Zaidlewicz, J. V. B. Kanth, H. C. Brown, J. Org. Chem. 2000,
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[16] Excluding acetylene cis-haloboration dominates on combining
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[17] Y. Uchikawa, K. Tazoe, S. Tanaka, X. Feng, T. Matsumoto, J.
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Manuscript received: October 13, 2016
Final Article published: && &&, &&&&
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Communications
Synthetic Methods
A. J. Warner, A. Churn, J. S. McGough,
M. J. Ingleson*
&&&& — &&&&
BCl3-Induced Annulative Oxo- and
Thioboration for the Formation of C3-
Borylated Benzofurans and
Benzothiophenes
C-3BO: These are the cyclizations you’re
looking for: BCl3-induced borylative cyc-
lization of aryl-alkynes possessing ortho-
EMe (E=S, O) moieties represents
a metal-free route to C3-borylated
a number of cases alkyne trans-halobora-
tion occurs alongside, or instead of
borylative cyclization and the factors
controlling the reaction outcome are
determined.
benzofurans and benzothiophenes. In
6
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