Formation of C(sp2) Boronate Esters by Borylative Cyclization of Alkynes Using BCl3

Article abstract of DOI:10.1002/anie.201505810

BCl₃is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp²) boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2-carboboration of alkynes is also achieved using BCl₃to generate trisubstituted vinyl boronate esters.

Full text of DOI:10.1002/anie.201505810

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505810
German Edition: DOI: 10.1002/ange.201505810
Isomerization
2
À
Formation of C(sp ) Boronate Esters by Borylative Cyclization of
Alkynes Using BCl3
Andrew J. Warner, James R. Lawson, Valerio Fasano, and Michael J. Ingleson*
Abstract: BCl3 is an inexpensive electrophile which induces the
borylative cyclization of a wide range of substituted alkynes to
regioselectively form polycycles containing synthetically ver-
2
À
satile C(sp ) boronate esters. It proceeds rapidly, with good
yields and is compatible with a range of functional groups and
substitution patterns. Intermolecular 1,2-carboboration of
alkynes is also achieved using BCl3 to generate trisubstituted
vinyl boronate esters.
2
À
C
(sp ) boronic acid and ester derivatives are ubiquitous in
modern synthetic chemistry because of their good ambient
À
stability, low toxicity, utility in C C bond formations, and
Scheme 1. BCl3-induced alkyne borylative cyclization. EDG=electron-
donating group, EWG=electron-withdrawing group.
facile transformation into other important functional
groups.[1] Classic synthetic approaches to forming C(sp ) B
bonds require Grignard or organolithium reagents, and thus
2
À
have issues with functional-group compatibility, and often
which are prevalent in (or are key precursors to) biologically
active molecules, pharmaceuticals (e.g. Nafoxidine;
Scheme 1) and/or organic materials.[7] Alkyne cyclization
with concomitant functional-group installation has been
principally limited to chalcogen and halogen electrophiles,[8]
and to the best of our knowledge, to date, the borylative
cyclization of alkynes to form polycyclic structures containing
2
À
require C(sp ) halide precursors and cryogenic temperatur-
es.[1b] Simple methods which are functional-group tolerant for
2
À
forming C(sp ) B bonds are highly desirable, particularly
reactions that proceed directly from hydrocarbon precursors.
The most notable recent breakthrough in this area is iridium-
[2]
À
catalyzed direct C H borylation. Whilst this reaction has
2
[9]
À
developed into a truly powerful transformation, the discovery
C(sp ) boronate esters requires transition-metal catalysis.
2
À
of new routes, particularly transition-metal-free methods, to
Metal-free borylative cyclization to form polycyclic C(sp )
B(OR)2 species is not documented. Furthermore, metal-free
borylative cyclization is distinct from the reactivity of
transition metals and heavier group 13 electrophiles (e.g.,
GaCl3), which catalyze the cycloisomerization of alkynes
2
À
efficiently generate C(sp ) boronate esters, which are chal-
lenging to access by iridium catalysis, remains desirable.
Recent advances in transition-metal-free borylation include
benzannulations,[3] radical mediated borylation,[4] electro-
philic borylation,[5] and carbanion-mediated borylation.[6]
without
concomitant
functional-group
installation
2
One underexplored approach to metal-free C(sp ) B
(Scheme 1).[10]
À
bond formation proceeding from simple hydrocarbon pre-
cursors is the borylative cyclization of alkynes, wherein
a boron electrophile activates an alkyne for intramolecular
Given the ubiquity of alkyne functionalization using
boron electrophiles (e.g., hydroboration) the cyclization of
unactivated[11] alkynes induced by any boron electrophile is
relatively scarce. Noteworthy exceptions use B(C6F5)3 to
initiate cyclization, thus generating a variety of borylated
polycyclic structures.[12] Whilst notable, these reactions use
the expensive (relative to BCl3) electrophile B(C6F5)3, which
can concomitantly install a C6F5 group, and more significantly,
precludes formation of the desirable boronic acid deriva-
tives.[12] Herein, we report borylative cyclization, using BCl3,
electrophilic cyclization with
a second p-system. This
approach represents step-economical reaction which
a
2
À
À
would simultaneously create new C(sp ) B and C C bonds
to generate new polycyclic frameworks such as borylated
dihydroquinolines, dihydronaphthalenes, and phenanthrenes,
[*] A. J. Warner, J. R. Lawson, V. Fasano, Dr. M. J. Ingleson
School of Chemistry, University of Manchester
Manchester, M13 9PL (UK)
as a method which is functional-group tolerant and rapidly
2
À
generates polycyclic structures containing C(sp ) boronate
À
À
esters. The reaction features concomitant C C and C B bond
formation under mild reaction conditions from simple starting
materials.
E-mail: Michael.ingleson@manchester.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505810.
Our studies into borylative cyclization started with 1,4-
diphenylbut-1-ynes (e.g., 1a; Scheme 2), thus targeting bory-
lated dihydronaphthalenes because of their importance in
pharmaceuticals.[7] BCl3 was utilized as it is inexpensive and
more electrophilic than BF3, with Et2O-BF3 previously shown
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
Angew. Chem. Int. Ed. 2015, 54, 11245 –11249
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11245

.
Angewandte
Communications
Scheme 2. Cyclization to produce 3-borylated 4-substituted 1,2-dihydronaphthalenes and derivatives.[28] [a] Reaction conditions A: 1) BCl3 (1.1
equiv); 2) pinacol (1.1 equiv), NEt3 (15 equiv). [b] Reaction conditions B: 1) BCl3 (2.1 equiv), TBP (1 equiv); 2) pinacol (1.1 equiv), NEt3 (15
equiv). [c] Reaction conditions C: 1) [Cl2B(2-DMAP)][AlCl4] (1 equiv); 2) pinacol (2.1 equiv), NEt3 (15 equiv). [d] Using unpurified CH2Cl2 and
2 equiv BCl3 in air. [e] Carried out on a 1.2 gram scale. [f] A reaction time of 12 h. g) 2 equiv of BCl3. [h] 2.5 h at 608C in a sealed vessel.
DMAP=2-(N,N-dimethylamino)pyridine, Ts =4-toluenesulfonyl.
not to cyclize aryl-substituted alkynes.[13] The addition of BCl3
(1.1 equiv) to 1a at 208C in CH2Cl2 led to complete
consumption of 1a within 10 minutes (Scheme 2). In the
1H NMR spectrum there was the appearance of a character-
istic doublet at d = 6.79 ppm, which is attributed to the
aromatic proton ortho to the newly formed carbon–carbon
bond. The appearance of a broad resonance at d = 54 ppm in
the 11B NMR spectrum was also consistent with a vinylBCl2
moiety. In situ formation of the pinacol boronate ester was
facile, and 2a was isolated as the desired cyclized product in
91% yield. Formation of the borylated dihydronaphthalene
presumably proceeds by activation of the alkyne by BCl3, with
the potentially competing haloboration reaction disfavored
for internal alkynes and BCl3.[14] The BCl3-activated alkyne
(as a vinyl cation or a p complex) is sufficiently electrophilic
at the carbon center to undergo an intramolecular SEAr
reaction by a 6-endo-dig cyclization; no 5-exo-dig cyclized
products were observed throughout this work. Loss of H+ as
HCl then leads to rearomatization, with no competing
protodeboronation of the newly formed vinylBCl2 moiety,
which is subsequently pinacol protected. The borylative
cyclization of 1a with BCl3 is distinct to the reactivity of
transition metals[10] and heavier group 13 Lewis acids (e.g.,
GaCl3 and InCl3),[15] which catalyze the cycloisomerization of
similar alkynes (Scheme 1, top). The disparity observed
between BCl3 and the heavier group 13 analogues presumably
to protodemetallation and thus enables isolation of the
2
À
desired C(sp ) boronate esters. Notably, protodeboronation
products are observed as a minor component for a number of
substrates (these may also form by HCl initiated cyclization),
but performing the reaction with the hindered base 2,4,6-tri-
tert-butylpyridine (TBP) enables cleaner borylative cycliza-
tion (this requires 2 equiv of BCl3 with 1 equiv consumed by
formation of [(TBP)H][BCl4]).
The functional-group tolerance of the BCl3 borylative
cyclization proved to be broad (Scheme 2). Substrates con-
taining halide substituents (1b–d) at the ortho- and para-
positions of the aryl alkyne, and the pentafluoro aryl alkyne
were all rapidly cyclized (reactions complete within 10 min)
and the corresponding pinacol boronate esters were isolated
in yields of greater than 70%. The alkyne 1b was successfully
cyclized open to air and using unpurified CH2Cl2, albeit using
excess BCl3 (due to H2O present in unpurified CH2Cl2). Only
a 7% decrease in the yield of the isolated product was noted
compared to the reaction under rigorously anhydrous con-
ditions, thus highlighting the robustness of this reaction.
À
Trifluoromethyl groups often undergo C F activation with
strong boron electrophiles, but in this case the CF3-containing
alkyne 1e was successfully cyclized and isolated in 97% yield.
2e was also produced on a gram scale, and isolated in a 96%
yield (1.74 g). The alkyne 1 f, containing a Lewis basic nitrile
functionality, reacted rapidly with BCl3 initially resulting in
the formation of two compounds. One compound was
À
arises from the less polar and stronger B C bond (relative to
À
À
Ga C), which makes the borylated compounds more resistant
consistent with a vinylBCl2 species and the other with a R
11246 www.angewandte.org
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 11245 –11249

Angewandte
Chemie
CN!BCl3 adduct (determined by 11B NMR spectroscopy).
After twelve hours the desired cyclized product was the only
species observed in the 11B NMR spectrum, with reversible
coordination between the nitrile group and BCl3 only slowing
the cyclization and not leading to any observable side
products. The structure of 2 f was also confirmed by X-ray
crystallography (Scheme 2). Other alkynes possessing oxo-
containing functional groups, including a tosyl-protected
amine linker (1g), nitro (1h), and ester (1i), were all cyclized
and isolated as the pinacol boronate ester in greater than or
equal to 70% yields. 1i required two equivalents of BCl3 for
complete cyclization with one equivalent of BCl3 coordinating
to the ester moiety and the second inducing the cyclization.
The ester!BCl3 adduct was subsequently cleaved on addi-
tion of NEt3 during the esterification step to yield 2i. The
ether-containing substrates 1j and 1k were also amenable to
cyclization with BCl3 with minimal ether cleavage observed,
presumably because of the rapid nature of the BCl3-induced
cyclization. The cyclization of 1g and 1k confirms that this
methodology is not limited to forming dihydronaphthalenes,
but is also applicable to dihydroquinoline and chromene
formation. Tolyl-substituted alkynes (1l–o) reacted with BCl3
to give a mixture of cyclized products attributable to Brønsted
acid initiated methyl group migration.[16] This migration was
avoided simply by repeating the cyclization in the presence of
TBP, which effectively sequesters the HCl by-product, and the
alkynes 1l–o all cleanly cyclized to give the desired product.
Previous work on electrophilic cyclization using iodonium
salts only succeeded when the alkyne substituent was capable
of electronically stabilizing the vinyl cation intermediate (e.g.,
p-MeOC6H4-functionalized alkynes).[17] Attempts at boryla-
tive cyclization of the terminal alkyne 4-phenyl-1-butyne
failed because of preferential haloboration with BCl3. In
contrast, upon addition of BCl3, the bromo-terminated alkyne
1p was converted into two products. Post esterification the
major product was the dihydronaphthalene, 2p (Scheme 2)
and the minor component was that derived from haloboration
of 1p (by GC-MS). Cyclization of an alkyl terminated alkyne,
1q, was achieved with BCl3, but in addition to the borylated
dihydronaphthalene product (2q) a borylated naphthalene
and tetralin were produced, consistent with transfer hydro-
genation proceeding under these reaction conditions.[18] When
cyclization was repeated with BCl3 and TBP, 2q was isolated
in a 67% yield, thus confirming that aryl groups for the
stabilization of vinyl cations are not essential for BCl3-
induced cyclization. TBP/BCl3 also enabled the cyclization of
alkynes substituted with naphthyl and vinyl groups to form 2r
and 2s, respectively. In the absence of TBP lower yields were
observed.
tolerant of methoxy groups during borylation reactions and it
does not haloborate internal alkynes.[19] Using 3 led to more
selective borylative cyclization reactions, thus enabling iso-
lation of 2t and 2u in moderate yields (36 and 57%). The
selectivity disparity between BCl3 and 3 is attributed to the
lower nucleophilicity of [AlCl4]À versus ClÀ (produced during
cyclization with BCl3), thus suppressing ether cleavage by
attack of the anion on the Med+ of Ar(Me)O!BCl3 adducts.
The more electrophilic (relative to BCl3) borocation 3 was
also essential for cyclizing substrates containing deactivated
internal aromatic nucleophiles. For example, the alkyne 1v
did not react when combined with BCl3 (at 208C or at raised
temperatures), but using 3 and heating at 608C for 2.5 hours
led to full cyclization. It is noteworthy that 1v (and 1o and 1u)
cyclized to produce only one regioisomer, thus borylative
cyclization is also highly regioselective. To the best of our
knowledge 4-R-1,2-dihydronaphthalenes borylated at the C3-
position are currently unknown and represent useful inter-
mediates because of the importance of this structure in
pharmaceuticals (e.g., Nafoxidine).
To demonstrate the utility of the borylated products 2e
was coupled with 4-bromotoluene to produce 4 in 75% yield
(Scheme 3). This proof-of-principle synthesis of a nafoxidine
analogue proceeds in an overall 63% yield over three steps
Scheme 3. Cross-coupling and oxidation of 2e. dba=dibenzylideneace-
tone, THF=tetrahydrofuran.
starting from the commercially available terminal alkyne and
haloarene precursors. The cross-coupled product 4 can be
readily oxidized using [Ph3C][BF4] to produce the 1,2-
disubstituted naphthalene in good yield upon isolation
(75% from 4; see the Supporting Information). Alternatively,
this oxidation procedure was adapted to allow dehydrogen-
ation of 2e, thus generating the borylated naphthalene 5 in
93% yield (Scheme 3). Regioselectively functionalized naph-
thalenes are useful in their own right or as precursors to
higher acenes.[20] To date the selective formation of 1-
substituted-2-borylated-naphthalenes by iridium catalysis
requires installation of directing groups at C1,[21] whereas
borylative cyclization/oxidation offers more versatility in the
nature of the C1 substituent.
In contrast to the borylative cyclization of 1i and 1k with
BCl3, it was observed that for other ether-containing sub-
strates (e.g., 1t and 1u), BCl3, with and without TBP, gave
extremely low yields of the isolated desired products
(Scheme 2). In situ NMR analysis showed that 1t reacted
rapidly with BCl3 to initially produce the cyclized product and
ether-cleavage products (ArylOBCl2 and chloromethane).
Previously, we reported that the borocation, [Cl2B(2-
DMAP)][AlCl4] (3), which can be readily produced and
handled in air as a solid for short periods, is reasonably
With the functional-group tolerance and utility of the
products from 4-aryl-1-alkyne borylative cyclization con-
firmed, our attention turned to identifying other systems
amenable to BCl3-induced cyclization guided by previously
reported metal-catalyzed cycloisomerization reactions. The
borylative cyclization of
a 2-alkynyl-1,1’-biphenyl was
selected as it undergoes cycloisomerization and represents
a more rigid analogue of 4-aryl-1-alkynes. 2-(p-tolylethynyl)-
1,1’-biphenyl underwent borylative cyclization using BCl3/
TBP. Whilst the reaction is slower than dihydronaphthalene
Angew. Chem. Int. Ed. 2015, 54, 11245 –11249
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11247

.
Angewandte
Communications
the reactivity observed by combining B(C6F5)3, phenylacety-
lene, and N-tBu-pyrrole.[27] 1-Phenyl-1-propyne shows no
reactivity with 2-methylthiophene or BCl3 alone, yet when all
three were combined in the presence of TBP, the product (12)
from a trans-1,2-carboboration was isolated after esterifica-
tion in a 41% yield (Scheme 6). Thus BCl3-induced alkyne
carboboration is not limited to intramolecular p-nucleophiles.
Scheme 4. Borylative cyclization of alkyne-functionalized biaryls.
formation, heating for 24 hours led to the isolation of 6 in
60% yield post esterification (Scheme 4). The disparity in
reaction outcome between BCl3 (borylative cyclization) and
GaCl3 (cycloisomerization) is again noteworthy.[22] The bor-
ylative cyclization of 2-alkynyl-1,1’-biphenyls forms versatile
intermediates (e.g., 6) which can be transformed into
desirable materials, such as dibenzochrysenes by established
methodologies.[23] The more functionality-rich precursor 7,
containing a heteroaromatic, a bromide, and a hexyl-substi-
tuted alkyne, was also amenable to borylative cyclization and
delivered 8 in 58% yield. This reactivity indicates the
applicability of this reaction to form pharmaceutically desir-
able borylated heterocycles, such as pyrrolo-[1,2a]-quinolines
(e.g., 8).[24]
Scheme 6. Intermolecular 1,2-carboboration.
In conclusion, BCl3 is an inexpensive and functional-
group-tolerant electrophile for the transition-metal-free bor-
ylative cyclization of alkynes. It provides rapid, high-yielding
2
À
access to hitherto unknown C(sp ) boronate esters which are
versatile synthetic intermediates. This methodology has been
exemplified by forming a range of important polycyclic
structures. The p-nucleophile is not limited to internal p-
systems, as intermolecular nucleophiles are also amenable.
These features, combined with the multitude of alkynes which
are reported to undergo metal-catalyzed cycloisomerizations,
indicates that borylative cyclization is a powerful, transition-
To further demonstrate the utility of borylative cycliza-
tion, we explored the cyclization of diynes. Again guided by
transition metal catalyzed cycloisomerization[25] we investi-
gated the reactivity of the diynes 9a and 9b with BCl3
(Scheme 5). In the presence of TBP the cyclization proceeded
metal-free route for accessing polycyclic structures containing
2
À
C(sp ) boronate esters.
Acknowledgements
We are grateful to the European Research Council (FP7
Grant No. 305868), the EPSRC (EP/J000973/1 and EP/
K039547/1), and the Royal Society for funding, and to Dr.
Ian A. Cade for the solid-state structure determination of 2 f.
In addition, we are grateful to M. Maqsood, G. Smith, and C.
Webb for mass spectrometry data.
Keywords: alkynes · boron · cyclizations · heterocycles ·
synthetic methods
Scheme 5. Electrophilic borylative cyclization of diynes.
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11245–11249
Angew. Chem. 2015, 127, 11397–11401
rapidly at 208C and produced the boronate esters 10a and
10b, which were isolated in 84 and 92% yield, respectively,
after pinacol protection. The mild reaction conditions and
compatibility with N-tosyl suggests a similar functional-group
tolerance to that in 2x formation. The reaction of the terminal
diyne, 1,6-heptadiyne, with BCl3 also proceeded rapidly, and
delivered a chlorinated cyclohexene possessing a vinyl BCl2
moiety exo to the newly formed ring. Upon esterification, 11
was isolated in 81% yield and characterized by a combination
of one- and two-dimensional NMR spectroscopy, as well as
comparison to the product from the reaction of 1,6-hepta-
diyne and iodosylbenzene.[26]
[1] a) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457; b) Boronic
Acids: Preparation and Applications (Ed.: D. Hall), Wiley-VCH,
Weinheim, 2011.
[2] a) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy,
J. F. Hartwig, Chem. Rev. 2010, 110, 890; b) S. Shimada, A. S.
Batsanov, J. A. K. Howard, T. B. Marder, Angew. Chem. Int. Ed.
2001, 40, 2168; Angew. Chem. 2001, 113, 2226; c) T. Ishiyama, J.
Takagi, K. Ishida, N. Miyaura, N. R. Anastasi, J. F. Hartwig, J.
Am. Chem. Soc. 2002, 124, 390; d) J.-Y. Cho, M. K. Tse, D.
Holmes, R. E. Maleczka, Jr., M. R. Smith III, Science 2002, 295,
305.
[3] For a recent example, see: D. F. P. CrØpin, J. P. A. Harrity, J.
Jiang, A. J. H. Meijer, A.-C. M. A. Nassoy, P. Raubo, J. Am.
Chem. Soc. 2014, 136, 8642, and references therein.
Finally, we investigated whether a BCl3-activated alkyne
could be intercepted by an external p-nucleophile, similar to
11248 www.angewandte.org
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 11245 –11249

Angewandte
Chemie
[4] F. Mo, Y. Jiang, D. Qiu, Y. Zhang, J. Wang, Angew. Chem. Int.
Ed. 2010, 49, 1846; Angew. Chem. 2010, 122, 1890.
Erker, Org. Biomol. Chem. 2015, 13, 764; c) R. Liedtke, F.
Tenberge, C. G. Daniliuc, G. Kehr, G. Erker, J. Org. Chem. 2015,
80, 2240; d) R. L. Melen, M. M. Hansmann, A. J. Lough, A. S. K.
Hashmi, D. W. Stephan, Chem. Eur. J. 2013, 19, 11928.
[13] N. Chatani, H. Inoue, T. Ikeda, S. Murai, J. Org. Chem. 2000, 65,
4913.
[14] M. F. Lappert, B. Prokai, J. Organomet. Chem. 1964, 1, 384.
[15] H. Inoue, N. Chatani, S. Murai, J. Org. Chem. 2002, 67, 1414.
[16] G. A. Olah, Friedel Crafts Chemistry, Wiley, Hoboken, 1973.
[17] A. J. Walkinshaw, W. Xu, M. G. Suero, M. J. Gaunt, J. Am.
Chem. Soc. 2013, 135, 12532.
[18] B. Michelet, C. Bour, V. Gandon, Chem. Eur. J. 2014, 20, 14488.
[19] J. R. Lawson, E. R. Clark, I. A. Cade, S. A. Solomon, M. J.
Ingleson, Angew. Chem. Int. Ed. 2013, 52, 7518; Angew. Chem.
2013, 125, 7666.
[20] For uses of acenes: a) J. E. Anthony, Angew. Chem. Int. Ed. 2008,
47, 452; Angew. Chem. 2008, 120, 460; For the borylation of
polycyclic aromatic hydrocarbons using iridium catalysts:
b) D. N. Coventry, A. S. Batsanov, A. E. Goeta, J. A. K.
Howard, T. B. Marder, R. N. Perutz, Chem. Commun. 2005,
2172.
[5] For recent reviews on electrophilic borylation, see: a) T. S.
De Vries, A. Prokofjevs, E. Vedejs, Chem. Rev. 2012, 112, 4246;
b) M. J. Ingleson, Top. Organomet. Chem. 2015, 49, 39.
[6] a) E. Yamamoto, K. Izumi, Y. Horita, H. Ito, J. Am. Chem. Soc.
2012, 134, 19997; b) E. Yamamoto, S. Ukigai, H. Ito, Chem. Sci.
2015, 6, 2943.
[7] D. Lednicer, S. C. Lyster, G. W. Duncan, J. Med. Chem. 1967, 10,
78.
[8] For select reviews, see: a) R. C. Larock in Acetylene Chemistry.
Chemistry, Biology and Material Science (Eds.: F. Diederich, P. J.
Stang, R. R. Tykwinski), Wiley-VCH, New York, 2005, pp. 51 –
99; b) B. Godoi, R. F. Schumacher, G. Zeni, Chem. Rev. 2011,
111, 2937; For select examples of cyclization, see: c) J. Bar-
luenga, J. M. Gonzalez, P. J. Campos, G. Asensio, Angew. Chem.
Int. Ed. Engl. 1988, 27, 1546; Angew. Chem. 1988, 100, 1604;
d) X. Zhang, M. A. Campo, T. Yao, R. C. Larock, Org. Lett.
2005, 7, 763; e) T. Yao, M. A. Campo, R. C. Larock, Org. Lett.
2004, 6, 2677; f) J. Barluenga, M. Trincado, M. Marco-Arias, A.
Ballesteros, E. Rubio, J. M. Gonzalez, Chem. Commun. 2005,
2008.
[9] For recent examples of transition-metal-catalyzed borylative
cyclization, see: a) K. Kubota, E. Yamamoto, H. Ito, J. Am.
Chem. Soc. 2013, 135, 2635; b) V. Pardo-Rodríguez, E. BuÇuel,
D. Collado-Sanz, D. J. Cµrdenas, Chem. Commun. 2012, 48,
10517; c) Y. Deng, T. Bartholomeyzik, J. E. Baeckvall, Angew.
Chem. Int. Ed. 2013, 52, 6283; Angew. Chem. 2013, 125, 6403;
d) A. K. A. Persson, T. Jiang, M. T. Johnson, J.-E. Baeckvall,
Angew. Chem. Int. Ed. 2011, 50, 6155; Angew. Chem. 2011, 123,
6279; e) J. Huang, S. J. f. Macdonald, J. P. A. Harrity, Chem.
Commun. 2010, 46, 8770; f) J. Marco-Martínez, V. López-
Carrillo, E. BuÇuel, R. Simancas, D. J. Cµrdenas, J. Am. Chem.
Soc. 2007, 129, 1874.
[10] For select reviews on metal-catalyzed cycloisomerizations, see:
a) Y. Luo, Z. Pan, X. Yu, J. Wi, Chem. Soc. Rev. 2014, 43, 834;
b) A. Das, S. M. Abu Sohel, R. S. Liu, Org. Biomol. Chem. 2010,
8, 960; c) For the use of electrophilic gold catalysts to form
dihydronaphthalenes by cyclization, see: D. Malhotra, L.-P. Liu,
M. S. Mashuta, G. B. Hammond, Chem. Eur. J. 2013, 19, 4043,
and references therein.
[11] B. Wrackmeyer and co-workers reported activating metalated
alkynes with a range of boron electrophiles, thus leading to
cyclization. See: B. Wrackmeyer, Heteroat. Chem. 2006, 17, 188.
[12] For a review on B(C6F5)3-catalyzed cyclizations, see : a) R. L.
Melen, Chem. Commun. 2014, 50, 1161; For select recent
publications, see: b) G. Chen, G. Kehr, C. G. Daniliuc, G.
[21] a) For a review, see: A. Ros, R. Fernandez, J. M. Lassaletta,
Chem. Soc. Rev. 2014, 43, 3229; b) For a recent example, see: B.
Ghaffari, S. M. Preshlock, D. L. Plattner, R. J. Staples, P. E.
Maligres, S. W. Krska, R. E. Maleczka, Jr., M. R. Smith III, J.
Am. Chem. Soc. 2014, 136, 14345.
[22] V. Mamane, P. Hannen, A. Fuerstner, Chem. Eur. J. 2004, 10,
4556.
[23] Y. Xia, P. Qu, Z. Liu, R. Ge, Q. Xiao, Y. Zhang, J. Wang, Angew.
Chem. Int. Ed. 2013, 52, 2543; Angew. Chem. 2013, 125, 2603.
[24] W. Kemnitzer, J. Kuenmerle, S. Jiang, N. Sirisoma, S. Kasibhatla,
C. Crogan-Grundy, B. Tseng, J. Drewe, S. X. Cai, Bioorg. Med.
Chem. Lett. 2009, 19, 3481.
[25] J. Lian, P. Chen, Y. Lin, H. Ting, R. Liu, J. Am. Chem. Soc. 2006,
128, 11372.
[26] M. Ochiai, M. Hirobe, A. Yoshimura, Y. Nishi, K. Miyamoto, M.
Shiro, Org. Lett. 2007, 9, 3335.
[27] M. A. Dureen, C. C. Brown, D. W. Stephan, Organometallics
2010, 29, 6422.
[28] CCDC 1408578 (2 f) contain the supplementary crystallographic
data for this paper. These data are provided free of charge by
The Cambridge Crystallographic Data Centre.
Received: June 24, 2015
Published online: July 31, 2015
Angew. Chem. Int. Ed. 2015, 54, 11245 –11249
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11249