Boryl (Hetero)aryne Precursors as Versatile Arylation Reagents: Synthesis through C-H Activation and Orthogonal Reactivity

Article abstract of DOI:10.1002/anie.201503152

(Pinacolato)boryl ortho-silyl(hetero)aryl triflates are presented as a new class of building blocks for arylation. They demonstrate unique versatility by delivering boronate or (hetero)aryne reactivity chemoselectively in a broad range of transformations. This approach enables the unprecedented postfunctionalization of fluoride-activated (hetero)aryne precursors, for example, as substrates in transition-metal catalysis, and offers valuable new possibilities for aryl boronate postfunctionalization without the use of specialized protecting groups. Ready for a complete makeover: As building blocks for arylation, (pinacolato)boryl ortho-silyl (hetero)aryl triflates (see structure; X=C, N) showed unique versatility by reacting chemoselectively as boronates or (hetero)arynes in a broad range of transformations. This approach offers valuable possibilities for the functionalization of both aryne precursors and aryl boronates without the use of specialized protecting groups.

Full text of DOI:10.1002/anie.201503152

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201503152
German Edition: DOI: 10.1002/ange.201503152
Synthetic Methods
Boryl (Hetero)aryne Precursors as Versatile Arylation Reagents:
À
Synthesis through C H Activation and Orthogonal Reactivity
Emilien Demory, Karthik Devaraj, Andreas Orthaber, Paul J. Gates, and Lukasz T. Pilarski*
Abstract: (Pinacolato)boryl ortho-silyl(hetero)aryl triflates
are presented as a new class of building blocks for arylation.
They demonstrate unique versatility by delivering boronate or
(hetero)aryne reactivity chemoselectively in a broad range of
transformations. This approach enables the unprecedented
postfunctionalization of fluoride-activated (hetero)aryne pre-
cursors, for example, as substrates in transition-metal catalysis,
and offers valuable new possibilities for aryl boronate
postfunctionalization without the use of specialized protecting
groups.
inefficient de novo syntheses, and enable iterative approaches
to their elaboration.[12] However, the same mild and versatile
reactivity that imparts aryl boronates and ortho-silyl aryl
triflates with such broad appeal makes their postfunctional-
ization commensurately more challenging.
For aryl boronates, responses to this problem have largely
relied on the use of protecting groups to mitigate the
reactivity of the boron center with respect to that of
(pseudo)halide,[13] stannane,[14] or orthogonally protected
boronate[15] groups present on the same arene. However,
À
this approach has been exploited almost exclusively in C C
T
he development of versatile (hetero)arylation strategies is
bond coupling; methods to introduce other valuable func-
a key pursuit in organic synthetic methodology.[1] Dramatic
advances on this front are exemplified by the use of aryl
boronate reagents[2] and aryne intermediates.[3] Both partic-
ipate in a seemingly inordinate range of reactions, including
tionality remain scarce.[16]
Remarkably, the postfunctionalization of ortho-silyl aryl
triflates has never been reported, despite their growing
popularity and the passing of three decades since the seminal
report by Kobayashi and co-workers on their use as aryne
precursors.[8] Variants requiring even ostensibly simple sub-
stitution patterns often need inconvenient, multistep prepa-
ration.
We envisaged that the unique reactivities of aryl boro-
nates and arynes could be harnessed for their mutual,
chemoselective postfunctionalization (Figure 1). This
approach would address both problems from common
intermediates and offer versatile building blocks for arylation.
Elegant recent studies by Akai and co-workers on the unusual
influence of 3-boryl substituents over the regioselectivity of
benzyne capture stands as the only previous juxtaposition of
aryne and boronate reactivities.[17] However, the requirement
that the boronate be placed at the benzyne C3 position in
these studies necessitated multistep preparation and revealed
an incompatibility with fluoride in the absence of extremely
robust protecting groups.[17c] It also seemingly precluded use
of the boronate prior to capture of the aryne and therefore the
prospect of postfunctionalizing the aryne precursor. We
À
À
C C, C N, carbon–chalcogen and carbon–halogen bond
formation. Aryl boronates have also attracted attention as
organocatalysts,[4] as well as for medicinal[5] and materials[6]
applications. Meanwhile, (hetero)arynes enable simultaneous,
regioselective functionalization at two adjacent carbon
atoms[7] and may be generated under mild conditions from
ortho-silyl (hetero)aryl triflates using fluoride.[8] Such advan-
tages have fueled their increasing popularity in the synthesis
of natural products,[9] functional materials,[10] and organome-
tallic compounds.[11]
The versatility of aryl boronates and ortho-silyl aryl
triflates renders the prospect of their selective postfunction-
alization exceptionally attractive. It promises greatly to
broaden the range of possible derivatives, obviate tedious/
[*] Dr. E. Demory, K. Devaraj, Dr. L. T. Pilarski
Department of Chemistry—BMC, Uppsala University
Box 576, 75-123 Uppsala (Sweden)
E-mail: lukasz.pilarski@kemi.uu.se
Homepage: http://www.pilarskigroup.org
reasoned that the remarkable functional-group tolerance of
[18]
À
iridium-catalyzed C H borylation and its preference for
Dr. A. Orthaber
Department of Chemistry—Šngstrçm Laboratories
Uppsala University
the least sterically hindered position of simple arenes[19] would
Box 523, 75-120 Uppsala (Sweden)
Dr. P. J. Gates
School of Chemistry, University of Bristol
Cantock’s Close, Clifton, Bristol, BS8 1TS (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503152.
ꢀ 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 Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial and no modifica-
tions or adaptations are made.
Figure 1. Underlying concept of this study. B(pin)=(pinacolato)boryl,
Tf =trifluoromethanesulfonyl.
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À
Scheme 1. Iridium-catalyzed C H borylation of aryne precursors 1.
[a] The reaction was carried out using 2 equivalents of B2(pin)2 in
a reaction time of 42 h. cod=1,5-cyclooctadiene.
provide a direct way to introduce a B(pin) substituent
selectively into precursors 1, thereby preserving various R
groups at C3, where they are known to influence aryne
reactivity most profoundly.[20]
At the outset of our study, we found that the treatment of
benzyne precursor 1a (R = H) with [{Ir(m-OMe)cod}2], 4,4’-
di-tert-butyl-2,2’-dipyridyl (dtbpy), and B2(pin)2 under the
conditions shown in Scheme 1 gave 2a quantitatively as a 3:1
mixture of regioisomers. The scope of the reaction was
extended to the synthesis of 2b–g with complete regioselec-
tivity and generally excellent yields on scales up to 2 g. The
reaction of 4,5-indolyne precursor 1h[21] under the same
conditions gave inseparable mixtures of C2- and C7-borylated
products; the use of more B2(pin)2 led exclusively to
diboronate 2h.[22]
We next explored chemoselective aryne generation from
2. We found that CsF (4 equiv) and 18-crown-6 (1 equiv) in
MeCN enabled a broad range of functionalization reactions
(Scheme 2), including attack by N- (products 4a–c), O-
(products 4e–g), and S-based nucleophiles (product 4i), as
well as [4+2] cycloaddition (product 4j), cyclization with an
aryl azide[23] (product 4k) or nitrone (product 4l,m), and
insertion into a cyclic urea[24] (product 4n). An exception was
substrate 2g, which invariably succumbed to rapid deboryla-
tion (to give 4d),[25] as is common for polyfluoroaryl
boronates.[26] Substrate 3a (R = H) underwent preferential
attack by heteroatoms at the carbon atom meta to the B(pin)
group (products 4a,l). Benzynes are usually biased by
electronegative substituents to favor attack by nucleophiles
at the distal carbon atom of the triple bond;[27] we attribute
the opposite selectivity in the formation of 4a and 4l to the
electropositive character of boron.[17a–d] Otherwise, the regio-
selectivity of aryne capture was governed by the R group. For
example, the OMe group of 2c directed nucleophilic addition
exclusively to C1 of the corresponding aryne 3c (products
4b,f,i), whereas 2e gave mixtures of regioisomers (products 4c
Scheme 2. Elaboration of boronates 2 through (hetero)aryne trapping.
[a] A mixture of regioisomers was obtained; the major regioisomer is
shown. [b] Crystal structure. Ellipsoids are drawn at 50% probability.
and 4g) arising from the competing steric and electronic
influences of the SiMe3 substituent.[28] The activation of 2d in
the presence of pyridine N-oxide gave exclusively the
3-substituted pyridine 4h through a rearrangement.[29]
In most cases, the trapping reagent for boryl (hetero)-
arynes 3 was chosen to highlight the difficulty in accessing the
anticipated products by established borylation methods in the
final step. Thus, the halogen substituents in 4e, 4h, 4k, and
4m would make problematic their synthesis by borylation
relying on lithiation or Pd catalysis.[15c,30] Similarly, the
À
presence of several sterically accessible Ar H bonds in 4e–
i and 4k–m precludes their selective preparation by iridium-
[19,31]
À
catalyzed C H borylation.
The presented method is
therefore a valuable and systematic new approach to aryl
boronate postfunctionalization with a wide variety of func-
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Scheme 3. Arylation of precursors 2 by SMC. [a] Reaction conditions in
this case: [Pd(dba)2] (4 mol%), XPhos (8 mol%), K2CO3 (3 equiv),
toluene/H2O, 808C, 20 h. dba=dibenzylideneacetone, XPhos=2-dicy-
clohexylphosphanyl-2’,4’,6’-triisopropylbiphenyl.
Scheme 5. Transformations of aryl boronic acids 6: i) palladium-cata-
lyzed allylation; ii) boronate conversion; iii) rhodium-catalyzed 1,4-con-
jugate addition; iv) palladium-catalyzed arylation with [Ar2I]X;
v) copper-catalyzed iodination; vi) Chan–Lam amination.
tional groups. Furthermore, no examples have been reported
previously in which the convenience of fluoride-induced
aryne generation was possible without concomitant loss of the
boronate or the use of specialized protecting groups.
tected by treatment with diethanolamine (DEA) and acid
hydrolysis.[32] This mild, expedient method furnished boronic
acids 6 (Scheme 4), which proved indefinitely stable at room
temperature in air.[25]
We next addressed the chemoselective manipulation of
the boronate group of 2 to effect further ortho-silyl (hetero)-
aryl triflate postfunctionalization. Suzuki–Miyaura coupling
(SMC) proceeded very well with various (hetero)aryl bro-
mides without compromising the ortho-silyl triflate moiety
(Scheme 3). We anticipate that this method will provide
a flexible route to novel substituted polyaromatic systems, for
which arynes are increasingly exploited as building blocks.[10]
Competitive activation of the bromide and/or ortho-silyl
triflate unit of 2d was circumvented by a complementary
strategy with diaryl iodonium reagents (see below).
Boronic acids 6 participated in a broad selection of high-
yielding transformations with complete preservation of the
ortho-silyl triflate (Scheme 5). The conversion of 6c through
palladium-catalyzed allylation into 7a or trifluoroborate[33] 7b
is notable for its use of KF (up to 4 equiv), which is also
commonly exploited to activate ortho-silyl aryl triflates. It
demonstrates the following practicable order of functional-
group reactivity with respect to fluoride: B(OH)2 > ortho-silyl
triflate > B(pin), and that this reactivity order can serve
selective arene functionalization. Rhodium-catalyzed 1,4-
conjugate addition[34] to chalcone afforded 7c,d in excellent
yields. The palladium-catalyzed oxidative coupling of 6b and
6d with diaryl iodonium reagents[35] also proved efficient.
Thus, bromide substituents on either coupling partner could
be preserved to give 7e,f, obviating problems with their
activation in SMC (Scheme 3) and leaving scope for further
derivatization. Acid 6 f underwent copper-catalyzed iodina-
tion with N-iodosuccinimide to give 7g in 93% yield. This
transformation is notable given the paucity of general
methods reported for the iodination of electron-poor hetero-
aryl boronic acids. Copper-catalyzed Chan–Lam amination[36]
(products 7h,i) further highlights the unique versatility of the
boryl (hetero)aryne building blocks: the same nucleophile
may be introduced by using either the electrophilic (aryne) or
nucleophilic (boronate) functionality under mild conditions
with meta selectivity with respect to R (compare 4b,c and
7h,i).
To extend the utility of the boryl group for aryne-
precursor postfunctionalization, compounds 2 were depro-
To the best of our knowledge, the transformations in
Schemes 1, 3, and 5 (reactions i, iii–vi) are the first to use
ortho-silyl (hetero)aryl triflates as substrates in catalysis not
involving activation of the aryne or mediation of its reactivity
by a transition metal.[11,37] This points to broad new possibil-
Scheme 4. Deprotection of 2 to give the parent boronic acids. In each
case the corresponding boroxine was formed as a minor by-product;
yields are given with respect to boron.
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ities for incorporating aryne precursors into more complex
chemical environments.
In summary, we have developed a class of arylation
reagents able to deliver orthogonally boronate or aryne
reactivity for a wide variety of transformations. They enable
the postfunctionalization of ortho-silyl (hetero)aryl triflate
aryne precursors and significantly expand the options avail-
able for decorating aryl boronates selectively. The generality
inherent in this approach offers a flexible new route to
diversely functionalized (hetero)aromatic compounds.
Acknowledgements
We thank the Swedish Research Council (Vetenskapsrådet)
and Carl Tryggers Stiftelse for funding. We are grateful to
Prof. Adolf Gogoll for help with NMR spectroscopy and Prof.
Lars Engman, Dr. Eszter Borbas, Dr. Johanna Larsson, and
Carina Sollert for manuscript proof-reading.
À
Keywords: arynes · boron · C H activation · chemoselectivity ·
synthetic methods
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Received: April 6, 2015
Revised: July 11, 2015
Published online: August 13, 2015
Angew. Chem. Int. Ed. 2015, 54, 11765 –11769
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11769