Hydroboration of arynes with N-heterocyclic carbene boranes

Article abstract of DOI:10.1002/anie.201408345

Arynes were generated in situ from ortho-silyl aryl triflates and fluoride ions in the presence of stable N-heterocyclic carbene boranes (NHC-BH₃). Spontaneous hydroboration ensued to provide stable B-aryl-substituted NHC-boranes (NHC-BH₂Ar). The reaction shows good scope in terms of both the NHC-borane and aryne components and provides direct access to mono-and disubstituted NHC-boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron-withdrawing group supports a hydroboration process with hydride-transfer character.

Full text of DOI:10.1002/anie.201408345

Angewandte
Chemie
DOI: 10.1002/anie.201408345
Hydroboration
Hot Paper
Hydroboration of Arynes with N-Heterocyclic Carbene Boranes**
Tsuyoshi Taniguchi* and Dennis P. Curran*
Abstract: Arynes were generated in situ from ortho-silyl aryl
triflates and fluoride ions in the presence of stable N-hetero-
À
cyclic carbene boranes (NHC BH3). Spontaneous hydrobo-
ration ensued to provide stable B-aryl-substituted NHC-
À
boranes (NHC BH2Ar). The reaction shows good scope in
terms of both the NHC-borane and aryne components and
provides direct access to mono- and disubstituted NHC-
boranes. The formation of unusual ortho regioisomers in the
hydroboration of arynes with an electron-withdrawing group
supports a hydroboration process with hydride-transfer char-
acter.
H
ydroboration is one of the bedrock reactions of alkenes
Scheme 1. Approaches to the hydroboration of benzyne and other
arynes. a) In the classical approach, the borane reagents and products
may not be compatible with the reagents and conditions needed to
generate benzyne. b) In the present approach, NHC-boranes are stable
to aryne formation yet still spontaneously hydroborate the arynes to
provide stable products.
and alkynes.[1] It often occurs rapidly even in nonpolar
settings. The primary hydroboration products are rarely
isolated but can be converted in situ into many kinds of
compounds.[2] Hydroboration reactions become increasingly
rapid as p bonds become more strained.
Given this background, it might seem surprising that there
are no methods for the hydroboration of benzyne and related
arynes (Scheme 1a). The hydroboration products of such
highly reactive species, phenyl- or aryl-substituted boranes,
have many potential uses.[2] The problem is that benzyne is
generated in situ,[3] but that benzyne-forming reagents and
reactive boranes are likely to be mutually incompatible. Even
if a product of benzyne hydroboration manages to form, it will
again be a reactive borane with the same survival problems as
the starting borane.
NHC-boranes would be stable to some of the common
reagents and conditions used to make benzynes in situ.[3] At
first glance, the hydroboration of benzynes does not look
promising because NHC-boranes do not hydroborate alkenes
and alkynes, even at high temperatures.[5] However, we felt
that spontaneous hydroboration might occur, because ben-
zynes often react as electrophiles[3] and because NHC-
boranes sometimes react as weak nucleophiles (that is,
hydride donors).[6] NHC-boranes are neutral, whereas the
nucleophiles used in reactions of benzyne are usually anions
or organometallic species. Along these lines, two attractive
new boryl metalation reactions of arynes provide aryl–boron
products in which the boron center has the same oxidation
state as in boronic acids.[7,8]
N-Heterocyclic carbene boranes (NHC-boranes) are
a class of ligated boranes with attractive features.[4] NHC
À
complexes of the parent borane (NHC BH3) are easy to
handle because they are stable to air, water, base, and even
mild acid. They show diverse reactivity[4] that is unencum-
bered by the release of a reactive borane. We suspected that
Herein we report that benzyne can be generated in the
presence of typical NHC-boranes, and that spontaneous
hydroboration ensues (Scheme 1b). The products of the
hydroboration, B-aryl NHC-boranes, are a little-known class
of molecules that have previously been prepared indirectly in
three steps from boronic acids.[9] Like the parent NHC-
boranes, these B-aryl NHC-boranes are stable compounds
that are readily purified by flash chromatography.
[*] Dr. T. Taniguchi
School of Pharmaceutical Sciences
Institute of Medical, Pharmaceutical and Health Sciences
Kanazawa University
Kakuma-machi, Kanazawa 920-1192 (Japan)
E-mail: tsuyoshi@p.kanazawa-u.ac.jp
Prof. Dr. D. P. Curran
Department of Chemistry, University of Pittsburgh
Pittsburgh, PA 15260 (USA)
E-mail: curran@pitt.edu
We initially conducted three scouting experiments for the
generation of benzyne in the presence of readily available 1,3-
dimethylimidazol-2-ylidene borane (diMe-Imd-BH3, 1):
1) metal–halogen exchange of 1,2-dibromobenzene with
nBuLi,[3] 2) thermolysis of benzenediazonium-2-carboxyl-
ate,[3] and 3) the exposure of 2-(trimethylsilyl)phenyl
trifluoromethanesulfonate (2) to fluoride ions.[3,10] Only the
third experiment provided the target product according to
11B NMR spectroscopic analysis, so we focused efforts on the
fluoride-mediated approach.
[**] We thank Swapnil Nerkar (University of Pittsburgh) for preparing
several NHC-borane starting materials and for the oxidation of 3.
This research was financially supported by the Ministry of
Education, Culture, Sports, Science and Technology of Japan
(MEXT). D.P.C. thanks the US National Science Foundation for
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408345.
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Table 2: Reactions of 2 with various boranes and fluoride sources.[a]
In a typical early 11B NMR spectroscopic experiment,
a mixture of 1 (1.0 equiv) and 2 (1.0 equiv) in THF was
treated with tetra-n-butylammonium fluoride (TBAF,
1.2 equiv) at 08C. After 1.5 h, the conversion of 1 plateaued
at about 50%. We assigned the three new peaks in the
spectrum to the phenyl, diphenyl, and triphenyl NHC-
boranes 3, 4, and 5 (d = À25.4 (t), À15.9 (d), and À9.6 ppm
(br s), respectively). The ratio of the products was about
83:15:2; Table 1, entry 1). The NHC-borane 1 was mostly
consumed when the amount of 2 was increased to 2.0 or
3.0 equivalents, and there was a corresponding increase in the
amount of diphenylborane 4 and triphenylborane 5 formed
(Table 1, entries 2 and 3). Apparently, both the NHC-borane
1 and its derived products 3 and 4 are capable of the
hydroboration of benzyne.
Entry
Borane
Conditions
Product (yield [%][b])
1
2
3
4
5
6
7
1
1
1
A
B
C
A
B
A
A
3 (58), 4 (5), 1 (50)
3 (61), 4 (4), 1 (60)
3 (60), 4 (4), 1 (52)
À
À
Me3N BH3
Me3N BH3
C5H5N BH3
Ph3P BH3
Me3N BH2Ph (32)
À
À
Me3N BH2Ph (38)
[c]
À
–
[c]
À
–
[a] Reaction conditions: borane (0.40 mmol), 2 (0.20 mmol), TBAF
(0.24 mmol) or KF (0.40 mmol)/[18]crown-6 (0.40 mmol) or CsF
(0.40 mmol), THF or MeCN (2 mL), 08C or room temperature, 1.5–20 h.
[b] Yield of the isolated product. [c] Starting material 2 was consumed,
but no target product was detected by 1H and 11B NMR spectroscopic
analysis of the crude material.
Table 1: Preliminary reactions between 1, 2, and TBAF.[a]
À
C-6. No diphenyl-substituted borane (Me3N BHPh2) was
isolated, but no starting material was recovered either
(Table 2, entries 4 and 5). In contrast, neither pyridine–
À
borane (C5H5N BH3) nor triphenylphosphine–borane
À
(Ph3P BH3) afforded resonances for the corresponding
phenylboranes in the 11B NMR spectra of the crude products
(Table 2, entries 6 and 7).
Entry
2 [equiv]
3/4/5[b]
Conversion [%][b]
1
2
3
1.0
2.0
3.0
83:15:2
60:28:12
45:26:29
50
85
95
We also tested reactions of 2 with reactive boranes not
À
shown in Table 2, including dimethylsulfide–borane (Me2S
BH3) and tetrahydrofuran–borane. In these experiments, the
benzyne precursor 2 remained largely intact, presumably
because the borane reacted directly with the fluoride source.
Pinacolborane (PinBH), catecholborane (CatBH), and 9-
borabicyclo[3.3.1]nonane (9-BBN) did not afford the hydro-
boration product for the same reason. On the basis of this
brief survey, we focused on the scope of the hydroboration of
arynes by NHC-boranes.
[a] Conditions: 1 (0.20 mmol), 2 (0.20–0.60 mmol), TBAF (0.24–
0.72 mmol), THF (2 mL), 08C, 1.5 h. [b] The product ratio was estimated
by 11B NMR spectroscopic analysis of the crude product. Tf=trifluoro-
methanesulfonyl, TMS=trimethylsilyl.
On the basis of these observations, we changed the
limiting reagent for preparative experiments to the benzyne
precursor 2. Reactions were conducted with NHC-borane
1 (2.0 equiv) and three different fluoride sources (Table 2,
entries 1–3). The 11B NMR spectrum of the crude product
from the reaction with TBAF (Table 2, entry 1) showed
primarily phenylborane 3 with a small resonance of diphe-
nylborane 4 (3/4 92:8); no triphenylborane 5 was detected.
Purification of this product by flash chromatography provided
pure 3 (58%) and 4 (5%) along with a substantial amount of
recovered 1 (50%, based on starting 1). Similar reactions
conducted with potassium fluoride/[18]crown-6 (KF/18-C-6)
and CsF gave comparable product ratios and yields (Table 2,
entries 2 and 3). Thus, the use of 2.0 equivalents of 1 is
convenient because it results in less diphenyl- and no
triphenyl-substituted product, and because unreacted 1 can
be readily recovered.
The structures and yields of products formed in reactions
of 1 (2.0 equiv) with various aryne precursors 6–16 (1.0 equiv)
are summarized in Scheme 2. In all cases, 11B NMR spectro-
scopic analysis of the crude product showed that the ratio of
the monophenylborane to the diphenylborane product
exceeded 90:10. The target monophenyl products 17–26
were isolated by flash chromatography along with near-
theoretical amounts of recovered 1 (see the Supporting
Information for full details). The minor diphenylborane
product was usually but not always fully separable from the
major product.[11]
The treatment of 1 and 3-methoxy-2-(trimethylsilyl)-
phenyl trifluoromethanesulfonate (6) with TBAF (condi-
tions A) gave 2-methoxyphenylborane 17 as a single regio-
isomer with the two aryl substituents in an ortho relationship
in 48% yield. A similar reaction with KF/18-C-6 (condi-
tions B) gave 17 in higher yield (70%), so we favored these
conditions for further experiments. The reaction of 2-bromo-
6-(trimethylsilyl)phenyl trifluoromethanesulfonate (7) also
provided 2-bromophenylborane 18 as the ortho regioisomer
We next tested the reactivity of several other classes of
stable borane complexes under these standard reaction
conditions (Table 2, entries 4–7). Trimethylamine-borane
was converted into the corresponding phenylborane Me3N
BH2Ph in 32% yield with TBAFand in 38% yield with KF/18-
À
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fluoromethanesulfonate (14), was the only substrate that
failed to provide a hydroboration product in this survey. In
contrast, the 3,4-pyridyne precursors 15 and 16 reacted with
1 to provide the corresponding pyridylborane compounds 25
and 26. The pyridylborane isomers 25 (C4/C3 85:15) were
formed in about 55% yield according to 11B NMR spectro-
scopic analysis, but were difficult to separate (see the
Supporting Information). Methoxypyridine 26 was obtained
as the ortho isomer in 69% yield. In analogy with the reaction
of 6, no meta isomer was observed.
Next we examined the scope of the reaction with respect
to the NHC-borane component with 2-(trimethylsilyl)phenyl
trifluoromethanesulfonate (2) as the benzyne precursor, KF/
18-C-6 as the fluoride source (conditions B), and five NHCs
of varying properties[13] on the borane. The structures and
yields of the products are shown in Scheme 3. Reactions of
Scheme 3. Synthesis of several NHC-phenylboranes. Conditions B
(Table 2) were employed unless otherwise noted. [a] TBAF (1.2 equiv)
was used instead of KF/18-C-6.
Scheme 2. Products of hydroboration reactions with NHC-borane 1.
The yields given are for the isolated product. Conditions B (Table 2)
were employed unless otherwise noted. The shown numbering of 6–13
was adopted for convenience. [a] Conditions A (Table 2) were used.
[b] The yield was estimated by 11B NMR spectroscopic analysis of the
crude material. TES=triethylsilyl.
dimethyltriazolylidene, tetramethylimidazolylidene, dime-
thylbenzimidazolylidene, and diisopropylimidazolylidene
borane were similar to that of dimethylimidazolylidene
borane 1 and gave the corresponding NHC-phenylborane
derivatives 27–30 in 58–68% yield. In these cases, 11B NMR
spectroscopic analysis of the crude product showed that the
ratio of the monophenylborane to the diphenylborane
product exceeded 90:10 (see the Supporting Information for
full details). The reaction of Dipp-protected imidazolylidene
borane (Dipp is 2,6-diisopropylphenyl) with 1 and KF/18-C-6
gave 31 in 38% yield, whereas a similar reaction with TBAF
provided 31 in 39% yield. With this bulky NHC-borane, the
ratio of monophenylborane to diphenylborane products in
the crude reaction product was excellent (ca. 96:4); however,
the reaction was not as clean as in the other examples.[11]
We next conducted two reactions of boron-substituted
NHC-boranes to make differentially disubstituted boranes
(Scheme 4). The treatment of phenylborane 3 with aryne
precursor 6 and KF/18-C-6 (Scheme 4) provided unsymmet-
rical phenyl aryl borane 32 in 55% yield as a single ortho
regioisomer. Furthermore, the ethyl 2-(phenylboryl)acetate
NHC complex 34 was obtained from ethyl borylacetate 33 in
30% yield. In turn, 33 was prepared in one step by a rhodium-
catalyzed insertion reaction between NHC-borane 1 and ethyl
2-diazoacetate (N2CHCO2Et).[14] Thus, the two-step synthesis
of 34 from 1 involves a (formal) 1,1-hydroboration of
in 45% yield, whereas that of 2-methyl-6-(trimethylsilyl)-
phenyl trifluoromethanesulfonate (8) gave an inseparable
mixture of two regioisomers of methylphenylborane 19 (meta/
ortho 70:30) in 72% yield.
4,5-Disubstituted borane products 20–22 were obtained
from the corresponding aryne precursors 9–11 with dime-
thoxy, difluoro, and dimethyl substituents in roughly com-
parable yield (50–62%). The precursors 1-(trimethylsilyl)-2-
naphthyl trifluoromethanesulfonate (13) and 3-(trimethyl-
silyl)-2-naphthyl trifluoromethanesulfonate (12) were
expected to give regioisomeric 1- and 2-naphthynes. The 2-
naphthyne precursor 12 provided 2-naphthylborane 23 in
57% yield, whereas the 1-naphthyne precursor 13 provided
an inseparable mixture of 1-naphthylborane 24 and 2-
naphthylborane 23 in good yield (79%) but with low
selectivity (55:45).
We extended this transformation to heterocycles and
examined the reactions of three pyridyne[12] precursors. The
precursor of 2,3-pyridyne, 3-(trimethylsilyl)pyridin-2-yl tri-
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a budding class of neutral nucleophiles that add spontane-
ously to arynes.
Received: August 18, 2014
Published online: && &&, &&&&
Keywords: arynes · boranes · hydroboration ·
.
main-group chemistry · N-heterocyclic carbenes
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H. Bonin, E. Lacꢀte, D. P. Curran, E. Vedejs, J. Am. Chem. Soc.
2012, 134, 12281 – 12288; b) X. Pan, A. Boussonniꢁre, D. P.
Curran, J. Am. Chem. Soc. 2013, 135, 14433 – 14437; c) A.
Boussonniꢁre, X. Pan, S. J. Geib, D. P. Curran, Organometallics
2013, 32, 7445 – 7450.
[6] a) Q. Chu, M. Makhlouf Brahmi, A. Solovyev, S.-H. Ueng, D. P.
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Scheme 4. Synthesis of two unsymmetrically disubstituted aryl boron
compounds.
a carbene, followed by a 1,2-hydroboration of benzyne. These
results suggest that the aryne hydroboration, either alone or
coupled with other functionalization reactions of NHC-
boranes, will provide unusually direct routes to diverse aryl-
substituted NHC-boranes.
We consider that nucleophilic addition to benzyne may be
occurring in these reactions, and that the nucleophile is
a “hydride” of the NHC-borane.[6] Most nucleophilic addition
reactions to benzynes bearing electron-withdrawing groups
involve anions and provide meta-substituted products.[3] In
contrast, this hydroboration reaction with neutral NHC-
boranes provides ortho-substituted products because the
hydride is the nucleophile (see Scheme 2, reactions of 6, 7,
and 16, and Scheme 4, first example). Tin hydrides also add
spontaneously to benzyne and provide regioselectivity con-
sistent with a hydride-addition model.[15,16] Still, it is not
prudent to settle on a mechanistic model based primarily on
results of preparative experiments. Radical chain mechanisms
for hydroboration deserve consideration, as do more sophis-
ticated models for addition reactions to arynes.
The ease of synthesis and stability of B-aryl NHC-boranes
set the stage for downstream applications in both boron
chemistry and organic synthesis. In the latter vein, the ready
oxidation of B-unsubstituted NHC-boranes to boric acid is
already known.[17] To validate this reaction with B-aryl NHC-
boranes, we treated the B-phenyl-substituted NHC-borane 3
with dibromine (1.0 equiv) and water. As expected, phenyl-
boronic acid (PhB(OH)2) was quickly and cleanly produced.
Thus, a preliminary bridge to Suzuki-type chemistry[18] is
already in place.
[7] For the transition-metal-catalyzed diborylation of arynes, see:
a) H. Yoshida, K. Okada, S. Kawashima, K. Tanino, J. Ohshita,
Chem. Commun. 2010, 46, 1763 – 1765; b) H. Yoshida, S.
Kawashima, Y. Takemoto, K. Okada, J. Ohshita, K. Takaki,
Angew. Chem. Int. Ed. 2012, 51, 235 – 238; Angew. Chem. 2012,
124, 239 – 242.
[8] For the borylzincation of arynes, see: Y. Nagashima, R. Takita,
K. Yoshida, K. Hirano, M. Uchiyama, J. Am. Chem. Soc. 2013,
135, 18730 – 18733.
In conclusion, we have reported the first direct hydro-
boration reaction of arynes. The generation of arynes by the
treatment of ortho-silyl aryl triflates with fluoride in the
À
presence of stable NHC-boranes led to B aryl NHC-boranes.
The stability of the NHC-borane precursors and products is
crucial to the success of the hydroboration. From the stand-
point of boron chemistry, the reaction shows good scope and
directly provides products that were previously only available
by multistep routes. From the standpoint of aryne chemistry,
the unusual formation of ortho products from arynes bearing
electron-withdrawing groups is valuable. Furthermore, rec-
ognition of the relationship between NHC-boranes and tin
hydrides suggests that they may be the first members of
[9] a) M. Makhouf Brahmi, J. Monot, M. Desage-El Murr, D. P.
Curran, L. Fensterbank, E. Lacꢀte, M. Malacria, J. Org. Chem.
À
2010, 75, 6983 – 6985; b) two products of the type NHC BH2Ar
have also been synthesized by the addition of a boryl anion to
two electron-poor arenes: J. Monot, A. Solovyev, H. Bonin-
Dubarle, ꢂ. Derat, D. P. Curran, M. Robert, L. Fensterbank, M.
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[10] Y. Himeshima, T. Sonoda, H. Kobayashi, Chem. Lett. 1983,
1211 – 1214.
4
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[11] In three cases (21, 23, and 31), it was difficult to completely
separate the diphenylborane product from the monophenylbor-
ane product because of similar retention factors. The reported
yields are corrected for small amounts (< 10%) of the insepa-
rable minor product (see the Supporting Information for
details).
[12] A. E. Goetz, N. K. Garg, Nat. Chem. 2013, 5, 54 – 60, and
references therein.
[13] K. Nagura, S. Saito, R. Frçhlich, F. Glorius, S. Yamaguchi,
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[16] In previous studies, the hydrostannylation of 4-methoxybenzyne
showed moderate meta selectivity.[15] The present hydroboration
showed a similar tendency (see the Supporting Information for
full details).
[17] A. Solovyev, Q. Chu, S. J. Geib, L. Fensterbank, M. Malacria, E.
Lacꢀte, D. P. Curran, J. Am. Chem. Soc. 2010, 132, 15072 – 15080.
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[14] X. Li, D. P. Curran, J. Am. Chem. Soc. 2013, 135, 12076 – 12081.
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Communications
Hydroboration
T. Taniguchi,*
D. P. Curran*
&&&& — &&&&
Hydroboration of Arynes with N-
Heterocyclic Carbene Boranes
A new partnership with a future: N-
Heterocyclic carbene (NHC) boranes
coupled with arynes in a hydroboration
reaction to produce various B-aryl-sub-
stituted NHC-boranes (see scheme),
which are stable and have many potential
uses. The proposed hydride-transfer
character of the mechanism was sup-
ported by the ortho regioselectivity
observed for the hydroboration of arynes
with an electron-withdrawing substituent.
6
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