Borenium-catalyzed hydroborations of silyl-substituted alkenes and alkynes with a readily available n-heterocyclic carbene-borane

Article abstract of DOI:10.1021/om400932g

Borenium-catalyzed hydroboration reactions of a stable, readily available N-heterocyclic carbene-borane with allyl-, alkenyl-, and alkynylsilane substrates provides either standard 1,2-hydroboration products or rearranged 1,1-hydroboration products, depending on the structure of the substrate. A competent catalyst can be generated in situ by addition of bis(trifluoromethane) sulfonimide or diiodine. In a typical 1,2-hydroboration, reaction of 1,3-dimethylimidazol-2-ylidine-borane (diMe-Imd-BH₃) with 1,2-bis(trimethylsilyl)ethene provides 1,3-dimethylimidazol-2-ylidine-(1,2- bis(trimethylsilyl)ethyl)borane (diMe-Imd-BH₂CH(TMS)CH ₂TMS)) as a stable product. In a typical 1,1-hydroboration, the reaction of diMe-Imd-BH₃ with bis(trimethylsilyl)ethyne provides 1,3-dimethylimidazol-2-ylidine-bis(2,2-bis(trimethylsilyl)ethenyl)borane (diMe-Imd-BH(CH=C(TMS)₂)₂ again as a stable product.

Full text of DOI:10.1021/om400932g

Article
pubs.acs.org/Organometallics
Borenium-Catalyzed Hydroborations of Silyl-Substituted Alkenes
and Alkynes with a Readily Available N‑Heterocyclic Carbene−
Borane
Anne Boussonniere,† Xiangcheng Pan, Steven J. Geib, and Dennis P. Curran*
̀
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
S
* Supporting Information
ABSTRACT: Borenium-catalyzed hydroboration reactions of a stable,
readily available N-heterocyclic carbene−borane with allyl-, alkenyl-, and
alkynylsilane substrates provides either standard 1,2-hydroboration
products or rearranged 1,1-hydroboration products, depending on the
structure of the substrate. A competent catalyst can be generated in situ by
addition of bis(trifluoromethane)sulfonimide or diiodine. In a typical 1,2-
hydroboration, reaction of 1,3-dimethylimidazol-2-ylidine−borane (diMe-
Imd-BH3) with 1,2-bis(trimethylsilyl)ethene provides 1,3-dimethylimida-
zol-2-ylidine−(1,2-bis(trimethylsilyl)ethyl)borane (diMe-Imd-BH2CH-
(TMS)CH2TMS)) as a stable product. In a typical 1,1-hydroboration, the reaction of diMe-Imd-BH3 with bis(trimethylsilyl)-
ethyne provides 1,3-dimethylimidazol-2-ylidine−bis(2,2-bis(trimethylsilyl)ethenyl)borane (diMe-Imd-BH(CHC(TMS)2)2
again as a stable product.
INTRODUCTION
■
N-heterocyclic carbene complexes of borane (NHC-BH3 or,
more generally, NHC−boranes) have a rich chemistry that
stems in part from their stability.1 Unlike most complexes of
borane (BH3) and neutral Lewis bases, NHC complexes of BH3
resist decomplexation even under forcing conditions. Classic
reactions of borane such as hydroboration cannot be directly
elicited. This means that NHC−boranes can be exposed to all
kinds of reagents that standard borane−Lewis base complexes
would not tolerate.1a
Spurred by the pioneering amine−borenium ion chemistry of
Vedejs,2 we and Vedejs recently showed that the prohibitive
barriers for direct hydroboration of alkenes by NHC−boranes
can be overcome by in situ generation of an NHC−borenium
3
+
ion catalyst (NHC-BH2 , Figure 1a). The borenium ion (or a
reactive equivalent2a) is a highly electrophilic species with a
potential vacant site for complexation. It can undergo rapid 1,2-
hydroboration reactions with alkenes (Figure 1a, step 1).
Turnover occurs when the resulting hydroborated borenium
ion (NHC-BH+CH2CH2R) abstracts hydride from the starting
NHC−borane to provide the neutral hydroborated product
(NHC-BH2CH2CH2R) while regenerating the catalyst (Figure
1a, step 2).
In a typical example (Figure 1b), 5−15% trifluoromethane
sulfonimide (hereafter triflimide or Tf2NH) is added to 1,3-
dimethylimidazol-2-ylidine−borane (1), followed by addition
of 3-hexene.3 Rapid dihydroboration occurs to give the primary
product, bis(3-hexylborane) 2. This evolves over a few hours,
presumably by reverse hydroboration, to provide the secondary
product, bis(2-hexylborane) 3. This product surprisingly resists
further rearrangement to the bis(1-hexylborane).
Figure 1. NHC−borenium-catalyzed hydroboration reactions.
A related procedure that involves addition of a small amount
of diiodine (I2) in place of triflimide has recently been
introduced.4 This procedure seems to be gentler, giving an
expanded range of products from more substituted and more
functionalized alkenes. The diiodine activation procedure often
selectively forms monohydroboration products when a 1/1
Received: September 30, 2013
© XXXX American Chemical Society
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ratio of NHC−borane and alkene is combined. This is an
advantage, because monoalkyl NHC−boranes tend to survive
standard chromatographic purification and are stable to storage.
In contrast, dialkyl NHC−boranes are stable for long periods in
solution but often cannot be further purified by chromatog-
raphy.5
We decided to study NHC−borenium-catalyzed hydro-
borations of silyl-substituted alkenes and alkynes because the
existing hydroboration chemistry of these compounds is so rich
and diverse. Borane and alkylboranes typically give 1,2-
hydroboration products with these species, with regioselectivity
often directed by the silicon atom.6 With alkynylsilanes, there
are rare examples of 1,1-hydroboration reactions with silicon
shifts in the carborane field.7 In contrast, 1,1-carboration
reactions with silicon shift (the Wrackmeyer reaction) are
rather common when alkylboranes are used.8 Products of
silicon migration have been observed with 9-BBN,9 and boron
migration can occur with bis(pentafluorophenyl)borane (HB-
(C6F5)2).10 Reactions of both alkenyl- and alkynylsilanes with
electrophilic trihaloboranes (BX3) often give silicon/boron
exchange products.11
We also reacted 1 first with 10% diiodine.4 This rapidly
provides 20% of the corresponding NHC−boryl iodide (NHC-
BH2I) and dihydrogen (H2).12 Addition of 2.2 equiv of
allyltrimethylsilane (4a) again provided the dihydroboration
product 5 rather cleanly in solution.
Hydroboration of 1,4-bis(trimethylsilyl)but-2-ene (4b; E/Z
mixture) was also conducted by the triflimide procedure. This
time, the monohydroboration product 6 predominated in the
11B NMR spectrum of the reaction mixture, even though 2.2
equiv of 4b was added. Monoalkylborane 6 was stable to flash
chromatography and was isolated in 59% yield and fully
characterized.
The results in Scheme 1 show similarities and differences in
comparison to the borenium-catalyzed hydroboration of simple
alkenes.3 On the one hand, the reaction of allyltrimethylsilane
resembles that of 1-hexene. The dihydroborated product 5 is
quickly formed in situ, but it is not stable to purification. On the
other hand, the results with disubstituted 4b are not so similar
to results with comparable alkenes such as 3-hexene and 4-
octene. The simple alkenes give dihydroboration products such
as 2 that are subject to further migration of boron (to give 3 in
Figure 1, for example). In contrast, 4b gives the mono-
hydroboration product 6, which is not prone to further
migration.
In short, there is no tendency for boron to migrate toward
silicon in either 1,2-hydroboration product 5 or 6. This is in
contrast with related hydroborations by the highly electrophilic
bis(pentafluorophenyl)borane ((C6H5)2BH), which show a
strong tendency for boron to migrate toward silicon as reaction
times are extended.10b
Here we show that allyl- and alkenylsilanes give products of
1,2-hydroboration in borenium-catalyzed hydroborations with
NHC−boranes. In contrast, rearranged 1,1-hydroboration
products are sometimes formed from alkynylsilanes. These
products are the first alkenyl and dialkenyl NHC−boranes, and
we find that they are rather stable. We also learn that activation
by diiodine (I2) is a valuable alternative to triflimide activation
for such substrates.
RESULTS AND DISCUSSION
Allylsilanes. The results of hydroboration reactions of two
typical allyl silanes by 1 are summarized in Scheme 1. Not
■
Alkenylsilanes. Scheme 2 summarizes the results of
borenium-catalyzed hydroborations of the three 1,2-disubsti-
tuted alkenylsilanes 7a−c. In preliminary 11B NMR experi-
ments, hydroboration of (Z)-1,2-bis(trimethylsilyl)ethene (7a)
Scheme 1. Results of Borenium-Catalyzed Hydroborations
of Allylsilanes
Scheme 2. Results of Borenium-Catalyzed Hydroborations
of Alkenylsilanes
surprisingly, nothing happened when 1 and allyltrimethylsilane
(4a) were mixed without a catalyst. In the catalytic method,
20% triflimide was first added to 1 to generate the catalyst, and
then 2.2 equiv of allylsilane 4a was added. Rapid dihydrobora-
tion occurred to give the major product 5, which was
1
characterized in situ by H, 11B, and 13C NMR spectroscopy.
While stable in solution, this product did not survive automated
flash chromatography.
B
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with 1 succeeded well with both the triflimide and the iodine
activation procedures. In both cases, the monohydroboration
product 8 was formed even though the alkene 7a was used in
excess (3 equiv in NMR experiments). In a preparative
experiment by the triflimide procedure, the hydroboration
product 8 was isolated in 39% yield after flash chromatography.
With (Z)-1-(trimethylsilyl)-1-pentene (7b) and (Z)-1-
(trimethylsilyl)-2-phenylethene (7c), the iodine procedure
gave cleaner reaction products in NMR experiments; therefore,
this was used for the preparative experiments. Both alkenes
again gave only monohydroboration products, but now two
regioisomers were formed. The reaction with 7b produced an
inseparable 83/17 mixture of α-borylsilane 9 and β-borylsilane
10. The structure of the major isomer 9 was confirmed by
oxidation of the crude product (H2O2, NaOH, MeOH) to give
1-(trimethylsilyl)-1-hexanol as the main product. The hydro-
boration reaction of 1 with 7c was unselective, giving the two
analogous regioisomers 11 and 12 in about a 50/50 ratio in
72% isolated yield.
Alkynylsilanes. Three of the alkynylsilanes selected for
study, 13a−c, have the same substituents as the alkenylsilanes
7a−c to facilitate comparison, while the fourth (13a′) is an
analogue of bis(trimethylsilyl)ethyne (13a) with two different
silyl groups. The results with the two bis(silyl)ethynes 13a and
13a′ are summarized in Scheme 3. Hydroboration of 13a (3
equiv) with 1 by the triflimide procedure gave rapid conversion
not to a monohydroboration product (as with alkenylsilane 7a)
but to a dihydroboration product. Flash chromatography of the
crude product provided pure 14 in 44% yield as a white solid,
mp 84−86 °C. This is the first alkenyl NHC−borane to be
isolated. Unlike many dialkyl NHC−boranes (see 5, for
example), this dialkenyl NHC−borane survives chromatog-
raphy and is stable to storage.
The spectra of 14 seemed at first glance consistent with the
standard 1,2-hydroboration product 15, but there was a glaring
problem. The alkenyl proton Ha resonated as a doublet at 7.93
1
ppm in the H NMR spectrum with a coupling constant (J) of
1
6.4 Hz. We recorded an 11B-decoupled H NMR spectrum of
14 to show that this was the coupling of the remaining proton
on boron (BH) to the alkenyl proton Ha. In this spectrum, the
proton on boron is a triplet, J = 6.4 Hz, at 3.08 ppm. On the
basis of the magnitude of the coupling constant, the C−Ha
bond and the B−H bond must be vicinal. Therefore, the
product is 14, the result of 1,1-hydroboration with silyl
migration, not 15, the result of standard 1,2-hydroboration.
To solidify this structure assignment, we prepared an
authentic sample of the 1,2-hydroboration product 15 by first
hydroboration of 13a with BH3·THF (2 equiv).13 After 1 h, the
resulting solution of dialkenylborane 16 was added to a solution
of 1,3-dimethylimidazol-2-ylidene (17). The resulting dialkenyl
NHC−borane 15 is a constitutional isomer of 14 that was again
stable to flash chromatography and was isolated as a while solid
in 34% yield, mp 68−70 °C. Now the lone alkenyl proton Hb is
not coupled to the H on boron (J < 1 Hz) and resonates at 6.49
ppm in the 1H NMR spectrum. There was no trace of
resonance of 14 in the 11B NMR spectrum of product 15 from
the borenium-catalyzed hydroboration of 1.
Adding in the step of complexation of 17 and BH3 to make
NHC−borane 1,14 the sequence of complexation followed by
borenium-catalyzed hydroboration gives the rearranged product
14. In contrast, the sequence of direct hydroboration to make
16 followed by complexation gives the standard product 15.
In a separate experiment, we added 20% triflimide to a
sample of pure 15. Although some decomposition occurred,15
much of 15 remained unreacted and no resonance for 14
appeared in the 11B NMR spectrum. These results suggest that
the borenium-catalyzed hydroboration of 13a by 1 does not
initially form the standard 1,2-hydroboration product 15, which
later rearranges. Instead, that reaction directly provides
rearranged product 14.
Scheme 3. Reactions of Bis(silyl)ethynes 13a,a′ Giving
Normal (15) or Rearranged (14 and 18) Products
Depending on the Order of Complexation and
Hydroboration Steps
Eventually we secured crystals of 1,1-hydroboration product
14 that were amenable to X-ray crystallography, and the
resulting structure is shown in Figure S1 (Supporting
Information). Likewise, we solved the X-ray structure of the
1,2-adduct 15, and this is shown in Figure S2 (Supporting
Information).16
To probe the stereochemistry of the silyl migration, we
hydroborated (triisopropylsilyl)(trimethylsilyl)ethylene with
13a′ by the triflimide procedure (Scheme 3). This time only
monohydroboration occurred, presumably due to the bulky
triisopropylsilyl (TIPS) group. The product 18 again is a formal
1,1-hydroboration product resulting from a silyl shift. It is also a
single stereoisomer, though the assignment was difficult. The
2D NOE spectrum of 18 was notable for its absence of cross
peaks. However, in a 1D experiment, irradiation of the alkenyl
proton Ha at 7.88 ppm produced no enhancement of the signal
for methyl protons on the trimethylsilyl group, but there was a
3% enhancement of the signal of the methyne protons of the
triisopropylsilyl group. This suggests that 18 has the TMS
group trans to the NHC−boryl group and the TIPS group cis.
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Results in the hydroborations of 1-(trimethylsilyl)-1-pentyne
(13b) and (trimethylsilyl)phenylethyne (13c) are summarized
in Scheme 4. Reactions of 13b and 1 by both the triflimide and
(Ha, 6.78 ppm in 20) in comparison to when it is vicinal (Hb,
5.86 ppm in 19 and 5.99 ppm in 20). The structure of 19 was
also confirmed by oxidation, which occurred with some
difficulty (19 is rather stable; see the Supporting Information)
but finally produced hexanoic acid. The Z geometries of the
alkenes in 20 and 21 are tentative and were inferred from the
structure of 18.
Scheme 4. Results of Borenium-Catalyzed Hydroborations
of Silylalkynes 13b,c
Hydroboration of 13c by 1 worked best by the iodine
activation method, providing the major monohydroboration
product 22, which was assigned as resulting from 1,2-
hydroboration with the boron atom located geminal to the
silyl group. The product was isolated by flash chromatography
in 34% yield, though it was only about 90% pure by 11B NMR
spectroscopy. The sample was further purified by crystalliza-
tion, and the structure of 22 was confirmed by solving the X-ray
crystal structure of this sample (see the Supporting
Information, Figure S3). The minor contaminant was also a
monoadduct (triplet in the 11B NMR spectrum), but it is not
clear whether this is the regioisomer of 22 (1,2-hydroboration
in the other direction) or a silyl migration product (1,1-
hydroboration).
The results show that the bis-silyl alkynes are highly prone to
silyl migration to give 1,1,-hydroboration products (Scheme 3),
while the monosilyl alkynes are less so, giving predominately
1,2-hydroboration products (Scheme 4). All of the alkenes,
whether allylsilanes or alkenylsilanes, gave 1,2-hydroboration
products (Schemes 1 and 2).
Figure 2 shows two possible pathways for formation of the
1,1-hydroboration product 18, which was isolated from the
the iodine activation procedures were followed by 11B NMR
spectroscopy. The triflimide procedure gave a mixture of three
products, the bis-1,2-hydroboration product 19 (71%), the bis-
1,1-hydroboration product 21 resulting from double migration
(8%), and the bis-hydroboration product 20 (21%), in which
one of the alkenyl groups has a migrated silyl group and the
other does not. No monohydroboration products were
detected by 11B NMR spectroscopy, and the ratio of products
19/20/21 did not evolve after the hydroboration reaction was
complete. By the diiodine activation procedure, the same three
products were formed more slowly but with somewhat better
selectivity for the bis-1,2-hydroboration product: 19, 86%; 20,
8%; 21, 6%.
Flash chromatographic purification of the crude product
from the triflimide reaction provided pure samples of the two
major products; 19 was obtained in 43% yield and 20 in 10%
yield. (The minor product 21 was not isolated; therefore, its
structure is tentative.) The structures of the two purified
Figure 2. Two plausible pathways for the 1,1-hydroboration reaction
of the unsymmetrical bis-silyl-ethyne 13a′.
1
products follow from both 11B NMR and H NMR spectros-
copy. In the 11B spectra, the boron atom in the 1,2-product 20
resonated upfield in comparison to the 1,1-product 19 (the
unsymmetrical silane 13a′ bearing TMS and TIPS groups. We
assume in both paths that the NHC−boryl group attaches itself
to the carbon bearing the smaller TMS group. Both paths also
avoid the intermediacy of the standard 1,2-hydroboration
products (such as 15), because the results in Scheme 3 suggest
that they do not rearrange to 1,1-products.
1
same trend is seen with 14 and 15 in Scheme 3). In the H
NMR spectra, 19 has two equivalent alkenyl protons (Hb)
while 20 has two different protons (Ha and Hb). The alkenyl
proton appears further downfield when it is geminal to boron
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In the “1,2-silyl shift” pathway (Figure 2a), the borenium ion
NHC-BH2 (or its reactive equivalent) is a strong electrophile
NHC group on silica gel, but dialkenylboranes are not. This
might be an indication that alkenylboranes are somewhat
stronger Lewis acids than the comparable alkylboranes;
therefore, they hold onto the NHC Lewis base more tightly.19
In the big picture, the results show that borenium ion
catalyzed hydroborations exhibit a number of attractive
features, including ease of reaction and workup. In addition,
reagent 1 is a stable solid that is easy to make, store, and handle.
However, the unique feature in comparison to most other
hydroboration methods is that many of the primary boron-
containing products are not especially reactive because the
NHC group remains complexed to the boron atom. This offers
a number of options, including direct isolation and storage of
the products.
+
that adds to the alkyne to form vinyl cation 23, which suffers a
standard 1,2-shift of the silyl group17 to give α-NHC-boryl vinyl
cation 24. This cation may abstract a hydride from NHC-BH3
to provide the product 18 directly. Alternatively, a 1,2-hydride
shift from boron to carbon could occur to form a rearranged
borenium ion (not shown, but this is analogous to the reaction
of 26 below), which in turn abstracts hydride from NHC-BH3
to continue the cycle. It is not immediately clear what the
stereochemistry-determining step in this mechanism is; thus,
accounting for the selective formation of 18 as the E isomer
(TIPS and NHC-boryl trans) is not straightforward. In
addition, if a pathway such as this was being followed, then
silylalkenes 7a−c might also be expected to suffer analogous
silyl shifts, but they do not (Scheme 2).
In the “boron−silicon exchange” pathway (Figure 2b), the
NHC−borenium ion exchanges with the silicon of the TMS
group to provide 25, which can be viewed as a silylenium ion
complex of an alkyne. Now a 1,2-shift of a hydride trans to the
TMS group provides the borenium ion 26, which abstracts
hydride from 1 to continue the cycle. Alternatively, 25 may
abstract hydride direct from NHC-BH3, again in a trans fashion,
to give product 18 and turn over the cycle.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and a CIF file giving complete
■
S
1
experimental details and compound characterization data, H,
13C, and 11B NMR spectra of hydroboration products, and a
crystal structure and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.P.C.: curran@pitt.edu.
■
CONCLUSIONS
■
In summary, the results herein with allyl-, alkenyl-, and
alkynylsilanes continue to solidify the generality of borenium-
catalyzed hydroborations of functionalized compounds. The
original triflimide-activation procedure3 is convenient because
reactions are faster, but the newer procedure with the less
expense diiodine activator4 seems more general because it gave
cleaner crude reaction products in a number of the examples in
Schemes 2−4.
Present Address
†Institut des Molec
́
ules et Mater
́
iaux du Mans (IMMM)-UMR
6283, Universite
́
du Maine, Le Mans, France.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Science Foundation for funding this
work. We thank Professor Edwin Vedejs, University of
■
These hydroborations continue to be unusual in that one
typically obtains selectively either the mono- or the
dihydroboration product. To date, the two are never mixed,
even when the stoichiometry is chosen to favor one or the
other. In early work,3 it looked as if mono- and disubstituted
alkenes favored dihydroboration, while tri- and tetrasubstituted
alkenes favored monohydroboration. However, as the studies
have expanded,4 it now looks like many disubstituted alkenes
(such as the allyl- and alkenylsilanes in this work) also favor
monohydroboration. Simple dialkyl-substituted alkenes are the
exceptions that favor dihydroboration, provided that the alkyl
groups are small. We suspect that this selectivity results from a
direct competition of the initial hydroborated borenium ion,
which can undergo either a second hydroboration (dihydrobo-
rated product results) or hydride transfer (monohydroborated
product results).
In the TMS−alkyne series, bis-hydroboration products are
produced when the second alkyne substituent is another TMS
group or a small alkyl group (Bu), while monohydroboration
occurs when the alkyne has a phenyl group or a bulky TIPS
group. The reactions with the bis-silylalkynes are rare examples
of formal 1,1-hydroboration reactions that occur by silyl shifts.
The alkenyl NHC−boranes isolated in this work are the first
examples of such compounds. Significantly, they are more
robust than the analogous dialkylboranes. All of the major
dialkenylboranes in this work survived chromatography and are
stable products,18 whereas most dialkyl NHC−boranes do not
survive chromatography.5 We suspect that this means that
dialkyl NHC−boranes are subject to decomplexation of the
̂ ́
Michigan, and Dr. Emmanuel Lacote, CNRS, Universite de
Lyon, for helpful discussions.
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dx.doi.org/10.1021/om400932g | Organometallics XXXX, XXX, XXX−XXX