Facile synthesis of electrophilic vinyl boranes: Reactions of alkynyl-borates and diazonium salts

Article abstract of DOI:10.1039/c2cc35553a

Reactions of alkynylborate salts, easily derived from reaction of frustrated Lewis pairs with terminal alkynes, with diazonium salts to induce 1,1-carboboration affording a facile and efficient route to substituted electrophilic vinyl boranes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc35553a

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COMMUNICATION
Facile synthesis of electrophilic vinyl boranes: reactions of
alkynyl-borates and diazonium saltsw
Xiaoxi Zhao, Liyuan Liang and Douglas W. Stephan*
Received 31st July 2012, Accepted 1st September 2012
DOI: 10.1039/c2cc35553a
Reactions of alkynylborate salts, easily derived from reaction of
frustrated Lewis pairs with terminal alkynes, with diazonium
salts to induce 1,1-carboboration affording a facile and efficient
route to substituted electrophilic vinyl boranes.
We have previously reported the rapid and facile synthesis
of alkynylborate salts of the form [R3PH][R0CRCB(C6F5)3]
from the reaction of the FLP tBu3P–B(C6F5)3 with a terminal
alkyne.28,29 In this fashion the species of [HPtBu3] [PhCR
CB(C6F5)3] 1a was prepared and subsequently reacted with an
equivalent of the diazonium salt [Cl(C6H4)N2][B(C6F5)4] 2a in
CH2Cl2, prepared by literature methods.30 This reaction was
stirred for 10 min, the solvent removed and the resulting
material was taken up in pentane. Following filtration and
solvent concentration, the new product 3 was isolated in 58%
yield as colorless crystals. NMR data revealed the formation
of two products in a ratio of approximately 19 : 1. The major
isomer of 3 gave a 11B NMR spectrum which showed a broad
singlet at 57.9 ppm, while the 1H NMR data showed arene
resonances attributable to both a phenyl ring and a ClC6H4
ring. The 19F NMR spectrum showed major and minor isomers
in a 94 : 6 ratio. The major product exhibited six resonances
attributable to two different C6F5 groups in a ratio of 2 : 1.
Peaks to ꢀ129.16, ꢀ145.80 and ꢀ160.84 were attributed to
B-bound fluoro-arene rings while the signals at ꢀ139.03,
ꢀ153.51 and ꢀ161.46 ppm were assigned to a C-bound C6F5
ring. The 13C{1H}NMR signals showed a signal at 144.66 ppm
suggesting the presence of an olefinic fragment, however
13C data for carbon atoms adjacent to B were not observable.
Collectively these data support the formulation of 3 as
(ClC6H4)(Ph)CQC(C6F5)(B(C6F5)2). Indeed this was subsequently
confirmed by X-ray crystallography (Fig. 1). Moreover these data
confirmed the major isomer of 3 to be the E-isomer. The B–C bond
length to the vinyl substituent was found to be 1.552(6) A, while the
vinylic CQC bond was 1.369(5) A.
Electrophilic boranes are important molecules that have been used
in a variety of applications that continue to draw attention.1–3
These species are perhaps best known for their utility as
alkyl-abstraction reagents and thus co-catalysts in olefin
polymerization,4–6 although they have also found broad
applications in Lewis acid catalysis. For example, Piers and
coworkers7–12 pioneered the use of electrophilic boranes in
the hydrosilylation of ketones, aldehydes, and imines. More
recently, we have demonstrated the use of electrophilic boranes
in ‘‘frustrated Lewis pair’’ (FLP) chemistry.13 FLP chemistry
enables the activation of a variety of small molecules, with the
most notable being dihydrogen.14 While the use of electrophilic
boranes continues to be of much interest, the limited synthetic
pathways and challenges of synthesizing modified electrophilic
boranes have limited abilities to explore the corresponding
structure–reactivity relationships. While metathetical routes
15
involving the replacement of the halide of (C6F5)BCl2 or
(C6F5)2BCl16 have been employed,15 such methods are plagued
by the challenging syntheses and low yields of the halide
precursors. In seeking new routes to electrophilic boranes, we
have developed the facile insertion reaction of diazomethane
derivatives into the B–C bonds of electrophilic boranes.17 In a
related sense, Erker et al.18–25 and Berke et al.26 have recently
developed 1,1-carboboration reactions of B(C6F5)3 with alkynes.2
Although in certain cases these reactions require thermolysis for
up to a week, this methodology offers a route to electrophilic
alkenyl boranes, reagents that are effective hydrogenation
catalysts for imines, and diaryl-substituted enones.27 In this
communication, we report an alternative synthetic pathway to such
alkenylboranes. Our method provides a rapid, facile, synthetically
flexible and high-yielding protocol for the preparation of
alkenylboranes.
In a similar fashion, reaction of the salt 1a with the
diazonium salt [MeO(C6H4)N2][B(C6F5)4] 2b resulted in the
formation of (MeOC6H4)(Ph)CQC(C6F5)(B(C6F5)2) 4 in 58%
yield. NMR data reveal that the two isomers of 2 are formed in
a 5 : 1 ratio. The major isomer, thought to be the E-isomer, gives
rise to a broad 11B NMR signal at 58.1 ppm while the 19F NMR
spectral data show the resonances attributable to the boron-bound
fluoro-arene rings at ꢀ130.6, ꢀ141.1 and ꢀ163.4 ppm and the
carbon-bound ring at ꢀ150.1, ꢀ157.1 and ꢀ163.9 ppm.
The corresponding reaction of 1a with [C6H5N2][BF4] 2c was
used to prepare the known species Ph2CQC(C6F5)(B(C6F5)2) 5
in 41% isolated yield. While this preparation is complete in the
course of an hour, the known thermolysis method required
reflux of diphenylacetylene and B(C6F5)3 at 110 1C for 7 days.
Department of Chemistry, University of Toronto, 80 St. George
Street, Toronto, Ontario M5S 3H6, Canada.
E-mail: dstephan@chem.utoronto.ca
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 893908–893910. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc35553a
c
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Chem. Commun., 2012, 48, 10189–10191 10189

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Fig. 1 POV-ray depiction of 3, C: black, F: pink, B: aqua, Cl: green;
H-atoms are omitted for clarity.
This approach to electrophilic vinyl boranes is conveniently
modular as variations in the alkynylborate or diazonium salt
precursors are readily achieved. For example, preparation of the
salt [tBu3PH][F3CC6H4CRCB(C6F5)3] 1b is readily achieved from
the reaction of the FLP tBu3P–B(C6F5)3 with the corresponding
terminal alkyne. A subsequent reaction with the diazonium
salt 2a affords the vinyl borane (F3CC6H4)(ClC6H4)CQ
C(C6F5)(B(C6F5)2) 6 in 78% isolated yield. The NMR data
are similar to those reported above for 3–5, with the addition
of the 19F NMR signal arising from the CF3 group at ꢀ64.3 ppm.
The NMR data were consistent with the formation of the
E and Z isomers of the product in a ratio of 25 : 1. The geometry
of the major isomer was subsequently confirmed by an X-ray
crystallographic study (Fig. 2(a)). In an analogous manner the
alkynylborate 1b reacts with 2b to provide (F3CC6H4)-
(MeOC6H4)CQC(C6F5)(B(C6F5)2) 7 in 40% isolated yield. The
E : Z isomer ratio in this case was determined to be 20 : 1. Again
the major isomer was confirmed to have the E geometry via a
crystallographic study (Fig. 2(b)).
Fig. 2 POV-ray depiction of (a) 6, (b) 7; C: black, F: pink, B: aqua,
Cl: green, O: red; H-atoms are omitted for clarity.
Mechanistically, these reactions are thought to proceed via
the interaction of the electron-deficient cation derived from the
diazonium salt and the alkyne fragment of the alkynylborate
generating a transient carbocation adjacent the borate center
(Scheme 1). This prompts migration of a C6F5 ring to the
alpha-carbon ring affording the vinyl borane. This mechanism
is consistent with the regiochemistry observed in which the
major isomer observed provides the substituent derived from
the diazonium salt cis to the B(C6F5)2 fragment.
The protocol described herein provides a new and facile
approach to the preparation of electrophilic vinyl boranes.
Moreover, this synthetic approach provides control of the
substitution pattern by judicious selection of the alkyne and
the diazonium reagent. Perhaps most importantly, this strategy
proceeds rapidly under mild conditions in contrast to established
thermal methodologies. It is noteworthy that this method is
remotely related to reactions of alkynylborate with chloro-
Scheme 1 Synthesis of 3–7.
grateful for the award of a NSERC of Canada postgraduate
scholarship.
phosphines previously described by Binger and Koster,31
Notes and references
¨
Grobe et al.32 and Balueva and coworkers.33,34 We are currently
pursuing further applications of these Lewis acidic vinyl
boranes in synthetic chemistry as well as their utility in FLP
chemistry.
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