The carboboration of Me3Si-substituted alkynes and allenes with boranes and borocations

Article abstract of DOI:10.1039/c5dt03003j

The 1,1-carboboration of 1-Me₃Si-1-alkynes is the dominant reaction observed using [PhBCl(2-DMAP)][AlCl₄], 1, and PhBCl₂ electrophiles, with highly substituted vinyl pinacol boronate esters isolated post esterification. Other aryl and heteroaryl congeners of both 1 and PhBCl₂ have a limited scope in the 1,1-carboboration of 1-Me₃Si-1-alkynes, with desilylboration more prevalent. PhBCl₂ converts Me₃Si-substituted allenes to allylboranes via a formal 1,3-carboboration with Me₃Si-migration. [Cl₂B(2-DMAP)][AlCl₄] reacts with a number of 1-Me₃Si-1-alkynes by desilylboration, whilst with Me₃Si-ethyne a 1,1-boroamination reaction proceeds, which with excess boron electrophile is followed by an intermolecular desilylboration to form a tricationic-borate. The use of excess 1-Me₃Si-1-propyne relative to 1 (and a thienyl congener of 1) formed 2-boradienes in low yields from the reaction with two equivalents of alkyne. Vinyl borocations ligated by 2,6-lutidine of the general formula, [(vinyl)BCl(2,6-lutidine)][AlCl₄] formed 1-boradienes with 1-Me₃Si-1-alkynes.

Full text of DOI:10.1039/c5dt03003j

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The carboboration of Me3Si-substituted alkynes
and allenes with boranes and borocations†
Cite this: DOI: 10.1039/c5dt03003j
James R. Lawson, Valerio Fasano, Jessica Cid, Inigo Vitorica-Yrezabal and
Michael J. Ingleson*
The 1,1-carboboration of 1-Me3Si-1-alkynes is the dominant reaction observed using [PhBCl(2-DMAP)]-
[AlCl4], 1, and PhBCl2 electrophiles, with highly substituted vinyl pinacol boronate esters isolated post
esterification. Other aryl and heteroaryl congeners of both 1 and PhBCl2 have a limited scope in the 1,1-
carboboration of 1-Me3Si-1-alkynes, with desilylboration more prevalent. PhBCl2 converts Me3Si-substi-
tuted allenes to allylboranes via a formal 1,3-carboboration with Me3Si-migration. [Cl2B(2-DMAP)][AlCl4]
reacts with a number of 1-Me3Si-1-alkynes by desilylboration, whilst with Me3Si-ethyne a 1,1-boro-
amination reaction proceeds, which with excess boron electrophile is followed by an intermolecular desi-
lylboration to form a tricationic-borate. The use of excess 1-Me3Si-1-propyne relative to 1 (and a thienyl
congener of 1) formed 2-boradienes in low yields from the reaction with two equivalents of alkyne. Vinyl
borocations ligated by 2,6-lutidine of the general formula, [(vinyl)BCl(2,6-lutidine)][AlCl4] formed 1-bora-
dienes with 1-Me3Si-1-alkynes.
Received 4th August 2015,
Accepted 8th September 2015
DOI: 10.1039/c5dt03003j
www.rsc.org/dalton
Introduction
The 1,1-carboboration of alkynes has received increased inter-
est in recent years following the discovery of the 1,1-carbo-
boration of terminal and internal alkynes with the strong
electrophile RB(C6F5)2 (R = C6F5, alkyl).1 Addition of RB(C6F5)2
to a terminal alkyne induces a 1,2-shift of H (or a hydrocarbyl
for internal alkynes) along the alkynyl backbone with sub-
sequent migration of the R group from boron to carbon result-
ing in 1,1-carboboration.2 This reaction has been developed
into a useful route to arylboranes by benzannulation,3 and to
highly substituted vinyl boranes that are complementary to
those obtained from hydroboration.4 1,1-carboboration was
pioneered by Wrackmeyer albeit with weaker boron electro-
philes, such as trialkylboranes, for the 1,1-carboboration of
activated alkynes, e.g., alkynes substituted with heavier group
14 substituents (Scheme 1, top).5 Wrackmeyer and co-workers
reported extensively on this topic and determined the relative
reactivity of a range of substituted alkynes toward BEt3 to be
R3Pb > R3Sn > R3Ge > R3Si (TMS), with R3C-substituted
alkynes not amenable.5 These studies predominantly used
trialkylboranes; in contrast, the utilisation of vinyl and aryl
Scheme 1 Relevant early 1,1-carboborations and the approach in this
work.
boranes (excluding B(C6F5)3) for the 1,1-vinyl-boration or 1,1-
aryl-boration of alkynes is rare with only limited examples
reported using BPh3 or PhB(C6F5)2.6,7 One report particularly
relevant to this work showed that Ph2BX (X = Cl or Br) effected
the 1,1-carboboration of 1-TMS-1-hexyne (Scheme 1, top).8 It
should be noted that with (chloro)x(aryl)3−xB compounds TMS
substitution appears essential for 1,1-carboboration, with
PhBCl2 and terminal alkynes reacting via 1,2-halo- or
1,2-carboboration instead.9 To the best of our knowledge the
outcome from combining TMS-alkynes and (hydrocarbyl)BCl2
has not been reported to date.
We envisaged combining arene borylation,10 or alkyne
haloboration,11 using BCl3 derived borocations (to produce
arylBCl2 and [vinylBCl(amine)]+, respectively) with 1,1-carbo-
boration to generate synthetically useful highly substituted
vinyl (or dienyl) boronate esters after esterification (Scheme 1,
bottom).12 This process may proceed directly from the organo-
School of Chemistry, University of Manchester, Manchester, M13 9PL, UK.
E-mail: Michael.ingleson@manchester.ac.uk
†Electronic supplementary information (ESI) available: Copies of NMR spectra,
coordinates for calculated structures and crystallographic data. CCDC 1416375
and 1416376. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c5dt03003j
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Table 1 1,1-Carboboration with 1,a followed by esterification with
pinacol
BCl2 or require enhancement of electrophilicity at boron by
formation of a borocation. We have previously demonstrated
that borocations are effective for the 1,2-haloboration and 1,2-
carboboration of alkynes, with no 1,1-elementoboration
observed.11,13 For example, terminal alkynes and the boronium
salt [(Ph)ClB(2-DMAP)][AlCl4] (2-DMAP
= 2-N,N-dimethyl-
amino-pyridine) react only by 1,2-chloroboration.11 Based on
the previous 1,1-carboboration studies with neutral boranes it
was hypothesised that TMS-substituted alkynes would prefer-
entially undergo 1,1-elementoboration over 1,2-elementobora-
tion when combined with organoBCl2 or borocation
compounds. Support for this comes from the work of Curran
and co-workers on the 1,1-hydroboration of 1-TMS-1-alkynes
using borenium equivalents such as (NHC)BH2(NTf)2.14
Herein is reported our studies using arylBCl2 and aryl and
vinyl containing borocations synthesised by electrophilic bory-
lation to effect the carboboration of TMS-substituted alkynes.
Compound
Entry Electrophile Ta (°C) no.
Isolated
yield (%)
R
1
2
3
4
5
1
1
1
1
1
60
60
60
60
60
2a
2b
2c
2d
2e
Me
Bu
63
61
C(Me)vCH2 55
Ph
p-Br-C6H4
65
56
a Performed in CH2Cl2 in sealed tubes fitted with J. Young valves.
indicated desilylboration and alkynylborane formation is the
dominant reaction outcome, the isolated yields post pinacol
esterification were consistently low due to the susceptibility of
alkynyl-B species to protodeboronation.11
Results and discussion
Vinylboronate ester formation
Studies commenced with the boronium cation [Ph(Cl)B(2-
DMAP)][AlCl4], 1.11 The combination of 1 and equimolar
1-TMS-1-propyne at 20 °C resulted in a slow reaction generat-
ing a single new silicon containing compound (δ29Si −6.05)
with minimal desilylboration observed (only a low intensity
TMSCl resonance present in the 1H NMR spectra). Heating the
reaction to 60 °C in CH2Cl2 in a sealed tube for extended
periods (>1 h) led to complex mixtures, however heating for
ð1Þ
In contrast, the combination of 3 and TMS-ethyne only
30 minutes at 60 °C led to one major new product possessing formed the alkynyl-borocation as a minor species (by NMR
identical resonances to that observed in the 20 °C reaction, spectroscopy). The major soluble product contained a vinylic
1
with unreacted 1 also remaining. Esterification of the reaction singlet at 6.65 ppm in the H NMR spectrum consistent with
mixture after 30 minutes with pinacol/Et3N led to two major alkyne elemento-boration. Crystalline solid spontaneously de-
boron containing products, PhBPin (derived from unreacted 1) posited from CH2Cl2 solutions as the reaction proceeded with
and a new product isolable by column chromatography. NMR a concomitant increase in the quantity of TMSCl and a
1
and mass spectroscopy confirmed this to be the product from decrease in the vinylic singlet (by H NMR spectroscopy). The
the 1,1-carboboration of 1-TMS-1-propyne, formed as the amount of precipitate was increased by using an excess of
E-isomer exclusively, 2a (Table 1). This reactivity was extended 3 (5 : 4 ratio of 3 : TMS-ethyne). X-ray diffraction studies on
to a number of other 1-TMS-1-alkynes to yield 2b to 2e. Using multiple crystals consistently produced poor quality data due
these conditions PhBPin was observed as a minor product to low crystal quality but an unambiguous connectivity map
from esterification of unreacted 1 in all cases and required sep- was obtained (Fig. 1, bottom left). This revealed the compound
aration by column chromatography. The reaction of 1 with to be the tricationic borate, 4 formed from 1,1-boroamination
TMS-ethyne and trimethyl(3-methylbut-1-yn-1-yl)silane under of TMS-ethyne and desilylboration. Due to the low data quality
analogous conditions led to complex intractable mixtures con- detailed discussion of structural metrics of 4 is not warranted.
taining significant TMSCl (by 1H NMR spectroscopy).
Compound 4 was confirmed as the major component of the
With 1,1-carboboration observed from the combination of 1 CH2Cl2 insoluble material by elemental microanalysis.
and 1-TMS-1-alkynes the reaction of 1-TMS-1-propyne and Furthermore, on dissolution of the crystalline solid in CD3CN
[Cl2B(2-DMAP)][AlCl4], 3, was explored to determine if any 1,1- two major resonances at +3.9 ppm and −15.4 ppm in the
chloroboration occurred. Instead, this led to formation of 11B NMR spectrum were observed consistent with four coordi-
TMSCl (by 1H and 29Si NMR spectroscopy) and a major new nate cationic and anionic boron centres, respectively. The for-
11B resonance at +6.5 ppm. Esterification enabled identifi- mation of 4 suggested that the CH2Cl2 soluble product formed
cation of the alkynyl pinacol boronate ester confirming desilyl- is 5, the product from 1,1-boroamination of TMS-ethyne prior
boration (eqn (1)). Desilylboration was observed also on to intermolecular desilylboration (Fig. 1). Whilst 5 could not
combination of 3 with 1-phenyl-2-TMS-acetylene and with be isolated analytically pure (due to contamination with 4, the
1-TMS-1-hexyne. Whilst in situ NMR spectra pre-esterification alkynyl-borocation and [H(2-DMAP)][AlCl4]) multinuclear NMR
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which presumably favours rapid TMS-migration leading to 5 as
the initial product and not the alkynyl-borocation from desilyl-
boration. It is noteworthy that the reaction of [Cl2B(2,6-
lutidine)][AlCl4], 6, a borocation where the amine does not
contain a pendant nucleophile, with 1-TMS-alkynes, including
TMS-ethyne, led predominantly to desilylboration in all cases.
Whilst the reactivity of 1-TMS-1-alkynes with BCl3 proceeds
by desilylboration15 the analogous reactivity with PhBCl2 has
not been explored to the best of our knowledge. To determine
if PhBCl2 and 1 react comparably with 1-TMS-1-alkynes equi-
molar 1-TMS-1-propyne and PhBCl2 were combined in CH2Cl2
at 20 °C. This resulted in a rapid reaction producing a single
new product identified by multinuclear NMR spectroscopy as
the product from 1,1-carboboration. Post esterification 2a was
isolated in a higher yield than when using 1. A substrate scope
exploration (Table 2) confirmed that the 1,1-carboboration of
1-TMS-1-alkynes with PhBCl2 consistently proceeds in higher
yield than when using 1 and does not require purification by
chromatography post esterification. Furthermore, the structure
of 2e was also confirmed by a single crystal X-ray diffraction
study (Table 2, right). It is noteworthy that trimethyl(4-phenyl-
but-1-yn-1-yl)silane (entry 7) resulted in only 1,1-carboboration
with no 6-endo-dig cyclisation as recently reported for related
alkynes and BCl3.16 The facile formation of 2a–2h by 1,1-carbo-
boration represents an alternative to transition metal catalysed
borosilyation of alkynes for accessing these versatile intermedi-
ates.17 Attempts to extend this reaction to 1-triisopropylsilyl-
1-propyne resulted in no reaction, whilst combining PhBCl2
and 1-(PhMe2Si)-1-propyne resulted predominantly in desilyl-
Fig. 1 Formation of the tricationic borate
Bottom left, solid state structure of 4 (anions and hydrogens omitted).
4 by 1,1-boroamination.
spectroscopy is fully consistent with this formulation with
NOE spectroscopy confirming the regio- and stereo-chemistry
and indicating that the desilylboration of 5 to form 4 occurs
with retention. The reactivity disparity between TMS-ethyne
and other 1-TMS-1-alkynes studied is attributed to a less stabil-
ised vinyl cation formed on interaction of 3 with TMS-ethyne
Table 2 1,1-Carboboration using (hetero)arylBCl2 (right, solid state structure of 2e, hydrogens and one component of the disordered pinacol
omitted for clarity)
Entry
Electrophile
t (h)
T (°C)
Compound no.
R
Isolated yield (%)
1
2
3
4
5
6
7
8
PhBCl2
PhBCl2
PhBCl2
PhBCl2
PhBCl2
PhBCl2
PhBCl2
PhBCl2
7a
1
6
24
20
20
2
1
1
1
0.5
1
20
20
20
20
20
20
20
60
20
20
20
20
2a
2b
2c
2d
2e
2f
2g
2h
2i
Me
Bu
C(Me)vCH2
Ph
88
79
77
85
68
76
37
30
p-Br-C6H4
iPr
CH2CH2Ph
H
Me
Me
Me
Me
a
9
—
—
a
10
11
12
7b
7c
7d
2j
2k
2l
68
61
1
a Isolated yield not obtained due to intractable minor contaminants of (heteroaryl)BPin.
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boration products (by observation of PhMe2SiCl by 1H and observed we attribute the reaction outcome to a 1,1-carbobora-
29Si NMR spectroscopy) and multiple other currently un- tion followed by a rapid intramolecular sigmatropic 1,3-boron
identified products.
shift to form the more thermodynamically stable less hindered
To increase the scope, variation of the aryl substituent on allylBCl2 species.18 This can be subsequently pinacol protected
the borane was explored. ArylBCl2 and heteroarylBCl2 species and the resultant boronate esters 9a and 9b isolated. This
are readily accessible by electrophilic arene borylation.10 Using enables access to complementary boronate ester isomers to
established methodologies 2-methylthiophene, 2-methylfuran, that produced by the hydroboration of closely related TMS-
chlorobenzene and triphenylamine were all borylated to allenes where TMS migration does not occur.19
produce the respective (hetero)arylBCl2 compounds 7a–7d
(Table 2) in good conversion as determined by multi-nuclear
Boradiene formation using borocations
NMR spectroscopy.10a,b Removal of reaction solvent (CH2Cl2 or
To explore potential scope expansion further [5-methyl-2-(BCl-
(2-DMAP))-thiophene][AlCl4], 10, was synthesised by addition
of 2-DMAP and AlCl3 to 7a. The addition of 1-TMS-1-propyne
to 10 resulted in a slow reaction at 20 °C that after 18 hours
1,2-Cl2C6H4) and extraction of 7a–d into hexanes was sufficient
to enable subsequent reaction with 1-TMS-1-propyne without
any additional purification steps. This led to the formation of
the desired 1,1-carboboration products which were esterified
to form a single regio- and stereo-isomer of the respective vinyl
pinacol boronate esters (entries 9–12). The products derived
from carboboration using 7a and 7b were repeatedly contami-
nated with minor quantities of heteroarylBPin (from esterfica-
produced four new TMS resonances one of which was attribu-
table to the 1,1-carboboration product (by 1H, 11B and 29Si
NMR spectroscopy). One of the other new compounds derived
from 10/7a could be formed as a greater component of the
reaction mixture when an excess of 1-TMS-1-propyne (5 equi-
valents) was used with heating to 60 °C for 18 h. Post esterifi-
tion of unreacted 7a and 7b) which in our hands proved
challenging to separate from 2i and 2j. The 1,1-carboboration
cation, isolation by column chromatography and analysis by
reaction using longer times (3 h) for 7a and 1-TMS-1-propyne
NMR and mass spectroscopy enabled it to be identified as the
led to considerably more complex NMR spectra and intractable
2-boradiene 11a, from reaction of 10 with two equivalents of
1-TMS-1-propyne (Scheme 3). Under identical conditions 1
products post esterification.
Attempts to extend 1,1-carboboration using 7a–7d to other
also reacted with excess 1-TMS-1-propyne to produce the 2-bor-
1-TMS-1-alkynes, specifically 1-TMS-2-phenylacetylene and tri-
adiene 11b as a minor product. In contrast, heating PhBCl2
methyl(3-methylbut-1-yn-1-yl)silane, instead led to desilylbora-
1
with excess (5 equiv.) 1-TMS-1-propyne led to no observable
2-boradiene after esterification with pinacol, instead complex
mixtures were produced with significant TMSCl observed
in situ indicating desilylboration. Under a range of conditions
11a and 11b were formed only as minor products from 10 and 1
(with a maximum 13 and 15% isolated yield, respectively) with
the 1,1-carboboration products, 2i and 2a, being the major
species isolated post esterification. 2-Boradienes related to 11x
have been previously synthesised by Wrackmeyer and co-
workers from the reaction of BEt3 with two equivalents of 1-
R3Sn-1-alkynes.20 Precise isomer assignment for 11x was based
on 1D and 2D NMR spectroscopy, most notably NOESY indi-
cated a trans-disposition of TMS and PinB moieties in the 2-bor-
tion being the dominant reaction pathway (by H, 11B and 29Si
NMR spectroscopy). To preclude the disparity between PhBCl2
and the four (hetero)arylBCl2 compounds 7a–d being due to
any impurities in commercially sourced PhBCl2 or impurities
in (hetero)arylBCl2 synthesised by electrophilic borylation,
benzene was borylated in 1,2-Cl2C6H4 using 4-N,N-trimethyl-
aniline, BCl3 and two equivalents of AlCl3 to form PhBCl2.10c
This reaction mixture was dried and PhBCl2 extracted into
hexane and found to form 2f on addition of trimethyl(3-
methylbut-1-yn-1-yl)silane, a substrate that 7a–d react with pre-
dominantly by desilylboration. Therefore the greater prevalence
for desilylboration using 7a–d is attributed to the modified
electrophilicity of the borane and the different migratory pro-
pensity of the (hetero)aryl group (relative to phenyl), indicating
that the 1,1-carboboration of 1-TMS-1-alkynes using (hetero)-
arylBCl2 compounds is somewhat limited in scope.
PhBCl2 was effective for the carboboration of silylated
allenes with 8a and 8b (Scheme 2) undergoing carboboration
with TMS migration producing only a single allylBCl2 product
(by multinuclear NMR spectroscopy). With no intermediates
Scheme 3 2-Boradiene formation from 1 and 10 and 1-TMS-1-propyne
(shown as vinyl cation intermediates, π complexes of {Me3Si}+ are also
feasible).
Scheme 2 The carboboration of TMS-allenes with PhBCl2.
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adiene. This is in contrast to that in 2x suggesting that 1,1-
carboboration is not the first step in 2-boradiene formation, a
hypothesis supported by the fact that resonances for the 1,1-
carboboration products increase in intensity as the reaction pro-
gresses, suggesting it is not an intermediate in diene formation.
Instead we propose that 1-TMS-1-propyne is activated to an
intermolecular attack by a second equivalent of alkyne by inter-
action with the borocation and this ultimately leads to the
observed 2-boradiene structures. Related borane activation of
alkynes towards external π nucleophiles have been reported pre-
viously.2b,21 As repeated attempts to crystallise 11a and 11b
failed in our hands to support the proposed diene structure,
particularly correlating the structure with the multiple NOE
interactions observed, the structure of 11b was optimised at the
M06-2X/6-311G(d,p)(PCM:DCM) level. This revealed that steric
bulk forces a significant dihedral angle in the diene (CvC–
CvC = 60.24°) and thus short distances (<4 Å) were observed in
the calculated structure for all the observed NOE interactions,
supporting this isomer assignment. Attempts to form other
2-boradienes using different (hetero)arylboronium cations or
different TMS-alkynes all led to complex mixtures and lower
conversions than that observed for 11a and 11b.
Scheme 4 1-Boradiene formation by haloboration and 1,2-carbobora-
tion. Electronic energies (Gibbs free energies at 293 K) shown in
kcalmol−1 for the model system where R = Me (all energies relative to
12Me and 1-TMS-1-propyne at infinite distance).
in significant TMSCl formation, whilst attempts to use greater
equivalents of 1-TMS-1-propyne also led to more TMSCl for-
mation; thus optimized conditions of 1.2 equivalents of
1-TMS-1-propyne and a 1 h reaction duration were found to
minimise the amount of unreacted haloboration compounds
(12a–b) remaining and TMSCl formation. Post esterification
the boradiene products 13a–b could be separated from the
vinyl-pinacol boronate esters (formed from esterification of
unreacted 12a–b) with NMR spectroscopy consistent with a
1-boradiene formulation formed from a 1,2-carboboration
ð2Þ
Subsequently, the one pot, two step reaction of 3 with term-
inal alkynes (proceeding by 1,2-haloboration as previously
reported to form 12x, eqn (2))11 followed by addition of
1-TMS-1-propyne was explored as an alternative route to 2-bor-
adienes. The addition of 1-TMS-1-propyne to 12a or 12b gave
no reaction (by NMR spectroscopy) at room temperature after
18 hours. When the reaction mixture was heated to 60 °C for
4
reaction (Scheme 4). Notably a JHH coupling of 1 Hz is
observed between the methyl and the vinyl-H in both 13a and
13b confirming the connectivity as this coupling would not be
observed in the 1,1-carboboration products. Whilst 1,2-carbo-
boration is less documented that 1,1-carboboration several
recent examples have been reported,22 including using boro-
cations.13,23 The diene structure (Scheme 4) expected from 1,2-
carboboration was confirmed by NOESY and in the absence of
crystalline material (which was unobtainable in our hands)
supported by optimising the structure of 13b at the M06-2X/
6-311G(d,p)(PCM:DCM) level. This indicated that 13b is a non-
planar diene (CvC–CvC = 39.93°) and that all observed NOE
interactions correspond to calculated H⋯H distances of <4 Å.
We attribute the reactivity disparity between 2-DMAP (2-bora-
dienes) and 2,6-lutidine (1-boradienes) borocations to the
greater steric demand of 2,6-lutidine which disfavours for-
mation of a more sterically hindered 2-boradiene. This is con-
sistent with calculations on a model complex (at the M06-2X/
6-311G(d,p)(PCM:DCM) level) which show that the 2-boradiene
B is 5 kcal mol−1 higher in energy than the 1-boradiene isomer
A (Scheme 4). Furthermore, in contrast to 1,1-carboboration
reactions with BEt3,5 the 1,2-vinylboration to form A is unlikely
to be reversible at 20 °C. This is indicated by the conversion
of the model compound 12Me (where R = Me) to A being
1
1 h multiple new species were observed in the H NMR spec-
trum, including [(2-DMAP)H]+, as well as four new 29Si reso-
nances (one corresponding to TMSCl) and new 11B resonances
at +55 and +66 ppm. Esterification and attempts to purify the
resultant complex mixture failed to deliver pure products in
our hands. A more electrophilic vinyl-borocation was targeted
to enable room temperature reactivity with 1-TMS-1-alkynes
and potentially avoid the complex mixtures observed with 12x
at 60 °C. Thus the reactivity of [(vinyl)BCl(2,6-lutidine)]+
cations, made via haloboration of alkynes with 6, with
1-TMS-1-alkynes was explored.
In a one pot two step reaction 6 was used to separately halo-
borate Bu-acetylene and phenylacetylene followed by addition
t
of one equivalent 1-TMS-1-propyne, which did not lead to any
significant TMSCl formation at short reactions times (<1 h by
NMR spectroscopy) in each case. The initial haloboration step
is rapid (complete in <5 minutes with both terminal alkynes)
whilst the subsequent reaction with 1-TMS-1-propyne is slower
it did proceed to form carboboration products (Scheme 4).
Running the reaction for longer times at 20 °C (≥2 h) resulted
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found to be exergonic by 7.7 kcal mol−1, thus the barrier to the cation system. All solvents were degassed and stored over mole-
reverse process (retro-vinylboration) will be significantly higher cular sieves (3 Å) under an inert atmosphere. Compounds
than that for the forward reaction (which requires at least 1 h 1 and 3 were synthesised according to the published pro-
for significant conversion).
cedures.11 All other materials were purchased from commer-
As bora-dienes are useful species for a range of subsequent cial vendors and used as received. NMR spectra were recorded
synthetic transformations,24 the broader applicability of this with a Bruker AV-400 spectrometer (400 MHz 1H; 100 MHz 13C;
reaction was explored initially looking at other terminal 128 MHz 11B; 376.50 MHz 19F; 104.3 MHz 27Al; 79.5 MHz 29Si).
alkynes. 12c–12e were all readily produced by haloboration 1H NMR chemical shifts are reported in ppm relative to
with 6 and underwent 1,2-carboboration to form 13c–13e, protio impurities in the deuterated solvents and 13C NMR
however the isolated yields of 13x are poor to moderate using the solvent resonances unless otherwise stated. 11B NMR
(23–59%), whilst 13d and 13e could not be separated from spectra were referenced to external BF3 : Et2O, 19F to Cl3CF, 27Al
reaction by-products. The propensity of other 1-TMS-1-alkynes to Al(NO3)3 in D2O (Al(D2O)63+), 29Si to Si(CH3)4. Resonances
to undergo 1,2-carboboration, specifically 1-TMS-2-phenyl- for the carbon directly bonded to boron are not observed
acetylene and 1-TMS-1-hexyne, were investigated using 12a, in the 13C{1H} NMR spectra due to quadrupolar effects.
however the reaction was slower (by in situ NMR spectroscopy) GC-MS analysis was performed on an Agilent Technologies
and resulted in lower conversions to the desired boradiene 7890A GC system equipped with an Agilent Technologies
and more unidentified by-products. Finally, the use of an 5975C inert XL EI/CI MSD with triple axis detector.
internal alkyne, 3-hexyne, was investigated, which as pre- The column employed was an Agilent J&W HP-5 ms ((5%-
viously reported underwent facile haloboration with 6,11 but phenyl)-methylpolysiloxane) of dimensions: length, 30 m;
subsequent reaction with 1-TMS-1-propyne resulted in a low internal diameter, 0.250 mm; film, 0.25 μm. Mass spectra
conversion to the 1-boradiene product which was isolated as were recorded on a Waters QTOF mass spectrometer. X-ray
the pinacol boronate ester, 13f in only 10% yield. The low con- data for compounds 2e and 4 was collected at a temperature of
versions with more substituted systems is presumably due to 150 K using Mo-Kα radiation on an Agilent Supernova,
the increased steric crowding resulting in the slower formation equipped with an Oxford Cryosystems Cobra nitrogen flow gas
of the 1,2-carboboration products and thus increased for- system. Elemental analysis of air sensitive compounds was per-
mation of by-products derived from desilylboration.
formed by London Metropolitan University service. Repeated
attempts to obtain satisfactory elemental analyses for a range
of the pinacol boronate esters repeatedly gave results with low
carbon content, even when using V2O5 as an additional
oxidant.
Conclusions
The carboboration of 1-TMS-1-alkynes with aryldichloro-
boranes and aryl-substituted and vinyl-substituted borocations
has been demonstrated to yield highly substituted vinyl and
General procedures for 1,1-carboboration reactions
With [PhBCl(2-DMAP)][AlCl4] (Route 1). To a suspension of
dienyl boronate esters post esterification. However, due to car- [PhBCl(2-DMAP)][AlCl4] (50 mg, 0.12 mmol, 1 eq.) in DCM
boboration occurring in competition with desilylboration and (0.5 ml) in a J. Youngs NMR tube was added 1-TMS-1-alkyne
diene formation, coupled with further reactions proceeding (0.12 mmol, 1 eq.). This was sealed and the mixture heated for
subsequent to the initial carboboration (e.g., desilylboration), 30 minutes after which time an excess of triethylamine
complex mixtures are often produced that limit the overall (0.1 ml) and pinacol (30 mg, 0.24 mmol, 2 eq.) were added and
utility of this reaction. Variation in borane and borocation the solvent was removed under reduced pressure to leave an
structure is therefore essential to preclude desilylboration to oil. Pentane was used to extract the product, which was passed
generate more general and higher yielding 1-TMS-1-alkyne car- through a 1 inch plug of silica to remove pinacol impurities.
boboration protocols.
Column chromatography (DCM : hexane, 1 : 1) was used to sep-
arate the desired product from the by-products.
With PhBCl2 (Route 2). To a solution of PhBCl2 (200 μl,
1.5 mmol, 1 eq.) in DCM (5 ml) in a Schlenk was added
1-TMS-1-alkyne (1.5 mmol, 1 eq.). After x hours an excess of tri-
ethylamine (0.5 ml) and pinacol (350 mg, 3.0 mmol, 2 eq.)
Experimental
General considerations
All manipulations of air and moisture sensitive species were were added, and the solvent was removed under reduced
performed under an atmosphere of argon or nitrogen using pressure to leave an oil. Filtration through a 1 inch plug of
standard Schlenk and glovebox techniques. Glassware was silica afforded the product in good purity without column
dried in a hot oven overnight and heated under vacuum before chromatography.
use. Hexane, ortho-dichlorobenzene, d1-chloroform, d2-dichlor-
(E)-(1-Phenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)prop-
omethane, 2,6-lutidine, Et3N and were dried over calcium 1-en-2-yl) trimethylsilane (2a). The product was isolated as a
hydride and distilled under vacuum. Pentane and dichloro- yellow oil (Route 1: 24 mg, 63%). (Route 2: 424 mg, 88%).
3
methane were dried by passing through an alumina drying
1H NMR (400 MHz, CDCl3) δ 7.30 (t, 2H, J(H,H) = 7.2 Hz),
3
3
column incorporated into an MBraun SPS800 solvent purifi- 7.18 (t, 1H, J(H,H) = 7.2 Hz), 7.06 (d, 2H, J(H,H) = 7.2 Hz),
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1.68 (s, 3H), 1.23 (s, 12H), 0.23 (s, 9H); 13C{1H} NMR NMR (79.5 MHz, CDCl3) δ −5.83 ppm. MS: (GC, M + Na+ m/z)
(100.06 MHz, CDCl3) δ 150.92, 143.54, 128.42, 127.86, 125.55, 367.4.
83.50, 25.05, 20.58, 0.00; 11B NMR (128.4 MHz, CDCl3): δ 30.2;
(E)-(1,4-Diphenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
29Si NMR (79.5 MHz, CDCl3) δ −4.22 ppm. MS: (M + Na+ m/z) but-1-en-2-yl)trimethylsilane (2g). Trimethyl(4-phenylbut-1-yn-
calculated for C18H29O2SiBNa = 339.1928 Found = 339.1923. 1-yl)silane (45 µL, 0.20 mmol, 1.0 eq.) was dissolved in DCM
(E)-(1-Phenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)hex- (0.6 ml) in a J. Youngs tube, PhBCl2 (29 µL, 0.22 mmol, 1.1 eq.)
1-en-2-yl) trimethylsilane (2b). The product was isolated as a was then added and the reaction sealed and rotated. After 1 h,
yellow oil (Route 1: 27 mg, 61%). (Route 2: 431 mg, 79%).
a cooled solution of pinacol (26 mg, 0.22 mmol, 1.1 eq.) and
1H NMR (400 MHz, CDCl3) δ 7.29 (t, 2H), 7.18 (t, 1H), 7.07 excess of triethylamine were added to the reaction mixture.
(d, 2H), 2.07–2.03 (m, 2H), 1.21 (s, 12H), 1.19–1.10 (m, 4H), The reaction mixture was then concentrated under reduced
0.72 (t, 2H), 0.26 (9H); 13C{1H} NMR (100.06 MHz, CDCl3) pressure, the resultant crude was dissolved in pentane, filtered
δ 155.70, 143.59, 128.16, 127.67, 125.35, 83.42, 33.49, 32.49, and concentrated. The residue was then purified by flash
24.90, 22.81, 13.77, 0.81; 11B NMR (128.4 MHz, CDCl3): δ 30.4; chromatography (petroleum ether : DCM 70 : 30), affording 2g
29Si NMR (79.5 MHz, CDCl3) δ −4.60 ppm. MS: (GC, M + H+, (30 mg, 37%) as a white solid.
m/z) 359.3.
1H NMR (400 MHz, CDCl3) δ 7.31 (t, J = 7.6 Hz, 2H), 7.22 (d,
(E)-(3-Methyl-1-phenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan- J = 7.34 Hz, 1H), 7.13–7.18 (m, 2H), 7.07–7.12 (m, 3H), 6.85 (d,
2-yl)buta-1,3-dien-2-yl)trimethyl silane (2c). The product was iso- J = 7.0 Hz, 2H), 2.40–2.47 (m, 2H), 2.28–2.36 (m, 2H), 1.22 (s,
lated as a yellow oil ((Route 1: 23 mg, 55%). (Route 2: 403 mg, 12H), 0.31 (s, 9H); 13C{1H} NMR (100.06 MHz, CDCl3): δ 154.4,
77%).
143.2, 142.4, 128.1, 128.0, 127.9, 127.8, 125.6, 125.5, 83.5, 36.6,
1H NMR (400 MHz, CDCl3) δ 7.24–7.15 (m, 5H), 4.69 (m, 36.2, 24.9, 0.7; 11B NMR (128.4 MHz, CDCl3): δ 30.9 (s) ppm;
1H), 4.38 (m, 1H), 1.42 (s, 1H), 1.23 (s, 12H), 0.24 (s, 9H); 29Si NMR (79.5 MHz, CDCl3): δ −4.4 (s) ppm; MS (GC,
13C{1H} NMR (100.06 MHz, CDCl3) δ 158.70, 147.58, 142.92, [M − CH3]+, m/z) 391.3; accurate mass: ([M + H]+ 407.2583).
134.70, 128.37, 127.19, 125.58, 83.72, 25.00, 24.17, 0.27; 11B
(Z)-Trimethyl(2-phenyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
NMR (128.4 MHz, CDCl3): δ 31.1; 29Si NMR (79.5 MHz, CDCl3) 2-yl)vinyl)silane
(2h). Trimethylsilylacetylene (48 μL,
δ −6.59 ppm. MS: (GC, M + Na+ m/z) 365.3.
0.33 mmol, 1.0 eq.) was dissolved in DCM in a J. Youngs tube,
(E)-(1,2-Diphenyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)- PhBCl2 (49 μL, 0.37 mmol, 1.1 eq.) was then added and the
vinyl)trimethylsilane (2d). The product was isolated as a yellow tube sealed. After 1 h at 60 °C a cooled solution of pinacol
oil (Route 1: 29 mg, 65%). (Route 2: 489 mg, 85%).
(44 mg, 0.37 mmol, 1.1 eq.) and excess of triethylamine were
1H NMR (400 MHz, CDCl3) δ 7.06–7.00 (m, 4H), 6.96–6.93 added to the reaction mixture. The reaction mixture was then
(m, 4H), 6.75–6.73 (m, 2H), 1.29 (s, 12H), 0.17 (s, 9H); 13C{1H} concentrated under reduced pressure and the crude was dis-
NMR (100.06 MHz, CDCl3) δ 156.92, 144.48, 142.51, 128.84, solved in pentane, filtered and concentrated. The residue was
128.10, 127.19, 127.11, 125.22, 124.57, 83.91, 25.11, 0.36; 11B then purified by flash chromatography (petroleum ether : DCM
NMR (128.4 MHz, CDCl3): δ 30.6; 29Si NMR (79.5 MHz, CDCl3) 70 : 30), affording 2h (30 mg, 30%) as a colourless oil.
δ −5.00 ppm. MS: (GC m/z) 378.2 MS: (M + Na+ m/z) 401.4.
1H NMR (400 MHz, CDCl3) δ 7.41 (d, J = 8.1 Hz, 2H), 7.30
(E)-(1-(4-Bromophenyl)-2-phenyl-2-(4,4,5,5-tetramethyl-1,3,2- (t, J = 7.8 Hz, 2H), 7.21–7.26 (m, 1H), 6.74 (s, 1H), 1.34 (s,
dioxaborolan-2-yl)vinyl)trimethylsilane (2e). The product was 12H), 0.23 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 150.6,
isolated as a yellow oil (Route 1: 31 mg, 56%). (Route 2: 145.5, 128.0, 126.9, 126.7, 83.8, 25.1, 0.3; 11B NMR (128 MHz,
102 mg, 68%, reaction performed on a 1/4 scale of the general CDCl3): δ 30.2 (s) ppm; 29Si NMR (79 MHz, CDCl3): δ −9.2 (s)
procedure).
ppm; MS (GC, [M − CH3]+, m/z) 287.1; accurate mass:
3
1H NMR (400 MHz, CDCl3) δ 7.17 (d, 2H, J(H,H) = 8.2 Hz), ([M − CH3]+ 287.1635).
7.07–6.91 (m, 5H), 6.63 (d, 2H, 3J(H,H) = 8.2 Hz), 1.28 (s, 12H),
(E)-(1-(5-Methylthiophen-2-yl)-1-(4,4,5,5-tetramethyl-1,3,2-dioxa-
0.17 (s, 9H); 13C{1H} NMR (100.06 MHz, CDCl3) δ 155.55, borolan-2-yl)prop-1-en-2-yl)trimethylsilane (2i). [BCl2(2-DMAP)]-
143.50, 142.16, 130.36, 129.80, 128.68, 128.08, 127.33, 125.47, [AlCl4] (100 mg, 2.7 mmol, 1 eq.) was combined with 2-methyl-
83.98, 25.08, 0.34; 11B NMR (128.4 MHz, CDCl3): δ 30.0; 29Si thiophene (26 μl, 2.7 mmol, 1 eq.) in DCM (0.5 ml) in a
NMR (79.5 MHz, CDCl3) δ −4.92 ppm. MS: (GC, M + H+ m/z) J. Youngs NMR tube which was then sealed and heated at
458.0.
60 °C for 1 hour. NMR spectroscopy confirmed formation of
(E)-(3-Methyl-1-phenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan- 2-methylthiophene-BCl2 which was extracted into hexane
2-yl)but-1-en-2-yl)trimethylsilane (2f). The product was isolated (10 ml). To this, 1-TMS-1-propyne (80 μl, 5.4 mmol, 2 eq.) was
as orange crystals (100 mg, 76%, reaction performed on a 1/4 added with the reaction mixture turning deep orange.
scale of the general procedure.
The reaction mixture was esterified after 1 hour by addition
3
1H NMR (400 MHz, CDCl3) δ 7.13 (t, 2H, J(H,H) = 7.0 Hz), of excess Et3N (0.1 ml) and pinacol (96 mg, 3 eq.). The
3
3
7.03 (t, 1H, J(H,H) = 7.0 Hz), 6.94 (d, 2H, J(H,H) = 7.0 Hz), crude product was extracted into pentane (20 ml) and
3
2.57 (septet, 1H, J(H,H) = 7.0 Hz), 1.02 (s, 12H), 0.81 (d, 6H, filtered through
a 1 inch plug of silica affording the
3J(H,H) = 7.0 Hz), 0.16 (s, 9H); 13C{1H} NMR (100.06 MHz, desired product contaminated with 2-methyl-5-BPin-thio-
CDCl3) δ 157.97, 143.70, 128.05, 127.89, 125.57, 83.56, 33.79, phene. The data below are for 2i with resonances for the
24.92, 22.28, 2.67; 11B NMR (128.4 MHz, CDCl3): δ 30.5; 29Si minor by-product omitted.
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1H NMR (400 MHz, CDCl3) δ 6.63 (m, 1H), 6.57 (d, 1H, product was extracted into pentane (20 ml) and filtered
3J(H,H) = 7.1 Hz), 2.46 (s, 3H), 1.93 (s, 3H), 1.29 (s, 12H), 0.22 through a 1 inch plug of silica affording the product. (85 mg,
(s, 9H); 13C NMR (100.06 MHz, CDCl3) δ 148.27, 139.11, 61%).
3
135.82, 126.00, 122.91, 83.22, 25.01, 20.37, 15.86, 0.02 ppm;
1H NMR (400 MHz, CDCl3) δ 7.03 (d, 2H, J(H,H) = 8.2 Hz),
11B NMR (128.4 MHz, CDCl3): δ 30.0; 29Si NMR (79.5 MHz, 6.75 (d, 2H, J(H,H) = 8.2 Hz), 1.44 (s, 3H), 1.00 (s, 12H), 0.00
3
CDCl3) δ −3.92 ppm. MS (GC, M+, m/z): 336.4.
(E)-(1-(5-Methylfuran-2-yl)-1-(4,4,5,5-tetramethyl-1,3,2-dioxabor- 131.30, 129.82, 128.03, 83.59, 24.99, 20.77, 0.00; 11B NMR
olan-2-yl)prop-1-en-2-yl)trimethylsilane
(2j). [BCl2(2-DMAP)]- (128.4 MHz, CDCl3): δ 30.0; 29Si NMR (79.5 MHz, CDCl3)
[AlCl4] (100 mg, 2.7 mmol, 1 eq.) was combined with 2-methyl- δ −4.04 ppm. MS: (GC, M+, m/z) 350.1.
(s, 9H); 13C{1H} NMR (100.06 MHz, CDCl3) δ 152.57, 136.11,
furan (24 μl, 2.7 mmol, 1 eq.) in DCM (0.5 ml) in a J. Youngs
Compound 4. A 5 : 4 ratio of [BCl2(2-DMAP)][AlCl4] (100 mg,
NMR tube. NMR spectroscopy confirmed formation of 0.27 mmol, 5 eq.), to trimethylsilylacetylene (31 μl, 0.22 mmol,
2-methylfuran-BCl2 in <5 min which was then extracted into 4 eq.) was dissolved in CH2Cl2 in a J. Young’s NMR tube. On
hexane (10 ml). To this 1-TMS-1-propyne (40 μl, 2.7 mmol, standing a crystalline solid precipitated out of solution.
1 eq.) was added with the reaction turning deep orange. The Removal of the solvent and washing with pentane allowed iso-
reaction mixture was esterified after 30 minutes with excess lation of the crystals 21 mg, 27% (based on boron content).
Et3N (0.1 ml) and pinacol (96 mg, 3 eq.). The crude product
1H NMR (400 MHz, CD3CN, −40 °C): δ 8.91 (d, 1H), 8.71 (t,
was extracted into pentane and filtered through a 1 inch plug 1H), 8.28 (d, 1H), 8.16 (t, 1H), 7.89 (s, 1H), 3.78 (s, 3H), 3.45 (s,
of silica affording the desired product contaminated with 3H); 11B NMR (128.4 MHz, CD3CN): δ 3.90, −15.44; 13C{1H}
2-methyl-5-BPin-furan. The data below are for 2j with reso- NMR (100.6 MHz, CD3CN): δ 154.09, 150.15, 143.80, 130.47,
nances for the minor by-product omitted.
119.85 ppm. Expected (%) for C36H44Al3B5N8Cl20 C = 30.18 H =
3
1H NMR (400 MHz, CDCl3) δ 6.17 (d, 1H, J(H,H) = 3.2 Hz), 3.10 N = 7.82, Found (%) C = 29.94, H = 2.97, N = 7.65.
5.99 (m, 1H), 2.28 (s, 3H), 2.02 (s, 3H), 1.37 (s, 12H), 0.21 (s,
Compound 5. To a 1 : 1 suspension of [BCl2(2-DMAP)][AlCl4]
9H); 13C{1H} NMR (100.06 MHz, CDCl3) δ 155.78, 153.22, (100 mg, 0.27 mmol, 1 eq.) in CH2Cl2 in a J. Young’s NMR
152.09, 109.07, 102.72, 82.98, 25.03, 21.25, 14.76, 0.98 ppm; tube was added trimethylsilylacetylene (38 μl, 0.27 mmol,
11B NMR (128.4 MHz, CDCl3): δ 30.2; 29Si NMR (79.5 MHz, 1 eq.) and tube was sealed and rotated at room temperature for
CDCl3) δ −3.92 ppm. MS (GC, M+, m/z): 320.3.
18 hours. The solvent was removed under reduced pressure,
(E)-N,N-Diphenyl-4-(1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- and the product redissolved in DCM.
yl)-2-(trimethylsilyl)prop-1-en-1-yl)aniline (2k). [2,6-lutBCl2]-
1H NMR (400 MHz, CD2Cl2): δ 8.99 (d, 1H), 8.88 (t, 1H),
[AlCl4] (80 mg, 2.2 mmol, 1 eq.) was generated in situ (from 8.56 (d, 1H), 8.25 (t, 1H), 6.65 (s, 1H), 3.80 (s, 6H), 0.38 (s, 9H);
lut-BCl3 and AlCl3) and combined with triphenylamine (55 mg,
11B NMR (128.4 MHz, CD2Cl2): δ 2.5 (s); 29Si NMR (79.5 MHz,
δ
2.2 mmol, 1 eq.) in DCM (0.5 ml) in a J. Youngs NMR tube CH2Cl2) δ −3.6 ppm. Due to difficulties in purifying this com-
which was then sealed and rotated at room temperature for pound, the 13C NMR spectra were complicated by numerous
2 hours resulting in the solution turning brown. NMR spectro- minor species. Accurate elemental analysis could not be
scopy confirmed formation of triphenylamine-BCl2, which was obtained due to the impure nature of the material from each
extracted into hexane (10 ml). To this 1-TMS-1-propyne (66 μl, attempt to isolate this product.
4.5 mmol, 2 eq.) was added. After 30 minutes the reaction
(E)-(3-phenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)but-
mixture was esterified with excess Et3N (0.1 ml) and pinacol 2-en-2-yl)trimethylsilane (9a). 3-(Trimethylsilyl)-1,2-butadiene
(78 mg, 3 eq.). The crude product was extracted into pentane (25 μL, 0.15 mmol, 1.0 eq.) was dissolved in DCM in a
(20 ml) and filtered through a 1 inch plug of silica afforded J. Youngs tube and PhBCl2 (22 μL, 0.16 mmol, 1.1 eq.) was
the product. (74 mg, 68%).
added. After 1 h a cooled solution of pinacol (19 mg,
1H NMR (400 MHz, CDCl3) δ 7.07–6.77 (m, 14H), 1.58 (s, 0.16 mmol, 1.1 eq.) and excess triethylamine were added to the
3H), 1.07 (s, 12H), 0.05 (s, 9H); 13C{1H} NMR (100.06 MHz, reaction mixture. The reaction mixture was then dried under
CDCl3) δ 150.41, 147.97, 145.25, 137.85, 129.38, 129.08, 123.97, reduced pressure and the crude was dissolved in pentane, fil-
123.69, 122.35, 83.49, 25.04, 20.63, −0.03; 11B NMR tered and concentrated. The residue was purified by flash
(128.4 MHz, CDCl3):
δ
30.3; 29Si NMR (MHz, CDCl3) chromatography (petroleum ether : DCM 60 : 40), affording 9a
δ −4.29 ppm. MS: (GC, M+, m/z) 483.3.
(21 mg, 43%) as a white solid.
(E)-(1-(4-Chlorophenyl)-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
1H NMR (400 MHz, CDCl3) δ 7.29 (t, J = 7.2 Hz, 2H), 7.19 (t,
2-yl)prop-1-en-2-yl)trimethylsilane (2l). DMT-BCl3 (100 mg, 1 eq.) J = 7.4 Hz, 1H), 7.14 (d, J = 7.0 Hz, 2H), 2.10 (s, 3H), 1.57 (s,
and AlCl3 (111 mg, 2.1 eq.) were combined in dichlorobenzene 2H), 1.23 (s, 12H), 0.23 (s, 9H); 13C{1H} NMR (100 MHz,
(0.5 ml) in a J. Youngs NMR tube which was then sealed and CDCl3): δ 146.0, 145.1, 131.3, 128.0, 127.6, 125.9, 82.9, 25.3,
heated to 100 °C for 18 h. NMR spectroscopy confirmed for- 24.8, 0.4; 11B NMR (128 MHz, CDCl3): δ 33.7 (s) ppm; 29Si NMR
mation of 4-chloro-phenyl-BCl2, which was extracted into (79.5 MHz, CDCl3): δ −5.8 (s) ppm. MS: (GC, M+, m/z) 330.2,
hexane (10 ml). To this hexane solution 1-TMS-1-propyne accurate mass: [M]+ 330.2181.
(117 μl, 2 eq.) was added turning the solution brown. After
(E)-(2-Phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pent-
40 minutes the reaction mixture was then esterified with 2-en-3-yl)trimethylsilane (9b). 2-(Trimethylsilyl)-2,3-pentadiene
excess Et3N (0.1 ml) and pinacol (140 mg, 3 eq.). The crude (25 μL, 0.13 mmol, 1.0 eq.) was dissolved in DCM in a
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J. Youngs tube and PhBCl2 (20 μL, 0.14 mmol, 1.1 eq.) was (s, 9H) ppm; 13C NMR (100.06 MHz, CDCl3) δ 144.00, 133.75,
added. The reaction mixture was heated at 60 °C and after 24 h 130.67, 129.96, 127.37, 126.72, 125.59, 83.08, 24.72, 22.40,
a cooled solution of pinacol (17 mg, 0.14 mmol, 1.1 eq.) and 20.86, 0.01 (TMS resonances coincident); 11B NMR
excess triethylamine were added to the reaction mixture. The (128.4 MHz, CDCl3):
δ
29.6; 29Si NMR (MHz, CDCl3)
reaction mixture was then dried under reduced pressure and the δ −6.00 ppm. MS: (m/z): Calculated [M]+ = 429.2811. Measured
crude was dissolved in pentane, filtered and concentrated. The = 429.2812.
residue was purified by flash chromatography (petroleum ether :
DCM 60 : 40), affording 9b (10 mg, 22%) as a colourless oil.
General procedure for vinylboration with 6
1H NMR (400 MHz, CDCl3) δ 7.29 (t, J = 7.2 Hz, 2H), 7.20 (t,
J = 7.3 Hz, 1H), 7.14 (d, J = 6.9 Hz, 2H), 2.08 (s, 3H), 2.04 (q, J = LutBCl3 (50 mg, 0.22 mmol) was suspended in anhydrous
7.5 Hz, 1H), 1.22 (s, 12H), 0.92 (d, J = 7.5 Hz, 3H), 0.26 (s, 9H); CH2Cl2 (5 ml) in a J. Young’s NMR tube and AlCl3 (30 mg,
13C{1H} NMR (100 MHz, CDCl3): δ 146.4, 145.8, 138.4, 128.0, 0.22 mmol) added causing dissolution to form a yellow solu-
127.8, 125.8, 82.9, 25.8, 25.3, 24.7, 16.3, 1.6; 11B NMR tion. To this the appropriate alkyne (0.22 mmol) was added.
(128 MHz, CDCl3): δ 34.5 (s) ppm; 29Si NMR (79 MHz, CDCl3): The reaction mixture was then sealed and rotated at room
δ −6.6 (s) ppm. MS: (GC, [M − CH3]+, m/z) 329.2, accurate temperature. After 10 min 1-TMS-1-propyne (40 µl, 0.26 mmol)
mass: [M]+ 344.2337.
was added and the tube resealed and rotated for a further
((2Z,4Z)-3-(5-Methylthiophen-2-yl)-4-(4,4,5,5-tetramethyl-1,3,2-
45 minutes. Then the solution was esterified with excess tri-
dioxaborolan-2-yl)hexa-2,4-diene-2,5-diyl)bis(trimethylsilane) (11a). ethylamine (0.5 ml) and pinacol (2 eq.). The solvent was
To a suspension of [Cl2B(2-DMAP)][AlCl4] (50 mg, 0.14 mmol, removed under reduced pressure, leaving a yellow oil. Hexane
1 eq.) in DCM (0.5 ml) in a J. Youngs NMR tube was added (30 ml) was used to extract the product, which was filtered.
2-methylthiophene (14 μl 0.15 mmol, 1.1 eq.). The reaction This left a mixture of the desired diene and the haloboration
was heated at 60 °C for 1 h when borylation was confirmed by derived vinyl by-product used to determine NMR conversion
NMR spectroscopy. Sequential addition of equimolar 2-DMAP before purification by column chromatography.
and AlCl3 then generated [ClB2-methylthiophene(2-DMAP)]-
((1E,3Z)-4-Chloro-2,5,5-trimethyl-1-(4,4,5,5-tetramethyl-1,3,2-
[AlCl4]. To this 1-TMS-1-propyne (90 μl, 0.6 mmol, 5 eq.) was dioxaborolan-2-yl)hexa-1,3-dien-1-yl)trimethylsilane (13a). This
addedand the tube was sealed and heated to 60 °C. After 18 h crude product (65% yield by NMR spectroscopy) was purified
an excess of triethylamine (0.1 ml) and pinacol (30 mg, with column chromatography using 2 : 1 hexane : DCM eluent
0.24 mmol, 2 eq.) were added and the solvent was removed and isolated as a yellow oil (18 mg, 23%).
4
under reduced pressure. Pentane (20 ml) was used to extract
1H NMR (400 MHz, CDCl3) δ 6.24 (q, 1H, J (H,H) = 1 Hz),
the product which was then passed through a 1 inch plug of 1.78 (d, 3H, 4J (H,H) = 1 Hz), 1.29 (s, 12H), 1.22 (s, 9H), 0.19 (s,
silica. Column chromatography (DCM : hexane, 1 : 1) was used 9H); 13C{1H} NMR (100.06 MHz, CDCl3) δ 154.05, 144.78,
to separate the desired diene from the 1,1-carboboration 123.72, 83.26, 38.72, 28.94, 25.36, 20.73, 0.28; 11B NMR
product. The product was isolated as a yellow oil (8 mg, 22%).
(128.4 MHz, CDCl3): δ 29.41; 29Si NMR (79.5 MHz, CDCl3)
3
1H NMR (400 MHz, CDCl3) δ 6.59 (d, 1H, J(H,H) = 3.5 Hz), δ −4.64 ppm. MS: (GC, M+ m/z) 356.3 g mol−1
.
6.51 (m, 1H), 2.42 (s, 3H), 1.96 (s, 3H), 1.81 (s, 3H), 1.20 (s, ((1E,3Z)-4-Chloro-2-methyl-4-phenyl-1-(4,4,5,5-tetramethyl-
6H), 1.17 (s, 6H), 0.08 (s, 9H), −0.03 (s, 9H); 13C{1H} NMR 1,3,2-dioxaborolan-2-yl)buta-1,3-dien-1-yl)trimethylsilane (13b).
(100.06 MHz, CDCl3) δ 147.42, 146.02, 144.67, 139.85, 135.77, This crude product (71% conversion by NMR spectroscopy)
126.83, 123.83, 83.06, 24.65, 24.59, 22.23, 20.49, 15.44, −0.13, was purified with column chromatography using 2 : 1 hexane :
−0.54; 11B NMR (128.4 MHz, CDCl3): δ 30.0; 29Si NMR DCM eluent and isolated as a yellow oil (49 mg, 59%).
(79.5 MHz, CDCl3) δ −5.98 ppm. MS: (GC, M+, m/z) 448.6 g
mol−1
1H NMR (400 MHz, CDCl3) δ 7.69 (m, 2H), 7.38–7.30 (m,
3H), 6.94 (q, 1H, J (H,H) = 1.2 Hz), 1.91 (d, 3H, J (H,H) = 1.2
4
4
.
((2Z,4Z)-3-Phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)- Hz), 1.33 (s, 12H), 0.25 (s, 9H); 13C{1H} NMR (100.06 MHz,
hexa-2,4-diene-2,5-diyl)bis(trimethylsilane) (11b). To a suspen- CDCl3) δ 155.01, 147.37, 136.44, 129.00, 128.31, 127.32, 120.10,
sion of [PhBCl(2-DMAP)][AlCl4] (50 mg, 0.12 mmol, 1 eq.) in 84.07, 24.25, 22.87, 0.41; 11B NMR (128.4 MHz, CDCl3): δ 29.16
DCM (0.5 ml) in a J. Youngs NMR tube was added 1-TMS-1- 29Si NMR (79.5 MHz, CDCl3) δ −4.18 ppm. MS: (GC, M+, m/z)
propyne (90 μl, 0.6 mmol, 5 eq.). The tube was sealed and then 376.2.
heated to 60 °C. After 18 h an excess of triethylamine (0.1 ml)
((1E,3Z)-4-Chloro-2-methyl-5-phenyl-1-(4,4,5,5-tetramethyl-
and pinacol (30 mg, 0.24 mmol, 2 eq.) were added and the 1,3,2-dioxaborolan-2-yl)penta-1,3-dien-1-yl)trimethylsilane (13c).
solvent was removed under reduced pressure leaving a yellow/ The crude product (70% conversion by NMR spectroscopy) was
orange oil. Pentane (20 ml) was used to extract the product purified with column chromatography using 2 : 1 hexane : DCM
which was then passed through a 1 inch plug of silica. eluent and isolated as a yellow oil (27 mg, 31%).
Column chromatography (DCM : hexane, 1 : 1) was used to sep-
arate the desired diene from the 1,1-carboboration product. 4J (H,H) = 1 Hz), 3.74 (s, 2H), 1.84 (d, 3H, 4J (H,H) = 1 Hz), 1.30
1H NMR (400 MHz, CDCl3) δ 7.33–7.23 (m, 5H), 6.37 (q, 1H,
The product was isolated as a yellow oil (8 mg, 15%).
(s, 12H), 0.20 (s, 9H); 13C{1H} NMR (100.06 MHz, CDCl3)
1H NMR (400 MHz, CDCl3) δ 7.22–7.20 (m, 5H), 1.98 (s, δ 156.72, 138.00, 132.80, 129.37, 128.85, 128.28, 126.56, 83.41,
3H), 1.85 (s, 3H), 1.16 (s, 6H), 1.11 (s, 6H), 0.09 (s, 9H), −0.15 45.64, 25.32, 21.43, 0.36; 11B NMR (128.4 MHz, CDCl3):
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δ 29.5 ppm; 29Si NMR (79.5 MHz, CDCl3) δ −4.40 ppm. MS:
(GC, M+, m/z) 390.2.
2010, 29, 125; (c) C. Fan, W. E. Piers, M. Parvez and
R. McDonald, Organometallics, 2010, 29, 5132; (d) C. Chen,
G. Kehr, R. Fröhlich and G. Erker, J. Am. Chem. Soc., 2010,
132, 13594.
2 For recent reviews see: (a) G. Kehr and G. Erker, Chem.
Commun., 2012, 48, 1839; (b) R. Melen, Chem. Commun.,
2014, 50, 1161.
3 For a recent synthetic use of 1,1-carboboration for benzan-
nulation see: (a) R. Liedtke, F. Tenberge, C. G. Daniliuc,
G. Kehr and G. Erker, J. Org. Chem., 2015, 80, 2240.
4 C. Chen, T. Voss, R. Fröhlich, G. Kehr and G. Erker, Org.
Lett., 2011, 13, 62.
((1E,3Z)-4-Chloro-2-methyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-4-(p-tolyl)buta-1,3-dien-1-yl)trimethylsilane (13d).
Attempts at purifying the crude product (70% conversion by
NMR spectroscopy) by column chromatography failed. NMR
data are given with the vinylboronate ester resonances
omitted.
1H NMR (400 MHz, CDCl3) δ 7.42–7.38 (m, 2H), 7.01–6.99
(m, 2H), 6.72 (m, 1H), 1.72 (m, 3H), 1.21 (s, 3H), 1.15 (s, 12H),
0.07 (s, 9H); 11B NMR (128.4 MHz, CDCl3): δ 29.5; 29Si NMR
(79.5 MHz, CDCl3) δ −4.24 ppm.
((1E,3Z)-4-Chloro-4-(4-methoxyphenyl)-2-methyl-1-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)buta-1,3-dien-1-yl)trimethylsilane
(13e). Attempts at purifying the crude product (67% conver-
sion by NMR spectroscopy) by column chromatography failed.
NMR data are given with the vinylboronate ester resonances
omitted.
5 (a) B. Wrackmeyer, Coord. Chem. Rev., 1995, 145, 125;
(b) B. Wrackmeyer, Heteroat. Chem., 2006, 17, 188.
6 For select reactions of BPh3 in 1,1-carboborations: With
1-TMS-1-alkynes see: (a) R. Köster, G. Seidel and
B. Wrackmeyer, Chem. Ber., 1989, 122, 1825. With stanny-
lated alkynes see: (b) B. Wrackmeyer, O. L. Tok and
W. Milius, Z. Naturforsch., B: Chem. Sci., 2007, 62, 1509;
(c) B. Wrackmeyer, P. Thoma, S. Marx, T. Bauer and
R. Kempe, Eur. J. Inorg. Chem., 2014, 2103. With Te-alkynes
see: (d) F. A. Tsao, A. J. Lough and D. W. Stephan, Chem.
Commun., 2015, 51, 4287; (e) F. A. Tsao and D. W. Stephan,
Dalton Trans., 2015, 44, 71.
7 For an example with PhB(C6F5)2: R. Liedtke, M. Harhausen,
R. Fröhlich, G. Kehr and G. Erker, Org. Lett., 2012, 14, 1448.
8 R.-J. Binnewirtz, H. Klingenberger, R. Welte and
P. Paetzold, Chem. Ber., 1983, 116, 1271.
9 M. Lappert and B. Prokai, J. Organomet. Chem., 1963, 1,
384.
1H NMR (400 MHz, CDCl3) δ 7.61–7.59 (m, 2H), 6.89–6.87
(m, 2H), 6.81 (m, 1H), 3.83 (s, 3H), 1.88 (m, 3H), 1.32 (s, 12H),
0.23 (s, 9H); 11B NMR (128.4 MHz, CDCl3): δ 30.0; 29Si NMR
(79.5 MHz, CDCl3) δ −4.28 ppm.
((1E,3Z)-4-Chloro-3-ethyl-2-methyl-1-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)hexa-1,3-dien-1-yl)trimethylsilane (13f). Fol-
lowing the general procedure but allowing 4 h for halobora-
tion. The crude product was then purified with column
chromatography using 2 : 1 hexane : DCM eluent and isolated
as a yellow oil (8 mg, 10%).
1H NMR (400 MHz, CDCl3) δ 2.50–2.39 (m, 2H), 2.26–2.16
(m, 2H), 1.74 (s, 3H), 1.24 (s, 12H), 1.14 (t, 3H), 0.95 (t, 3H),
0.19 (s, 9H); 13C{1H} NMR (100.06 MHz, CDCl3) δ 153.05, 10 (a) A. Del Grosso, M. D. Helm, S. A. Solomon, D. Caras-
139.87, 128.54, 82.65, 27.98, 26.10, 24.62, 24.39, 20.72, 12.65,
12.30; 11B NMR (128.4 MHz, CDCl3): δ 29.30; 29Si NMR
(79.5 MHz, CDCl3) δ −4.75 ppm.
Quintero and M. J. Ingleson, Chem. Commun., 2011, 47,
12459; (b) V. Bagutski, A. Del Grosso, J. Ayuso-Carrillo,
I. A. Cade, M. D. Helm, J. R. Lawson, P. J. Singleton,
S. A. Solomon, T. Marcelli and M. J. Ingleson, J. Am. Chem.
Soc., 2013, 135, 474; (c) A. Del Grosso, J. Ayuso Carrillo and
M. J. Ingleson, Chem. Commun., 2015, 51, 2878.
Acknowledgements
11 J. R. Lawson, E. R. Clark, I. A. Cade, S. A. Solomon and
M. J. Ingleson, Angew. Chem., Int. Ed., 2013, 52, 7518.
The research leading to these results has received funding
from the University of Manchester, the EPSRC (EP/J000973/1 12 For synthetic applications of boronic acid derivatives see:
and EP/K03099X/1) and the European Research Council (FP/
2007–2013/ERC Grant Agreement 305868). MJI acknowledges
the Royal Society (for the award of a University Research
Boronic Acids: Preparation, Applications in Organic Synthesis
and Medicine, ed. D. G. Hall, Wiley-VCH, Weinheim,
New York, 2011.
Fellowship) The authors would also like to acknowledge the 13 I. A. Cade and M. J. Ingleson, Chem. – Eur. J., 2014, 20,
use of the EPSRC UK National Service for Computational 12874.
Chemistry Software (NSCCS) at Imperial College London in 14 A. Boussonniere, X. Pan, S. J. Geib and D. P. Curran,
carrying out this work.
Organometallics, 2013, 32, 7445.
15 D. A. Singleton and S.-W. Leung, J. Organomet. Chem., 1997,
544, 157.
16 A. J. Warner, J. R. Lawson, V. Fasano and M. J. Ingleson,
Angew. Chem., Int. Ed., 2015, 54, 11245.
Notes and references
1 For initial reports on 1,1-carboboration using fluorinated 17 For
metal
catalysed
alkyne
borosilylation
see:
boranes see: (a) C. Chen, F. Eweiner, B. Wibbeling,
R. Fröhlich, S. Senda, Y. Ohki, K. Tatsumi, S. Grimme,
G. Kehr and G. Erker, Chem. – Asian J., 2010, 4711;
(b) C. Jiang, O. Blacque and H. Berke, Organometallics,
(a) T. Ohmura, K. Oshima, H. Taniguchi and
M. Suginiome, J. Am. Chem. Soc., 2010, 132, 12194;
(b) J. Jiao, K. Hyodo, H. Hu, K. Nakajima and Y. Nishihara,
J. Org. Chem., 2014, 79, 285 and references therein.
Dalton Trans.
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Dalton Transactions
Paper
18 A related 1,1-carboboration followed by a 1,3-boron shift has 21 (a) M. A. Dureen, C. C. Brown and D. W. Stephan, Organo-
been observed previously with B(C6F5)3: M. M. Hansmann,
R. L. Melen, F. Rominger, A. S. K. Hashmi and
D. W. Stephan, J. Am. Chem. Soc., 2014, 136, 777.
metallics, 2010, 29, 6422; (b) B.-H. Xu, G. Kehr, R. Fröhlich
and G. Erker, Organometallics, 2011, 30, 5080.
22 S. Roscales and A. G. Csaky, Org. Lett., 2015, 17, 1605.
19 K. K. Wang, Y. G. Gu and C. Liu, J. Am. Chem. Soc., 1990, 23 M. Devillard, R. Brousses, K. Miqueu, G. Bouhadir and
112, 4424. D. Bourissou, Angew. Chem., Int. Ed., 2015, 54, 5722.
20 B. Wrackmeyer and R. Zentgraf, J. Chem. Soc., Chem. 24 L. Eberlin, F. Tripoteau, F. Carreaux, A. Whiting and
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This journal is © The Royal Society of Chemistry 2015
Dalton Trans.