Rhodium catalysed dehydrogenative borylation of alkenes: Vinylboronates via C-H activation

Article abstract of DOI:10.1039/b715584k

We present herein a high yield, highly selective catalytic synthesis of vinylboronate esters (VBEs), including 1,1-disubstituted VBEs, from alkenes without significant hydrogenation or hydroboration, using the simple catalyst precursor, trans-[RhCl(CO)(PPh₃)₂] (1), and the diboron reagents B₂pin₂ (2a, pin = pinacolato = OCMe ₂CMe₂O) or B₂neop₂ (2b, neop = neopentylglycolato = OCH₂CMe₂CH₂O), or the monoboron reagent HBpin, all of which are commercially available. The reactions were conducted at 80 °C using conventional heating, or in a microwave reactor at 150 °C. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715584k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Rhodium catalysed dehydrogenative borylation of alkenes: Vinylboronates via
C–H activation†
Ibraheem A. I. Mkhalid,a R. Benjamin Coapes,a S. Natasha Edes,a David N. Coventry,a Fabio E. S. Souza,b
Rhodri Ll. Thomas,a Jonathan J. Hall,a Si-Wei Bi,c Zhenyang Lin*c and Todd B. Marder*a
Received 10th October 2007, Accepted 22nd November 2007
First published as an Advance Article on the web 20th December 2007
DOI: 10.1039/b715584k
We present herein a high yield, highly selective catalytic synthesis of vinylboronate esters (VBEs),
including 1,1-disubstituted VBEs, from alkenes without significant hydrogenation or hydroboration,
using the simple catalyst precursor, trans-[RhCl(CO)(PPh3)2] (1), and the diboron reagents B2pin2 (2a,
pin = pinacolato = OCMe2CMe2O) or B2neop2 (2b, neop = neopentylglycolato = OCH2CMe2CH2O),
or the monoboron reagent HBpin, all of which are commercially available. The reactions were
conducted at 80 ◦C using conventional heating, or in a microwave reactor at 150 ◦C.
alkene)2RhCl]2 complex gave the trans-VBE and ethyl anisole
Introduction
in a 1 : 1 ratio. Several reports followed using Rh,8a,18a–c,g,h,j
Ti,18d,f Ru,18e,j,l Pd18i and Pt18i catalysts and (RO)2BH reagents,
but only in one case18b was the VBE produced in the absence
of significant hydrogenation, and a 1,1-disubstituted alkene was
employed. The main problem is that most catalysts used in
these reactions are efficient hydrogenation and/or hydroboration
catalysts. Thus, in order to overcome competing hydrogenation
and/or hydroboration of the alkene substrate, catalyst systems
must be capable of carrying out the dehydrogenative borylation
reaction, but must show little or no catalytic activity in alkene
hydrogenations or hydroborations under the conditions employed.
We present herein a high yield, highly selective catalytic synthesis
of VBEs, including 1,1-disubstituted VBEs, from alkenes without
significant hydrogenation or hydroboration using the simple
catalyst precursor, trans-[RhCl(CO)(PPh3)2] (1), and the diboron
reagents B2pin2 (2a) or B2neop2 (2b), or the monoboron reagent
HBpin, all of which are commercially available. Preliminary results
of this study have been communicated.18k
Vinylboronate esters (VBEs) are useful synthetic intermediates in
many reactions, including C–C bond formation via Pd-catalysed
Suzuki–Miyaura cross-coupling.1 VBEs are also important for
homologation to prepare chiral allylboronates,2 in Diels–Alder
reactions,3 in multicomponent chiral amine synthesis,4 in the
preparation of chiral cyclopropanones,5 and in Heck reactions.6
They can be prepared by uncatalysed7 or metal catalysed8 hydrob-
oration of alkynes, eqn (1). Recently, VBEs have been synthesised
by Pd-catalysed cross-coupling of alkenyl halides or triflates with
B2pin2,9 eqn (2), by reaction of 1-halo-1-lithioalkenes and HBpin
at −110 ◦C,10 eqn (3), by Pt-catalysed 1,2-diboration of alkynes,11
eqn (4), by Pd-catalysed diboration of methylenecyclopropanes,12
eqn (5) and by Pd-catalysed borylsilylation or borylstannylation of
1,2-dienes,13 eqn (6). Other interesting procedures for the forma-
tion of VBEs include Miyaura’s unusual trans-hydroboration of
alkynes,14 presumably taking place via a vinylidene intermediate,
eqn (7), the Ru-catalysed olefin cross-metathesis of vinyl boronates
and alkenes,15 eqn (8), catalytic boryl group transfer from a
VBE to an alkene,16 eqn (9), and the enyne cross metathesis
reaction between alkynylboronates and terminal alkenes,17 eqn
(10). An exciting alternative is the catalytic dehydrogenative
borylation of alkenes,8a,18 eqn (11), a C–H bond functionalisation
process which would allow the direct synthesis of 1,1-disubstituted
VBEs that cannot be made via the hydroboration of alkynes.
However, conditions must be found wherein the H2 produced is
not consumed via hydrogenation of half of the alkene substrate.
In 1992, Brown et al.18a,c showed that reaction of 4-vinyl anisole
(1)
(2)
2
(VA) with N-isopropyl oxazaborolidine in the presence of an [(g -
(3)
(4)
aDepartment of Chemistry, Durham University, Durham, UK DH1 3LE.
E-mail: todd.marder@durham.ac.uk; Fax: +44 (0)191 384 4737; Tel: +44
(0)191 334 2037
bDepartment of Chemistry, University of Waterloo, Waterloo, Ontario,
Canada N2L 3G1
cDepartment of Chemistry, Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China. E-mail: chzlin@ust.hk
† This paper is dedicated to Prof. Ken Wade, FRS, on the occasion of his
75th birthday, in recognition of his numerous contributions to boron (and
other!) chemistry.
(5)
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results to those which were obtained from the reactions in toluene–
acetonitrile (3 : 1 T–A), and we found no difference between
toluene and benzene as solvents. The results presented herein are
those obtained via in situ NMR spectroscopy. In general, this data
is, as expected, quite similar to that obtained by GC-MS, with
the notable exception that hydroboration products appear to have
somewhat larger MS response factors than expected, such that
amounts of these were slightly overestimated in the preliminary
studies.
(6)
(7)
(8)
(9)
Dehydrogenative borylations using conventional heating
Firstly, we examined the mono substituted alkenes, styrene and
4-vinylanisole (VA). The reaction of styrene with ◦0.67 equiv. of
2a and 3 mol% of 1 in 2 ml of (3 : 1 d6–d3) at 80 C gave 100%
conversion in 5 days by NMR spectroscopy (Table 1, entry 1)
with 86% selectivity for trans-VBE (i.e. (E)-VBE) and 11 and
3% selectivity for the 2,2-vinylbis(boronate) ester (VBBE) and
hydroboration, respectively. On the other hand, the conversion
by GC-MS was 100% with 66% selectivity for VBE, 19 and 4%
selectivity for two hydroboration isomers and 11% selectivity for
VBBE. Thus, as noted above, the hydroboration products appear
to have a higher response factor than the VBE or VBBE in
the GC-MS. It is also worth noting that the (E)-VBE isomer is
produced in all of the dehydrogenative borylations of the styrene
derivatives.
(10)
(11)
The reaction of VA with 0.67 equiv. of 2a and 3 mol% of 1
gave 99% conversion during 2 days by NMR spectroscopy with
93% selectivity for VBE and 7% hydroboration. It appears, from
1H NMR spectroscopy, that small amounts of both terminal and
internal hydroboration isomers are produced, with the internal
one predominating (Table 1, entry 2). As only 0.67 equiv. of
2a was used, it is evident that both boron atoms of 2a are
being incorporated into the product. As will be shown below
from studies with disubstituted alkenes, it is likely that most of
the hydroboration byproduct arises after the diboron reagent is
consumed, at which point HBpin becomes the boron source.
In an investigation of 2b as a source of boron, VA was reacted
with 0.67 equiv. of 2b and 3 mol% of 1, giving 44% conversion
over 2 days and 65% over 4 days by NMR spectroscopy with
complete selectivity for VBE (Table 1, entries 3 and 4). When
the concentration of 2b was increased to 1 equiv. under the
above conditions, after 2 days NMR spectroscopy showed 100%
conversion and excellent selectivity for the VBE (Table 1, entry
5), although small amounts of what appears to be the 2,2-
vinylbis(boronate) ester (VBBE), vide infra, are also formed. Thus,
2b is also an effective reagent for the diboration of alkenes, and
indeed, is somewhat more reactive than 2a with this substrate.
Interestingly, reaction of VA with 2 equiv. of 2a and 5 mol% of
1 (Table 1, entry 6) gave 93% selectivity for VBBE and 7% VBE
Results and discussion
In the course of our studies on alkene diboration,19 we examined
the reaction of VA with B2pin2 catalysed by 1 in a variety of
solvents. Toluene, THF and 1,4-dioxane all gave complicated
mixtures containing dehydrogenative borylation, diboration, hy-
droboration and hydrogenation products and, in some cases,
vinylbis(boronate) esters (VBBEs). In contrast, the reaction in
CH3CN was clean, but the rate was much slower than that in
toluene, for example. We therefore examined the reaction in 3 :
1 toluene–acetonitrile (3 : 1 T–A) which proved an excellent
compromise between selectivity and rate. In our preliminary
study,18k the yields and selectivities were determined by in situ GC-
MS which, for truly quantitative analysis, requires a knowledge
of the response factors for the starting materials and products,
which in turn, requires their isolation and calibration against
standards. However, as some of the compounds or isomers were
only produced in very small quantities, this was not possible in
all cases. We relied on our previous experience in which related
reaction mixtures had been studied by a combination of in situ
NMR spectroscopy and GC-MS, which showed that isomeric
hydroboration products have quite similar response factors. In
order to improve the accuracy of our measurements in the current
study, we re-examined all of the previous reactions, and carried out
all new reactions, in deuterated solvents in order to obtain both
in situ GC-MS and NMR spectra of the crude reaction mixtures,
using long enough relaxation times to insure accurate integrations.
To this end, we chose to examine these reactions in 3 : 1 C6D6–
CD3CN (3 : 1 d6–d3). For completeness, we also examined the
reactions in protio benzene–acetonitrile (3 : 1) and compared these
=
by NMR spectroscopy. Thus, both hydrogens of the CH2 group
can be replaced by Bpin in a single catalytic reaction.
The reaction of 1-octene with 0.67 equiv. of 1a and 3 mol%
catalyst (Table 1, entry 7) gave 100% conversion with 67% selec-
tivity for a mixture of VBBEs with 33% mono-dehydrogenative
borylation product (VBE) produced. The GC-MS showed 100%
conversion with 66% selectivity for VBBEs, of which 60% is
=
the C6H13CH C(Bpin)2 isomer, with 34% of a mixture of VBEs
produced. This distribution of products likely results from the
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Table 1 Product distribution and conversion for the dehydrogenative borylation of alkenes with B2pin2, B2neop2 and HBpin via conventional heatinga
Hydroboration
Boron reagent (%)
Total VBEb (major Total
isomer) (%)
Conversion
(%)
Entry Substrate
VBBEc %
Time/days Solventd
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Styrene
4-Vinyl anisole
4-Vinyl anisole
B2pin2
B2pin2
B2neop2
B2neop2
B2neop2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
HBpin
B2neop2
B2neop2
B2neop2
B2neop2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
HBpin
B2neop2
3
7
86
93
100
100
ca. 90
7
11
5
2
2
4
2
4
3
6
4
2
6
2
2
2
4
2
3
4
1
4
3
3
2
2
3
5
5
3 : 1 B–A 100
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
99
44
65
4-Vinyl anisole
4-Vinyl anisolee
4-Vinyl anisolef
Octene
ca. 10
93
67
3 : 1 B–A 100
3 : 1 B–A 100
3 : 1 B–A 100
33
Indene
100
100
96
100
97
3 : 1 B–A
3 : 1 B–A
B
19
90
54
52
86
55
42
53
96
a-Methyl styrene
a-Methyl styrene
a-Methyl styrene
a-Methyl styreneg
a-Methyl styrene
a-Methyl styrene
a-Methyl styrene
a-Methyl styrenee
a-Methyl styrenee
1,1-Diphenylethylene
1,1-Diphenylethylene
1,1-Diphenylethylene
1,1-Diphenylethyleneh
1,1-Diphenylethyleneg
1,1-Diphenylethylene
1,1-Diphenylethylene
4
A
3
5
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
95
100
100
100
100
100
94
100
100
100
90
100
100 (91)
100
100
3 : 1 B–A 100
3 : 1 B–A
B
A
67
71
48
70
6
3 : 1 B–A
3 : 1 B–A 100
10
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
44
29
96
81
96
Methylene cyclopentane B2pin2
Methylene cyclohexane B2pin2
Methylene cyclohexaneg B2pin2
a For detailed reaction conditions, see the Experimental section. Reactions were carried out in deuterated solvents. Conversion and product distributions
1
were determined by in situ H-NMR spectroscopy (see text). b VBE = vinylboronate ester. c VBBE = vinylbis(boronate) ester. d B = benzene-d6, A =
acetonitrile-d3. e 1 equiv. of B2neop2 used. f 2 equiv. of B2pin2 and 5 mol% of catalyst used. g 1 equiv. of B2pin2 used. h 5 mol% of catalyst used.
isomerisation of the double bond along the hydrocarbon chain
during the reaction. As such, the catalytic dehydrogenative bo-
rylation reaction appears best suited for systems which cannot
undergo facile double bond isomerisation.
Investigation of solvent effects on the reactions of 1,1-
diphenylethylene and a-methylstyrene showed that the reac-
tion in neat C6D6 was faster than in neat CD3CN. For 1,1-
diphenylethylene in C6D6, 71% conversion was observed in 1 day,
with 94% VBE and 6% of the hydroboration side product (Table 1,
entry 19). On the other hand, the reaction in CD3CN was very
clean, giving 100% selectivity for VBE, but only 48% conversion
over 4 days, as indicated by both NMR and GC-MS (Table 1, entry
20). In addition, a-methylstyrene gave 54% conversion in 2 days
when C6D6 was used, with 94% VBE and 6% of the hydroboration
byproduct (Table 1, entry 9). The reaction in CD3CN gave only
52% conversion in 6 days with 100% VBE (Table 1 and 2, entry 10).
Why the reaction of 1,1-diphenylethylene in C6D6 is more efficient
than that of a-methylstyrene is not clear.
The substrate 1,1-diphenylethylene was reacted with 1 equiv.
of 2a in the presence of 3 mol% of 1 and (3 : 1 d6–d3) to
examine the effect of providing a stoichiometric quantity of the
diboron reagent, 2a, and to see whether or not the reaction went
to completion. After 3 days, 100% conversion to the VBE was
observed (Table 1, entry 22). Similarly, using 1 equiv. of 2a, 86%
conversion of a-methylstyrene was observed within 2 days (Table 1,
entry 12); surprisingly, a small amount (3%) of hydroboration was
found under these conditions. Thus, whereas conversions greater
than 67% can be achieved for a-methylstyrene with prolonged
heating, using 0.67 equiv. of 2a, the reaction is clearly more efficient
when a stoichiometric quantity of the diboron reagent is used. For
1,1-diphenylethylene, conversions of greater than ca. 70% were
only observed when 1 equiv. of 2a was used, indicating that only
The 1,2-disubstituted alkene, indene, and the 1,1,2-trisubs-
tituted alkenes 2-methyl-2-butene and 3,4,4-trimethyl-2-pentene
were also examined under the above conditions. Indene showed
lower reactivity than the mono-substituted alkenes giving 100%
selectivity for VBE but with only 19% conversion by NMR
spectroscopy after 6 days (Table 1, entry 8). NMR spectroscopy
unambiguously identified the VBE as the 2-Bpin isomer (see
the Experimental section), which requires further comment from
a mechanistic point of view, vide infra. No reaction at all
was observed with either 2-methyl-2-butene or 3,4,4-trimethyl-
2-pentene. Consequently, this system appears to be generally
unsuitable for dehydrogenative borylation of 1,1,2-trisubstituted
alkenes.
In contrast, 1,1-disubstiuted alkenes such as a-methylstyrene
and 1,1-diphenylethylene show higher activity than indene and
1,1,2-trisubstituted alkenes. The reaction of 0.67 equiv. of 2a
with a-methylstyrene in the presence of 3 mol% of 1 gave, by
NMR spectroscopy, 100% of the (E)–VBE (Table 1, entry 9) with
90% conversion in 4 days. Under the same reaction conditions,
1,1-diphenylethylene proved less reactive than a-methylstyrene,
most likely due to increased steric bulk, giving 67% conversion
in 4 days, with 100% selectivity for VBE as determined by both
techniques (GC-MS and 1H NMR spectroscopy), and only traces
of hydroboration (Table 1, entry 18).
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Table 2 Product distribution and conversion for the dehydrogenative borylation of alkenes with B2pin2, B2neop2 and HBpin via microwave heatinga
Entry
Substrate
Boron reagent
Hydroboration (%)
VBEb (%)
VBBEc (%)
10
Solventd
Conversion (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Styrenee
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
HBpin
B2neop2
90
100
100
87
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
B
100
77
100
100
100
5
55
65
50
48
44
4-Vinyl anisole
4-Vinyl anisolef,g
4-Vinyl anisoleg,h,i
Octenef
13
65
35
Indene
100
100
100
100
95
100
100
100
100
100
a-Methyl styrene
a-Methyl styreneg,h
1,1-Diphenylethylene
1,1-Diphenylethylene
1,1-Diphenylethylene
1,1-Diphenylethylenei
1,1-Diphenylethylenej
1,1-Diphenylethylene
1,1-Diphenylethylene
5
A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
3 : 1 B–A
62
83
39
29
a For detailed reaction conditions, see the Experimental section. Reactions were carried out in deuterated solvents. Conversion and product distributions
1
were determined by in situ H-NMR spectroscopy (see text). b VBE = vinylboronate ester. c VBBE = vinylbis(boronate) ester. d B = benzene-d6, A =
acetonitrile-d3. e 180 ◦C used. f 10 min heating. g 2 equiv. of B2pin2 used. h 30 min heating. i 5 mol% of catalyst used. j 1 equiv. of B2pin2 used.
one of the two available borons is being incorporated with this
substrate.
(4 days), the conversion was only increased to 53%. However,
increasing the concentration of 2b to 1 equiv. instead of 0.67 equiv.
gave 96% conversion of a-methylstyrene with 100% selectivity for
VBE within 2 days, and 100% conversion after 3 days (Table 1,
entries 16 and 17). Thus, in no case was the second boron
incorporated when 2b was employed, presumably a result of the
fact that HBneop is much less thermally stable than HBpin.
Methylene cyclopentane and methylene cyclohexane were ex-
amined as 1,1-disubstituted alkenes which do not contain aro-
matic substituents. The borylation of methylene cyclopentane
was achieved during 3 days with 0.67 equiv. of 2a giving 96%
conversion with 91% selectivity for the parent VBE and 9%
constituting isomeric VBE (Table 1, entry 25). The GC-MS showed
the same result, and that the VBE consist of 3 isomers, the parent
constituting 92% of the total VBEs. Two other isomeric VBEs
(5% and 3%) presumably result from double bond isomerisation
into the ring, but their small quantities precluded detailed analysis
by NMR spectroscopy, as the aliphatic protons resonances are
overlapped. Finally, methylene cyclohexene was reacted with 0.67
and 1 equiv. of 2a giving 86 and 96% conversions, respectively, in
5 days with complete selectivity for a single VBE (Table 1, entries
26 and 27).
To investigate HBpin as a source of boron, both of the alkenes
1,1-diphenylethylene and a-methylstyrene were reacted with 1.34
equiv. of HBpin in place of 2a under the above conditions. All
reactions gave VBE with modest amounts of hydroboration. The
reaction of 1,1-diphenylethylene gave 44% conversion over 2 days
with 90% selectivity for VBE and 10% for hydroboration (Table 1,
entry 23) whereas a-methylstyrene gave 55% conversion by NMR
spectroscopy during 2 days with 95% selectivity for VBE and 5%
for hydroboration (Table 1, entry 13). These experiments indicate
that HBpin can indeed be used in these reactions with fairly
limited hydroboration taking place, although the amount of this
competing reaction is greater than when the diboron reagents are
employed. Hydrogenation was not significant in any case. The
above results, when taken together, also indicate that whilst HBpin
is indeed formed from 2a after initial dehydrogenative borylation
takes place, high initial concentrations of this reagent do lead to
some hydroboration. Also, if formed from 1,1-diphenylethylene
dehydrogenative borylation using 2a, HBpin is not effective,
presumably due to catalyst and/or HBpin decomposition during
the prolonged heating periods. In addition, the reaction of 1,1-
diphenylethylene was examined with 5 mol% of catalyst and 0.67
equiv. of B2pin2, (Table 1, entry 21), which gave 70% conversion
during 3 days with 100% selectivity for VBE. While the reaction
proceeds somewhat faster with the higher catalyst loading, no
significant effect on total conversion was observed for this system,
so the key factor is increasing the amount of B2pin2 to 1 equiv.
Diboron compound 2b was then investigated as a source
of boron with the substrates 1,1-diphenylethylene and a-
methylstyrene. The reaction of 1,1-diphenylethylene with 0.67
equiv. of 2b gave 29% conversion during 2 days by both GC-MS,
1H NMR spectroscopy, with 100% selectivity for VBE (Table 1,
entry 24). Reaction with a-methylstyrene gave 42% conversion
during 2 days with 100% selectivity for VBE (Table 1, entry 14).
This shows that 2b has a lower reactivity than 2a in this reaction.
When the reaction of a-methylstyrene with 2b was carried out
under the above conditions but with an extended reaction time
Dehydrogenative borylations using microwave heating
The thermal reactions discussed above were re-examined using
microwave heating in the presence of 3 mol% of 1 with 0.67 equiv.
of 2a in a mixture of deuterated solvents (3 : 1 d6–d3) under a
variety of conditions. Selectivities and conversions were measured
1
by in situ H NMR spectroscopy, and these can be compared
with those obtained from conventional thermal reactions. It
should be noted that the reactions of styrene, 1-octene, indene
and 1,1-diphenylethylene were carried out in a CEM microwave
reactor at 150 W maximum microwave power and 150 ◦C, unless
otherwise stated. Reactions of 4-vinylanisole and a-methylstyrene
were carried out in a Biotage Personal Chemistry Optimizer
microwave reactor with a maximum input power 300 W to reach
the required temperature of 150 ◦C. Once the required temperature
1058 | Dalton Trans., 2008, 1055–1064
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is reached, monitored internally by an infrared sensor, the reactor
automatically adjusts the microwave input power to maintain the
setpoint temperature.
With 0.67 equiv. of 2a, 1,1-diphenylethylene gave 67% conver-
sion after 3 days at 80 C and 50% conversion by the microwave
◦
reaction after 1 hour with the same selectivity for VBE. Moreover,
increasing the amount of 2a to 1 equiv. in the microwave reaction
gave 83% conversion after only 1 hour. Similar and relatively low
conversion but excellent selectivity was observed in the reaction
of 1,1-diphenylethylene with 2b by either heating technique, but
the higher temperature of the microwave reactions again allowed
the reaction times to be reduced from days to 1 hour. For a-
methylstyrene, we were only able to achieve 65% conversion in the
microwave reaction when the amount of 2a was increased to 2
equiv.
The reaction of styrene (Table 2, entry 1) with 0.67 equiv. of 2a
and 3 mol% of 1 was carried out at a higher temperature, 180 ◦C,
than the other microwave reactions. The conversion was 100%,
with 90% selectivity for VBE and 10% for VBBE after only 1 hour.
Reaction of VA at 150 ◦C gave 77% conversion after 1 hour, with
100% selectivity for VBE (Table 2, entry, 2). The reaction was re-
examined with 2 equiv. of 2a to attempt to improve the selectivity
for VBBE; 100% conversion occurred within only 10 minutes,
giving 100% selectivity for VBE (Table 2, entry 3). Increasing the
reaction time to either 30 or 60 minutes, using 5 mol% of 1, gave
13% VBBE and 87% VBE, with 100% conversion (Table 2, entry 4).
The reaction of 1-octene with 0.67 equiv. of 2a (150 W) at 150 ◦C
for 10 minutes gave 100% conversion, with 35% VBE and 65%
VBBE (Table 2, entry 5). Indene gave only 5% conversion during
1 hour, but with 100% selectivity for VBE (Table 2, entry 6).
Preliminary theoretical studies of and comments on some possible
reaction mechanisms
Given the presence of the strong p-acceptor CO ligand and the
relatively weak PPh3 donor ligand, it is extremely unlikely that
the C–H activation process involves a direct oxidative addition
of the vinylic-C–H bond of the substrate, and we are not aware
of any such C–H oxidative additions involving 1. With this
in mind, a more likely scenario involves formation of a Rh–
boryl complex followed by insertion of alkene into the Rh–
B bond and subsequent b-hydride elimination giving the VBE
product and a rhodium hydride. Indeed, the results of such a
process were observed20 in the stoichiometric reaction of VA
with [RhCl(Bcat)2(PPh3)2], which also led to the diborated alkene
via competing reductive elimination following the initial alkene
insertion process. In addition, the [RhClH(Bcat)(PPh3)2] formed
via b-hydride elimination led to hydroborated products. As neither
significant amounts of hydroboration nor any diboration products
were observed in the current study, a related mechanism would
require that b-hydride elimination be much faster than B–C
reductive elimination and that subsequent insertion of alkene into
the resulting [RhClH(Bpin)(PPh3)(CO)] would take place only
into the Rh–B bond leading again to rapid b-hydride elimination
rather than C–H reductive elimination (giving hydroboration), and
would still fail to explain the influence of acetonitrile on greatly
improving selectivity to VBE. For a host of reasons, this type of
mechanism thus seems unlikely. If acetonitrile were to coordinate
to the ‘vacant’ site in a putative alkene insertion product of
the form [RhClH(CHRCH2Bpin)(PPh3)(CO)], this would inhibit
coordination of a Bpin oxygen atom at this site (Scheme 1), which
has been predicted theoretically to enhance reductive elimination
in a similar system, due to ring strain.21 However, this would also
block the site required for b-hydride elimination. We therefore
considered the possibility (Scheme 1) of an unconventional b-
hydride elimination pathway which could potentially occur in
a coordinatively-saturated complex, involving direct B–H bond
formation via b-hydride transfer to the electron-deficient cis-boryl
ligand on an acetonitrile-coordinated boryl–alkyl Rh intermediate
(1) in Fig. 1. We investigated this possibility with the aid of DFT
calculations at the B3LYP level. Indeed, we were able to locate a
transition state for the direct B–H bond formation (see TSA in
Fig. 1). However, the corresponding reaction free energy barrier
(43.30 kcal mol−1) is extremely high, suggesting that the reaction
path (path A in Fig. 1) is unlikely. Alternatively, we found that a
two-step reaction pathway (path B in Fig. 1), which involves b-
hydride elimination giving a Rh(III) intermediate (INTB) followed
◦
The reaction of a-methylstyrene was examined at 150 C with
0.67 equiv. of 2a in the presence of 3 mol% of 1. After 1 hour,
the reaction gave 100% selectivity for VBE with 55% conversion
(Table 2, entry 7). Using 2 equiv. of 2a, increased the conversion
slightly to 65% after 30 minutes with 100% selectivity for VBE
(Table 2, entry 8). Similar conversions and selectivities were
observed when the reaction time was extended to 60 minutes. The
reaction of 1,1-diphenylethylene was examined with 0.67 equiv.
of 2a in the presence of 3 mol% of 1 at 150 ◦C for 1 hour. The
conversion was 50% with 100% selectivity for VBE (Table 2, entry
9). Increasing the concentration of catalyst to 5 mol%, with 0.67
equiv. of 2a only increased the conversion to 62% (Table 2, entry
12). Using 1 equiv. of 2a with 3 mol% of 1, however, gave 83%
conversion with 100% selectivity for VBE (Table 2, entry 13). To
investigate the solvent effect on the dehydrogenative borylation
of 1,1-diphenylethylene under microwave heating, the reaction
was carried out in neat C6D6 and neat CD3CN at 150 ◦C for
1 hour. The reaction in neat C6D6 gave 95% selectivity for VBE
with 5% hydroboration and 48% conversion (Table 2, entry 10),
whereas CD3CN gave 44% conversion with 100% selectivity for
VBE (Table 2, entry 11). Using 1.34 equiv. of HBpin in place of
B2pin2 resulted in only 39% conversion after 1 hour, but with 100%
selectivity for VBE (Table 2, entry 14), whereas using 0.67 equiv.
of 2b gave 29% conversion after 1 hour with 100% selectivity for
VBE (Table 2, entry 15).
Comparison between thermal and microwave reactions
The conversions of styrene by thermal or microwave heating were
similar, although the reaction time under microwave irradiation at
180 ◦C, compared with the thermal reaction at 80 ◦C, was reduced
from 5 days to 1 hour. VA was converted completely to its VBE in
only 10 minutes using 2 equiv. of 2a at 150 ◦C. Even with 2 equiv.
of 2a, larger amounts of catalyst, and heating for 1 hour, relatively
little progression to the VBBE was observed. The reaction of 1-
octene with 0.67 equiv. of 2a gave the same conversion (100%) and
the same selectivities for VBE and VBBE by both thermal and
microwave heating, but the reaction time was reduced from 3 days
at 80 ◦C to only 10 minutes at 150 ◦C. Indene showed even lower
conversion at 150 ◦C than at 80 ◦C.
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the electron count by two around the metal centre. The ligand dis-
sociation is necessary in order to maintain an acceptable electron
count, i.e. 18 electrons or 16 electrons depending on the structure
considered. Therefore, in the determination of the transition state
structures, we considered the acetonitrile as a separate entity.
As pointed out above, a process such as that in path B seems
unlikely to account for the lack of any diboration products, the
overall selectivities observed or the unusual solvent effects. Thus,
the overall mechanism of the current reaction remains unclear.
However, the fact that TSA was located at all is interesting in its
own right, and suggests the possibility that diverse and unconven-
tional mechanism may operate in appropriate circumstances. A di-
rect B–C bond formation path, considered as a r-bond metathesis
path, was found to be feasible, for example, in alkane borylations
mediated by Cp-containing Fe and W boryl complexes.22 The
results here suggest that different coordination environments
and metal centres have significant influences in the selection of
favourable reaction pathways, and that a variety of mechanisms
are worth investigating, even if they involve unusual steps.
It remains to address the fact that only (E)-VBEs are formed
in these reactions. We believe that this is a result of the fact that,
following alkene insertion into a Rh–B bond (which no doubt is a
syn-addition process), either of the two diastereotopic b-hydrogens
could be transferred to the Rh. As this b-hydride elimination
process also requires a syn-disposition of the hydride and Rh
groups, rotation around the C–C bond is required (Fig. 2). The
least hindered rotamer, illustrated for the a-methylstyrene case,
would thus lead to the observed (E)-product. Clearly this would
be even more prevalent in styrene and VA, in which the a-Me group
is replaced by an even smaller H atom. The case of indene requires
additional discussion. In this case, syn-addition of Rh–B across
the double bond leads to a ring system in which the Rh and b-H
groups are mutually trans. If both occupy pseudo axial positions,
an anti disposition would make b-hydride elimination impossible.
However, a conformation in which the two groups are pseudo-
equatorial would bring them much closer together, although the
Rh–C–C–H group could not attain complete planarity. As a
result, b-hydride elimination might be expected to be quite slow,
consistent with the observed low conversions for this substrate.
Finally, it is worth pointing out that the insertion of alkenes
into M–B bonds can be a reversible process, as b-boryl elimination
can also have a relatively small barrier.23
Scheme 1 Some possible reaction pathways.
Fig. 1 Computed energies for some possible intermediates and tran-
sition states for conventional and unconventional b-hydride elimination
pathways.
Fig. 2 Conformation leading to (E)-VBE.
by reductive elimination leading to the B–H bond formation, has
a much lower barrier. The b-hydride elimination step is the rate-
determining step and requires a free energy barrier of 23.83 kcal
mol−1. In the calculations of TSA and TSB1, the coordinated
acetonitrile was found to be dissociating. This phenomenon is
understandable because the conversion from I to II or I to INTB
in the related b-hydride elimination steps involves an increase of
Conclusions
Mono and 1,1-disubstituted alkenes can be converted into VBEs
in high yield and with excellent selectivity via dehydrogenative
borylation of C–H bonds using the simple catalyst, 1, and
commercially available diboron reagents, 2a and 2b, which give
better selectivities than HBpin.
1060 | Dalton Trans., 2008, 1055–1064
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1
In situ H NMR spectroscopy, which was used to determine
pathways are required to elucidate the mechanism and to develop
higher activity catalyst systems capable of carrying out the
reactions at lower temperatures and with reduced reaction times.
In addition, it can be envisaged that more stable catalyst systems
would allow for higher conversions with very short reaction times
at the high temperatures employed in the microwave reactions.
The excellent selectivity of the reactions, especially with 1,1-
disubstituted substrates, makes this technology suitable for appli-
cations involving the conversion of hydrocarbons into high-value
added species via dehydrogenative borylation/Suzuki–Miyaura
cross-coupling sequences, and such studies are in progress in our
laboratory.
the conversions of the reactions, was slightly more accurate than
GC-MS due to the fact that the NMR technique does not
depend on a response factor for each compound; however, it
1
is difficult to estimate, by H NMR, the side products such as
those arising from hydroboration and hydrogenation if they are
present in very small quantities. To improve the accuracy of the
GC-MS method, it is necessary to obtain response factors for
the starting materials and every product; however, this is not
always practical, especially when many different substrates are
employed, as it would entail isolation of every possible product and
isomer. What is clear, however, is that, in this study, closely related
isomeric products have very similar response factors, and that
hydroboration products have somewhat larger response factors
than VBEs, which in turn have larger response factors than the
hydrocarbons. In any event, the differences are not very large, and
for this chemistry, GC-MS, even without predetermined response
factors, provides reasonable quality data which can be considered
as at least semi-quantitative.
Microwave heating is a useful technique to reduce the time of
the reaction from days to minutes. Noting that the ◦temperatures
used in the two types of reactions were 80 and 150 C, and that,
as a rule of thumb, one may expect an approximate doubling of
reaction rates with every 10 ◦C increase in temperature, we might
expect rate increases of ca. 27 = 128 in the microwave reactions.
Thus, 60 minutes at 150 ◦C is roughly comparable to 5 days at
80 ◦C. With this in mind, we can suggest that there is nothing
unusual about the microwave results to imply any special effect
rather than the ability to heat the sample to 150 ◦C safely and
rapidly, well above the solvent boiling points in a sealed system,
allowing for convenient, rapid reactions. As these are carried out
in sealed tubes, the higher temperature results in a much higher
pressure which could inhibit reactions which produce gaseous
products such as H2. Obviously, care needs to be taken to calculate
the maximum potential pressure to avoid explosions, although the
reactor monitors pressure in situ and shuts off the heating when
a preselected maximum (safe) pressure is reached. In addition,
the reactor is designed to withstand explosion of the thick-walled,
glass reaction tubes, although this was not encountered.
It can be seen that the selectivities were fairly similar but the
overall conversions were often significantly higher in the thermal
reactions compared to the microwave ones. We attribute the
somewhat lower conversions obtained with less reactive substrates
to significant catalyst decomposition after 1 hour at the higher
temperatures employed in the microwave reactions, whereas the
more reactive substrates, such as VA, allowed complete conversion
within 10 minutes during which time catalyst decomposition is
apparently not significant.
Finally, the detailed mechanism of the reaction remains un-
known, although with the CO-containing catalyst system em-
ployed, it most likely involves insertion of the alkene into a Rh–B
bond, followed by b-hydride elimination, as opposed to direct C–H
bond oxidative addition. In view of the preliminary calculational
results, we feel that the reaction mechanism involved in the
dehydrogenative borylation reactions is more complicated and
probably very different from that which we initially thought. The
role of CH3CN in greatly enhancing the selectivity towards VBE
with styrene substrates remains unclear. Further experimental
studies as well as theoretical studies of various possible reaction
Experimental
All reactions were carried out under a dry nitrogen atmosphere
using standard Schlenk techniques or in an Innovative Technology,
Inc. System 1 glove box. Glassware was oven dried before transfer
into the glove box. Toluene was dried and deoxygenated by
passage through columns of activated alumina and BASF-R311
catalyst under Ar pressure using a locally modified version of the
Innovative Technology, Inc. SPS-400 solvent purification system.24
The solvents CH3CN, CD3CN, C6D6 and CDCl3 were dried over
calcium hydride, and 1,4-dioxane, THF and C7D8 were dried over
sodium–benzophenone; all were distilled under nitrogen. Alkenes
were purchased from Aldrich Chemical Company, Lancaster
Synthesis or Avocado Research Chemicals, and were checked
for purity by NMR and GC-MS techniques and distilled from
25
calcium hydride under nitrogen. The boron reagents B2pin2 and
25b,26
B2neop2
were generously donated by Frontier Scientific Inc.,
NetChem Inc. and AllyChem Co. Ltd. and were checked for
purity by NMR and GC-MS techniques. HBpin was purified as in
ref. 27. The catalyst precursor [Rh(CO)(Cl)(PPh3)2] was prepared
using published procedures28 and checked for purity by NMR
spectroscopy. NMR spectra were recorded at 25 ◦C on Varian
1
Inova 500 (1H, 13C{ H}, HSQC, NOESY), Bruker Avance (1H,
1
1
1
13C{ H}, 11B{ H}), Varian Mercury 400 (1H, 13C{ H}), Varian
Unity 300 (11B and 11B{ H}) and Bruker AC200 (11B{ H}) in-
struments. Proton and carbon spectra were referenced to external
SiMe4 via residual protons in the deuterated solvents or solvent
resonances, respectively. 11B chemical shifts were referenced to
external BF3·OEt3. Elemental analyses were conducted in the
Department of Chemistry at the University of Durham using
an Exeter Analytical Inc. CE-440 Elemental Analyzer. GC-MS
analyses were performed on a Hewlett-Packard 5890 Series II gas
chromatograph equipped with a 5971A mass selective detector
and a 7673 autosampler. A fused silica capillary column (10
or 12 m cross-linked 5% phenylmethylsilicone) was used, and
the oven temperature was ramped from 50 to 280 ◦C at a rate
1
1
◦
of 20 C min−1. UHP grade helium was used as the carrier
gas. The screw-cap autosampler vials used were supplied by
Thermoquest Inc. and were fitted with teflon/silicone/teflon septa
and 0.2 ml micro inserts. Microwave reactions were carried out in
a CEM Corporation or a Biotage Personal Chemistry Optimizer
microwave reactor, in the latter case, using an automated sample
changer. The reactions were carried in thick-walled, glass reaction-
tubes with septum seals, specifically designed to withstand the
pressures generated.
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+
+
+
=
Typical catalytic reaction conditions
[M ], 371 (0.2) [M -Me], 84 (100) [Me2C CMe2] . The NOESY
NMR spectrum shows a correlation between the ortho CH on the
Conventional heating. All reactions were carried out in deuter-
ated solvents and were prepared in a nitrogen-filled glove
box (Innovative Technology, Inc.). To a solution of trans-
[Rh(Cl)(CO)(PPh3)2] (3 mol% unless otherwise indicated) in 1 ml
of solvent was added a mixture of boron reagent (0.4 mmol total
boron unless otherwise indicated) and alkene (0.3 mmol) in 1 ml
of solvent (2 ml total solvent volume). The mixture was shaken
vigorously to ensure complete mixing, transferred to ampoules
=
arene ring and the CH of the alkene.
CH3(CH2)5CH C(Bpin)2. 1H NMR (400 MHz, C6D6): d 0.76
=
(t, 3JH–H = 6.95, 3H, CH3), 0.89–1.13 (m, 32H, (Bpin)2 +(CH2)4),
3
=
2.25 (q, JH–H = 7.32, 2H, CHCH2(CH2)4), 6.62 (m, 1H,
CH C(Bpin)2); 13C{ H} NMR (100 MHz, C6D6): d 14.0 (s, CH3),
1
=
22.7 (s, CH3CH2(CH)4), 24.6 (s, C3H6CH2CH3), 24.7, 24.8 (s,
BO2C2(CH3)4), 29.4 (s, C3H6CH2CH3), 31.6 (s, C3H6CH2CH3),
◦
=
32.8 (s, CH2CH C(Bpin)2), 83.0 (s, BO2C2(CH3)4), 163.7 (s,
sealed with a Teflon Young’s tap and then heated to 80 C. The
=
CH2CH C(Bpin), the resonance for the carbon attached to boron
reaction was monitored by either GC-MS or a combination of
GC–MS and 1H NMR spectroscopy. In the latter case, an aliquot
of the solution was removed by syringe under nitrogen and was
then diluted with additional dry, O2-free C6D6 in an NMR tube.
1
was not observed; 11B{ H} NMR (128.4 MHz, C6D6): d 28.3 (s,
br); MS (EI): m/z (rel. int.): 364 (0.31) [M+], 349 (2) [M+-Me], 84
(100).
Microwave heating. In a nitrogen-filled glove box, a mixture of
boron reagent (0.4 mmol total boron unless otherwise indicated)
and alkene (0.3 mmol) in 1 ml of deuterated solvent was added to
a solution of trans-[Rh(Cl)(CO)(PPh3)2] (3 mol% unless otherwise
indicated) in 1 ml of solvent (2 ml total solvent volume). The mix-
ture was shaken vigorously to ensure complete mixing, transferred
to a microwave reactor tube (5 ml) which was then crimp sealed and
heated in the microwave reactor at 150 or 300 W, unless otherwise
stated, and 150 ◦C for 60 min. Reactions of styrene, 1-octene,
indene and 1,1-diphenylethylene were carried out in the CEM
microwave reactor (150 W) at 150 ◦C, unless otherwise stated.
Reactions of 4-vinylanisole and a-methylstyrene were carried out
in a Biotage Personal Chemistry Optimizer reactor, (300 W) at
150 ◦C. Pressure and temperature were monitored automatically,
in situ, to assure that safe limits were maintained. The crude
reaction solutions were examined by either NMR spectroscopy
or a combination of NMR spectroscopy and GC–MS.
Inden-2-yl(Bpin). 1H NMR (400 MHz, C6D6): d 1.26 (s, 12H,
Bpin), 3.28 (s, 2H, b), 7.14–7.42 (m, 4H, e, f, g, h), 8.14 (s, 1H,
1
a);13C{ H} NMR (100 MHz, C6D6): d 22.7 (s, BO2C2(CH3)4), 38.9
(s, b), 83.3 (s, BO2C2(CH3)4), 124.6, 125.8, 126.1, 126.3 (s, e, f, g,
h) 134.2, 134.8 (s, c, d), 149.4 (s, a); the resonance for the carbon
1
attached to boron was not observed 11B{ H} NMR (128.4 MHz,
C6D6): d 28.3 (s, br); MS (EI): m/z (rel. int.): 242 (4) [M+], 227 (2)
[M+-Me], 142 (100).
(E)-PhC(Me) CH(Bpin). 1H NMR (300 MHz, CDCl3) d 1.3
=
3
=
(s, 12H, Bpin), 2.41 (d, J(H,H) = 1 Hz, 3H, CH3PhC ), 5.77
3
=
(q, J(H,H) = 1 Hz, 1H, CHBpin), 7.31 (m, 2H, C6H5), 7.48
1
(m, 3H C6H5); 13C{ H} NMR (126 MHz, CDCl3) d 20.1 (s,
=
(E)-(Ph)CH CH(Bpin). 1H NMR (500 MHz, C6D6): d 1.12
CH3PhC ), 24.8 (s, BO2C2(CH3)4), 82.9 (s, BO2C2(CH3)4), 115.5
=
=
=
(s, 12H, Bpin), 6.44 (d, 3J(H,H) = 18 Hz, 1H, CHBpin),
(s, br, CHBpin), 125.8, 128.0, 128.2, 143.8, 157.8 (MePhC );
=
1
11B{ H} NMR (96 MHz, CDCl3) d 29.0 (s, br); anal. calcd (%) for
7.00 (m, 3H, C6H5), 7.24 (m, 2H, C6H5), 7.75 (d, 3J(H,H) =
1
18 Hz, 1H, ArCH ); 13C{ H} NMR (126 MHz, C6D6): 24.8 (s,
C15H21O2B: C 73.79, H 8.67; found C 73.21, H 8.67; MS (EI): m/z
(rel. int.): 244 (21) [M+], 229 (8) [M+-Me], 105 (100). The NOESY
NMR spectrum shows correlations consistent with this molecular
geometry.
=
=
BO2C2(CH3)4), 83.3 (s, BO2C2(CH3)4), 116.9 (s, br, CHBpin)
1
127.4, 128.8, 129.0, 137.9 (s, C6H5), 150.4 (s, ArCH ); 11B{ H}
=
NMR (96 MHz, C6D6): 30.4 (s, br); MS (EI) m/z (rel. int.): 230
(10) [M+], 215 (7) [M+-Me], 120 (100).
Ph2C CH(Bpin). 1H NMR (400 MHz, C6D6): d 1.04 (s,
=
=
12H, Bpin), 6.01 (s, 1H, CHBpin), 7.06–7.8 (m, 10H, Ph2);
(E)-(4-MeO–C6H4)CH CH(Bpin). 1H NMR (500 MHz,
=
1
13C{ H} NMR (100 MHz, C6D6): d 24.5 (s, BO2C2(CH3)4), 83.1
C6D6): d 1.14 (s, 12H, Bpin), 3.23 (s, 3H, CH3O), 6.34 (d,
(s, BO2C2(CH3)4), 128.2, 128.3, 130.1 143.2 (s, C6H4), 160.3 (s,
3
=
J(H,H) = 18 Hz, 1H, CHBpin), 6.62 (m, 2H, C6H4), 7.28 (d,
=
Ph2C ), the resonance for the carbon attached to boron was not
2H, C6H4), 7.76 (d, J(H,H) = 18 Hz, 1H, ArCH ); 13C{ H}
3
1
=
1
observed; 11B{ H} NMR (128.4 MHz, C6D6): d 30.25 (s, br); MS
(126 MHz, C6D6): d 24.8 (s, BO2C2(CH3)4), 54.6 (s, CH3O), 83.0
(EI): m/z (rel. int.): 306 (21) [M+], 291 (3) [M+-Me], 190 (100).
=
(s, BO2C2(CH3)4), 114.2 (s, br, CHBpin), 116.0 (s, C6H4), 128.7
C6H10 CH(Bpin). 1H NMR (500 MHz, C6D6): d 1.24 (s, 12H,
=
(s, C6H4), 130.8 (s, C6H4), 145.1 (s, ArCH ), 160.7 (s, C6H4);
=
11B{ H} NMR (64 MHz, C6D6) 30.9 (s, br); MS (EI): m/z (rel.
1
Bpin), 1.35 (m, 2H, CH2), 1.43 (m, 2H, CH2), 1.51 (m, 2H, CH2),
int.): 260 (100) [M+], 245 (16) [M+-Me].
2.10 (t, 3JH–H = 6 Hz, 2H, CH2), 2.68 (t, 3JH–H = 6 Hz, 2H, CH2),
1
5.29 (s, 1H, CHBpin); 13C{ H} NMR (126 MHz, C6D6): d 25 (s,
=
4-MeO–C6H4CH C(Bpin)2. 1H NMR (500 MHz, C6D6): d
=
BO2C2(CH3)4), 26.8 (s, CH2), 28.9 (s, CH2), 29.1 (s, CH2), 33.5 (s,
1.24 (s, 24H, Bpin), 3.31 (s, 3H, CH3O), 6.70 (d, 3J(H,H) =
=
CH2), 40.5 (s, CH2), 83 (s, BO2C2(CH3)4), 112 (s, br, CHBpin),
8 Hz, 2H, o-C6H4), 7.49 (d, 3J(H,H) = 8 Hz, 2H, m-C6H4),
1
167.19 (s, C CHBpin); 11B{ H} NMR (128.4 MHz, C6D6): d 30.2
=
1
7.98 (s, 1H, ArCH ); 13C{ H} NMR (100 MHz, C6D6): d 24.8
=
(s, br); MS (EI): m/z (rel. int.): 222 (1) [M+], 207 (0.006) [M+-Me],
(s, BO2C2(CH3)4), 54.6 (s, CH3O), 83.4 (s, BO2C2(CH3)4), 127.2,
55 (100).
=
128.8, 129.3, 137.8 (s, C6H4), 145.1 (s, ArCH ), the resonance for
1
the carbon attached to boron was not observed; 11B{ H} NMR
C5H8 CH(Bpin). 1H NMR (500 MHz, C6D6): d 1.09 (s,
=
(128.4 MHz, C6D6): d 30.25 (s, br); MS (EI): m/z (rel. int.): 386 (2)
1062 | Dalton Trans., 2008, 1055–1064
12H, Bpin), 1.41 (m, 2H, CH2), 1.51 (m, 2H, CH2), 2.24
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(t, 3JH–H = 7.32 Hz, 2H, CH2), 2.69 (t, 3JH–H = 7.32 Hz, 2H,CH2),
a Journals Grant for International Authors. Z. L. thanks the
Research Grants Council of Hong Kong for financial support
(HKUST601507). I. A. I. M. thanks the Saudi Arabian Cultural
Attache (London) for a postgraduate scholarship. We thank
Frontier Scientific, Inc., NetChem Inc. and AllyChem Co. Ltd.
for generous gifts of diboron reagents, Mr I. H. McKeag, Mrs
C. F. Heffernan, and Dr A. M. Kenwright for assistance with
some of the NMR spectra, and Mrs J. Dostal for carrying out the
elemental analyses.
1
5.58 (s, 1H, CHBpin); 13C{ H} NMR (126 MHz, C6D6): d 25.14
=
(s, BO2C2(CH3)4), 26.34 (s, CH2), 27.34 (s, CH2), 33.87 (s, CH2),
=
37.39 (s, CH2), 82.64 (s, BO2C2(CH3)4), 110 (s, br, CHBpin),
11
1
=
171.67 (s, C CHBpin); B{ H} NMR(128.4MHz, C6D6): d 30.18
(s, br); MS (EI): m/z (rel. int.): 207 (6) [M+], 192 (8) [M+-Me], 55
(100).
(E)-(MeOC6H4)CH CH(Bneop). 1H NMR (500 MHz,
=
C6D6): d 0.63 (s, 6H, Bneop-CH3), 3.23 (s, 4H, Bneop-CH2), 3.40
3
=
(s, 3H, CH3O), 6.41 (d, JH–H = 18 Hz, 1H, CHBneop), 7.01
3
References
(m, 2H, C6H4), 7.37 (m, 2H, C6H4), 7.79 (d, JH–H = 18 Hz, 1H,
ArCH ); 13C{ H} NMR (126 MHz, C6D6): 21.5 (s, Bneop-CH3),
1
=
1 (a) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 49; (b) A. Suzuki,
J. Organomet. Chem., 1999, 576, 147.
30.1 (s, Bneop-C(CH3)2), 70.6 (s, Bneop-CH2), 71.9 (s, CH3O),
2 R. W. Hoffmann and B. Landman, Chem. Ber., 1986, 119, 2013.
3 (a) P.-Y. Reynard, Y. Six and J.-Y. Lallemand, Tetrahedron Lett., 1997,
38, 6589; (b) A. Zhang, Y. Kan and B. Jiang, Tetrahedron, 2001, 57,
2305.
4 (a) R. A. Batey, D. B. MacKay and V. Santhakumar, J. Am. Chem.
Soc., 1999, 121, 5075; (b) N. A. Petasis and S. Boral, Tetrahedron Lett.,
2001, 42, 539.
5 J. E. A. Luithle and J. Pietruszka, Liebigs Ann., 1997, 2297.
6 N. He´naff and A. Whiting, Tetrahedron, 2000, 56, 5193.
7 (a) H. C. Brown and S. K. Gupta, J. Am. Chem. Soc., 1975, 97, 5249;
(b) C. F. Lane and G. W. Kabalka, Tetrahedron, 1976, 32, 981.
8 (a) K. Burgess, W. A. van der Donk, S. A. Westcott, T. B. Marder, R. T.
Baker and J. C. Calabrese, J. Am. Chem. Soc., 1992, 114, 9350; (b) K.
Burgess and M. J. Ohlmeyer, Chem. Rev., 1991, 91, 1179; (c) S. Pereira
and M. Srebnik, Organometallics, 1995, 14, 3127; (d) X. He and J. F.
Hartwig, J. Am. Chem. Soc., 1996, 118, 1696; (e) I. Beletskaya and A.
Pelter, Tetrahedron, 1997, 53, 4957; (f) T. Ohmura, Y. Yamamoto and
N. Miyaura, J. Am. Chem. Soc., 2000, 122, 4990.
=
115.9 (s, C6H4), 119.0 (s, br, CHBneop), 128.8, 131.3 (s, C6H4),
1
147.6 (s, ArCH ), 160.5 (s, C6H4); 11B{ H} NMR (96 MHz,
=
C6D6): d 27.1 (s, br); MS (EI) m/z (rel. int.): 256 (100) [M+], 231
(3) [M+-Me].
(E)-PhC(Me) CH(Bneop). 1H NMR (500 MHz, C6D6): d
=
3
=
0.60 (s, 6H, neop-CH3), 2.50 (d, JH–H = 1 Hz, 3H, CH3PhC ),
3.38 (s, 4H, Bneop-CH2), 6.06 (q, 3JH–H = 1 Hz, 1H, CHBneop),
=
7.14 (m, 2H, C6H5), 7.31 (m, 3H, C6H5); 13C{ H} NMR (126 MHz,
1
=
CDCl3): d 21.8 (s, CH3PhC ), 21.9 (s, neop-CH3), 31.6 (s, neop-
=
C(CH3)2), 72.1 (s, neop-CH2), 119.8 (s, br, CHBpin), 126.5,
128.1, 128.5, 143.5 (C6H5), 155.4 (s, MePhC ); 11B{ H} NMR
1
=
(96 MHz, C6D6): d 26.7 (s, br); MS (EI) m/z (rel. int.): 230
(100) [M+], 215 (3) [M+-Me]. The NOESY NMR spectrum shows
correlations consistent with this molecular geometry.
9 (a) K. Takahashi, J. Takagi, T. Ishiyama and N. Miyaura, Chem. Lett.,
2000, 126; (b) T. Ishiyama and N. Miyaura, J. Organomet. Chem., 2000,
611, 392.
10 T. Hata, H. Kitagawa, H. Masai, T. Kurahashi, M. Shimizu and T.
Hiyama, Angew. Chem., 2001, 113, 812, (Angew. Chem., Int. Ed., 2001,
40, 790).
Ph2C CH(Bneop). 1H NMR (500 MHz, C6D6): d 0.58 (s, 6H,
=
=
neop-CH3), 3.21 (s, 4H, neop-CH2), 6.24 (s, 1H, CHBneop),
7.11 (m, 4H, Ph), 7.27 (m, 6H, Ph); 13C{ H} NMR (126 MHz,
1
C6D6): 21.5 (s, neop-CH3), 31.4 (s, neop-CMe2), 71.7 (s, neop-
11 (a) T. Ishiyama, N. Matsuda, N. Miyaura and A. Suzuki, J. Am. Chem.
Soc., 1993, 115, 11018; (b) T. Ishiyama, N. Matsuda, M. Murata, F.
Onazawa, A. Suzuki and N. Miyaura, Organometallics, 1996, 15, 713;
(c) G. Lesley, P. Nguyen, N. J. Taylor, T. B. Marder, A. J. Scott, W.
Clegg and N. C. Norman, Organometallics, 1996, 15, 5137; (d) C. N.
Iverson and M. R. Smith, III, Organometallics, 1996, 15, 5155; (e) R. Ll.
Thomas, F. E. S. Souza and T. B. Marder, J. Chem. Soc., Dalton Trans.,
2001, 1650.
=
CH2), 123.0 (s, br, CHBneop), 127.9, 128.1, 128.4, 130.1, (s, Ph)
1
158.2 (s, Ph2C ); 11B{ H} NMR (96 MHz, C6D6): 26.7 (s, br); MS
=
(EI) m/z (rel. int.): 292 (100) [M+], 277 (3) [M+-Me].
Theoretical studies
Density functional theory calculations at the Becke3LYP (B3LYP)
level have been performed to optimise all of the model species in
which the phosphine ligands were modeled with PH3. Frequency
calculations at the same level of theory have also been performed
to identify all stationary points as minima (zero imaginary
frequencies) or transition states (one imaginary frequency). In the
B3LYP calculations, the Lan2DZ29 basis set was used to describe
Rh, P and Cl, whereas the 6–31G basis set30 was used for C, B, O
and H atoms. Polarisation functions were added for P (fd = 0.34)
and Cl (fd = 0.514).31 All calculations were performed using the
Gaussian 98 package.32
12 T. Ishiyama, S. Momato and N. Miyaura, SYNLETT, 1999, 11, 1790.
13 S.-Y. Onozawa, Y. Hatanaka and M. Tanaka, Chem. Commun., 1999,
1863.
14 T. Ohmura, Y. Yamamoto and N. Miyaura, J. Am. Chem. Soc., 2000,
122, 4990.
15 (a) H. E. Blackwell, D. J. O’Leary, A. K. Chatterjee, R. A. Washenfelder,
D. A. Bussmann and R. H. Grubbs, J. Am. Chem. Soc., 2000, 122, 58;
(b) C. Morrill and R. H. Grubbs, J. Org. Chem., 2003, 68, 6031; (c) C.
Morrill, T. W. Funk and R. H. Grubbs, Tetrahedron Lett., 2004, 45,
7733.
16 (a) B. Marciniec, M. Jankowska and C. Pietraszuk, Chem. Commun.,
2005, 663; (b) K. C. Lam, Z. Lin and T. B. Marder, Organometallics,
2007, 26, 3149.
17 M. Kim and D. Lee, Org. Lett., 2005, 7, 1865.
18 (a) J. M. Brown and G. C. Lloyd-Jones, J. Chem. Soc., Chem.
Commun., 1992, 710; (b) S. A. Westcott, T. B. Marder and R. T. Baker,
Organometallics, 1993, 12, 975; (c) J. M. Brown and G. C. Lloyd-Jones,
J. Am. Chem. Soc., 1994, 116, 866; (d) D. H. Motry and M. R. Smith, III,
J. Am. Chem. Soc., 1995, 117, 6615; (e) R. T. Baker, J. C. Calabrese, S. A.
Westcott and T. B. Marder, J. Am. Chem. Soc., 1995, 117, 8777; (f) D. H.
Motry, A. G. Brazil and M. R. Smith, III, J. Am. Chem. Soc., 1997,
119, 2743; (g) M. Murata, S. Watanabe and Y. Masuda, Tetrahedron
Lett., 1999, 40, 2585; (h) C. M. Vogels, P. G. Hayes, M. P. Shaver and
S. A. Westcott, Chem. Commun., 2000, 51; (i) D. E. Kadlecek, P. J.
Carroll and L. G. Sneddon, J. Am. Chem. Soc., 2000, 122, 10868; (j) M.
Murata, K. Kawakita, T. Asana, S. Watanabe and Y. Masuda, Bull.
Acknowledgements
We thank EPSRC for a postgraduate studentship (R.B.C.) and
for research grants GR/M23038 and GR/R61598 (T.B.M.).
T.B.M. also thanks NSERC (Canada) for research support, the
Leverhulme Trust for a Study Abroad Fellowship, the Hong Kong
University of Science and Technology for a Visiting Professorship,
the University of Durham for a Sir Derman Christopherson
Foundation Fellowship, and the Royal Society of Chemistry for
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Chem. Soc. Jpn., 2002, 75, 825; (k) R. B. Coapes, F. E. S. Souza, R. Ll.
Thomas, J. J. Hall and T. B. Marder, Chem. Commun., 2003, 614; (l) A.
Caballero and S. Sabo-Etienne, Organometallics, 2007, 26, 1191.
19 (a) R. T. Baker, P. Nguyen, T. B. Marder and S. A. Westcott, Angew.
Chem., Int. Ed. Engl., 1995, 34, 1336, (Angew. Chem., 1995, 107, 1451);
(b) T. B. Marder, N. C. Norman and C. R. Rice, Tetrahedron Lett.,
1998, 39, 155; (c) C. Y. Dai, E. G. Robins, A. J. Scott, W. Clegg, D. S.
Yufit, J. A. K. Howard and T. B. Marder, Chem. Commun., 1998, 1983;
(d) P. Nguyen, R. B. Coapes, A. D. Woodward, N. J. Taylor, J. M. Burke,
J. A. K. Howard and T. B. Marder, J. Organomet. Chem., 2002, 652, 77;
(e) for other examples of catalysed alkene diboration, see: C. N. Iverson
and M. R. Smith, III, Organometallics, 1997, 16, 2757; (f) T. Ishiyama,
M. Yamamoto and N. Miyaura, Chem. Commun., 1997, 689; (g) J. B.
Morgan, S. P. Miller and J. P. Morken, J. Am. Chem. Soc., 2003, 125,
8702; (h) S. P. Miller, J. B. Morgan, F. J. Nepveux and J. P. Morken,
Org. Lett., 2004, 6, 131; (i) D. M. Kalendra, R. A. Duen˜es and J. P.
Morken, J. P., SYNLETT, 2005, 1749; (j) S. Trudeau, J. B. Morgan,
M. Shrestha and J. P. Morken, J. Org. Chem., 2005, 70, 9538; (k) J.
Ram´ırez, R. Corbera´n, M. Sanau, E. Peris and E. Ferna´ndez, Chem.
Commun., 2005, 3056; (l) J. Ram´ırez, A. M. Segarra and E. Ferna´ndez,
Tetrahedron: Asymmetry, 2005, 16, 1289; (m) R. Corbera´n, J. Ram´ırez,
M. Poyatos, E. Peris and E. Ferna´ndez, Tetrahedron: Asymmetry, 2006,
17, 1759; (n) V. Lillo, J. Mata, J. Ram´ırez, E. Peris and E. Ferna´ndez,
Organometallics, 2006, 25, 5829; (o) V. Lillo, M. R. Fructos, J. Ram´ırez,
A. A. C. Braga, F. Maseras, M. Mar D´ıaz-Requejo, P. J. Pere´z and
E. Ferna´ndez, Chem.–Eur. J., 2007, 13, 2614; (p) for reviews, see: T. B.
Marder and N. C. Norman, Top. Catal., 1998, 5, 63; (q) T. Ishiyama and
N. Miyaura, J. Synth. Org. Chem., Jpn., 1999, 57, 503; (r) T. Ishiyama
and N. Miyaura, J. Organomet. Chem., 2000, 611, 392; (s) V. M.
Dembitsky, H. Abu, Ali and M. Srebnik, Appl. Organomet. Chem.,
2003, 17, 327; (t) T. Ishiyama and N. Miyaura, Chem. Rec., 2004, 3,
271.
Commun., 2005, 663; (c) N. Miyaura and A. Suzuki, J. Organomet.
Chem., 1981, 213, C53; (d) K. Nilsson and A. Hallberg, Acta Chem.
Scand., 1987, 38, 569.
24 A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J.
Timers, Organometallics, 1996, 15, 1518.
25 (a) D. Manning and H. No¨th, J. Chem. Soc., Dalton Trans., 1985,
1689; (b) F. J. Lawlor, N. C. Norman, N. L. Pickett, E. G. Robins, P.
Nguyen, G. Lesley, T. B. Marder, J. A. Ashmore and J. C. Green, Inorg.
Chem., 1998, 37, 5282; (c) T. Ishiyama, M. Murata, T.-A. Ahiko and
N. Miyaura, Org. Synth., 1999, 77, 176.
26 (a) P. Nguyen, G. Lesley, N. J. Taylor, T. B. Marder, N. L. Pickett, W.
Clegg, M. R. J. Elsegood and N. C. Norman, Inorg. Chem., 1994, 33,
4623; (b) W. Clegg, M. M. J. Elsegood, F. J. Lawlor, N. C. Norman,
N. L. Pickett, E. G. Robins, A. J. Scott, P. Nguyen, N. J. Taylor and
T. B. Marder, Inorg. Chem., 1998, 37, 5289.
27 S. Shimada, A. S. Batsanov, J. A. K. Howard and T. B. Marder,
Angew. Chem., Int. Ed., 2001, 40, 2168, (Angew. Chem., 2001, 113,
2226).
28 (a) D. Evans, J. A. Osborn and G. Wilkinson, Inorg. Synth., 1966, 8,
215; (b) J. A. McLeverty and G. Wilkinson, Inorg. Synth., 1968, 11,
99.
29 (a) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270; (b) W. R.
Wadt and P. J. Hay, J. Chem. Phys., 1985, 82, 284; (c) P. J. Hay and
W. R . Wa d t , J. Chem. Phys., 1985, 82, 299.
30 P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213.
31 S. Huzinaga, Gaussian Basis Sets for Molecular Calculations, Elsevier
Science Pub. Co., Amsterdam, 1984.
32 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V.
Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M.
Head-Gordon, E. S. Replogle and J. A. Pople, GAUSSIAN 98 (Revision
A.9), Gaussian, Inc., Pittsburgh, PA, 1998.
20 R. T. Baker, J. C. Calabrese, S. A. Westcott, P. Nguyen and T. B. Marder,
J. Am. Chem. Soc., 1993, 115, 4367.
21 C. Widauer, H. Gru¨tzmacher and T. Ziegler, Organometallics, 2000, 19,
2097.
22 C. E. Webster, Y. Fan, M. B. Hall, D. Kunz and J. F. Hartwig, J. Am.
Chem. Soc., 2003, 125, 858.
23 (a) K. C. Lam, Z. Y. Lin and T. B. Marder, Organometallics, 2007,
26, 3149; (b) B. Marcinec, M. Jankowska and C. Pietraszuk, Chem.
1064 | Dalton Trans., 2008, 1055–1064
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