Highly efficient monophosphine platinum catalysts for alkyne diboration

Article abstract of DOI:10.1039/b010225n

Automated parallel screening using a series of in situ generated platinum(0) phosphine complexes has allowed the identification of improved catalysts for the diboration of alkynes using bis(pinacolato)diboron (B₂(pin)₂, pin = OCMe₂CMe₂O). A selection of phosphines were added to [Pt(NBE)₃] (NBE = norbornene), which contains only labile mono-olefin ligands, and the activity of the resulting solutions as catalysts for the diboration of 4-CF₃C₆H₄C≡CC₆H ₄CF₃-4′ 1 by B₂(pin)₂ was investigated by in situ GC-MS and/or NMR spectroscopy. This allowed the optimum phosphine : platinum stoichiometry to be identified as 1 : 1, and the large differences in catalyst activity depending on the nature of the phosphine to be quantified. The best phosphines employed in the study, PCy₃ and PPh₂(o-Tol) (o-Tol = C₆H₄Me-o), give activities orders of magnitude greater than the worst, such as P(C₆F₅)₃ and PBu^t₃. The monophosphine catalysts function much more efficiently than previous catalysts for a range of alkynes allowing diborations to be performed at ambient temperatures. The diboration of strained cyclic alkenes and some vinyl- and allyl-arenes proceeded well, although the catalysts were inactive for other olefinic systems examined. As a result of these studies, the isolable and stable compound [Pt(PCy₃)(η₂-C₂H₄) ₂] was identified as an excellent catalyst for alkyne diboration even at room temperature.

Full text of DOI:10.1039/b010225n

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Highly efficient monophosphine platinum catalysts for alkyne
diboration
Rhodri Ll. Thomas,a Fabio E. S. Souzab and Todd B. Marder*ab
a Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
E-mail: Todd.Marder@durham.ac.uk
b Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Received 21st December 2000, Accepted 21st March 2001
First published as an Advance Article on the web 19th April 2001
Automated parallel screening using a series of in situ generated platinum(0) phosphine complexes has allowed
the identification of improved catalysts for the diboration of alkynes using bis(pinacolato)diboron (B2(pin)2,
pin = OCMe2CMe2O). A selection of phosphines were added to [Pt(NBE)3] (NBE = norbornene), which
contains only labile mono-olefin ligands, and the activity of the resulting solutions as catalysts for the diboration
᎐
of 4-CF C H C᎐CC H CF -4Ј 1 by B (pin) was investigated by in situ GC-MS and/or NMR spectroscopy. This
᎐
3
6
4
6
4
3
2
2
allowed the optimum phosphine : platinum stoichiometry to be identified as 1 : 1, and the large differences in catalyst
activity depending on the nature of the phosphine to be quantified. The best phosphines employed in the study, PCy3
and PPh2(o-Tol) (o-Tol = C6H4Me-o), give activities orders of magnitude greater than the worst, such as P(C6F5)3 and
PBut3. The monophosphine catalysts function much more efficiently than previous catalysts for a range of alkynes
allowing diborations to be performed at ambient temperatures. The diboration of strained cyclic alkenes and some
vinyl- and allyl-arenes proceeded well, although the catalysts were inactive for other olefinic systems examined. As a
result of these studies, the isolable and stable compound [Pt(PCy3)(η2-C2H4)2] was identified as an excellent catalyst
for alkyne diboration even at room temperature.
[Pt(dba)2]26 (NBE = norbornene; COD = 1,5-cyclooctadiene;
dba = dibenzylideneacetone) catalyse the diboration of alkenes,
Introduction
The utility of organoboranes as intermediates in organic syn-
thesis is well documented.1,2 Metal catalysed borylation reac-
tions, including hydroboration,3 diboration,4,5 dehydrogenative
borylation,6,7 borylation of aryl and vinyl halides and tri-
flates,2,8,9 and additions of B–Si,10 B–Sn11 and B–S bonds12 pro-
vide new routes to important organoboron compounds. We
have reported rhodium13,14 and gold14 complexes as catalysts
for the diboration of styrenes using B2(cat)2 (cat = 1,2-O2C6H4)
to give the expected alkyl 1,2-bis(boronate) esters, and the use
of platinum and palladium compounds for catalytic boron-
ations with B2(OR)4 compounds has been reviewed.4,5 Recently,
it has been shown that copper based compounds also catalyse
borylation reactions.15
cis-Alkene-1,2-bis(boronate) esters, the product of di-
boration of alkynes, are a useful class of organoboron com-
pounds. They can be employed as substrates for the Suzuki–
Miyaura cross-coupling reaction with aryl halides,2,16 and have
been used in the synthesis of biologically active molecules such
as the anti cancer agent Tamoxifen.17 Their preparation via a
platinum catalysed diboration reaction was first reported by
Miyaura, Suzuki and co-workers,16 using [Pt(PPh3)4] as a
catalyst. Subsequent studies showed that this catalyst may be
used in the diboration of allenes,18 methylcyclopropanes19 and
1,3-dienes.20 These diboration reactions typically required heat-
ing at 80 ЊC for 24 hours in DMF, and so an improved catalyst
was sought. Simultaneous work by our group21 and by Iverson
and Smith22 showed that using bis(phosphine)platinum based
catalysts, either [Pt(PPh3)2(η2-C2H4)] or [Pt(PPh3)2{B(OR)2}2],
reduced the reaction times required and allowed these reactions
to be performed in less polar solvents such as toluene. These
compounds also catalyse the diboration of enones23 as well as
the formation of the novel hexaboryl- and tetraboryl-ethanes
and tetraborylethenes from bis(boryl) alkynes.24 Phosphine-
free platinum(0) species such as [Pt(NBE)3], [Pt(COD)2]25 or
and show very different selectivities from the catalysts that con-
tain phosphines, with, for example, different products found in
the diboration of dienes.20
Baker and co-workers have recently reported that the com-
mercially available platinum() compound [Pt(COD)Cl2] shows
catalytic activity, after an induction period, for the diboration
of a range of substrates, including imines;27 however, this
catalyst requires the more active diboron reagent B2(cat)2 and
the reaction failed when B2(pin)2 was used (pin = pinacolate,
OCMe2CMe2O). As was the case for the phosphine free
platinum(0) complexes, alkenes, such as 1-octene and 4-vinyl-
anisole, were diborated far more easily at room temperature
than alkynes such as 1-octyne and di(p-tolyl)acetylene, which
required a temperature of 55 ЊC. In light of the selectivity dis-
played and the induction period required by this catalyst, it is
likely that the catalytically active species is a phosphine free
platinum(0) species generated in situ.
For bis(triphenylphosphine)platinum catalysts, alkyne di-
boration was inhibited by added PPh3,21 and the rate was in fact
shown to be inverse first order in [PPh3].22 Chelating phosphines
such as dppe or dppb [Ph2P(CH2)4PPh2] greatly reduced the
catalyst efficiency.21 The fact that phosphine free platinum
complexes are far more active for alkenes than alkynes27 sug-
gests that the presence of phosphine may be required for effi-
cient alkyne diboration, and that a P : Pt ratio in solution of
1 : 1 would provide the most active catalyst. This could be
achieved via either an isolable monophosphine platinum(0)
complex or in situ formation of a similar species. Recent
reports by Miyaura and co-workers on the platinum catalysed
diboration of allenes18 have highlighted the marked effect of
the nature of the phosphine ligand on the activity of a related
catalytic system, with yields varying from 7 to 85% as the
phosphine is changed from PMe3 to PCy3, and with significant
variations in the regioselectivity of the reactions being noted. In
1650
J. Chem. Soc., Dalton Trans., 2001, 1650–1656
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b010225n

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addition, the development of highly efficient palladium/
phosphine based catalysts for a range of coupling reactions
by the group of Fu,28,29 using PBut3, and Buchwald and co-
workers,30 using bulky biphenyl based phosphines, led us to
consider how the nature of the phosphine might affect platinum
catalysed diboration of alkynes.
column chromatography on silica gel (4 cm diameter, 10 cm
1
high), using hexane as eluent. Yield = 2.4 g, 84%. NMR: H
δ 7.02 (d, J = 7, 4H), 7.15 (d, J = 7, 4H); 19F-{1H} δ Ϫ63.3 (s).
MS: m/z 314 (Mϩ, 100), 295 (Mϩ Ϫ F, 17).
Typical diboration of 1
In this paper we report the use of automated parallel screen-
ing techniques for the identification and optimisation of
improved catalysts for the diboration of alkynes and simul-
taneous collection of kinetic data. The effects of the nature and
amount of phosphine ligands have been investigated, as have
the lifetimes of the catalysts produced.
To a sample of [Pt(NBE)3] (2.4 mg, 5 µmol) dissolved in toluene
(0.1 ml) was added a solution of PCy3 in toluene (0.1 ml,
0.05 M), giving a pale orange-yellow solution. This solution
was added to a mixture of B2(pin)2 (28 mg, 0.11 mmol) and 1
(31 mg, 0.1 mmol) dissolved in toluene (2 ml), and shaken
vigorously to ensure complete mixing. Aliquots of the solution
(ca. 50 µL) were transferred to micro inserts in GC autosampler
vials at ambient temperature. The vials were sealed under N2,
removed from the glove box and GC-MS samples taken at 19
minute intervals, each from a fresh autosampler vial, until the
reaction was complete, or no further reaction was apparent.
Experimental
General
All reactions were performed under an atmosphere of dry
nitrogen, in a double length glove box (Innovative Technology,
Inc., System One), in sealed Young’s tap vessels using standard
Schlenk line techniques, or in sealed GC autosampler vials, vide
infra. Solvents were distilled from sodium prior to use or dried
and deoxygenated using a modified Innovative Technology Inc.
SPS-solvent purification system containing activated alumina
followed by an activated BASF R311 copper catalyst under
argon pressure, similar to that described by Grubbs and co-
workers.31
Diboration of 1
Using [Pt(NBE)3]/PPh2(o-Tol). A solution of B2(pin)2 (7 mg,
0.028 mmol) and 1 (8 mg, 0.025 mmol) in C6D6 (0.5 ml) was
placed in a 5 mm NMR tube sealed with a Teflon tap. This
NMR tube was placed in a Mercury 200 NMR spectrometer,
and the shim settings were optimised. The tube was removed to
a glove box, and to it was added a C6D6 solution of a mixture of
PPh2(o-Tol) (o-Tol = C6H4Me-o) and [Pt(NBE)3] (0.025 ml,
0.05M in each component). The NMR tube was replaced in the
spectrometer, and 19F-{1H} NMR spectra were collected for
30 seconds every 2 minutes until the reaction was complete.
NMR spectra were collected on Varian Mercury 200 (1H,
19F) and Unity 300 (11B) spectrometers at the following fre-
quencies: 1H, 200; 19F, 188; 11B, 96 MHz. Proton NMR spectra
were referenced to residual protic solvent resonances, 19F and
11B to external standards (CFCl3 and BF3ؒOEt2 respectively).
All spectra were recorded as solutions in C6D6 or mixed C6D6/
toluene unless stated otherwise. Chemical shifts are reported in
ppm and coupling constants in Hz.
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 capil-
lary column (12 m cross-linked 5% methylphenylsilicone) was
used, and the oven temperature was ramped from 70 to 270 ЊC
at a rate of 20 ЊC minϪ1. UHP grade helium was used as the
carrier gas. The screw-cap autosampler vials used were supplied
by National Scientific Inc. and were fitted with Teflon/silicone/
Teflon septa and 0.2 ml micro inserts.
Using a low catalyst loading. To a solution of 1 (0.62 g,
2.0 mmol) and B2(pin)2 (0.50 g, 2.0 mmol) in toluene (2 ml)
was added a catalyst solution containing [Pt(NBE)3] (5 mg,
10 µmol, 0.5 mol% relative to alkyne) and PPh2(o-Tol) (2.8 mg,
10 µmol, 0.5 mol%) in toluene (1 ml). The solution was stirred
for 10 hours, at which time GC-MS analysis indicated that no 1
remained, with 2 being the only product.
Characterisation of cis-4-CF3C6H4C(Bpin)᎐C(Bpin)C6H4CF3-4Ј
᎐
2
After completion of the above reactions, the combined solu-
tions were filtered through Celite, and the solvent was
removed under reduced pressure leaving an orange oily solid,
which was washed with cold hexane to give the product as a
white powder. NMR: 1H δ 1.32 (s, 12H), 7.03 (d, J = 8, 2H), 7.35
All compounds were either obtained commercially or pre-
pared by literature methods or variants thereof ([Pt(NBE)3],32
[Pt(dba)2],33 [Pt(PCy3)(η2-C2H4)2],34 B2(pin)2,35 4-CF C H C᎐
᎐
᎐
3
6
4
CH36), with the exception of 1 which was prepared as described
below. All commercial materials were purified by distillation or
recrystallisation prior to use, and their purities were confirmed
by GC-MS and NMR spectroscopy. Stock solutions of
[Pt(NBE)3] and phosphines were prepared immediately prior to
use, and were employed in order to achieve accuracy when
small quantities of reagent were required, as well as uniformity
from run to run.
(d, J = 8, 2H); 19F-{1H} δ Ϫ62.7 (s); 11B-{1H} δ 25.3 (br s).
ϩ
,
MS: m/z 568 (Mϩ, <1%), 549 (Mϩ Ϫ F, 3), 84 (Me C᎐CMe
᎐
2
2
100). Microanalysis: calculated for C28H32B2F6O4 59.19% C,
5.68% H; Found 58.92% C, 5.64% H.
Diboration of other substrates using [Pt(NBE)3]/PR3 catalyst
To a sample of [Pt(NBE)3] (2.4 mg, 5 µmol) dissolved in toluene
(0.1 ml) was added a solution of PPh3 in toluene (0.1 ml,
0.05 M), giving a pale orange-yellow solution. This solution
was added to a mixture of B2(pin)2 (28 mg, 0.11 mmol) and a
substrate (0.1 mmol) dissolved in toluene (2 ml) at ambient
temperature. Aliquots were removed at intervals of 30 minutes
initially, then at intervals of several hours for the reactions that
required significant reaction time. The product mixtures were
analysed by GC-MS and 1H NMR spectroscopy, by com-
parison with previously reported data,21,22,25,37 except for the
diboration products of 1 and 4-allylanisole, which are given
below.
᎐
Synthesis of 4-CF C H C᎐CC H CF -4Ј 1
᎐
3
6
4
6
4
3
To a mixture of [Pd(PPh3)2Cl2] (65 mg, 0.092 mmol) and CuI
(17.5 mg, 0.092 mmol) in triethylamine (150 ml) was added
᎐
4-CF C H I (2.5 g, 9.2 mmol) and 4-CF C H C᎐CH (1.7 g, 10.1
᎐
3
6
4
3
6
4
mmol). Formation of a solid deposit began immediately, and
vigorous stirring at room temperature was required for 45 min,
at which time GC-MS analysis indicated that the reaction
was complete. After filtration through silica, the solvent was
removed from the dark solution under reduced pressure. The
oily solid produced was dissolved in Et2O (50 ml) and
washed with dilute aqueous HCl (0.2 M, 100 ml) three times
[CAUTION: if much amine remains, the heat generated may
cause a build up of pressure]. The ether was removed from the
residue under reduced pressure, and the solid purified by
Characterisation of 4-MeOC6H4CH2CH(Bpin)CH2(Bpin).
1
NMR: H δ 1.31 (s, 12H, Bpin), 1.37 (s, 12H, Bpin), 1.70 (m,
2H, CH2Bpin), 2.20 (apparent quintet, J = 7, 1H, CH2CH-
BpinCH2Bpin), 2.75 (dd, 1H, J = 7, 12, CH2CHBpinCH2Bpin),
J. Chem. Soc., Dalton Trans., 2001, 1650–1656
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2.75 (dd, J = 7, 12, 1H, CH2CHBpinCH2Bpin), 3.24 (s, 3H,
CH3O), 6.98 (m, 2H), 7.33 (m, 2H); 11B-{1H} δ 32.3 (br s, 1B),
35.2 (br s, 1B). MS: m/z 402 (Mϩ, 5), 387 (Mϩ Ϫ Me, 68), 84
(Me C᎐CMe ϩ, 100).
᎐
2
2
Diboration of other substrates using [Pt(PCy3)(ꢀ2-C2H4)2]
catalyst
These studies were conducted in an identical manner to the
above, except that a sample of [Pt(PCy3)(η2-C2H4)2] (5.3 mg, 10
µmol) was dissolved in toluene (0.2 ml) and half of this solution
was used to catalyse the diboration reaction.
General procedure for competitive diboration of alkynes
Fig. 1 Effect of [Pt(NBE)3] : PPh3 stoichiometry on the catalytic
diboration of 1. [1] = 7.4 mM, [B2(pin)2] = 8.0 mM, [Pt(NBE)3] = 0.37
mM (5 mol%), [PPh3] = 0 (᭿), 0.37 mM (᭹) and 0.75 mM (ꢀ).
The diboron reagent (B2(pin)2 or B2(cat)2, 0.15 mmol), alkynes
(0.15 mmol of each) and [Pt(PPh3)2(η2-C2H4)] (6 mg, 7.5 µmol,
5 mol%) were placed in a vial to which was added toluene
(4 ml); samples containing B2(cat)2 were warmed to ensure
complete dissolution. The solutions were transferred to
ampoules sealed with a Young’s Teflon tap and heated to 80 ЊC,
with regular sampling by GC-MS until the reaction was
complete. The solvent was removed under reduced pressure
1
and the remaining solids were analysed by H and 19F NMR
spectroscopy as CDCl3 solutions.
Data analysis
The relative amounts of the species present were determined by
integration of the GC-MS total ion chromatogram trace. The
relative response factors for the compounds studied were
determined from the analysis of several mixtures of these com-
pounds of known concentrations. The proportion of substrate
remaining was defined as the amount of substrate/(amount
of substrate ϩ amount of product), which assumes no by-
products are produced from the substrate. This assumption was
confirmed by study of the GC-MS and NMR analysis of the
final reaction mixtures.
Scheme 1
spectroscopy is efficient for fast reactions,39 but was not
employed in the current work.
The catalysts to be studied were all phosphineplatinum(0)
complexes. To generate these species in situ it was decided to use
a labile platinum(0) precursor complex to which different phos-
phines could be added in various amounts. Initial studies were
attempted using [Pt(dba)2] as a source of platinum; however,
this proved problematic. Few of the test reactions went to com-
pletion, and deposition of a dark residue was apparent. This
was presumably platinum metal due to catalyst decomposition,
as seen in previous studies by Miyaura and co-workers.18
Analysis of the final mixtures also revealed several by-products
arising from diboration, hydroboration and dehydrogenative
borylation of the dba ligand. The relatively low solubility of
this compound also precluded its use for quantitative studies
under our conditions.
The platinum(0) complex thus chosen was [Pt(NBE)3] as it
contains only very labile mono-olefin ligands. This complex and
the desired phosphine ligand were mixed in solution prior to
addition to the substrate mixture to provide ample time for
generation of the phosphineplatinum complex before exposure
to the reactants which ensured consistent, comparable data.
Slightly different data were obtained when the platinum and
phosphine were added sequentially to the reaction mixture, pre-
sumably due to the increased time required for the phosphine
ligand and platinum atom to interact with each other in this
more dilute solution.
Results and discussion
Automated parallel screening
The development of optimal catalysts is a difficult process.
Although general features of a good catalyst may be identified
by logical considerations such as kinetics, the fine tuning of the
system developed may require the optimisation of many
parameters. Automated parallel screening allows the investi-
gation of a range of catalyst precursors to attempt to identify
promising catalysts more quickly than by traditional methods.
Such approaches to homogeneous catalysis are becoming the
subject of more widespread interest.38 For the process to be
successful, it is necessary that one can identify a suitable test
reaction, and a method for it to be monitored. It is useful to
choose a catalyst system that allows for modification in a
straightforward manner, such as a precursor to which different
ligands can be added.
The reaction chosen for testing the diboration of alkynes was
᎐
that of 4-CF C H C᎐CC H CF -4Ј 1 with B (pin) in an aro-
᎐
3
6
4
6
4
3
2
2
matic solvent (Scheme 1). This reaction may be monitored by
GC-MS, as both reagents and the desired product are readily
identified and quantified by this method and, importantly, there
are no side products to complicate the analysis. Alternatively,
NMR spectroscopy of either the 19F resonance of the alkyne
or the 1H resonance of the boron reagent may be used to
monitor the reaction, as both nuclei give sharp singlets for
both starting material and product. The time frame of the
reaction will govern the technique used: GC requires a
modest time delay between each aliquot taken for analysis,
typically 19 minutes in our case. For reactions that proceed
somewhat more rapidly, NMR spectroscopy is the technique
of choice. The use of UV-vis absorbance or fluorescence
Phosphine : platinum stoichiometry in the catalyst
The effect of varying the ratio of phosphine to platinum from
2 : 1 to 1 : 1 and then to 0 : 1 was investigated using PPh3.
The amount of alkyne remaining was monitored by automated
GC-MS, and the results are displayed in Fig. 1. It is apparent
that while the use of a catalyst formulated as ‘Pt(PPh3)2’
is known to be effective at elevated temperatures, at room
temperature the reaction is too slow to be of general use, with
only a few percent reaction in seven hours. The reaction does
proceed to completion; however, this takes fifteen days. The
reaction using [Pt(NBE)3] is much more rapid in the absence of
added phosphine compared with 2 molar equivalents of added
1652
J. Chem. Soc., Dalton Trans., 2001, 1650–1656

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Table 1 Effect of phosphine size and basicity on the [Pt(NBE)3]/
phosphine catalysed diboration of 1
Cone anglea/Њ
pKa
keff c/10Ϫ5
b
Phosphine
PBut3
182
194
184
—
145
136
122
132
11.40
2.8
9.2
9.6
13
15
19
P(o-Tol)3
P(C6F5)3
No phosphine
PPh3
3.08
d
—
2.73
4.57
6.50
8.43
PMePh2
PMe2Ph
61
63
PBun
3
PCy3
PPh2(o-Tol)
170
9.70
82
232
d
d
Fig. 3 Effect of [catalyst] on the [Pt(NBE)3]/phosphine catalysed
diboration of 1. [1] = 7.4 mM; [B2(pin)2] = 8.0 mM; [Pt(NBE)3] = [phos-
phine] = 0.14 (2, ꢀ), 0.37 (5, ᭹), 0.69 mM (10 mol%, ᭿).
Reaction conditions: 1.5 µmol of 1 (7.4 mM), 1.1 equivalent B2(pin)2
(8.0 mM) in 0.2 ml toluene with 5 mol% of an equimolar mixture of
[Pt(NBE)3] and PR3 at ambient temperature. a See reference 47. b pKa of
the conjugate acid. See reference 48. c Rate = keff[alkyne]/[alkyne]0. d No
value reported for this parameter.
differences in the catalytic activity of PCy3 and PBut3 illustrate
how sensitive catalysts may be to small changes in phosphine
size and basicity, with phosphines larger than a threshold value
being inactive. Similar differences in activity have been seen in
the palladium catalysed Suzuki–Miyaura coupling reaction,
where Fu and co-workers28 have shown that, in the reaction of
TfOC6H4Cl (TfO = CF3SO3) with aryl boronic acids, different
products are obtained when PCy3 is used (TfO substitution) in
place of PBut3 (Cl substitution).
The phosphine which generated the most active catalyst was
PPh2(o-Tol), which is not expected to have significantly greater
basicity than triphenylphosphine, although the additional
methyl group would increase the cone angle of the phosphine
significantly. The substantially enhanced activity of this catalyst
was therefore surprising. It is possible that there is an alterna-
tive mechanism in action in this case, as has been found for the
Heck reaction, where the introduction of ortho-tolyl groups
into the phosphines used also increases the catalytic activity of
systems dramatically.40 This has been postulated to be due to
the formation of metallacyclic intermediates, and indeed such
metallacycles41 and related compounds containing ‘PCP pincer’
ligands42 have been shown to be very active catalysts in that
reaction.
If similar alkyne diboration activity were observed for tri-
(o-tolyl)phosphine, which is known to form palladacycles
readily,43 the suggestion of platinacyclic catalysts would be
reinforced. The activity found with this phosphine, however, is
comparable to that with P(C6F5)3, both of which show no
greater rate for diboration than is found for the phosphine free
platinum species. The lower basicity and large size of P(o-Tol)3
and P(C6F5)3 may disfavour the formation of a catalytically
active platinum–phosphine complex in the presence of other
competing ligands in the reaction mixture. This would give the
same phosphine-free catalytically active species in the presence
or absence of these two phosphines. The reason for the
enhanced activity of the PPh2(o-Tol) catalyst remains elusive.
Fig. 2 Effect of phosphine on the [Pt(NBE)3]/phosphine catalysed
diboration of 1. [1] = 7.4 mM, [B2(pin)2] = 8.0 mM, [Pt(NBE)3] = [phos-
phine] = 0.37 mM (5 mol%). Phosphine = PPh2(o-Tol) (a, ᮀ), PCy3
(b, ᭹), PMe2Ph (c, ×), PBun3 (d, ᭛), PMePh2 (e, ᭿), PPh3 (f, ᭺), None
(g, *), P(C6F5)3 (h, ᭝), P(o-Tol)3 (i, ꢀ), PBut3 (j, ᭜).
PPh3; however, the reaction does not proceed to completion,
and the formation of colloidal platinum is apparent in the reac-
tion mixture. The most effective catalyst stoichiometry was
found to be phosphine : platinum = 1 : 1, with the reaction
being more rapid than either of the other systems tested, and
proceeding to completion without any obvious precipitation
of platinum metal. Similar results were found using other
phosphines such as PCy3 and PPh2(o-Tol), with the ‘Pt(PR3)1’
stoichiometry producing a significantly faster reaction than
that using ‘Pt(PR3)2’ as the catalytic species.
Nature of phosphine added
Since the optimal catalyst appeared to be a monophosphine
platinum species, as predicted by the previous studies, this
stoichiometry was investigated for a range of commercially
available phosphines to determine the effects of basicity and/or
cone angle on the rate of diboration. The results are displayed
in Fig. 2. It is apparent from the linearity of the plots that
the reaction is overall first order with respect to the product
[alkyne][B2(pin)2], as had been reported.21,22
Concentration of catalyst
The above studies were conducted using identical catalyst con-
centrations of 0.37 mM, corresponding to 5 mol% with respect
to the alkyne. At this concentration only 20 catalyst turnovers
are required for complete reaction; however, higher turnover
numbers and hence lower catalyst loadings would be prefer-
able for preparative use. Using concentrations of [Pt(NBE)3]/
PPh2(o-Tol) of 0.14 and 0.69 mM, corresponding to 2 and 10
mol% respectively, we obtained the kinetic data shown in Fig. 3.
From the gradients of the three lines (Table 2), we obtain keff for
the reactions, demonstrating that keff ∝ [catalyst], i.e. the reac-
tion is first order in catalyst concentration. This also illustrates
that the reaction can proceed with only 2 mol% catalyst, equiv-
alent to 50 turnovers per platinum atom. In further studies the
reaction proceeded to completion in 10 hours using only 0.5
mol% catalyst, with a consequent turnover number of ca. 200.
The wide variation in activity with different phosphines is
apparent. The effective rate constants, given in Table 1, vary by
two orders of magnitude. The order of activity of the phos-
phines may be summarized as: PPh2(o-Tol) > PCy3 > PBun
≈
3
PMe2Ph > PMePh2 > PPh3 > no phosphine ≈ P(C6F5)3 ≈ P(o-
Tol)3 > PBut3. A general trend is apparent, with the catalyst
being improved by use of basic phosphines; however there are
two notable exceptions. The catalytic system based on PBut3,
the most basic phosphine tested, showed little activity. This may
be due to the extreme bulk of the phosphine, preventing the
approach or reaction of the catalyst and reagents. The major
J. Chem. Soc., Dalton Trans., 2001, 1650–1656
1653

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Reactions with other substrates
reactivity of the former diboron reagent.21,44 Stoichiometric
diborations of similar para substituted diphenylacetylenes
using [Pt(PPh3)2(Bcat)2] led to a similar order of reactivity, with
rates in the order R = CF3 ӷ H > OMe.22
Interestingly, the order of reactivity of the diphenylacetyl-
enes is apparently reversed from that of the para-substituted
terminal phenylacetylenes, vide supra, where donor moieties
To test the general applicability of monophosphine catalysts, as
well as the relative reactivity of various unsaturated substrates,
the diboration of a selection of alkynes and alkenes was
attempted. The catalytic system used was [Pt(NBE)3]/PPh3, as
the reactions involving this catalyst were slow enough for
relative reaction rates to be determined with greater confidence
than would be the case for the extremely active catalysts. Table 3
lists the substrates utilised and the time required for completion
of the diboration reaction.
enhance the reaction rate. The reasons for this are still not clear,
᎐
although it seems reasonable that ᎐C–H activation processes
᎐
may become an important factor when the pKa of the alkynyl
proton drops below a certain level, with such processes present
in phenylacetylene (pKa = 29.145) but absent in 4-methoxy-
phenylacetylene (pKa = 29.745).
Amongst the alkynes it is seen that the diboration reaction
of phenylacetylene is unusually slow, as has been observed
previously with both ‘Pt(PPh3)2’21 and Pt(PPh3)4 catalysts.16
Alkene substrates were also examined (Table 3), and it was
found that diboration of NBE proceeded rapidly and cleanly.
This was as expected as diborated NBE was observed in some
of the slower alkyne diborations reactions, derived from
the ligands of the catalyst used. It was most apparent in the
diboration of 1-heptyne and 4-octyne, although traces were
observed in the diborations of the arylacetylenes. The di-
boration of styrene derivatives could also be catalysed at room
temperature; however, these reactions were not clean, with
some hydroboration and dehydrogenative borylation6,7 also
occurring giving the monoborylated alkane and the vinyl
boronate ester respectively in equal amounts. Surprisingly,
4-allylanisole was cleanly and rapidly diborated. Other alkenes,
including cis- and trans-stilbene, trans-4-octene and 1-heptene,
failed to react under these conditions.
᎐
The π-acceptor substituted 4-NCC H C᎐CH substrate proved
even less reactive, but the π-donor substituted 4-MeOC H C᎐
᎐
6
4
᎐
᎐
4
6
CH was readily diborated. All other alkyne substrates were
diborated cleanly at room temperature, i.e. the catalyst is suc-
cessful for both alkyl- and aryl-alkynes, and for terminal and
internal alkynes. From the comparison of the reactions of the
substituted diphenylacetylenes, 1–3, we can see that for these
compounds the reaction rate is enhanced by electron withdraw-
ing substituents such as CF3, and retarded by those that are
electron donating, such as Me or OMe.
These results are in agreement with a series of competition
experiments using [Pt(PPh3)2(η2-C2H4)] as a catalyst, in which
solutions of equimolar amounts of differently substituted
diphenylacetylenes were treated with sufficient diboron reagent
to react with only half of the triple bonds present. The results
of these studies are listed in Table 4, and it can be seen that
diboration of the CF3 substituted compound is faster than the
reaction of the more electron rich substrate. This preference is
much less marked when B2(cat)2 is used in place of B2(pin)2,
with the lower selectivity presumably due to the greater
The [Pt(NBE)3]/PCy3 mixture has been shown to be a much
better catalyst than [Pt(NBE)3]/PPh3 in our test reaction, the
diboration of 1. This catalyst was then tested for a range of
Table 4 Product distributions in the competitive diboration studies of
alkyne mixtures
Table 2 Effect of [catalyst] on [Pt(NBE)3] : PPh2(o-Tol) catalysed
diboration of 1
Substrates
4-RC6H4C᎐CC6H4R-4Ј
᎐
᎐
Diboron
reagent
Reaction
time
Diboration
ratio
Catalyst
concentration/mM
Catalyst
loading/mol%
R
R
keff/10Ϫ3
krel
B2(pin)2
B2(pin)2
B2(cat)2
B2(cat)2
CF3
CF3
CF3
CF3
H
OMe
H
2 days
1 week
1 day
6 : 4
9 : 1
1 : 1
7 : 5
0.14
0.37
0.69
2
5
10
0.82
2.32
4.63
1.8
5.0
10
OMe
1 week
Reaction conditions: 1.5 µmol of 1 (7.4 mM), 1.1 equivalents B2(pin)2
(8.0 mM) in 0.2 ml toluene solution of given concentration in
[Pt(NBE)3]/PPh2(o-Tol) (1 : 1).
Reaction conditions: 0.15 mmol of each alkyne, 0.15 mmol diboron
reagent in 4 ml toluene with 7.5 µmol [Pt(PPh3)2(η2-C2H4)] as catalyst
at 80 ЊC.
Table 3 Time required and product distribution for the catalysed reaction of B2(pin)2 with selected substrates
Reaction
time/
hours
Product distributiona (%)
Substrate
4-CF3C6H4C᎐CC6H4CF3-4Ј 1
Diboration
Hydroboration
Dehydrogenative borylation
᎐
᎐
4.3
5.0
5.5
9
12
1.3
36
65
2
2.5
3
5
36
36
36
36
100
100
100
100
100
100
30
11
100
78
74
98
0
᎐
4-MeC6H4C᎐CC6H4Me-4
᎐
᎐
4-MeOC6H4C᎐CC6H4OMe-4
᎐
4-Octyne
1-Heptyne
᎐
4-MeOC6H4C᎐CH
᎐
᎐
C6H5C᎐CH
᎐
᎐
4-NCC6H4C᎐CH
᎐
Norbornene
Styrene
12
14
1
10
13
1
4-Vinylanisole
4-Allylanisole
trans-Stilbene
cis-Stilbene
trans-4-Octene
1-Heptene
0
0
0
Reaction conditions: 0.1 mmol of substrate, 1.1 equivalents B2(pin)2, 2.3 µmol [Pt(NBE)3], 2.3 µmol PPh3 in 2.2 ml toluene at ambient temperature.
a Does not include NBE diboration product arising from [Pt(NBE)3].
1654
J. Chem. Soc., Dalton Trans., 2001, 1650–1656

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Table 5 Reaction times required for the catalytic diboration of a range of substrates using [Pt(NBE)3]/PPh3, [Pt(NBE)3]/PCy3 and [Pt(PCy3)-
(η2-C2H4)2] as catalysts
Time required for complete reaction/h
Substrate
4-CF3C6H4C᎐CC6H4CF3-4Ј 1
[Pt(NBE)3]/PPh3
[Pt(NBE)3]/PCy3
[Pt(PCy3)(η2-C2H4)2]
᎐
᎐
4.3
5.0
5.5
9.0
12
1.3
2.0
1.4
1.6
1.6
3.0
3.5
0.4
0.8
1.2
1.6
1.8
2.5
3.3
0.6
0.7
᎐
4-MeC6H4C᎐CC6H4Me-4
᎐
᎐
4-MeOC6H4C᎐CC6H4OMe-4
᎐
4-Octyne
1-Heptyne
MeOC6H4C᎐CH
᎐
᎐
Norbornene
Reaction conditions: 0.1 mmol of substrate, 1.1 equivalents B2(pin)2 in 2.2 ml toluene with 2.4 µmol [Pt(NBE)3] and 2.4 µmol PR3 or 2.4 µmol
[Pt(PCy3)(η2-C2H4)2] as catalyst.
alkyne substrates to ensure that the effect was not specific to
just one compound. The results, given in Table 5, show that the
greater activity of the PCy3 containing catalyst in our test reac-
tion was repeated for all the alkynes studied, and also for the
strained alkene, NBE. It can be seen that diboration reactions
using the PCy3 system typically take one third of the time
needed using PPh3 as the phosphine.
equivalents of phosphine were present. The above results con-
firm this finding and demonstrate conclusively that a PR3 : Pt
ratio of 1 : 1 is more efficient than that of 0 : 1 which in turn is
much more efficient than 2 : 1. Parallel screening of a series
of in situ generated monophosphineplatinum catalysts using
0.2 ml sample size and automated GC-MS techniques allowed
identification of two particularly effective phosphines, namely
PCy3 and PPh2(o-Tol). While a general trend of enhanced activ-
ity with increased phosphine basicity was observed, PPh2(o-Tol)
does not fit this trend, pointing to the importance of using
empirical screening methods as a complement to rational
design based on kinetic and mechanistic study. These studies
led to the hypothesis that the known stable, isolable mono-
phosphine complex [Pt(PCy3)(η2-C2H4)2] would also be an
excellent alkyne diboration catalyst. This has now been verified,
and both the isolated PCy3 compound and the in situ generated
PPh2(o-Tol) system provide high catalytic activity for alkyne
diboration at room temperature.
Preformed catalysts
The use of [Pt(NBE)3] for the in situ preparation of diboration
catalysts is hampered by the thermal instability of the com-
pound; it must be stored in an inert atmosphere at low temper-
atures. An ideal catalyst would be a thermally stable, and
preferably air-stable, solid that matched the properties elucid-
ated above. A literature search reveals that the compounds
[Pt(PR3)(η2-C2H4)2], which match the desired catalyst stoichio-
metry, were reported in the 1970s by the group of Stone.46
Importantly, [Pt(PCy3)(η2-C2H4)2] was reported to be suffi-
ciently stable to be handled in air and stored under an inert
atmosphere at room temperature. This stability was greater
than that reported for compounds containing the less basic
phosphines, such as PPh3, and so the cyclohexyl compound was
chosen for study (Scheme 2).
Acknowledgements
We thank the EPSRC for a research grant GR/M23038 and
R. T. Baker for a generous gift of PPh2(o-Tol).
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