The active role of NHC ligands in platinum-mediated tandem hydroboration-cross coupling reactions

Article abstract of DOI:10.1039/b700800g

Stable N-heterocyclic platinum-carbene complexes are the first example of platinum-mediated regioselective H-B addition to vinylarenes and alkynes, allowing consecutive cross coupling reactions with the same catalytic system. The Royal Society of Chemistry.

Full text of DOI:10.1039/b700800g

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The active role of NHC ligands in platinum-mediated tandem
hydroboration–cross coupling reactions
Vanesa Lillo,a Jose A. Mata,b Anna M. Segarra,a Eduardo Peris*b and Elena Fernandez*a
Received (in Cambridge, UK) 18th January 2007, Accepted 6th February 2007
First published as an Advance Article on the web 2nd March 2007
DOI: 10.1039/b700800g
pre-step to allow the formation of the hydridoboryl platinum
Stable N-heterocyclic platinum–carbene complexes are the first
example of platinum-mediated regioselective H–B addition to
vinylarenes and alkynes, allowing consecutive cross coupling
reactions with the same catalytic system.
complex, as has been suggested previously.9 Phosphine-free
platinum catalytic systems, such as Pt(COD)Cl2, which have
proved to be very effective in the catalytic diboration of terminal
alkenes, alkynes and aldimines,10 show much higher conversion
under the same reaction conditions (Table 1, entry 2), although the
system lacks selectivity. Addition of PPh3 to this catalyst clearly
decreases the activity. The extent of this decrease depends on the
amount of phosphine added (entries 2–5), in agreement with
analogous catalytic B–B transformations.11 Palladium complexes
modified with electronically different phosphines and phosphites
provided less than 25% of branched isomer (entries 6 and 7), while
diphosphines (dppm and dppb) resulted in complete loss of
activity. Taking all this into consideration, we explored the
possibility of performing H–B addition of alkenes with
N-heterocyclic carbene (NHC) Pt(0) complexes, since they are an
effective alternative to phosphines in numerous catalytic exam-
ples.12–14 We recently reported that compounds 1 and 2 were very
efficient in the diboration of alkenes, alkynes and allylic sulfones.15
The catalytic insertion of unsaturated moieties into a B–H bond
through the hydroboration protocol is an effective strategy for
selectively obtaining organoboron derivatives,1 an important class
of compounds used as synthetic intermediates2 especially for
constructing carbon frameworks.3 Although hydroboration can
also be performed under non-catalytic conditions, the search for
optimised selectivity on the target organoborane compounds
requires metal-mediated catalysis to be used. The most efficient
catalysts for hydroboration appear to be rhodium complexes,4
especially for asymmetric B–H addition to alkenes.5 However,
iridium, palladium, ruthenium, niobium, titanium, zirconium and
lanthanide-based catalysts are an interesting alternative.1 We were
aware that hydroboration with platinum-based catalysts and
catecholborane had not been reported even though they had been
successfully used in catalytic B–B addition to alkenes and alkynes.6
To the best of our knowledge, there are only two significant
precedents: Pt(II)-catalysed hydroboration of terminal olefins with
polyboranes7 and Pt(0)-catalysed hydroboration of allenes with
pinacolborane.8 To verify whether Pt(0) could be a better
alternative to the hydroboration/oxidation of vinylarenes
(Scheme 1), we first used the readily available Pt(PPh3)4 complex,
which provided 18% conversion from styrene with catecholborane,
under standard reaction conditions after 3 h at room temperature
(Table 1, entry 1). This poor result, in comparison with the
uncatalysed reaction (entry 0), was accompanied by the lack of
regioselectivity, hence producing the Markovnikov and the anti-
Markovnikov alcohol derivatives, together with phenylethane.
This preliminary result may explain why phosphine-based Pt
catalysts for the hydroboration of alkenes are so scarce. B–H
addition to alkenes probably requires a phosphine dissociative
When complexes 1 and 2 were initially tested in our model
reaction with styrene as the substrate, the conversion was complete
within 3 h and the regioselectivity was surprisingly high for the
branched alcohol, 85–90% (Table 1, entries 8 and 10). No
hydrogenated byproduct was detected. It is worth mentioning that
the catalytically active species are still active at the end of the
reaction, because subsequent addition of a second and third
amount of styrene and catecholborane leads to the desired
products without any measurable decrease in catalytic activity.
Even more important is the observation that after several days at
ambient temperature the platinum–NHC complexes are still active
in sharp contrast to many of the rhodium–phosphine precursors of
catalysts. The use of catecholborane as the hydroborating reagent
was beneficial because the addition of pinacolborane provided a
lower conversion but also a low regioselectivity (Table 1, entry 9).
Alternative NHC ligands with more basic properties in complexes
3 and 4, resulted in activity comparable to that displayed by 1 and
2 but lead to lower selectivity.
Scheme 1 Pt-based catalytic hydroboration/oxidation of vinylarenes.
Complex 2 (0.025 mmol) was dissolved in 1 mL of thf-d8 in a
sealable NMR tube, and freshly distilled HBcat (0.025 eq.) was
added via syringe. Despite the immediate change to a brownish
colour, no specific signals were detected in the hydride region of
the 1H NMR spectra. Then 0.5 mmol of styrene and 0.5 mmol of
aDepartament de Qu´ımica F´ısica i Inorga`nica. Universitat Rovira i
Virgili, Pc¸a, Imperial Ta`rraco 1, 43005, Tarragona, Spain.
E-mail: elenaf@quimica.urv.es; Fax: 34 977 559563; Tel: 34 977 558046
bDpt. Qu´ımica Inorga`nica i Orga`nica, Universitat Jaume I, Av Vicente
Sos Baynat s/n, 12080, Castello´n, Spain. E-mail: eperis@qio.uji.es
2184 | Chem. Commun., 2007, 2184–2186
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withdrawing properties of the aryl substituent (entries 1, 2 and 9,
10). Internal vinylarenes also favour the formation of the branched
isomer, up to values about 97% for the 1-phenyl-1-propanol.
However, the conversion is lower than with terminal substrates,
because the CLC bond is more hindered (entries 3, 4 and 11, 12).
Complex 2 also provided small amounts of hydrogenated product
in the hydroboration of indene and trans-b-methylstyrene.
Interestingly, the catalytic hydroboration of vinylalkenes inverts
the selectivity with respect to the other vinylarenes used (entries 5
and 6). Vinylcyclohexane and tert-butylethene mainly afforded the
linear alcohol with complex 1, probably because a g3-metal–alkene
intermediate cannot be formed in the catalytic cycle.16 Finally, 1
and 2 are efficient catalysts in the hydroboration of phenyl allyl
sulfones, although the regioselectivity is moderate because the
hydrogenated product is also formed (entries 7, 8 and 13).
Table 1 Platinum-catalysed hydroboration/oxidation of styrene with
catecholboranea
Conv.b Branchedb Linearb Hydrog.b
Entry Catalytic system
(%)
(%)
(%)
(%)
0
1
2
3
4
5
6
7
8
9
10
11
12
a
—
Pt(PPh3)4
Pt(COD)Cl2
Pt(COD)Cl2 + 1PPh3
Pt(COD)Cl2 + 2PPh3
Pt(COD)Cl2 + 4PPh3
Pt(COD)Cl2 + 1PCy3
14
18
100
92
0
Trace
17
49
15
—
—
25
19
85
52
90
71.5
65
99.9
18
51
85
—
—
59
28
15
48
10
28.5
35
—
65
—
—
—
—
15
53
—
—
—
—
—
0
71
c
Pt(COD)Cl2 + 1P(OR)3 80
1
100
19
100
100
82
1d
2
3
4
Standard conditions: Styrene : catechol borane : Pt complex = 0.5 :
The versatility of the platinum-catalyzed hydroboration reaction
is further substantiated by the H–B addition to alkynes. In this
case, complex 2 provided branched and linear alkenylboronic
esters in moderate selectivity, within 3 h at room temperature in
THF (Scheme 2). Styrene was also formed under these reaction
conditions. In comparison, the uncatalysed hydroboration of
0.55 : 0.025; solvent: THF; T = 25 uC, 3 h. Determined by 1H
NMR and GC. P(OR)3 = Tris(2,4-di-tert-butylphenyl)phosphite.
Hydroborating reagent: pinacolborane.
b
c
d
HBcat were added to monitor the product formation under these
conditions. Fig. 1 shows the styrene consumption within the first
3.5 h following the disappearance of the doublet at 5.7 ppm in the
1H NMR spectra. Similarly, Fig. 2 shows the branched and linear
product formation under these reaction conditions, attending to
the increasing doublet at 1.5 ppm for the branched boronate ester,
and the triplet at 1.6 ppm for the linear boronate ester.
Table 2 Platinum catalysed hydroboration–oxidation of alkenes with
catecholboranea
Pt
system (%)
Conv.b Branchedb Linearb Hydrog.b
Entry Substrate
1
(%)
(%)
(%)
In order to check whether the applicability of 1 and 2 could be
extended to other substrates, we studied the hydroboration of
other terminal and internal vinylarenes and vinylalkanes, as well as
allylic systems such allylsulfones (Table 2). As can be seen, the
selectivity on the branched derivative increases with the electron
1
1
100
100
90
10
—
2
74
26
—
3
4
1
1
60
77
87
92
13
8
—
—
5
6
1
1
100
100
5
95
85
—
—
15
7
1
1
2
2
41
72
69
53
92
70
21
29
8
10
18
—
9
8c
9
Fig. 1 Monitoring the disappearance of styrene.
100
95
10
21
11
12
13c
a
2
2
2
70
77
73
77
97
64
9
3
14
—
15
21
Standard conditions: Alkene : catecholborane : Pt complex = 0.5 :
0.55 : 0.025; solvent: THF. T = 25 uC, 3 h. Determined by 1H
NMR and GC 3 eq. catecholborane.
b
Fig. 2 Monitoring the appearance of branched (r) and linear (&)
boronate esters.
c
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The authors are grateful for the financial support from the
CICYT of Spain (CTQ2004-04412/BQU and CTQ2005-05187/
BQU) and Bancaixa (P1.1A2004-05). V. L. thanks the MEC and
J. M. thanks the program ‘‘Ramon y Cajal’’.
Notes and references
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2 H. C. Brown, Organic Synthesis via Boranes, Wiley-Interscience, 1975,
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Scheme 2 Pt-based catalytic hydroboration of arylalkynes.
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ed. P. A. Evans, John Wiley and Sons, NY, 2005.
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Matteson, Stereodirected in the Synthesis with Organoboranes, 1995,
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Scheme 3
alkynes required elevated temperatures17 and the rhodium(I)
mediated hydroboration of phenylethylene in the presence of
variable amounts of PPh3, resulted in a more complex mixture of
byproducts.18 However, the regioselectivity towards the alkenyl
boronate isomer was optimal with alternative catalytic systems,
such as Cp2Ti(CO)2, Cp2ZrHCl and nickel complexes modified
with phosphines.19
One significant advantage of Pt-based catalysts in the hydro-
boration of olefins over the catalysts such as Rh-based, is their
potential ability to perform the tandem H–B addition/cross
coupling reaction, under the same catalytic system. In
a
preliminary study, we tested the catalytic activity of complex 2 in
the one-pot hydroboration of alkynes/Suzuki–Miyaura coupling
reaction. Addition of 4-iodoanisole to the 1-ethynyl-4-methoxy-
benzene hydroboration mixture in presence of K2CO3, afforded
the coupling product in 34% yield, as a 1 : 1 mixture of isomers,
after heating to 110 uC for 16 h in dioxane (Scheme 3). The
platinum catalyst affects both the hydroboration and the Suzuki–
Miyaura coupling. It should be noted that there are few examples
in the literature of Pt-catalysing this type of cross coupling
reaction.20
In summary, we have shown that platinum(0)–carbene com-
plexes afford an effective alternative for the catalytic regioselective
hydroboration of alkenes, and that they can be applied in the one-
pot tandem H–B addition/Suzuki–Miyaura coupling reaction,
under the same catalytic system.
2186 | Chem. Commun., 2007, 2184–2186
This journal is ß The Royal Society of Chemistry 2007