Platinum catalysts for Suzuki biaryl coupling reactions

Article abstract of DOI:10.1021/om0202524

A study of platinum catalysts for Suzuki biaryl coupling reactions was carried out. The assignment of stereochemistry was supported by a comparison of the IR and NMR spectroscopic data. The results indicate that the platinum complexes show unexpectedly good activity in Suzuki biaryl coupling reactions with aryl bromide substrates.

Full text of DOI:10.1021/om0202524

Organometallics 2002, 21, 2599-2600
2599
P la tin u m Ca ta lysts for Su zu k i Bia r yl Cou p lin g Rea ction s
Robin B. Bedford* and Samantha L. Hazelwood†
School of Chemistry, University of Exeter, Exeter EX4 4QD, U.K.
David A. Albisson
Department of Chemistry, Trinity College Dublin, Dublin 2, Ireland
Received April 1, 2002
Sch em e 1. Su zu k i Bia r yl Cou p lin g Rea ction
Summary: Platinum complexes with π-acidic, ortho-
metalated triaryl phosphite and phosphinite ligands
show unexpectedly good activity in Suzuki biaryl cou-
pling reactions with aryl bromide substrates.
Sch em e 2a
The coupling of aryl halides with arylboronic acids,
the Suzuki reaction (Scheme 1), is one of the most
powerful methods for the synthesis of biaryls.1 While
palladium complexes have enjoyed enormous success as
catalysts, the application of platinum complexes to
biaryl coupling reactions has received almost no atten-
tion.2 To the best of our knowledge, the only reported
use of a platinum complex in such a reaction was the
[Pt(PPh3)4]-catalyzed Stille coupling of aryl triflates with
vinyltributylstannane.3 In this case the catalyst showed
very poor activity, giving yields of only 30-37% after 2
days at 5 mol % Pt catalyst loading. It is probable that
the slow rate of reductive elimination of the biaryl group
from platinum(II) complexes compared with the rate for
the analogous palladium(II) complexes accounts for the
lack of activity displayed by platinum.2 If this is the
case, then the introduction of more π-acidic ligands
should increase the rate of reductive elimination and
thus open up the possibility of using platinum complexes
in coupling reactions.4 We recently found that the ortho-
palladated complexes 1 and 2, with π-acidic triaryl
a
Conditions: (i) (for 3a ) [PtCl2(NCPh)2], CH2Cl2, 17 h; (ii)
2-methoxyethanol or o-xylene, ∆, 14-17 h.
The assignment of stereochemistry was supported by a
comparison of the IR and NMR spectroscopic data7 with
those reported for the trans and cis isomers of [PtCl2-
{P(OC6H4-2-Me)3}2].8 Presumably the high steric profile
of the phosphite ligands in 4a overrides the electronic
advantages associated with the cis arrangement, in
which the π-acidic triaryl phosphite ligands would be
trans to the π-basic chlorides. For comparison purposes
we also prepared [PtCl2{P(OC6H5)3}2] (4b) according to
a literature method.8
Heating a mixture of K2[PtCl4] and 1 equiv of the
ligand 3a in 2-methoxyethanol at reflux temperature
led to the formation of the ortho-platinated dimer [{Pt-
(µ-Cl){κ2-P,C-P(OC6H2-2,4-tBu2)(OC6H3-2,4-tBu2)2}}2] (5a)
as a mixture of cis and trans isomers in 83% yield. The
31P NMR spectrum of 5a shows a singlet with platinum
satellites at 81.2 ppm (J ) 7750 Hz) corresponding to
the major isomer and a second singlet with platinum
satellites at 79.9 ppm (J ) 7875 Hz) corresponding to
the minor isomer. These data are consistent with those
reported previously for the related complex [{Pt(µ-Cl)-
{κ2-P,C-P(OC6H4)(OC6H5)2}}2] (5b).9 The syntheses of
the related ortho-platinated phosphinite complexes 5c,d
by reaction of K2[PtCl4] with 1 equiv of the ligands 3b,c,
respectively, were somewhat low yielding (30% and 4%
for 5c,d , respectively).
phosphite and phosphinite ligands, act as highly active
catalysts in biaryl coupling reactions5,6 and were inter-
ested to see whether such ligands would be sufficiently
electron withdrawing to enable the use of analogous
platinum complexes as catalysts in these reactions. The
initial findings of this study are reported below.
The reaction of 2 equiv of the bulky phosphite ligand
tris(2,4-di-tert-butylphenyl) phosphite (3a ) with [PtCl2-
(NCPh)2] in dichloromethane gives the bis(phosphite)
complex trans-[PtCl2(3a )2] (4a ) in 71% yield (Scheme 2).
* To whom correspondence should be addressed. Fax: +44 (0) 1392
263434. E-mail: r.bedford@ex.ac.uk.
† Ne´e Welch.
(1) For recent reviews see: (a) Miyaura, N.; Suzuki, A. Chem. Rev.
1995, 95, 2457. (b) Stanforth, S. P. Tetrahedron 1998, 54, 263. (c)
Suzuki, A. J . Organomet. Chem. 1999, 576, 147.
(2) Clarke, M. L. Polyhedron 2001, 20, 151.
(3) Mateo, C.; Ferna´ndez-Rivas, C.; Ca´rdenas, D. J .; Echavarren,
A. M. Organometallics 1998, 17, 3661.
(4) Merwin, R. K.; Schnabel, R. C.; Koola, J . D.; Roddick, D. M.
Organometallics 1992, 11, 2972.
(5) Albisson, D. A.; Bedford, R. B.; Scully, P. N. Chem. Commun.
1998, 2095.
(7) See the Supporting Information.
(8) Ahmad, N.; Ainscough, E. W.; J ames, T. A.; Robinson, S. D. J .
Chem. Soc., Dalton Trans. 1973, 1148.
(9) Albinati, A.; Affolter, S.; Pregosin, P. S. Organometallics 1990,
9, 379.
(6) Bedford, R. B.; Welch, S. L. Chem. Commun. 2001, 129.
10.1021/om0202524 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/31/2002

2600 Organometallics, Vol. 21, No. 13, 2002
Communications
Ta ble 1. Su zu k i Cou p lin g of Ar yl Br om id es a n d
Ch lor id es w ith P h en ylbor on ic Acid a
can be seen that the order of activity is 5c > 5d > 5a .
The fact that 5a shows the lowest activity indicates that,
if anything, the phosphite ligand is too π-acidic and that
the oxidative-addition step is being retarded with
respect to that seen when 5c is used. Since the platinum
center in 5d is more electron rich than that in 5c, it
would be expected that the rate of oxidative addition
would be higher, but the increased electron density at
the platinum appears to be deleterious. This suggests
that in this case the rate-determining step is probably
not oxidative addition but rather either nucleophilic
attack of the base-activated boronate11 or reductive
elimination of the biaryl product. Catalyst 5c seems to
have about the best balance of electronic properties for
the coupling of 4-bromoacetophenone and respectably
high activity results. The maximum TON of 2,500,000
is well in excess of that shown by classical catalysts such
as [Pd(PPh3)4] under the same conditions (entry 16) and
is comparable with or better than the activity reported
for most palladacyclic catalysts.5,12 This activity is
maintained even under air. When the performances of
the catalysts 5a ,c in the coupling of the nonactivated
substrate bromobenzene are compared, it can again be
seen that 5c shows better activity. Catalyst 5c also
shows reasonable activity with electronically deacti-
vated and sterically hindered aryl bromide substrates
(entries 20-25). Gratifyingly, 5c also shows activity in
the coupling of an activated aryl chloride substrate. It
is only recently that palladium catalysts have made
inroads into the activation of this technically interesting
class of substrates in Suzuki coupling reactions.13
In summary, we have shown not only that, by judi-
cious choice of ligand sets, the use of platinum com-
plexes in Suzuki coupling reactions becomes viable but
also that these catalysts can show activity far in excess
of “classical” palladium catalysts such as [Pd(PPh3)4]
and comparable with many of the new high-activity
palladacyclic-based catalysts. This unexpected activity,
coupled with the fact that platinum has catalytic
properties that are distinct from those of palladium,2
makes the whole area of platinum-catalyzed coupling
reactions ripe for further investigation.
con-
TON (mol
cat. ([Pt],
mol % Pt)
versn of product/
entry
aryl halide
(%)b
mol of Pt)
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
BrC6H4C(O)Me-4
BrC6H4C(O)Me-4
BrC6H4-4-C(O)Me 5c (0.1)
4a (0.1)
5a (0.1)
63
100
100
100
9.5
6
26
26
16
100
74
100
100
24
25
35
98
39
48
72
630
1 000
1 000
1 000
9 500
BrC6H4-4-C(O)Me 5d (0.1)
BrC6H4-4-C(O)Me 4a (0.001)
BrC6H4-4-C(O)Me 4b (0.001)
BrC6H4-4-C(O)Me 5a (0.001)
BrC6H4-4-C(O)Me 5a (0.001)c
BrC6H4-4-C(O)Me 5b (0.001)
BrC6H4-4-C(O)Me 5c (0.001)
BrC6H4-4-C(O)Me 5d (0.001)
BrC6H4-4-C(O)Me 5c (0.0001)
BrC6H4-4-C(O)Me 5c (0.0001)d
BrC6H4-4-C(O)Me 5c (0.00001)
BrC6H4-4-C(O)Me 5c (0.00001)d
BrC6H4-4-C(O)Me [Pd(PPh3)4] (0.001)
BrC6H5
BrC6H5
BrC6H5
BrC6H4-4-OMe
BrC6H4-4-OMe
BrC6H4-4-Me
BrC6H4-4-Me
BrC6H4-2-Me
BrC6H4-2-OMe
6 000
26 000
26 000
16 000
100 000
74 000
1 000 000
1 000 000
2 400 000
2 500 000
35 000
980
5a (0.1)
5a (0.001)
5c (0.001)
5c (1.0)
5c (0.1)
5c (0.5)
5c (0.5)
5c (0.5)c
5c (0.5)
39 000
48 000
72
13
72
88
87
51
29
130
144
176
174
102
58
ClC6H4-4-C(O)Me 5c (0.5)
a
Reaction conditions: 10 mmol of aryl halide, 15 mmol of
PhB(OH)2, 20 mmol of K3PO4, 30 mL of 1,4-dioxane, 100 °C, 18 h
b
(not optimized). Conversion to product determined by GC, based
on aryl bromide, with hexadecane internal standard. c Reaction
d
time 2 h. In air.
For the catalytic studies we initially performed a brief
solvent/base optimization for the coupling of 4-bromoac-
etophenone with phenylboronic acid and found that
potassium phosphate in 1,4-dioxane gave the best
results.7 These conditions were used throughout the
Suzuki test reactions with the catalysts 4a ,b and 5a -
d .10 The results of these catalytic studies are sum-
marized in Table 1.
From a comparison of the activities of the ortho-
metalated phosphite complexes 5a ,b with those of the
nonmetalated complexes 4a ,b (entries 1, 2, and 5-9)
in the coupling of 4-bromoacetophenone, two conclusions
can be drawn. First, it can be seen that ortho-metalation
leads to greatly enhanced activity, and second, increas-
ing the size of the OAr function leads to higher activity.
We have previously observed similar patterns of activity
with palladium complexes.6 In the palladium case we
reasoned that reductive elimination of the ortho-meta-
lated ring and an aryl substituent introduced by the
boronic acid leads to the formation of a catalytically
active, low-coordinate Pd(0) species. It is possible that
similar reductive processes occur with the platinum
complexes; however, at this stage we are not able to rule
out the possibility of a Pt(II)/Pt(IV) cycle, although the
use of strongly π-acidic ligands tends to mitigate against
this. Preliminary investigations to determine whether
the catalysts enter a Pt(0)/Pt(II) or a Pt(II)/Pt(IV)
manifold have been inconclusive, and we are currently
exploring this area further.
Ack n ow led gm en t. We thank the EPSRC for fund-
ing and J ohnson Matthey Chemicals for funding and
the loan of platinum salts.
Su p p or tin g In for m a tion Ava ila ble: Text giving details
of the synthesis and characterization of the new complexes
and catalytic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0202524
(11) This process is often referred to as “transmetalation”.
(12) (a) Gibson, S.; Foster, D. F.; Eastham, G. R.; Tooze, R. P.; Cole-
Hamilton, D. J . Chem. Commun. 2001, 779. (b) Zim, D.; Gruber, A. S.;
Ebling, G.; Dupont, J .; Monteiro, A. L. Org. Lett. 2000, 2, 2881. (c)
Bedford, R. B.; Draper, S. M.; Scully, P. N.; Welch, S. L. New J . Chem.
2000, 24, 745. (d) Weissman, H.; Milstein, D. Chem. Commun. 1999,
1901. (e) Beller, M.; Fischer, H.; Herrmann, W. A.; O¨ fele, K.; Brossmer,
C. Angew. Chem., Int. Ed. Engl. 1995, 34, 1848.
(13) For recent examples of palladium-catalyzed Suzuki coupling
reactions with aryl chloride substrates see: (a) Botella, L.; Na´jera, C.
Angew. Chem., Int. Ed. 2002, 41, 179. (b) Liu, S.-Y.; Choi, M. J .; Fu,
G. C. Chem. Commun. 2001, 2408. (c) Bedford, R. B.; Cazin, C. S. J .
Chem. Commun. 2001, 1540. (d) Zapf, A.; Ehrentraut, A.; Beller, M.
Angew. Chem., Int. Ed. 2000, 39, 4153. (e) Andreu, M. G.; Zapf, A.;
Beller, M. Chem. Commun. 2000, 2475. (f) Old, D. W.; Wolfe, J . P.;
Buchwald, S. L. J . Am. Chem. Soc. 1998, 121, 9722. (g) Wolfe, J . P.;
Singer, R. A.; Yang, B. H.; Buchwald, S. L. J . Am. Chem. Soc. 1999,
121, 9550. (h) Bei, X.; Turner, H. W.; Weinberg, W. H.; Guram, A. S.
J . Org. Chem. 1999, 64, 6797. (i) Zhang, C.; Huang, J .; Trudell, M. L.;
Nolan, S. P. J . Org. Chem. 1999, 64, 3804. (j) Littke, A. F.; Fu, G. C.
Angew. Chem., Int. Ed. 1998, 37, 3387.
When the activities of the complexes 5a ,c,d in the
couplings with 4-bromoacetophenone are compared, it
(10) In most cases a nonoptimized standard reaction time of 18 h
was used; however, it can be seen from a comparison of entry 7 with
8 and entry 23 with 24 that the reactions are probably over in a much
shorter space of time. Indeed, a plot of conversion against time for the
former reaction (see the Supporting Information) showed it to be
essentially over by 80 min.