New approaches to high-activity transition-metal catalysts for carbon-carbon bond forming reactions. Rhenium-containing phosphorus donor ligands for palladium-catalyzed Suzuki cross-couplings

Article abstract of DOI:10.1039/b201512a

The rhenium complexes (η⁵-C₅H₅)Re(NO)(PPh₃)- ((CH₂)_nPR₂:) (n/R = 0/Ph, 0/t-Bu, 0/Me, 1/Ph, 1/t-Bu), which contain electron-rich and sterically congested phosphido moieties, give

Full text of DOI:10.1039/b201512a

New approaches to high-activity transition-metal catalysts for
carbon–carbon bond forming reactions. Rhenium-containing phosphorus
donor ligands for palladium-catalyzed Suzuki cross-couplings
Sandra Eichenseher, Klemenz Kromm, Olivier Delacroix and J. A. Gladysz*
Institut für Organische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestrasse 42,
91054 Erlangen, Germany. E-mail: gladysz@organik.uni-erlangen.de
Received (in Cambridge, UK) 11th February 2002, Accepted 3rd March 2002
First published as an Advance Article on the web 18th April 2002
The
rhenium
complexes
(h5-C5H5)Re(NO)(PPh3)-
Phenyl bromide disappeared as depicted in Fig. 1, and biphenyl
appeared at a similar rate. On the time scale of 0.5–1.0 h, the
catalysts derived from ligands 1a,b were distinctly more
reactive than those derived from 1c (less bulky) and 2a,b (less
bulky and electron-rich). The tert-butyl-substituted ligand 1b
gave the most active catalyst, and this rate was also monitored
at 80 and 60 °C to improve time resolution. It was generally
close to that of the catalyst from the related organophosphine
P(t-Bu)3,15b which is one of the best for Suzuki couplings
[conversions, 1b/P(t-Bu)3: 80 °C, 84%/88% (0.25 h), 99%/96%
(0.5 h); 60 °C, 21%/79% (0.25 h), 74%/86% (0.5 h)].3b The
phenyl-substituted ligand 1a gave a somewhat less active
catalyst than PPh3. Curiously, the catalyst derived from 2b was
less reactive than unmodified Pd(OAc)2. As a possible contrib-
uting factor, we speculate that ReCH2PR2PdX systems might
equilibrate with reactive +ReNCH2 and [R2PPdX]2 species.
However, 31P NMR spectra of 1b/Pd(OAc)2 mixtures showed
only pairs of coupled signals, consistent with intact Ph3PReP(t-
Bu)2 linkages. The initial rate with 1c was also low, but high
yields were achieved with long reaction times (Table 1).
The two best rhenium-containing ligands, 1a,b, were applied
to other Suzuki reactions. As summarized in Table 2, > 99–78%
yields of the corresponding biaryls were obtained. All reactions
with 1b were complete in < 1 h at 100 °C. Entry 8 in Table 2
was repeated, but on a five-fold greater scale. Chromatography
gave the biaryl product in 97% yield. Entries 7 and 8 were
repeated, but with reduced palladium and rhenium loadings (0.1
and 0.4 mol%). After 1.0 and 0.5 h, quantitative aryl bromide
((CH2)nPR2:) (n/R = 0/Ph, 0/t-Bu, 0/Me, 1/Ph, 1/t-Bu),
which contain electron-rich and sterically congested phos-
phido moieties, give active catalysts for the title reaction;
typical conditions (toluene, 60–100 °C): aryl bromide (1.0
equiv.), PhB(OH)2 (1.5 equiv.), K3PO4 (2.0 equiv.), Pd(OAc)2
(1 mol%), and
a Re(CH2)nPR2: species or a 1+2
[Re(CH2)nPR2H]+X2/t-BuOK mixture (4 mol% rhenium).
Phosphine adducts of transition metals catalyze an extensive
variety of useful carbon–carbon bond forming reactions.1,2
Over the past five years, the activities of many such systems
have been improved by substituting phosphines that are bulkier
and/or more electron-rich.3 Despite these advances, there
remains a distinct need for even more active and long-lived
catalysts, as well as improvements in selectivities.4 Although
many clever design and high-throughput screening strategies
are being assayed,3,5 the next generation of breakthroughs may
require new paradigms far removed from past approaches.
Accordingly, we have sought to develop new catalysts from
monodentate phosphorus donor ligands featuring a novel,
untested design element: an 18-valence-electron transition-
metal center a or b to the phosphorus atom, which would not
directly participate in bond breaking/making steps of the
catalytic cycle. Such coordinatively saturated LnMPR2: and
LnMCH2PR2: species have an extensive literature. They, and/or
nitrogen or sulfur analogs,6–9 have been shown to be much more
basic and nucleophilic than model compounds without the
metal, and the underlying reasons have been analyzed in
detail.6,8 A potential concern is that the ‘spectator’ metal
fragment might somehow compromise catalyst lifetimes or
Table 1 Survey of Suzuki coupling conditions
stabilities. However, metal-containing ligands of all types10,11
–
not just the familiar ferrocenes12 – are playing rapidly
increasing roles in catalysis. Hence, we set out to probe whether
effective catalysts can be generated from rhenium-containing
5
phosphido species, with LnM equal to (h -C5H5)Re(NO)(PPh3).
Chelating diphosphines derived from this chiral template give
long-lived rhodium hydrogenation catalysts.11,13
5
The racemic complexes (h -C5H5)Re(NO)(PPh3)(PR2) (1;
5
R
=
Ph, a; t-Bu, b; Me, c) and (h -C5H5)Re-
(NO)(PPh3)(CH2PR2) (2a,b) shown in Table 1 were isolated via
5
deprotonations of the corresponding phosphonium salts [(h -
C5H5)Re(NO)(PPh3)((CH2)nPR2H)]+X2 (n/R
=
0/Ph,
3a+TfO2; 0/t-Bu, 3b+TfO2; 0/Me, 3c+BF42; 1/Ph, 4a+PF6
;
2
1/t-Bu, 4b+BF42) with t-BuOK, as previously described in most
cases.6,7,11,14 New candidates for high-activity phosphine
ligands are often evaluated in palladium-catalyzed Suzuki
cross-couplings of aryl bromides and aryl boronic acids.2,5
Indeed, 1–2 gave effective catalysts. However, such species are
easily oxidized to phosphine oxides.6 For convenience, sub-
sequent experiments utilized 1–2 that had been generated in situ
from t-BuOK (2.0 equiv.) and the phosphonium salts, which
have shelf-lives of years. The similar use of protonated
organophosphines in several palladium-catalyzed reactions was
recently reported.15
Conversion
(BrPh/%)c
Yield
Entrya
Ligandb
(PhPh/%)c
Time/h
1
2
3
4
5
6
7
8
1a
1b
1c
2a
100
100
100
100
76
100
100
66
93
97
96
90
57
95
97
59
2
0.5
96
24
168
1
2b
PPh3
P(t-Bu)3
No ligand
0.5
168
a Reaction scale: 0.448 mmol of bromobenzene. b The ligands in entries
1–5, 7 were generated in situ from protonated precursors as described in the
text. c Determined by GC, vs. tridecane as internal standard.
The coupling of phenyl bromide and phenylboronic acid was
studied at 100 °C as indicated in Table 1, with K3PO4 as the
boron-activating base and Pd(OAc)2 as the palladium source.16
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CHEM. COMMUN., 2002, 1046–1047
This journal is © The Royal Society of Chemistry 2002

conversions and biaryl yields were again obtained, establishing
that turnover numbers of !1000 can be realized.
to O. D.), and Johnson Matthey PMC (palladium loan) for
support.
Ligand 1b also effected Suzuki couplings of chlorobenzene,
but not as rapidly as P(t-Bu)3 (conversions, 100 °C: 40%/168 h
vs. 83%/96 h). Since K3PO4 is only moderately less basic than
the t-BuOK used to deprotonate the [Re(CH2)nPR2H]+X2
ligand precursors [DpKa(H2O) ca. 6],17 we wondered whether
the latter was needed at all. Accordingly, when a toluene
solution of 3b+TfO2 was treated with K3PO4 (2.0 equiv) at 100
°C, the characteristic orange-red color of 1b was generated.
Entry 2 of Table 1 was repeated with this sample. After 0.5 h,
the conversion and yield were 100 and 99%. An identical
reaction was conducted, but with all components mixed
simultaneously. The rate and yield data were essentially
identical.
The above data clearly establish the viability of transition-
metal-containing monodentate phosphorus donor ligands such
as 1 and 2 for palladium-catalyzed Suzuki reactions. The most
bulky and electron-rich ligand (1b) often comes close to the
activity of the corresponding organophosphine. Although the
other ligands are less effective, we consider these to be highly
promising lead results, amenable to further optimization and
extendable to other metal fragments as well as related carbon–
carbon bond forming reactions. Furthermore, 1 and 2 are easily
obtained in enantiomerically pure form.6,11 Hence, applications
to enantioselective catalysis can be anticipated.18 The utiliza-
tion of 1 and 2 in additional types of transition-metal-catalyzed
reactions will be reported in the near future.
Notes and references
1 Metal-Catalyzed Cross-Coupling Reactions, F. Diederich and P. J.
Stang, ed., Wiley-VCH, Weinheim, Germany, 1998.
2 A. Suzuki, ch. 2 in ref. 1; A. Suzuki, J. Organomet. Chem., 1999, 576,
147; H. Gröger, J. Prakt. Chem., 2000, 342, 334.
3 This literature is extensive. See the following representative papers, and
refs. cited therein: (a) J. P. Wolfe, R. A. Singer, B. H. Yang and S. L.
Buchwald, J. Am. Chem. Soc., 1999, 121, 9550; (b) A. F. Littke, C. Dai
and G. C. Fu, J. Am. Chem. Soc., 2000, 122, 4020; (c) A. Zapf, A.
Ehrentraut and M. Beller, Angew. Chem., Int. Ed., 2000, 39, 4153; (d) G.
Y. Li, Angew. Chem., Int. Ed., 2001, 40, 1513; (e) M. L. Clarke, D. J.
Cole-Hamilton and J. D. Woolins, J. Chem. Soc., Dalton Trans., 2001,
2721; (f) S. Lee, N. A. Beare and J. F. Hartwig, J. Am. Chem. Soc., 2001,
123, 8410.
4 For an essay on the ‘ideal catalyst’ (an unattainable limit), see: J. A.
Gladysz, Pure Appl. Chem., 2001, 73, 1319.
5 Approaches to high-activity Suzuki catalysts besides those in refs. 3a–f,
see: (a) C. Zhang, J. Huang, M. L. Trudell and S. P. Nolan, J. Org.
Chem., 1999, 64, 3804; (b) X. Bei, H. W. Turner, W. H. Weinberg, A.
S. Guram and J. L. Petersen, J. Org. Chem., 1999, 64, 6797; (c) H.
Weissman and D. Milstein, Chem. Commun., 1999, 1901; (d) V. P. W.
Böhm, C. W. K. Gstöttmayr, T. Weskamp and W. A. Herrmann, J.
Organomet. Chem., 2000, 595, 186; (e) A. Fürstner and A. Leitner,
Synlett, 2001, 290; (f) R. B. Bedford and C. S. J. Cazin, Chem.
Commun., 2001, 1540; (g) M. Feuerstein, H. Doucet and M. Santelli,
Tetrahedron Lett., 2001, 42, 5659; (h) S.-Y. Liu, M. J. Choi and G. C.
Fu, Chem. Commun., 2001, 2408; (i) L. Botella and C. Nájera, Angew.
Chem., Int. Ed., 2002, 41, 179; (j) C. Rocaboy and J. A. Gladysz,
Tetrahedron, 2002, 58, in press.
We thank the Deutsche Forschungsgemeinschaft
(GL 300/4-1), the von Humboldt Foundation (fellowship
6 W. E. Buhro, B. D. Zwick, S. Georgiou, J. P. Hutchinson and J. A.
Gladysz, J. Am. Chem. Soc., 1988, 110, 2427.
7 See also: B. D. Zwick, M. A. Dewey, D. A. Knight, W. E. Buhro, A. M.
Arif and J. A. Gladysz, Organometallics, 1992, 11, 2673.
8 M. A. Dewey, D. A. Knight, A. M. Arif and J. A. Gladysz, Chem. Ber.,
1992, 125, 815.
9 F. B. McCormick, W. B. Gleason, X. Zhao, P. C. Heah and J. A.
Gladysz, Organometallics, 1986, 5, 1778.
10 See literature cited in ref. 11, and the following papers: C. Bolm and K.
Muñiz, Chem. Soc. Rev., 1999, 28, 51; G. Jones and C. J. Richards,
Organometallics, 2001, 20, 1251; C. Pasquier, L. Pélinski, J. Brocard,
A. Mortreux and F. Agbossou-Niedercorn, Tetrahedron Lett., 2001, 42,
2809; S. U. Son, K. H. Park, S. J. Lee, Y. K. Chung and D. A. Sweigart,
Chem. Commun., 2001, 1290; C. Bolm, M. Kesselgruber, N. Hermanns,
J. P. Hildebrand and G. Raabe, Angew. Chem., Int. Ed., 2001, 40,
1488.
11 K. Kromm, B. D. Zwick, O. Meyer, F. Hampel and J. A. Gladysz, Chem.
Eur. J., 2001, 7, 2015.
12 A. Togni, N. Bieler, U. Burckhardt, C. Köllner, G. Pioda, R. Schneider
and A. Schnyder, Pure Appl. Chem., 1999, 71, 1531.
13 See also: L. J. Alvey, O. Delacroix, C. Wallner, O. Meyer, F. Hampel,
S. Szafert, T. Lis and J. A. Gladysz, Organometallics, 2001, 20,
3087.
2
14 Complexes 1c, 2b, 3c+BF42, and 4b+BF4 have not been reported
earlier, but all preparative and spectroscopic details are analogous to the
other compounds.
15 (a) M. R. Netherton and G. C. Fu, Org. Lett., 2001, 3, 4295; (b) The P(t-
Bu)3 used in our study was generated in situ from [HP(t-Bu)3]+BF42 and
t-BuOK.
Fig. 1 Rate of consumption of phenyl bromide under the conditions of
Table 1.
Table 2 Survey of aryl bromides
16 General procedure: an oven-dried Schlenk flask was charged with the
phosphonium salt or phosphine (0.0179 mmol, 4.0 mol%) and dry
toluene (4 mL). A 1.0 M THF solution of t-BuOK (0.036 mL, 0.036
mmol) was added with stirring (for phosphonium salts only; rhenium
systems turn from yellow to orange-red). After 5 min, a 0.0045 M
toluene solution of Pd(OAc)2 (1.0 mL, 0.0045 mmol, 1 mol%),
phenylboronic acid (0.082 g, 0.67 mmol, 1.5 equiv.), K3PO4 (0.190 g,
0.897 mmol, 2.0 equiv.), an internal standard [0.204–0.205 mmol of
tridecane (0.050 mL), hexadecane (0.060 mL), or eicosane (0.058 g)],
and the aryl halide (0.448 mmol, 1.0 equiv.) were added. The suspension
was stirred (80 or 100 °C) and monitored by GC. The product was
identified by comparison of its GC retention time to that of a commercial
sample.
Conversion Yield
(ArBr/%) (ArPh%) Time/h
Entry ArBr
Ligand
1
2
1a
1b
100
100
86
78
0.25
0.25
3
4
1a
1b
100
100
100
100
4
1
5
6
1a
1b
100
100
88
88
4
17 D. D. Perrin, Ionisation Constants of Inorganic Acids and Bases in
Aqueous Solution, Pergamon, New York, 2nd edn., 1982.
18 Enantioselective syntheses of chiral biaryls via the Suzuki reaction: A.
N. Cammidge and K. V. L. Crépy, Chem. Commun., 2000, 1723; J. Yin
and S. L. Buchwald, J. Am. Chem. Soc., 2000, 122, 12 051.
0.25
7
8
1a
1b
100
100
100
100
0.25
0.5
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