Catalysis for catalysis: Synthesis of mixed phosphine-phosphine oxide ligands via highly selective, Pd-catalyzed monooxidation of bidentate phosphines

Full text of DOI:10.1021/ja990841+

J. Am. Chem. Soc. 1999, 121, 5831-5832
5831
attractive strategy for the preparation of BPMOs. However, the
direct oxidation of Ph2P(CH2)nPPh2 with conventional oxidants
(e.g., O2, H2O2, Br2/H2O) is nonselective, always leading to
mixtures of the unreacted diphosphine, its monoxide, and its
dioxide.9-11 Tedious column separations of such mixtures and
poor isolated yields are inevitable when preparing BPMOs via
the conventional diphosphine oxidations.10 In sharp contrast with
this, our anaerobic Pd-catalyzed biphasic oxidation with 1,2-
dibromoethane/alkali is remarkably selective, giving rise to the
desired mono-oxidized product (eq 1).
Catalysis for Catalysis: Synthesis of Mixed
Phosphine-Phosphine Oxide Ligands via Highly
Selective, Pd-Catalyzed Monooxidation of Bidentate
Phosphines
Vladimir V. Grushin†
Department of Chemistry, Wilfrid Laurier UniVersity
Waterloo, Ontario N2L 3C5, Canada
ReceiVed March 15, 1999
Ph2P-Y-PPh2 + BrCH2CH2Br +
Pd(OAc)2
In this paper, we report the first metal-catalyzed, highly
selective, and efficient mono-oxidation of bidentate phosphines
to bis-phosphine monoxides (BPMOs) of the general formula R2P-
(O)-Y-PR2, where Y is a divalent bridging group. A number of
BPMOs have proven to be valuable soft/hard ligands for
inorganic/organometallic synthesis1 and especially catalysis with
transition metals.2-5 The use of BPMOs for the hydroformylation2
and hydroxycarbonylation5 of olefins resulted in one of the highest
linear-to-branched product ratios ever observed for such reactions.
The temperature (200 °C) and pressure (500 psi) normally required
to run the Monsanto process (MeOH + CO to AcOH)6 can be
brought down to as low as 80 °C and 50 psi, with no loss in
catalytic turnover frequency, by simply activating the conventional
Rh catalyst with Ph2P(O)(CH2)2PPh2.3a,d Despite its great potential
and anticipated diversity, the coordination and catalytic chemistry
of BPMOs still remains in its infancy due to the lack of a
convenient, general method to synthesize these ligands.7
2NaOH
8 Ph2P(O)-Y-PPh2 +
water/1,2-C2H4Cl2 or CH2Cl2
CH2dCH2 + 2NaBr + H2O (1)
Reaction 1 occurs under mild conditions, affording various
BPMOs in up to 90% isolated yield (Table 1). Our research was
focused on aromatic substrates containing PPh2 moieties, which
cannot be mono-oxidized selectively, using the “protonation
followed by oxidation” technique developed by Ma¨ding and
Scheller.11 All reactions were run on a 0.1-100 g scale, with the
substrate-to-catalyst ratio being in the range of 1000-100.12
(7) (a) The literature methods to prepare BPMOs with one1m,p,7b,c or two7d,e
carbon atoms separating the P centers employ active organometallic compounds
or toxic and expensive secondary phosphines and vinylphosphine oxides. These
methods cannot be extended to the synthesis of BPMOs with bridges other
than -CHR- and -(CH2)2-. The only general method to synthesize BPMOs is
a two-step process that involves the monobenzylation of bidentate phosphine
with benzylic halides, followed by recrystallization of the phosphonium salt
and its alkaline hydrolysis.7f Some research groups have experienced difficulties
preparing dppmO1m and dpppO,8 following the two-step method.7f (b) Seyferth,
D. U.S. Patent 3426021, 1969. (c) Rudomino, M. V.; Tsvetkov, E. N. Synthesis
1991, 125. (d) Kabachnik, M. I.; Medved’, T. Ya.; Pisareva, S. A.; Ignat’eva,
T. I.; Lomakina, L. N.; Kozachenko, A. G.; Matrosov, E. I.; Petrovskii, P.
V.; Komarova, M. P. IzV. Akad Nauk SSSR, Ser. Khim. 1980, 673. (e)
Bondarenko, N. A.; Rudomino, M. V.; Tsvetkov, E. N. IzV. Akad. Nauk SSSR,
Ser. Khim. 1990, 2180. (f) Abatjoglou, A. G.; Kapicak, L. A. Eur. Pat. Appl.
EP 72560, 1983; U.S. Patent 4429161, 1984.
A selective mono-oxidation reaction of readily available
bidentate phosphines would apparently be the simplest and most
† Present Address: DuPont CR&D, E328/306, Experimental Station,
Wilmington, DE 19880-0328.
(1) (a) Coyle, R. J.; Slovokhotov, Yu. L.; Antipin, M. Yu.; Grushin, V. V.
Polyhedron 1998, 17, 3059. (b) Brassat, I.; Englert, U.; Keim, W.; Keitel, D.
P.; Killat, S.; Suranna, G.-P.; Wang, R. Inorg. Chim. Acta 1998, 280, 150.
(c) Hipler, B.; Doering, M.; Dubs, C.; Goerls, H.; Huebler, T.; Uhlig, E. Z.
Anorg. Allg. Chem. 1998, 628, 1329. (d) Vicente, J.; Arcas, A.; Bautista, D.;
Jones, P. G. Organometallics 1997, 16, 2127. (e) Mecking, S.; Keim, W.
Organometallics 1996, 15, 2650. (f) Al-Jibouri, S. A.; Al-Hayaly, L. J.; Al-
Allaf, T. A. K. Asian J. Chem. 1995, 7, 471. (g) Colton, R.; Harrison, K. L.;
Mah, Y. A.; Traeger, J. C. Inorg. Chim. Acta 1995, 231, 65. (h) Al-Jibouri,
S. A.; Kalay, Z. M.; Al-Allaf, T. A. K. Transition Met. Chem. (London) 1994,
19, 293. (i) Blagborough, T. C.; Davis, R.; Ivison, P. J. Organomet. Chem.
1994, 467, 85. (j) Visseaux, M.; Dormond, A.; Baudry, D. Bull. Soc. Chim.
Fr. 1993, 130, 173. (k) Rossi, R.; Marchi, A.; Marvelli, L.; Magon, L.;
Peruzzini, M.; Casellato, U.; Graziani, R. Inorg. Chim. Acta 1993, 204, 63.
(l) Berry, D. E.; Browning, J.; Dixon, K. R.; Hilts, R. W. Can. J. Chem.
1988, 66, 1272. (m) Higgins, S. J.; Taylor, R.; Shaw, B. L. J. Organomet.
Chem. 1987, 325, 285. (n) Bao, Q. B.; Landon, S. J.; Rheingold, A. L.; Haller,
T. M.; Brill, T. B. Inorg. Chem. 1985, 24, 900. (o) Grim, S. O.; Walton, E.
D.; Inorg. Chem. 1980, 19, 1982. (p) Grim, S. O.; Satek, L. C.; Tolman, C.
A.; Jesson, J. P. Inorg. Chem. 1975, 14, 656.
(2) Hydroformylation: (a) Huang, I. D.; Westner, A. A.; Oswald, A. A.;
Jermansen, T. G. PCT Int. Appl. 8001690, 1980. (b) Abatjoglou, A. G.; Bryant,
D. R. Eur. Pat. Appl. EP 73398, 1983; U.S. Patent 4,491,675, 1985; U.S.
Patent 4,593,011, 1986. (c) Abatjoglou, A. G.; Billing, E. Eur. Pat. Appl. EP
73961, 1983; U.S. Patent 4,400,548, 1983; U.S. Patent 4522933, 1985. (d)
Oswald, A. A.; Jermansen, T. G.; Westner, A. A.; Huang, I. D. U.S. Patent
4,687,874, 1987. (e) Terekhova, M. I.; Kron, T. E.; Noskov, Yu. G.; Petrov,
E. S. Zh. Obshch. Khim. 1994, 64, 1966. (f) Abu-Gnim, C.; Amer, I. J.
Organomet. Chem. 1996, 516, 235.
(3) Carbonylation of alcohols: (a) Wegman, R. W.; Schrek, D. J. Eur. Pat.
Appl. EP 173170, 1986. (b) Wegman, R. W. Eur. Pat. Appl. EP 171804, 1986.
(c) Wegman, R. W.; Abatjoglou, A. G. PCT Int. Appl. 8600888, 1986; U.S.
Patent 4,670,570, 1987. (d) Wegman, R. W.; Abatjoglou, A. G.; Harrison, A.
M. J. Chem. Soc., Chem. Commun. 1987, 1891.
(4) Carbonylation of esters: (a) Wegman, R. W. U.S. Patent 4,563,309,
1984. (b) Wegman, R. W. PCT Int. Appl. 8600889, 1986.
(5) Hydroxycarbonylation of olefins: Terekhova, M. I.; Kron, T. E.;
Bondarenko, N. A.; Petrov, E. S.; Tsvetkov, E. N. IzV. Akad. Nauk SSSR,
Ser. Khim. 1992, 2003.
(6) The Monsanto acetic acid process accounts for ca. 55% of all acetic
acid produced worldwide. See: Weissermel, K.; Arpe, H.-J. Industrial Organic
Chemistry, 3rd ed.; Wiley-VCH: Weinheim, 1997.
(8) Shcherbakov, B. K., private communication.
(9) Berners-Price, S. J.; Norman, R. E.; Sadler, P. J. J. Inorg. Biochem.
1987, 31, 197.
(10) (a) Despite very poor yields, e.g., 13%,10b and the tediuos isolation
procedure, the direct nonselective oxidation is sometimes preferred over other
literature methods.7 (b) Brock, S. L.; Mayer, J. M. Inorg. Chem. 1991, 30,
2138.
(11) (a) Protonated forms of much more basic Alk2P(CH2)nPAlk2 (e.g., Alk
) Me; n ) 2)10b may be mono-oxidized in a selectiVe manner. This technique
is not applicable to less basic Ph2P(CH2)2PPh2 and other aromatic diphosphines.
(b) Ma¨ding, P.; Scheller, D. Z. Anorg. Allg. Chem. 1988, 567, 179.
(12) All reactions were run under N2 and monitored by TLC and/or 31P
NMR. The products were isolated in air. Representative procedures for the
synthesis of dppeO and dppmO follow. dppeO: (a) Aqueous NaOH (10%,
20 mL) was added to a solution of Pd(OAc)2 (10 mg, 4.45 × 10-2 mmol),
dppe (4.00 g, 10.05 mmol), and 1,2-dibromoethane (2.85 g, 15.2 mmol) in
1,2-dichloroethane (30 mL). The biphasic system was vigorously stirred under
reflux for 7 h, until the originally yellow mixture turned pale yellow or almost
colorless. The organic phase was filtered through a silica plug which was
then washed with 60 mL of CH2Cl2/AcOEt (5:3 by volume). After the
combined organic solutions were evaporated and treated with ether, white
crystals of dppeO were washed with ether and dried under vacuum. The yield
was 3.62 g (87%). 1H NMR (CDCl3, 20 °C), δ 2.3 (m, 4H, CH2); 7.2-7.7
(m, 20H, Ph). (b) Under similar conditions (21.5 h), dppeO was obtained
(40.75 g, 78%) from Pd(OAc)2 (50 mg, 22.3 × 10-2 mmol), dppe (50.00 g,
125.6 mmol), 1,2-dibromoethane (36 g, 191.5 mmol) in 1,2-dichloroethane
(200 mL) and 20% NaOH (125 mL). dppmO: 20% NaOH (6 mL) and NaI
(40 mg, 0.27 mmol) were added to a solution of Pd(OAc)2 (5 mg, 2.2 × 10-2
mmol), dppm (2.00 g, 5.21 mmol), and 1,2-dibromoethane (2.0 g, 10.64 mmol)
in 1,2-dichloroethane (10 mL). After the biphasic system was vigorously stirred
under reflux for 4 h, the organic phase was filtered through a silica plug which
was then washed with CH2Cl2/AcOEt (3:1 by volume). The combined organic
solutions were evaporated to dryness to produce crude solid dppmO which
was redissolved in a small amount of boiling CH2Cl2 and precipitated with
100 mL of ether. After standing for 2 h at room temperature, fluffy needles
of dppmO were filtered off, washed with ether, and dried under vacuum. The
yield was 1.53 g (74%). 1H NMR (CDCl3, 20 °C), δ 3.1 (d, 2H, J ) 12.7 Hz,
CH2); 7.1-7.9 (m, 20H, Ph).
10.1021/ja990841+ CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/05/1999

5832 J. Am. Chem. Soc., Vol. 121, No. 24, 1999
Communications to the Editor
Table 1. Pd-catalyzed Mono-Oxidation of Bidentate Phosphines
a A: 1,2-dichloroethane, 10-20% NaOH, 1.5-2 equiv C2H4Br2, reflux. B: in the presence of NaI (8-10 mol equiv/Pd). C: CH2Cl2, 20%
NaOH, 2 equiv C2H4Br2, room temperature. D: CH2Cl2, 20% NaOH, 3 equiv C2H4Br2, reflux. E: CH2Cl2, 4.5% NaOH, 4 equiv C2H4Br2, room
temperature for 48 h, then under reflux with 8% NaOH for 1 more day.
Although Pd(OAc)2 is the most convenient catalyst to use, a wide
variety of Pd and Pt compounds, e.g., PdCl2, Na2PdCl4, K2PtCl4,
phosphine complexes, and so forth, may be used as well. In
general, Pd catalysts are more efficient than their Pt counterparts.13
Importantly, to obtain high yields of BPMOs, reaction conditions
should be thoroughly optimized for each particular diphosphine
substrate to be oxidized. For instance, the oxidation of dppp to
dpppO occurs with the highest yield when conducted at 80 °C,
whereas room temperature and longer reaction times benefit the
clean formation of dppbO from dppb (Table 1). In some cases
(dppm, dppfc), the mono-oxidation is much more efficient when
run in the presence of small quantities of iodide anion. At this
point, the effect of the I- is not entirely understood.14
Catalytic process 1 was realized by means of an elaborate
design stemming from fundamentals of organic and modern
organometallic chemistry, rather than from screening techniques.
The mechanism of the catalytic oxidation involves the hard base-
promoted selective intramolecular redox process Pd(II)/P(III) f
Pd(0)/P(V),15 followed by reoxidation of the resulting Pd(0)
species back to the catalytically active Pd(II). The latter forms
via the oxidative addition of BrCH2CH2Br to give a bromoethyl
complex, [LPd(Br)(CH2CH2Br)], which then loses ethylene as a
result of â-halogen elimination.16 Detailed mechanistic studies
of reaction 1 will be reported separately.
ligands can (i) stabilize the transition metals in both low and high
oxidation states and (ii) form labile chelates which easily generate
highly reactive, coordinatively unsaturated species, thus providing
the metal complex with low activation energy paths to isomeri-
sation, oxidative addition, migratory insertion, reductive elimina-
tion reactions, and so forth. It is hoped that due to the simple
and efficient catalytic route to BPMOs described herein, these
valuable ligands will soon enjoy many new applications in
synthesis and catalysis.
Acknowledgment. Wilfrid Laurier University and DuPont are
acknowledged for support.
JA990841+
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Due to the presence of both the soft (P) and hard (O)
nucleophilic centers within one molecule, hemilabile17 BPMO
(13) In the absence of a metal catalyst, a slow, nonselective oxidation
occurs.
(14) It is conceivable that the iodide somehow facilitates oxidative addition
of BrCH2CH2Br to the Pd(0) species involved in the catalytic cycle due to
the formation of electron-enriched anionic Pd iodo species and/or generation
of more electrophilic ICH2CH2Br because of the equilibrium BrCH2CH2Br
+ I- a ICH2CH2Br + Br-.
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