Novel sterically congested chiral tripodal phosphite ligands: Catalytic asymmetric hydrosilylation of ketones with a Rh(I)-TRISPHOS catalyst

Article abstract of DOI:10.1016/S0957-4166(98)00018-4

A new chiral trisphosphite ligand, (S,S,S)-2,2',2_-tris(2,4,8,10-tetra- tert-butyl-dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl-6-oxy)tri-2-propylamine, (S,S,S)-TRISPHOS, was synthesized and coordination chemistry investigated. The Rh(I)-(S,S,S)-TRISPHOS complexes were found to be effective catalysts for the enantioselective hydrosilylation of ketones.

Full text of DOI:10.1016/S0957-4166(98)00018-4

Pergamon
Tetrahedron: Asymmetry 9 (1998) 543–546
Novel sterically congested chiral tripodal phosphite ligands:
catalytic asymmetric hydrosilylation of ketones with a
Rh(I)–TRISPHOS catalyst
Stephen D. Pastor ∗ and Sai P. Shum
Ciba Specialty Chemicals Corporation, Additives Research Department, PO Box 2005, 540 White Plains Road, Tarrytown,
New York, NY 10591, USA
Received 15 December 1997; accepted 5 January 1998
Abstract
A new chiral trisphosphite ligand, (S,S,S)-2,20,200-tris(2,4,8,10-tetra-tert-butyl-dibenzo[d,f][1,3,2]dioxaphos-
phepin-6-yl-6-oxy)tri-2-propylamine, (S,S,S)-TRISPHOS, was synthesized and coordination chemistry investi-
gated. The Rh(I)–(S,S,S)-TRISPHOS complexes were found to be effective catalysts for the enantioselective
hydrosilylation of ketones. © 1998 Elsevier Science Ltd. All rights reserved.
Rationally designed sterically hindered phosphite ligands are a recent tool in the synthetic chemists
arsenal for achieving novel stereoselectivity in transition metal catalyzed reactions.1–8 Relatively little
attention, however, has been paid to the synthesis of tripodal ligands with phosphite functionality.9,10
The phosphite ligand 1a was prepared by the reaction of the seven-membered phosphorochloridite 211
with triethanolamine (3a) using triethylamine as an acid acceptor. Although three stereoaxes are present
in a molecule of 1a, rotation about the stereoaxis in the seven-membered ring is known to be rapid
at room temperature.1a,11a,12,13 In the 31P{1H} NMR (CD2Cl2) spectrum of 1a at 26°C, a singlet was
observed at δ 140.6, which was assigned to three equivalent phosphorus atoms. In the VT 31P{1H} NMR
spectrum of 1a below −55°C, the coalescence temperature, signals were observed that were assigned to
the (M*,M*,M*)- and (M*,M*,P*)-diastereoisomers of 1a.
∗
Corresponding author. E-mail: steve.pastor@cibasc.com
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00018-4

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In an analogous manner, the chiral ligand 1b, (S,S,S)-TRISPHOS,14 was prepared by the reaction of 2
with (S,S,S)-triisopropanolamine (3b).15 In the 31P{1H} NMR (CDCl3) spectrum of (S,S,S)-TRISPHOS,
a singlet was observed at δ 145.2 that is consistent with the C3 symmetry of the molecule.
Quite interestingly, the Rh(I) complex formed by the reaction of either 1a or 1b with chloro(1,5-
cyclooctadiene)Rh(I) dimer showed evidence for the formation of the dimeric Rh(I) complexes 4ab
illustrated with coordination of only two of the phosphorus atoms of the ligand to rhodium.16 For
example, in the 31P{1H} NMR (CDCl3) spectrum of complex 4b formed by the reaction of (S,S,S)-
TRISPHOS with chloro(1,5-cyclooctadiene)Rh(I) dimer, a singlet and a doublet were observed at δ 146.6
and δ 125.5 (1JPRh=323 Hz), respectively, whose peak areas integrated to a 1:2 ratio, respectively.
Consistent with this interpretation, the addition of pyridine to 4b resulted in the formation of the
monomeric Rh(I) complex 5b with three nonequivalent phosphorus atoms in the 31P{1H} NMR
spectrum.16 Similarly, the coordination of only two phosphorus atoms of (S,S,S)-TRISPHOS to
Rh(I) were observed both in the neutral complex formed with (bicyclo[2.2.1]hepta-2,5-diene)(2,4-
pentanedionato)Rh(I) and the cationic complex formed with bis(bicyclo[2.2.1]hepta-2,5-diene)Rh(I)
perchlorate.16
Although Rh-catalyzed hydrosilylations of acetophenone with chiral amine and phosphine17,18 ligands
have been reported to give the corresponding alcohol with high enantioselectivity, the use of chiral
phosphites in this reaction gave the corresponding alcohol in only 60% ee.3b,19 The Rh(I)-catalyzed
hydrosilylation of acetophenone with the achiral ligand 4a prepared in situ followed by hydrolysis
gave sec-phenethyl alcohol in 60% isolated yield. The asymmetric hydrosilylation of acetophenone
using (S,S,S)-TRISPHOS was investigated using dimeric, monomeric, and cationic catalyst precursors
(Table 1). All three catalyst precursors gave the same sense of chiral induction. The highest rate

S. D. Pastor, S. P. Shum / Tetrahedron: Asymmetry 9 (1998) 543–546
545
Table 1
Rh(I)-catalyzed hydrosilylation of acetophenone with TRISPHOS ligand
and conversion were obtained using a cationic Rh(I) catalyst and an unprecedented 81% ee of (R)-
sec-phenethyl alcohol was obtained for a catalyst prepared from a chiral phosphite ligand. As noted
previously by van Leeuwen20 for hydroformylation reactions using chiral seven-membered phosphite
ligands, the high reaction stereoselectivity observed suggests that that there is a cooperative effect
between the stereocenters and stereoaxes in the coordinated ligand, a manifestation of the concept of
chiral cooperativity.21
In a typical procedure for the determination of reaction enantioselectivity a catalyst solution
prepared in situ from 300 mg (0.20 mmol) of (S,S,S)-TRISPHOS and 49 mg (0.20 mmol) of
bis(bicyclo[2.2.1]hepta-2,5-diene)Rh(I) perchlorate in 10 mL of benzene was added to a solution of 1.2
mL (10 mmol) of acetophenone and 1.9 mL (10 mmol) of diphenylsilane in 11 mL of benzene. After
stirring for 16 h at room temperature (complete disappearance of acetophenone), a mixture of 5 mL
of 2.5 N aqueous sodium hydroxide and 7 mL of methanol was added to the reaction mixture. After
stirring for 10 min, the reaction mixture was extracted with diethyl ether (3×3 mL) and the combined
organic extracts were dried over anhydrous sodium sulfate. The volatiles were removed in vacuo and
1
the ee of the resultant sec-phenethyl alcohol was determined both by H NMR spectroscopy using the
chiral solvating agent (R)-(−)-2,2,2-trifluoro-1-(9-anthryl)ethanol and GLC using a Chiraldex™ꢀ β-PM
chiral column (30 m×0.25 mm). The crude alcohol was purified by dissolution in 3 mL of hexane and
filtration of any precipitate formed followed by flash chromatography (silica gel; hexane eluent) to give
1.0 g (65%) of (R)-sec-phenethyl alcohol.
The scope of hydrosilylation reactions using the (S,S,S)-TRISPHOS ligand and the extension to other
transition metal catalyzed reactions is currently being pursued.
References
1. (a) Pastor, S. D.; Shum, S. P.; DeBellis, A. D.; Burke, L. P.; Rodebaugh, R. K.; Clarke, F. H.; Rihs, G. Inorg. Chem. 1996,
35, 949. (b) Pastor, S. D.; Rogers, J. S.; NabiRahni, M. A. Inorg. Chem. 1996, 35, 2157.
2. (a) Cuny, G. D.; Buchwald, S. L. J. Am. Chem. Soc. 1993, 115, 2066. (b) Johnson, J. R.; Cuny, G. D.; Buchwald, S. L.
Angew. Chem. Int. Ed. Engl. 1995, 34, 1760.
3. (a) Billig, E.; Abatjoglou, A. G.; Bryant, D. R. U.S. Patent 4,748,261 (1988); Chem. Abstr. 1987, 107, 7392. See also (b)
Babin, J. E.; Whiteker, G. T. U.S. Patent 5,360,938 (1994); Chem. Abstr. 1995, 122, 186609.
4. (a) Buisman, G. J. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Tetrahedron: Asymmetry 1993, 4, 1625. (b) van Leeuwen,
P. W. N. M.; Buisman, G. J. H.; van Rooy, A.; Kamer, P. C. J. Recl. Trav. Chim. Pays-Bas 1994, 113, 61. (c) see also

546
S. D. Pastor, S. P. Shum / Tetrahedron: Asymmetry 9 (1998) 543–546
Jongsma, T.; Fossen, M.; Challa, G.; van Leeuwen, P. W. N. M. J. Molecular Cat. 1993, 83, 17. (d) van Rooy, A.; Orij, E.
N.; A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Organometallics 1995, 14, 34. (e) Buisman, G. J. H.; Martin, M. E.; Vos,
E. J.; Klootwijk, A.; Kamer, P. C. J.; van Leeuwen P. W. N. M. Tetrahedron: Asymmetry 1995, 6, 719. (f) Kranenburg, M.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Chaudret, B. J. Chem. Soc., Chem. Commun. 1997, 373.
5. (a) Moasser, B.; Gross, C.; Gladfelter, W. L. J. Organomet. Chem. 1994, 471, 201. (b) Moasser, B.; Gladfelter, W. L.; Roe,
D. C. Organometallics 1995, 14, 3832.
6. (a) Sakai, N.; Nozaki, K.; Mashima, K.; Takaya, H. Tetrahedron: Asymmetry 1992, 3, 583. (b) Sakai, N.; Mano, S.; Nozaki,
K.; Takaya, H. J. Am. Chem. Soc. 1993, 115, 7033. (c) Nozaki, K.; Sakai, N.; Nanno, T.; Higashijima, T.; Mano, S.; Horiuchi,
T.; Takaya, H. J. Am. Chem. Soc. 1997, 119, 4413 and references therein. (d) Horiuchi, T.; Ohta, T.; Shirakawa, E.; Nozaki,
K.; Takaya, H. J. Org. Chem. 1997, 62, 4285.
7. (a) Bedford, R. B.; Chaloner, P. A.; Hitchcock, P. B. J. Chem. Soc., Chem. Commun. 1995, 2049. (b) Bedford, R. B.;
Castillòn, S.; Chaloner, P. A.; Claver, C.; Fernandez, E.; Hitchcock, P. B.; Ruiz, A. Organometallics 1996, 15, 3990.
8. Cserépi-Szücs, S.; Bakos, J. J. Chem. Soc., Chem. Commun. 1997, 635.
9. (a) Scherer, J.; Huttner, G.; Büchner, M.; Bakos, J. J. Organomet. Chem. 1996, 520, 45. (b) Scherer, J.; Huttner, G.; Büchner,
M. Chem. Ber. 1996, 129, 697. (c) Scherer, J.; Huttner, G.; Walter, O.; Janssen, B. C.; Zsolnai, L. Chem. Ber. 1996, 129,
1603.
10. Baker, M. J.; Pringle, P. G. J. Chem. Soc., Chem. Commun. 1993, 314.
11. (a) Pastor, S. D.; Shum, S. P.; Rodebaugh, R. K.; Debellis, A. D.; Clarke, F. H. Helv. Chim. Acta 1993, 76, 900. (b) For
preparation of 2 in situ, see Malen, A. H.; NabiRahni, M. A.; Pastor, S. D.; Stevens, E. D.; Snyder, J. A. Phosphorus, Sulfur,
Silicon Relat. Elem. 1993, 82, 1.
12. Pastor, S. D.; Richardson, C. F.; NabiRahni, M. A. Phosphorus, Sulfur, Silicon Relat. Elem. 1994, 90, 95.
13. Whiteker, G. T.; Harrison, A. M.; Abatjoglou J. Chem. Soc., Chem. Commun. 1995, 1805.
14. General procedure for ligand preparation: 1b [(S,S,S)-TRISPHOS]; to a solution of 1.5 g (9.1 mmol) of (S,S,S)-tris-2-
propanolamine and 3.8 mL (27 mmol) of triethylamine in 20 mL of toluene was added dropwise a solution of 13.0 g (27
mmol) of 2 in 50 mL of toluene. The reaction mixture was stirred for 16 h at rt and then the resulting precipitate removed by
filtration. The volatiles were removed in vacuo and the residue purified by trituration with 100 mL of acetonitrile followed
by crystallization from a 8:2 mixture of acetonitrile:toluene to give 7.0 g (51%) as a white crystalline solid, mp 187–188°C;
1
[α]25 −72.489 (c 1.0, CHCl3); 31P{1H} NMR (CDCl3, 202.36 MHz) δ 145.2; H NMR (CDCl3, 499.85 MHz) δ 1.16 (d,
9H), 1.33 (s, 27H), 1.34 (s, 27H), 1.45 (s, 27H), 1.46 (s, 27H), 2.30 (dd, 2JHCH=12.8 Hz, 3H), 2.64 (dd, 2JHCH=12.8 Hz, 3
3
3
H), 4.28 (dd, J=10.1 Hz, J=3.8 Hz, 3H), 7.14 (d, 3 H), 7.15 (d, 3 H), 7.41 (d, 6 H). Anal. calcd for C93H138NO9P3: C,
74.12; H, 9.23; N, 0.93. Found: C, 73.90; H, 9.29; N, 0.86.
15. Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc. 1994, 116, 6142.
16. The coupling between P atoms in 4ab and 5ab was verified by the appropriate 2D homonuclear 31P COSY experiment.
1
4b: 31P{1H} NMR (C6D6, 202.3578 MHz) δ 146.6 (s), 125.5 (d, JPRh=323 Hz). 5b: 31P{1H} NMR (C6D6, 202.3578
1
2
1
2
MHz) δ 146.5 (s), 133.1 (d, JPRh=288 Hz, JPP=73 Hz), 124.9 (d, JPRh=325 Hz, JPP=73 Hz. Complex of 1b with
bis(bicyclo[2.2.1]hepta-2,5-diene)Rh(I) perchlorate: 31P{1H} NMR (C6D6, 121.47 MHz) δ 146.3 (s), 124.1 (d, 1JPRh=278
Hz).
17. For a general review of hydrosilylation, see Ojima, I; Hirai, K. Asymmetric Synthesis, Vol 5; Morrison, J. D., Ed.; Academic
Press: New York, 1985, pp 103–146.
18. Ojima, I The Chemistry of Organic Silicon Compounds, Patai, S.; Rappoport, Z., Eds.; Wiley: New York, 1989, pp.
1479–1526.
19. Sakaki, J.; Seebach, D. Helv. Chim. Acta 1993, 76, 2654.
20. Buisman, G. J. H.; van der Veen, L. A.; Klootwijk, A.; de Lange, W. G. J.; Kamer, P. C. J.; van Leeuwen P. W. N. M.; Vogt,
D. Organometallics 1997, 16, 2929.
21. (a) Pastor, S. D.; Togni, A. J. Am. Chem. Soc. 1989, 111, 2333. (b) Togni, A.; Pastor, S. D. J. Org. Chem. 1990, 55, 1649.
(c) Pastor, S. D.; Togni, A. Helv. Chim. Acta 1991, 74, 907. (d) Togni, A.; Pastor, S. D. Chirality 1991, 3, 331. (e) Pastor, S.
D. Trends in Organometal. Chem. 1994, 1, 63.