[1088-00-2]  · C18H13P  · 5-Phenyl-5H-benzophosphindole  · (MW 260.28)

(synthesis of phosphorus compounds required for Wittig reactions;1 ring expansion to the dibenzophosphorin system;2 synthesis of ligands used in asymmetric hydroformylation,3a-c hydrosilylation,4 and hydrogenation3a,5)

Alternate Name: 5-phenyldibenzophosphole.

Physical Data: mp 95-97 °C.6

Solubility: sol THF, benzene.

Form Supplied in: not commercially available.

Preparative Methods: prepared in one step from commercially available tetraphenylphosphonium bromide which has been vacuum dried at 100 °C prior to use.6 Drying: no particular requirements.

Handling, Storage, and Precautions: best kept in an O2-free container. Slow phosphine oxide formation will occur upon exposure to O2.

Wittig and Wittig-like Reactions.

Heterolytic C-P bond cleavage of 5-phenyldibenzophosphole (DBP-Ph) with lithium gives lithium dibenzophosphole (DBP-Li). Alkylation of lithium dibenzophosphole with alkyl halides generates the corresponding phosphonium salts. Deprotonation of these phosphonium salts under salt-free conditions (i.e. Na+ or K+ derived base) and treatment of the resulting phosphorus ylide with an aldehyde provides trans-alkenes predominantly (eq 1).1 A 5-phenyldibenzophosphole-derived ylide has been used in the synthesis of (+)-dictyopterene C for the generation of an intermediate alkene with >99:1 trans selectivity.7

The stability of the intermediate 5-phenyldibenzophosphole-derived oxaphosphetanes has been exploited to show that the only observable intermediates in Wittig reactions are oxaphosphetanes1 and to demonstrate that betaines are unlikely intermediates in the Wittig reaction.1c The reaction of trans-stilbene epoxide with DBP-Li (as in eq 1) generates a phosphine oxide.8 After base treatment of the phosphine oxide, cis-stilbene is obtained.

Ring Expansion to Dibenzophosphorin Oxides.

Dibenzophosphorins are obtained after treatment of 5-dibenzophosphole with Benzoyl Chloride in the presence of Triethylamine and subsequent hydrolysis (eq 2).2a,b A similar reaction occurs with Methyl Propiolate.2c

Preparation of Catalysts for Asymmetric Reactions.

5-Phenyldibenzophosphole has been used to prepare bidentate ligands for rhodium3a and platinum complexes.3b-c Platinum complexes with dibenzophosphole-derived ligands are readily obtained (eq 3), and they can be used as catalysts for the hydroformylation of styrene with high ee's (>95%).3b These complexes give the highest ratio of branched to nonbranched aldehydes; however, the conversion is low (eq 4). For rhodium complexes, the 5-phenyldibenzophosphole-based catalysts give the best ee's (40%) in the hydroformylation reaction.3a

Other alkenes have been hydroformylated with poor to average ee's with similar 5-phenyldibenzophosphole-derived platinum complexes.3c A rhodium catalyst with 5-phenyldibenzophosphole ligands does not increase the diastereoselectivity during hydrosilylation of 4-t-butylcyclohexanone (eq 5).4 Hydrogenation of a-acetamidocinnamic acid3a,5 and 2-phenyl-1-butene3a with 5-phenyldibenzophosphole-derived rhodium complexes gives poor ee's (eq 6).

Related Reagents.


1. (a) Vedejs, E.; Marth, C. TL 1987, 28, 3445. (b) Vedejs, E.; Marth, C. F.; Ruggeri, R. JACS 1988, 110, 3940. (c) Vedejs, E.; Marth, C. F. JACS 1990, 112, 3905.
2. (a) Allen, D. W.; Oades, A. C. JCS(P1) 1976, 2050. (b) Hughes, A. N.; Amornraksa, K.; Phisithkul, S.; Reutrakul, V. JHC 1976, 13, 937. (c) Richards, E. M.; Tebby, J. C. JCS(C) 1971, 1064.
3. (a) Hayashi, T.; Tanaka, M.; Ikeda, Y.; Ogata, I. BCJ 1979, 52, 2605. (b) Stille, J. K.; Su, H.; Brechot, P.; Parrinello, G.; Hegedus, L. S. OM 1991, 10, 1183. (c) Consiglio, G.; Nefkens, S. C. A.; Borer, A. OM 1991, 10, 2046.
4. Felföldi, K.; Kapocsi, I.; Bartók, M. JOM 1989, 362, 411 (CA 1989, 111, 134 259k).
5. Dang, T. P.; Poulin, J.-C.; Kagan, H. B. JOM 1975, 91, 105.
6. Cornforth, J.; Cornforth R. H.; Gray, R. T. JCS(P1) 1982, 2289.
7. Grandjean, D.; Pale, P.; Chuche, J. T 1991, 47, 1215.
8. Roberts, T. G.; Whitham, G. H. JCS(P1) 1985, 1953.

Thomas J. Fleck

The Upjohn Company, Kalamazoo, MI, USA

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