Dihydroboronium derivatives of (S,S)-1,2-bis(t-butylmethylphosphino)ethane as convenient chiral ligand precursors Takehiro Miyazaki, a Masanobu Sugawara, b Hiroshi Danjo a,  and Tsuneo Imamoto a, * a Department of Chemistry, Faculty of Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b Fine Chemicals Division, Kaneka Corporation, 3-2-4 Nakanoshima, Kita-ku, Osaka 530-8288, Japan Received 30 September 2004; revised 21 October 2004; accepted 22 October 2004 Available online 6 November 2004 Abstract—Dihydroboronium derivatives of (S,S)-1,2-bis(t-butylmethylphosphino)ethane (t-Bu-BisP*) were prepared and used as chiral diphosphine ligand precursors in Rh-catalyzed asymmetric hydrogenation of methyl (Z)-acetamidocinnamate to afford the hydrogenation product in up to 94% enantioselectivity. Ó 2004 Elsevier Ltd. All rights reserved. Previously, we have reported the preparation of P-chiral trialkylphosphine ligands and their use in transition- metal-catalyzed asymmetric reactions. 1 The chirality at the donor atoms is expected to provide an asymmetric environment that is very close to the reaction center. This has led to the realization of high enantio-induction in some representative catalytic organic transforma- tions. 2 However, as electron-rich phosphines are easily oxidized in air, they must be stored in a protected form, such as a phosphine–borane complex. This has com- pelled researchers to perform a complicated deprotection process prior to the use of these phosphines. 1,3,4 There- fore, the development of effective protecting groups for the electron-rich phosphines is one of the important sub- jects in the field of organophosphorus chemistry. 5 We report herein the preparation of new P-chiral diphosphine ligand precursors for the transition-metal- catalyzed asymmetric transformation. In order to pre- pare a phosphine–borane complex that can be easily cleaved under mild conditions, catecholborane was cho- sen as the new candidate for the protecting group. Cate- cholborane exhibits weaker Lewis acidity than borane owing to the electronic feature of the catecholate moiety. Previously, it has been revealed that the reaction of tri- methylphosphine with catecholborane resulted in the formation of [bis(trimethylphosphine)]boronium bis- (catechol)borate rather than trimethylphosphine–cate- cholborane complex. 6 According to this fact, we tried to prepare new compounds 1 and/or 2 by the reaction of t-Bu-BisP* with catecholborane (see Fig. 1). The reaction sequence is shown in Scheme 1. 7 Debora- nation of phosphine–borane 3 was carried out by succes- sive treatments with trifluoromethanesulfonic acid and aqueous sodium hydroxide. The obtained free diphosphine 4 was mixed with 2equiv of catecholborane in THF at 50 °C. The reaction was completed within 12h and the product was obtained as a white powder. 11 B NMR measurement revealed that there were two different signals at À58.0 and À4.9ppm, implying the 0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2004.10.119 Keywords: (Diphosphine)boronium salts; P-chiral diphosphine; Cate- cholborane; Rh-catalyzed asymmetric hydrogenation. * Corresponding author. Tel./fax: +81 43 290 2791; e-mail: imamoto@ faculty.chiba-u.jp   Present address: Faculty of Pharmaceutical Science at Kagawa Campus, Tokushima Bunri University, Shido, Sanuki, Kagawa 769- 2193, Japan. P P t-Bu Me t-Bu Me BH O O HB O O P P t-Bu Me t-Bu Me H 2 B + B O O O O – 1 2 Figure 1. Two possible products obtained in the reaction of cate- cholborane and t-Bu-BisP*. Tetrahedron Letters 45 (2004) 9341–9344 Tetrahedron Letters