Optically Active 1,1′-Di-tert-butyl-2,2′-diphosphetanyl and Its Application in Rhodium-Catalyzed Asymmetric Hydrogenations

Abstract

(1S,1′S,2R,2′R)-1,1′-Di-tert-butyl-2,2′-diphosphetanyl was prepared from tert-butylphosphine via phosphine-boranes as intermediates. The rhodium complex of the ligand was used as a highly efficient catalyst in asymmetric hydrogenations of α-acetyl­aminoacrylates and α-substituted enamides.

Key words

chiral phosphine ligand - chiral phosphetane derivative - rhodium complex - asymmetric hydrogenation - phosphine-borane

References

• For representative reviews and accounts, see:
• 1a Tang W. Zhang X. Chem. Rev.  2003,  103:  3029 
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• 9a

  Various optically active phosphetane ligands have been reported. However, to our knowledge, P-stereogenic phosphetane ligands have been rarely synthesized

• 9b See: Marinetti A. Jus S. Genet J.-P. Tetrahedron Lett  1999,  40:  8365 
• 9c See also: Berens U. Burk MJ. Gerlach A. Hems W. Angew. Chem. Int. Ed.  2000,  39:  1981 
• 9d See also: Marinetti A. Carmichael D. Chem. Rev.  2002,  102:  201 ; and references cited therein
• 10a

  DiSquareP* closely resembles TangPhos (optically active 1,1′-di-tert-butyl-2,2′-diphosphoranyl) which has proved to be an outstanding phosphine ligand in transition metal-catalyzed asymmetric reactions

• 10b See: Tang W. Zhang X. Angew. Chem. Int. Ed.  2002,  41:  1612 
• 10c See also: Tang W. Zhang X. Org. Lett.  2002,  4:  4159 
• 10d See also: Tang W. Zhang X. Org. Lett.  2003,  5:  205 
• 11a

  We independently attempted to prepare optically active 1,1′-di-tert-butyl-2,2′-diphosphoranyl using 1-tert-butyl-phospholane-borane. However, the deprotonation of 1-tert-butylphospholane-borane with sec-BuLi/(-)-sparteine complex proceeded with very low enantioselectivity and only (1S,1′R,2R,2′S)-1,1′-di-tert-butyl-2,2′-diphosphoranyl-borane was isolated as an oxidatively coupled product.

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• 16a

  This type of reverse stereoselectivity was previously observed when DuPhos, BisP*, and MiniPHOS were used as the ligand

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• 16d

  See also: Ref. 6.

15

The purification of the C ₂-compound to obtain enantiomerically pure 8 by recrystallization was not easily accomplished. The major reason is that the two enantiomers [(1R,1′R,2R,2′R) and (1S,1′S,2S,2′S)] readily associated to form a racemic compound (mp 250-252 °C; HPLC analysis: Daicel Chiracel OD-H, hexane-i-PrOH (95:5), 0.5 mL/min, t = 13.6 min; Anal. Calcd for C₂₈H₅₆P₄S₄: 52.15, H, 8.75. Found: C, 51.83; H, 8.79) which was less soluble than enantiomerically pure 8.