Vaulted Biaryls: Efficient Ligands for the Aluminum-Catalyzed Asymmetric Baeyer-Villiger Reaction

Abstract

Vaulted biaryls (VANOL and VAPOL) have been applied in the aluminum-catalyzed asymmetric Baeyer-Villiger ­reaction of prochiral 3-substituted cyclobutanones. Optically active γ-butyrolactones are obtained in high yields with enantioselectivities of up to 84% ee.

aluminium - asymmetric catalysis - Baeyer-Villiger ­reaction - biaryls - lactones

References

1a Krow GR. Org. React.  1993,  43:  251 

1b Krow GR. In Comprehensive Organic Chemistry   Vol. 7:  Trost BM. Pergamon Press; Oxford: 1991.  p.671 

1c Renz M. Meunier B. Eur. J. Org. Chem.  1999,  737 

2a Bolm C. Beckmann O. Luong TKK. In Transition Metals for Organic Synthesis   Vol. 2:  Beller M. Bolm C. Wiley-VCH; Weinheim: 1998.  p.213 

2b Bolm C. In Advances in Catalytic Processes   Vol. 2:  Doyle PM. JAI Press; Greenwich: 1997.  p.43 

2c Strukul G. Angew. Chem. Int. Ed.  1998,  37:  1199 

Reviews:

3a Bolm C. Luong TKK. Beckmann O. In Asymmetric Oxidation Reactions   Katsuki T. Oxford University Press; New York: 2001.  p.147 

3b Bolm C. In Peroxide Chemistry   Adam W. Wiley-VCH; Weinheim: 2000.  p.494 

3c Bolm C. Beckmann O. In Comprehensive Asymmetric Catalysis   Vol. 2:  Jacobsen EN. Pfaltz A. Yamamoto H. Springer-Verlag; Stuttgart: 1999.  p.803 

3d Bolm C. Med. Res. Rev.  1999,  19:  348 

4a Bolm C. Beckmann O. Palazzi C. Can. J. Chem.  2001,  79:  1593 

4b Bolm C. Beckmann O. Kühn T. Palazzi C. Adam W. Rao PB. Saha-Möller CR. Tetrahedron: Asymmetry  2001,  12:  2441 

5 Chen Y. Yekta S. Yudin AK. Chem. Rev.  2003,  103:  3155 

6 Bao J. Wulff WD. Rheingold J. J. Am. Chem. Soc.  1993,  115:  3814 

7 Zhang Y. Yeung S.-M. Wu H. Heller DP. Wu C. Wulff WD. Org. Lett.  2003,  5:  1813 

8a Loncaric C. Wulff WD. Org. Lett.  2001,  3:  3675 

8b Antilla JC. Wulff WD. Angew. Chem. Int. Ed.  2000,  39:  4518 

8c Antilla JC. Wulff WD. J. Am. Chem. Soc.  1999,  121:  5099 

9 Xue S. Yu S. Deng Y.-H. Wulff WD. Angew. Chem. Int. Ed.  2001,  40:  2271 

10

After 72 h at -40 °C the reaction was quenched to give the lactone with 84% ee in 40% yield.

11

Experimental Procedure for the Al-Catalyzed Asymmetric Baeyer-Villiger Reaction.
To a solution of (R)-VANOL (26.4 mg, 0.06 mmol) in toluene (2.5 mL) was added dropwise Me₂AlCl (0.06 mmol, 60 µL, 1 M solution in hexanes) at r.t. The mixture was stirred for 30 min and then cooled at -30 °C. After 15 min at this temperature, a solution of the ketone (0.3 mmol) in toluene (0.5 mL) was added. Stirring was continued for 15 min, cumene hydroperoxide (technical grade 80%, 1.2 equiv, 75 µL) was then added in one portion. Subsequently, the reaction mixture was stirred at -30 °C until full conversion of the ketone (as monitored by GC). The solvent was then evaporated, and the crude product absorbed onto silica gel. Extraction with Et₂O afforded the crude product, which was analyzed by GC or HPLC using chiral columns (for details of the enantiomeric ratio determinations and assignments of absolute configurations, see ref. 4 and references therein).

12

Slightly better results can be obtained for benzyl-substituted substrates when 50 mol% of the BINOL/Me₂AlCl catalyst system are applied (comp. ref. 4a). Thus, conversions of 6c and 6d give lactones with 73% and 58% ee, respectively.

For other recent examples of metal-catalyzed asymmetric Baeyer-Villiger reactions focusing on the conversions of cyclobutanones, see:

13a Uchida T. Katsuki T. Ito K. Akashi S. Ishii A. Kuroda T. Helv. Chim. Acta  2002,  85:  3078 

13b Ito K. Ishii A. Kuroda T. Katsuki T. Synlett  2003,  643 ; and references therein

14 To the best of our knowledge there is only a single report with a higher ee. Katsuki et al. found a zirconium catalyst bearing a complex salen ligand to give 87% ee in the conversion of 4 to 5. See: Watanabe A. Uchida T. Ito K. Katsuki T. Tetrahedron Lett.  2002,  43:  4481