ASYMMETRIC HALOGENATION AND HYDROHALOGENATION OF TRANS-2-BUTENOIC ACID IN A CRYSTALLINE ALPHA-CYCLODEXTRIN COMPLEX

trans-2-Butenoic acid was asymmetrically hydrohalogenated and halogenated in a crystalline a-cyclodextrin complex. Exposure to gaseous hydrogen bromide at 20 °C and to hydrogen chloride at 0 °C gave (S)-(+)-3-bromobutanoic acid in 58% and (S)-(-)-3-chlorobutanoic acid in 64% enantiomeric excesses, respectively. At 45–50 °C, the guest in the cavity of the cyclodextrin reacted with gaseous bromine or chlorine to produce erythro-dihalides with extremely low optical activities; no products were obtained on treatment with bromine for 50 h at the lower temperature of 20 °C. The crystal structure of the complex was determined to be: C36H60O30-C4H6O2.5H2O; FW = 1149.0; orthorhombic; space group, P212121; Z = 4; a = 14.406 (5), b = 38.174 (12), and c = 9.430 (3) A; V = 5185.9 A3; Dx= 1.472, Dm= 1.475 g/cm2. A mechanism for the observed chiral induction in the present gas-solid reaction is discussed in terms of the crystal structure of the complex. © 1990, American Chemical Society. All rights reserved.