ASYMMETRIC SOLID-GAS HYDROHALOGENATION OF UNFUNCTIONALIZED OLEFINS VIA FORMATION OF CRYSTALLINE CYCLODEXTRIN COMPLEXES

Asymmetric solid-gas hydrohalogenation of styrene, beta-methylstyrene, allylbenzene, and 2-norbornene as unfunctionalized olefins was carrie d out by using their chiral crystalline alpha- and beta-cyclodextrin c omplexes by exposing them to gaseous HCl and HBr in the dark at room t emperature. The optical purities of the Markovnikov products obtained from the ionic addition of HCl to the included olefins appear consider ably higher than those from the reaction with HBr. The highest enantio selectivities of 58% and 62% enantiomeric excess tee) were obtained fo r the hydrochlorination of 3-phenyl-1-propene (allylbenzene) in the cr ystalline alpha- and beta-cyclodextrin complexes, respectively, and bo th reactions, which had little danger of racemization, gave (S)-(+)-2- chloro-1-phenylpropane as the same predominant product in moderate che mical yields. A much lower enantioselectivity (<10% ee) was observed i n the hydrobromination of the same olefin in the solid alpha- and beta -cyclodextrin complexes involving a racemization reaction. The enantio facial selection provided the (S)-enantiomer similarly during hydrochl orination.