Hydrohalogenation asymmetric

Asymmetric hydrohalogenation of fraws-2-butenoic acid has been achieved in a crystalline a-cyclodextrin complex using gaseous HBr at 20 °C and HC1 at 0 °C. The products were formed with 58% and 64% e.e., respectively, and were of (S )-configuration81. This contrasts with the low enantioselectivity of halogenation attempted in the same paper (vide supra). 

Hydrohalogenation of crotonic acid in a crystalline a-cyclodextrin complex gives ( + )-(S)-3-bromobutanoic acid with 58% enantiomeric excess146. Asymmetric reactions catalyzed by cyclodcxtrin have been attempted in solution but all of these reactions afforded the products in low optical yields. Exposure of crystalline cyclodextrin complexes to gaseous hydrogen bromide and hydrogen chloride at 20 °C gives better optical yields. 

Tranv-1,4- and 1,2-polybutadiene can be hydrohalogenated under mild conditions with gaseous HCl. The same is true of copolymers of butadiene with piperylene and also of isotactic transAA-piperylene. The addition of HCl to the asymmetric double bond is trans for polypiperylene and occurs in a stereoselective way, judging from the NMR spectra. 

Y. TANAKA, H. SAKURABA, Y. OKA, and H. NAKANISHI / Asymmetric Halogenation and Hydrohalogenation of Ethyl tpans-Cinnamate in Crystalline Cyclodextrin Complexes... 

ASYMMETRIC HALOGENATION AND HYDROHALOGENATION OF tranS-CINNAMIC ACID IN CRYSTALLINE CYCLODEXTRIN COMPLEXES... 

Table III. Asymmetric Halogenation and Hydrohalogenation of trans-Cinnamic Acid in Crystalline Cyclodextrin Complexes...

Key words Asymmetric halogenation, hydrohalogenation, styrene, cyclodextrin complex. 

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