Cyclopentene halohydrins

Cleavage of epoxides. As with simpler haloboranes, these reagents cleave epoxides of cycloalkenes to give, after nonoxidative workup, halohydrins in 65-90% yield. When carried out at -78 to -100°, the cleavage can show high en-antioselectivity. Thus the halodiisopinocampheylboranes derived from (+ )-pinene react with the oxide of cyclohexene or of cyclopentene to furnish (1R,2R) halo-... 

Stereochemistry of Halohydrin Formation Because the mechanism involves a halonium ion, the stereochemistry of addition is anti, as in halogenation. For example, the addition of bromine water to cyclopentene gives fran.v-2-bromocyclopentanol, the product of anti addition across the double bond. 

Halohydrins are easily generated by treating alkenes with aqueous solutions of halogens. Bromine water and chlorine water add across double bonds with Markovnikov orientation (Section 8-11). The following reaction shows cyclopentene reacting with chlorine water to give the chlorohydrin. Treatment of the chlorohydrin with aqueous sodium hydroxide gives the epoxide. 

Vicinal halo alcohols (halohydrins) can be S5mthesi2ed by treating epoxides with HX. (a) Show how you would use this method to synthesize 2-chlorocydopentanol from cyclopentene. (b) Would you expect the product to be OT-2-chlorocyclopentanol or trans-2-chlorocyclopentanol that is, would you expect a net syn addition or a net anti addition of —Cl and —OH Explain. 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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