【正文】 HH++ O HHC COOHHH O HH+? If the epoxide is unsymmetrical, the nucleophile attacks primarily at the more substituted carbon atom in acidcatalyzed ring opening. C C H 2OC H 3H 3 CC H 3 O H + H A C C H 2 O HC H 3H 3 CO C H 3? Bonding in the protonated epoxide is unsymmetrical, which the more highly substituted carbon atom bearing a considerable positive charge。 the reaction is SN1 like. C C H 2OC H 3H 3 CC H 3 O H + C C H 2 O HC H 3H 3 CO C H 3H? +? +T h i s c a r b o n r e s e m b l e s a 3o c a r b o c a t i o nH+P r o t o n a t e d e p o x i d e馬式加成 ? BaseCatalyzed Ring Opening of an Epoxide ? If the epoxide is unsymmetrical, the nucleophile attacks primarily at the less substituted carbon atom in basecatalyzed ring opening. C COR O ? C CR O O ? C CR O O H++H O R S t r o n g n u c l e o p h i l e E p o x i d e A n a l k o x i d e i o n R O ?H 2 C C H C H 3OH 3 C H 2 C O ?C H 2 C HC H 3H 3 C H 2 C O O ?C H 2 C HC H 3H 3 C H 2 C O O H++H O C H 2 C H 3M e t h y l o x i r a n e H3 C H 2 C O ?1 E t h o x y 2 p r o p a n o l1o C a r b o n a t o m i s l e s s h i n d e r e d反馬式加成 Anti Hydroxylation of Alkenes via Epoxides ? Epoxidation of cyclopentene ? Acidcatalyzed hydrolysis of 1,2epoxycyclopentane HHH HOC y c l o p e n t e n e 1 , 2 E p o x y c y c l o p e n t a n e+R OCOOH+R OCOHH HO++HOHHH HOH+HOH+ H OH O Ht r a n s 1 , 2 C y c l o p e n t a n e d i o l+HH OH O HHHHOHe n a n t i o m e r? Epoxidation followed by acidcatalyzed hydrolysis constitutes a method for anti hydroxylation of a double bond. CC H3H3CCH HOc i s 2 , 3 D i m e t h y l o x i r a n eCC H3H3CCH HOHH AHOH+H2OCC H3H3CCHHO HOHH+CH3CC H3CHHH OOHH+CC H3H3CCHHO HH OCH3CC H3CHHH OO HA?A?( 2 R , 3 R ) 2 , 3 B u t a n e d i o l( 2 S , 3 S ) 2 , 3 B u t a n e d i o l( a )( b )CC H 3HCH 3 C HO? PhaseTransfer Catalysis(PTC) ? SN2 reactions take place much more rapidly in polar aprotic solvents (DMSO and DMF) . ? In some ways the ideal solvent for an SN2 reaction would be a nonpolar aprotic solvent such as a hydrocarbon or a relatively nonpolar chlorinated hydrocarbon. ? low boiling points ? they are inexpensive ? they are relatively stable ? Phasetransfer catalysts are used with two immiscible phases in contact ––– often an aqueous phase containing an ionic reactant and an anic phase (benzene, CHCl3, etc.) containing the anic substrate. ? The phasetransfer catalyst (Q+X–) is usually a quaternary ammonium halide (R4N+X–) such as tetrabutylammonium halide Bu4N+X–. ? The phasetransfer catalyst causes the transfer of the nucleophile (. CN–) as an ion pair [Q+CN–] into the anic phase. Q+ : lipophilic ? An example of phasetransfer catalysis: C H 3 ( C H 2 ) 6 C H 2 C l ( i n d e c a n e ) C H 3 ( C H 2 ) 6 C H 2 C Na q u e o u s N a C N , 1 0 5 o CR 4 N+ B r ?? Many other types of reactions using PTC: Oxidation of alkenes ( i n b e n z e n e ) a q u e o u s K M n O 4 , 3 5 o C( 9 9 % )C H 3 ( C H 2 ) 5 C H C H 2R 4 N + B r ? C H3 ( C H 2 ) 5 C O 2 H + H C O 2 HAnother important PTC: Crown ethers They can transport ionic pounds into an anic phase high efficiency. Crown Ethers: ? Crown ethers are cyclic polymers of ethylene glycol OOOOOOK +1 8 C r o w n 6OOOOOOK +? named as xcrowny ? X=the total number of atoms in the ring ? Y=the number of oxygen atoms. hostguest plexation Hostguest chemistry Supramolecular chmistry: “ molecular recognition‖ ? When crown ethers coordinate with a metal cation, they thereby convert the metal ion into a species with a hydrocarbonlike exterior. OOOOON a +1 5 C r o w n 5OOOOON a? Crown ethers render many salts soluble in nonpolar solvents. +d i c y c l o h e x a n o 1 8 c r o w n 6b e n z e n e( 9 0 % )K M n O 4H O 2 CO? P532 ? (b)(d)(e)(f) ? (c)(d) ? (a)(d) ? ? (a)(d) ? ? (a)(d