MOR分子筛12元环孔道对α-丙氨酸手性转变反应的限域影响

The Confined Effect of the MOR Zeolite 12-membered Ring Channel on theα-Ala Chiral Transition Reaction

采用量子力学与分子力学相结合的ONIOM方法,研究了α-丙氨酸限域在MOR分子筛12元环孔道内的手性转变.反应通道研究发现:手性转变反应有a,b和c 3个通道.a通道上,手性C上的H以氨基N作为迁移桥梁;b通道上,手性C上的H先后以羰基O和氨基N作为迁移桥梁;c通道上,先是在羧基内实现H迁移,而后手性C上的H再以羰基O为桥梁迁移,进而实现手性转变.反应势能面计算发现:相对于孤立环境,α-Ala限域在MOR分子筛12元环孔道,在各通道的手性转变能垒被不同程度地降低.在c通道,羧基内H迁移和手性C上的H向羰基迁移的能垒分别为124.4和298.2 kJ·mol-1,比单体此过程的能垒195.1和316.5kJ·mol-1明显降低.结果表明:MOR分子筛12元环孔道对α-Ala的手性转变反应具有催化作用,对羧基内H迁移反应的限域催化作用明显.

The chiral transition ofα-Ala confined in the MOR zeolite 12-membered ring channel is studied by using the combined method of quantum mechanics and molecular mechanics.The study of reaction channel showed that there were three channels a,b and c in the chiral transition reaction.In the channel a,the amino N was uesed as a transfer bridge of the hydrogen in the chiral carbon.In the channel b,the carbonyl O and amino N were successively used as a transfer bridge of the hydrogen in the chiral carbon.In the channel c,the hydrogen migration was firstly achieved,secondly the carbonyl O was used as a transfer bridge of the hydrogen on the chiral carbon,and then the chiral transition was realized.Through calculating potential energy surface of the reaction we find whenα-Ala confinded in the MOR zeolite 12-membered ring channel,compared to the isolated environment the energy barrier of chiral transition in each channel can be reduced in different degree.In the channel c,the energy barriers of hydrogen migration in carbonyl and hydrogen migration in the chiral carbon to carbonyl were respectively 124.4and 298.2kJ·mol-1 which were significantly lower than that of the single body 195.1and 316.5kJ·mol-1.The results show that the MOR zeolite 12-membered ring channel has a catalytic effect on the chiral transition ofα-Ala,and the catalytic effect is obvious for the hydrogen migration reaction in the carboxyl.