锌胺模型化物催化苯乙酮的烯丙基化反应机制

Study on the Mechanism of Allylation of Acetophenone Catalyzed by Zinc Amide Model Compound

在M05-2X(SMD,pentane)/6-311++G**水平下,研究锌胺模型化物Zn(HHDS)2催化苯乙酮与烯丙基硼酸酯的烯丙基化反应机制。计算结果表明,无催化剂存在时,底物硼酸酯的B中心作为Lewis酸活化苯乙酮。反应按协同机制进行,需要的能垒高达119.7 k J·mol-1。在Zn(HHDS)2催化作用下,烯丙基硼酸酯通过B与Zn的金属置换,生成具有较强亲核性的烯丙基锌胺物种,实现苯乙酮的烯丙基化。B与Zn的金属交换为速率控制步骤,需要的能垒为63.2 k J·mol-1。计算发现形成烯丙基锌胺物种比二烯丙基锌物种在动力学上更占优势。通过B-N键断裂实现Zn(HHDS)2的再生在能量上比B与Zn的金属交换形成烯丙基锌胺活性物种低33.9k J·mol-1,Zn(HHDS)2为反应中起实际催化作用的活性物种。

The reaction mechanism of allylation of acetophenone catalyzed by zinc amide model compound is studied at the M05-2X( SMD,pentane) /6- 311 + + G** level. The calculation result indicates that the non- catalyzed reaction proceeds concertedly via a six- membered- ring transition state,which requires a high energy barrier of 119. 7 k J · mol- 1. In the presence of Zn( HHDS)2,the reaction takes place via the stepwise mechanism,which involves the B to Zn transmetalation,allylation of acetophenone and the regeneration of Zn( HHDS)2. The B to Zn transmetalation with an energy barrier of 63. 2 k J·mol- 1is rate- determining step.The calculation result shows that the formation of the allyl zinc amide species is kinetically more favorable than the generation of diallylzinc species. The recovery of Zn( HHDS)2via the cleavage of the B- N bond is 33. 9 k J·mol- 1lower than the regeneration of the allyl zinc amide species via B to Zn transmetalation in free energy,and Zn( HHDS)2is the actual active species in the present system.