Oh Keunchan, Jeong Kyu-Sung, Moore Jeffrey S
Departments of Chemistry and Material Science & Engineering, University of Illinois, Urbana, Illinois 61801, USA.
J Org Chem. 2003 Oct 31;68(22):8397-403. doi: 10.1021/jo035007o.
Imine metathesis between m-phenylene ethynylene oligomers of various lengths was performed in acetonitrile, a solvent in which oligomers containing eight or more repeat units adopt a compact helical conformation. The equilibrium constants and corresponding free energy change for the imine metathesis reactions were estimated. The results showed that the magnitude of equilibrium shifting measured by the free energy change for the formation of imine-containing oligomers increases linearly below a critical product chain length and grows asymptotically above it. The linear region is ascribed to the constant increase in contact area between monomer units of adjacent helical turns as the product chain grows to the 12-mer. Once the ligation product is 12 units in length, full contact is made between adjacent helical turns. On the other hand, for imine metathesis between oligomers leading to products having more than 12 units, the driving force is the difference between the folding energy of products and that of reactants. The additional stabilizing energy is roughly constant, regardless of the chain length, since the contact area between adjacent helical turns is unchanged. Consistent with the notion that the imine bond only minimally destabilizes the helical conformation, the position of the imine bond in the ligation product has been observed to have no significant effect on the folding stability. The magnitudes of equilibrium shifting are similar for ligation products of the same length but having the imine at various positions along the sequence. This suggests that the imine bond is compatible with the m-phenylene ethynylene backbone, regardless of the position in the sequence. Imine metathesis of m-phenylene ethynylene oligomers could allow a quick access to an unbiased, dynamic library of oligomer sequences joined by imine linkages.
在乙腈中进行了不同长度的间亚苯基乙炔低聚物之间的亚胺复分解反应，在这种溶剂中，含有八个或更多重复单元的低聚物会呈现紧密的螺旋构象。估算了亚胺复分解反应的平衡常数和相应的自由能变化。结果表明，通过形成含亚胺低聚物的自由能变化来衡量的平衡移动幅度，在临界产物链长度以下呈线性增加，而在其之上则渐近增长。线性区域归因于随着产物链增长至12聚体，相邻螺旋圈的单体单元之间的接触面积不断增加。一旦连接产物长度达到12个单元，相邻螺旋圈之间就会实现完全接触。另一方面，对于导致产物具有超过12个单元的低聚物之间的亚胺复分解反应，驱动力是产物和反应物的折叠能之差。由于相邻螺旋圈之间的接触面积不变，额外的稳定能大致恒定，与亚胺键仅使螺旋构象略微不稳定的观点一致，已观察到连接产物中亚胺键的位置对折叠稳定性没有显著影响。对于相同长度但亚胺在序列中不同位置的连接产物，平衡移动幅度相似。这表明亚胺键与间亚苯基乙炔主链兼容，与序列中的位置无关。间亚苯基乙炔低聚物的亚胺复分解反应可以快速构建一个由亚胺键连接的无偏差、动态的低聚物序列库。
