Effect of the nO → π*CO Interaction on the Conformational Preference of 1,3-Diketones: A Case Study of Riolozatrione Derivatives

The cyclopropane ring-opening reaction of riolozatrione, a natural product obtained from Jatropha dioica, afforded a 2,2-disubstituted 1,3-cyclohexandione displaying an alkyl methyl ether group at position 5. The conformational analysis of this product showed a high preference for the trans-diaxial conformation in both solution and solid state. Such conformation was possible from the noncovalent intramolecular nX → π*CO interactions (X = an element having an unshared electron pair), allowing the determination of the interaction energies. Since the nX → π*CO interactions can be regarded as additive, the energy values ranged from 4.52 to 6.51 kcal mol–1 for each carbonyl group with a strong dependency on the interatomic distances. The rigorous analysis of the electron density in the topological theory of atoms in molecules framework clearly shows that the origin of O–CO interactions are through the nO → π*CO electron transfer mechanism. Such interactions are slightly weaker than a canonical hydrogen bond but seemingly stronger than a van der Waals interaction. This interaction must be considered as a stereoelectronic effect due the electronic transfer between the interacting groups, which are limited by their relative stereochemistry and can be represented by a bond–no bond interaction, causing the pyramidalization of the carbonyl, which is the charge acceptor group.

从迪氏麻风树（Jatropha dioica）中分离得到的天然产物里奥洛扎特酮（riolozatrione）经环丙烷开环反应（cyclopropane ring-opening reaction）后，得到了5位带有烷基甲醚基团的2,2-二取代1,3-环己二酮。对该产物的构象分析结果显示，其在溶液与固态两种状态下均强烈倾向于反式双直立键构象（trans-diaxial conformation）。该构象可通过非共价分子内nX → π*C=O相互作用（X代表带有未共享电子对的元素）得以稳定，借此可测定该类相互作用的能量值。由于nX → π*C=O相互作用可被视为可加和的，每个羰基的相互作用能取值范围为4.52至6.51 kcal·mol⁻¹，且与原子间距离存在极强的相关性。在分子中原子拓扑理论（topological theory of atoms in molecules）框架下对电子密度开展的严谨分析明确显示，O–C=O相互作用的起源为nO → π*C=O电子转移机制。此类相互作用的强度略弱于标准氢键（canonical hydrogen bond），但显著强于范德华相互作用（van der Waals interaction）。由于相互作用基团间的电子转移受限于二者的相对立体化学构型，该相互作用应被归为一类立体电子效应，可被表述为“键-非键”相互作用，其会导致作为电荷受体的羰基发生锥型化（pyramidalization）。