Arene Substitution Design for Controlled Conformational Changes of Dibenzocycloocta-1,5-dienes

We
report that an agile eight-membered cycloalkane can be stabilized
by fusing a benzene ring on each side, substituted with proper functional
groups. The conformational change of dibenzocycloocta-1,5-diene (DBCOD),
a rigid–flexible–rigid organic moiety, from its Boat
to Chair conformation requires an activation energy of 42 kJ/mol,
which is substantially lower than those of existing submolecular shape-changing
units. Experimental data corroborated by theoretical calculations
demonstrate that intramolecular hydrogen bonding can stabilize Boat,
whereas electron repulsive interaction from opposing ester substituents
favors Chair. Intramolecular hydrogen bonding formed by 1,10-diamide
substitution stabilizes Boat, spiking the temperature at which Boat
and Chair can readily interchange from −60 to 60 °C. Concomitantly
this intramolecular attraction raises the energy barrier from 42 kJ/mol
for unsubstituted DBCOD to 68 kJ/mol for diamide-substituted DBCOD.
Remarkably, this value falls within the range of the activation energy
of highly efficient enzyme-catalyzed biological reactions. With shape
changes once considered only possible with high energy, our work reveals
a potential pathway exemplified by a specific submolecular structure
to achieve low-energy-driven shape changes for the first time. The
intrinsic cycle stability and high-energy output systems that would
incur damage under high-energy stimuli could particularly benefit
from this new kind of low-energy-driven shape-changing mechanism.
This work has laid the basis to construct systems for low-energy-driven
stimuli-responsive applications, hitherto a challenge to overcome.

本研究报道，一种可灵活构象变化的八元环烷烃，可通过在其两侧各稠合一个带有合适官能团取代的苯环来实现稳定。作为刚-柔-刚型有机结构单元，二苯并环辛-1,5-二烯（dibenzocycloocta-1,5-diene，DBCOD）从船式（Boat）构象转变为椅式（Chair）构象的活化能为42 kJ/mol，远低于已报道的亚分子级形状变换单元的活化能。结合理论计算验证的实验数据表明，分子内氢键可稳定船式构象，而对位酯基取代物产生的电子排斥作用则更有利于椅式构象的稳定。通过1,10-二酰胺取代形成的分子内氢键可稳定船式构象，使船式与椅式构象间的快速互变温度从-60℃提升至60℃。与此同时，这种分子内相互作用将未取代DBCOD的能垒从42 kJ/mol提升至二酰胺取代DBCOD的68 kJ/mol。值得注意的是，该能垒数值处于高效酶促生物反应的活化能区间内。此前人们认为构象变化仅能通过高能量驱动实现，本研究首次通过特定亚分子结构范例，揭示了一条可实现低能量驱动构象变化的潜在路径。对于那些在高能量刺激下易发生损伤的本征循环稳定型高能量输出体系而言，这种新型低能量驱动的形状变换机制尤为适用。本研究为构建低能量驱动的刺激响应应用体系奠定了基础，而此类体系在此前一直是难以攻克的挑战。