The Application Of Aromaticity In Small Molecules Activation By Frustrated Lewis Pairs

Frustrated Lewis pairs(FLP)is a common strategy in the field of metal-free tansformations.Due to the sterically congested Lewis acid and Lewis base pair,the unquenched cooperativity in frustrated Lewis pairs can promote small molecules activaition(H2,CO2,NO,CS2,etc).Compared with acyclic systems,electron delocalization in aromatic species results in high thermodynamic stability which has a tendency to retain the structural type in the course of chemical transformations.Aromaticity is an important concept in organic chemistry which can be utilized to evaluate the degree of electron delocalization in the closed systems.Aromaticity,as a chemical concept,is not a directly measurable quantity.Aromatic species usually have unique electronic properties(equalized bond length,magnetic properties,enhanced thermodynamic stability).The application of the aromaticity concept to the transition state can facilitate the understanding of reaction mechanisms.However,combining these two fundamental concepts together in small molecules activation is less developed.Herein,we investigated the effect of aromaticity in FLPs in activating CO2、CS2 and H2 by density functional theory calculations.The specific contents are summarized as follows:1.A study of carbon dioxide activation by a series of hetero-cyclopentadienes connected P/N-based FLPs was discussed,addressing the importance of aromaticity in the reactivities of FLPs.The results show that when Lewis acid and Lewis base substitutes with unsaturated bonds of hetero-cyclopentadienes,the FLPs which using aromatic furan and pyrrole as linker ring show high reactivity.Whereas the anti-aromatic heterocycles(borole and aluminacyclopentadiene)bridged FLPs lead to relative higher activation energies for CO2 capture.It can be attributed to the gradual gain of(anti)aromaticity in the process of activation.The FLPs,Lewis acid and Lewis base replaced by the symmetry b-C in heterocycles,show relatively higher activation barriers towards CO2 capture mainly caused by the additional energy in breaking the weaker interaction.2.Based on the capture of CO2 by P/N-based FLPs,a follow-up study focuses on the influence of aromacity of connecting ring in CS2 fixation.According to the results,strong correlation between activation barriers and NICS(1)zz values of FLPs,NICS(1)zz values of the corresponding transition states and ΔNICS(1)zz values(the diffenerce between FLPs and transition states)is achieved,indicating the gain of aromacity promotes CS2 capture.Compared with CO2 activation,the reaction of CS2 activation shows relative higher activation barrier as reported in most literature.The higher electronegativity of oxygen atom and positively charged carbon atom in CO2 allow a good charge alternation in transition state,while the reversed polarity of C=S bond in CS2 makes activation reaction more difficult.3.The anti-aromatic borole(as Lewis acid)and aromatic pyridine(as Lewis base)were introduced into cyclooctetraene skeleton.Due to the geometric constraints,such systems can be classified as FLPs.In the process of hydrogenation,breaking anti-aromatic boron-containing ring can decrease the reaction barrier,whereas breaking aromatic of B-containing heterocycle makes hydrogenation reaction both kinetically and thermodynamically unfavorable.Further investigation reveals that more rigid anti-aromatic B-containing ring results in smaller deformation degree of FLP moiety in comparison with non-aromatic ring,which is one factors for its lower energy barrier.