| Cyclopropene is highly reactive due to its high ring strain,and the Diels-Alder reactions of substituted cyclopropenes have been a long-standing research topic in bioorthogonal and organic chemistry.Our previous studies indicated that substituted 2-borabicyclo(1.1.0)but-1(3)-enes(2BB)are a kind of novel molecules and structurally analogous to cyclopropene.The theoretical analyses of the derivatives of 2BB confirmed the stability and likely existence of 2BB.It is interesting to note that the LUMOs of substituted 2BB molecules are similar to those of ethene and cyclopropene,therefore,it can be anticipated that substituted 2BB potentially provides rich opportunities in the rational design of novel molecules via Diels-Alder reactions.In this paper,we firstly investigated the structure,stability,aromaticity and bonding of substituted 2BB,then we extended the study of 2BB to the Diels-Alder reaction with butadiene and 5-substituted cyclopentadiene and explored the factors affecting Diels-Alder reactions.The research content includes the following three parts:1.The structure,stability,aromaticity,bonding of twenty-four substituted 2BB have been investigated by B3LYP-D3/6-311G**method.Calculated results show that the 2BB derivative contains a C=C bond whose lengths are in the range of 1.465-1.497A,and their WBIs are close to 1.33,indicating the inverted C=C bond are between single and double bond.Yet ab initio valence bond(VB)method computations of three derivatives of 2BB showed that the inverted bonds therein are charge-shifted and contains a conjugated π bond,the ionic resonance structures arising from charge-shifting are of carbene characteristics as well.Thus,the derivatives of 2BB can also be considered as B-heterocyclic carbenes(BHCs).The values of nucleus-independent chemical shifts indicating that there is significant aromatic stability in these three-membered rings.2.Since the LUMOs of the BHCs are similar to those of ethene and cyclopropene,here we investigated the Diels-Alder reactions of the BHCs with butadiene through B3LYP-D3/6-311G**method.The results show that the cyclization reaction is an exothermic reaction.The reaction mechanism indicates that the endo addition is more favorable than the exo addition,so the predicted product is 7-bora-oct-3-ene[4.1.1]propellane.Notably,there is a good correlation between the endo reaction barriers and the hyperconjugative aromatic stabilization enthalpies.3.The Diels-Alder reaction of 8 kinds of BHCs and 5 kinds of 5-substituted cyclopentadiene was futher studied by B3LYP/6-311G**method.The results have shown that 40 cycloaddition reactions are exothermic.The investigation of reaction mechanism has shown that the reaction have a specific endo-exo stereoselectivity,which tends to endo addition.Analysis shows that the secondary orbital interaction(SOI),steric hindrance are main factors that affect the stereoselectivity of the Diels-Alder reaction.In addition,the electrostatic interaction also affects the endo-exo stereoselectivity of the above Diels-Alder reaction.The syn-anti stereoselectivity of the reactions are determined by both steric hindrance and electrostatic interaction.Hyperconjugative aromatic stabilization enthalpies(ΔHASE)of 3-substituted 2-borabicyclo(1.1.0)but-1(3)-ene and 5-substituted cyclopentadiene has a certain correlation with the reaction energy barriers,indicating the hyperconjugative aromatic stabilization have a certain influence on the reactivity of the reactants.Therefore,SOI,steric hindrance,electrostatic interactions,and hyperconjugative aromatic stabilization energy(ΔHASE)are all factors that affect the Diels-Alder reaction.The BHCs have novel structures and well reactivity,it will be of great significance to field such as organic chemistry,and new materials. |
