Neutral Homoaromaticity in Some Heterocyclic Systems

摘要

Neutral homoaromaticity has been evaluated in heterocyclic systems related to the bicyclo[3.2.1]-octane skeleton with replacement of CH_2 at C-2 in bicyclo[3.2.1]octa-3,6-diene with X - BH.A1H,Be,Mg,O,S,PH,NH (12);replacement of CH at C-3 in bicyclo[3.2.1]octa-3,6-dien-2-yl anion with PH,S,NH,0 (13);and replacement at C-2 and C-3 with N and 0 (14).Stabilization energies (SE) are evaluated using density functional theory and homodesmotic equations at the B3LYP/6-311+G-(3df,2p)//B3LYP/6-31G(d) level for series 12-14.Stabilization energies are compared with diamagnetic susceptibility exaltations,A,CSGT-B3LYP/6-3lG(d)//B3LYP/6-31G(d),and nucleus-independent chemical shifts (NICS),GIAO-B3LYP/6-311+GC2d;p)//B3LYP/6-31G(d).Analysis of frontier orbitals and geometries,B3LYP/6-31G(d)//B3LYP/6-31G(d),and proton affinities of 2-azabicyclo[3.2.1]octa-3,6-diene,pyrrole,and related model systems,B3LYP/6-311+G(2d.2p)// B3LYP/6-3lG(d),provide complementary evidence supporting the division of the substrates evaluated into antihomoarornatic (12,X = BH,A1H,and Be),nonhomoaromatic (12,X = O,S,NH,PH),and homoaromatic (13,X = S,PH,NH,0 and 14 X = ON),with 12 (X = Mg) appearing as transitional between anti- and nonhomoaromatic.