Tandem Pseudopericyclic Reactions: [1,5]-X Sigmatropic Shift/6pi-Electrocyclic Ring Closure Converting N-(2-X-Carbonyl)phenyl Ketenimines into 2-X-Quinolin-4(3H)-ones

摘要

N-(2-X-Carbonyl)phenyl ketenimines undergo,under mild thermal conditions,[1,5]-migration of the X group from the carbonyl carbon to the electron-deficient central carbon atom of the ketenimine fragment,followed by a 6pi-electrocyclic ring closure of the resulting ketene to provide 2-X-substituted quinolin-4(3H)-ones in a sequential one-pot manner.The X groups tested are electron-donor groups,such as alkylthio,arylthio,arylseleno,aryloxy,and amino.When involving alkylthio,arylthio,and arylseleno groups,the complete transformation takes place in refluxing toluene,whereas for aryloxy and amino groups the starting ketenimines must be heated at 230 deg C in a sealed tube in the absence of solvent.The mechanism for the conversion of these ketenimines into quinolin-4(3H)-ones has been studied by ab initio and DFT calculations,using as model compounds N-(2-X-carbonyl)vinyl ketenimines bearing different X groups (X = F,Cl,OH,SH,NH_2,and PH_2) converting into 4(3H)-pyridones.This computational study afforded two general reaction pathways for the first step of the sequence,the [1,5]-X shift,depending on the nature of X.When X is F,Cl,OH,or SH,the migration occurs in a concerted mode,whereas when X is NH_2 or PH_2,it involves a two-step sequence.The order of migratory aptitudes of the X substituents at the acyl group is predicted to be PH_2 > Cl > SH > NH_2 > F> OH.The second step of the full transformation,the 6pi-electrocyclic ring closure,is calculated to be concerted and with low energy barriers in all the cases.We have included in the calculations an alternative mode of cyclization of the N-(2-X-carbonyl)vinyl ketenimines,the 6pi-electrocyclic ring closure leading to 1,3-oxazines that involves its 1-oxo-5-aza-1,3,5-hexatrienic system.Additionally,the pseudopericyclic topology of the transition states for some of the [1,5]-X migrations (X = F,Cl,OH,SH),for the 6pi-electrocyclization of the ketene intermediates to the 4(3H)-pyridones,and for the 6pi-electrocyclization of the starting ketenimines into 1,3-oxazines could be established on the basis of their geometries,natural bond orbital analyses,and magnetic properties.The calculations predict that the 4(3H)-pyridones are the thermodynamically controlled products and that the 1,3-oxazines should be the kinetically controlled ones.