[chemical substances]
   reactive intermediates
     Class dipolar compounds
       Class ylides [[RmX+-YRn [<-> RmX=YRn] (X=N,O,P,S,etc.; Y=C, etc.)] or [Z=X+-YRn <-> Z-+X=YRn] or [RmX+-Y <-> RmX=Y]]
         Class phosphorus ylides [RmP+-CR2 <-> RmP=CR2]
           Class phosphonium ylides [R3P+-CR2 <-> R3P=CR2]