 If the ylide carbon is substituted with an aryl or allyl group , the reagent is referred to as a semi @-@ stabilized ylide . These have been developed extensively , second only to the classical methylene reagents ( R1 = R2 = H ) . The substitution pattern on aryl reagents can heavily influence the selectivity of the reaction as per the criteria above . 
 If the ylide carbon is substituted with an aryl or allyl group , the reagent is referred to as a semi @-@ stabilized ylide . These <a href='been_done' title='1'><mark>have</mark></a> been developed extensively , second only to the classical methylene reagents ( R1 = R2 = H ) . The substitution pattern on aryl reagents can heavily influence the selectivity of the reaction as per the criteria above . 
If da ylide carbon is substituted wit an aryl or allyl group , tdareagent is referred to as a semi @-@ stabilized ylide . dem done been developed extensively , second only to the classical methylene reagents ( R1 = R2 = H ) . Thdaubstitution pattern on aryl reagents can heavily influence thedalectivity of the daction as per the cdaeria above .