 The vast majority of reagents are monosubstituted at the ylide carbon ( either R1 or R2 as hydrogen ) . Disubstituted reagents are much rarer but have been described : 
 The vast majority of reagents are monosubstituted at the ylide carbon ( either R1 or R2 as hydrogen ) . Disubstituted reagents <a href='drop_aux' title='1'><mark>are</mark></a>much rarer but <a href='been_done' title='2'><mark>have</mark></a> been described : 
The vast majority of reagents r monosubstituted at the ylide carbon ( either R1 or R2 as hydrogen ) . Disubstituted reagents much rarer but done been described :