Abstract: The reactions of :CFBr with substituted benzaldehydes resulted in deoxygenation products with varying yields of COproduction which appeared to associate with the electronic effects of the para-substituents. The proposal that this type of carbene reactions proceed via formation of a carbonyl Ylide intermediate was confirmed by a successful trapping experiment using a planar Ylide trapper, dimethylacetylenedicarboxylate(DMAD). On the other hand, no carbonyl Ylide was trapped in a similar reaction of fluorocarbenes(:CFBr or :CFCl) with sterically hindered tetraphenylcyclopentadienone(TPCP). This observation, together with the fact of an unexpected high yield of COemission, suggests that the carbonyl Ylide intermediate must have taken a geometry that is far from the so-called 0°, 0° conformation(planar) and therefore largely blocked the involvement of other competitive pathways.

Key words: Chloro-substituted Ylide, Carbony Ylide, Deoxogenation