Reaction of benzylic α-hydroxythioamides with thionyl chloride

9-Hydroxy-9-(N,N-dimethylthiocarbamyl)fluorene,4, reacts very rapidly with 1 equiv of thionyl chloride to give the corresponding alpha-chlorothioamide derivative, 2. However reaction of 4 with 2 equiv of thionyl chloride gives the desulfurized product 9-chloro-9-(N,N-dimethylcarbamyl)fluorene 6. This unusual desulfurization reaction, which is neither completely general nor completely understood at the present time, occurs less readily than initial formation of the alpha-chlorothioamide. The a-chlorothioamide 2 is highly reactive in hydroxylic solvents. The substantially less reactive alpha-chloroamide 6 undergoes reaction in hydroxylic solvents via the intermediacy of the 9-(N,N-dimethylcarbamyl)-9-fluorenyl cation, 15, at rates comparable to those of the or-H analogue, 9-chlorofluorene, 14. Computational studies indicate that the cl-carbonyl cation 15 prefers a conformation in which the carbonyl group is rotated 90 degrees with respect to the fluorenyl system. The relatively rapid rate of solvolysis of 6 is attributed to relief of ground-state strain as the cation 15 forms, as well as delocalization involving the fluorene system. Carbonyl conjugation as a cation-stabilizing feature does not appear to operate in the alpha-carbonyl cation 15.