To clarify the reaction mechanism, some controlled experiments
were carried out under the optimized conditions as
follows: (1) By adding a radical scavenger (ascorbic acid) to the
reaction,12 this reaction was inhibited, and the rate of reactions
was greatly suppressed. Only a trace amount of the desired
product was detected (Scheme 1a). (2) Benzaldehyde was
obtained in 7% yield from the oxidization of toluene under the
standard reaction condition in the absence of palladium catalyst
and azobenzene (Scheme 1b). 3) With the use of benzaldehyde
as an acylation reagent instead of toluene under the standard
reaction conditions, the desired product 3a was obtained in
45% yield, and benzoic acid was detected (Scheme 1c). Part of
benzaldehyde 2b was oxidated into benzoic acid, whereas
toluene gave higher yield, and an unwanted side reaction was
prevented under the same reaction conditions. Moreover,
toluene and its derivatives are less toxic and more easily
available compared to benzaldehyde. In view of the above
merits, toluene was used as an ideal and efficient acylation
reagent. The results obtained demonstrated that a radical
process was involved in the catalytic system and that the
azobenzene reacted with the benzoyl radical rather than the
benzyl radical