Our initial plan was to realize Cp
*Co(Ⅲ)-catalyzed C(aryl)–H amidation of sulfoxonium ylides with dioxazolones as the amidating reagents. Much to our surprise, when we treated sulfoxonium ylide
1a (0.2 mmol) with dioxazolone
2a (0.3 mmol) in the presence of Cp
*Co(CO)I
2 (0.02 mmol), and AgSbF
6 (0.05 mmol) in DCE (2 mL) under N
2 at 130 ℃ for 18h, benzoxazinone derivative
3aa was obtained in 69% yield (
Table 1, entry 1). Encouraged by this result, we further studied other reaction conditions for the transformation. It was found that introduction of additives such as NaOAc, Zn(OAc)
2, KOAc and PivOH could not improve the yield of
3aa (entries 2–5). Subsequent screening of solvents suggested that DCE was the most efficient medium for the transformation, while other solvents such as 1, 4-dioxane, CH
3CN, and TFE were totally ineffective (entries 6–8). Other cobalt catalysts such as Co
2(CO)
8 and Co(acac)
3 were also examined and found to be ineffective for this reaction (entries 9 and 10). It was observed that the choice of Ag salt had an important effect on the reaction. When AgOTf was used in place of AgSbF
6,
3aa could be isolated in 43% yield (entry 11). In sharp contrast, no desired product was detected when AgOAc was employed (entry 12). The yield of
3aa was not improved under elevated (67%, 140 ℃) or lower (58%, 120 ℃) reaction temperatures (entries 13 and 14). It is worthwhile to note that when the reaction temperature was lowered to 70 ℃, only
ortho C–H amidation product of sulfoxonium ylide was obtained, and no cyclization product
3aa was detected. When the reaction was carried out under an air atmosphere, the yield of
3aa could be improved to 71% (entry 15). Control experiments revealed that both Cp
*Co(CO)I
2 and AgSbF
6 were necessary for the formation of
3aa (entries 17 and 18). It should be mentioned that when another amidating reagent 5-phenyl-1, 3, 2, 4-dioxathiazole 2-oxide was employed in place of
2a, no desired product was detected (not shown in
Table 1). Therefore, we decided to set reacting
1a with 1.5 equiv. of
2a in the presence of 10 mol% of Cp
*Co(CO)I
2 and 25 mol% of AgSbF
6 in DCE under air at 130 ℃ for 18h as our optimized reaction conditions. It should be noted that only 5 mol% of cobalt catalyst is enough to obtain similar yields when the reaction is run at larger scale, details see Supporting information.