Figure 2 High-resolution ESI-MS/MS of [M –OTf]+: (a ) 5 , (b ) 6 , (c ) 7 and (d )8 .
Scheme 4 Proposed rearrangement patterns of [M –OTf]+ of salts5 ~7 .
Based on the observation of O-atom transfer product ion [t Bu-C6H4O]+at m/z 149 from nitro-substituted diaryliodonium cation [4 –OTf]+, we extended to more diaryliodonium cations with salts 5 ~7as shown in Scheme 1 , and the spectra of the salts5 ~7 with nitro substituent in different position (ortho , meta and para ) of Ar2 were depicted in Figure 2a ~c . To our delight, [Mes-I+-(NO2-C6H4)] from salts 5 ~7 all exhibited the O-atom transfer product ion [MesO]+ atm /z 135 with high abundances in their respective MS/MS. As a comparison, we also tested cations from salt 8 simultaneously (Figure 2d ). However, the product ion [MesO]+ at m /z 135 could not be observed in the MS/MS of [Mes-I+-Mes] atm/z 365. The accurate mass measurements (in Table S1 andS2 ) supported our explanations. These results collectively demonstrated that the introduction of a nitro group into Ar2 of [Ar1-I+-Ar2] could give rise to new type fragmentation ions [Ar1O]+ by O-atom transfer process from the nitro group in Ar2 to Ar1 in ESI-MS/MS of nitro-substituted diaryliodonium cations.
Most importantly, when the nitro group was in ortho -position of Ar2, the fragmentation ion [MesO]+ at m /z 135 dominated the tandem mass spectrum of [Mes-I+-(o -NO2- C6H4)] at m /z 368 from salt 5 , with only a minimal presence of [Mes]+ at m/z 119 (Figure 2a ). Two possible reaction pathways were proposed for the gas-phase O-atom transfer rearrangement of [5 –OTf]+ (Scheme 4 ):path (a ) Smiles rearrangement via a six-membered ring transition state (TS 5a ) to intermediate 5b[Mes-O-(o -NO-C6H4I)]+, which finally dissociated to [MesO]+ atm /z 135; path (b ) Cleavage of the C–I bond to form another transition state (TS 5a ) and ion-neutral intermediate 5d , which could directly give to [Mes]+ at m/z 119 or further lead to intermediate 5b and finally forming [MesO]+ at m /z 135 (Scheme 4 ). Due to the distance problems, the [Mes-I+-(m -NO2-C6H4)] and [Mes-I+-(p -NO2-C6H4)] from salt 6 or 7 might follow a stepwise process involving ion-neutral intermediates [Mes+ & (m -NO2-C6H4I)] or [Mes+ & (p -NO2-C6H4I)] to generate the intermediate 6b[Mes-O-(m -NO-C6H4I)]+or 7b[Mes-O-(p -NO-C6H4I)]+, respectively. These intermediates ultimately resulted in the O-atom transfer product ion [MesO]+ at m /z135 (here Ar1 = Mes) with loss of neutral species [m -NO-C6H4I] or [p -NO-C6H4I] (Scheme 4 ).