Mass spectral and theoretical (AM1) study of cations derived from janusene. Evidence for interannular proton transfer

A previous failure to find clear evidence for protonation of janusene in superacid solutions, coupled with evidence for radical cation formation, led to the current mass spectral and theoretical study. The janusene radical cation JAN .+ generated by EI/MS undergoes cycloreversion to give the anthracene radical cation with high selectivity over the dibenzobarrelene cation. Cycloreversion of tetrafluorojanusene radical cation F 4 JAN .+ is also selective toward AN .+ formation, with lesser amounts of F 4 N .+ , DBB .+ , and F 4 DBB .+ cations being formed in order of decreasing relative abundance. In constrast, protonated janusenium ion (JAN.H) + and (F 4 JAN.H) + undergo cycloreversion with a preponderance of (DBB.H) + as the charged fragment. Acetylation (with MeCO + ), trimethylsilylation (with TMS + ), and trifluoroacetylation (with CF 3 CO + ) of JAN and F 4 JAN, and fragmentation of their resulting cations, were also studied by tandem mass spectrometry. Remarkably, the acylated cations also gave predominantly (DBB.H) + as the cycloreversion cation, thus indicating interannular proton transfer. The stabilities of protonated and oxidized janusenes, and several model compounds, were calculated by the AM1 molecular orbital method. For JAN, β-facial protonation is preferred over lateral protonation , whereas in F 4 JAN preference for facial versus lateral protonation is less. The AM1 calculations lend support to the concept of rapid interannular proton transfer between facial rings, which may help to explain the observation of broad NMR lines in attempts to protonate janusene in superacid