Skeletal rearrangements of siliconium ions in the gas phase: mass spectra of some phenoxasilins

The molecular ions of both 10,10-dimethyl-(Ia) and 10,10-diphenyl-phenoxasilin (Ib) undergo consecutive elimination of the two substituents from position 10, followed by ejection of SiO from the centre ring. The (M– R)+ species in (Ia) fragments by competitive expulsion of CH2O, SiH2O, and SiHO·, which involves complex molecular reorganizations. An abundant m/e 91 (C7H7) peak, generated through C(alkyl)–C(aryl) bond formation, is noted. The unique extrusion of the oxygen bridge in conjunction with two aromatic hydrogen atoms (H2O) from the (M– R)+ peak in (Ib) leads to an abundant even-electron siliconium ion at m/e 255. The expulsion of SiHO· from the (M– R)+ species is also observed in compound (Ib). The elements of dibenzofuran are lost from the molecular ion to give divalent siliconium ions SiR2˙+ in both compounds (Ia) and (Ib).