Desilylation of β-Silyl Pyridinium Ions: Gas-Phase and Solution-Phase Studies in Conjunction with DFT Calculations

Gas-phase chemistry of N-methyl 2- and 4-trimethylsilylmethylpyridinium ions 8 and 20 and the deuterated derivatives 17 and 21 was investigated using electrospray ionization (ESI) and tandem mass spectrometry. Collision-induced ionization (CID) of the derived ions m/z 180 from both 8 and 20 gave rise to (CH3)(3)Si+ at m/z 73 in addition to ions at m/z 108 and 106 arising from competing proton transfer and hydride transfer pathways from within the initially formed ion-molecule complexes. CID of the derived ions m/z 189 from the deuterated derivatives 17 and 21 gave rise to (CD3)(3)Si+ at m/z 82, in addition to ions at m/z 109 and 106. Formation of the m/z 108 ions from 8 and 20 provides evidence to support the proposed mechanism for desilylation of beta-silyl carbenium ions in solution. Computational studies at B3LYP/6-311g(d,p) support the MS studies, as it was found that the favored pathway is the proton transfer from the (CH3)(3)Si+ ion to the basic enamine intermediate, giving rise to N-methylpicoline and silylethene, followed closely by dissociation of the ion-molecule complex, with the least energetically favored pathway being hydride abstraction. Crystals of the cation 17 as its triflate salt. when heated at 160 degrees C in a sealed tube for 2 days, gave rise to it mixture of N-methyl 4-CH3-pyridinium ion and N-methyl 4-CH2D-pyridinium ion, indicating that the proton transfer step also occurs in the condensed phase.