Group and Site Selective σ-Bond Metathesis Reactions of CH_3ScCH_2CH_3^+ with [2,2-D_2]Propane, [1,1,1,4,4,4-D_6]-n-Butane, [2-D]-Isobutane, and n-Pentane

Abstract

Fourier transform ion cyclotron resonance mass spectrometry has been used to examine the reactions of CH_3ScCH_2CH_3^+ with [D_4]methane, ethane, [2,2-D_2]propane, [1,1,1,4,4,4-D_6]-n-butane, [2-D]isobutane, n-pentane, and [D_12]-n-pentane. CH_3ScCH_2CH_3^+ is not observed to react with methane or ethane, but for propane, n-butane, isobutane, and n-pentane σ-bond metathesis with ethane elimination is the initial and dominant reaction observed, with further dehydrogenation of the resulting products occurring as additional reaction channels. For propane, n-butane, and isobutane, no methane elimination is observed. For n-pentane, methane elimination is a minor reaction channel. For propane, n-butane, and isobutane, the initial σ-bond metathesis involves predominantly the primary C-H bonds of the hydrocarbon. These processes are facile at room temperature and occur with little or no activation energy. Measured total bimolecular rate constants with [2,2-D_2]propane, [1,1,1,4,4,4-D_6]-n-butane [2-D]isobutane, and n-pentane are 0.87 x x 10^(-10), 0.98 x 10^(-10), 1.7 x and 6.4 x 10^(-10) cm^3 s^(-1) molecule^(-1), respectively. With the butanes and pentane a second intramolecular metathesis reaction follows the initial addition to yield a metallacycle product. In accordance with earlier theoretical predictions these metathesis reactions appear to proceed via an allowed four-center mechanism similar to that of a 2_σ + 2_σ cycloaddition. The higher reactivity of the metal-ethyl bond compared to the metal-methyl bond and the observed C-H bond specificity are also in agreement with theoretical predictions.