Determination of the β-Silicon Effect from a Mass Spectrometric Study of the Association of Trimethylsilylium with Alkenes

Article abstract of DOI:10.1021/ja00197a013

A mass spectrometer equipped with a high-pressure ion source and pulsed electron beam has been used to study the association of alkenes with trimethylsilylium: Me3Si(1+) + alkene <*> Me3Si-alkene(1+) (7).The equilibrium constants for reaction 7 have been measured for a series of alkenes over a range of temperatures, and from the resulting van't Hoff plots the following -ΔH₇⁰ (kcal mol^-1) and -ΔS₇⁰ (in brackets; cal K^-1 mol^-1) values are obtained: ethene, 23.6+/-0.3 (38.5+/-0.8); propene, 30.6+/-0.4 (42.5+/-0.8); trans-2-butene, 30.9+/-0.6 (41.5+/-1.5);isobutene, 36.5+/-1.1 (42.7+/-2.1); 2-methyl-2-butene, 38.2+/-0.5 (48.0+/-1.1); cyclohexene, 32.9+/-1.0 (45.6+/-2.1); styrene, 36.6+/-0.7 (42.4+/-1.4).The β-silicon effect, defined as the stabilization energy resulting from a silicon atom in a position β to a carbenium carbon, is given by ΔH⁰ for the isodesmic reaction Me3Si*X(1+) + XH2 <*> Me3SiXH + XH(1+) (X = alkene). ΔH_f⁰(Me3SiX(1+)) is obtained from ΔH₇⁰ using ΔH_f⁰(Me3Si(1+)) = 141 kcal mol^-1; ΔH_f⁰(Me3SiXH) is estimated.The stabilization energies (kcal mol^-1) are the following: X = ethene, 48.2; propene, 38.4; trans-2-butene, 38.2; isobutene, 28.1; 2-methyl-2-butene, 28.8; 2,3-dimethyl-2-btene, 25.8; styrene, 21.8.These values are grouped according to the number of substituents α to the carbenium carbon in classically drawn structures and are consistent with the theoretically predicted large hyperconjugative interaction between the Si-C ?-bond and the formally empty p orbital on the carbenium carbon.