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8604J . Am. Chem. SOC.119 yields the final product of the dehydration reaction. The same is true for the ion of m/z 87 (Table 11): at low excitation energies, this ion yields an abundant fragment ion of m/z 59 which further decomposes by loss of water to yield an ion of m/z 41 (a small amount of an ion of m/z 43 is also obtained at very high excitation energies). These findings suggest that some of the intermediates c are generated with an insufficient amount of energy for further reaction but that the reaction can be induced by collisional activation. Reaction of CH3O-I0B+-OCH3 with diethyl ether yields an intermediate ion of m/z 118 instead of m/z 119. This demonstrates that the proposed intermediate contains boron. Further, reaction of CH3O-I1B+-OCH3 with dlo-diethyl ether yields an ion corresponding to c with six deuterium atoms (m/z 125; Figure 3). Thus, all the hydrogen atoms in the first eliminated ethylene must originate from the ether. Finally, an ion corresponding to the intermediate c must be generated upon collisions of CH30-"B+-OCH3 with ethanol (Scheme 11). This ion decomposes spontaneously by loss of ethylene ( Table I) to yield the ions e and/or f shown in Scheme 11.Alcohols that cannot lose an alkene according to Scheme I1 are not dehydrated by the boron cations. For example, methanol does not undergo any apparent reaction with CH30-B+-OCH3. Further, ethers that cannot lose two alkenes according to Scheme I are not dehydrated. Only slow condensationza is observed for anisole and tetrahydrofuran ( Table I). n-Butyl methyl ether undergoes a facile reaction (efficiency 0.9) with CH,O-'lB+-OCH3 to yield an ion corresponding to the intermediate c:of this product ion (Table 11) is in agreement with the structural assignment: loss of methanol dominates the fragmentation.The reaction of CH30-B+-OCH3 with n-butyl methyl ether yielding (CH30)zB-O(H)CH3+ and neutral butene is analogous to the first elimination step in Scheme I. In order to obtain an estimate for the enthalpy change associated with this reaction, the proton affinity of trimethyl borate was determined. Protonated trimethyl borate was allowed to react with n-propanol (proton affinity 190.8 kcal/molI3), sec-butanol (proton affinity 195.0 kcal/mol13), tetrahydropyran (proton affinity 199.7 kcal/m0ll3), and diethyl ether (proton affinity 200.2 kcal/mol"). Proton transfer was observed to occur only to tetrahydropyran and diethyl ether. This brackets the proton affinity of trimethyl borate between that of sec-butanol and tetrahydropyran (195.0-199.7 kcal/mol). Hence, the elimination of butene from n-butyl methyl 1992, 11 4, 8604-8606 ether by the ion CH30-B+-OCH3 is estimatedI6 to be exothermic by at least 50 kcal/mol. For diethyl ether, the elimination of two ethylenes by CH30-B+-OCH3 is estimated to be exothermic by at least 40 kcal/mo1.I6J7The hydrogen-bonded structures b and d, if they occur as distinct intermediates on the reaction path, are not likely to be long-lived (they may actually correspond to transition states). This is indicated b...

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A 6 initio molecular orbital calculations at the MP2/6-31G*//6-31G* + ZPVE level of theory suggest that the ion (CH,),S+--CH,'is 75 k J mol-' higher in energy than ionized ethyl methyl sulfide. However, this distonic ion is stable toward isomerization to the conventional structure: the two ions show distinctly different reactivity in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. Molecular orbital calculations further indicate that the charge site of the ion is on the sulfur atom whereas the odd-spin density is localized on the CH, group. The chemical properties of the ion reflect this description. The coordinatively saturated sulfonium charge site is chemically inert. This radical cation possesses unique chemical reactivity in that its reactions are associated with the radical site. Most of these reactions involve a homolytic bond cleavage in the neutral molecule, resulting in abstraction of an atom or a radical by the ion. The reaction efticiency correlates with the homolytic bond dissociation energy of the bond being broken in the neutral molecule, suggesting a simple radical abstraction mechanism for these reactions. Hence, the methylenedimethylsulfonium ion is best described as on electrophilic radical with an inert charge site.

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