Reactivity and regioselectivity in the reaction of allylsilanes and some analogous all‐carbon alkenes with sulfur trioxide

Abstract

AbstractSulfonations of the 1‐(trimethylsilyl)alk‐2‐enes 1a‐12a, the 1‐(trimethylsilyl)‐ω‐phenyl‐alk‐2‐enes 13a–18a, the triisopropylsilylalk‐2‐enes 19a–21a, 1‐(trimethylsilyl)penta‐2,4‐diene (22a), and the allylsilane 23a with sulfur trioxide were studied in the temperature range ‐60 to 25°C using dichloromethane‐d2 as solvent and 1.5 mol‐equiv. of dioxane‐d8 relative to the amount of SO3 as reactivity moderator. For comparison, the sulfonation of the all carbon analogues 24a–27a were also studied. Reaction of the 1‐(trialkylsilyl)alk‐2‐enes 1a–11a and 13a–17a with SO3 at −60°C all afford the corresponding trialkylsilyl‐alk‐1‐ene‐3‐sulfonate esters c within 20 min in yields of > 95%. 1‐(Trimethylsilyl)‐2‐cycloheptylideneethane (12a) at −60°C gives a mixture of the trimethylsilyl sulfonate ester 12c and the isomeric 1‐(trimethylsilyl)‐2‐(1‐cycloheptenyl)ethane‐2‐sulfonic acid (12f) in a molar ratio of 4:6. With 1‐(trimethylsilyl)‐2‐(1,2,3,4‐tetrahydro‐1‐naphthylidene)‐ ethane (18a) the exclusive product is 1‐(trimethylsilyl)‐2‐(3,4‐dihydro‐1‐naphthyl)ethane‐2‐sulfonic acid (18f). Direct NMR evidence for the formation of β‐sultones as initial intermediates was observed in the sulfonation of the 1‐(trimethylsilyl)‐2‐cycloalkylideneethanes 11a and 12a. Upon sulfonation of the 1‐(triisopropylsilyl)‐alk‐2‐enes 19a–21a at — 60°C the initial products are the β‐sultone 20b‐II and the carbyl sulfates 19h‐II and 21h–II, of which the relative yields depend on the amount of SO3 employed. The SO3 sulfonation of 1‐(trimethylsilyl)penta‐2,4‐diene (22a) gives trimethylsilyl penta‐1,3‐diene‐5‐sulfonate (22i). The three homologous non‐silicon containing 1‐ cycloalkylidenehexanes 24a–26a at — 60°C afford the respective 1‐cycloalkylhexane‐1,1′‐sultones 24b‐III–26b‐III which, at higher temperatures, isomerize to give the 1‐(cycloalk‐1′‐enyl)hexane‐1‐ sulfonic acids 24m–26m. 1‐Methylene‐1,2,3,4‐tetrahydronaphthalene (27a) reacts with SO3 at −60°C to give (3,4‐dihydro‐1‐naphthyl)methanesulfonic acid (27o). Treatment of the various products containing a trialkylsilyl sulfonate ester substituent with aqueous KOH gives quantitatively the corresponding potassium sulfonate salts.Mechanisms for the formation of the various products are suggested and the observed selectivities are discussed.