Exploring the cyclization reaction channel for the gas-phase elementary reaction between the silicon nitride radical (SiN, X2Σ+) and isoprene (C5H8, X1A′) to prepare methylazasilacyclohexadienylidenes (SiNC5H7, X1A′)

Abstract

Silicon-containing heterocyclic molecules have emerged as promising candidates in medicinal and agrochemical research. However, the synthesis of silicon-containing heterocycles has remained highly challenging. In this work, we employed the crossed molecular beams technique to elucidate the underlying reaction pathways for the synthesis of a unique class of six-membered cyclic organosilicon molecules in which silicon and nitrogen atoms occupy adjacent positions: methyl-azasilacyclohexadienylidenes – silicon and nitrogen substituted benzenes functionalized with a methyl group. This class was accessed via the reaction of ground-state silicon nitride radicals (SiN, X²Σ⁺) with isoprene (C₅H₈, X¹A′) under single-collision conditions at a collision energy of 25 ± 1 kJ mol⁻¹. Integration of experimental results with electronic structure calculations revealed the formation of at least two cyclic products: 4-methyl-1-aza-2-silacyclohexa-3,5-dien-2-ylidene and 5-methyl-1-aza-2-silacyclohexa-3,5-dien-2-ylidene. The underlying mechanism shows strong similarities to the previously studied reaction of the silicon nitride radicals (SiN, X²Σ⁺) with 1,3-butadiene (C₄H₆, X¹A_g), with the methyl group in isoprene classified as a spectator, thus advancing our fundamental understanding of the organosilicon chemistry through the delivery of synthetic pathways to a distinct class of silicon-containing heterocyclic molecules: methylazasilacyclo-hexadienylidenes.