The mechanism of silver(I) and copper(I) catalyzed cycloaddition between 1,2-diazines and siloxy alkynes remains controversial. Here we explore the mechanism of this reaction with density functional theory. Our calculations show that the reaction takes place through a metal (Ag+, Cu+) catalyzed [2+2] cycloaddition pathway and the migration of a silylium ion [triisopropylsilyl ion (TIPS+)] further controls the reconstruction of four-member ring to give the final product. The lower barrier of this silylium ion mediated [2+2] cycloaddition mechanism (SMC) indicates that well-controlled [2+2] cycloaddition can obtain some poorly-accessible IEDDA (inverse-electron demand Diels-Alder reaction) products. Strong interaction of d10 metals (Ag+, Cu+) and alkenes activates the high acidity silylium ion (TIPS+) in situ. This п-acid (Ag+, Cu+) and hard acid (TIPS+) exchange scheme will be instructive in silylium ion chemistry. Our calculations not only provide a scheme to design IEDDA catalysts but also imply a concise way to synthesise 1,2-dinitrogen substituted cyclooctatetraenes (1,2-NCOTs).
The ultrafast excited-state dynamics of 2,4-dimethylpyrrole following excitation at wavelengths in the range of 255.8-199.7 nm are studied using the time-resolved photoelectron imaging method. It is found that excitation at longer wavelengths (255.8, 250.0, 246.0 and 242.0 nm) results in population of the S(πσ*) state, which decays out of the photoionization window in less than 30 fs. At 237.7 nm, the second πσ* state is excited, which decays in about 130 fs. At shorter pump wavelengths (231.8, 224.8, 217.5 and 199.7 nm), the assignments are less clear-cut. We tentatively assign the initially photoexcited states to theπ3p Rydberg states, which decay in about 60 fs, with internal conversion to the S(πσ*) state as one of the decay channels. The lifetimes of these π3p Rydberg states vary little with the pump wavelengths in this wavelength range.
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