Direct Detection of 1,1-Diphenyl-2-neopentylsilene and the Effects of Solvent Polarity on Its Reactivity with Nucleophiles

Abstract

The reactivity of the transient silene 1,1-diphenyl-2-neopentylsilene (4) has been studied in various solvents by laser flash photolysis methods, using trans-1,1,3,3-tetraphenyl-2,4dineopentyl-1,3-disilacyclobutane as the precursor. Silene 4 exhibits a lifetime of ca. 250 µs and a UV absorption maximum of λ max ) 335 nm ( ) 10000 ( 2900 M -1 cm -1 ) in dry, deoxygenated hexane, where it undergoes head-to-tail dimerization and reacts with oxygen with absolute rate constants of k dim ) (5 ( 2) × 10 8 M -1 s -1 and k O2 ) (6.5 ( 0.8) × 10 5 M -1 s -1 , respectively. Identical absorption maxima are exhibited by 4 in 1,2-dichloroethane, tetrahydrofuran, and acetonitrile solution, indicating that the silene does not form detectable Lewis acid-base complexes with these solvents, thus making it possible to study the effects of solvent polarity on silene reactivity for the first time. Accordingly, absolute rate constants for reaction of 4 with acetone, methanol, acetic acid, 2,2,2-trifluoroethanol, and n-butylamine have been determined as a function of temperature in two or more of the four solvents. The results are compared to previously reported data for 1,1-diphenylsilene (2a) and show that the 2-neopentyl substituent in 4 enhances the reactivity of the SidC bond with oxygen, but reduces its reactivity with nucleophilic reagents and toward [2+2]-dimerization by as much as a factor of 10 3 . As with 2a, the Arrhenius parameters for the reactions of 4 are consistent with stepwise mechanisms, initiated by reversible complexation between the nucleophile and the silene, followed by H-transfer. The absolute rate constants at 25 °C vary by only a factor of 3-10 with solvent in each case, but there is a general trend toward increasingly positive Arrhenius activation energies and pre-exponential factors with increasing solvent polarity. The mechanistic ramifications of these results are discussed.

Substituent effects on silene reactivity — Reactive silenes from photolysis of phenylated tri- and tetrasilanes

“…Comparison of the kinetic data for the two groups of compounds 6a/6d/6f and 6c/6e/6f allows a more detailed assessment of the effects of sequential trimethylsilyl substitution on the kinetics of the six silene-substrate reactions that have been studied in this work. In general, increasing TMS substitution in both series of compounds leads to a reduction in the rate constant for all six of these reactions, which is consistent with previously established trends for other silene derivatives (8,50). However, the effect varies tremendously with the specific reaction type.…”

Reactivity of a Polar Silene toward Terminal Alkynes: Preference for C−H Insertion over Cycloaddition

“…Oxonium complexes have long been proposed and since confirmed as intermediates in the addition of alcohols to silenes . Evidence for the reversible formation of an acid−base complex between naturally polarized silenes and carbonyl compounds has been provided by Leigh and co-workers. , Interestingly, the lifetime of the Lewis acid−base complex formed in the reaction between silene 4 and trans -2-phenylcyclopropanecarbaldehyde was sufficient to allow ester formation via the Lewis acid-catalyzed aldehyde dimerization (Tishchenko reaction) . Hence, the initial formation of a complex in the addition of ethoxyacetylene to 4 is well precedented.…”

Competing Germene and Germylene Extrusion from Photolysis of 1,1-Diarylgermacyclobutanes. Substituent Effects on Germene Reactivity

“…There has been great interest in the chemistry of homo- and heteronuclear heavier group 14 ethylene analogues, focusing principally on the synthesis and structural characterization of derivatives stabilized kinetically with sterically bulky substituents. − Much less attention has been paid to the study of simpler derivatives that are typically transient species in fluid solution, whose direct detection and study typically require the use of fast time-resolved spectroscopic methods and a photochemical reaction for the formation of the transient species of interest. While a significant body of experimental data now exists on the kinetics and mechanisms of the reactions of transient silenes and disilenes in solution, − relatively little has been done with their organogermanium counterparts, germenes and digermenes. − …”