Kinetics and Mechanism of the Addition of Aliphatic Amines to Transient Silenes

Abstract

Primary and secondary amines such as n- butyl-, t-butyl-, and N,N-diethylamine add across the SidC bond of transient silenes such as 1,1-diphenylsilene (1a) and 1,1-bis(4-trifluoromethylphenyl)silene (1b) to yield the corresponding amino(methyl)diarylsilanes as the only products of reaction. The kinetics and mechanism of reaction of these three amines with the two 1,1-diarylsilene derivatives have been studied in hexane, acetonitrile, and tetrahydrofuran (THF) solution by laser flash photolysis techniques, using the corresponding 1,1-diarylsilacyclobutanes as photochemical precursors to the silenes. The reactions proceed with clean secondorder kinetics and bimolecular rate constants in excess of 5 × 10 8 M -1 s -1 in hexane and MeCN, with 1b being up to four times more reactive than 1a. Arrhenius plots for reaction in hexane and/or acetonitrile solution show strong curvature over the 0-60°C temperature range, consistent with an addition mechanism involving the intermediacy of a zwitterionic silene-amine addition complex, which collapses to product by intramolecular proton transfer from nitrogen to the silenic carbon. The reactions are substantially slower in THF, where rate reductions on the order of 5-10-fold and 50-70-fold are observed for 1a and 1b, respectively, as compared to MeCN solution. This is due to the effects of complexation of the silenes with the ether solvent, the equilibrium constant for which enters the expression for the reaction rate constant in the complexing solvent. In contrast to the situation in hexane and MeCN, Arrhenius plots for reaction of n-BuNH 2 with 1a,b in THF solution are linear and lead to positive activation energies. Addition of amines in THF solution is proposed to occur predominantly via the free silenes but with minor contributions from a pathway involving nucleophilic displacement of the solvent from the silene-solvent complex.

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

“… a From ref . b The limiting ([AcOH] bulk → 0) slopes of plots of k decay vs [(AcOH) bulk ]. c From ref . d From ref . e From ref . f From ref , in isooctane solution. …”

“…The rate constants for quenching of 1 , 6 , 7 , and 1,1-diphenylsilene ( 3 ; see Table ) by n -BuNH 2 provide a final point of comparison between the electrophilicities of germenes and silenes of homologous structures. Like alcohols, primary and secondary amines react with 3 by 1,2-addition, again via a stepwise mechanism initiated by nucleophilic attack at silicon followed by rate-controlling proton transfer; the reaction of 3 with n -BuNH 2 is the fastest reaction that we have yet encountered with this silene . While the course of its reaction with 1 has not been determined, the amine is the one substrate for which it is most reasonable to expect nucleophilic attack at germanium to be the initial step; indeed, the increase in the quenching rate constants throughout the series 1 < 6 < 7 is consistent with this.…”

“…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. − …”

Steric Effects on Silene Reactivity. The Effects ofortho-Methyl Substituents on the Kinetics and Mechanisms of the Reactions of 1,1-Diarylsilenes with Nucleophiles

“…c Linear dependence of k decay on [Q] over the concentration range studied. d From ref . e Calculated from the rate constants for reaction in dry hexane solution; k MeOH = (5.1 ± 0.2) × 10 8 M -1 s -1 .…”

“…The rate constants for reaction with MeOD and AcOD are not listed as such, but rather as k H / k D values calculated from the corresponding data for the protiated and deuterated reagents. For comparison, the table also includes previously reported rate constants for reaction of 5a , b with the same three reagents under similar conditions. , …”