Kinetics and mechanisms of the reactions of transient silylenes with amines

Abstract

The N–H insertion reactions of dimethyl‐, diphenyl‐, and dimesitylsilylene (SiMe2, SiPh2, and SiMes2, respectively) with n‐butylamine (BuNH2) and diethylamine (Et2NH) were studied in hexanes by steady‐state and laser photolysis methods. The process begins with the formation of the corresponding Lewis acid–base complexes, which decayed with second‐order kinetics at rates that show modest sensitivity to silylene and amine structures. The complexation process, which was also studied using triethylamine (Et3N), proceeds at rates close to the diffusion limit, but the rate constants vary systematically with steric bulk in the amine. Equilibrium constants were determined for the complexation of Et2NH and Et3N with SiMes2, which proceeds reversibly. The complexes of SiMe2 and SiPh2 with BuNH2 and Et2NH decayed with pseudo‐first‐order rate coefficients in the 104–105 s–1 range. This is consistent with upper limits of about 106 M–1 s–1 for the rate constants for amine‐catalyzed H‐migration, which is thought to be the dominant mechanism for product formation from the complexes. The results are compared to published kinetic data for the O–H insertion reactions of these silylenes with alcohols, which also proceeds via initial complexation followed by catalytic proton transfer. The results indicate that catalyzed H‐transfer in the amine complexes is at least 104 times slower than the analogous process in silylene–MeOH complexes. The experimental data are compared to the results of theoretical calculations of the SiMe2 + NH2Me and SiMe2 + MeOH potential energy surfaces, carried out at the Gaussian‐4 and B3LYP/6‐311 + G(d,p) levels of theory. Copyright © 2011 John Wiley & Sons, Ltd.

Reactions of m-Terphenyl-Stabilized Germylene and Stannylene with Water and Methanol: Oxidative Addition versus Arene Elimination and Different Reaction Pathways for Alkyl- and Aryl-Substituted Species

“…The insertion into the O–H bond was found to take place catalytically, aided by a second molecule of water (Figure , left). This parallels the reactivity of transient tetrylenes with water/alcohols or amines as well as the mechanism calculated for the insertion of Ge­(Ar Me 6 ) 2 into the N–H bond in NH 3 . The Gibbs energy of activation for the formation of (Ar Me 4 ) 2 E­(H)­OH was found to be 46 and 73 kJ mol –1 for E = Ge and Sn, respectively.…”

Electronic and Steric Effects on the Lewis Acidities of Transient Silylenes and Germylenes: Equilibrium Constants for Complexation with Chalcogen and Pnictogen Donors

“…Early fast kinetic studies of the complexation of SiMe 2 with various donors in solution , and with ethers in the gas phase established the first absolute rate constants for silylene Lewis acid–base complexation reactions and showed them to be essentially diffusion- or encounter-controlled in all cases, consistent with an enthalpically barrierless process. This has been extended by our own, more recent solution-phase studies of the reactivities of SiMe 2 , GeMe 2 , and the corresponding diphenyl and dimesityl derivatives, , which show that complexation with THF, alcohols, and amines is invariably quite fast, the rates varying only modestly with variations in structure or substitution in either the tetrellylene or the donor or with the overall thermodynamics of the complexation process. Equilibrium constants have been measured or estimated in only a handful of casesfor the complexation of the three germylene derivatives with methanol (MeOH) and THF and of SiMes 2 with diethylamine (Et 2 NH) in hexanes and of SiMe 2 with dimethyl ether (Me 2 O) and tetrahydrofuran (THF) in the gas phase. , The available thermodynamic data are thus fairly sparse but are nevertheless consistent with the indication from theory that silylenes are stronger Lewis acids than germylenes of homologous structures and that both species form stronger complexes with amines than with O donors.…”

Photochemical Generation and Characterization of Di-tert-butylsilylene (SitBu2) in Solution

“…As with Et 2 O and THF, laser photolysis of 5 or 7 in the presence of THT and Et 2 NH led to the formation of new transient absorptions at shorter wavelengths, which could also be assigned to the corresponding Lewis acid–base complexes with Si t Bu 2 based on comparisons to the spectra of the corresponding complexes with SiMe 2 . ,, Transient UV–vis spectra were recorded with 5 in hexanes containing 5.0 mM of the sulfide or amine, and are shown in Figures S2b and S3b, respectively. No new transient products were observed with any of the other silylene substrates except MeOH, which will be discussed in more detail below.…”

“… a In hexanes at 25 °C, unless otherwise noted. Errors are quoted as twice the standard error from the least-squares analyses. b Determined using 7 as Si t Bu 2 precursor unless otherwise noted. c Determined using 5 as Si t Bu 2 precursor. d Ref . e Ref . f Ref . g Ref . h Ref . i Ref j Cyclohexane, 23 °C; ref . k Cyclohexane, 22 °C; ref . …”

“…No new transient products were observed with any of the other silylene substrates except MeOH, which will be discussed in more detail below. Table summarizes the UV–vis absorption maxima of each of the Lewis acid–base complexes studied in this work, along with those reported previously for the corresponding complexes with SiMe 2 . ,,, As can be seen, the spectra of the Si t Bu 2 -donor complexes are in most cases red-shifted slightly relative to those of the corresponding SiMe 2 -donor complexes, to a similar extent (in terms of transition energy) as is observed with the spectra of the free silylenes.…”