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

Abstract

The effects of ortho-methyl substitution on the reactivity of 1,1-diarylsilenes are assessed, through the study of a series of transient 1,1-diarylsilenes bearing ortho-methyl substituents in various numbers and positions on the aryl rings. The five silenes studied in this work (ArAr′SidCH 2 ; 5c-g) were prepared by photolysis of the corresponding 1,1-diarylsilacyclobutanes (6c, Ar ) Ar′ ) 2-MeC 6 H 4 ; 6d, Ar ) Ph, Ar′ ) 2,6-Me 2 C 6 H 3 ; 6e, Ar ) Ph, Ar′ ) 2,4,6-Me 3 C 6 H 2 ; 6f, Ar ) 2-MeC 6 H 4 , Ar′ ) 2,6-Me 2 C 6 H 3 ; 6g, Ar ) Ar′ ) 2,6-Me 2 C 6 H 3 ). Isomeric 1,1-diarylsilacyclobutane(s) arising from phototransposition reaction of one of the aryl rings are also formed in significant amounts in the photolyses, but do not interfere with the analysis. UV absorption spectra and lifetimes of 5c-g were determined in acetonitrile and hexane solution at 25 °C by laser flash photolysis methods, along with absolute rate constants for their reactions with methanol, n-butylamine, and acetic acid in acetonitrile solution. Deuterium kinetic isotope effects were also determined for the reactions with methanol and acetic acid. The rate constants for decay of the five silenes and for their reaction with the three reagents vary over 3-4 orders of magnitude and in the order 5c . 5d ∼ 5e > 5f . 5g. The decline in reactivity throughout the series is accompanied by a change in the mechanism for methanol addition, from one that is purely first order in alcohol in the case of 5c to one that is purely second order in alcohol in the case of 5g. The results are consistent with a mechanism involving initial complexation of the nucleophile with silene, followed by proton transfer by competing unimolecular and bimolecular pathways, the latter via a protonation-deprotonation sequence. The rate constants for acetic acid addition parallel those for n-butylamine addition, verifying that both reactions proceed via a mechanism involving initial nucleophilic attack at silicon followed by proton transfer.

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

“…The effect on the activation energy seems consistent with a mechanism in which the second molecule of alcohol acts as a general acid, of enhanced acidity compared to that in non H-bonding solvents, protonating the zwitterionic complex from the opposite side of the SiC bond to the site of complexation. This is the mechanism first proposed by Kira and co-workers to explain the increased yields of anti relative to syn addition products from reaction of a cyclic transient silene with alcohols at high concentrations, and we have recently reported corroborating evidence for the mechanism from an analysis of the kinetics of alcohol addition to a series of sterically hindered 1,1-diarylsilenes related to 2a . However, it seems difficult to rationalize the large difference in entropic factors between H-bond acceptor and nonaccepting solvents in the context of this mechanism.…”

“…It would thus appear, on the surface at least, that the stabilizing influence of the 2-neopentyl substituent in 4 is insufficient to cause a change in the gross features of the mechanism for reaction of the SiC bond with nucleophilic reagents. In the case of AcOH addition, we previously proposed a stepwise mechanism analogous to that of eq 2 (with the complexation step being rate-determining rather than the proton-transfer step) on the basis of the fact that the reactions of 2a and other 1,1-diarylsilenes with AcOD exhibit the same rate constants as with the protiated isotopomer and the assumption that an isotope effect would be measurable if the reaction proceeded by either a concerted mechanism or one involving initial protonation of the SiC bond. , Similarly, no isotope effect is evident on the rate constant for reaction of AcOH with 4 , leading to the conclusion that this reagent also reacts with 4 by the same mechanism as that with 2a and 2b .…”

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…The Ph-Si C -conjugation can also be attenuated by twisting the phenyl rings out of the plane of the silene moiety. Increased number of ortho-methyl groups bonded to the phenyl substituents of Ph 2 Si CH 2 (59) weakened the conjugation as indicated by a blueshift in λ max of 30 nm when going from 59 to 61a-61f [101,102].…”

“…In the context of bulk, Leigh and co-workers also studied the combinations of electronic and steric effects on the kinetics and mechanism of addition of methanol, n-butylamine, and acetic acid to the 1,1-diarylsilenes 61a-61f (Scheme 28) [102]. These species had ortho-methyl groups on the phenyl substituents at Si.…”

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. − …”