Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene

Abstract

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (−Si t Bu 3 ) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C−C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Buchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC 6 rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC 5 rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C−C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.

Terminal methylene phosphonium ions: precursors for transient monosubstituted phosphinocarbenes

“…The analogous Buchner ring expansion reactions, involving benzene derivatives and transient carbenes or complexes thereof, has been widely used to introduce cycloheptatriene rings into natural products and materials. 62–69 Similar intramolecular ring expansions were reported for other transient main-group carbene analogues, such as silylenes, 70–76 and phosphinidenes. 77 Attempts to trap the transient carbene by performing the deprotonation at −40 °C in fluorobenzene in the presence of 2 bar dihydrogen or carbon monoxide, both known to react with electrophilic carbenes, 78–80 were unsuccessful.…”

Facile Bond Activation of Small Molecules by an Acyclic Imino(silyl)silylene

“…Similar to the formation of siloxysilylene VIII, silylene 1 reacted with triethylphosphine oxide to afford a more polarized silanone intermediate, due to the electronic effect of the methylated backbone NHI, which more easily undergoes oxygen migration to the silicon-silicon single bond, resulting in siloxysilylene 10. The Si1À O1 distance of 1.613(3) Å, and N1À Si1À O1 angle of 104.33 (15)°a re also similar with VIII. [13] Next, 1 : 1 reaction of 1 with S, Se, and Te were conducted, to afford the desired monomeric Si=E compounds 11 (E=S), 12 (E=Se), 13 (E=Te), respectively (Scheme 4, b).…”

“…We also reported the equilibrium of silepin and silylene, in which both isomers can be observed at ambient conditions [14] . Very recently, we reported the synthesis and isolation of the acyclic imino(silyl)silylene 1 bearing a methylated backbone NHI ligand, which reflects both high σ‐donor and π‐acceptor abilities [15] . Furthermore, 1 prevents the intramolecular C=C bond activation of its aromatic framework.…”

“…[14] Very recently, we reported the synthesis and isolation of the acyclic imino(silyl)silylene 1 bearing a methylated backbone NHI ligand, which reflects both high σ-donor and π-acceptor abilities. [15] Furthermore, 1 prevents the intramolecular C=C bond activation of its aromatic framework. Instead, it exhibits intermolecular dearomatization of arenes at ambient temperature forming corresponding silepins, which possess almost planar geometry.…”

Reversible Boron‐Insertion into Aromatic C−C Bonds

“…There are some examples of ring‐enlargement reactions of arenes with divalent silicon (Si II ) [6,7] and monovalent aluminum (Al I ) [8,9] species via the Buchner‐type ring expansion. As a recent example, Inoue reported intra‐ and intermolecular dearomatization reactions of arenes with isolable iminosilylenes to afford silepins via generation of silanorcaradiene intermediates followed by an electrocyclic reaction (Figure 1‐b) [7e,f] . Aldridge and Goicoechea reported reversible formation of seven‐membered aluminacycle by the reaction of an aluminyl compound with benzene [8a] .…”