Zwitterionic Silenes: Interesting Goals for Synthesis?

Abstract

Properties of silenes, as a function of increased reversal of the Si=C bond polarity, have been examined through quantum-chemical calculations. The aim of this study was to identify silenes that can be of general interest for organic synthesis. The calculations were carried out primarily with the B3LYP hybrid density functional method, but also with the CASSCF, MP2, MP4(SDQ), and CCSD(T) methods. The study was performed on Z(2)Si=CXY compounds which were divided into three sets that differ with regard to their Si substituents (Z), and with their C substituents (X and Y) varying from weakly to strongly pi-electron-donating groups. The charge at the Si atom (q(Si)) was used as a measure of the extent of reversed silicon-carbon bond polarity. For each of the three sets, the variation in silicon-carbon bond lengths (r(Si=C)) and extent of Si pyramidalization (SigmaSi) in relation to q(Si) follow three separate curves. Silenes with strongly pi-electron-donating X and Y groups are completely described by zwitterionic (reverse-polarized) resonance structures. Such zwitterionic silenes are singly (Si=C) rather than doubly bonded (Si=C), and have a distinctly pyramidal Si atom due to negative charge localization. These silenes also have much lower heats of dimerization than the parent silene. Finally, inversion barriers of zwitterionic silenes are increased by electron-withdrawing substituents, and this enables computational design of silenes with their Si atoms as chiral centers. It is hoped that such chiral zwitterionic silenes can find use in organic synthesis.

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…The breakpoint corresponded approximately to the point where E ST ≈ f(E + ) according to Eq. (3.5) [64]. Moreover, for the Z 2 Si CXY silenes of Ottosson, the E ST correlated with the bond dissociation energy E diss , similar as Carter and Goddard found for substituted alkenes [26].…”

“…Very recently, Cheng and Chu published a quantum chemical B3LYP/cc-pVDZ study of disubstituted silenes similar to those investigated by Ottosson [64], but augmented to include silenes that contained Si(NH) 2 (CH) 2 and/or C(NH) 2 (CH) 2 cycles [51]. An interesting observation was that the geometries of the SiH 2 fragments of H 2 Si CXY silenes with planar equilibrium geometries was similar to the optimal geometry of triplet state SiH 2 .…”

“…The geometric features of species such as H 2 Si C(NH 2 ) 2 (19) suggested that zwitterionic resonance structures with negative charge at Si and positive at the C side were dominant. A plot of the Si C bond length versus q(Si) of the various Si C bonded compounds revealed two linear parts separated by a breakpoint at which Si pyramidalization sets in [64]. The breakpoint corresponded approximately to the point where E ST ≈ f(E + ) according to Eq.…”

“…Ottosson performed a computational study on the effects of increasingly reversed Si C bond polarity, as measured by the charge at the Si atom (q(Si)), on the structural properties of silenes [64]. Silenes of the general type Z 2 Si CXY (Z = H, Me or H 3 Si; X = H, SH, Cl, F, OH or NH 2 ; Y = H, SH, Cl, F, OH, NH 2 , O − , S − or NH − ) were investigated.…”

Reversible, vollständige Spaltung von SiSi‐Bindungen durch Isocyanidinsertion

“…Der Si2-C01-Abstand beträgt 1.735(2) und gehçrt damit zu den kleineren bei donorsubstituierten Silenen gefundenen Abständen. [20] Obwohl die Anwesenheit des p-donierenden Stickstoffatoms am Kohlenstoffzentrum zu einer Verlänge-rung der Si = C-Bindung führen sollte, [21] kann durch die elektronegativen Arylgruppen am Siliciumatom eine Über-kompensation dieses Effektes erwartet werden. [22] Dennoch weist die Winkelsumme von SSi2 = 355.85(18)8 auf eine leichte Pyramidalisierung hin, die bisher nur bei einem cyclischen Silen des Brook-Typs beobachtet wurde.…”