Synthesis of Norbornene-Based Phosphine-Stabilized Silylium Ions Behaving as Masked Frustrated Lewis Pairs

Abstract

A novel architecture of phosphine-stabilized silylium ions is described. The peculiar moderate flexible framework allows an efficient donation of the phosphine group toward the electron-deficient silicon center, thus providing a very good stabilization of the corresponding silylium ions. The P–Si interaction was studied in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis, respectively. The reactivity of silylium ions is easily improved using less donating phosphine ligands. Of particular interest, thanks to the flexible ligand framework, the phosphine-stabilized silylium ion 5f behaves as a frustrated Lewis pair instead of a simple Lewis acid and exhibits an original ambiphilic reactivity with carbonyl derivatives that allows the synthesis of seven- to nine-membered-ring heterocycles.

Labile Base‐Stabilized Silyliumylidene Ions. Non‐Metallic Species Capable of Activating Multiple Small Molecules

“…The 31 P NMR signal of 5 (37.6 ppm) is significantly shifted to high field compared to that of 3 a (67.0 ppm). Similar chemical shifts have been previously observed for related phosphine‐stabilized silylium ions (22–40 ppm) [38] . In the case of NHC‐stabilized cation 3 d , no reaction was observed even upon heating at 100 °C.…”

“…Similar chemical shifts have been previously observed for related phosphine-stabilized silylium ions (22 -40 ppm). [38] In the case of NHC-stabilized cation 3d, no reaction was observed even upon heating at 100 °C. Silyliumylidene ion 3a also immediately reacts with diphenylacetylene to give the corresponding [2+1]cycloadduct 6-A at RT.…”

“…ion, [38] with an appropriate nucleophilic partner, it can form an efficient FLP able to activate H2 in relatively mild conditions.…”

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

“…In the presence of neutral main-group element compounds of groups 15–17, Lewis adduct formation leads to silylated onium ions that are often difficult-to-access using conventional silyl transfer reagents. In this way, numerous silylonium salts of type [R n E–SiR 3 ]­[WCA] have been isolated and structurally characterized, including (per)­silylated pnictonium (E = N, ,,– ,,,,,,,,,,,,,,,,– P, ,,,,,– As, , and Sb), chalconium (E = O, ,,,,,,,,,,,– , S ,,– , Se ,, , and Te ,…”

Computational insights into the reactivity for the [2+5] cycloaddition reactions of norbornene-linked group 14 element/P-based and Si/group 15 element-based frustrated Lewis pairs with benzaldehyde

“…Consequently, its three-coordinated silyl ion and phosphine center can be considered as a Lewis acidic and a Lewis basic center, respectively, thus making it behave as a frustrated Lewis pair (FLP)-assisted molecule. Moreover, according to the available experimental report, 18 the norbornene-based phosphine-stabilized silylium ion ( 1 ), considered as an intramolecular FLP based on silicon and phosphorus with a larger spatial separation, can react with carbonyl derivatives for the synthesis of seven- and nine-membered-ring heterocycles, as shown in Scheme 1. Notably, this finding shows that the new strategy of the FLP concept can be effectively used to design and synthesize novel chemical heterocyclic molecules possessing a ring with greater than six members.…”