Synthesis and Characterization of an Isolable Base‐Stabilized Silacycloprop‐1‐ylidene

Abstract

The chemistry of stable silylenes has been the subject of considerable research since the isolation of silicocene [1] and N-heterocyclic silylene. [2] To date, several stable silylenes, [3][4][5] including kinetically stabilized dialkylsilylene, [6] have been synthesized and their chemical properties have been extensively investigated. [7] A novel silylene stabilized by ylidic carbon centers has also recently been reported. [8] However, the structural modifications of stable silylenes are still far more limited than those of their lighter analogues, the carbenes. [9] Recently, silylenes I stabilized by the coordination of a donating ligand to the silicon(II) atom have attracted much attention owing to their high reactivity, which is strongly related to the nature of the ligands. [10][11][12][13] In particular, this stabilization method allows for easy functionalization of the Si(II) atom. Indeed, silylenes with reactive SiÀX bonds II (X = Cl, Br) [11,14] and even dihalogenosilylene complexes III [12] have been isolated as stable crystalline materials and have been used as precursors for preparing unique molecules. [14,15] We have also successfully synthesized the first basestabilized silicon(II) hydride species IV, [16] which are difficult to prepare in the absence of steric protection. [17] In this series, the small highly strained cyclic silylenes, such as the silacycloprop-1-ylidenes V, remain challenging target molecules. [18] Herein, we report the first synthesis of a base-stabilized silacycloprop-1-ylidene 3.We recently reported the synthesis of the first phosphinestabilized silicon(II) hydride 1 and studied the reactivity of the species with olefins. [16] Compound 1 similarly reacts at room temperature with diphenylacetylene through a [2+1] cycloaddition reaction to give the corresponding pentacoordinate silirene 2, which was fully characterized by NMR spectroscopy in solution. A 29 Si NMR analysis of silirene 2 shows a high-field doublet (À135.2 ppm, 1 J SiP = 58.9 Hz), which is characteristic of this type of structure. [19] In the 1 H NMR spectrum, the Si À H fragment appears as a doublet at 6.25 ppm ( 2 J PH = 96.4 Hz) with the characteristic 29 Si satellites (ca. 4.7 %, 1 J HSi = 286.1 Hz), confirming a direct siliconhydrogen bond in 2 (Scheme 1).Silirene 2 is thermally labile and quantitatively isomerizes in toluene at 80 8C into silacycloprop-1-ylidene 3, in which the Si II atom is stabilized by the intramolecular coordination of the imine fragment. Compound 3 was successfully isolated as colorless crystals in 92 % yield from pentane at room temperature (Scheme 1). Although the mechanism of the reaction is still unclear, this transformation formally involves two 1,2shifts of silicon ligands, hydride and phosphine, to each of the carbon atoms of the silirene ring. Contrary to the previous cases, neither an alkyne-insertion into the Si II À H bond [15] nor tricyclic phosphine formation [19] was observed. In the 31 P NMR spectrum, 3 displays two singlets (46.5 and 42.8 ppm), which is in agreement wit...

A Base‐Stabilized Sila‐β‐Lactone and a Donor/Acceptor‐Stabilized Silanoic Acid

“…[14] Moreover, the significantly larger Wiberg bond order for the silanone function (1.194) than that for the single Si1-O2 bond (0.446) also indicates its multiplebond character. The two phenyl groups are syn to each other like in the case of the starting silacycloprop-1-ylidene 1, [12] thereby confirming that the oxidation takes place with retention of stereochemistry.…”

“…[11] Similarly, the base-stabilized silacycloprop-1-ylidene 1 [12] readily reacts with one equivalent of dioxygen at 5 8C to give selectively the corresponding base-supported sila-b-lactone 2 (Scheme 1).…”

Synthesis of a Donor‐Stabilized Silacyclopropan‐1‐one

“…The 29 Si NMR spectrum of 3 exhibited a lower-field chemical shift (d = À7.1 ppm, 2 J PSi = 23.9 Hz) relative to that observed for 2 (d = À69.1 ppm), close to the region for silene-Lewis base adducts (d = À18 to À39 ppm). [23] In the 13 C NMR spectrum, the signal for the Si = C atom appeared as a doublet at d = 48.3 ppm ( 2 J PC = 127.8 Hz), in the chemical-shift range typical for carbon atoms in base-stabilized silenes (d = 40-52 ppm). [23] The 1 H NMR spectrum revealed a singlet for the SiCH atom (d = 3.82 ppm), in agreement with CÀC bond cleavage of the three-membered ring.…”

Durch ein N‐heterocyclisches Carben koordinierte neutrale und kationische schwere Cyclopropylidenanaloga

“…[12] Verbindung 3 gehçrt zur Klasse der SchwerelementVinylidenanaloga ("schwere Vinylidene"), die wir als neuartige Bausteine in die Chemie der Gruppe 14 eingeführt haben. [13] Unabhängig voneinander berichteten Robinson et al [14] und Baceiredo et al [15] von basenkoordinierten Silacycloprop-1-ylidenen, in denen nur ein schweres Gruppe-…”