Toward a Silicon Version of Metathesis: From Schrock-Type Titanium Silylidenes to Silatitanacyclobutenes

Abstract

Olefin metathesis is one of the most important industrial processes for the production of alkenes. In contrast, silicon versions of metathesis are unknown given the lack of available silylene transition-metal complexes suitable for [2 + 2] cycloaddition with unsaturated substrates. Here, we report the synthesis of 18-electron titanium silylene complexes featuring different Lewis base ligands and classified on the basis of structural, computational, and reactivity studies as Schrock-type silylene complexes. Because of the presence of loosely bound Lewis base ligands, such silylene complexes readily undergo reaction with simple unsaturated hydrocarbons, such as alkynes, forming the corresponding [2 + 2] cycloaddition products.

[2+2] Cycloadduct of Titanium Silylidene and Benzonitrile

“…Crystal structure of 2 is shown in Figure . As is typical for the [2 + 2] cycloadducts with 1 ,[6a] the geometry around the Ti‐bound silicon atom is almost unchanged upon the cycloaddition reaction. Thus, the sum of the bond angles around Si1 (ignoring the N1 atom) comes to 358.9° which value is quite comparable to that of the starting titanium silylidene 1 (359.6° for the isocyanide complex of 1 ,[6a] crystallographic data for the thf complex of 1 are not available).…”

“…It is worth mentioning that the latter signal appeared at much higher fields than that of the previously reported [2 + 2] cycloadduct of 1 with trimethylsilylacetylene Me 3 Si–C≡CH (+125.4 ppm). [6a] As a diagnostic feature of titanacyclobutenes, the Ti‐bound olefinic cyclic C atom in 2 resonated at exceptionally low‐field at +242.3 ppm [calculated values +259.8(+246.7) ppm at the DFT1(DFT2) levels; calculated values for the potential regioisomer 3 (R = SiMe t Bu 2 ) with the Si‐bound olefinic carbon atom are greatly different, +210.9(+197.4) ppm], being even more deshielded than that of the above‐mentioned [2 + 2] cycloadduct of 1 with trimethylsilylacetylene (+221.1 ppm). [6a]…”

“…However, to date none of them were applied as catalysts for the silicon version of the metathesis process. As the approach towards this goal, we recently reported synthesis of the novel Schrock‐type silylidenes of the group 4 metals and their subsequent [2+2] cycloaddition with terminal alkynes forming the corresponding metallasilacyclobutenes, as the “frozen” intermediates of the metathetical catalytic cycle . Extending the range of applicability of our Schrock silylidenes towards unsaturated compounds, we report in this communication [2+2] cycloaddition of titanium silylidene 1 with benzonitrile forming unprecedented azasilatitanacyclobutene derivative 2 that was isolated and fully characterized, and which structure was determined by X‐ray crystallography and verified by DFT computations ,…”

Reaktionen auf die Vollsperrung der Schiersteiner Brücke auf der A643

“…Lee, Sekiguchi et al stellten eine siliciumbasierte Schrock‐Alkyliden‐Verbindung [(η5‐C5H5)2Ti(thf) (=Si(Si3(SitBu3)4)] dar, die mit Alkinen irreversibel in einer [2+2]‐Cycloaddition zum Titanasilacyclobuten reagiert (Abbildung 3) 1_3…”

Titanium tales

“…Even the early days of olefin metathesis used titanium carbenoids, long before the now familiar molybdenum and ruthenium systems. Titanium may even make a name for itself in silicon metathesis -scientists in Japan and France recently characterized stable silacyclobutenes 3 . The low cost and high abundance of titanium obviates any guilt over throwing away the relatively non-toxic salts produced as by-products in the reactions described above.…”