Reversible Cycloaddition of Isocyanates to Ruthenium Silylene Complexes

Reactions of ruthenium silylene complexes of the type [Cp*(PMe 3 ) 2 Ru SiR 2 ] + with unsaturated compounds were investigated. Nonpolar, unsaturated substrates such as ethylene, acetylene, and 2-butyne do not react with [Cp*(PMe 3 ) 2 Ru SiR 2 ]B(C 6 F 5 ) 4 ( 1a, R = Me; 1b, R = Ph). However, methyl isocyanate inserts into an Si−S bond of the silylene complex [Cp*(PMe 3 ) 2 Ru Si(STol) 2 ][BPh 4 ] ( 5 ) to give the 1,2-dipolar addition product {Cp*(PMe 3 ) 2 RuSi(STol)[η 2 −O(MeN)C(STol)]}[BPh 4 ] ( 6a ) in 87% yield. This product was characterized by X-ray crystallography as possessing a base-stabilized silylene ligand with thiolate and thiocarbamate substituents. Compound 1a reacts with methyl and phenyl isocyanate to give the 2 + 2 R = Me; 8, R = Ph). The analogous triflate R = Ph; 10, R = Me) were prepared by reaction of the appropriate ruthenium silyls Cp*(PMe 3 ) 2 RuSiR 2 OTf with methyl isocyanate. Heating 9 to 100 °C in toluene resulted in dissociation of the isocyanate and regeneration of the triflate Cp*(PMe 3 ) 2 RuSiPh 2 OTf. Competition reactions of various para-substituted phenyl isocyanates with 1b show that the rate of cycloaddition increases with the electron-donating ability of the incoming isocyanate. This is consistent with a stepwise cycloaddition mechanism involving initial coordination of the isocyanate nitrogen atom to the Lewis acidic silylene silicon atom.