Reversible Cycloaddition of Isocyanates to Ruthenium Silylene Complexes

Abstract

Reactions of ruthenium silylene complexes of the type [Cp*(PMe3)2RuSiR2]+ with unsaturated compounds were investigated. Nonpolar, unsaturated substrates such as ethylene, acetylene, and 2-butyne do not react with [Cp*(PMe3)2RuSiR2]B(C6F5)4 (1a, R = Me; 1b, R = Ph). However, methyl isocyanate inserts into an Si−S bond of the silylene complex [Cp*(PMe3)2RuSi(STol)2][BPh4] (5) to give the 1,2-dipolar addition product {Cp*(PMe3)2RuSi(STol)[η2−O(MeN)C(STol)]}[BPh4] (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*(PMe3)2RuSiR2OTf with methyl isocyanate. Heating 9 to 100 °C in toluene resulted in dissociation of the isocyanate and regeneration of the triflate Cp*(PMe3)2RuSiPh2OTf. 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.

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

“…Prominent examples include the formation of base-stabilized silylenes [54–55], insertion of olefins into hydrosilylenes [56], and bimolecular redistribution of thiolates between ruthenium silyl and silylene complexes [57]. Reactivity that involves metal-ligand cooperation (in the sense described in this article) has been reported in the formal [2 + 2] cycloaddition of isocyanates to ruthenium(II) silylenes [58] (Scheme 8). These complexes do not react with nonpolar substrates (although a possible cycloaddition with azobenzene was reported), and the overall cycloaddition was found to proceed through initial nucleophilic attack at an electrophilic silylene, indicating that the metal center is not itself very reactive.…”

Chemistry of metal – boron double bonds

“…While the insertion of carbodiimides or related heteroallenes into B¼ ¼N bonds, or into a silicon-heteroatom linkage of a cationic terminal silylene complex {such as [Cp*Ru(PMe 3 ) 2 Si(S-4-tol) 2 ] þ [BPh 4 ] 7 } has previously been reported [82], and a [2 þ 2] cycloaddition mechanism proposed by Bu¨rger and coworkers for the former transformation [83], insertion into M¼ ¼B bonds is extremely rare. Given that dicyclohexylcarbodiimide has been shown by low temperature NMR measurements to coordinate to the boron centre of 18 prior to insertion into the Fe¼ ¼B bond, a potential mechanism involves migration of the [CpFe(CO) 2 ] fragment from boron to the quaternary carbon of the carbodiimide, accompanied by the formation of a N!B coordinate bond.…”

“…Thus, alkenes, ketones, and even CO will displace the mesitylborylene ligand from the iron coordination sphere to generate the corresponding cationic complexes [Cp*Fe(CO) 2 L] þ ½BAr f 4 À (scheme 21) [43,44]. Such chemistry finds a precedent of sorts in the reactivity of cationic ruthenium silylene complexes towards nonpolar unsaturated substrates (such as alkenes), which has been shown to proceed via silylene ligand displacement [82]. Furthermore, a degree of generality for this chemistry can be demonstrated; the reactions of cationic aminoborylene complexes with CO have been shown to result in analogous ligand substitution chemistry (Kays et al, unpublished results).…”

Theoretical investigation on possible mechanisms on regioselective formation of (η3-siloxyallyl)tungsten complexes

“…Similar to transition metal carbene complexes [34], the important cycloaddition reactions of silylene complexes with unsaturated substrates were developed. For instance, [2 + 2]-cycloaddition was developed by reaction of cationic ruthenium silylene complexes [Cp*(PMe 3 ) 2 Ru@SiR 2 ]BPh 4 (R = Me or Ph) with methyl and simple aryl isocyanates [35]. Thereafter, the first [2 + 4] cycloaddition reactions were developed, where the neutral hydrido(hydrosilylene) tungsten complexes Cp 0 (CO) 2 …”