Silylenes intramolecularly coordinated

It is an intriguing idea to stabilize low-valent silicon species, such as silyl cations, silylenes, silenes, and disilenes using intramolecularly coordinating ligands. Corriu et al. succeeded in the preparation of the first hypervalent silyl cation [(8-Me2NCioH6)2SiH]+l/2[l8]z 782 by the reaction of the hexacoordinated diorganosilane... 

Not surprising, an intramolecularly coordinated silylene 25 exhibits a comparable shift29 When tetrasilane 24 was photolyzed in a 3-MP matrix at 77 K, a band at Amax = 478 nm was detected. Because the noncoordinated silylene 26 absorbs at Amax = 662 nm, this hypsochromic shift was taken as evidence for the formation of silaylide 25 (Scheme 7). 

Belzner64 discovered a mild access to an intramolecularly coordinated silylene by the thermolysis of cyclotrisilanes 105 at moderate temperatures (40-60°C). It was furthermore shown73 that cyclotrisilane 105 and silylene... 

Corriu et al.29 presented several attempts to generate intramolecularly coordinated silylenes either chemically or photochemically. One approach used to generate a silylene employs the metal-mediated reductive dehalo-genation of dihalosilanes 112 and 115, respectively. Trapping experiments... 

Silylene complexes with two intramolecular coordinating arms can participate in dynamic behavior, involving a flip-flop alternating coordination, illustrated in... 

Remarkably facile thermal generation of a silylene has recently been demonstrated for the pentacoordinate alkoxydisilane 92416. Intramolecular N—>-Si coordination in 92 was shown by X-ray crystallography. This disilane underwent thermolysis at 110°C in toluene, or 90 °C in DMF, producing the silylene 93, which was trapped with 2,3-dimethylbutadiene and diphenylacetylene (Scheme 26). The 2 1 adduct with diphenylacetylene was shown to have one nitrogen intramolecularly coordinated to silicon, even though the silicon atom lacks any electronegative substituents. 

Presumably, this reaction proceeds through intermediate formation of the silylene 96, stabilized by intramolecular coordination from nitrogen (equation 133). 

The research on silylenes with intramolecular coordination shows beyond doubt that such electron donation from nitrogen greatly stabilizes the silylenes. A persistent silylene stabilized in this manner has not yet appeared, but it seems possible that one might be made if just the right substituents are employed. 

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

It is tempting to assume, that the facile formation of silylene 2 from cyclotrisilane 1 is due to the effective stabilization of 2 by intramolecular coordination of the dimethylamino group to the silicon centre [10], which should lower the activation energy of a dissociation process from 1 to 2. Reaction of 1 with benzylvinylether resulted in a complex reaction mixture, from which 12 % of vinylsilane 4 was isolated 4 is presumably formed by rearrangement of the unstable oxy-substituted silacyclopropane 3c. 

Summary The reactivity of silylene 4 towards halogenated alkanes, diphenylacetylenes, chlorosilanes, and a stable, nucleophilic carbene was investigated. Product studies, kinetic measurements, and calculations provide good evidence for 4 being a nucleophilic species due to an intramolecular coordination of the dimethylamino groups to the silicon center... 

Further confirmation of thermodynamic stabilization of 4 by intramolecular coordination is expected to arise from the investigation of its reactivity. The inherent electron demand of a silylene may be satisfied in a highly coordinated species such as 4a or 4b by the intramolecular coordination of one or two dimethylamino groups accordingly, such species should show more nucleophilic character. Here we will present experimental and theoretical results that show that 4 indeed acts as a nucleophile towards appropriate substrates these results corroborate the hypothesis of 4 being an intramolecularly coordinated silylene. 

It was reported that a substituent such as 8-dimethylaminonaphth-l-yl group, which may be able to coordinate with the generated silylene intramolecularly, facilitates the nickel and palladium-catalyzed extraction of silylene species from trisilane 96 (Eq. 50) [87]. 

Corriu et al. have reported that the coupling reaction of 2-(iV,iV-dimethylaminomethyl)phenyllithium with (McvSi)vSiCI 53 affords 2-(iV,iV-dimethylaminomethyl)-l-[tris(trimethylsilyl)silyl]benzene 894. No evidence has been found that the intramolecular iV-ligand coordinates to the silicon atom of 894. Upon UV irradiation, the trisilane forms a transient silyene 895, which has been trapped with 2,3-dimethyl-2,3-butadiene and triethylsilane to give the oligosilanes 896 and 897 as well as 898-900, (Scheme 126).859 Apparently, the bulk on the two ligands is insufficient to provide kinetic stabilization of the silylene intermediate 895. 

Attempts to stabilize a sila-ylide (hypercoordinated silylene) by intramolecular Lewis-base coordination did not result in sufficiently stable compounds for actual isolation. Formation of the sila-ylide was demonstrated by trapping with 2,3-dimethylbutadiene123. 

A silacyclopropene intermediate 56 was trapped by a reaction with 2-propanol during the thermal degradation of pentacoordinate disilanes 55 bearing an 8-thiona- or 8-seleno-l-naphthyl group and a methoxy group on the same silicon center. This reaction generates intramolecular sulfur- or selenium-coordinated silylenes 57 (Scheme 10) <2003OM3299>. 

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