N-heterocyclic silylenes

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

Table 2 Experimental and calculated geometries of N-heterocyclic silylenes using different methods (bond lengths in pm, bond angles in degrees)...

Reactivity of Carbene Complexes with N-Heterocyclic Silylenes... 

N-Heterocyclic silylenes have the advantage of kinetic as well as thermodynamic stahilization achieved through the coordination of silicon to the nitrogen atoms possessing bulky aryl substituents as well as with rr-donation from nitrogen to silicon. 

NICKEL(II) COMPLEX OF N-HETEROCYCLIC SILYLENE (NHSi)-CARBENE (NHC) CHELATE LIGAND [L i(CH2)(NHC)NiBr2] L = CH(MeC = NAr>2. NHC = 3,4,5-TRIMETHYLIMIDAZ0L-2-YLIDEN-6-YL ... 

Preparation of (CH)2(tBuNl2SiHCI 120 Preparation of N-heterocyclic silylene 120 References 121... 

Since N-heterocyclic silylenes are air and moisture-sensitive, the experiments must be carried out under an atmosphere of dry argon or nitrogen. All solvents were freshly distilled from Na and degassed immediately prior to use. 

This new method can be applied for the generation of stable and transient N-heterocyclic silylenes with a range of different substituents on the cyclic backbones under very mild conditions in high yields. 

N-heterocyclic silylene (HCNAr)2Si and terphenyl azide Ai N3 were synthesized according to published procedures (7,8). 

THF (50 mL) was added to a mixture containing the N-heterocyclic silylene (HCNAr)2Si (0.81 g, 2 mmol) and the ter-phenyl azide Ar Na (0.71 g, 2 mmol) at -78°C. The mixture was warmed to room temperature and stirred for 1 h. The solvents were removed, and the remaining solid was recrystallized from n-hexane at —40°C to yield yellow crystals of (HCNAr)2Si=NAr (yield 1.25 g, 85%). 

Stabilized silylium ions can be also used to transfer silyl groups to singlet carbene analogues. As outlined previously, silylation of N-heterocyclic silylenes gives access to aromatic silaimidazolium ions [27]. Likewise, the reaction of stannylene 104 with arene-stabilized silylium borates results in the formation of stable... 

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