1.3- Diisopropyl-4,5-dimethylimidazol-2-ylidene

Reactions of silylenes with alkynes present an alternative approach for the silylation of alkynes without the aid of transition-metal catalysts (4). The bis-silylation reaction has been accomplished in a stereospecific manner via a 1,4-silyl migration with the easily available NHC-stabilized silylaminosilylene Ar SiMe3)N(Cl)Si- hPr) (Ar = 2,6-iPr2C6H3, hPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) under metal-free conditions (5), representing the first successful approach for the selective bis-silylation of alkynes with a donor-supported silylene as the silylation reagent. Furthermore, the alcoholysis of these bis-silylated alkenes gave trimethoxylsilyl-substituted alkenes. 

Treatment of l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene with SC12 and S0C12 resulted in 87 and 88, respectively (67). Complex 87 can be converted into the corresponding fluoride (89) via reaction with AgF. Surprisingly, however, reduction of the sulfur center was ob-... 

Scheme 1. Reactions of indium halides with l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene.

Singlet carbenes can be viewed as the conjugate bases of secondary carbo-cations, and so the pKgS and proton affinities of diaminocarbenes are of fundamental interest. As mentioned in section 1.2, we were able to measure the pKg of 2,4-diisopropyl-3,4-dimethylimidazol-2-ylidene on the DMSO scale as 24. Unfortunately attempts to measure the pKg of non-aromatic diaminocarbenes on the same scale have not been successful, due to side reactions (see Scheme 1). 

The A-heterocyclic carbene (NHC), l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, is an efficient and selective catalyst for the dehydrogenative coupling of a wide range of silanes and hydroxyl groups to form Si-O bonds under mild and solvent-free conditions. Mechanistic studies indicate that the activation of hydroxyl groups by the NHC is the most plausible initial step for the process." " ... 

The robust Cp Ir(NHC) moiety (NHC = l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) has also been used to stabilize [Cp (NHC)Ir=PMes] complex 552 which contains an unusually stable phosphinidine ligand (Scheme 11) ... 

In a related study, Hedrick, Waymouth and coworkers reported that NHCs were also able to act as catalysts for the ROP of trimethylene carbonate (target DP 50) [31]. The appHcation of l,3-bis(2,6diisopropylphenyl)-imidazol-2-ylidene resulted in >99% monomer conversion within 30 min, producing polymers with predictable molecular weights and a narrow PDl (1.06). However, the less-hindered and more basic l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene produced polymers with much broader polydispersities (>2), albeit within a few seconds. 

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