2,6-Diisopropylphenyl substituents

Once the NHC was introduced onto the ruthenium center, the addition of pyridine (10 equiv.) led to the 16-electron, monopyridine indenylidene complexes 54a-f (Scheme 14.24) [55, 56, 65, 66]. Interestingly, from a synthetic point of view, this preparation can also be achieved starting from (triphenylphos-phine)(indenylidene)ruthenium complexes, thereby avoiding the use of the costly and sensitive tricyclohexylphosphine [50,66]. The monopyridine complexes bearing mesityl substituents on the nitrogens were not very active at room temperature for the RCM of dienes and enynes, nor in the CM of methyl acrylate [66]. However, the controlled ROMP of norbornene-type monomers was allowed [65]. In contrast, the indenylidene complexes 54b and 54f, which bear 2,6-diisopropylphenyl substituents on the NHC and either a pyridine or 3-bromopyridine ligand, were... 

With steiically crowded substituents it is possible to obtain stannaimines, as shown in reaction 44. If R is a 2,6-diisopropylphenyl group, then compound 82 of reaction 37 is produced241. 

Dilithiosilanes with bulky substituents such as Tbt(Dip)SiLi2 (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl] phenyl, Dip = 2,6-diisopropylphenyl) and ()-Pr3Si)2SiLi2 have been reported recently. 

---