Ruthenium carbene ligand conformation

Davies and co-workers have explored the role of ligand conformation in the ruthenium(II)-catalyzed cyclopropanation of styrene.10 This study was based on results reported by Nishiyama in which the catalyst prepared in situ from pyridine-bis(oxazoline) 62 and RuCI2(/>cymene) 2 was found to be highly active and selective in the reaction of ethyl diazoacetate with styrene (66% yield, 84% de, and 89% ee of major trans-isomer).52 Several ligands hindered on the oxazoline ring, including 3, were tested and poorer yields and selectivities were obtained (for 3, 50% yield, 81% de, and 59.5% ee of major trans-isomer), which indicated unfavorable steric interactions between styrene and the Ru(in-pybox) carbene complex (Scheme 17.22).10... 

Chiral ruthenium complexes, 8.391 and 8.392, have been used in a variety of asymmetric metathesis reactions, including ring-opening metathesis (Scheme 8.107, compare with Scheme 8.98) and cross-metathesis (Scheme 8.108). The chirality of the complexes resides both in the backbone of the carbene ligand by the placement of substituents, and also in the conformation of the aryl substituents on the carbene nitrogen atoms. 

For the catalytic activity of ruthenium-carbene complexes, the relative stability of the carbene conformations plays a pivotal role. After the dissociation of a victim ligand, followed by alkene coordination, the carbene has to adopt an active conformation. Only then can the new C-C bond in the ruthena(IV)cyclobutane evolve (green traffic light row in Figure 5.2) [19, 20]. 

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