Bottom-bound metallacycles

In both side-bound and bottom-bound metallacycle orientations, disubstituted metallacycles can adopt one of two stereochemical configurations one where the substituents are syn to each other, which would lead to formation of the Z-alkene and... 

If one considers the bottom-bound metallacycle orientation observed in previous studies (A, Figure 4), kinetic selectivity for the -isomer can be rationalized on the basis of minimizing steric interactions both between the two metallacycle substituents and between the substituents and the chloride ligands. The difference in the initial E/Z ratio between the first- and the second-generation catalysts is less well understood. A potential explanation lies between the different distribution of steric bulk in the tricyclohexylphosphine ligand and the NHC ligand. This was invoked to rationalize the... 

Figure 19 (a) Electronic factors stabilizing side-bound over bottom-bound metallacycle. 

Computational studies have provided a valuable working hypothesis for the Z-selectivity exhibited by cyclometalated catalyst 6 [31, 71, 72], In contrast to the bottom-bound metallacycles observed with previous generaticMis of ruthenium metathesis catalysts (cf. Sect. 2.1), it is proposed that ruthenacycles derived from 4 and 6 adopt a side-bound conformation (7a). The ratimiale for the preferential formation of side-bound ruthenacycles is twofold (Fig. 1) first, there are significant steric interactions present between the developing metaUacylobutane and the adamantyl moiety in the bottom-bound conformation (7b) that are alleviated in the side-bound conformation. Moreover, the bottom-bound conformation is destabilized as it requires back-donation from the same ruthenium d-orbital that is back-donating into the NHC this competition is alleviated in the side-bound conformation, as two separate metal d-orbitals are now available for back-donation into both the NHC and alkylidene carbon p-orbitals (Fig. 1) [71]. 

Two distinct pathways have been proposed for the formation of the metallacyclobutane that differ in the orientation of the metallacycle with respect to the other ligands around Ru (Figure 2). In the bottom-bound pathway, metallacycle formation takes place with an olefin bound trans to the NHC, leading to a metallacycle on the opposite face to the NHC and the two anionic ligands (X) being trans to each other. Alternatively, in the side-bound pathway, metallacycle formation takes place with an olefin bound cis... 

Figure 5 Syn and anti geometries of bottom- and side-bound metallacycles...

Figure 4.5 Transition structures of the metallacycle formation for the side- and bottom-bound pathways, and representation of the backdonation. Reproduced with permission from R Liu et al.,J. Am. Chem. Soc., 2012, 134,1464-1467. Copyright (2012) American Chemical Society. ...

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