Hoveyda-Blechert ruthenium

The Grubbs pyridine solvates are the fastest initiators of alkene metathesis and are valuable as synthetic intermediates to prepare other ruthenium carbene complexes. In particular, the 18-electron pyridine solvates 4a,b are very fast initiators that were developed to catalyze difficult alkene metatheses (e.g., the cross metathesis of acrylonitrile) [6]. The rates of initiation for several complexes are provided in Table 9.9. The pyridine solvate 4a has been found to initiate about 105 times faster than the parent Grubbs complex 2 and at least 100 times faster than the second-generation triphenylphosphine variant 26. When compared with the Hoveyda-Blechert complex 3a, 4a initiated about 100 times faster (c entry 3 vs. entry 5). The bromopyridine solvate 4b exceeded all of these in its initiation rate it was at least 20 times more reactive than 4a. 

Blechert et al. successfully introduced a desymmetrization approach. An asymmetric ring-opening cross metathesis of norbornenedicarboxylic anhydride was rendered possible by a Hoveyda- Blechert type ruthenium- carbene complex with a chiral N-heterocyclic carbene (NHC) ligand (Scheme 5.6). The chiral iirforma-tion of the NHC backbone was translated into ruthenium s active coordination sphere by the steric repulsion of an isopropyl group with a skewed ort/ro-biphenyl substituent [17]. 

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

A family of phosphine-free ruthenium-based olefin metathesis catalysts has been developed over the last few years. First, work done independently by Hoveyda and Blechert resulted in the H2Mes isopropoxybenzylidene (4b), a highly active air-stable ruthenium (pre)catalyst for olefin metathesis (Scheme 4). Hoveyda described (4b) as a recyclable monomeric catalyst also with high activity for ring-opening, ring-closure, and cross metathesis that tolerates... 

Synthesis of the phosphine free NHC-ruthenium complex 11 by the Hoveyda [34] and Blechert groups [35] in 2000 was another remarkable contribution (Figure 11.4). These complexes not only displayed improved tolerance to air, moisture, various functional groups, and thermal stabiUtjr compared to the previous ruthenium catalysts, but also showed better activitjr toward electron-deficient alkenes. 

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