Asymmetric ring-opening metathesis reactions

Asymmetric ring opening metathesis reactions can also be performed.67-66 In these cases, it is common for the reaction to be coupled with a cross metathesis or other reaction to avoid polymerization of the product.51 The catalysts are constantly being improved to provide higher selectivities and faster reactions. The catalyst 35 is recyclable and air-stable (Scheme 28.24).60-71... 

The appreciable levels of asymmetric induction observed in the catalytic ARCM reactions mentioned above suggest a high degree of enantiodifferentiation in the association of olefinic substrates and chiral complexes. This stereochemical induction may also be exploited in asymmetric ring-opening metathesis (AROM). Catalytic ROM transformations [20] offer unique and powerful methods for the preparation of complex molecules [2d, 2g]. The chiral Mo-alkyli-denes that are products of AROM reactions can be trapped either intramolecu-larly (RCM) or intermolecularly (cross metathesis, CM) to afford a range of optically enriched adducts. 

The molybdenum complex Mo(NAr)(CHCMe2Ph)[(3)-Me2SiBiphen] 43 was used for catalytic asymmetric olefin metathesis reactions such as desymmetrization of trienes, kinetic resolution of allylic ethers, tandem catalytic asymmetric ring-opening metathesis/cross-metathesis. Interestingly, tandem catalytic asymmetric ring-opening... 

Recently, Floveyda and Schrock have developed molybdenum-based catalysts (i.e. 744a,b) for an asymmetric ring-opening metathesis (AROM) reaction (Scheme 121) (98JA4041, 98JA9720, 99JA11603). Using a tandem AROM/RCM sequence, they examined the asymmetric synthesis of several heterocyclic compounds. For example,... 

Attempting to further enhance activity, various modifications on the naphtho-late and chelating isopropoxy benzylidene moiety were introduced and studied (Figure 11.33) [119]. Indeed, complexes 161-166 were found to be more efficient than 160 at low catalyst loadings for certain asymmetric ring-opening metathesis/cross-metathesis (AROM/CM) reactions. 

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. 

Crowe and Goldberg and Grubbs et al. have found conditions under which cross-metathesis (CM) of two different monoalkenes can give good selectivity for the desired cross-dimer over other products. Equation 14.95 shows an example that relies on the fact that acrylonitrile is inactive for self-metathesis but t es part in the cross-reaction. Differential reactiviQr of different carbene intermediates is also responsible for the selectivity of ring-opening metathesis Eq. 14.96 shows a ROM example by Snrqrper et al. Asymmetric ROM catalysis is also possible. ... 

---