Alkene metathesis, catalytic enantioselective

The enantioselective synthesis of azabicyclic y-lactams starting from 2-azanorbornenones after treatment of a catalytic amount of RuCl2(PCy3)2 (= CHPh) in the presence of ethylene or allyl acetate proceeds also via ring rearrangement—alkene metathesis (ROM-CM-RCM) [41] (Scheme 19). If n = 0 or 3, no RCM occurs and a cyclic dialkenyl compound is formed by cascade ROM-CM reactions. 

Hoveyda and co-workers have developed chiral catalysts for asymmetric alkene metathesis. They have demonstrated that with their chiral molybdenum catalyst asymmetric syntheses of dihydrofurans through catalytic kinetic resolution by RCM and enantioselective desymmetrization by RCM are feasible processes (Scheme 40) <1998JA9720>. The use of Schrock s molybdenum catalysts for asymmetric alkene metathesis has been reviewed <2001CEJ945>. 

Catalytic enantioselective alkene metathesis has recently been developed as a powerful method for the synthesis of complex natural products [86]. The availability of various chiral catalysts for olefin metathesis provides more flexible and concise means to construct efficiently highly functionalized and enantiomerically pure frameworks than using achiral catalytic complexes with chiral nonracemic substrates. 

Hoveyda and Schrock s laboratories have pioneered in this field and developed a number of effective chiral Mo-based catalysts for enantioselective alkene metathesis [28]. A recent apphcation of catalytic enantioselective alkene ROM/RCM in the total synthesis of (+)-africanol (135) was reported by Hoveyda et al. (Fig. 36) [87]. Treatment of meso tertiary TBS ether (136) with 3 mol% chiral alkylidene [Mo] catalyst (137) smoothly afforded the desired bicycle (138) in 97% yield and 87% ee. 

In summary, the development of catalytic enantioselective alkene metathesis has become a fascinating new direction for olefin metathesis. In this rapidly emerging field, several elegant applications in complex natural product synthesis have been reported to date. We can certainly expect that more active and robust catalysts will be developed and applied to target-oriented synthesis in the near future. 

Catalytic ring-closing metathesis makes available a wide range of cyclic alkenes, thus rendering a number of stereoselective olefin functionalizations practical. The availability of effective metathesis catalysts has also spawned the development of a variety of methods that prepare specially-outfitted diene substrates that can undergo catalytic ring closure. The new metathesis catalysts have already played a pivotal role in a number of enantioselective total syntheses. 

As an alternative, iridium complexes show exciting catalytic activities in various organic transformations for C-C bond formation. Iridium complexes have been known to be effective catalysts for hydrogenation [1—5] and hydrogen transfers [6-27], including in enantioselective synthesis [28-47]. The catalytic activity of iridium complexes also covers a wide range for dehydrogenation [48-54], metathesis [55], hydroamination [56-61], hydrosilylation [62], and hydroalkoxylation reactions [63] and has been employed in alkyne-alkyne and alkyne - alkene cyclizations and allylic substitution reactions [64-114]. In addition, Ir-catalyzed asymmetric 1,3-dipolar cycloaddition of a,P-unsaturated nitriles with nitrone was reported [115]. 

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