Iridium-Catalyzed Enantioselective Cycloaddition

Shibata, T., Arai, Y., Takami, K. et al. (2006) Iridium-catalyzed enantioselective [24-24-2] cycloaddition of diynes and monoalkynes for the generation of axial chiralities. Advanced Synthesis Catalysis, 348(16-17), 2475-2483. 

Takeuchi and coworkers reported an iridium-catalyzed cycloaddition of a,o -diynes and nitriles to give pyridines in 2012 [57]. With [Ir (cod)Cl]2/DPPF or BINAP as the catalytic system, pyridines were formed effectively (Scheme 3.24). A wide range of nitriles (aliphatic and aromatic nitriles) can be applied and reacted smoothly with Q ,a -diynes to give the pyridines. In the case of unsymmetrical diyne bearing two different internal alkyne moieties, high regioselectivity can be achieved which can be explained by the different reactivities of the a-position in iridacyclopentadiene. Terpyridine and quinquepyridine were prepared as well. [Ir(cod)Cl]2/chiral diphosphine catalyst can be applied to enantioselective synthesis. Kinetic resolution of the racemic... 

C2-symmetric axially chiral biaryl skeletons were constructed successfully via neutral iridium(I)/bisphosphine complex-catalyzed enantioselective double [2-I-2-I-2] cycloaddition [13]. The intermolecular cycloaddition of tetraynes 24,... 

Shibata T, Takasaku K, Takesue Y, Hirata N, Takagi K (2002) Iridium complex-catalyzed enantioselective intramolecular [4 - - 2] cycloaddition of dieneynes. Synlett 1681-1682... 

The synthesis of chiral racemic atropisomeric pyridines by cobalt-catalyzed [2 + 2 + 2] cycloaddition between diynes and nitriles was reported in 2006 by Hrdina et al. using standard CpCo catalysts [CpCo(CO)2, CpCo(C2H4)2, CpCo(COD)] [34], On the other hand, chiral complexes of type II were used by Gutnov et al. in 2004 [35] and by Hapke et al. in 2010 [36] for the synthesis of enantiomerically enriched atropisomers of 2-arylpyridines (Scheme 1.18). This topic is described in detail in Chapter 9. It is noteworthy that the 2004 paper contains the first examples of asymmetric cobalt-catalyzed [2 - - 2 - - 2] cycloadditions. At that time, it had been preceded by only three articles dealing with asymmetric nickel-catalyzed transformations [37]. Then enantioselective metal-catalyzed [2 -i- 2 - - 2] cycloadditions gained popularity, mostly with iridium- and rhodium-based catalysts, as shown in Chapter 9. 

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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