Iridium catalysts alkenes, chiral complexes

Pfaltz has recently reported a new class of chiral phosphanodihydrooxazole-iridium catalysts 34 for the enantioselective hydrogenation of imines [38]. Based on Crabtree s success using similar achiral catalysts for the hydrogenation of normally unreactive tri- and tetrasubstituted alkenes [39], Pfaltz has now found that chiral phosphanodihydrooxazole-iridium complexes 34a-g will hydrogenate phenyl-substituted alkenes with high enantioselectivity [40]. As shown in Table 7, the trisubstituted alkene 35 can be hydrogenated under mild conditions (50 bar H2 at 23 C) with superior results using complex 34f (333 1 substrate to catalyst, >99% conversion and 98% ee, entry 6 ). 

The catalytic asymmetric hydroboration of 1,1-disubstituted alkenes R R C=CH2 with pinacolborane has been attained (<92% ee) by using complexes of iridium with chiral phospine-oxazolidines as catalysts (2.5 mol%). The same class of compounds has been shown to facilitate hydroboration (<98% ee) catalysed by chiral complexes of CuCl with a bidentate A)-heterocyclic carbene (NHC) containing SOg" as an additional ligating group. ... 

Over the last years, one of the most studied DCR has been the asymmetric version of the cycloaddition of nitrones with alkenes. This reaction leads to the construction of up to three contiguous asymmetric carbon centers (Scheme 4). The resulting five-membered isoxazolidine derivatives may be converted into amino alcohols, alkaloids, or p-lactams. Several chiral metal complexes have been used as catalysts for this process [13-15, 18-22]. However, the employment of iridium derivatives is very scarce. 

Iridium The intermolecular hydroamination of unactivated C=C bonds in ct-olefins (RCH=CH2) and bicycloalkenes (norbornene and norbornadiene) with arylamides (ArCONH2) and sulfonamides has been attained upon catalysis by chiral iridium complexes (PP)IrHCl(NHCOAr)(NH2COAr) [PP = chiral bidentate diphosphine]. Mechanistic studies identified the product of N-H bond oxidative addition and coordination of the amide as the resting state of the catalyst. Rapid, reversible dissociation of the amide precedes reaction with the alkene, but an intramolecular, kinetically significant rearrangement of the species occurs before the reaction with alkene. ... 

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