Alkene Hydrogenation with Iridium Catalysts

N -heterocyclic carbenes as ligands in this process. In particular complex (2.115) where the oxazoline moiety has been replaced with an imidazolylidine group has been shown to be as effective as some PHOX ligands in the hydrogenation of ( )-arylethenes.  

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Tetrasubstituted alkenes are challenging substrates for enantioselective hydrogenation because of their inherently low reactivity. Crabtree showed that it was possible to hydrogenate unfunctionalized tetrasubstituted alkenes with iridium catalysts [46]. Among the iridium catalysts described in the previous section, several were found to be sufficiently reactive to achieve full conversion with al-kene 77 (Table 30.14). However, the enantioselectivities were significantly lower than with trisubstituted olefins, and higher catalyst loadings were necessary. 

Another series of achiral iridium catalysts containing phosphine and heterocyclic carbenes have also been tested in the hydrogenation of unfunctionalized alkenes [38]. These showed similar activity to the Crabtree catalyst, with one analogue giving improved conversion in the hydrogenation of 11. 

Aided by these developments, the past five years has seen a rapid growth in this area. A breakthrough was the introduction of iridium catalysts with chiral P,N ligands. A large number of new P,N and other ligands have been synthesized and applied to the hydrogenation of unfunctionalized alkenes. This chapter details the catalysts, conditions and substrates used in the enantiomeric hydrogenation of unfunctionalized alkenes. 

Only one paper has reported on catalytic asymmetric hydrogenation. In this study by Corma et al., the neutral dimeric duphos-gold(I)complex 332 was used to catalyze the asymmetric hydrogenation of alkenes and imines. The use of the gold complex increased the enantioselectivity achieved with other platinum or iridium catalysts and activity was very high in the reaction tested [195] (Figure 8.5). 

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 ). 

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