Alkenes iridium catalysts

The reversal of hydrogenation is also possible, as evidenced by the many iridium catalysts for alkane dehydrogenation to alkenes or arenes, though to date this area is of mainly academic interest rather than practical importance [19]. 

Transition metals can display selectivities for either carbonyls or olefins (Table 20.3). RuCl2(PPh3)3 (24) catalyzes reduction of the C-C double bond function in the presence of a ketone function (Table 20.3, entries 1-3). With this catalyst, reaction rates of the reduction of alkenes are usually higher than for ketones. This is also the case with various iridium catalysts (entries 6-14) and a ruthenium catalyst (entry 15). One of the few transition-metal catalysts that shows good selectivity towards the ketone or aldehyde function is the nickel catalyst (entries 4 and 5). Many other catalysts have never been tested for their selectivity for one particular functional group. 

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. 

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. 

A key feature of the mechanism of Wilkinson s catalyst is that catalysis begins with reaction of the solvated catalyst, RhCl(PPh3)2S (S=solvent), and H2 to form a solvated dihydride Rh(H)2Cl(PPh3)2S [1], In a subsequent step the alkene binds to the catalyst and then is transformed into product via migratory insertion and reductive elimination steps. Schrock and Osborn investigated solvated cationic complexes [M(PR3)2S2]+ (M=Rh, Ir and S= solvent) that are closely related to Wilkinson s catalyst. Similarly to Wilkinson s catalyst, the mechanistic sequence proposed by Schrock and Osborn features initial reaction of the catalyst with H2 followed by reaction of the dihydride with alkene for the case of monophosphine-ligated rhodium and iridium catalysts [12-17]. Such mechanisms commonly are characterized... 

In spite of the success of asymmetric iridium catalysts for the direct hydrogenation of alkenes, there has been very limited research into the use of alternative hydrogen donors. Carreira and coworkers have reported an enantioselective reduction of nitroalkenes in water using formic acid and the iridium aqua complex 69 [66]. For example, the reduction of nitroalkene 70 led to the formation of the product 71 in good yield and enantioselectivity (Scheme 17). The use of other aryl substrates afforded similar levels of enantioselectivity. 

The Dihydrido Iridium Triisopropylphosphine Complex [lrH2(NCMe)3(PPr3)]BF4 as Alkene Hydrogenation Catalysts... 

Iridium-phosphine complexes were found to be efficient carbonylative alkyne-alkene coupling catalysts [62]. Although frequently applied in other transformations, the dimeric complex [ Ir( x-Cl)(cod) 2] appeared to be a very active catalyst in the coupling of silylated diynes with CO [63], giving bicyclic products with a carbonyl moiety (Scheme 14.12). 

In contrast to reactions with vinyl epoxides and palladium catalysts, the reactions with rhodium retain the stereochemistry of the alkene fragment during the reaction [20]. This is illustrated by the reactions of trans-37a/h and cis-37a/b, which give only one product possessing the same olefin geometry as the starting epoxides (Eqs. 4 and 5). The retention of olefin stereochemisty has also been documented in allylic functionalizations with iridium catalysts, indicating that similar modes of action may be present [21, 22]. 

Mononuclear oxazolines are among the most effective ligands for enantioselective hydrogenation of nonfunctionalized alkenes." " The styrene substrate 597 is one of the most studied nonfunctionahzed alkenes used to evaluate the efficiency of new chiral ligands (Scheme 8.185). Selected examples of enatioselective hydrogenation of 597 using iridium catalysts are shown in Table g jg 359,425,426,457-459... 

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). 

A number of silyl enol ethers of acyl silanes have been produced from alkenes by subjection to 50 atmospheres of carbon monoxide in the presence of 0.1 equivalents of trialkylsilane and 2 mol% of an iridium catalyst (Scheme 26)102. Hydrolysis to the acyl silanes was achieved using hydrochloric acid-acetone. 

The hydroboration of stilbenes and related disubstituted alkenes catalysed by QUINAP complexes may proceed with high enantio- and regio-selectivity [(48) (49)] rhodium and iridium catalysts give the same regioisomer but opposite enantiomers.58... 

The rearrangement of aliyl ethers is not without its complications. For example, with less active catalysts that require elevated temperatures, adjacent azides can undergo [3+2]-cycloaddition on the alkene of the ally ether function before isomerisation occurs [Scheme 4.220],419 but this can be avoided by using a more active iridium catalyst that isomerises the alkene at room temperature, Iridium catalysts are also preferred over Rh owing to problems of competing reduction of the alkene which can occur with the latter (see above). 

There are transition-metal catalyzed addition reaction of alkyl units to alkenes, often proceeding with metal hydride elimination to form an alkene. An intramolecular cyclization reaction of an A-pyrrolidino amide alkene was reported using an iridium catalyst for addition of the carbon ot to nitrogen to the alkene unit. OS I, 229 IV, 665 VII, 479. 

This chemistry was extended to a number of bicyclic alkenes and dienes utilizing various chelating axially chiral bisphosphine iridium catalysts (Scheme 11.5) [29]. Further synthetic transformations of the chiral hydroamination product 13 provide access to functionally substituted chiral cyclopentylamines with multiple stereocenters, such as 14 and IS. It should be noted that alkylamines, such as octylamine or N methyl aniline, and sterically encumbered aniline derivatives, such as 0 toluidine or o anisidine did not undergo hydroamination reactions under these conditions. 

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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