Iridium-catalyzed dehydrogenation transfer reaction

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of benzene (184) has also been observed. Iridium compounds have also been shown to catalyze hydrogenation (185) and isomerization of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Alkane dehydrogenation took a step forward in 1996 with the report of rhodium and iridium pincer complexes that could catalyze transfer hydrogenation. While the rhodium complex was found to be active but unstable, the iridium complex was stable even after a week at 200 °C. This permitted it to efficiendy catalyze the transfer hydrogenation of cyclooctane to cyclooctene (12 t.o./min, Scheme The reaction is inhibited by high concentrations of olefin, either the... 

This reaction is also a transfer dehydrogenative reaction, as two reactant hydrogen atoms are not incorporated into the enol silyl ether product but instead serve to hydrogenate another molecule of starting alkene. For example, in the reaction of vinylcyclohexane, ethylcyclohexane is obtained in equal amounts to the silylated product. Both iridium complexes effectively catalyze the reaction. Various silanes can be used, including di-ethylmethyl-, triethyl-, and dimethylphenylsilane. The reaction is successful for a range of terminal alkenes, even those bearing cyano, acetal, and epoxide functionalities. The E isomer of the product is predominantly formed. 

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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