In the project an iridium-catalyzed degradation of primary alcohols, where hydrogen and carbon monoxide were liberated as gases, was developed.
Homogeneous transition metal catalysis has revolutionized organic chemistry over the past 40 years. The fact that three of the last 12 Nobel Prizes in chemistry have been awarded to scientists in this field illustrates its importance. However, iridium has been in less focus in comparison with other transition metals. In this project a novel iridium-catalyzed pathway for degradation of primary alcohols was developed. The end product is mixed hydrogen and carbon monoxide, which is important in many industrial contexts.
In industry, the mixture of hydrogen and carbon monoxide is known as syngas. As carbon monoxide is a versatile carbon based feedstock, syngas is an interesting component in organic chemistry.
In the project an iridium-catalyzed degradation of primary alcohols, where hydrogen and carbon monoxide were liberated as gases was developed. The reaction was carried out with the complex [Ir(coe)2Cl]2 and the phosphine ligand BINAP in refluxing mesitylene. The catalytic system showed strong functional group tolerance as many different primary alcohols could be converted selectively. The reaction combined two known iridium-catalyzed transformations in the same sequence, i.e. dehydrogenation of an alcohol and decarbonylation of an aldehyde in independent catalytic cycles.
In a near stoichiometric reaction of [Ir(cod)Cl]2, (rac-)BINAP and benzyl alcohol, the complex IrCl(CO)(rac)-BINAP was formed and isolated in high yield. The complex was catalytically active and kinetically very close to the in situ formed system. The complex was applied to study the individual steps in the catalytic cycles. The cod ligand had been shown to thwart KIE experiments due to scrambling. The premade catalyst was devoid of the cod ligand and could therefore be applied to determine a KIE of 1.0 in the decarbonylation cycle and 1.42±0.07 in the overall reaction.
The complex was also applied to synthesize expected catalytic intermediates such as the dihydride isomers IrH2Cl(CO)(rac)-BINAP which were proven to reductively eliminate hydrogen under the reaction conditions. Furthermore, convincing results indicated that the carbon monoxide was liberated from a dicarbonyl complex.
An overall mechanism was suggested where HCl was formed in the dehydrogenation of the alcohol and could be oxidatively added to another IrCl(CO)(rac)-BINAP molecule forming IrHCl2(CO)(rac)-BINAP.
The project also investigated a two-chamber method with promising results where syngas is released from the iridium-catalyzed reaction in one chamber and then reacted in a palladium-catalyzed transformation in the other chamber.