Experimental and Theoretical Mechanistic Investigation of the Iridium-Catalyzed Dehydrogenative Decarbonylation of Primary Alcohols

Abstract

ABSTRACT:The mechanism for the iridium-BINAP catalyzed dehydrogenative decarbonylation of primary alcohols with the liberation of molecular hydrogen and carbon monoxide was studied experimentally and computationally. The reaction takes place by tandem catalysis through two catalytic cycles involving dehydrogenation of the alcohol and decarbonylation of the resulting aldehyde. The square planar complex IrCl(CO)(rac-BINAP) was isolated from the reaction between [Ir(cod)Cl] 2 , rac-BINAP and benzyl alcohol. The complex was catalytically active and applied in the study of the individual steps in the catalytic cycles. One carbon monoxide ligand was shown to remain coordinated to iridium throughout the reaction and release of carbon monoxide was suggested to occur from a dicarbonyl complex. IrH 2 Cl(CO)(rac-BINAP) was also synthesized and detected in the dehydrogenation of benzyl alcohol. In the same experiment, IrHCl 2 (CO)(rac-BINAP) was detected from the release of HCl in the dehydrogenation and subsequent reaction with IrCl(CO)(rac-BINAP). This indicated a substitution of chloride with the alcohol to form a square planar iridium alkoxo complex that could undergo a β-hydride elimination. A KIE of 1.0 was determined for the decarbonylation and 1.42 for the overall reaction. Electron rich benzyl alcohols were converted faster than electron poor alcohols, but no electronic effect was found when comparing aldehydes of different electronic character. The lack of electronic and kinetic isotope effects implies a rate determining phosphine dissociation for the decarbonylation of aldehydes.

Iridium-based hydride transfer catalysts: from hydrogen storage to fine chemicals

“…Interestingly, we do not observe decarbonylation of glyceraldehyde with the same catalyst. Tandem iridium catalyzed dehydrogenative decarbonylation of alcohols is known, 131,132 but we do not observe this potential side-reaction, probably due to the presence of KOH, which promotes conversion of the aldehyde intermediate to the corresponding hemiacetal. Another structural feature is the catalyst’s ortho -metalated pyridyl moieties.…”

Ruthenium‐Catalyzed Dehydrogenative Decarbonylation of Primary Alcohols

“…This rather modest value suggests that C–H cleavage in the aldehyde (e.g., by oxidative addition to ruthenium) is not the rate‐determining step and the KIE for the overall transformation therefore relates to the alcohol dehydrogenation. Although, these kinetic experiments do not lead to a detailed mechanism, the most likely scenario is a ruthenium(II) phosphine complex as the catalytically active species responsible for both the dehydrogenation and the decarbonylation in two separate catalytic cycles as determined for the corresponding iridium‐catalyzed transformation …”

“…The iridium‐catalyzed reaction was carried out with a 5 % catalyst loading in mesitylene at 164 °C while the rhodium‐catalyzed procedure used a 10 % loading in benzene at room temperature with a 450 W medium pressure Hg lamp . Recently, we presented a thorough mechanistic study of the iridium‐catalyzed dehydrogenative decarbonylation where the reaction was shown to go through two catalytic cycles (dehydrogenation and decarbonylation) with the square‐planar complex IrCl(CO)BINAP as the catalytically active species in both cycles . The [Ir(COD)Cl] 2 /BINAP system was also used for releasing dihydrogen and carbon monoxide, i.e., syngas, from diols and polyols in a two‐chamber system where the liberated syngas was utilized for hydroformylation of olefins or reductive carbonylation of aryl halides in the second chamber …”

Catalytic Asymmetric Synthesis of Cyclohexanes by Hydrogen Borrowing Annulations

“…Following an extensive search for possible binding modes of an enone to a model Ir I complex (for full details, see Supporting Information) the most stable was found to have both the carbonyl and alkene bound to the Ir center. The most stable [IrH( R ‐BINAP) 4 ] complex was then located (see Figures and S5 and Table S1 in the Supporting Information) . Si ‐coordination of 4 ( Si ‐INT0) is computed to be favoured by 4.8 kcal mol −1 over its Re counterpart ( Re ‐INT0).…”