Several transition metals are known for their ability to catalyze chemical reactions. The right choice of both the transition metal and the specific synthesis method for the catalyst is crucial to the efficiency for any given reaction. The project suggests suitable catalysts for synthesis of three groups of important compounds in organic chemistry; namely carboxylic acids, imines, and biaryls.
The efficiency of a catalyst is expressed through its capability to enhance both the kinetics of the reaction and the selectivity towards the desired product. A further property in high demand is the ability of the catalyst to be swiftly regenerated.
Firstly, a new methodology for the synthesis of carboxylic acids from primary alcohols and hydroxide was developed. The reaction is catalyzed by the ruthenium N-heterocyclic carbene complex [RuCl2(I/iPr)(p-cymene)] where dihydrogen is generated as the only by-product. Various substituted benzyl alcohols smoothly undergo transformation. Fast conversion to the carboxylic acids was achieved. The kinetic isotope effect of the reaction was determined to be 0.67 using 1-butanol as the substrate. A plausible catalytic cycle was characterized by DFT/B3LYP-D3 and involved coordination of the alcohol to the metal, β-hydride elimination, hydroxide attack on the coordinated aldehyde, and a second β-hydride elimination to furnish the carboxylate.
Secondly, the reaction between aryl halides and aryl Grignard reagents catalyzed by MnCl2 was extended to several methyl-substituted aryl iodide reagents by performing the reaction at 120 °C. A limitation of the hetero-coupling process is the concomitant dehalogenation of the aryl halide and homo-coupling of the Grignard reagent, which result in low to moderate yields of the desired hetero-coupling product. The mechanism of the cross-coupling process was investigated in two radical trap experiments. The employment of radical scavengers such as 1,4-cyclohexadiene and 4-(2-bromophenyl)-but-1-ene revealed the presence of an aryl radical intermediate. This leads to the proposal of an SRN1 pathway for the coupling.
Finally, a study of the dehydrogenative synthesis of imines catalyzed by simple and commercially available manganese complexes was conducted. Originally, the low valent CpMn(CO)3, Mn(CO)5Br, and Mn2(CO)10 complexes were employed for the coupling reaction between benzyl alcohol and cyclohexylamine. These displayed poor or no reactivity. Surprisingly, when the Jacobsen complex is used as the catalyst, the reaction between benzyl alcohol and cyclohexylamine resulted in 77 % yield of the corresponding imine. Moreover, gas evolution confirmed that the reaction occurs by dehydrogenation.
Illustration:
Dehydrogenation of a primary alcohol to the carboxylic acid.