Computational insight into the mechanism of selective imine formation from alcohol and amine catalyzed by the ruthenium(II)-PNP pincer complex

We have used density functional theory computations to investigate the mechanism of the reaction of an amine with a primary alcohol catalyzed by the ruthenium(II)-PNP pincer complex [PNP = 2,6-bis(di-tert-butylphosphanylmethyl) pyridine]; the reaction produces an imine as the major product. The catalytic cycle includes four stages: (stage I) alcohol dehydrogenation to aldehyde, (stage II) coupling of aldehyde with amine to form hemiaminal, (stage III) hemiaminal dehydration to give imine, and (stage IV) catalyst regeneration by means of H₂ elimination of the trans ruthenium dihydride complex produced in stage I. The mechanism is similar to that for amide formation from amine and alcohol that was catalyzed by the Ru^II-PNN pincer complex [PNN = 2-(di-tert-butylphosphanylmethyl)-6-(diethylaminomethyl)pyridine], the only difference being in stage III. Alcohol dehydrogenation (stage I) occurs by a bifunctional double hydrogen transfer mechanism and alcohol can facilitate stages II and III. The selectivity of imine over ester is governed by stage II: the formation of hemiaminal by means of aldehydeamine coupling is kinetically much more favorable than the alternative aldehydealcohol coupling reaction that yields hemiacetal. Furthermore, the hemiaminal dehydration to give an imine is also kinetically more favorable than the hemiacetal dehydrogenation to give an ester. The selectivity of imine over amide is determined by stage III: the hemiaminal dehydration to give an imine is kinetically much more favorable than the hemiaminal dehydrogenation to give an amide. The essential difference between the Ru^II-PNP-catalyzed imine synthesis and the Ru ^II-PNN-catalyzed amide formation is that the former prefers hemiaminal dehydration whereas the latter prefers hemiaminal dehydrogenation. In addition, water produced during hemiaminal dehydration can catalyze stages II and III more effectively than alcohol can. By contrast, the water-catalyzed hemiaminal formation does not happen in the Ru^II-PNN-catalyzed synthesis of an amide because no water is produced in any stage of the reaction.