Bidentate Ruthenium(?)-NC Complexes As Catalysts For Dehydrogenation/Borrowing Hydrogen Reaction Of Alcohols

The development of new catalysts to promote actom ecomomy and avoid byprodcut is one of the research hotpots in organic synthesis,which is great significance for development of green chemistry.In recently years,the dehydrogenation/borrowing hydrogen reaction of alcohols has been focus of metal catalytic field and this strategy is environmental friendly with high atom economey.However,most of the dehydrogenation/borrowing hydrogen reaction is in relatively harsh catalytic conditions with poor reaction selectivity.The mojority of ligands are sensitive to air and difficult to be synthesized.Based on hereinbefore problems,a series of Ru(?)-NC catalysts have been designed and synthesized,which show highly catalytic activity in dehydrogenation/borrowing hydrogen reaction of alcohols.Five Ru(?)-NC complexes were synthesized by one-step processes from the corresponding NC ligands with RuHCl(CO)(PPh₃)₃.These complexes(1a-1e)were tested as catalysts for alcohol dehydrogenative reactions,and complex{(C₅H₄N)-(C₆H₄)}RuCl(CO)(PPh₃)₂(1a)showed the highest activity(0.5 mol%,6 h).With KOH as the nucleophile and 0.5 mol%catalyst loading,a series of carboxylic acids were synthesized in toluene without any oxidant.Compared with other reported catalysts,1a showed the highest TOF in toluene.Catalyst 1a could be transformed to complex{(C₅H₄N)-(C₆H₄)}RuH(CO)(PPh₃)₂(1f)when treated with KOH and benzyl alcohol.Complex 1f further reacted with Ph CHO and H₂O to generate product{(C₅H₄N)-(C₆H₄)}Ru(OCOPh)(CO)(PPh₃)₂(1g).Complexes 1f and 1g exhibited similar efficiency as complex 1a,suggesting they can be regarded as the catalytic intermediates of 1a.This strategy used primary alcohols as alkylating agents and water was released as the only by-product,which consistented with the requirements of green chemistry.Based on dehydrogenative reaction from primary alcohols to carboxylic acids,five Ru(?)-NC complexes(1a-1e)were tested as catalysts for?-alkylation of unactivated amides using alcohols as alkylating agents,and complex 1a showed the highest activity.With 0.5 mol%catalyst loading,a series of?-alkylated amides were isolated at 80 ^oC within 6 hours.Under similar conditions,complex 1a was also active for?-alkylation of unactivated esters with alcohols,and the reaction time was shortened to 1.5 hours.Furthermore,the TOF of 1a increased to two-(amides)and eight-fold(esters)compared to the reported best catalyst.Furthermore,five Ru(?)-NC complexes(1a-1e)were tested as catalysts for?-alkylation of secondary alcohols with primary alcohols.With 0.5 mol%catalyst loading at 120 ^oC for 1.5 h,complex 1a showed the highest activity.In this catalytic system,a series of secondary alcohols were synthesized using 1a as catalyst.At last,the Rucomplexes(1a-1d)were tested as catalysts for semihydrogenation of internal alkynes to E-alkenes with ethanol,and complex 1a also showed the highest activity.The reactions proceeded well with 1 mol%catalyst loading and 0.1equivalent of t-Bu ONa at 110 ^oC for 1 h,and 32 alkenes were synthesized with excellent E/Z selectivity.This is the first ruthenium-catalyzed semihydrogenation of internal alkynes to E-alkenes using ethanol as the hydrogen donor.These results suggest that bidentate Ru(?)-NC complex is a promising ruthenium catalyst and offers a fertile field for study.