Study On A Novel Metal-free Reductive Esterification Of N-tosylhydranoes With Carboxylic Acids

N-tosylhydrazones are valuable synthetic intermediates, versatile reagents usedin transition-metal catalyzed cross-coupling reactions and general reagents to builduseful functionalities for synthetic compounds. N-tosylhydrazones involved twocomponents crosss-coupling reactions under metal-free conditions have attractedincreasing attention since they are more eco-benign and economical than that oftransition metal catalyzed reactions. To the best of our knowledge, only four reportsconcerning the use of tosylhydrazones with pinacolborane, arylboronic acids,alcohols, and thiols were documented. Inspired from those pioneer work, wehypothesized that carboxylic acids could be a decent nucleophile to attack the diazointermediate. Through this novel and straightforward insertion process，esters couldbe obtained under the metal-free conditions.To test our hypothesis,3-phenoxybenzoic acid and N-tosylhydrazones freshlyprepared1-（4-methoxyphenyl）ethanone were refluxed in1,4-dioxane with thepresence of K₂CO₃and1-（4-methoxyphenyl） ethyl3-phenoxybenzoate wassuccessfully prepared. We found that2equivalents of tosylhydrazones,4equivalents K₂CO₃, refluxed dioxane as solvent and18hrs continuous reacting timeoffered the best yield.We then screened the substrate scopes from aromatic to aliphatic carboxylicacids, found that aromatic carboxylic acids bearing electro-donating groupsexhibited better yields than that of with electro-withdrawing groups, aliphaticcarboxylic acids afforded medium yields to corresponding esters; the couplingreactions of carboxylic acids with N-tosylhydrazones generated from thecondensation of p-Toluenesulfonhydrazide with carbonyl compounds were carriedout to give medium to good yields, electro-donating effect promoted this novelcoupling reaction, and tosylhydrazones with aliphatic skeletons were also proven tobe good substrates for this esterification reaction. Proposal reaction mechanism,substrate scope and functional group compatibility of this reaction were alsoinvestigated on the basis of results. Analysis showed that a novel method forreductive coupling of N-tosylhydrazones with carboxylic acids under metal-freeconditions to access esters was developed. Various functional groups were found tobe tolerable under the reaction conditions to afford the corresponding esters withlow to good yields. This protocol provides a potential novel asymmetric synthesisapproach to chiral esters.