Abstract
A simple method for the synthesis of formate esters and formamides is presented based on the Au/TiO2-catalyzed aerobic oxidative coupling between alcohols or amines and formaldehyde. The suitable form of formaldehyde is paraformaldehyde, as cyclic trimeric 1,3,5-trioxane is inactive. The reaction proceeds via the formation of an intermediate hemiacetal or hemiaminal, respectively, followed by the Au nanoparticle-catalyzed aerobic oxidation of the intermediate. Typically, the oxidative coupling between formaldehyde (2 equiv) and amines occurs quantitatively at room temperature within 4 h, and there is no need to add a base as in analogous coupling reactions. The oxidative coupling between formaldehyde (typically 3 equiv) and alcohols is unprecedented and occurs more slowly, yet in good to excellent yields and selectivity. Minor side-products (2–12%) from the acetalization of formaldehyde by the alcohol are also formed. The catalyst is recyclable and can be reused after a simple filtration in five consecutive runs with a small loss of activity.
Highlights
Supported Au nanoparticles (Au NPs) have emerged during the past two decades as potent catalysts in several organic transformations
The key role of the support is to stabilize and protect Au clusters from leaking, aggregation, or against oxidation, which deactivates them in certain cases, but, in many reports, it is proposed that there is a synergy in the catalytic activity among metal oxide and Au NPs
Following the Au NPs-catalyzed aerobic oxidation of CO, the first major step towards their recognition as powerful catalysts was triggered by their application in the aerobic oxidation of organic compounds, such as alcohols, aldehydes, amines, alkenes, etc. [3,4,5,6,7,8]
Summary
Supported Au nanoparticles (Au NPs) have emerged during the past two decades as potent catalysts in several organic transformations. The surprising initial observation by Haruta’s group, that Au NPs are capable of oxidizing CO into CO2 even at −70 ◦C by atmospheric air [1,2], a reaction with great environmental significance, was the starting point of the extended growth of the applications of Au NPs in catalysis This first breakthrough observation established a metal (Au) that no one could foresee having any major catalytic activity, into the first row of exploration by many research groups worldwide. The second major step in the field of catalysis using Au NPs, which essentially expanded during the first decade of this century, was rather unexpected, as nanogold catalysts found catalytic applications in organic transformations beyond aerobic oxidations. Ahesmaisaecceotanldistasnkti,cwipeateexdte. nAdseadstehciosncdontacsekp,twtoe tehxetemndileddatnhdisfcaocinlecespytnttohtehseismofilfdoarmndamfaicdileessfyrnomthetshies ocof rfroersmpoanmdiidnegs afmroimnesthweitchoorurtesupsoinngdianngeaxmterinneasl bwaistehoasutinupsrinevgioauns erexpteorrntaeld baansaeloagsouins opxriedvaitoiuves rceopuoprltiendg apnroaltogcooluss. oxidative coupling protocols
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