Direct Atom-Efficient Esterification between Carboxylic Acids and Alcohols Catalyzed by Amphoteric, Water-Tolerant TiO(acac)₂

Abstract

A diverse array of oxometallic species were examined as catalysts for a test direct condensation of benzoic acid and 2-phenylethanol in 1:1 stoichiometry. Besides group IVB MOCl2-xH2O and TiOX2-xH2O, group VB VOCl2-xTHF and group IVB TiO(acac)2 were found to be the most efficient and water-tolerant catalysts for the test reaction. The new neutral catalytic protocol with the optimal TiO(acac)2 tolerates many stereo/electronic structural variations in both (di)acid (1 degree -3 degrees alkyl and aryl) and (di)alcohol (1 degree, 2 degrees alkyl, and aryl) components with high chemoselectivity.

A TFAA–H3PO4-mediated direct, metal-free and high-speed synthesis of aryl carboxylate esters from phenols

“…This can be achieved either via conversion of carboxylic acid to more reactive functional groups, such as anhydride [5][6][7] and acyl halide [8][9][10], or via in situ activation in the presence of coupling reagents. These include trifluoroacetic anhydride (TFAA) [11], 2-chloro-1-methylpyridinium iodide [12], BOP [13], DCC [14], PPE [15], N,N-bis(2-oxo-3-oxazolidinyl)phosphordiamidic chloride [16], CCl 4 /PPh 3 [17], diphenyl(1,2-benzisoxazol-3-yl)phosphate [18], Me 2 NSO 2 Cl [19], montmorillonite-Ti 4+ [20], metal triflates in [bmim]PF 6 [21], Mn(OAc) 3 [22], TiO(acac) 2 [23], diarylammonium arensulfonate [24], and several other condensing agents [25][26][27][28][29][30]. Some of these methods however, are associated with several drawbacks such as moderate yields, long reaction times, use of expensive reagents, and the use of volatile and environmentally harmful organic solvents.…”

Efficient method for the direct preparation of aryl carboxylates from carboxylic acids using tosyl chloride under solvent-free conditions

“…There are several reagents for the direct esterification of phenols under liquid-phase conditions, such as CCl 4 /PPh 3 [6], 2-chloro-1-methylpyridinium iodide [7], N,Nbis(2-oxo-3-oxazolidinyl)phosphordiamidic chloride [8], PPE *Corresponding author. E-mail: khalafi@chem.susc.ac.ir [9], TFAA (trifluoroacetic anhydride) [10], DCC [11], BOP [12], Me 2 NSO 2 Cl [13], diphenyl(1,2-benzisoxazol-3-yl) phosphate [14], montmorillonite-Ti 4+ [15], TiO(acac) 2 [16], Mn(OAc) 3 [17], metal triflates in [bmim]PF 6 [18], diarylammonium arensulfonate [19], and several other condensing agents [20]. These reported methods are associated with one or more drawbacks including moderate yields, long reaction times, use of relatively expensive reagents, and no agreement with green chemistry protocols by using volatile organic solvents.…”

One for Many: A Universal Reagent for Acylation Processes

“…Thec omplete removal of the DCPN carrier from the toluene solutionw as confirmed by 1 HNMR spectroscopy.T he DCPN carrierc ould be also isolated, at least in part, through recrystallization by placing the toluene solutioni nafreezer (see the Supporting Information for details). Thee asy isolation of the DCPN carrier makes the acylating agent reported in this workp romising for the developmento fa tom-efficient acylation reactions.A tom-efficiency is ac hallenging issue for acylation reactions: [15] for example, developing as ynthetic method for the formation of peptide bonds with high atom-efficiency is one of the most challenging issues to be resolved for pharmaceutical applications. [16] Them ethod reported herein has several attractive features: (i)i ti ss uitable for av ariety of substratesi ncluding amines,c arboxylic acids,a lcohols,a nd thiols; (ii)i tu ses carboxylic acids,w hich are easily accessible and can be synthesized straightforwardly;( iii)i td oes not require additives or catalysts;( iv) it generates the DCPN carrier as ab y-product, which is easily isolated by simple extraction with basic aqueous solution, and is thus recyclable.T he reported 2-acyloxy DCPN acylating agent facilitates coupling reactions for aw ide range of substrates.T his is particularly attractive for designingo ne-pot reactions comprising as erieso fr eactions (e.g.,c ascade reactions), andf or applications in flow chemistry [17] and combinatorial chemistry.…”