A flexible Pinner preparation of orthoesters: the model case of trimethylorthobenzoate

 

 

Dipartimento di Scienze Molecolari e Nanosistemi dell’Universita Ca’ Foscari Venezia, Centre for Sustainable Technologies, calle Larga S. Marta, 2137-30123 Venezia, Italy 
E-mail: marco.noe@unive.it

Green Chem., 2013,15, 2252-2260

DOI: 10.1039/C3GC40774H
Received 24 Apr 2013, Accepted 10 Jun 2013

 
The Pinner preparation of orthoesters was revisited achieving remarkable improvements. Outstandingly an unprecedented synthesis of trimethylorthobenzoate starting from benzonitrile has been described.
In the absence of additional solvents,
a novel procedure was implemented for the synthesis oftrimethylorthoesters through the Pinner reaction. At 5 °C, the reaction of both aliphatic and aromatic nitriles (RCN; R = Et, Bu, Ph) with a moderate excess of MeOH and gaseous HCl gave the corresponding imidate hydrochlorides [RC([double bond, length as m-dash]NH)OR′·HCl]
 
in excellent yields (>90%). At 25–65 °C, the methanolysis of alkyl imidate salts provided trimethylortho-propionate and valerate, while only traces of trimethylorthobenzoate (TMOB) were observed. However, the aromatic hydrochloride could be readily converted into the hydrogenphosphate salt [PhC([double bond, length as m-dash]NH)OR′·H3PO4] which, in turn, underwent a selective (>80%) reaction with MeOH to produce TMOB in a 62% isolated yield. This allowed for an unprecedented Pinner-type synthesis of TMOB starting from benzonitrile, rather than from the highly toxic trichloromethylbenzene. Overall, remarkable improvements in safety and process intensification were achieved.
 
 
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