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    Process for the selective synthesis of salicylaldehydes
    4.
    发明授权
    Process for the selective synthesis of salicylaldehydes 失效

    公开(公告)号：US3972945A

    公开(公告)日：1976-08-03

    申请号：US535895

    申请日：1974-12-23

    申请人： Charles F. Albright

    发明人： Charles F. Albright

    IPC分类号： C07C45/00

    CPC分类号： C07C45/008

    摘要： The classical Reimer-Tiemann reaction for the synthesis of phenolic aldehydes has been modified from an aqueous to a non-aqueous system to provide an improved route for the formation of salicylaldehydes and, preferably, 3-substituted salicylaldehydes, e.g. 3-fluorosalicylaldehyde. Heretofore, compounds such as 3-substituted salicylaldehydes have proven to be extremely difficult to prepare in other than small laboratory quantities from the corresponding ortho-substituted phenol, since, in the final salicylaldehyde, each position ortho to the hydroxyl group contains substitution. In particular, with respect to 3-fluorosalicylaldehyde, one of the positions is occupied by the strongly electronegative fluorine atom so that the principal reaction product in the aqueous Reimer-Tiemann reaction has always been the para-isomer (3-fluoro-4-hydroxy-benzaldehyde), only negligible quantities of the desired ortho-isomer being obtained. In the process of the present invention, o-fluorophenol is caused to react with sodium hydroxide and chloroform in a hydrocarbon diluent (preferably benzene), maintaining the reaction under essentially anhydrous conditions by taking up the water of reaction with excess sodium hydroxide. It is necessary that an aprotic solvent, such as N,N-dimethylformamide, be employed as a catalyst. The use of boron oxide, while not absolutely necessary to the reaction, has been found to be advantageous in that the reaction proceeds more smoothly if the phenoxyboroxine is formed initially. The boron oxide also acts as a dehydrating agent and aids in removing the water of reaction. The preferential reaction product is the desired 3-fluorosalicylaldehyde, no paraisomer having been found. After hydrolysis and neutralization, the 3-fluorosalicylaldehyde can be recovered by azeotropic distillation along with a substantial quantity of unreacted o-fluorophenol which can be purified for recycle in subsequent preparations.