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Selective Syntheses Using Cyclodextrin as Catalyst. 1. Control of Orientation in the Attack of Dichlorocarbene at Phenolates
Komiyama, Makoto,Hirai, Hidefumi
, p. 2018 - 2021 (2007/10/02)
4-Hydroxybenzaldehydes, 2,4-dihydroxybenzaldehyde, and 4-(dichloromethyl)-2,5-cyclohexadienones were synthesized in virtually 100percent selectivities and high yields from the corresponding phenols and chloroform in alkaline aqueous solutions by using α- or β-cyclodextrin (α- or β-CD) as catalyst.The regulation of the molar ratio of chloroform to CD below unity throughout the reaction time, by controlling the rate of addition of chloroform, was definitely required to attain high selectivity.Under these conditions, dichlorocarbene, prepared in situ from chloroform and sodium hydroxide, attacked overwhelmingly at the para carbon of phenols with almost perfect suppression of the reaction at the ortho carbon.The structure of the ternary molecular complex composed of β-CD, chloroform, and phenol, formed in the reaction mixture for the selective synthesis of 4-hydroxybenzaldehyde, was determined by NMR spectroscopy.The selective catalysis by CDs was attributed to the regulation of molecular conformation of phenols with respect to chloroform, and thus to dichlorocarbene, in the ternary complex.
