Benzoylphosphonates with alcohols

The reaction of acylphosphonate diesters with alcohols was reported early to lead to dialkyl hydrogenphosphonates and carboxylate esters In a more recent systematic study, conditions were developed to use acylphosphonates for the acylation of alcohols. It was found that 1,5-diazabicyclo [5.4.0] undec-7-ene (dbu) is a highly effective catalyst for acylation of alcohols by acylphosphonates. Two special aspects deserve mention (1) /er butyl alcohol could be acylated with diethyl benzoylphosphonate, in the presence of dbu and 4-dimethylaminopyridine, to give r r/-butyl benzoate in 57% yield (2) the primary hydroxy group of a diol (e.g. butane 1,3-diol) could be acylated fairly selectively in the presence of a secondary hydroxy group by this methodology (ratio of mono to diacyl product =88 12). 

Benzoylation of alcohols.2 This reaction with 1 is markedly catalyzed by DBU, but not DMAP. Tertiary alcohols react slowly and it is possible to effect selective benzoylation of primary alcohols in the presence of secondary alcohols. The selectivity can be enhanced by use of diisopropyl benzoylphosphonate. 

Dialkyl acylphosphonates are used for acylation of alcohols. Kabachnik [236] reported that dimethyl benzoylphosphonate formed crystalline 1 1 addition products with aUphatic alcohols, which can be converted in the presence of dry hydrogen chloride into carboxylic esters and dimethyl H-phosphonate in moderate yields. Sakurai reported that dialkyl acylphosphonates underwent slow acylation with a large excess amount of alcohols to give esters [237]. It was shown that triethylamine catalyzed acylation of alcohols with dialkyl acylphosphonates [238]. It was found that use of l,5-diazabicyclo[5,4,0]undec-5-ene (DBU) increased the rate of the acylation of alcohols with diethyl benzoylphosphonate dramatically [239]. When one equivalent of DBU was used in CHjClj, the acylation was completed within 10 min and the corresponding ester 1 was obtained in 99% yield. The use of 0.1 equivalent of DBU gave 1 in 95% yield. Under this condition, the formation of 3 was suppressed remarkably and only a trace of 3 was formed. 

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