Synthetic and Mechanistic Investigations of Some Novel Organophosphorus Reagents

Abstract

The alkoxytriphenylphosphonium ion intermediate of the Mitsunobu reaction for the esterification and inversion of configuration of an alcohol can be generated using the Hendrickson reagent, triphenylphosphonium anhydride trifluoromethanesulfonate, 27. While 27 was used in place of the Mitsunobu reagents (triphenylphosphine and a dialkyl azodicarboxylate) for the esterification of primary alcohols, the reaction failed with secondary alcohols such as (-)-menthol giving predominately elimination rather than the desired SN2 displacement. The difference between the two reactions was shown to be related to the more 'ionic' conditions generated when the Hendrickson reagent 27 was employed. An extreme sensitivity of the Mitsunobu reaction to the presence of salts was discussed and may indicate a mechanism involving ion pair clustering. Five-, six- and seven-membered cyclic analogues of the Hendrickson reagent 90-92 were prepared. A kinetic comparison of the cyclic analogues 90-92 revealed that a considerable increase in the rate of esterification could be achieved when the five-membered ring analogue 90 was used in a non-polar solvent such as toluene. Selected acyclic analogues of the Hendrickson reagent 27 possessing tributyl 118, tricyclohexyl 130 and diphenyl-2-pyridyl 137 functionalities were synthesised. However when 118, 130 and 137 were used for the attempted esterification of (-)-menthol, elimination was the major reaction pathway. Diphenyl-2-pyridylphosphonium anhydride triflate 137 was found to be a useful reagent for the synthesis of acyclic dialkyl ethers from primary alcohols. A polymeric version of the five-membered ring analogue 56, prepared by reaction of the polymer-supported 1,2-bis(diphenylphosphinyl)ethane 57 with triflic anhydride, was used for the preparation of simple esters and amides. A new dehydrating agent, polymer-supported triphenylphosphine ditriflate 157, was readily prepared from the oxidised form of commercially available polymer-supported triphenylphosphine and triflic anhydride. A wide range of dehydration-type reactions, such as ester, amide, anhydride, peptide, ether and nitrile formation, were performed in high yield using polymer-supported triphenylphosphine ditriflate 157. The reagent 157 was easily recovered and re-used several times without loss of efficiency. The use of 4-dimethylaminopyridine allowed the esterification of secondary alcohols with 157 to proceed without elimination and gave esters in high yield but with retention of configuration. Both reagents 56 and 157 provide an alternative to the Mitsunobu reaction, where the use of azodicarboxylates and chromatography to remove the phosphine oxide by-product can be avoided. However, the Mitsunobu reaction retains its supremacy for the inversion of configuration of a secondary alcohol. Preliminary investigations on the phosphityation of alcohols via the Hendrickson reagent 27, 1,3-benzodioxole formation using the Mitsunobu reaction and azodicarboxylate alternatives in the Mitsunobu reaction are described.