Pinacol–Pinacolone Rearrangement Promoted by Polyphosphoric Acid Trimethylsilyl Ester (PPSE)

Abstract

Pinacols afforded the corresponding pinacolones in high yields in the presence of the polyphosphoric acid trimethylsilyl ester (PPSE) at a temperature above 80 °C. Tetraphenylethylene oxide, in addition to benzopinacolone, was obtained in the reaction of benzopinacol under milder conditions. The solvent effect suggested that the rearrangement proceeded via the carbonium-ion intermediate.

Synthesis and characterisation of pure isomers of iodoacetamidotetramethylrhodamine

“…Trimethylsilyl polyphosphate has been used previously as a combined Lewis acid and dehydrating agent (see ref. 13 and references therein) and has been particularly recommended to promote intramolecular Friedel-Crafts reactions. The crude 5-acetamidorhodamine 10 was hydrolysed with hot HCI-HOAc and the crude amine was treated with chloroacetyl chloride-DMF.…”

A new solid acid catalyst: the first phosphonate and phosphonic acid functionalised microporous polysilsesquioxanes

“…A range of solid acid catalysts have been reported to catalyse pinacol-pinacolone rearrangements; examples include metal substituted aluminophosphates, 8 lanthanide substituted zeolites, 9 heteropolytungstates, 10 various SAPOs (silicoaluminophosphates) 11 and polyphosphoric acid. 12 A reaction between P2 (0.2 g) and pinacol (6.0 g) at 140 °C (no solvent) resulted in 80% conversion to pinacolone after 12 h. Control experiments∑ using mesoporous or microporous silicas under the same conditions afforded no rearranged product. Attempted dehydration reactions using P2 (0.25 g) and cyclohexanol (25 cm 3 ) (C 6 H 11 OH+PO(OH) 2 = 60+1) at 120 °C resulted in about 15% conversion to cyclohexene ( 1 H NMR of the cooled reaction mixture) after 12 h. In control experiments using mesoporous or microporous silica under the same conditions no cyclohexene was produced.…”

Polyphosphoric Acid Trimethylsilylester: A Useful Reagent for Organic Synthesis

“…Several pinacolones 32 were prepared from the corresponding pinacols 30 in high yields in the presence of PPSE at a temperature above 80°C in 1,2-dichloroethane. 4 Additionally, tetraphenylethylene oxide (R 1 = R 2 = R 3 = R 4 = Ph) 31, was obtained with benzopinacolone in the reaction of benzopinacol under milder conditions (80°C).…”

“…This method Scheme 2 Proposed mechanism for the Pinacolic rearrangement of several pinacols in the presence of PPSE. 4 was also applied to the direct preparation of 3-trifluoromethyl-2H-1,2,4-benzothiadiazine 1,1-dioxide 49 from 5-bromo-2-aminobenzenesulfonamide 48, and gave a 6:4 mixture of the desired heterocycle 49 and the trifluoroacetylated benzenesulfonamide 50.…”

“…PPSE is essentially an aprotic liquid which possess a strong dehydrating power as well as being a solvent. 2 It is a synthetically useful reagent for the induction of a diversity of reactions such as: dehydration of amides into nitriles, 3 pinacolic rearrangement, 4 synthesis of heterocycles, 5,6 aldol-like condensations, 2 preparation of alkyl iodides from alcohols, 7 synthesis of dithioketals 8 and the formation of enynes and enediynes from propargyl alcohol. 9 Now commercially available, this flammable reagent, which is a mild irritant, is synthesised from phosphorus pentoxide and hexamethyldisiloxane (HMDS) in solvents such as benzene, chloroform, dichloromethane or toluene under reflux in an inert atmosphere.…”