New α-substituted alkylbenzene- and dialkylbenzene-1,2-diphosphonates: side-chain metalation of tetraethyl 4-methyl- and 4,5-dimethylbenzene-1,2-diphosphonates

Carbocyclic 1,2-diphosphonates (1a, 1b) are prepared by the Diels–Alder reaction of classical donor alka-1,3-dienes (isoprene and 2,3-dimethyl-1,3-butadiene) with tetraethyl acetylene bisphosphonate. Their aromatization by the KMnO4–Al2O3 system affords 4-methyl and 4,5-dimethylbenzene-1,2-diphosphonates (2a, 2b), used as precursor for the generation of benzyl-type carbanions (3a, 3b) by lithiation with lithium isopropylamide in THF at −80 °C. The carbanions react with electrophilic reagents (chlorotrimethylsilane, p-fluorobenzaldehyde, and ethyl trifluoroacetate) in situ to form corresponding α-substituted monoalkyl- and dialkylbenzenediphosphonates in good yields.