Alkylphosphonium methylcarbonates, ylide precursors for halide- and base-free Wittig reactions

Abstract

The synthesis of a category of new phosphonium salts by methylation of phosphines is described. In particular triphenylmethylphosphonium methylcarbonate [PΦ,Φ,Φ,1][OCOOCH3] was obtained by methylation of triphenylphosphine (Ph3P) with dimethylcarbonate, while a set of alkylphosphonium methylcarbonate [PR,R,R,1][OCOOCH3] ionic liquids were synthesized by the methylation of the corresponding trialkylphosphines (R3P). The [PΦ,Φ,Φ,1][OCOOCH3] phosphonium salt was observed to possess significant P-CH3 proton acidity. In fact, deuterium exchange experiments showed the formation of the analogous PhP3-CD3 phosphonium salt. Spontaneous deprotonation of the methyl group lead to formation of the corresponding phosphorus ylide, Ph3P=CH2. This Ph3P=CH2 ylide was tested for the Wittig reaction with benzaldehyde PhCHO, generating the desired PhC=CH2 olefination product. It was noteworthy that this Wittig reaction protocol did not require an alkyl halide or a strong base for the formation of the ylide, and could be conducted in air, making it a greener procedure. The scope of the olefination reaction was extended to a number of carbonyl substrates, both aldehydes and ketones, with high conversions and selectivity. It was performed under mild conditions (34 – 80 °C), using a ratio ylide:carbonyl between 1.0 -3.0, in 2-methyl tetrahydrofuran (2-Me-THF) as solvent. The study was also extended to the other alkylphosphonium methylcarbonate ionic liquids [PR,R,R,1][OCOOCH3]. It was demonstrated that, depending on the reaction conditions, it was possible to achieve not only the transfer of a =CH2 fragment, but also the selective transfer of the bulkier alkyl group e.g. =CH(CH2)nCH3, giving access to a variety of olefins. Cis-trans selectivity was in the range 20-80.