One-step methylation of aromatic phosphorus heterocycles: synthesis and crystallographic characterization of a 1-methyl-phosphininium salt

Abstract

For the first time the direct synthesis of 1-methyl-phosphininium salts has been achieved by reacting aromatic λ3,σ2-phosphinines with the readily available dimethyl chloronium salt [(CH3)2Cl]+[Al(OTeF5)4]-. The remarkably high electrophilicity of...

Borane Adducts of Aromatic Phosphorus Heterocycles: Synthesis, Crystallographic Characterization and Reactivity of a Phosphinine‐B(C6F5)3 Lewis Pair

“…This compound shows a resonance at δ =156.8 ppm in the 31 P NMR spectrum, which is corroborated in the corresponding 1 H NMR spectrum with a coupling constant of 2 J P‐H =27.0 Hz to the alkyl CH. This is in line with the presence of the 1‐ R ‐phosphininium salt 12 , as the 31 P{ 1 H} NMR signal of the related 1‐methyl‐phosphininium cation can be observed at δ =160.2 ppm [9,21] . Most intriguingly, phosphininum salts can currently only be prepared in a multistep synthesis, or alternatively, with the very strong methylation reagent [(CH 3 ) 2 Cl] + [Al(OTeF 5 ) 4 ] − due to the low nucleophilicity of the phosphorus atom (see above, Figure 1).…”

“…Most intriguingly, phosphininum salts can currently only be prepared in a multistep synthesis, or alternatively, with the very strong methylation reagent [(CH 3 ) 2 Cl] + [Al(OTeF 5 ) 4 ] − due to the low nucleophilicity of the phosphorus atom (see above, Figure 1). [9,21,29] Our first results, depicted in Scheme 8, thus impressively show that various novel phosphininium salts might be easily formed by making use of phosphinine–borane adducts in combination with aromatic alkyl esters. To this end, 1‐ R ‐phosphininium salts can be generated and further explored, which are otherwise synthetically inaccessible.…”

“…[9,21] Most intriguingly, 1). [9,21,29] Our first results, depicted in Scheme 8, thus impressively show that various novel phosphininium salts might be easily formed by making use of phosphinine-borane adducts in combination with aromatic alkyl esters. To this end, 1-R-phosphininium salts can be generated and further explored, which are otherwise synthetically inaccessible.…”

“…This is in line with the presence of the 1-R-phosphininium salt 12, as the 31 P{ 1 H} NMR signal of the related 1-methyl-phosphininium cation can be observed at δ = 160.2 ppm. [9,21] Most intriguingly, 1). [9,21,29] Our first results, depicted in Scheme 8, thus impressively show that various novel phosphininium salts might be easily formed by making use of phosphinine-borane adducts in combination with aromatic alkyl esters.…”

“…[8] Recently we have shown direct methylation of phosphinine 2 by using the strong alkylating reagent [(CH 3 ) 2 Cl] + [Al(OTeF 5 ) 4 ] À (2-CH 3 ). [9] According to DFT calculations by Erhardt and Frenking, the bond dissociation energy of a phosphinineborane adduct is about 70 % that of a pyridine-borane adduct (25.8 vs. 35.6 kcal mol À 1 ). [10] Based on this calculated value, the formation of phosphinine-borane adducts should consequently be possible.…”