The enamino(triphenyl)phosphonium salts [Ph₃PCHCMeNR¹R²]⁺ Br^– [where R¹ = R² = (CH₂)₄, (CH₂)₅; (S)-CHMePh; R¹ = H, R² = (S)-CHMePh; R¹ = H, R² = (S)-(C>>|  >>H₂)₃-C>>|  >>H(CH₂OH)] have been synthesised and have been shown to react with an excess of borane·tetrahydrofuran to give novel azaboretidinium salts—the first examples of four-membered C–B–N–C heterocycles. The structure of [(1S,3R,4S,1′S)-4-methyl-1-(1-phenylethyl)-1,2-azaboretidin-1-ium-2-uid-3-yl]triphenylphosphonium bromide has been established by X-ray crystallography. Borane does not result in any significant stereoselectivity in these reductions and the azaboretidinium salts are mixtures of diastereomers. In contrast, the similar reduction with (R)-(+)-monoisopinocampheylborane or (S)-(–)-monoisopinocampheylborane leads to single diastereomers in high yields when R¹ and R² are non-bulky; with sterically demanding groups the azaboretidinium salts are unstable and decompose on work-up. Heating these azaboretidinium salts with aqueous sodium hydroxide in methanol, or better, aqueous sodium hydroxide alone, results in the direct formation of the phosphine oxides, Ph₂P(O)CH₂CHMeNR¹R² which, in the case of compounds derived from Ipc₂BH₂, have an ee value of >75%. In some cases, particularly when R¹ and R² are bulky, the use of sodium hydroxide in methanol results in an appreciable amount of rac-Ph₂PCH₂CHMe(OMe) as a by-product, but this can be avoided by carrying out the reactions in the absence of methanol.