Comments on the ring-opening polymerization of morpholine-2,5-dione derivatives by various metal catalysts and characterization of the products formed in the reactions involving R₂SnX₂, where X = OPrⁱ and NMe₂ and R = Buⁿ, Ph and p-Me ₂NC₆H₄

(3S,6S)-3-Isopropyl-6-methyl-morpholine-2,5-dione (1), and (3S,6S)-3,6-dimethyl-morpholine-2,5-dione (2), do not enter into ring-opening polymerization reactions with metal catalyst precursors commonly employed for lactides, and with Sn(ii) octanoate, only low molecular weight oligomers are obtained. Reactions with R₂SnX₂ compounds, where R = Ph, Buⁿ and p-Me₂NC₆H₄ and X = OPr ⁱ or NMe₂, reveal that ring-opening of the morpholine-2,5-diones does occur, but that polymerization is terminated by the formation of kinetically-inert products such as (Ph₂Sn[μ, η³-OCH(Me)CONCH(Prⁱ)COOPrⁱ])₂ (3), and ([Buⁿ)₂Sn[μ,η³-OCH(Me)CONCH(Me) CONMe₂])₂ (4), with elimination of HX. Ph ₃SnOPrⁱ is seen to react reversibly with morpholine-2,5-diones in toluene-d₈ by ¹H NMR spectroscopy while (Buⁿ)₃SnNMe₂ reacts by ring opening to give (Buⁿ)₃SnOCH(Me)C(O)NHCHMeC(O)NMe₂. The new organotin compounds have been characterized by ¹H, ¹³C(¹H) and ¹¹⁸Sn NMR spectroscopy and compounds 1, 2, 3 and 4 by single crystal X-ray crystallography.