A study of the ring-opening of lactides and related cyclic esters by Ph2SnX2 and Ph3SnX compounds (X = NMe2, OR)

Abstract

Ph₃SnX reacts with L-lactide in the order X = NMe₂ ∼ OMe ≫ OPrⁱ > OBu^t in the initial ring-opening event. The rate of ring-opening of methyl substituted 1,4-dioxane-2,5-diones decreases with methyl substitution and ring-opening of 3,3,6,6-tetramethyl-1,4-dioxane-2,5-dione is not observed. From studies of the reaction between Ph₃SnOPrⁱ and L-lactide the activation parameters ΔH^≠ = 13(1) kcal mol⁻¹ and ΔS^≠ = −37(3) eu have been determined. Ph₃SnNMe₂ reacts with cyclic esters and propylene carbonate at low temperatures to give isolable ring-opened products. The compound Ph₃Sn[OCHMeC(O)OCHMeC(O)X] (where X = NMe₂, OMe) are labile in solution at room temperature, yielding Ph₃Sn[OCHMeC(O)X] and Ph₃Sn[OCHMeC(O)]_nX, where n ≥ 3, by transesterification, along with other minor products due to phenyl/OR group transfer (i.e., Ph₄Sn). Ph₂Sn(NMe₂)₂ and L-lactide react to give Ph₂Sn[OCHMeC(O)NMe₂]₂ by ring-opening of L-lactide followed by rapid amidation. A related compound, Ph₂Sn[OCHMeC(O)OPrⁱ]₂, is also formed in the reaction between Ph₂Sn(OPrⁱ)₂ and L-lactide but is more labile toward further ring-opening of L-lactide.