Catalysis of Organic..

332Rearrangement of 3,4-Epoxy-1-Butene to 2,5-DihydrofuranTable 2 Tri(organo)tin iodides tested.Tri(organo)tin iodide M.W. M.P. (°C) B.P. (°C (mm))Ph 3 SnI 476.91 121 253 (13.5)(n-Bu) 3 SnI 416.94 168 (8)(n-Oct) 3 SnI, TOT 585.26 215 (5)Thus the selected Lewis acid catalyst, TOT, has good catalytic activity,selectivity and stability. It is a non-viscous liquid which is compatible with TOP18and is miscible with 2,5-DHF and non-polar alkane solvents. It has a very low vaporpressure so it is not lost during product distillation and catalyst recovery operations.TOT is easily synthesized at low cost and it has low toxicity (Oral LD-50 (rat),>2000 mg/kg; Dermal LD-50 (rat), >2000 mg/kg).Final Catalyst SystemThe optimized catalyst system consisted of a 1:1 mole ratio of TOP18 and TOT(16). Fresh TOP18 is charged to the reactor as a melt or a THF solution (THF isstripped during start-up). No additional solvents are used in the reaction stage of the2,5-DHF process. The reaction mixture therefore consists of only reactant 1, 2,5-DHF product, catalysts and side products (crotonaldehyde and oligomer). Indeedbecause of the high loading of catalysts, the reaction mixture may be considered asan ionic liquid. Analysis of the reaction mixture is best performed using X-rayFluorescence Spectrometry (XFS, for % P, Sn, I, and Cl) and by quantitative carbon-13 NMR (for % catalysts and oligomer). Carbon-13 NMR is especially useful forobserving the condition of the organotin catalyst (17).The use of organotin reagents and reactions is well entrenched in organicsynthesis (19). Organotin alkoxides have been shown to be valuable in thepreparation of ethers (20). Heating 4-haloalkoxytributyltin compounds givesquantitative yields of tetrahydrofurans (21). In view of this precedent, Scheme 2shows the presumed catalytic mechanism of 1 rearrangement to 2,5-DHF.Scheme 2R 3 SnIO R 4 P + I -O+-1 R 3 Sn ICHOCrotonaldehydeI -IOSnR 3I - I -OSnR 3IIOSnR 3O2,5-DHFOmOligomerO n