The Reductive Cleavage of 1,3-Oxathiolanes With Lithium Aluminum Hydride in the Presence of Aluminum Chloride or Boron Trifluoride

Abstract

.A number of 1,3-oxathiolanes and l,3-dithiolanes were subjected t o reduction in ether solution by lithium aluminum hydride in the presence of either aluminum chloride or boron trifluoride. Both of these Lewis acids not only isomerized 1,3-oxathiolanes but also catalyzed their hydrogenolysis to the hydroxy thioethers. Under the same conditions neither aluminum chloride nor boron trifluoride had any effect upon 1,3-dithiolanes. However, in methylene chloride solution, 1,3-dithiolanes are isomerized by both Lewis acids although their hydrogenolysis was not achieved, probably due t o the insolubility of the hydride in methylene chloride.A mechanistic interpretation is discussed to explain the results obtained.

The major reducing species operative when an equimolar proportion of AlCl3 is added over a short period of time to an ether solution of equimolar proportions of LiAlH4 and acetal or ketal

“…The ~rocedure used (4)(5)(6) was to add over a shori period of time ( e l 0 min) an ether solution of AlC13 to a 'Taken in part from the theses of U. E. Diner and H. A. Davis to be submitted to the Department of Chemistry, University of Alberta, Edmonton, Alberta, in partial fulfilment of the requirements for the Ph.D. and M.Sc. degrees, respectively.…”

“…There has been considerable interest in the use of the "mixed reagent" AlCl3-LiAlH4 as a means of selective reduction of cyclic acetals and ketals (1)(2)(3)(4)(5)(6)(7). The nature of the reducing species in this "mixed reagent" has been uncertain.…”

The Number of Hydride Ions in AlH2Cl Available for Hydrogenolysis of 1,3-Dioxolanes

“…In support of this proposal, it is found that 1 is converted to 4 to the extent of 50% in 1 h by reaction with an equimolar proportion of (2-methy1thio)ethoxyhydridoaluminum chloride, the homolog and sulfur analog of 3. On the basis of the finding that 1'3-oxathiolanes are reductively cleaved at the C--0 but not the C-S bond by mixed hydrides (7,8) presumably because of the great preference of the Lewis acid to complex with the oxygen rather than the sulfur atom (8)' the reagent (2-methy1thio)ethoxyhydridoaluminum chloride is believed to prefer complex formation with the oxygen atom of 1 rather than with its own sulfur atom to form an internal complex such as A(X = S). Experimental…”