Reductions with Metal Hydrides. XIII. Hydrogenolysis of Hemithioacetals and Hemithioketals with Lithium Aluminum Hydride-Aluminum Chloride

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

“…Our results corroborate the findings of the previous workers (1,2) that in the reduction of 1,3-oxathiolanes the C-0 but not the C-S bond is ruptured. No base-soluble mercaptans were isolated.…”

“…I t is noteworthy that both isomers of 4-t-butylcyclohexanone ethylene hemithioketal, when reduced by A1Cl3-LiAlH4, were found t o give a high (86 to 92%) yield of the same isomer, trans-4-t-butylcyclohexyl /3-hydroxyethyl sulphide (1). This indicates that if a-halogenation were in fact a preliminary step, then either the a-halo ether must lose the halogen t o become a carbonium ion, a common intermediate from both isomers, which is then reduced stereospecifically, or a rapid equilibration of the oxathiolanes occurs (cf.…”

“…On the basis of this information, a mechanism for the hemithioketal reduction was proposed (1) involving first the formation of the resonance-stabilized sulphocarbonium ion which could not be reduced by the hydride but did permit isomerization. Reaction of this ion with the more nucleophilic chloride ion from the aluminum chloride, but not with the less nucleophilic fluoride ion from BF3, gave the a-halo thioether which in turn could be reduced easily by the hydride to the hydroxy thioether (equation (c)).…”

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

“…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.…”

Asymmetric Synthesis by the Reaction of Mercuric Acetate With Optically Active Thioacetals

“…Preferential C-0 cleavage of hemithioacetals and lcetals of the type has been demonstrated by Eliel and co-workers (13,14) and Brown (15) recently applied the reaction to other substrates and found that a1uminul.n chloride cleaved dioxolanes faster than oxathiolanes and dithiolanes not a t all. Preferential bonding of A13+ to oxygen to give C-0 cleavage is expected; I-Ig2+ would be expected to preferentially bond to divalent sulphur but the C-S bond in our 1-ethylthio-1-acetoqr I compounds was not cleaved.…”