Intramolecular Stereospecific Pummerer Reactions of Aryl (Substituted-methyl) Sulfoxides Bearing Electron-withdrawing Groups with Acetic Anhydride

Article abstract of DOI:10.1246/bcsj.56.257

The Pummerer reaction of optically active cyanomethyl aryl sulfoxides with acetic anhydride gave the corresponding α-acetoxy sulfides which were induced with a partial asymmetry nearly 30percent at α-carbon, while the 18O-label of the original sulfoxides was retained in more than 85percent in the resulting ester, the Pummerer reaction product.Kinetic experiments with cyanomethyl (p-substituted phenyl) sulfoxides and α,α-dideuterated cyanomethyl p-tolyl sulfoxide in the reaction with acetic anhydride containing a small amount of acetic acid revealed that the rates were correlated with Hammett ?-values and ρ-value of -0.70 was obtained, while the kinetic isotope effect was practically nil, i.e., k_H/k_D=1.01.These observations indicate clearly that the rearrangement is intramolecular and proceeds via forming a very intimate ion-pair and the rate-determining step is believed to be the S-O bond cleavage after the initial reversible acylation and deprotonation.Elimination of acetic acid proceeds through the Eleb type process.The uneven distribution of 18O in the two oxygens of the resulted ester, i.e., ether and carbonyl oxygens, suggests the recombination of the α-sulfenyl carbonium ion and acetate ion to be very rapid.The pummerer reaction of optically active N,N-dimethyl-p-tolyl-sulfinylacetamide with excess acetic anhydride also gave the corresponding α-acetoxy sulfide in optically active form.In the presence of di-cyclohexylcarbodiimide (DCC), the Pummerer reaction was found to be highly stereoselective affording the corresponding α-acetoxy sulfide with 65percent e.e.Other optically active α-carbonyl-substituted alkyl sulfoxides, i.e., ethyl p-tolylsulfinylacetate and ω-(p-tolylsulfinyl)acetophenone also gave the corresponding highly optically active Pummerer rearrangement products in the treatment with acetic anhydride/DCC system.The Pummerer reaction of the 18O-labeled sulfoxides with acetic anhydride/DCC system gave the corresponding α-acetoxy sulfides which retained much of the original 18O-labels of the sulfoxides.These observations reveal clearly that in the Pummerer reaction of these compounds the acetoxyl migration is intramolecular and takes place through intimate ion-pairs.