I. Studies directed towards the synthesis of vicinal quarternary centers via trapping reactions of transient silenes. II. Synthesis and applications of chiral benzimidazolium salts in phase transfer catalysis.

摘要

The first object of this project was to investigate the reactivity of silene to create quaternary carbon centers. The strategy involved an in situ generation of silenes and their intramolecular trapping with pendant alkenes. The intermolecular trapping studies provided evidence of silenes generation under photochemical conditions. These studies included trapping reactions of silene with alkenes, carbonyls and alcohols. The intramolecular silene-alkene cycloaddition studies failed to provide the silacyclobutane due to unreactive nature of pendant alkene. These studies resulted in cis-trans isomerization of starting alkene and in competitive dimerization reactions. In intramolecular silene-carbonyl cycloadditions rate of silene trapping was higher as compare to silene-alkene cycloadditions due to dipolar nature of carbonyl compounds.; The second part of this project dealt with the asymmetric phase transfer reactions using chiral benzimidazolium salts as catalysts. For these studies chiral benzimidazolium salts were synthesized via chiral 1,2-benzenediamines. BINAP-Pd₂dba₃ catalyzed amination of o-dibromobenzene and monobromobenzene provided N,N′-disubstituted 1,2-benzenediamines in good to excellent yields. The amination was carried out stepwise and in one pot to give unsymmetrically and symmetrically substituted 1,2-benzenediamines. These chiral 1,2-benzenediamines have been successfully converted to benzimidazolium salts. It was found that these chiral heterocyclic salts were good for phase transfer reactions but did not provide satisfactory enantiomeric excess. The problem of low enantiomeric excess would be overcome by synthesizing new catalysts. Synthesis of these catalysts has been proposed in future studies.