Palladium(II)-Catalyzed Oxidative Cyclization Strategies Selective Formation of New C-C and C-N Bonds
Abstract: The main focus of this thesis has been directed towards preparation and oxidative carbocyclization of en-, dien- and aza-enallenes.In the first part of this thesis, a stereoselective oxidative carbocyclization of dienallenes was realized. By employing cheap and readily available palladium trifluoroacetate we were able to efficiently cyclize a variety of dienallenes into hydroxylated carbocycles in high yield and high selectivity. This oxidative process was compatible with two different reoxidation protocols: one relying on p-benzoquinone (BQ) as the oxidant and the other employing molecular oxygen as the oxidant.In the second part of the thesis the carbocyclization methodology was extended to include carbocyclization of aza-enallenes. This was achieved in two distinct steps. First, a copper-catalyzed coupling of allylic sulfonamides with bromoallenes was developed, giving access to the corresponding aza-enallenes. Subjecting these substrates to catalytic amounts of palladium acetate, along with BQ as the oxidant, rendered N-heterocycles in good yield. The reactivity of these N-heterocycles towards activated dienophiles was later exploited in a tandem (aerobic) oxidative carbocyclization/Diels-Alder reaction.The third topic involves efficient oxidative arylative/borylative carbocyclization of enallenes. These reactions, catalyzed by palladium acetate, relies on transmetallation of a (?-alkyl)palladium(II) intermediate with diboranes or arylboronic acids. With this novel methodology we were able to obtain an array of arylated or borylated carbocycles, as single diastereomers, in high yield.Finally, we developed a palladium(II)-catalyzed cyclization of allylic carbamates. This mild, operationally simple, and scalable catalytic reaction opens up access to an array of oxazolidinones in high yield and excellent diastereoselectivity.
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