Chemistry - Dissertation Defense

Old Dominion University

Room: Planetarium

 

 

 

 

 

 

 

 

 

Several challenges exist in the field of synthetic chemistry which often address problems of sustainability. The first portion of this presentation discusses the advancements made in sustainable oxidations of hydrazines to diazenes, as well as thiols to disulfides in the presence of alcohols and amines using Bobbitt’s salt. Computational investigations and experimental Hammett studies suggest that the oxidation of hydrazines with Bobbitt’s salt proceeds through a hydride transfer mechanism. In contrast, the observed chemoselectivity in the oxidation of thiols was found to arise from a novel mechanistic pathway. Combined experimental and computational studies suggests that the oxidation takes place via a polar two-electron process with concomitant and unexpected deoxygenation of the oxoammonium cation through homolysis of the N−O bond, which differs from prototypical radical-based thiol couplings.

            Organoboranes are versatile reagents in organic synthesis, and the second portion of this talk involves an operationally simple method to access 1,3-bis(boryl) dienes from parent 1,3-diones; and applying boronic acids and related derivatives in safer, sustainable cobalt-catalyzed carbonyl additions. The reactivity of the prepared 1,3-bis(boryl) dienes in [4+2] cycloadditions were investigated with common dienophiles in the presence of lanthanum(III) chloride bis(lithium chloride) and afforded diastereoselective access to borylated bicyclo[2.2.2]octenes in high yields. A chemoselective method for the 1,2-addition of organoboranes into aldehydes and ketones catalyzed by a bench-stable cobalt(III) complex was developed under mild aqueous conditions and will also be discussed to include: the scope of this catalytic transformation, key mechanistic underpinnings, and applications for the late-stage functionalization of bioactive molecules.

            The final portion of this talk details synthetic efforts toward the first total synthesis of the meloyunannines A-C and ent-sodwanone H. The meloyunnanines A-C contain an unprecedented 6/6/5/6/5/5-hexacyclic scaffold, and a novel synthetic strategy leveraging a key [4+2] cycloaddition of a masked enamine with a tethered cyclopentadienyl system is explored to provide rapid access to this scaffold. A biomimetic approach for the synthesis of ent-sodwanone H, a highly potent natural product, is pursued via a convergent route through coupling two key oxepane-cycloalkane and trans-decalin fragments. The oxepane-cycloalkane scaffold is forged through a boron trifluoride etherate-mediated cyclization and a titanocene-mediated radical cascade cyclization is employed to afford the trans-decalin fragment.

 

 

 

 

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