Oxidative Transformations of Allenes (Part I) and Enyne Ring-Closing Metathesis (Part II)
thesisposted on 01.07.2016, 00:00 authored by Venkata Reddy Sabbasani
Exploration of new reactivity of silyl-substituted allenes along with the corresponding alkyl-substituted allenes under conditions with various oxidants and/or enophiles to induce nitration, Alder ene reaction, copper-catalyzed oxidative dimerization, and iron-catalyzed oxidative transformations. More specifically, in nitration chemistry; nitration of silylallenes with nitrogen dioxide radical, generated from NaNO2 and AcOH, to form α-nitro-α,β-unsaturated silyl oximes and also using Fe(NO3)3·9H2O and FeCl3·6H2O, to form regioisomeric chloride-trapped products depending on the steric bulk of the silyl group. Another novel class of compound isooxazolidinones were obtained upon treating the initially formed α-nitro-α,β-unsaturated silyl oximes with TBAF. In dimerization chemistry; silylallenes with a catalytic system of copper(I) chloride and N-hydroxyphthalamide (N-Hpth) along with a stoichiometric amount of a terminal oxidant diacetoxyiodobenzene were favored mainly for the formation of dimer products with or without 1,3-enynes, and N-Hpth adducts. In Alder ene reaction, activation of allenic C(sp2)–H bond over an allylic C(sp3)–H bond was described. In this ene reaction linear silylallenes preferentially engage an allenic C(sp2)–H bond with high selectivity but cycloalkyl-substituted silylallenes showed low or reversed selectivity. On the other hand, non-silylated allenes engage allylic C(sp3)–H bonds favorably regardless of their structural feature. DFT calculations were provided for further insight into the selectivity trend. In iron-catalyzed oxidative transformations substituent and oxidant-dependent transformations of allenes are described using alkyl- and silyl-substituted allenes with DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) and TBHP (tert-butyl hydroperoxide). The reactions of non-silylated allenes involve allylic cation intermediate via forming C–O bond at the sp-hybridized C2 carbon and the reactions of silylated allenes favor the formation of propargylic cation intermediates via transferring the allenic hydride to the oxidant, generating 1,3-enynes or propargylic TBHP ethers. In addition, structure and reactivity of sulfonamide-, acetate-, alcohol, and alkene-chelated ruthenium alkylidene complexes derived from enyne RCM were investigated. All 5-membered sulfonamide chelates, hydroxy/ether chelates and 6-membered acetate chelates have the chelated oxygen and the NHC ligand in cis relationship. On the other hand, alkene-chelated ruthenium alkylidenes have N-heterocyclic carbene and chelated alkene in trans relationship. These newly generated variety of chelated complexes were further tested their metathesis activity as well as in some cases their behavior in presence of carbon monoxide were described. Also, the unexpected thermal bicyclization of metathesis substrates ynamide tethered triynes to generate novel class of tetrahydropyranopyridines were explored. Where the ynamide tethered triynes smoothly underwent 6-exo-mode ring-closure reaction by heating at 85 oC. The reaction has been explored by varying anion- and cation-stabilizing groups, including the ether side chain, and the alkyne substituent to provide broad range of tetrahydropyranopyridines.