Department of Chemistry
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Item Rhodium-Catalyzed Annulation of Vinylated Tyrosines with Internal Alkynes to Access Oxepine-Mounted Unnatural Tyrosines and Its Peptide Late Stage Functionalization(ACS, 2022-09) Sakhuja, RajeevA Rh(III)-catalyzed [5+2] annulation of vinyl tyrosines with symmetrical and unsymmetrical internal alkynes was achieved, furnishing a series of oxepine-mounted tyrosine-based unnatural amino acids. In addition, the chemical applicability of the developed strategy was exemplified by stapling amino acid/peptide-appended alkynes with vinyl tyrosines and late stage functionalization of tyrosine-containing dipeptides and tripeptide with internal alkynes.Item Photophysical Impact of Diacetylenic Conjugation on Classical Donor–Acceptor Electronic Energy Pair(ACS, 2018-12) Pati, Avik K.Organic fluorophores with extended π-conjugation are important for their widespread applications. The present work provides photophysical insights into a diacetylene bridged classical donor–acceptor electronic energy pair, naphthalene–pyrene, in comparison with its constituents’ molecular structures, naphthyl and pyrenyl acetylenes, as well as parent naphthalene and pyrene chromophores. The diacetylenic dye loses the individual spectral identities of the donor and acceptor fluorophores exhibiting a locally excited (LE) emission (∼411 nm) from the overall molecular entity with high fluorescence quantum yields (0.55–0.84) in nonaqueous media. In contrast to the parent pyrene, the hybrid derivative shows a strongly allowed S0 → S1 transition. In mixed-aqueous media, the dye forms aggregates displaying a new red-shifted absorption (∼425 nm) as well as emission (∼510 nm) band. Unlike the hybrid dye, the naphthyl and pyrenyl acetylenes do not form aggregates. In the aggregate state of the hybrid fluorophore, electronic energy transfer takes place from the naphthyl moiety to pyrenyl ring. The excited-state photophysical properties of the dye are exploited in vapor sensing in the solid state.Item Traceless Directing Groups in Radical Cascades: From Oligoalkynes to Fused Helicenes without Tethered Initiators(ACS, 2014-12-24) Banerjee, TanmayWe report the first example of a traceless directing group in a radical cascade. The chemo- and regioselectivity of the initial attack in skipped oligoalkynes is controlled by propargyl OR moiety. Radical translocations lead to the boomerang return of the radical center to the site of initial attack where it assists the elimination of the directing functionality via β-scission in the last step of the cascade. The Bu3Sn moiety continues further via facile reactions with electrophiles as well as Stille and Suzuki cross-coupling reactions. This selective radical transformation opens a new approach for the controlled transformation of skipped oligoalkynes into polycyclic ribbons of tunable dimensions.Item Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions for the Preparation of Expanded Polyaromatics(ACS, 2015-04-23) Banerjee, TanmayChemoselective interaction of aromatic enynes with Bu3Sn radicals can be harnessed for selective cascade transformations, yielding either Sn-substituted naphthalenes or Sn-indenes. Depending on the substitution at the alkene terminus, the initial regioselective 5-exo-trig cyclizations can be intercepted at the 5-exo stage via either hydrogen atom abstraction or C–S bond scission or allowed to proceed further to the formal 6-endo products via homoallylic ring expansion. Aromatization of the latter occurs via β-C–C bond scission, which is facilitated by 2c,3e through-bond interactions, a new stereoelectronic effect in radical chemistry. The combination of formal 6-endo-trig cyclization with stereoelectronically optimized fragmentation allows the use of alkenes as synthetic equivalents of alkynes and opens a convenient route to α-Sn-substituted naphthalenes, a unique launching platform for the preparation of extended polyaromatics.Item Gas-Phase Spectroscopy of Synephrine by Laser Desorption Supersonic Jet Technique(ACS, 2011-08-06) Chakraborty, ShamikIn our previous work, we found that synephrine has six conformers in the gas phase, while adrenaline, which is a catecholamine and has the same side chain as synephrine, has been reported to have only two conformers. To determine the conformational geometries of synephrine, we measured resonance enhanced multiphoton ionization, ultraviolet–ultraviolet hole burning, and infrared dip spectra by utilizing the laser desorption supersonic jet technique. By comparing the observed infrared spectra with theoretical ones, we assigned geometries except for the orientations of the phenolic OH group. Comparison between the determined structures of synephrine and those of 2-methylaminno-1-phenylethanol, which has the same side chain as synephrine but no phenol OH group, leads to the conclusion that the phenolic OH group in synephrine does not affect the conformational flexibility of the side chain. In the case of adrenaline, which is expected to have 12 conformers if there are no interactions between the catecholic OH groups and the side chain, some interactions possibly exist between them because only two conformations are observed. By estimation of the dipole–dipole interaction energy between partial dipole moments of the catecholic OH groups and the side chain, it was concluded that the dipole–dipole interaction stabilizes specific conformers which are actually observed.Item Cyclization of Oxa-Bicyclic Alkenes with β-Iodo-(Z)-propenoates and o-Iodobenzoate Catalyzed by Nickel Complexes: A Simple Efficient Route to Annulated Coumarins(ACS, 2003) Shukla, ParitoshIn the presence of Ni(dppe)Br2 and Zn powder in acetonitrile at 80 °C, oxa-bicyclic olefins undergo cyclization with o-iodobenzoate and with β-iodo-(Z)-propenoates to give the benzocoumarin derivatives in moderate to good yields. This methodology offers a simple efficient way for the synthesis of structurally complicate coumarins in one pot.Item Diazapentacene Derivatives as Thin-Film Transistor Materials: Morphology Control in Realizing High-Field-Effect Mobility(ACS, 2009) Shukla, Paritosh5,7,12,14-Tetrachloro-6,13-diaza-6,13-dihydropentacene (TCDAHP) and 5,7,12,14-tetrachloro-6,13-diazapentacene (TCDAP) were synthesized and assessed as the active channel materials for thin-film transistor applications. Analyses of the crystal structures of these molecules revealed that both exhibited slipped π−π stacking of the long and fused aromatic moiety. Although the packing features of the two compounds are basically identical, their highest occupied molecular orbitals, which are relevant to hole transport, are very different. Better mobility was predicted for TCDAHP over TCDAP based on the dimeric structure in the X-ray coordinates. The morphologies of thin films of TCDAHP and TCDAP prepared by thermal evaporation depend critically on the substrate on which the molecules were deposited: from the amorphous state on a SiO2/Si surface to the crystalline state on a pentacene buffer layer surface. The performance of thin-film transistors prepared on various substrate surfaces was studied. While no field-effect mobility was observed for these films deposited on SiO2/Si, a high mobility of 1.4 cm2/(V s) for the TCDAHP film was achieved when deposited on a pentacene buffer layer prepared on a rubbed monolayer of n-nonyltrichlorosilane on a SiO2/Si surface. A similar device prepared from TCDAP gave a mobility of 0.13 cm2/(V s).Item Microwave-Accelerated Solvent-Free Synthesis of Thioketones, Thiolactones, Thioamides, Thionoesters, and Thioflavonoids(RSC, 1999-07-29) Kumar, DalipAn expeditious, solvent-free, and high yield conversion of ketones, flavones, isoflavones, lactones, amides, and esters to the corresponding thio analogues is described utilizing Lawesson's reagent in a process that circumvents the use of dry solvents and excess of the reagent.Item A Simple and Efficient Synthesis of 2,3-Diarylnaphthofurans Using Sequential Hydroarylation/Heck Oxyarylation(ACS, 2013) Kumar, AnilAn efficient and simple strategy has been developed for the synthesis of 2,3-diarylnaphthofurans using sequential hydroarylation of naphthols and alkynes in the presence of In(OTf)3 under microwave irradiation followed by one-pot Heck-oxyarylation of generated 1-substituted-α-hydroxy styrenes.Item NEXT Access to Substituted Dihydrothiopyrano[2,3-b]indoles via Sequential Rearrangements During S-Alkylation and Au-Catalyzed Hydroarylation on Indoline-2-thiones(ACS, 2020-04-20) Kumar, AnilAn efficient methodology for the synthesis of indole-fused dihydrothiopyrans has been developed from indoline-2-thiones. The protocol involves the synthesis of conjugated ene-yne-substituted indole-sulfides, a gold(III)-catalyzed rearrangement of the ene-yne side chain followed by intramolecular hydroarylation via C3–H functionalization of the indole core. This new synthesis of functionalized tricyclic indole derivatives through sequential rearrangements is quite general in nature