Mechanistic insights on N-heterocyclic carbene-catalyzed annulations: the role of base-assisted proton transfers

The density functional theory investigation on the mechanism of NHC-catalyzed cycloannulation reaction of the homoenolate derived from butenal with pentenone is studied. The M06-2X/6-31+G** and B3LYP/6-31+G** levels of theory, including the effect of continuum solvation in dichloromethane and tetrahydrofuran, are employed. Several mechanistic scenarios are examined for each elementary step by identifying the key intermediates and the corresponding transition states interconnecting them on the respective potential energy surfaces. Both assisted and unassisted pathways for important proton transfer steps are considered, respectively, with and without the explicit inclusion of base (DBU) in the corresponding transition states. The barrier for the crucial proton transfer steps involved in the formation of the Breslow intermediate as well as in the subsequent steps is found to be significantly lowered by explicit inclusion of DBU. The energetic comparison between two key pathways, depicted as path A and path B, respectively, leading to cyclopentene and cyclopentanone derivatives, is performed. The major mechanistic bifurcation has been identified as emanating from the site of enolization of the initial zwitterionic intermediate resulting from the addition of a homoenolate equivalent to enone. If the enolization occurs nearer to the NHC moiety, the reaction is likely to proceed through path A, leading to cyclopentene. The enolization away from NHC leads to cyclopentanone product through path B. The computed results are generally in good agreement with the reported experimental results.     

Total synthesis of (+)-(2'S,3'R)-zoapatanol exploiting the B-alkyl Suzuki reaction and the nucleophilic potential of the sulfinyl group

A stereoselective synthesis of the diterpenoid oxepane (+)-zoapatanol is described. The key steps include a B-alkyl Suzuki cross-coupling reaction for the stereoselective synthesis of trisubstituted alkenes, creation of the two stereogenic centers on the oxepane ring by heterofunctionalization of an alkene through substrate control exploiting the nucleophilic potential of an intramolecular sulfinyl group, and transformation of a β-hydroxy sulfoxide into a terminal alkene.     

EPR and magnetic properties of vapour phase grown Zn1−xCrxTe crystals

The EPR and magnetic properties were investigated on vapour phase grown Zn_1−xCr_xTe (0.001?x?0.005) crystal samples at room temperature. The EPR spectra were observed for samples with x=0.001, 0.002 only. The simulations of the spectra confirm Cr³⁺ charge state of the dopant ion at tetrahedral symmetry. The magnetic behaviour of the samples with x=0.001 and 0.002 is neither that of Brillouin paramagnets nor Van Vleck systems while the samples with compositions x=0.003, 0.004 and 0.005 exhibited hysteresis behaviour.     

Ultracold Rydberg atoms for efficient storage of terahertz frequency signals using electromagnetically induced transparency

Quantum communication with terahertz (THz) frequency signals has many advantages like reduced attenuation and scintillation effects in certain atmospheric conditions along with very high level of data security. In this work, we propose a scheme to realize Quantum Memory (QM) for efficient storage of terahertz (THz) frequency signals using Electromagnetically Induced Transparency (EIT) in an ultracold atomic medium of ⁸⁷Rb Rydberg atoms prepared in a Two Dimensional Magneto Optical Trap (2D-MOT). The uniqueness of our scheme lies in the choice of the energy levels involved in the EIT process, all three of which have been chosen to be the Rydberg levels (enabling signal beam to be in THz) in a lambda type arrangement. This first of its kind proposal reveals that atomic media are a potential candidate for devising QMs which can store THz frequency signals. We have estimated that the Optical Depth (OD) in our scheme can reach a very high value of 690, very high maximum obtainable storage efficiency (η) of ～99%, the group velocity ($v_g$) can be as low as 5.07 × 10³ m/s, and the Delay Bandwidth Product (DBP) can be as high as 9.5. All of these estimates emphasize the feasibility of our scheme as a QM device for efficient storage of THz pulses.