Quantum stochastic resonance in parity violating chiral molecules

In order to explore parity violating effects in chiral molecules, of interest in some models of evolution towards homochirality, quantum stochastic resonance (QSR) is studied for the population difference between the two enantiomers of a chiral molecule (hence for the optical activity of the sample), under low viscous friction and in the deep quantum regime. The molecule is described by a two-state model in an asymmetric double well potential where the asymmetry is given by the known predicted parity violating energy difference (PVED) between enantiomers. In the linear response to an external driving field that lowers and rises alternatively each one of the minima of the well, a signal of QSR is predicted only in the case that the PVED is different from zero, the resonance condition being independent on tunneling between the two enantiomers. It is shown that, at resonance, the fluctuations of the first order contribution to the internal energy are zero. Due to the small value of the PVED, the resonance would occur in the ultracold regime. Some proposals concerning the external driving field are suggested.     

Pseudotetrahedral polyhalocubanes: synthesis, structures, and parity violating energy differences

All possible pseudotetrahedral, stable polyhalocubanes were prepared, and their structures were proven by NMR spectroscopy and X-ray crystallography. Parity violating energy differences (DeltaEpv) and vibrational frequency shifts were computed. The DeltaEpv values are predicted to be one to two orders of magnitude smaller than those for the corresponding polyhalomethanes. However, the DeltaEpv energy ordering is the same as that for the methane analogues. For both substance classes, the (S) isomers are, with the exception of the bromochlorofluoroiodo derivatives, more stable than the (R) forms.     

On the measurement of the parity violating energy difference between enantiomers

An experiment is outlined for measuring the small energy difference between two enantiomers due to the parity-violating weak neutral current perturbation. The method is based on the violation of the selection rules for the time evolution of states of well defined initial parity in isolated molecules. It could confirm or reject recent quantitative theoretical estimates of parity-violating energy differences.     

An efficient route to chiral alpha- and beta-hydroxyalkanephosphonates

Enzymatic kinetic resolution of alpha- and beta-hydroxyphosphonates in combination with ruthenium-catalyzed alcohol isomerization led to a successful dynamic kinetic resolution. A variety of racemic hydroxyphosphonates were efficiently transformed to the corresponding enantiomerically pure acetates (ee up to 99% and yield up to 87%).     

An alternative route to 4-benzoyl-2-azetidinones

Cyclodehydrohalogenation by terminal N₁-C₄ bond formation affords N-aryl-4-benzoyl-2-azetindinones from 3-benzoyl-3-bromopropionanilides in an unequivocal annelation process.     

An expeditious chiral route to analogs of mevinolin and compactin

A new and novel approach to the preparation of optically active analogs of mevinolin and compactin, in which the β-hydroxy-δ-lactone moiety is derived from D-glucose, is described.     

An alternative synthesis of Dolby-Weinreb enamine en route to cephalotaxine

A novel alternative synthesis of the Dolby-Weinreb enamine (2) was achieved from readily available amino dione 6 by a mild transannular Clemmensen-Clemo-Prelog-Leonard reductive rearrangement, which thus constitutes a formal total synthesis of cephalotaxine.     

An alternative synthetic route to compactin via a michael-alkylation sequence

The $\text{cis}$-octalinone 1, derived from p-benzoquinone and butadiene, was transformed via a Michael-alkylation sequence to the key triene precursor 5b in a total synthesis of the HMG-CoA reductase inhibitor compactin.     

High resolution spectroscopy of methyltrioxorhenium: towards the observation of parity violation in chiral molecules

Originating from the weak interaction, parity violation in chiral molecules has been considered as a possible origin of biohomochirality. We have proposed the observation of molecular parity violation using the two-photon Ramsey fringes technique on a supersonic beam. As a first step in this direction, a detailed spectroscopic study of methyltrioxorhenium (MTO) has been undertaken. It is an ideal test molecule as the achiral parent molecule of chiral candidates for a parity violation experiment. For the (187)Re MTO isotopologue, a combined analysis of Fourier transform microwave and infrared spectra as well as ultra-high resolution CO(2) laser absorption spectra enabled the assignment of 28 rotational lines and 71 rovibrational lines, some of them with a resolved hyperfine structure. A set of spectroscopic parameters in the ground and first excited state, including hyperfine structure constants, was obtained for the ν(as) antisymmetric Re=O stretching mode of this molecule. This result validates the experimental approach to be followed once a chiral derivative of MTO is synthesized, and shows the benefit of the combination of several spectroscopic techniques in different spectral regions, with different set-ups and resolutions. The first high resolution spectra of jet-cooled MTO, obtained on a set-up being developed for the observation of molecular parity violation, are shown, which constitutes a major step towards the targeted objective.     

Catalytic dissymmetrization of meso-2-imidazolidinones: alternative route to chiral synthons for 1,2-diamines

The chiral functionalization of a simple heterocycle, 1,3-dihydro-2-imidazolone, was achieved by the highly enantioselective monodeacylation of meso-1,3-diacetyl-2-imidazolidinones via an oxazaborolidine-catalyzed borane reduction. This kinetically controlled dissymmetrization is sufficiently effective to provide a synthetic route to either enantiomer of (4S, 5S)- or (4R, 5R)-4,5-dimethoxy-2-imidazolidinone derivatives, which serve as chiral synthons for threo-1,2-diamines.     

An alternative channel of reductive condensation of trichloromethylarenes with hydrazines

The reductive condensation of trichloromethylarenes with hydrazines can proceed without intermediate formation of pyridinium salts and without participation of pyridine in the reduction act. Variants of reductive condensation using hydrazines as reducting agents and α-chlorobenzylhydrazines and hydrazonoyl chlorides, nitrile imines, or hydrazonoylpyridinium salts as intermediates are considered, α-Chlorobenzylhydrazines and hydrazonoyl chlorides are shown to be the most probable intermediates.     

An alternative route to tethered Ru(II) transfer hydrogenation catalysts

A new route towards a series of tethered η⁶-arene/Ru(II) catalysts for use in the transfer and pressure hydrogenation of ketones and aldehydes to alcohols is reported. The route proceeds through the formation of an amide from the diamine precursor, followed by reduction, rather than the direct alkylation of the diamine. This has the advantage that dialkylation of the amine is avoided during the synthesis. Through this new route, both racemic and enantiomerically-pure η⁶-arene/Ru(II) tethered catalysts can be prepared in high yield.     

Condensation of enolates with O-ethyl pyrrolidonium salts: An alternative to the orthoamide condensation

O-Ethyl-N-substituted pyrrolidonium fluoroborate salts were condensed under basic conditions with active methylene compounds to give vinylogous carbamates and cyanamides.     

An enantioselective route to pyrrolidines: removal of the chiral template from homochiral pyrroloimidazoles

Two-step reductive removal of the chiral template from optically active pyrroloimidazoles, available from 1,3-dipolar cycloaddition of homochiral 4,5-dihydroimidazolium ylides, gives optically active substituted pyrrolidines. Selective manipulation of the substituents affords, e.g., naturally occurring and other optically active proline derivatives, and optically active pyrrolizidines and indolizidines.     

Chemoenzymatic dynamic kinetic resolution of beta-halo alcohols. An efficient route to chiral epoxides

Enzymatic resolution of beta-chloro alcohols in combination with ruthenium-catalyzed alcohol isomerization led to a successful dynamic kinetic resolution (conversion up to 99% and ee up to 97%). The efficiency of the DKR is dramatically reduced when beta-bromo alcohols are used. The presence of the bromo substituent causes decomposition of the ruthenium catalysts, which triggers the progressive deactivation of the enzyme. The synthetic utility of this procedure has been illustrated by the practical synthesis of different chiral epoxides.     

Theoretical determination of parity-violating vibrational frequency differences between the enantiomers of chiral molecules

A perturbation treatment has been used to compute the leading first- and second-order parity-violating corrections to the vibrational energy levels of a chiral molecule. Assuming the molecular equilibrium geometry as expansion point of both parity-violating and parity-conserving potential-energy surfaces, it is shown that these corrections, i.e., harmonic and anharmonic contributions, are of the same order of magnitude and that none of them can be neglected for a realistic determination of vibrational frequency differences. Numerical tests based on ab initio MP2 force fields and quantum-relativistic calculations of the parity-violating potential for each normal mode of PHBrF and AsHBrF molecules confirm this conclusion. In particular, it is shown that a normal mode of AsHBrF is characterized by one of the largest vibrational frequency difference ever found--the prediction is approximately 0.1 Hz--only one order of magnitude less than the presumed resolution limit of current experimental investigations.     

Base-catalyzed cross coupling of secondary alcohols and aryl-aldehydes with concomitant oxidation of alcohols to ketones: An alternative route for synthesis of the Claisen-Schmidt condensation products

Base-catalyzed C–C cross coupling of secondary alcohols and aryl-aldehydes was achieved, when an alcoholic solution of an aryl-aldehyde was stirred under reflux for 45 h in the presence of a catalytic (20 mol%) amount of K₂CO₃. The consistent formation of α,α′-bis-(benzylidene) alkanones was obtained in moderate to good yields using various secondary alcohols and substituted aryl-aldehydes. Herein, α,α′-bis-(benzylidene)alkanones, which are the classical products of Claisen-Schmidt (cross aldol) condensation, have been synthesized via an alternative strategy using secondary alcohols. Bis-(benzylidene) alkanones are an integral part of various drug regimes and the production of bis-(benzylidene) alkanones without using any precious metal is a major outcome of the present reaction.     

An alternative aqueous synthetic route to preparing CdTe quantum dots with tunable photoluminescence

An aqueous synthetic route has been developed for the preparation of mercaptosuccinic acid（MSA）-capped CdTe quantum dots （QDs） using TeO₂ as tellurium source and sodium borohydride as reductant.The size and the emission color of CdTe QDs can be tuned by varying the reflux time.The obtained QDs were characterized by photoluminescence（PL） spectroscopy,X-ray powder diffraction（XRD） and high-resolution transmission electron microscopy（HRTEM）.The results show that the CdTe QDs were of zinc-blende crystal structure in a sphere-like shape.     

An application of correlation energy density functionals to atoms and molecules

Four density functionals — including that recently introduced by Perdew ((1986) Phys Rev B33: 8822)—are tested for first-row atoms, hydrides and dimers. Calculated contributions of the correlation energy to the ionization potentials and electron affinities of atoms and to the dissociation energies of molecules are compared with empirical values which were reevaluated for this purpose. An improvement over Hartree-Fock is found in all cases if the self-interaction or the gradient correction are included in the density functional, although there is a rather large variation in the accuracy of the predictions.