R.W J Zijlstra

Light-driven monodirectional molecular rotor

Attempts to fabricate mechanical devices on the molecular level have yielded analogues of rotors, gears, switches, shuttles, turnstiles and ratchets. Molecular motors, however, have not yet been made, even though they are common in biological systems. Rotary motion as such has been induced in interlocked systems and directly visualized for single molecules, but the controlled conversion of energy into unidirectional rotary motion has remained difficult to achieve. Here we report repetitive, monodirectional rotation around a central carbon–carbon double bond in a chiral, helical alkene, with each 360° rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system. We find that axial chirality and the presence of two chiral centres are essential for the observed monodirectional behaviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180° rotation around the carbon–carbon double bond and are each followed by thermally controlled helicity inversions, which effectively block reverse rotation and thus ensure that the four individual steps add up to one full rotation in one direction only. As the energy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic components for ‘molecular machinery’ driven by light.

Asymmetric 1,3-dipolar cycloadditions to 5-(R)-menthyloxy-2(5H)-furanone

Various diazo compounds, nitrile oxides, nitrones and azomethine ylides were examined in 1,3-dipolar cycloadditions to enantiomerically pure 5-(R)-menthyloxy-2(5H)-furanone 1a. Pyrazoline 9 was obtained in 100% c.y. as a mixture of 2 diastereoisomers in ratios up to 72 : 28, whereas pyrazoline 16 was obtained in 100 % c.y. as a single enantiomer. Photochemically pyrazolines 9 and 10 have been converted to cyclopropanes 11 and 13. Under thermal conditions pyrazoline 9 is converted to 4-methyl-5-menthyloxy-2(5H)-furanone. Isoxazoles 21a-24a were obtained enantiomerically pure via nitrile oxide addition to 1a in 64-67% yield. Nitrone addition afforded isoxazolidines 27, 28 and 34 with complete anti-facial- and regiochemistry, but with endo-exo selectivities up to 76%. Enantiomerically pure isoxazolidines were obtained in 25-75% yield. Pyrrolidine 36 was obtained diastereomerically pure in 81% c.y. Pyrrolidines 42 and 45, however, were obtained as diastereomeric mixtures in 37% resp. 6% yield.

Iodine-benzene charge-transfer complex: Potential energy surface and transition probabilities studied at several levels of theory

We present the results of detailed studies of the potential energy surfaces of the iodine)benzene charge-transfer complex obtained from fully counterpoise . . corrected ab initio calculations at the second-order Moller)Plesset MP2 level and from .semi- classical calculations. The most stable conformations found were the above-bond and the above-carbon conformations. The axial conformation was found to be somewhat less stable. The remarkable difference in intermolecular distance for different orientations of the iodine is explained in terms of the polarization anisotropy. This feature makes the construction of an accurate classical force field rather difficult because of the marked dependence of the repulsion parameter}usually the radius}for iodine on both orientation and polarization of the iodine. Investigation of the oscillator strengths of different complex geometries shows that there are many conformations in which the charge-transfer excitation can take place. Q 1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 709)723, 1999

(S)-2H-2-oxo-5,5-dimethyl-4(R)-phenyl-1,3,2-dioxaphosphorinane, a new reagent for the enantiomeric excess determination of unprotected amino acids using 31P NMR

Diastereomeric amide derivatives of title phosphorinane 2 and unprotected amino acids are easily prepared in aqueous solutions, showing well separated signals in the 31P NMR spectra allowing accurate e.e. determination.

A New 31P NMR Method for the Enantiomeric Excess Determination of Alcohols, Amines and Amino Acid Esters.

Diastereoisomeric ester and amide derivatives of phosphoric acid chloride 2 show well separated signals in the 31P NMR spectra allowing accurate e.e. determination of chiral alcohols, esters of amino acids and amines.

Many-body interactions calculated with the direct reaction field model

Abstract The direct reaction field (DRF) model was developed for calculations of electronic properties of molecules in the condensed phase. In the DRF approach the electrons of (part(s) of a system is described with wavefunctions, the larger parts classically with point charges and polarizabilities. Leaving out the quantum-mechanical part(s) leads naturally to a polarizable force field. In this paper we demonstrate the usefulness of the DRF model for the study of many-body interactions in polar systems. We have calculated the many-body interactions in clusters of HF, H2O and urea in our classical polarization model and compared the results to ab initio calculations using large basis sets. We find that the results obtained using the classical model compare excellently to ab initio results.

Unusual conformational aspects of some novel chiral non-racemic pyridinyl-2-phosphonates

Abstract Reaction of pyridinyl-2-phosphonyl dichloride (6) with 1-phenyl-2,2-dimethylpropane-1,3-diol (9) leads to the two epimeric 2-oxo-2-(2-pyridinyl)-4-phenyl-5,5-dimethyl-1,3,2-dioxaphosphorinanes (10a,b). These can be separated and the stereochemistry assigned on the basis of 31P NMR spectroscopy. For 10a the pyridinyl substituent is arranged axially at phosphorus. Arguments derived from 2D NMR experiments indicated that the nitrogen of pyridine is locked in a conformation whereby the pyridinyl nitrogen points over the six-membered ring; in other words it is locked between the two ring oxygen substituents. This conclusion is substantiated by an X-ray crystal determination. Oxidation of 10a with hydrogen peroxide leads to the N-oxide (12). The crystal structure of 12 reveals that despite serious steric overcrowding the NO bond is also oriented over the six-membered ring. Methylation of 10a with methyl trifluoromethanesulfonate affords the N-methyl pyridinium salt (13). NMR experiments indicate that in this case the methylated nitrogen has turned “outside” of the six-membered ring. The borane adduct of 10a appears on the basis of NMR data to have a conformation wherein the complexed borane is located just outside of the six-membered ring. Although crystal structures have not been obtained the pyridinyl-2-thiophosphonates (15a,b) obtained from treatment of 10a and 10b with [(4-MeOC6H4)2PS]2 appear to have the same conformational properties as 10a and 10b. Restricted Hartree-Fock geometry optimizations have been carried out to aid in clarifying this unexpected conformational behaviour. These calculational results are in excellent accord with the experimental observations, and provide insight into the reasons for the conformational behaviour.

Polarization of the excited states of twisted ethylene in a non-symmetrical environment

The polarization behavior of the low lying excited states in the vicinity of the perpendicularly twisted (D-2d) ethylene has been investigated in a quantum mechanical CISD approach, in which the quantum system was embedded in a polarized dielectric continuum modeling a non-symmetrical distribution of the solvent around the solute. The results show a strong polarization in the two lowest lying excited states in the region where the two vacuum energy surfaces of those states intersect, which strongly suggests that the lowering of the symmetry of the solvent shell can provide the adiabatic coupling for the avoided crossing between both potential energy surfaces. All examined values of epsilon, with the exception of the smallest investigated value(epsilon = 2.0), showed the polarization to keep intact on progressive twisting towards the perpendicular geometry.