Temperature dependence of the pitch in chiral lyotropic chromonic liquid crystals.

One of the most simple cases in which chirality at the microscopic level produces a chiral macroscopic structure is the chiral nematic liquid crystal phase. In such a phase, the preferred direction of molecular orientation rotates in helical fashion, with the pitch of the helix in different systems ranging from around 100 nm to as large as can be measured (∼10 mm). For almost all thermotropic and lyotropic liquid crystals, the ordered entities are formed from strong bonds, so the pitch varies in accordance with how the interactions between these largely immutable entities are affected by changing conditions. A unique exception are lyotropic chromonic liquid crystals (LCLCs) that spontaneously form weakly bound assemblies in solution, the size of which depends strongly on experimental parameters. While the temperature dependence of the pitch has been measured for chiral LCLCs formed by short strands of DNA (DNA-LCLCs), such is not the case for chiral LCLCs formed by small molecules. Polarized optical microscopy experiments on small molecule chiral LCLCs reveal the changing assembly size through a temperature dependence of the pitch not typical for many other systems, including the most recent measurements on DNA-LCLCs. In fact, the pitch measurements in small molecule chiral LCLCs strongly increase in value as the temperature is increased and the assemblies shrink in size. Theoretical considerations provide some help in understanding this phenomena, but leave much to be explained.

Diastereoselective spiroketalization: stereocontrol using an iron(0) tricarbonyl diene complex.

It has been demonstrated that an element of planar chirality can influence the formation of an adjacent spiroketal stereocenter. Appropriately functionalized enantiomerically pure 1- and 2-sulfinyl 1,3-dien-5-ones and their corresponding iron(0) tricarbonyl complexes (7, 17) have been prepared, and the derived spiroketals (8, 18) are made in good to excellent diastereoselectivity. A preliminary exploration of the combined effects of planar and central chirality upon the diastereoselectivity revealed matched and mismatched combinations (14).

Diastereoselective intramolecular pinacol couplings of sulfinyl iron(0) diene complexes.

3-(2-Formylaryl)-1-sulfinyl-(1Z,3E)-pentadien-5-al iron tricarbonyl complexes were prepared to examine the feasibility and diastereoselectivity of intramolecular pinacol couplings on such substrates. It was found that the pinacol coupling, promoted by VCl(3) x (THF)(3)/Zn, proceeded in good yield and with high diastereoselectivity (>23:1 dr), provided the 2-formylaryl unit remained unsubstituted at the aryl C3 position (ortho to the formyl group). In these latter cases the pinacol coupling was diastereorandom. A 3-formyl-4-(2-formylaryl)-1-sulfinyl-(1Z,3E)-butadiene iron tricarbonyl complex also underwent diastereoselective pinacol coupling (22:1 dr). 3-(3-Formylindolyl)-1-sulfinyl-(1Z,3E)-pentadien-5-al iron tricarbonyl complexes were also prepared, though pinacol coupling of these substrates proceeded in, at best, modest yield for two of the four examples tested. All cases described herein represent the first intramolecular pinacol couplings performed on the periphery of an iron(0) diene tricarbonyl complex.

Computational modeling of a stereoselective epoxidation: reaction of carene with peroxyformic acid.

Electronic structure theory was used to model the epoxidation of 3-carene by peroxyformic acid. Reactants, products, and transition states were optimized at the B3LYP/6-31G* level of theory, followed by B3LYP/6-311+G** and MP2/6-311+G** single point calculations. The reaction pathway yielding the trans-epoxide product was found to have a significantly lower reaction barrier (7.8 kcal/mol) than that leading to the cis-epoxide product (9.4 kcal/mol), in agreement with expectations. Magnetic shieldings of the two isomeric carene epoxides were also calculated, using the GIAO method, and compared to experimental (1)H and (13)C NMR spectra. Although the calculated carbon spectra proved inconclusive, the proton shieldings calculated for the trans-epoxide correlated much more closely to the experimental values for the major epoxidation product than did the shieldings calculated for the cis-epoxide, serving to verify the identity of the major product.

Simultaneous and stereoselective formation of planar and axial chiralities in enantiopure sulfinyl iron diene complexes.

[reaction: see text] Enantiopure (1Z,3E)-1-sulfinyl dienes bearing an o-dithianylphenyl group can be prepared and complexed with (bda)Fe(CO)(3) to afford the corresponding sulfinyl diene iron(0) tricarbonyl complexes. This diastereoselective complexation introduces planar and axial chirality simultaneously, with a high degree of facial selectivity as well as atropselectivity. Dynamic kinetic resolution is likely to be the origin of the atropselectivity.