Helical aromatics

A related approach has been used for the synthesis of poly(thiaacene)s, which are novel helical aromatic polymers comprised of fused benzothiophene rings, poly(thiaheterohelicene) <20050L755>. 

DFT investigation of the eight-electron electrocyclization reactions of 1,8-disubsti-tuted (3Z,5Z)-octa-l,3,5,7-tetraenes has found that these reactions proceed via a Mobius helical aromatic transition state. It was also found that outward substituents were preferred to inward ones, regardless of the electronic nature of the substituents, and that torquoelectronic effects are overridden by secondary orbital, electrostatic,... 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

Tanaka [96] described the preparation of helical aromatic compounds by electrophilic aromatic substitution of p-phenylenediamines with carboxylic acids, catalyzed by ZnCl2 (Scheme 5.49). Microwave irradiation led to a reduction in the reaction time from 9 h to 5 min, and a small increase in yield (10%) - except when low-boiling carboxylic acids such as acetic and propionic acid were used. On starting with optically pure (S)-174, the conventional heating reaction gave a racemic... 

The one-pot synthesis of helical aromatics, 13,14-dialkyldibenzo[fc,/] [4,7]-phenanthrolines (78) together with their stability against racemiza-tion and the assignment of absolute configuration assisted by experimental and theoretical circular dichroism were reported by Tanaka et al. (04JOC7794). For R = CH3, AH = 87.5kJ moC and AS = -24.1J mol (and not kJ moC as reported). The absolute configuration of these molecules was established on the basis of the CD exciton chirality method (exciton-coupled CD) together with theoretically calculated spectra. 

One-pot synthesis of helical aromatics 423 was achieved under MWI by the reaction of WA -diphenyl-p-phenylenediamine (421) and dicarboxylic acids 422 under solvent-free conditions in the presence of a Lewis acid. Compounds 423 were obtained within 5 min in 34-84% yields (Scheme 83), but conventional heating required 9h to give 18-80% yields (04JOC7794). 

Tanaka and co-workers in their work on helical aromatic compounds were able to decrease reaction time and control racemization when heated in a commercial microwave oven. When (5)-2-methylglutaric acid was reacted under classic heating conditions complete epimerization of the chiral methyl groups was observed. Using microwave heating a 50% enantiomeric excess was observed. The retention of stereochemistry was attributed by the authors to the markedly decreased reaction times. 

Watanabe K, Suda K, Akagi K (2013) Hierarchically self-assembled helical aromatic conjugated polymers. J Mater Chem C l(16) 2797-2805. doi 10.1039/c3tc00045a... 

Another major area of application of TDDFT involves clusters, large and small, covalent and metallic, and everything in between,as, for example, Met-Cars. Several studies include solvent effects, one example being the behavior of metal ions in explicit water. TDDFT in the realm of linear response can also be used to examine chirality,including calculating both electric and magnetic circular dichroism, " and it has been applied to both helical aromatics and to artemisinin complexes in solution. " There exist applications in materials and quantum dots, but, as discussed below, the optical response of bulk solids requires some nonlocal... 

One-Pot Synthesis of Helical Aromatics Stereoselectivity, Stability Against Racemization, and Assignment of Absolute Configuration Assisted by Experimental and Theoretical Circular Dichroism. 

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