Bromination of endo-7-norbornene derivatives revisited: failure of a computational NMR method in elucidating the configuration of an organic structure.

Previously we reported on the bromination of endo-7-bromonorbornene at different temperatures yielding mixtures of addition products. The structural elucidations of the formed compounds were achieved by NMR spectroscopy. Particularly, the γ-gauche effect and long-range couplings were instrumental in assigning the stereochemistry of the adducts. However, in a recent paper, Novitskiy and Kutateladze claimed that based on an applied machine learning-augmented DFT method for computational NMR that the structure of the product, (1R,2R,3S,4S,7s)-2,3,7-tribromobicyclo[2.2.1]heptane was wrong. With the aid of their computational method, they revised a number of published structures, including ours, and assigned our product the structure (1R,2S,3R,4S,7r)-2,3,7-tribromobicyclo[2.2.1]heptane. To fit their revised structure, they proposed an alternative mechanism featuring a skeletal rearrangement without the intermediacy of a carbocation. Herein, we are not only confirming the structure originally assigned by us through crucial NMR experiments, we also present the ultimate structural proof by means of X-ray crystallography. Moreover, we disprove the mechanism proposed by the aforementioned authors based on sound mechanistic reasoning and point to an oversight by the authors that led them to an erroneous mechanistic pathway.

Triplet Distribution in a Symmetrical Zinc(II) Porphyrin-BODIPY Pentameric Array.

A symmetrical molecular array has been synthesized comprising a central zinc(II) 5,10,15,20-tetraphenylporphyrin with identical boron dipyrromethene (BODIPY) units appended at each of the meso sites. Excitation of any subunit causes a cascade of electronic energy-transfer steps, ultimately leading to the BODIPY triplet-excited state in high yield. Coincidentally, the triplet energy levels of the zinc(II) porphyrin and BODIPY appendage are closely balanced such that an equilibrium is established at both 77 K and room temperature. This fast equilibration allows global distribution of the triplet exciton around the array, leading to a significantly increased capture volume for bimolecular quenching and a substantial increase in the rate constant for bimolecular triplet-triplet annihilation. The corresponding free-base porphyrin analogue does not favor triplet exciton decentralization because of the large disparity in the electronic energy levels.

Mutagenic assessment of three synthetic pyridine-diaryl ketone derivatives.

Nowadays, there are increasing numbers of studies about synthetic chemicals according to the supply demands of bioactive chemicals. The current study aims to investigate genotoxic potential of bioactive synthetic pyridine compounds, phenyl-3-pyridinylmethanone (1), p-tolyl-3-pyridinylmethanone (2), and 4-methoxyphenyl-3-pyridinylmethanone (3), using Ames/Salmonella and Escherichia coli WP2 bacterial reversion mutagenicity test systems. The mutant bacterial tester strains sodium azide-sensitive Salmonella typhimurium TA1535, 9-aminoacridine-sensitive S. typhimurium TA1537, and N-methyl-N'-nitro-N-nitrosoguanidine-sensitive E. coli WP2uvrA were used to detect the mutagenic potential of the test compounds. The results indicated that none of the test substances showed significant mutagenic activity on S. typhimurium TA1535, TA1537, and E. coli WP2uvrA bacterial strains up to 1 µg/plate concentrations.