Aromatic ouroboroi: heterocycles involving a σ-donor-acceptor bond and 4n + 2 π-electrons

Structural Factors That Determine the Activity of the Xenobiotic Reductase B Enzyme from Pseudomonas putida on Nitroaromatic Compounds

Xenobiotic reductase B (XenB) catalyzes the reduction of the aromatic ring or nitro groups of nitroaromatic compounds with methyl, amino or hydroxyl radicals. This reaction is of biotechnological interest for bioremediation, the reuse of industrial waste or the activation of prodrugs. However, the structural factors that explain the binding of XenB to different substrates are unknown. Molecular dynamics simulations and quantum mechanical calculations were performed to identify the residues involved in the formation and stabilization of the enzyme/substrate complex and to explain the use of different substrates by this enzyme. Our results show that Tyr65 and Tyr335 residues stabilize the ligands through hydrophobic interactions mediated by the aromatic rings of these aminoacids. The higher XenB activity determined with the substrates 1,3,5-trinitrobenzene and 2,4,6-trinitrotoluene is consistent with the lower energy of the highest occupied molecular orbital (LUMO) orbitals and a lower energy of the homo orbital (LUMO), which favors electrophile and nucleophilic activity, respectively. The electrostatic potential maps of these compounds suggest that the bonding requires a large hydrophobic region in the aromatic ring, which is promoted by substituents in ortho and para positions. These results are consistent with experimental data and could be used to propose point mutations that allow this enzyme to process new molecules of biotechnological interest.

On Neutral Unsaturated Ouroboric Borylenes

The search is on for stable isolated borylenes. Potential roles in modern synthetic chemistry for boron analogues of carbenes continue to motivate interest in locating them. Using density functional and ab initio methods, we posit and examine the thermochemistry, and chemical bonding, including aromaticity, of several classes of 5- and 6-membered borylenic rings. In these systems, cyclization relies on dative bonding (ouroboric coordination) and π donation to a monovalent boron center from an adjacent O center. Certain neutral five-membered rings (heterocyclic cyclopentadienyl analogues) in particular are found to exhibit exceptionally strong preferences for the singlet multiplicity, each with singlet-triplet (S-T) gaps in excess of 40 kcal·mol^-1. The singlet five-membered rings with the largest S-T gaps and some of the six-membered rings show evidence of weak aromaticity. Relationships of the form N = A·r^-b, in line with Gordy's and other functions linking bond order, N, and covalent bond length, are identified for dative B←O contacts, r, reinforced in rings by π-delocalization.

Resonance-assisted intramolecular triel bonds

The possibility that the intramolecular Tr⋯S triel bond is strengthened by resonance is examined by quantum chemical calculations within the planar five-membered ring of TrH₂-CRCR-CRS (Tr = Al, Ga, In; R = NO₂, CH₃). This internal bond is found to be rather short (2.4-2.7 Å) with a large bond energy between 12 and 21 kcal mol^-1. The pattern of bond length alternation and atomic charges within the ring is consistent with resonance involving the conjugated double bonds. This resonance enhances the triel bond strength by some 25%. The electron-withdrawing NO₂ group weakens the bond, but it is strengthened by the electron-donating CH₃ substituent. NICS analysis suggests the presence of a certain degree of aromaticity within the ring.

Searching for double σ- and π-aromaticity in borazine derivatives

Inspired by the double-aromatic (σ and π) C6H3 +, C6I6 2+, and C6(SePh)6 2+ ring-shaped compounds, herein we theoretically study their borazine derivative analogues. The systems studied are the cation and dications with formulas B3N3H3 +, B3N3Br6 2+, B3N3I6 2+, B3N3(SeH)6 2+, and B3N3(TeH)6 2+. Our DFT calculations indicate that the ring-shaped planar structures of B3N3H3 +, B3N3I6 2+, and B3N3(TeH)6 2+ are more stable in the singlet state, while those of B3N3Br6 2+ and B3N3(SeH)6 2+ prefer the triplet state. Besides, exploration of the potential energy surface shows that the ring-shaped structure is the putative global minimum only for B3N3I6 2+. According to chemical bonding analysis, B3N3H3 +, B3N3I6 2+, and B3N3(TeH)6 2+ have σ and π delocalized bonds. The number of delocalized σ/π electrons is 2/6 for the first, and 10/6 for the second and third, similar to what their carbon analogs exhibit. Finally, the analysis of the magnetically induced current density allows B3N3H3 +, B3N3I6 2+, and B3N3(TeH)6 2+ to be classified as strongly σ aromatic, and poorly π aromatic compounds.