Impact of Conjugation and Hyperconjugation on the Radical Stability of Allylic and Benzylic Systems: A Theoretical Study

Synthesis of Borylated Carbocycles by [2 + 2]-Cycloadditions and Photo-Ene Reactions

Saturated bicyclic compounds make up a valuable class of building blocks in the development of agrochemicals and pharmaceuticals. Here, we present the synthesis of borylated bicyclo[2.1.1]hexanes via crossed [2 + 2]-cycloaddition. Due to the presence of the C-B bond, a variety of structures can be easily prepared that are not accessible by other methods. Moreover, a rare photo-ene reaction is also disclosed, allowing for the diastereoselective synthesis of trisubstituted borylated cyclopentanes.

Chemical bonding in potential PFAS products from the thermal degradation of energetic devices, a DFT analysis

This work investigates stability and chemical bonding in possible per- and polyfluoroalkyl substances (PFAS) generated through the disposal of munitions in controlled detonations and open burns. Density functional theory (DFT) calculations were used to determine bond dissociation enthalpies (BDEs), activation energies, and other chemical properties. Calculated parameters were used to determine the functional groups most likely to be present based on the level of fluorination and the position of fluorines. In compounds that form C-O bonds, the presence of α-fluorines significantly strengthens the C-O bond by ∼4-18 kcal/mol. The results of this study indicate that fluoroalkyl alcohols are a very likely product of the disposal of munitions. This work was designed to expedite the analytical process of confirming that PFAS are created from current disposal methods of energetic devices by providing insight as to of what types of compounds should be expected. The PFAS generated in such reactions are expected to contain some functional groups (i.e., nitro and nitrite) that have not been known to exist as a result of the environmental degradation of industrially relevant PFAS, therefore, they may have been overlooked before. These initial results imply that PFAS with nitro functionalities may be formed in these conditions considering the abundance of NO2 radicals expected to be present as well as the strength of the C-N bond that can form (∼40-50 kcal/mol) whereas with nitroso functionalities are not expected to be found since the bonds formed are much weaker (∼25-35 kcal/mol), and nitrosoalkanes are known to decompose under mild conditions. Although these results are promising, analytical work is needed to assess the conclusions of this study in real systems.

Theoretical Study of Hydroxylation of α- and β-Pinene by a Cytochrome P450 Monooxygenase Model

Previous studies on biocatalytic transformations of pinenes by cytochrome P450 (CYP) enzymes reveal the formation of different oxygenated products from a single substrate due to the multistate reactivity of CYP and the many reactive sites in the pinene scaffold. Up until now, the detailed mechanism of these biocatalytic transformations of pinenes have not been reported. Hereby, we report a systematic theoretical study of the plausible hydrogen abstraction and hydroxylation reactions of α- and β-pinenes by CYP using the density functional theory (DFT) method. All DFT calculations in this study were based on B3LYP/LAN computational methodology using the Gaussian09 software. We used the B3LYP functional with corrections for dispersive forces, BSSE, and anharmonicity to study the mechanism and thermodynamic properties of these reactions using a bare model (without CYP) and a pinene-CYP model. According to the potential energy surface and Boltzmann distribution for radical conformers, the major reaction products of CYP-catalyzed hydrogen abstraction from β-pinene are the doublet trans (53.4%) and doublet cis (46.1%) radical conformer at delta site. The formation of doublet cis/trans hydroxylated products released a total Gibbs free energy of about 48 kcal/mol. As for alpha pinene, the most stable radicals were trans-doublet (86.4%) and cis-doublet (13.6%) at epsilon sites, and their hydroxylation products released a total of ~50 kcal/mol Gibbs free energy. Our results highlight the likely C-H abstraction and oxygen rebounding sites accounting for the multi-state of CYP (doublet, quartet, and sextet spin states) and the formation of different conformers due to the presence of cis/trans allylic hydrogen in α-pinene and β-pinene molecules.

Alcohols as Alkylating Agents: Photoredox-Catalyzed Conjugate Alkylation via In Situ Deoxygenation

The rapid exploration of sp3 -enriched chemical space is facilitated by fragment-coupling technologies that utilize simple and abundant alkyl precursors, among which alcohols are a highly desirable, commercially accessible, and synthetically versatile class of substrate. Herein, we describe an operationally convenient, N-heterocyclic carbene (NHC)-mediated deoxygenative Giese-type addition of alcohol-derived alkyl radicals to electron-deficient alkenes under mild photocatalytic conditions. The fragment coupling accommodates a broad range of primary, secondary, and tertiary alcohol partners, as well as structurally varied Michael acceptors containing traditionally reactive sites, such as electrophilic or oxidizable moieties. We demonstrate the late-stage diversification of densely functionalized molecular architectures, including drugs and biomolecules, and we further telescope our protocol with metallaphotoredox cross-coupling for step-economic access to sp3 -rich complexity.

Spontaneous Stereoselective Oxidation of Crystalline Avermectin B1a to Its C-8a-(S)-Hydroperoxide

Prolonged storage of technical abamectin as well as avermectin B_1a samples yielded a previously unknown derivative, designated here as compound 1. Detailed NMR analysis and X-ray crystallography allowed us to determine the structure of this compound and revealed the presence of a hydroperoxide group (-OOH) attached stereoselectively with configuration S to the C-8a carbon. This surprising result involves the formation of the peroxide bond in solid crystalline avermectin B_1a upon exposure to air with no involvement of light or recognized catalytic factors and is consistent with a topotactic mechanism for the oxidation reaction. Compound 1 is stable in the absence of reducing agents and has potential as a starting point in structural modification of the tetrahydrofuran ring of avermectin B_1a. It could also serve as a marker in assessing the quality of stored technical abamectin.