This year marks the 50th anniversary of Baird’s rules of aromaticity — a set of perturbational molecular orbital theory analyses that has garnered considerable attention in the past ten years in ...light of its many real-world applications in photochemistry.
Secondary metal cations, such as alkali and transition metal ions, have been shown to enhance the catalytic performance of nickel and palladium olefin polymerization catalysts. Their beneficial ...effects can manifest in different ways, such as increasing rates of polymerization, altering polymer microstructures, enhancing catalyst thermal stability, or a combination of these effects. We have systematically quantified secondary metal ion influences on nickel phenoxyphosphine polyethylene glycol (PEG) complexes. We demonstrate that cation tuning could readily achieve three-dimensional structures and electronic environments that are not easily accessible through conventional ligand tuning. This study led to the development of extremely active ethylene polymerization catalysts. For example, the nickel–lithium complex gave activity and turnover number as high as 7.0 × 104 kg PE/mol Ni·h and 2.5 × 106 mol ethylene/mol Ni, respectively, and the nickel–cesium complex showed unusual thermal stability up to 90 °C (activity = 2.3 × 104 kg/mol h, turnover number = ∼4.1 × 105 mol ethylene/mol Ni, and M n = 1.6 × 104 g/mol). We provide both experimental and computational data showing that secondary metals impact the relative stability of cis and trans isomers, which is a phenomenon not shown previously. Unlike in our earlier work, which was limited by poor nuclearity control and/or secondary metals that were too far from the catalyst center, the nickel phenoxyphosphine–PEG complex is an ideal platform for future studies of cation-controlled polymerization.
Whether tetra‐tert‐butyl‐s‐indacene is a symmetric D2h structure or a bond‐alternating C2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical ...shifts based on minima structures optimized at the M06‐2X, ωB97X‐D, and M11 levels confirm a bond‐localized C2h symmetry, which is consistent with the expected strong antiaromaticity of TtB‐s‐indacene.
The structure of tetra‐tert‐butyl‐s‐indacene is a computational challenge. Highly correlated methods and popular DFT functionals predict a bond‐delocalized D2h symmetry, but excellent agreement between experimental and computed proton chemical shifts suggests a true C2h geometry.
We propose a carbonyl umpolung strategy for activating tropone as a normal-electron-demand Diels–Alder diene. Tropone has low reactivity for Diels–Alder reactions because of its 4n+2 π-aromaticity. ...Conversion of the carbonyl group into a hydrazone ion (N–NR–) reverses the polarity of the exocyclic double bond, increases the 4n ring π-antiaromaticity, and raises the HOMO energy. Computed gas-phase activation free energies for a Diels–Alder reaction with maleimide suggest a billion-fold rate increase when the tropone CO is replaced by N–NR– (R = H or SO2CH3). Other nonbenzenoid aromatics can be activated as normal-electron-demand Diels–Alder dienes in the same way.
Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following ...electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π‐conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (CH…O, OH…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance‐assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented.
This article is categorized under:
Structure and Mechanism > Molecular Structures
Structure and Mechanism > Reaction Mechanisms and Catalysis
Hydrogen bonds are chemical interactions that can bind molecules and molecular fragments together to create elaborate structures and functions. In the past 30 years, many hydrogen bond design principles have emerged, and together they propel the field of supramolecular chemistry.
Many popular organic chromophores that catalyze photoinduced proton-coupled electron transfer (PCET) reactions are aromatic in the ground state but become excited-state antiaromatic in the lowest ππ* ...state. We show that excited-state antiaromaticity makes electron transfer easier. Two representative photoinduced electron transfer processes are investigated: (1) the photolysis of phenol and (2) solar water splitting of a pyridine–water complex. In the selected reactions, the directions of electron transfer are opposite, but the net result is proton transfer following the direction of electron transfer. Nucleus-independent chemical shifts (NICS), ionization energies, electron affinities, and PCET energy profiles of selected 4n and 4n + 2 π-systems are presented, and important mechanistic implications are discussed.
The selective modification of nitrogen heteroaromatics enables the development of new chemical tools and accelerates drug discovery. While methods that focus on expanding or contracting the skeletal ...structures of heteroaromatics are emerging, methods for the direct exchange of single core atoms remain limited. Here, we present a method for 14N → 15N isotopic exchange for several aromatic nitrogen heterocycles. This nitrogen isotope transmutation occurs through activation of the heteroaromatic substrate by triflylation of a nitrogen atom, followed by a ring-opening/ring-closure sequence mediated by 15N-aspartate to effect the isotopic exchange of the nitrogen atom. Key to the success of this transformation is the formation of an isolable 15N-succinyl intermediate, which undergoes elimination to give the isotopically labeled heterocycle. These transformations occur under mild conditions in high chemical and isotopic yields.
The Watson-Crick A·T and G·C base pairs are not only electronically complementary, but also photochemically complementary. Upon UV irradiation, DNA base pairs undergo efficient excited-state ...deactivation through electron driven proton transfer (EDPT), also known as proton-coupled electron transfer (PCET), at a rate too fast for other reactions to take place. Why this process occurs so efficiently is typically reasoned based on the oxidation and reduction potentials of the bases in their electronic ground states. Here, we show that the occurrence of EDPT can be traced to a reversal in the aromatic/antiaromatic character of the base upon photoexcitation. The Watson-Crick A·T and G·C base pairs are aromatic in the ground state, but the purines become highly antiaromatic and reactive in the first
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ππ* state, and transferring an electron and a proton to the pyrimidine relieves this excited-state antiaromaticity. Even though proton transfer proceeds along the coordinate of breaking a N-H σ-bond, the chromophore is the π-system of the base, and EDPT is driven by the strive to alleviate antiaromaticity in the π-system of the photoexcited base. The presence and absence of alternative excited-state EDPT routes in base pairs also can be explained by sudden changes in their aromatic and antiaromatic character upon photoexcitation.
The Watson-Crick A·T and G·C base pairs are not only electronically complementary, but also photochemically complementary.
We examine the effects of fusing two benzofurans to s‐indacene (indacenodibenzofurans, IDBFs) and dicyclopentab,gnaphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity ...and diradical character of these core units. Synthesis via 3‐functionalized benzofuran yields syn‐IDBF and syn‐IIDBF. syn‐IDBF possesses a high degree of paratropicity, exceeding that of the parent hydrocarbon, which in turn results in strong diradical character for syn‐IIDBF. In the case of the anti‐isomers, synthesized via 2‐substituted benzofurans, these effects are decreased; however, both derivatives undergo an unexpected ring‐opening reaction during the final dearomatization step. All the results are compared to the benzothiophene‐fused analogues and show that the increased electronegativity of oxygen in the syn‐fused derivatives leads to enhancement of the antiaromatic core causing greater paratropicity. For syn‐IIDBF increased diradical character results from rearomati‐zation of the core naphthalene unit in order to relieve this paratropicity.
Fusion of benzofuran to either s‐indacene or dicyclopentab,gnaphthalene results in two dramatically different outcomes depending upon heterocycle orientation. Whereas the “anti” isomers are unstable and hydrolyze readily to ring‐opened products, the “syn” isomers are quite stable and afford molecules that either possess a high degree of antiaromaticity or exhibit pronounced diradical character.
A first-of-its-kind enantioselective aromatic Finkelstein reaction is disclosed for the remote desymmetrization of diarylmethanes. The reaction operates through a copper-catalyzed C–I bond-forming ...event, and high levels of enantioselectivity are achieved through the deployment of a tailored guanidinylated peptide ligand. Strategic use of transition-metal-mediated reactions enables the chemoselective modification of the aryl iodide products; thus, the synthesis of a diverse set of otherwise difficult-to-access diarylmethanes with excellent levels of selectivity is realized from a common intermediate. A mixed experimental/computational analysis of steric parameters and substrate conformations identifies the importance of remote conformational effects as a key to achieving high enantioselectivity in this desymmetrization reaction.
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