Nico J. R. van Eikema Hommes

The effect of perfluorination on the aromaticity of benzene and heterocyclic six-membered rings

Despite having six highly electronegative Fs, perfluorobenzene C(6)F(6) is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C(6)F(6) and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the Fs induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C(6)F(n)H((6-n)), n = 1-6) have similar ring-LMO-NICS(pi zz) values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C(5)H(5)Y heterocycles (Y = BH(-), N, NH(+)) are as aromatic as benzene, based on their ring-LMO-NICS(pi zz) and ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O(+)) are involved.

Rational Design of Cation Hosts – Prediction of Cation Selectivity by Quantum Chemical Calculations

Summary Structures of [M ⊂ 2.2.2]n+ obtained with MNDO, AM1, PM3, PM3/SPASS, and PM5, are evaluated against X-ray and DFT (B3LYP/LANL2DZp) geometries. DFT and X-ray structures agree well, followed by PM3/SPASS. The ion selectivity calculated with DFT and PM3/SPASS for a simple model reaction, agrees satisfactorily with the available complex-formation constants (log KS). The applied theoretical treatment has a good predictive potential.

A Domino Aldol Addition/Hemiketal Formation/Hemiketal Formation/Epimerization Route to a Heteroadamantane. A Crystal-Structure, NMR, and Computational Study

It has been found by serendipity during the attempted synthesis of the potential tris-bidentate ligand 7 that this compound undergoes multiple ring-closure reactions to form the heterodamantane derivative 12. This reaction involves a domino aldol addition/hemiketal formation/hemiketal formation/epimerization sequence. Compound 12 was studied intensively by X-ray crystal-structure analysis, NMR, and AM1 computations. Complete assignment of all 1H- and 13C-NMR signals was achieved by a combination of HMQC, HMBC, DPFGSE-NOE, COSY, and long-range-COSY experiments. The NMR data agreed well with the crystallographic and computational results. Accordingly, 12 is present as the thermodynamically most-stable diastereoisomer with relative u-configuration at centers C(8) and C(9). In summary, five stereogenic centers were created starting from an achiral precursor in an efficient cascade reaction under thermodynamic control.

Protonen on Tour – DFT-Studie zur H+-Wanderung in [1.1.1]- und [2.2.2]-Cryptanden / Protons on Tour – DFT-study of H+-Migration in [1.1.1]- and [2.2.2]-Cryptands

DFT-calculations (RB3LYP/LANL2DZp) show that the migration of a proton inside [1.1.1]- and [2.2.2]-cryptand from one nitrogen atom to the other follows different paths. While the proton in [H⊂1.1.1]-cryptand moves via an ether oxygen atom (activation energy: 19.2 kcal/mol), the proton in [H⊂2.2.2]-cryptand moves directly from one nitrogenatom to the other (activation energy: 16.1 kcal/mol). Our calculations rule out the application of doubly protonated [2.2.2]-cryptands as anion hosts.

Novel Nine-Membered Titanaheterocycles – Structure, ab initio Calculations, and Preparative Use towards the Selective Synthesis of Substituted Cyclopentanols

Reactions of Cp2Ti(CO)2 (3) with two equivalents of α,β-unsaturated ketones 4 yield the novel titana-2,9-dioxacyclonona-3,7-dienes 5. Cross-coupling of 3 with two different ketones 4 and 6to give 7 can be achieved under certain reaction conditions. Hydrolysis of 5 or 7 may generate diketones 9, cyclopentanols 10/11, or cyclopentenes 12/13, depending on substituents and conditions. The X-ray crystal structure of 5a, the first nine-membered bis(η5-cyclopentadienyl)-substituted titanaheterocycle containing carbon, is presented. Ab initio calculations were performed for 5a and for titana-2-oxacyclopentene 1, a conceivable intermediate in the coupling reaction.

HCN exchange on [Cu(HCN)4]+: a quantum chemical investigation

Density functional (B3LYP, B3PW91, X3LYP, BP86, PBEPBE, PW91PW91, and M06) and ab initio (MP2, MP4sdq, CCSD, and CCSD(T)) calculations with extended basis sets (6-311+G**, TZVP, LANL2DZ+p, and SDD+p, the latter including extra polarization and diffuse functions) indicate that HCN exchange on [Cu(HCN)4]+ proceeds via an associative interchange (Ia) mechanism and a D3h transition structure {[Cu(HCN)5]+}‡. The activation barrier, relative to the model complex [Cu(HCN)4]+·HCN, varies modestly, depending on the computational level. Typical values are 8.0 kcal M−1 (B3LYP/6-311+G**), 6.0 kcal M−1 (M06/6-311+G**), and 4.8 kcal M−1 (CCSD(T)/6-311+G**//MP2(full)/6-311+G**). Inclusion of an implicit solvent model (B3LYP(CPCM)/6-311+G**) leads to an activation barrier of 5.8 kcal mol−1. Comparison of the HCN exchange mechanisms on [Li(HCN)4]+ (limiting associative, A) and [Cu(HCN)4]+ (associative interchange, Ia) reveals that π back donation in the equatorial Cu–N bonds in the transition state determines the mechanism. Graphical Abstract Computed structural and energetic data indicate an associative interchange (Ia) mechanism for HCN exchange on [Cu(HCN)4]+.