Jonathan Becker

London Dispersion Enables the Shortest Intermolecular Hydrocarbon H···H Contact

Neutron diffraction of tri(3,5-tert-butylphenyl)methane at 20 K reveals an intermolecular C-H···H-C distance of only 1.566(5) Å, which is the shortest reported to date. The compound crystallizes as a C3-symmetric dimer in an unusual head-to-head fashion. Quantum chemical computations of the solid state at the HSE-3c level of theory reproduce the structure and the close contact well (1.555 Å at 0 K) and emphasize the significance of packing effects; the gas-phase dimer structure at the same level shows a 1.634 Å C-H···H-C distance. Intermolecular London dispersion interactions between contacting tert-butyl substituents surrounding the central contact deliver the decisive energetic contributions to enable this remarkable bonding situation.

Reactions of Copper(II) Chloride in Solution: Facile Formation of Tetranuclear Copper Clusters and Other Complexes That Are Relevant in Catalytic Redox Processes

Mixing CuCl2⋅2 H2O with benzylamine in alcoholic solutions led to an extremely colorful chemistry caused by the formation of a large number of different complexes. Many of these different species could be structurally characterized. These include relatively simple compounds such as [Cu(L(1))4Cl2] (L(1) = benzylamine) and (HL(1))2[CuCl4]. Most interestingly is the easy formation of two cluster complexes, one based on two cluster units Cu4OCl6(L(1))4 connected through one [Cu(L(1))2Cl2] complex and one based on a cubane-type cluster ([Cu4O4](C11H14)4Cl4). Both clusters proved to be highly reactive in a series of oxidation reactions of organic substrates by using air or peroxides as oxidants. Furthermore, it was possible to isolate and structurally characterize ([Cu(L(1))Cl]3 and [Cu(benz2mpa)2]CuCl2 (benz2mpa = benzyl-(2-benzylimino-1-methyl-propylidene)-amine), two copper(I) complexes that formed in solution, demonstrating the high redox activity of the cluster systems. In addition, it was possible to solve the molecular structures of the compounds Cu4OCl6(MeOH)4, [Cu(MeOH)2Cl2], [Cu(aniline)2Cl2], and an organic side product (HC13 H19 NOCl). In fact all determined structures are of a known type but the chemical relation between these compounds could be explained for the first time. The paper describes these different compounds and their chemical equilibria. Some of these complexes seem to be relevant in catalytic oxidation reactions and their reactivity is discussed in more detail.

The Enantioselective Dakin–West Reaction

Here we report the development of the first enantioselective Dakin-West reaction, yielding α-acetamido methylketones with up to 58 % ee with good yields. Two of the obtained products were recrystallized once to achieve up to 84 % ee. The employed methylimidazole-containing oligopeptides catalyze both the acetylation of the azlactone intermediate and the terminal enantioselective decarboxylative protonation. We propose a dispersion-controlled reaction path that determines the asymmetric reprotonation of the intermediate enolate after the decarboxylation.

[2](1,3)Adamantano[2](2,7)pyrenophane: A Hydrocarbon with a Large Dipole Moment

The fusion of the sp(3) -hybridized parent diamondoid adamantane with the sp(2) -hybridized pyrene results in a hybrid structure with a very large dipole moment which arises from bending the pyrene moiety. Presented herein is the synthesis, study of the electronic and optical properties, as well as the dynamic behavior of this new hydrocarbon.

Copper Chloride Catalysis: Do μ4-Oxido Copper Clusters Play a Significant Role?

Copper chloride catalysis is a well-established field in organic and inorganic chemistry. However, in most cases a detailed mechanistic understanding of the individual reaction steps and identification of reactive intermediates are still missing. The present study reports the results of spectroscopic and spectrometric measurements that support formation of copper agglomerates during catalytic processes. The composition of CuCl2·2H2O in several coordinating solvents and the influence of basic coreagents such as NaO(t)Bu and K2CO3 on the structure in the solid state as well as in solution were investigated. Several experiments involving crystal structure determination, IR spectroscopy, and ultra-high-resolution cryospray-ionization mass spectrometry were performed. The crystal structures of [CuCl2(H2O)]·0.5(CH3)2CO (1), [Cu2(CH3CN)2Cl4] (2), [Cu3(CH3CN)3Cl6] (3), [Cu3Cl6(THF)4] (4), [Cu(DMSO)2Cl2] (5), (H2N(CH3)2)2[CuCl3] (6), and [Cu4OCl6(THF)(urea)3]·3THF·urea (8) are reported herein. It can be clearly demonstrated that μ4-oxido copper clusters of the formula [Cu4OCl6(solvent)4] are the main product from the reactions of CuCl2·2H2O and basic coreagents. As a final result of these experiments, it can be stated that μ4-oxido copper clusters most likely play an important role in the mechanism of copper chloride-catalyzed reactions.

Sulfur versus Dioxygen: Dinuclear (Trisulfido)copper Complexes: Sulfur versus Dioxygen: Dinuclear (Trisulfido)copper Complexes

In contrast to dinuclear (disulfido)copper complexes related trisulfido compounds have not been reported so far. Here we describe two new complexes of this type, [Cu2(tmpa)2S3](SbF6)2 {tmpa = tris(2-pyridylmethyl)amine} and [Cu2(tet b)2S3](OTf)2 (tet b = rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), that have been investigated and could be structurally characterized. Furthermore, the reactivity of these complexes towards dioxygen showed that the corresponding (peroxido)copper complexes formed. Depending on the stoichiometric conditions (excess of sulfur) mononuclear end-on (sulfido)copper complexes were detected spectroscopically. DFT calculations were performed and supported the experimental results.

CCDC 1844941: Experimental Crystal Structure Determination

Related Article: Afsaneh Pilevar, Abolfazl Hosseini, Marina Sekutor, Heike Hausmann, Jonathan Becker, Kevin Turke, and Peter R. Schreiner|2018|J.Org.Chem.|||doi:10.1021/acs.joc.8b01392

Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives

The long-known, ubiquitously present, and always attractive London dispersion (LD) interaction was probed with hexaphenylethane (HPE) derivatives. A series of all- meta hydrocarbyl [Me, iPr, tBu, Cy, Ph, 1-adamantyl (Ad)]-substituted triphenylmethyl (TPM) derivatives [TPM-H, TPM-OH, (TPM-O)2, TPM•] was synthesized en route, and several derivatives were characterized by single-crystal X-ray diffraction (SC-XRD). Multiple dimeric head-to-head SC-XRD structures feature an excellent geometric fit between the meta-substituents; this is particularly true for the sterically most demanding tBu and Ad substituents. NMR spectra of the iPr-, tBu-, and Cy-derived trityl radicals were obtained and reveal, together with EPR and UV-Vis spectroscopic data, that the effects of all- meta alkyl substitution on the electronic properties of the trityl scaffold are marginal. Therefore, we concluded that the most important factor for HPE stability arises from LD interactions. Beyond all- meta tBu-HPE we also identified the hitherto unreported all- meta Ad-HPE. An intricate mathematical analysis of the temperature-dependent dissociation constants allowed us to extract Δ Gd298(exptl) = 0.3(5) kcal mol-1 from NMR experiments for all- meta tBu-HPE, in good agreement with previous experimental values and B3LYP-D3(BJ)/def2-TZVPP(C-PCM) computations. These computations show a stabilizing trend with substituent size in line with all- meta Ad-HPE (Δ Gd298(exptl) = 2.1(6) kcal mol-1) being more stable than its tBu congener. That is, large, rigid, and symmetric hydrocarbon moieties act as excellent dispersion energy donors. Provided a good geometric fit, they are able to stabilize labile molecules such as HPE via strong intramolecular LD interactions, even in solution.

From mononuclear to polynuclear: Copper and zinc complexes obtained from polypyridylamine ligands related to tris(2-pyridylmethyl)-amine (tmpa)

Abstract Copper(I)/(II) complexes and a zinc compound with polypyridylamine ligands (related to the tripodal ligand tris(2-pyridylmethyl)amine, tmpa) were synthesized. Crystallographic characterization was possible for most of the complexes obtained. The different structures of the complexes allowed some insight into the basic understanding of the design of coordination polymers. Depending on ligand, solvent or anion, either mononuclear, dinuclear or polynuclear complexes were obtained. Graphical Abstract

Aerobic Aliphatic Hydroxylation Reactions by Copper Complexes: A Simple Clip‐and‐Cleave Concept

A simple imine clip-and-cleave concept has been developed for the selective hydroxylation of non-activated CH bonds in aliphatic aldehydes with dioxygen through a copper complex. The synthetic protocol involves reaction of a substrate aldehyde with N,N-diethyl-ethylendiamine to give the corresponding imine, which is used as a bidentate donor ligand forming a copper(I) complex with [Cu(CH3 CN)4 ][CF3 SO3 ]. After exposure of the reaction mixture to dioxygen acidic cleavage and aqueous workup liberates the corresponding β-hydroxylated aldehyde. The concept for the hydroxylation of trimethylacetaldehyde as well as adamantane and diamantane 1-carbaldehydes was investigated and the corresponding β-hydroxy aldehydes were obtained with high selectivities. The results of low temperature stopped-flow measurements indicate the formation of a bis(μ-oxido)dicopper complex as reactive intermediate. According to density functional theory (DFT, RI-BLYP-D3/def2-TZVP(SDD)/ COSMO(CH2 Cl2 )//RI-PBE-D3/def2-TZVP(SDD)) computations CH bonds suitably predisposed to the [Cu2 O2 ]2+ core undergo hydroxylation in a concerted step with particularly low activation barriers, which explains the experimentally observed regioselectivities.