C3-Symmetrie in der asymmetrischen Katalyse und der chiralen Erkennung

Uncovering the Hidden Landscape of Tris(4-pyridyl) Ligands: Topological Complexity Derived from the Bridgehead

Supramolecular main group chemistry is a developing field which parallels the conventional domain of metallo-organic chemistry. Little explored building blocks in this area are main group metal-based ligands which have the appropriate donor symmetry to build desired molecular or extended arrangements. Tris(pyridyl) main group ligands (E(py)₃ , E=main group metal) are potentially highly versatile building blocks since shifting the N-donor arms from the 2- to the 3-positions and 4-positions provides a very simple way of changing the ligand character from mononuclear/chelating to multidentate/metal-bridging. Here, the coordination behaviour of the first main group metal tris(4-pyridyl) ligands, E(4-py)₃ (E=Sb, Bi, Ph-Sn) is explored, as well as their ability to build metal-organic frameworks (MOFs). The complicated topology of these MOFs shows a marked influence on the counter anion and on the ability of the E(4-py)₃ ligands to switch coordination mode, depending on the steric and donor character of the bridgehead. This structure-directing influence of the bridgehead provides a potential building strategy for future molecular and MOF design in this area.

Tricoordinate Coinage Metal Complexes with a Redox-Active Tris-(Ferrocenyl)triazine Backbone Feature Triazine-Metal Interactions

2,4,6-Tris(1-diphenylphosphanyl-1'-ferrocenylene)-1,3,5-triazine (1) coordinates all three coinage metal(I) ions in a 1:1 tridentate coordination mode. The C3 -symmetric coordination in both solid state and solution is stabilised by an uncommon cation-π interaction between the triazine core and the metal cation. Intramolecular dynamic behaviour was observed by variable-temperature NMR spectroscopy. The borane adduct of 1, 1BH3 , displays four accessible oxidation states, suggesting complexes of 1 to be intriguing candidates for redox-switchable catalysis. Complexes 1Cu, 1Ag, and 1Au display a more complicated electrochemical behaviour, and the electrochemical mechanism was studied by temperature-resolved UV/Vis spectroelectrochemistry and chemical oxidation.

Synthetic Approaches to Star-Shaped Molecules with 1,3,5-Trisubstituted Aromatic Cores

Herein, we summarize the synthetic approaches that have been developed for the synthesis of star-shaped molecules. Typically, to design such highly functionalized molecules, simple building blocks are first assembled through trimerization reactions, starting from commercially available starting materials. Then, these building blocks are synthetically manipulated to generate extended star-shaped molecules. We also discuss the syntheses of star-shaped molecules that contain 2,4,6-trisubstituted 1,3,5-triazine or 1,3,5-trisubstituted benzene rings as a central core and diverse substituted styrene, phenyl, and fluorene derivatives at their periphery, which endows these molecules with extended conjugation. A variety of metal-catalyzed reactions, such as Suzuki, Buchwald-Hartwig, Sonogashira, Heck, and Negishi cross-coupling reactions, as well as metathesis, have been employed to functionalize a range of star-shaped molecules. The methods described herein will be helpful for designing a wide range of intricate compounds that are highly valuable in the fields of supramolecular chemistry and materials science. Owing to space limitations, we will not cover all of the publications on this topic. Instead, we will focus on examples that were reported by our research group and other relevant recent literature. Apart from the trimerization sequence, this Minireview has been structured based on the key reactions that were used to prepare the star-shaped molecules and other higher analogues. Finally, some examples that do not fit into this classification are discussed.

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

Although a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main-group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris-2-pyridyl ligands E(2-py)₃ (E=C-R, N, P), which are closely related to archetypal tris-pyrazolyl borates. With the 6-methyl substituted ligands E(6-Me-2-py)₃ (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.

C3 -Symmetric Tricyclo[2.2.1.02,6 ]heptane-3,5,7-triol

A straightforward access to a hitherto unknown C₃ -symmetric tricyclic triol both in racemic and enantiopure forms has been developed. Treatment of 7-tert-butoxynorbornadiene with peroxycarboxylic acids provided mixtures of C₁ - and C₃ -symmetric 3,5,7-triacyloxynortricyclenes via transannular π-cyclization and replacement of the tert-butoxy group. By refluxing in formic acid, the C₁ -symmetric esters were converted to the C₃ -symmetric formate. Hydrolysis gave diastereoisomeric triols, which were separated by recrystallization. Enantiomer resolution via diastereoisomeric tri(O-methylmandelates) delivered the target triols on a gram scale. The pure enantiomers are useful as core units of dopants for liquid crystals.

Cobalt(III) Werner Complexes with 1,2-Diphenylethylenediamine Ligands: Readily Available, Inexpensive, and Modular Chiral Hydrogen Bond Donor Catalysts for Enantioselective Organic Synthesis

In the quest for new catalysts that can deliver single enantiomer pharmaceuticals and agricultural chemicals, chemists have extensively mined the "chiral pool", with little in the way of inexpensive, readily available building blocks now remaining. It is found that Werner complexes based upon the D3 symmetric chiral trication [Co(en)3](3+) (en = 1,2-ethylenediamine), which features an earth abundant metal and cheap ligand type, and was among the first inorganic compounds resolved into enantiomers 103 years ago, catalyze a valuable carbon-carbon bond forming reaction, the Michael addition of malonate esters to nitroalkenes, in high enantioselectivities and without requiring inert atmosphere conditions. The title catalysts, [Co((S,S)-dpen)3](3+) ((S,S)-3 (3+)) 3X(-), employ a commercially available chiral ligand, (S,S)-1,2-diphenylethylenediamine. The rates and ee values are functions of the configuration of the cobalt center (Λ/Δ) and the counteranions, which must be lipophilic to solubilize the trication in nonaqueous media. The highest enantioselectivities are obtained with Λ and 2Cl(-)BArf (-), 2BF4 (-)BArf (-), or 3BF4 (-) salts (BArf (-) = B(3,5-C6H3(CF3)2)4 (-)). The substrates are not activated by metal coordination, but rather by second coordination sphere hydrogen bonding involving the ligating NH2 groups. Crystal structures and NMR data indicate enthalpically stronger interactions with the NH moieties related by the C3 symmetry axis, as opposed to those related by the C2 symmetry axes; rate trends and other observations suggest this to be the catalytically active site. Both Λ- and Δ-(S,S)-3 (3+) 2Cl(-)BArf (-) are effective catalysts for additions of β-ketoesters to RO2CN=NCO2R species (99-86% yields, 81-76% ee), which provide carbon-nitrogen bonds and valuable precursors to α-amino acids.

Synthesis, π-face-selective aggregation, and π-face chiral recognition of configurationally stable C(3)-symmetric propeller-chiral molecules with a π-core

The C3 -symmetric propeller-chiral compounds (P,P,P)-1 and (M,M,M)-1 with planar π-cores perpendicular to the C3 -axis were synthesized in optically pure states. (P,P,P)-1 possesses two distinguishable propeller-chiral π-faces with rims of different heights named the (P/L)-face and (P/H)-face. Each face is configurationally stable because of the rigid structure of the helicenes contained in the π-core. (P,P,P)-1 formed dimeric aggregates in organic solutions as indicated by the results of (1) H NMR, CD, and UV/Vis spectroscopy and vapor pressure osmometry analyses. The (P/L)/(P/L) interactions were observed in the solid state by single-crystal X-ray analysis, and they were also predominant over the (P/H)/(P/H) and (P/L)/(P/H) interactions in solution, as indicated by the results of (1) H and 2D NMR spectroscopy analyses. The dimerization constant was obtained for a racemic mixture, which showed that the heterochiral (P,P,P)-1/(M,M,M)-1 interactions were much weaker than the homochiral (P,P,P)-1/(P,P,P)-1 interactions. The results indicated that the propeller-chiral (P/L)-face interacts with the (P/L)-face more strongly than with the (P/H)-face, (M/L)-face, and (M/H)-face. The study showed the π-face-selective aggregation and π-face chiral recognition of the configurationally stable propeller-chiral molecules.

Novel supramolecular affinity materials based on (-)-isosteviol as molecular templates

The readily available ex-chiral-pool building block (-)-isosteviol was combined with the C 3-symmetric platforms hexahydroxytriphenylene and hexaaminotriptycene providing large and rigid molecular architectures. Because of the persistent cavities these scaffolds are very potent supramolecular affinity materials for head space analysis by quartz crystal microbalances. The scaffolds serve in particular as templates for tracing air-borne arenes at low concentration. The affinities of the synthesized materials towards different air-borne arenes were determined by 200 MHz quartz crystal microbalances.

The entrapment of chiral guests with gated baskets: can a kinetic discrimination of enantiomers be governed through gating?

The capacity of gated hosts for controlling a kinetic discrimination between stereoisomers is yet to be understood. To conduct corresponding studies, however, one needs to develop chiral, but modular and gated hosts. Accordingly, we used computational (RI-BP86/TZVP//RI-BP86/SV(P)) and experimental (NMR/CD/UV/Vis spectroscopy) methods to examine the transfer of chirality in gated baskets. We found that placing stereocenters of the same kind at the rim (R(1) =CH3, so-called bottom) and/or top amide positions (R(2) =sec-butyl) would direct the helical arrangement of the gates into a P or M propeller-like orientation. With the assistance of (1)H NMR spectroscopy, we quantified the intrinsic (thermodynamic) and constrictive (kinetic) binding affinities of (R)- and (S)-1,2-dibromopropane 5 toward baskets (S3b/P)-2, (S3t/M)-3, and (S3bt/P)-4. Interestingly, each basket has a low ( ≤1.3 kcal mol(-1)), but comparable (de<10%) affinity for entrapping enantiomeric (R/S)-5. In terms of the kinetics, basket (S3b/P)-2, with a set of S stereocenters at the bottom and P arrangement of the gates, would capture (R)-5 at a faster rate (kin(R)/kin(S) =2.0±0.2). Basket (S3t/M)-3, with a set of S centers at the top and M arrangement of the gates, however, trapped (S)-5 at a faster rate (kin(R)/kin(S) =0.30±0.05). In light of these findings, basket (S3bt/P)-4, with a set of S stereocenters installed at both top and bottom sites along with a P disposition of the gates, was found to have a lower ability to differentiate between enantiomeric (R/S)-5 (kin(R)/kin(S) =0.8). Evidently, the two sets of stereocenters in this "hybrid" host acted concurrently, each with the opposite effect on the entrapment kinetics. Gated baskets are hereby established to be a prototype for quantifying the kinetic discrimination of enantiomers through gating and elucidating the electronic/steric effects on the process.

The more gold--the more enantioselective: cyclohydroaminations of γ-allenyl sulfonamides with mono-, bis-, and trisphospholane gold(I) catalysts

A series of chiral mono-, di-, and trinuclear gold(I) complexes have been prepared and used as precatalysts in the asymmetric cyclohydroamination of N-protected γ-allenyl sulfonamides. The stereodirecting ligands were mono-, di-, and tridentate 2,5-diphenylphospholanes, which possessed C(1), C(2), and C(3) symmetry, respectively, thereby rendering the catalytic sites in the di- and trinuclear complexes symmetry equivalent. The C(3)-symmetric trinuclear complex displayed the highest activity and enantioselectivity (up to 95 % ee), whilst its mono- and dinuclear counterparts exhibited considerably lower enantioselectivities and activities. A similar trend was observed in a series of mono-, di-, and trinuclear 2,5-dimethylphospholane gold(I) complexes. Aurophilic interactions were established from the solid-state structures of the trinuclear gold(I) complexes, thereby raising the question as to whether these secondary forces were responsible for the different catalytic behavior observed.

(+)-syn-Benzotriborneol an enantiopure C3-symmetric receptor for water

(+)-syn-Benzotriborneol forms stable complexes with one molecule of water. This is due to the ability of the host to form three hydrogen bonds with water, to act simultaneously as a hydrogen-bond acceptor and donor, and to a perfect geometrical match between the pair. We report experimental (X-ray and neutron diffraction, VT NMR, DSC, TGA) and stereochemical studies carried out to elucidate and quantify the molecular and thermodynamic aspects of this supramolecular complex.

Synthesis and preferred all-syn conformation of C3-symmetrical N-(hetero)arylmethyl triindoles

A new series of C(3)-symmetrical N-(hetero)arylmethyl triindoles has been synthesized in a straightforward procedure. The structure and conformation in the solid state have been determined for three derivatives (3, 4, and 6) by X-ray crystallographic analysis. In all three cases, the molecules adopt a tripodal conformation with all of the flexible arms directed towards the same side, thereby delimiting an inner cavity. Compound 6 crystallizes and forms C(3)-symmetric dimeric cagelike complexes. Guest molecules of chloroform and water are confined within the resulting cavities with stabilization by different intermolecular interactions; this highlights the potential of these compounds in the construction of supramolecular systems. A computational analysis has been performed to predict the most stable conformers. As a general trend, a preference for a conformation with all branches directed to the same side has been predicted. Comparison between theoretical and experimental results indicates that the computational level selected for the present study, B3LYP/6-31G*, is able to reproduce both the minimum energy conformations and the rotation barriers about the N--CH(2) bond.