The chirality of dendrimer-based supramolecular complexes

Boc-protected L-phenylalanine has been connected to a spacer-armed ureido-acetic acid derivative, which can form multiple supramolecular complexes with urea-adamantyl modified poly(propylene imine) dendrimers in chloroform. Complexes of this guest with several generations of urea-adamantyl dendrimers were prepared. The dendrimer-guest complexes were characterized in detail by (1)H-NMR, (1)H-(1)H-NOESY spectroscopy and mass spectrometry to prove their formation. Optical rotation experiments performed on different generations of dendrimer-guest complexes showed a constant positive value. These observations indicate that, though guest molecules decrease the flexibility at the periphery of the dendrimer upon binding, the amino acid at the terminus of the guest molecule retains its high local conformational freedom. This is in agreement with values found for covalently modified spacer-armed dendrimers.     

Expression of chirality in salicyloxazolinate complexes of zirconium

Treatment of chiral non-racemic salicyloxazoline proligands HL with tetrabenzylzirconium(IV) gave species [L₂Zr(CH₂Ph)₂]. Reactions of KL with ZrCl₄(THF)₂ gave similar chloro complexes. One example of a benzyl complex was shown to exist as the Λ-trans,cis,cis diastereomer by X-ray crystallography. DFT calculations showed the observed isomer to be the most stable by 35 kJ mol⁻¹, indicating that thermodynamic diastereoselection for this species is excellent. Examination of the chiral environment about the benzyl co-ligands indicates however that the degree of expression of the chirality of the structure in what would be the site of metal-based reactions is poor in comparison to related systems. Variable temperature ¹H NMR data are consistent with this in that the low temperature spectrum exhibits a very small chemical shift difference between the chemically inequivalent benzylic CH atoms, and at higher temperatures dissociation of the portion of the ligand that contains the chiral information, i.e., the oxazoline unit, leads to apparent equivalence.     

Unprecedented coordination of dithiocarbimate in multinuclear and heteroleptic complexes

An uncommon coordination protocol induced by the p-tolylsulfonyl dithiocarbimate ligand (L) [L = p-CH(3)C(6)H(4)SO(2)N[double bond, length as m-dash]CS(2)(2-)] in conjunction with PPh(3) allowed the formation of novel homodimetallic, Cu(2)(PPh(3))(4)L (1), trinuclear heterometallic Cu(2)Ni(L)(2)(PPh(3))(4) (2) and heteroleptic complexes of general formula cis-[M(PPh(3))(2)L] [M = Pd(ii) (3), Pt(ii) (4)]. The complexes have been characterized by microanalysis, mass spectrometry, IR, (1)H, (13)C and (31)P NMR and electronic absorption spectra and single-crystal X-ray crystallography. 2 uniquely consists of square planar, trigonal planar and tetrahedral coordination spheres within the same molecule. In both heteroleptic complexes 3 and 4 the orientation of aromatic protons of PPh(3) ligand towards the Pd(ii) and Pt(ii) center reveals C-HPd and C-HPt rare intramolecular anagostic or preagostic interactions. These complexes exhibit photoluminescent properties in solution at room temperature arising mainly from intraligand charge transfer (ILCT) transitions. The assignment of electronic absorption bands has been corroborated by time dependent density functional theory (TD-DFT) calculations. Complexes 1 and 2 with σ(rt) values ～ 10(-6) S cm(-1) show semi-conductor properties in the temperature range 313-403 K whereas 3 and 4 exhibit insulating behaviour.     

Enantiomerization of chiral uranyl-salophen complexes via unprecedented ligand hemilability: toward configurationally stable derivatives

In the search for configurationally stable inherently chiral uranyl-salophen complexes, the newly synthesized compound 3 featuring a dodecamethylene chain was expected to be a promising candidate. Unexpectedly, dynamic HPLC on a enantioselective column showed that it still undergoes enantiomerization at high temperature. By comparison with the dynamic behavior of compounds 4 and 5, it was found that the enantiomerization rate is independent of the size of the ligand. This finding definitely rules out a jump rope-type mechanism for the enantiomerization process and points to reaction pathways involving preliminary rupture of one of the O...U coordinative bonds. This provides unprecedented evidence of the occurrence of ligand hemilability in metal-sal(oph)en complexes. Such findings inspired the synthesis of compound 6 endowed with a more rigid spacer, i.e., that derived from 4,4'-(1,4-phenylenediisopropylidene)bisphenol. DHPLC investigations showed that the new structural motif imparts a higher configurational stability, thus raising the half-life for the enantiomerization to more than 2 months at room temperature. This clearly establishes that this compound represents the first member of a new class of inherently chiral receptors, whose potential in chiral recognition and catalysis now can be feasibly explored.     

Synthesis of a linearly linked triscalixarene consisting of calix[4]arene units with combined axial chirality and inherent chirality

A linearly linked triscalix[4]arene, in which each calix[4]arene unit possesses both axial chirality and inherent chirality, was synthesized by repeated use of two stereoselective Williamson etherification on calix[4]arene. In the crystal lattice of the biscalix[4]arene intermediate, there are interesting alternate layers of biscalixarene molecules and solvent molecules in the ab plane.     

Unprecedented optically induced long-lived intramolecular electron transfer in cobalt-dioxolene complexes

The observation of very long lifetimes of the metastable states of two cobalt-dioxolene complexes undergoing photoinduced and high T(c) thermally-induced valence tautomer interconversion opens new research perspectives.     

New chiral phosphoramidite allenylidene complexes of ruthenium obtained with chirality transfer

The synthesis and structural characterization of the first ruthenium phosphoramidite allenylidene complexes that are chiral at the metal are described. The precursor complex [RuCl(Ind)(PPh₃)₂] (Ind = indenyl anion) was reacted with 1 equiv of different chiral phosphoramidite ligands L to give complexes of the general formula [RuCl(Ind)(PPh₃)L]. These complexes are stereogenic at the metal and at the ligand L. One of these complexes was obtained in diastereomeric purity, and was subsequently converted to allenylidene complexes of the general formula [RuCCCR′R(Ind)(PPh₃)L]⁺PF₆ (R = R′ = Ph; R = Ph, R′ = Me) in diastereomeric purity. As shown by X-ray, the chiral information is completely transferred from the precursor complex to the allenylidenes, which is of importance for potential catalytic applications.     

Amylose's recognition of chirality in polylactides on formation of inclusion complexes in vine-twining polymerization

Amylose selectively includes poly(L-lactide) (PLLA) among the poly(lactide)s (PLAs) to produce an inclusion complex when the phosphorylase-catalyzed polymerization of α-D-glucose 1-phosphate is performed in the presence of PLLA, poly(D-lactide) (PDLA), or poly(DL-lactide) (PDLLA) (vine-twining polymerization). This result indicates that amylose recognizes the chirality in PLAs on the formation of an inclusion complex in vine-twining polymerization. Modeling calculations support the amylose's chiral recognition in favor of PLLA and the atomistic details of the inclusion complex which involved the preferred orientation of the constituent molecular chains with respect to their fiber axis is proposed.     

Azacalix[4]arene tetramethyl ether with inherent chirality generated by substitution on the nitrogen bridges

The first inherently chiral azacalix[4]arene has been prepared by introducing three benzyl groups onto the nitrogen bridges. The highly enantioenriched compound was easily obtained via a moderately enantioselective cyclization followed by a simple crystallization procedure. NMR and X-ray crystallographic studies revealed that easy access to the enantiomer was permitted by the non-racemizable 1,3-alternate conformation in solution, up to 110 °C, as well as by the preferential crystallization of a racemic compound.     

Unprecedented bending and rearrangement of f-element sandwich complexes induced by superbulky cyclooctatetraenide ligands

The use of the superbulky cyclooctatetraenide dianion ligand [C(8)H(6)(SiPh(3))(2)](2-) (= COT(BIG)) in organo-f-element chemistry leads to unprecedented effects such as the formation of a significantly bent anionic Ce(III) sandwich complex, a novel cerocene formed by sterically induced SiPh(3) group migration, as well as the first example of a bent uranocene.     

2d-CUSUM for the detection of tendencies inherent in noisy bivariate data

Noisy analytical data dependent on 2 different variables (bivariate data) can be evaluated by an extended (2-dimensional) CUSUM-algorithm. Even at very high random fluctuations this 2d-CUSUM allows to detect long-term instationarities even from short series of measurements. The model of 2d-CUSUM as well as examples (distribution analysis, time-series analysis, treatment of calibration functions) are shown in this paper.Dedicated to Professor Dr. R. Geyer (Naumburg/Saale) on the occasion of his 80th birthday on April 14, 1994     

Theoretical evidence of persistent chirality in D3 homoleptic hexacoordinate complexes with monodentate ligands

A theoretical study of the enantiomer interconversion pathway relevant to racemization reactions of hexacoordinate transition-metal complexes is presented based on density functional calculations. The potential-energy surface for the trigonal twist pathway of the [Zr(SH)(6)](2-) model compound has been explored. The optimum structure reproduces, to a very good approximation, the experimental geometry of the analogous compound in which the thiolato groups have C(6)H(4)-4-OMe substituents instead of H atoms. A barrier of about 19 kcal mol(-1) is estimated for the racemization of [Zr(SH)(6)](2-) and exploratory calculations for [Zr(SC(6)H(4)-4-OMe)(6)](2-) indicate that a larger barrier should be expected. For the chiral homoleptic organometallic complexes [ZrMe(6)](2-) and [RhMe(6)](3-) no significant racemization barrier is expected.     

Unprecedented rearrangement during the formation of P-P homoatomic N-phosphino formamidine complexes

A variety of homoatomic P-P donor-acceptor homoleptic (R = R′) and heteroleptic (R ≠ R′) N-phosphino formamidine complexes [iPr₂N-C(H)N-PR₂-PR′₂]Cl were synthesized from the addition of N-phosphino formamidine (phosfam) donor reagent iPr₂N-C(H)N-PR₂ on halogenophosphane compounds R′₂PCl which are synthetic sources for the corresponding phosphenium derivatives R₂P⁺. We have demonstrated that the dynamic equilibrium observed between the different species is shifted either completely to the side of the free species or to the side of the donor-acceptor adduct [iPr₂N-C(H)N-PPh₂-PPh₂]Cl by changing the solvent or by varying the temperature. Activation parameters of ΔS^≠ = (−130 ± 7.2) J mol⁻¹ K⁻¹, ΔH^≠ = (8.4 ± 0.6) kJ mol⁻¹ and ΔG^≠ (298.15 K) = (53.6 ± 2.3) kJ mol⁻¹ were determined by an Eyring analysis over the temperature range of 193-293 K. The negative entropy of activation is consistent with an associative pathway and the low value of ΔH^≠ suggests that the energy barrier for this reaction is entropically controlled. Phosphine-phosphenium adducts is the most appropriate term to describe the dynamic process observed at variable temperature for complexes [iPr₂N-C(H)N-PR₂ → PR′₂]⁺, but the ³¹P NMR chemical shift and the calculated electronic charges are more in favor of a phosphinophosphonium Lewis drawing [iPr₂N-C(H)N-PR₂-PR′₂]⁺. Formation of the homoatomic P-P heteroleptic formamidine complexes [iPr₂N-C(H)NPR′₂PR₂]Cl (R = Ph, R′ = Et, iPr) results in the formal insertion of the phosphino group of the corresponding alkyl chlorophosphanes R′₂PCl into the N-P bond of the starting phosfam ligand iPr₂N-C(H)N-PR₂. Computed data are in agreement with the transient formation of a heteroatomic N-P intermediate [iPr₂N-C(H)N(PR₂)PR′₂]Cl, which then rearranges to the more thermodynamically favored homoatomic P-P compound [iPr₂N-C(H)N-PR₂-PR′₂]Cl.     

Unprecedented copper(I) bifluoride complexes: synthesis, characterization and reactivity

To be or not to bifluoride: Two synthetic pathways to unprecedented N-heterocyclic carbene copper(I) bifluoride complexes have been developed. Catalytic tests demonstrated that copper(I) bifluorides are very efficient catalysts, which do not require any additional activating agent. The first Cu-catalyzed diastereoselective allylation of (R)-N-tert-butanesulfinyl aldimines was also established. The method enables efficient, simple and general synthesis of enantiomerically enriched homoallylic amines at room temperature in high yields.     

Versatile switching in substrate topicity: supramolecular chirality induction in di- and trinuclear host complexes

Supramolecular chirality effects have been achieved both for ditopic and monotopic substrates by using a programmable bis-salphen scaffold that incorporates either two or three Zn nuclei. The dinuclear host shows preferential chirogenesis in the presence of ditopic systems, whereas effective chirality transfer to the trinuclear complex is realized through monotopic binding. The mode of binding in the trinuclear host has been investigated through X-ray crystallography, CD measurements, UV/Vis spectroscopy, and DFT analysis. The bis-salphen scaffold holds promise for the development of substrate-specific host systems useful for determination of the absolute configuration of various types of organic molecules.