On the electronic origins of structural isomerism in the iron-sulfur cubane, [(C(5)H(5))(4)Fe(4)S(4)](2+)

Density functional theory provides new insights into the structural isomerism observed in the cyclopentadienyl-capped iron-sulfur cluster, [(C(5)H(5))(4)Fe(4)S(4)](2+). Two distinct, closely spaced minima have been located, a triplet with D(2) symmetry and a C(2)-symmetric singlet, both of which correspond closely to the structure of one of the known crystal forms of the cation. Thus, the structural diversity in these species reflects genuine molecular bistability rather than simple solid-state packing effects. In contrast, no stable D(2)(d)()-symmetric minimum has been located, suggesting that the reported D(2)(d)() symmetry of the cation in [(C(5)H(5))(4)Fe(4)S(4)][PF(6)](2) may be a crystallographic artifact. In the ruthenium analogue, the more diffuse 4d orbitals stabilize the C(2)-symmetric singlet, which is unambiguously the ground state, but the D(2)-symmetric potential energy surface provides a viable low-energy pathway for the dynamic exchange of the Ru-Ru bonds.     

A New Method of Resolution of Racemic 6,6''''-Dihydroxy-5,5''''- biquinoline Using (S)-( + )-Camphorsulfonyl Chloride

The chiral ligand bearing C2-symmetric binaphthyl backbonce (e. g. BINOL, BINAP) is one of the widely used chiral auxiliaries in catalytic asymmetric synthesis. [1] Numerous synthetic approaches to these two compounds have been developed. In contrast, the C2-symmetric ligands containing heterocyclic moiety has been relatively less explored.     

Supramolecular double helical Cu(I) complexes for asymmetric cyclopropanation

Chiral double-stranded helicates, formed between Cu(I) ion and C(2)-symmetric oligopyridines, were used for catalytic asymmetric cyclopropanation of alkenes; low catalyst loadings (0.2 mol%), high TONs (up to 404) and short reaction times (30-60 min) were achieved with [Cu(2)L(2)]OTf(2)(L = chiral C(2)-symmetric terpyridine).     

Absolute configuration of D(2)-symmetric fullerene C(84)

The D(2)-symmetric isomer of fullerene C(84) predominantly found in soots is inherently chiral. We determine its absolute configuration by comparison of the experimental electronic circular dichroism (CD) spectrum with time-dependent density functional calculations.     

Synthesis of C3-symmetric tris(beta-hydroxy amide) ligands and their Ti(IV) complex-catalyzed enantioselective alkynylation of aldehydes

[reaction: see text]. A series of new chiral C3-symmetric tris(beta-hydroxy amide) ligands have been synthesized via the reaction of 1,3,5-benzenetricarboxylic chloride and optically pure amino alcohols (up to 96% yield). The asymmetric catalytic alkynylation of aldehydes with these new C3-symmetric chiral tris(beta-hydroxy amide) ligands and Ti (O(i)'Pr)4 was investigated. Ligand 4c synthesized from (1R,2S)-(-)-2-amino-1,2-diphenylethanol is effective for the enantioselective alkynylation of various aldehydes, and high enantioselectivity was obtained with aromatic aldehydes and alpha,beta-unsaturated aldehyde (up to 92% ee).     

Zn(II)-induced conformational control of amphiphilic cavitands in langmuir monolayers

The reversible switching from the C(4v)-symmetric vase to the C(2v)-symmetric kite conformation of an amphiphilic resorcin[4]arene cavitand was induced by Zn(II) ion coordination. Langmuir monolayers were obtained of both conformers with the area per molecule increasing dramatically from 120 A(2) for the vase to 270 A(2) for the kite form. (1)H NMR spectroscopy in chloroform solution supports the formation of a stoichiometric kite-cavitand[radical dot](Zn(II))(2) complex, with the metal ions coordinating to pairs of neighbouring quinoxaline N-atoms.     

The self-assembly between C2-symmetric (methanol)6 or S6-symmetric (ethanol)6 cyclohexamers and paddle-wheel dinuclear copper units leads to unique 1D polymer chains

The first crystallographic evidences of the theoretically suggested C2-symmetric cyclohexameric form of liquid methanol and the S6-symmetric cyclohexameric form of liquid ethanol are reported, trapped inside a hydrophilic pocket shielded by tert-butyl groups.     

Conformational studies of cyclo(L-Phe-L-Pro-Gly-L-Pro)2 by 13C nuclear magnetic resonance

The 13C-NMR spectrum (Fig. 2,1) of cyclooctapeptide cyclo(L-phe-L-Pro-Gly-L-Pro)2 (A) in CDC13 suggested that its conformation involved the coexistence of two kinds of C2-symmetric conformation with trans-trans-trans-trans and cis-trans-trans-trans forms. Adding 0.5 equivalent of CsSCN or one equivalent of DL-Phe-OMe.HCl to the solution of cyclopeptide (A) in CDC13 yielded 13C-NMR spectra (Fig. 2,2 and Table I) which suggested a single C2-symmetric conformation with trans-trans-trans-trans form, resulting from the formation of complexes with CsSCN or DL-Phe-OMe.HCl. The 13C-NMR spectrum of complexes of A with DL-Phe-OMe.HCl displayed separate resonances for C(gamma), C(o), C(m), C(alpha), and C(beta) of D-Phe-OMe.HCl and L-Phe-OMe.HCl (Table I).     

Asymmetric Henry reaction catalyzed by C2-symmetric tridentate bis(oxazoline) and bis(thiazoline) complexes: metal-controlled reversal of enantioselectivity

[reaction: see text] C(2)-symmetric tridentate bis(oxazoline) and bis(thiazoline) ligands with a diphenylamine backbone have been investigated in the catalytic asymmetric Henry reaction of alpha-keto esters with different Lewis acids. Their Cu(OTf)(2) complexes furnished S enantiomers, while Et(2)Zn complexes afforded R enantiomers, both of them with higher enantioselectivities (up to 85% ee). Reversal of enantioselectivity in asymmetric Henry reactions was achieved with the same chiral ligand by changing the Lewis acid center from Cu(II) to Zn(II). The results show that the NH group in C(2)-symmetric tridentate chiral ligands plays a very important role in controlling both the yields and enantiofacial selectivity of the Henry products.     

Asymmetric Michael addition of nitroalkanes to nitroalkenes catalyzed by C2-symmetric tridentate bis(oxazoline) and bis(thiazoline) zinc complexes

The first asymmetric synthesis of 1,3-dinitro compounds through Michael addition of nitroalkanes to nitroalkenes catalyzed by C2-symmetric chiral tridentate bis(oxazoline) and bis(thiazoline) zinc complexes was achieved with high enantioselectivities (up to 95% ee).     

Monohelical Iron(II) and Zinc(II) complexes of a (1R,2R)-cyclohexyl salen ligand with benz[a]anthryl sidearms

An enantiomerically pure C2-symmetric salen ligand with benz[a]anthracene siderams produces M helical complexes (FeII and ZnII) in solution. Interconversion to produce a 1:1 mixture of helical conformers in the solid state is possible, despite overlapped sidearms.     

Enantioselective Henry reaction catalyzed by a C2-symmetric bis(oxazoline)-Cu(OAc)2.H2O complex

A C(2)-symmetric diethyl (i)Pr-bis(oxazoline)-Cu(OAc)(2).H(2)O was found to be an efficient catalyst for catalyzing an enantioselective Henry reaction between nitromethane and various aldehydes to provide beta-hydroxy nitroalkanes with high chemical yields (up to 95%) and enantiomeric excesses (up to 97%).     

Synthesis and reactivity of a C3-symmetric trinuclear zinc(II) hydroxide catalyst efficient at phosphate diester transesterification

Inspired by trinuclear Zn(ii) sites in enzymatic systems, a ligand system containing three preorganized (2-pyridyl)methyl piperazine moieties anchored onto a rigid C(3)-symmetric triphenoxymethane platform has been developed for preorganizing three zinc ions into an environment conducive to intramolecular interaction. Zinc(ii) binding by this ligand has been analyzed by means of potentiometric measurements in 50% (v/v) CH(3)CN-H(2)O solutions. Subsequently a C(3)-symmetric trinuclear Zn(ii) hydroxide complex of the C(3)-symmetric ligand was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. This complex induces a 16 900-fold rate enhancement in the catalytic cyclization of the RNA model substrate, 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP, pH 6.7, 25 degrees C) over the uncatalyzed reaction with multiple catalyst turnovers. The observed differences in the pH-rate profile can be attributed to the varying concentration of various trinuclear zinc species. The trinuclear Zn(ii) catalyst exhibits a higher hydrolytic activity compared to its mononuclear analogue. The reactivity and structural features of this trinuclear Zn(ii) complex will be discussed.     

Stereoselective total synthesis of (-)-kumausallene

A stereoselective total synthesis of (-)-kumausallene was completed in 12 steps from acetylacetone. The hidden symmetry of (-)-kumausallene was recognized, and its skeleton was constructed efficiently from a C(2)-symmetric diol by a palladium-catalyzed cascade reaction. High diastereoselectivity was observed for the DMF-promoted biomimetic 1,4-bromocyclization of a conjugated enyne.     

Highly stable chiral (A)6-B supramolecular copolymers: a multivalency-based self-assembly process

A novel type of chiral layered supramolecular copolymer with high molecular weight has been assembled from a hydrogen bonded C(6)-symmetric zinc porphyrin hexamer and chiral C(3)-symmetric pyridine hexadentate linkers driven by multivalent zinc porphyrin-pyridine coordination. UV-vis, circular dichroism, and static light scattering experiments revealed that the formation of the layered supramolecular copolymers is at first dynamically controlled and then becomes thermodynamically controlled.     

Cobalt-mediated, enantioselective synthesis of C(2) and C(1) dienes

The asymmetric C-H functionalization of norbornene and norbornadiene with five-, six-, and seven-membered cyclic enones mediated by the reactive intermediate [{η(5)-((t)BuMe(2)Si)C(5)H(4)}Co(NO)(2)] is reported. A novel base mixture derived from enantiopure ammonium salts and NaHMDS was used as a source of chirality, and this enantioselective desymmetrization of C(s) alkenes has been applied to the asymmetric synthesis of C(2)- and C(1)-symmetric diene ligands in high regioselectivity (3.7-20:1 anti/syn), near perfect diastereoselectivity (>99:1 dr), and high enantioselectivity (90-96% ee).     

Rational design and assembly of M(2)M'(3)L(6) supramolecular clusters with C(3h) symmetry by exploiting incommensurate symmetry numbers.

A rational approach to heterometallic cluster formation is described that uses incommensurate symmetry requirements at two different metals to control the stoichiometry of the assembly. Critical to this strategy is the proper design and synthesis of hybrid ligands with coordination sites selective toward each metal. The phosphino-catechol ligand 4-(diphenylphosphino)benzene-1,2-diol (H(2)L) possesses both hard catecholate and soft phosphine donor sites and serves such a role, using soft (C(2)-symmetric) and hard (C(3)-symmetric) metal centers. The ML(3) catecholate complexes (M = Fe(III), Ga(III), Ti(IV), Sn(IV)) have been prepared and characterized as C(3)-symmetry precursors for the stepwise assembly (aufbau) of heterometallic clusters. While the single-crystal X-ray structure of the Cs(2)[TiL(3)] salt shows a C(1) mer-configuration in the solid -state, room-temperature solution NMR data of this and related complexes are consistent with either exclusive formation of the C(3)-fac-isomer with all PPh(2) donor sites syn to each other or facile fac/mer isomerization. Coordination of these [ML(3)](2)(-) (M = Ti(IV), Sn(IV)) metallaligands via their soft P donor sites to C(2)-symmetric PdBr(2) units gives exclusively pentametallic [M(2)Pd(3)Br(6)L(6)](4)(-) (M = Ti, Sn) clusters. These clusters have been fully characterized by spectral and X-ray structural data as C(3h) mesocates with Cs(+) or protonated 1,4-diazabicyclo[2.2.2]octane (DABCO.H(+)) cations incorporated into deep molecular clefts. Exclusive formation of this type of supramolecular species is sensitive to the nature of the counterions. Alkali cations such as K(+), Rb(+), and Cs(+) give high-yield formation of the respective clusters while NEt(3)H(+) and NMe(4)(+) yield none of the desired products. Extension of the aufbau assembly to produce related [M(2)Pd(3)Cl(6)L(6)](4)(-), [M(2)Pd(3)I(6)L(6)](4)(-), and [M(2)Cr(3)(CO)(12)L(6)](4)(-) (M = Ti, Sn) clusters has also been realized. In addition to this aufbau approach, self-assembly of several of these [M(2)Pd(3)Br(6)L(6)](4)(-) clusters from all eleven components (two M(IV), three PdBr(2), six H(2)L) was also accomplished under appropriate reaction conditions.     

Synthesis of dimeric Re(I) amino acid conjugate complexes

The aqueous one-step reaction of Re(CO)(3)(H(2)O)(3)(+) with pyridine-2-carboxyaldehyde and glycine or alanine produces a new class of cyclic dimeric products. The ligands of the chiral C(2)-symmetric products are bound via the diimine at one rhenium centre and via the pendant carboxylate to a second rhenium center.     

Synthesis of Novel Chiral Dibenzo [ a, c ] cycloheptadiene Bis(oxazoline) and Catalytic Asymmetric Reactions

Over the last decade, C2-symmetric chiral oxazoline metal complexes have been recognized as an effective classof chiral catalyst in a variety of transition metal catalyzed asymmetric reactions. [1] High catalytic activities and enantiomeric excesses have been obtained using C2-symmetric chiral ligands in conjunction with suitable transition metal ion, for example, the hydrosilylation of ketone, allylic alkylation, Michael addition, Diels-Alder cycloaddition, and cyclopropanation. Thus, the design and synthesis of new chiral oxazoline ligands have inspired many scientists to work with great efforts.     

Synthesis, Reactivity, and Resolution of a C(2)-Symmetric, P-Stereogenic Benzodiphosphetane, a Building Block for Chiral Bis(phosphines)

Although the pyramidal inversion barriers in diphosphines (R(2)P-PR(2)) are similar to those in phosphines (PR(3)), P-stereogenic chiral diphosphines have rarely been exploited as building blocks in asymmetric synthesis. The synthesis, reactivity, and resolution of the benzodiphosphetane trans-1,2-(P(t-Bu))(2)C(6)H(4) are reported. Alkylation with MeOTf followed by addition of a nucleophile gave the useful C(2)-symmetric P-stereogenic ligand BenzP* and novel analogues.