Helical Enantiomers Research Articles

Coordination‐Driven Self‐Assembly, Structures, and Dynamic Properties of Diplatinum Hexatriynediyl and Butadiynediyl Complexes in which the sp Carbon Chains are Shielded by sp3 Carbon Chains: Towards Endgroup–Endgroup Interactions

Sequential reactions of trans-(C6F5)(p-tol3P)2Pt(C[triple chemical bond]C)3SiEt3 (PtC6SiEt3) with nBu4N+ F(-) (THF/methanol), PtCl, KPF6/tBuOK, and CuCl give trans,trans-[(C6F5){(p-tol3P)2}Pt(C[triple chemical bond]C)3Pt{(Pp-tol3)2}(C6F5)] (PtC6Pt) in 95 % yield on multigram scales. Reactions of PtC6Pt and Ar2P(CH2)mPAr2 afford substitution products trans,trans-[(C6F5){(Ar2P(CH2)mPAr2)}Pt(C[triple chemical bond]C)3Pt{(Ar2P(CH2)mPAr2)}(C6F5)] (PtC6Pt-m/Ar; m/Ar=8/p-tol, 78 %; 10/Ph, 82 %; 11/Ph, 69 %; 12/Ph, 57 %; 14/p-tol, 57 %; 14/p-C6H4-tBu, 71 %), in which the diphosphines span the square planar platinum endgroups. An analogous reaction with PEt3 gives a tetrakis PEt3 complex Pt'C6Pt' (72 %). The crystal structures of PtC6Pt, Pt'C6Pt', PtC6Pt-10/Ph, PtC6Pt-11/Ph, and PtC6Pt-14/p-tol or solvates thereof are compared. In PtC6Pt, the endgroups can avoid van der Waals contact, and define angles of 0 degrees . In PtC6Pt-14/p-tol, the sp3 chains twist around the sp chain in a chiral double-helical motif, with an endgroup/endgroup angle of 189 degrees . The sp3 chains are too short to adopt analogous conformations in the other complexes, but laterally shield the sp chain. NMR spectroscopy shows that the helical enantiomers of PtC6Pt-14/p-tol rapidly interconvert in solution at low temperature. A crystal structure of PtC4Pt shows endgroups that are in van der Waals contact and define an angle of 41 degrees . Reactions with Ar2P(CH2)8PAr2 give PtC4Pt-8/Ar (Ar=Ph, 53 %; p-tol, 87 %). Low-temperature NMR spectroscopy establish non-helical chiral conformations. Electrochemical oxidations of the diplatinum complexes are analyzed, the reversibilities of which decrease with increasing sp chain length.

The First Enantiomerically Pure Helical Noncovalent Tripod for Assembling Nine-Coordinate Lanthanide(III) Podates

Decomplexation of the trivalent lanthanide, Ln(III), from the racemic bimetallic triple-stranded helicates [LnCr(L8)(3)](6+) provides the inert chiral tripodal nonadentate receptor [Cr(L8)(3)](3+). Elution of the latter podand with Na(2)Sb(2)[(+)-C(4)O(6)H(2)](2).5H(2)O through a cation exchange column allows its separation into its inert helical enantiomers M-(+)(589)-[Cr(L8)(3)](3+) and P-(-)(589)-[Cr(L8)(3)](3+), whose absolute configurations are assigned by using CD spectroscopy and exciton theory. Recombination with Ln(III) restores the original triple-stranded helicates [LnCr(L8)(3)](6+), and the associated thermodynamic parameters unravel the contribution of electrostatic repulsion and preorganization to the complexation process. Combining M-(+)(589)-[Cr(L8)(3)](3+) with Eu(III) produces the enantiomerically pure d-f helicate MM-(-)(589)-[EuCr(L8)(3)](CF(3)SO(3))(6).4CH(3)CN, whose X-ray crystal structure (EuCrC(113)H(111)N(25)O(21)S(6)F(18), monoclinic, P2(1), Z = 2) unambiguously confirms the absolute left-handed configuration for the final helix. The associated ligand-centered and metal-centered chiro-optical properties recorded for the complexes MM-[LnCr(L8)(3)](6+) and PP-[LnCr(L8)(3)](6+) (Ln = Eu, Gd, Tb) show a strong effect of helicity on specific rotary dispersions, CD and CPL spectra.

Quantum control of molecular chirality: optical isomerization of difluorobenzo[c]phenanthrene.

The results of a theoretical study are presented on quantum control of a chiral exchange reaction of a polyatomic molecule by using infrared laser pulses. Difluorobenzo[c]phenanthrene was chosen to be the simplest model for its helical chirality exchange reaction. This molecule has two stable configurations: M and P forms. From the viewpoint of chemical reaction dynamics, isomerization is regarded as the movement of one of the two representative points that initially correspond to the two forms to the position of the other representative point, while the other representative point remains in its initial position. The ground-state potential energy surface and dipole moment functions required to control this reaction were evaluated at the MP2/6-31+G(d,p) and MP2/TZV+(d,p) levels of molecular orbital (MO) theory. An effective potential energy surface (PES) that is a function of twisting motion of the benzene rings and wagging motion of the CF(2) group was constructed on the basis of the MO results. An analytical expression for the effective PES and that for the dipole moment functions were prepared to make the isomerization control tractable. A quantum control method in a classical way was applied to the isomerization of preoriented difluorobenzo[c]phenanthrene in low temperature limits. The time evolution of the representative point of the M form and that of the P form are separately evaluated to determine the optimal laser fields. This means that the laser control produces pure helical enantiomers from a racemic mixture. Representative points are replaced by the corresponding nuclear wave packets in this treatment. The derived control laser field consists of two linearly polarized E(x)() and E(z)() components that are perpendicular to each other. These components are pi-phase-shifted when the representative point is in the transition-state regions. Under the irradiation of this laser pulse, one of the two representative points of the isomerization is transferred to the target position along the intrinsic reaction path between the enantiomers, while the other representative point remains in its initial potential well. This results in one-way isomerization control, that is, the M(P) to P(M) form. The isomerization is completed with yields of ca. 70% within a few picoseconds. Temporal behaviors of the nuclear wave packet whose center corresponds to the representative point are drawn to see how the desired chiral exchange reaction proceeds in the presence of the control field, while its reverse process is suppressed.

Intramolecular DNA coiling mediated by metallo-supramolecular cylinders: differential binding of P and M helical enantiomers.

We have designed a synthetic tetracationic metallo-supramolecular cylinder that targets the major groove of DNA with a binding constant in excess of 10(7) M(-1) and induces DNA bending and intramolecular coiling. The two enantiomers of the helical molecule bind differently to DNA and have different structural effects. We report the characterization of the interactions by a range of biophysical techniques. The M helical cylinder binds to the major groove and induces dramatic intramolecular coiling. The DNA bending is less dramatic for the P enantiomer.

Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited: a new assignment of structural changes and crystal-field effects 25 years later.

Variable-temperature (1)H and (13)C NMR measurements of the D(3)-symmetrical triple-helical complexes [Ln(L1-2H)(3)](3)(-) (L1 = pyridine-2,6-dicarboxylic acid; Ln = La-Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)(3)](3)(-) (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P <==> M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La-Eu) and one for small lanthanides (Ln = Tb-Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A(0)2<r (2)> parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb-Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La-Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.

Conformational studies by dynamic NMR. 80.(1) cog-wheel effect in the stereolabile helical enantiomers of dimesityl sulfoxide and sulfone.

The (1)H NMR solution spectra of the title compounds display anisochronous lines for the o-methyl substituents below -170 degrees C, due to the existence of two propeller-like M and P conformational enantiomers. The free energies of activation for the interconversion were determined to be 4.5 and 5.0 kcal mol(-)(1), respectively, for dimesityl sulfoxide and dimesityl sulfone. Molecular mechanics calculations indicate that the enantiomerization process occurs via a correlated rotation (cog-wheel effect) entailing a one-ring flip (gear-meshing) pathway. (13)C NMR (CP-MAS) spectra and X-ray diffraction show that these helical enantiomers are stable in the crystalline state.

Conformational studies by dynamic NMR. 78. Stereomutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: dimesitylketone, dimesitylthioketone, and dimesitylethylene.

The free energies of activation for the enantiomerization of the title compounds (Mes2C = X, Mes = 2,4,6-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol-1 for X = O, S, and CH2, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization barrier in the crystal must be much higher (at least 19 kcal mol-1) than that in solution. Contrary to the case of dimesitylketone, the calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.

Biomimetic copper(I)--CO complexes: a structural and dynamic study of a cali.

Four novel calix[6]arene-based cuprous complexes are described. They present a biomimetic tris(imidazole) coordination core associated with a hydrophobic cavity that wraps the apical binding site. Each differs from the other by the methyl or ethyl substituents present on the phenoxyl groups (OR1) and on the imidazole arms (NR2) of the calix[6]arene structure. In solution, stable CO complexes were obtained. We have investigated their geometrical and dynamic properties with respect to the steric demand. IR and NMR studies revealed that, in solution, these complexes adopted two distinct conformations. The preferred conformation was dictated only by the size of the OR1 group. When R1 was an ethyl group, the complex preferentially adopted a flattened C3-symmetrical structure. The corresponding helical enantiomers were in conformational equilibrium, which, however, was slow on the 1H NMR time scale at -80 degrees C. When R1 was a methyl group, the low-temperature NMR spectra revealed the partial inclusion of one tBu group. The complex wobbled between three dissymmetric but equivalent conformations. Hence, small differences in the steric demand of the calixarene's skeleton changed the geometry and dynamics of the system. Indeed, this supramolecular control was promoted by the strong conformational coupling between the metal center and the host structure. Interestingly, this was not only the result of a covalent preorganization, but also stemmed from weak interactions within the hydrophobic pocket. The vibrational spectra of the bound CO were revealed to be a sensitive gauge of this supramolecular behavior, similar to copper proteins in which allosteric effects are common.

Effect of chiral helical force field on molecular helical enantiomers and possible origin of biomolecular homochirality

Biomolecular homochirality means that the amino acids in proteins, and sugars in nucleic acids, occur in one form: amino acids in L-form and sugars in D-form. So far, the reason why these molecules have such uniform chirality is not known. In this paper, it is suggested that a net natural chiral right-handed helical force field, produced by the Earth's orbital chirality (EOC), could affect the stability of molecular helical enantiomers and make the right-handed helical enantiomers more stable than their left-handed enantiomers, so terrestrial biological systems must select both right-handed helical nucleic acids based on D-sugars, and proteins based on L-amino acids. In given conditions, the maximum energy difference between biomolecular helical enantiomers ( ΔE max ⋎E left − E right ∥ ) caused by the EOC may be calculated theoretically to be in the order: B-DNA > A-RNA > A-DNA ⪢ α-protein > β-protein. Our experimental results strongly supported the hypothesis that the EOC could cause the spontaneous selection and amplification of right-handed helical enantiomers and could be the origin of homochirality on the Earth.

A novel calix[6]arene-based mononuclear copper(I) complex that exhibits chirality at low temperature

A calix[6]arene functionalized in alternate positions at the lower rim with three picolyl groups acts as a biomimetic N3 ligand with cuprous ion. The stable C3 symmetrical complex derived from CuCl exists as a pair of helical enantiomers. A low temperature 1H NMR study revealed that the chirality is transmitted to the calixarene skeleton, thereby providing a chiral cavity around the apical binding site of the metal center.

Trinuclear Double Helicates of Iron(II) and Nickel(II): Self‐assembly and resolution into helical enantiomers

AbstractThe synthesis of the linear tris[terpyridine] 1 with three tridentate binding sites is described. The reaction with metal ions of octahedral coordination geometry, such as FeII or NiII, leads to the self‐assembly of trinuclear complexes [M3(1)2]6+, which display properties in agreement with a double helical structure. The trinuclear iron (II) helicate has been resolved into its enantiomers.

The first [2.2]phane diagonally hetero-substituted in the bridges

The novel sulphide (1a) is much less stable than its isomer (2a), and the stable sulphone (1b) could be resolved into the helical enantiomers (t½ 92.5 min at 100 °C; free enthalpy of racemisation 127 kJ/mol; n.m.r. and c.d. data and results of molecular mechanics calculations are compared to those of isomeric helical molecules.

Synthesis and X-ray structure of [{Li(thf)2PHMes} n ](thf = tetrahydrofuran; Mes = 2,4,6-Me3C6H2)

The reaction between Ph2Mes (Mes = 2,4,6-Me3C6H2) and LiBun in tetrahydrofuran (thf) at ambient temperature affords the crystalline polymeric lithium phosphide [{Li(thf)2PHMes}n], whose X-ray structure shows a helical arrangement of four-co-ordinate P and Li atoms and the presence of both helical enantiomers (P and M), which are related by two crystallographic glide planes.