Steric effects control self-sorting in self-assembled clusters

Endohedrally functionalized bis(pyridine) ligands show the ability to self-discriminate when treated with coordinating metals to form self-assembled clusters. Self-sorting between components is controlled by substitution on the interior of the complex. Tuning the size of the internal substituent allows selective heterocluster formation, determined by noncovalent and space-filling interactions. This novel method of self-sorting allows discrimination between ligands of identical geometry and donor type.     

Steric control of the asymmetric synthesis of N-substitued 2-methyl-4-piperidones

Transmission of the iodomethylate of 1,2-dimethyl-4-piperidone by (S)-sec-butylamine gives 1-(S-sec-butyl)-2S-methyl-4-piperidone in 33% optical yield while transamination by (S)-l-methyl-2-phenylethylamine gives a 1:1 diastereomeric mixture of 1-(l-methyl-2-phenylethyl)-2 -methyl-4-piperidone.The decrease in the optical yield is related to the facile opening of the piperidone ring at the C-N bond with subsequent recyclization. The¹³C MIR data indicate that all the diastereomers of the 4-piperidones obtained are in the chair conformation with predominantly equatorial orientation of the methyl group at C₍₂₎. The chiral optical properties were studied and the absolution configurations of the 4-piperidones obtained were established.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1648–1655, December, 1985.     

Steric control in the reactions of 3-pyrazolecarboxylic acid with diorganotin dichlorides

Reaction of 3-pyrazolecarboxylic acid (LH₂) with dibenzyltin dichloride (Bn₂SnCl₂) in the presence of potassium hydroxide afforded a 2D-coordination polymer, [{(Bn₂Sn)₂(μ-L)(μ-LH)(μ-OH)}₂]_n (1), which crystallizes as an ethanol solvate. The repeat unit of the coordination polymer 1 is a tetrameric motif which contains two pairs of centrosymmetrically related dimeric stannoxanes; in the latter the two tin atoms are connected to each other by a μ-OH and a η²-μ-pyrazole ligand. The tetrameric units are linked to each other by the bridging coordination action of the pyrazolecarboxylate ligand to generate a 2D-coordination polymer. An analogous reaction of LH₂ with the sterically encumbered t-Bu₂SnCl₂ afforded the monomeric dinuclear compound [{(t-Bu₂Sn)₂(μ-L)(LH)(μ-OH)}]·H₂O (2). In the latter the two tin atoms are linked to each other by a bridging hydroxide as well as a pyrazole ligand affording a 6-membered ring. The steric hindrance of the tert-butyl groups seems to prevent the elaboration of this unit into a coordination polymer. Both 1 and 2 have been characterized by single crystal X-ray, NMR and ESI-MS studies.     

Steric control of substituted phenoxide ligands on product structures of uranyl aryloxide complexes

A series of uranyl aryloxide complexes has been prepared via metathesis reactions between [UO(2)Cl(2)(THF)(2)](2) and di-ortho-substituted phenoxides. Reaction of 4 equiv of KO-2,6-(t)()Bu(2)C(6)H(3) with [UO(2)Cl(2)(THF)(2)](2) in THF produces the dark red uranyl compound, UO(2)(O-2,6-(t)()Bu(2)C(6)H(3))(2)(THF)(2).THF, 1. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo groups, two cis-aryloxides, and two THF ligands. A similar product is prepared by reaction of KO-2,6-Ph(2)C(6)H(3) with [UO(2)Cl(2)(THF)(2)](2) in THF. Single-crystal X-ray diffraction analysis of this compound reveals it to be the trans-monomer UO(2)(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), 2. Dimeric structures result from the reactions of [UO(2)Cl(2)(THF)(2)](2) with less sterically imposing aryloxide salts, KO-2,6-Cl(2)C(6)H(3) or KO-2,6-Me(2)C(6)H(3). Single-crystal X-ray diffraction analyses of [UO(2)(O-2,6-Cl(2)C(6)H(3))(2)(THF)(2)](2), 3, and [UO(2)Cl(O-2,6-Me(2)C(6)H(3))(THF)(2)](2), 4, reveal similar structures in which each U atom is coordinated by seven ligands in a pseudopentagonal bipyramidal fashion. Coordinated to each uranium are two apical oxo groups and five equatorial ligands (3, one terminal phenoxide, two bridging phenoxides, and two nonadjacent terminal THF ligands; 4, one terminal chloride, two bridging phenoxides, and two nonadjacent terminal THF ligands). Apparently, the phenoxide ligand steric features exert a greater influence on the solid-state structures than the electronic properties of the substituents. Emission spectroscopy has been utilized to investigate the molecularity and electronic structure of these compounds. For example, luminescence spectra taken at liquid nitrogen temperature allow for a determination of the dependence of the molecular aggregation of 3 on the molecular concentration. Electronic and vibrational spectroscopic measurements have been analyzed to examine trends in emission energies and stretching frequencies. However, comparison of the data for compounds 1-4 reveals that the innate electron-donating capacity of phenoxide ligands is only subtly manifest in either the electronic or vibrational energy distributions within these molecules.     

Steric control over arene coordination to beta-diiminate rhodium(I) fragments

The bulky ligands L(X)- (L(X) = (2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2), X =Cl, Me) can be used to generate fluxional mononuclear arene complexes [L(X)Rh(eta4-arene)] (arene = benzene, toluene, m-xylene, mesitylene), which for X = Me disproportionate to fluxional dinuclear complexes [[L(Me)Rh]2(anti-mu-arene)]. For both mononuclear and dinuclear complexes, steric interactions do not stop the fluxionality but govern the preferred orientation of the methyl-substituted arenes, thus allowing indirect determination of the static NMR parameters. For the mu-arene complexes, two distinct types of fluxionality are proposed on the basis of calculations: ring rotation and metal shift. In the solid state, the toluene complex has an eta4(1,2,3,4):eta4(3,4,5,6)-bridged structure; the NMR analysis indicates that the benzene and m-xylene complexes have similar structures. The mesitylene complex, however, has an unprecedented eta3(1,2,3):eta3(3,4,5)-bridged structure, which is proposed to correspond to the transition state for arene rotation in the other cases. Steric factors are thought to be responsible for this reversal of stabilities.     

Local control of antibody binding to hapten-presenting interfaces: Steric and electrostatic interaction

The binding of labeled antibodies to hapten substituted monolayers at the air/water interface has been studied by means of fluorescence microscopy. Haptens with various spacer lengths between the epitope and a hydrocarbon chain, anchoring the molecule to the interface, have been synthesized. With DMPC,^a unspecific binding has been shown to predominate over specific binding due to electrostatic interactions. At high surface pressures the bound antibody is detached because of steric interference with the lipid head groups. Due to a reduction of electrostatic interactions, no unspecific binding is observed to monolayers of cholesterol, which carries a small dipole moment. Mixed monolayers of cholesterol and DMPC separate into two fluid phases, with preferential antibody binding to the cholesterol-enriched phase.     

Steric control in the oligomerisation of phosphazane dimers; towards new phosphorus-nitrogen macrocycles

The reaction of [ClP(mu-NtBu)]2 (1) with H2O (1 : 2 equivalents) in the presence of excess Et3N gives the new chain compound [(mu-O)[P(mu-NtBu)2P(H)=O]2] (3), consisting of two P2N2 rings linked by a mu-O atom and terminating in P(V)(H)=O groups. A similar chain species is obtained from the reaction of the lithiate of [(tBuNH)P(mu-NtBu)2P(H)=O] (5) with [ClP(mu-NtBu)2P(NHtBu)] (2), the product being [(mu-O)[P(mu-NtBu)2P(NHtBu)]2] (6). Compounds 3 and 6 are the first examples of O-bridged chain phosphazanes and potential precursors to new phosphorus-nitrogen macrocycles. The syntheses and X-ray structures of 3, 5 and 6 are reported.     

Steric stabilization II

Summary A theory of steric stabilization is developed that is essentially an extension ofFischer's solvency theory. It treats both interpenetration and compression using a latice approach. This permits the introduction of segment density distribution functions. One virtue of the new theory is that it is in good qualitative agreement with the results of experiment. Some limitations of the theory are also noted.     

Steric stabilization I

Summary A comparison is presented between the qualitative predictions of the various theories of steric stabilization with the results of experiments. It is shown that with the notable exception ofFischer's solvency theory, all of the theories advanced to-date are at odds with results of experiments.Possible reasons for these discrepancies are discussed.     

Steric control of regiochemistry in the reactions of methyl substituted pentadienyl cations with isobutene

The dienylic chlorides 1–5 react with isobutene in the presence of zinc chloride/ether in dichloromethane to give the acyclic adducts 6–10 in 64–91% yield. The orientation effects are in contrast to the predictions of perturbational molecular orbital theory and can be explained on the basis of steric effects.     

Steric enhancement of resonance

The rate constants and the activation energies for the alkaline hydrolysis of some substituted ethyl benzoates furnish evidence for the phenomenon of steric enhancement of resonance. For 3-substituted-4-alkoxybenzoic esters the rate constants calculated on the basis of the principle of additivity of substituent effects are appreciably higher than the observed values, indicating that the 3-substituent does not sterically inhibit the resonance interaction of the alkoxy and ester groups but actually enhances it. The cause of this steric enhancement of resonance is explained. The rates of quaternization of some N,N-dimethylanilines with methyl iodide have also been determined. The observed rate constants of 4-alkoxy-3-methyl-N,N-dimethylanilines are significantly higher than the values predicted on the basis of additivity of group effects. Thus there is enhanced resonance interaction of the alkoxy group with the dimethylamino group increasing the nucleophilicity of the dimethylamino group. A similar steric enhancement of resonance is observed in the case of 3-methyl-4-methylthio-N,N-dimethylaniline also.