Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Preparation of 2‐Alkylidene‐Substituted 1,3,4,5‐Tetramethylimidazolines and Their Reactivity Towards RhI Complexes and B(C6F5)3

Abstract: In addition to the known 1,3,4,5-tetramethyl-2-methyleneimidazoline (1a), which exhibits a highly polarized exocyclic C–C bond, a series of novel 2-alkylidene-substituted 1,3,4,5-tetramethylimidazolines 1b–e were synthesized and characterized. The molecular structures of 1b, 1d, and 1e were determined by X-ray diffraction analysis and revealed an increase in the polarization of the exocyclic C–C bond with increasing steric demand of the 2-substituent. On the basis of their ylidic nature, 1a–e show enhanced basicity and reactivity towards Lewis acidic centers. Treatment of 1a and 1b with [{RhCl(cod)}2] or B(C6F5)3 afforded complexes of the type [(L)RhCl(cod)] (4a,b), [(L)RhCl(CO)2] (7a,b) (L = 1a,b) or classical Lewis acid/base adducts [(1a)B(C6F5)3] (8a) and [(1b)B(C6F5)3] (8b). In contrast, complexes [(L)RhCl(cod)] with 1c and 1d as ligands are not stable, and imidazolium dichlororhodate salts 6a and 6b were isolated instead. Rhodium–alkyl complexes 5a and 5b are assumed to be intermediates in this decomposition process, and 5a was characterized by X-ray diffraction analysis. Furthermore, treatment of 1c and 1d with B(C6F5)3 did not afford classical Lewis adducts, and instead imidazolium hydridoborate salts 9a and 9b are formed by hydride abstraction. Surprisingly, we found that 1e does not react with [{RhCl(cod)}2] and forms an abnormal Lewis adduct 10 when treated with B(C6F5)3.

A Zwitterionic Carbanion Frustrated by Boranes – Dihydrogen Cleavage with Weak Lewis Acids via an “Inverse” Frustrated Lewis Pair Approach

Abstract: The synthesis, structural characterization, and acid–base chemistry of [C(SiMe2OCH2CH2OMe)3]Na (2), a sterically encumbered zwitterionic organosodium compound, is reported. 2 is a strong Bronsted base that forms frustrated Lewis pairs (FLPs) with a number of boron-containing Lewis acids ranging from weakly Lewis acidic aryl and alkyl boranes to various alkyl borates. These intermolecular FLPs readily cleave H2, which confirms that even poor Lewis acids can engage in FLP-mediated H2 cleavage provided that the present bulky base is of sufficiently high Bronsted basicity.

Stereoselectivity of Triplet Photocycloadditions:1 Diene−Carbonyl Reactions and Solvent Effects

Abstract: The diastereoselectivity of the photocycloaddition of benzaldehyde to furan was determined (exo/endo = 212:1) and compared with the reaction of other carbonyl compounds and carbonyl analogues. These results were compared with Paterno−Buchi reactions of cycloalkenes and cyclic enol ethers. An increase in steric demand of the α-substituent in benzoyl compounds led to a change in exo/endo-selectivity for furan cycloadditions that was not observed for cycloalkenes or cyclic enol ethers. Different ISC-reactive conformers with enhanced spin−orbit coupling are postulated as a reasonable explanation for the stereoselectivities observed. Additionally, solvent effects were studied, demonstrating the influence of photoinduced electron-transfer steps on the regio- and diastereoselectivity of Paterno−Buchi reactions with 2,3-dihydrofuran in polar solvents. Two bicyclic oxetanes (8 and 10) were characterized by X-ray structure analysis.

The family of ferrocene-stabilized silylium ions: synthesis, 29Si NMR characterization, Lewis acidity, substituent scrambling, and quantum-chemical analyses.

Abstract: The purpose of this systemat- ic experimental and theoretical study is to deeply understand the unique bond- ing situation in ferrocene-stabilized si- lylium ions as a function of the sub- stituents at the silicon atom and to learn about the structure parameters that determine the 29 Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene- stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and char- acterized by multinuclear 1 H and 29 Si NMR spectroscopy. A closer look at the NMR spectra revealed that addi- tional minor sets of signals were not impurities but silylium ions with substi- tution patterns different from that of the initially formed cation. Careful as- signment of these signals furnished ex- perimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanis- tic insight into that substituent transfer in which the migrating group is ex- changed between two silicon fragments in a concerted process involving a ferro- cene-bridged intermediate. Moreover, the quantum-chemical analysis of the 29 Si NMR chemical shifts revealed a linear relationship between dA 29 Si) values and the Fe···Si distance for sub- sets of silicon cations. An electron lo- calization function and electron localiz- ability indicator analysis shows a three- center two-electron bonding attractor between the iron, silicon, and C'ipso atoms, clearly distinguishing the silicon cations from the corresponding carbe- nium ions and boranes. Correlations between 29 Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non-intuitive trends, indicating a com- plicated interplay of steric and elec- tronic effects on the degree of the Fe···Si interaction.