Steric Effects Of Substituents Research Articles

Understanding α,β-Unsaturated Imine Formation from Amine Additions to α,β-Unsaturated Aldehydes and Ketones: An Analytical and Theoretical Investigation

A combination of in situ IR spectroscopy (ReactIR) and DFT calculations have been used to understand what factors govern the selectivity in the addition of primary amines to α,β-unsaturated aldehydes and ketones, i.e., 1,2- versus 1,4-addition. It has been found that the 1,2-addition products (α,β-unsaturated imines following addition and elimination) usually predominate for most systems. However, exceptions, such as methyl vinyl ketone, selectively give 1,4-addition products. This has been rationalized by DFT calculations that show that major conformational effects are involved, controlled mainly by steric effects of carbonyl substituents, resulting in a model that provides simple and predictable preparation of α,β-unsaturated imines for in situ utilization in synthesis.

Efficient photoluminescence from the encumbered platinum(II) porphyrins

Completely enclosed platinum(II) porphyrins exhibit the dramatic improvements in photoluminescence properties since the intermolecular π–π interactions and intramolecular vibrations are effectively suppressed by the encapsulating and steric effect of peripheral bulky and saturated substituents. The absolute photoluminescence quantum yields (PLQY) is up to 0.46–0.55 and significantly improved over their parent PtTPP.

Exploring the Wittig Reaction: A Collaborative Guided-Inquiry Experiment for the Organic Chemistry Laboratory

A two-week guided-inquiry Wittig reaction experiment was developed for the undergraduate organic laboratory. In week one, groups are assigned a research topic to investigate the Wittig reaction, such as the effect that substituents on the carbonyl compound may have on E/Z selectivity. A hypothesis, controlled by a provided chemical list, is developed along with a series of experiments to test predictions. For example, more Z isomer might be expected from a steric effect of methyl substituents on o-, m-, and p-tolualdehydes. In the second week, students perform reactions by adapting a general procedure and obtain E/Z selectivities from 1H NMR. Compiled data are shared online as groups aim to support or refute their hypotheses. As a result of the experiment, students gain insights into the nature of science and the analysis of data as well as a better understanding of the research process.

ChemInform Abstract: Ruthenium‐Catalyzed Olefin Metathesis Accelerated by the Steric Effect of the Backbone Substituent in Cyclic (Alkyl)(amino) Carbenes.

AbstractThe metathesis catalyst shows very high activity and selectivity towards the ring closing metathesis of (I), as well as in the ethenolysis of methyl oleate (III) compared to readily available Grubbs‐Hoveyda catalysts.

Monomethylaluminum and dimethylaluminum pyrrolylaldiminates for the ring-opening polymerization of rac-lactide: effects of ligand structure and coordination geometry

Two series of aluminum alkyl complexes supported by pyrrolylaldiminate ligands, LAlMe2 (1a-7a) and L2AlMe (1b-7b), were successfully synthesized and characterized by NMR spectroscopy and elemental analysis. Reactions of trimethylaluminum with the corresponding pyrrolylaldiminate ligands in the molar ratios of 1 : 1 and 1 : 2 yielded dimethylaluminum pyrrolylaldiminates (1a-7a) and monomethylaluminum pyrrolylaldiminates (1b-7b), respectively, in good yields. The structure of 3b, determined by single-crystal X-ray diffraction, displayed a distorted trigonal bipyramidal geometry with the τ value of 0.65. Upon addition of 1 equivalent of benzyl alcohol, all complexes promoted the living ring-opening polymerization of rac-lactide with a good control over molecular weights and polydispersities. Complexes 6a and 7a were found to efficiently mediate the immortal polymerization in the presence of excess equivalents of benzyl alcohol (up to 5 equivalents), as evidenced by the narrow PDI values and the good agreement between the experimental M(n) values and monomer/benzyl alcohol ratios. The steric and electronic effects of the imine nitrogen substituents had a strong influence on the polymerization activities both in catalytic activity and polymer microstructure. The catalytic activity decreased as follows: 4-Me-C6H4 (3) > C6H5 (1) ≈ 4-F-C6H4 (2) ≈ 2-Me-C6H4 (5) > 4-OMe-C6H4 (4) ≫ 2-(t)Bu-C6H4 (6) > adamantyl (7). In comparison, the catalytic activity of the monomethylaluminum complex is slightly higher than that of the dimethylaluminum counterpart. The polymerization of rac-lactide by 6b yielded heterotactically enriched polylactide (P(r) = 0.60) whereas the isotactic-enriched polymer (P(m) = 0.74) was obtained from 7b.

QSAR analysis of the catalytic asymmetric ethylation of ketone using physical steric parameters of chiral ligand substituents

We have demonstrated that a validated QSAR (quantitative structure–activity relationship) model can be constructed between sterimol steric parameters of the N-substituents of chiral 1,2-amino-phosphoramide ligands and the enantiomeric ratios of alcohol products produced in the asymmetric additions of diethylzinc to acetophenone, which is powerful for predicting the steric effects of ligand substituents on enantioselectivities and instructive for ligand optimization.

Regioselectivity in the Nucleophile Trapping of Arynes: The Electronic and Steric Effects of Nucleophiles and Substituents

The regioselectivity in nucleophile trapping is investigated with arynes generated directly from bis-1,3-diynes. The regioselectivity is profoundly influenced by not only the nature of nucleophiles but also the substituents on the arynes, which is the consequence of both the unfavorable steric interaction between the incoming nucleophile and the nearby substituent and the inherent electronic bias induced by different substituents on the arynes.

Influence of the position of the substituent on the efficiency of lipase-mediated resolutions of 3-aryl alkanoic acids

Hydrolase-catalysed kinetic resolutions to provide enantioenriched α-substituted 3-aryl alkanoic acids are described. (S)-2-Methyl-3-phenylpropanoic acid (S)-1a was prepared in 96% ee by Pseudomonas fluorescens catalysed ester hydrolysis, while, Candida antarctica lipase B (immob) resolved the α-ethyl substituted 3-arylalkanoic acid (R)-1b in 82% ee. The influence of the position of the substituent relative to the ester site on the efficiency and enantioselectivity of the biotransformation is also explored; the same lipases were found to resolve both the α- and β-substituted alkanoic acids. Furthermore, the steric effect of substituents at the C2 stereogenic centre relative to that for their C3 substituted counterparts on the efficiency and stereoselectivity is discussed.

ChemInform Abstract: Rhodium(III)‐Catalyzed Indazole Synthesis by C—H Bond Functionalization and Cyclative Capture.

AbstractRegioselectivity of the reaction is mainly controlled by the steric effects of the aromatic substituent at nitrogen [cf.

Electronic and steric effects of substituents in series of Zn2+ asymmetrical bis-Schiff-base ligands complexes on catalytic ring-opening copolymerization of CHO and MA

Based on the half-unit Schiff-base ligand HL and series of asymmetrical bis-Schiff-base ligand H2Ln (n = 1–6) in situ synthesized from the reaction of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP), o-phenylenediamine, and/or different salicylaldehyde derivatives, complexes [Zn(L)2] (1) and series of [Zn(Ln)] (n = 1–6, 2–7) are obtained, respectively. Complex 1 proves to be inactive, resulting from its saturated octahedral coordination environment around the central Zn2+ ion, while in complexes 2–7, the unsaturated coordination environment for the catalytic active centers (Zn2+ ions) permits the monomer insertion for the effective bulk or solution copolymerization of CHO (cyclohexene oxide) and MA (maleic anhydride). As to the electron and steric effect in complexes 2–7 on the catalytic behaviors, the copolymerization results show that the introduction of withdrawing –Br substituent para to the phenoxide group endows higher reactive activity, while the bulkiness effect of the encumbering substituent (–Br, –OMe or tert-butyl) ortho to the phenoxide groups is in favor of the chain growth of the copolymers. Moreover, all the bulk copolymerizations in 2-7/DMAP (4-(dimethylamino)pyridine) systems afford poly(ester-co-ether)s, while lower catalyst and co-catalyst concentration for 3/DMAP system is helpful for the formation of perfectly alternating polyester in solution. Of the three co-catalysts, DMAP is found to be the most efficient, while an excess thereof is detrimental for the chain growth of the copolymers.

An assessment of the intercarbon stretching phenomenon in C-substituted “pseudocloso” {3,1,2-RuC2B9} metalladicarbaboranes

DFT calculation of cluster shapes and their related boron-vertex 11B nuclear shieldings for a range of substituted 12-vertex metalladicarbaborane cluster compounds [3,1,2-R2-3-(η6-C6H6)-3,1,2-RuC2H9H9] confirms that the array of experimentally observed C–C distances in the group of progressively more open closo, ‘semipseudocloso’ and ‘pseudocloso’ clusters results from the inherent propensity of the [(η6-C6H6)RuC2H9H9R2] cluster to occupy, in the absence of steric drivers, either closo-structured or so-called ‘pseudocloso’-structured fundamental local minima on the overall potential energy surface. The shallow profiles of the potential wells, together with the electronic and steric influences of substituents on the cluster carbon vertices, alter the relative energies of the closo and ‘pseudocloso’ minima, and alter the energy profile between them, thereby accounting for the observed variations in the distances in the crystallographically measured C–C separations. We propose that these differ fundamentally due to either the closo or ‘pseudocloso’ configuration being electronically preferred, with some instances of the ‘pseudocloso’ configuration being compressed towards closo by packing forces, or stretched further beyond ‘pseudocloso’ by steric effects of bulky substituents on carbon. This contrasts with one view postulated in the literature which holds that steric repulsion between the C-bound substituents is the sole driver that forces an inherently fundamental closo cluster to open into an anomalous ‘pseudocloso’ configuration. In two reported cases, the cluster flexibility is evident in solution and, in these cases, GIAO calculation of the 11B chemical shieldings explains the measured 11B chemical shifts, which can be interpreted in terms of the dynamic interchange of the closo and ‘pseudocloso’ species. A currently held view is that the ‘weighted average’ of 11B chemical shifts reflects the inherent degree of electronic ‘pseudocloso’ character in a particular cluster can therefore often equally be rationalised in terms of lability between similarly stable fundamentally closo and ‘pseudocloso’ forms.

Molecular Tailoring: Substituent Design for Hexagermabenzene

The preparation of new, heavy element compounds with an unusual bonding situation is still a real challenge since, with the lack of solid chemical knowledge, every synthesis is unpredictable and eventual. In this paper, we analyze the general reaction scheme of reductive dehalogenation of halogermanes, which has led to the formation of several interesting structures, for example, digermyne, tetragermahedrane, hexagermaprismane, and octagermacubane, and point out that all of these syntheses can be explained by the principle of energy minimum and the special steric effect of bulky substituents. We have found that the proper choice of the bulky substituent can result in only one stable minimum on the potential energy surface, which could be realized in the synthetic works. We also demonstrate that this recognition can be exploited to design appropriate substituents for the synthesis of new organometallic structures. On the basis of our approach, we suggest Rind-type and terphenyl-type substituents for the synthesis of tetragermacyclobutadiene and hexagermabenzene, respectively.

Nanothallium(III) sulfide from dithiocarbamate precursors: Synthesis, single crystal X-ray structures and characterization

Nanothallium sulfide was formed from Tl(chmdtc)3(1), Tl(chedtc)3(2) and Tl(dchdtc)3(3) (where chmdtc=cyclohexyl methyl dithiocarbamate, chedtc=cyclohexylethyl dithiocarbamate, dchdtc=dicyclohexyl dithiocarbamate). Single crystal structures of the complexes are reported. The ease of formation was the highest for (3) and is in line with the longest mean Tl–S and thioureide C–N bonds observed for (3). The nano metal sulfide formed is spherical in nature and has been characterized by PXRD, EDX, TEM. Tl4S3 was stable up to 300°C. Thioureide stretching bands are observed at 1471, 1468, 1448cm−1 for (1), (2) and (3) respectively. Thioureide stretching band of (3) is significantly lower than others due to steric effect of cyclohexyl substituents. Thallium(III) complexes exhibit fully allowed charge transfer transitions(CT) which appeared as intense absorptions. 1H NMR shows that the α-CH and α′-CH3 protons are affected to a maximum effect on complexation. 13C NMR spectra show thioureide carbon signal around 200ppm.

Construction of Optically Active Quaternary Propargyl Amines by Highly Enantioselective Zinc/BINOL‐Catalyzed Alkynylation of Ketoimines

A general and efficient method for the highly enantioselective alkynylation of ketoimines through a zinc/1,1'-bi-2-naphthol (BINOL)-catalyzed process has been developed. A variety of ketoimines, including α-fluoroalkyl α-imine esters, α-aryl α-imine esters, and trifluoromethyl aryl ketoimines, are applicable and provide their corresponding quaternary propargyl amines in excellent yields with high ee values (up to 99 % ee). Both the steric and electronic effects of substituents at the 3,3' positions of BINOL are critical for the reaction efficiency and enantioselectivity. To demonstrate the usefulness of the method, (R)-α-CF3 α-proline has been prepared in a highly efficient manner. The notable features of this protocol are its broad substrate scope, high reaction efficiency (up to 99 %) and enantioselectivity (up to 99 % ee), low catalyst loading (5 mol % of BINOL derivative), and mild reaction conditions.

Bond lengths in organometallic and coordination complexes: Substituent effects

The literature data on substituent influence on the bond lengths obtained from X-ray crystallography or quantum chemical calculations for 32 series of the intermolecular, intramolecular as well as transition metal complexes have been considered, using the correlation analysis. Generally the bond lengths were first established to depend on the joint influence of the inductive, resonance, polarizability, and steric effects of substituents. The presence or otherwise of one or the other effects as well as the relation between their contributions to the overall change in bond length are determined by the type of series. The reason for the occurrence of the polarizability effect (an electrostatic ion–dipole interaction) is the appearance of an excess charge on the atoms of bond studied as a result of the charge transfer from the donor centre of the complex to the acceptor one. The contribution of the polarizability effect may be as great as 55%.

Relation between the N-NO2 bond length and stability of the secondary nitramines

The fact of the constancy of activation entropy of N-NO2 bond homolysis in a series of secondary nitramines was utilized for correction of the experimental values of activation energy E of this process proceeding from the reliable data for the rate constants of the nitramines decomposition in solutions. When comparing the refined values of E (kJ mol−1) with the N-N bond length dN-N (A) the following correlations were obtained: for cyclic and framework nitramines E = 663 − 356dN-N, and for the aromatic nitramines E = 1810 − 1227dN-N. A linear relationship between E and d is observed in the series of similar compounds. It depends on the electronic and steric effects of substituents.

DFT study of 1-, 4-, and 5-substituted 2-oxo-1,2,3,4-tetrahydropyrimidines: substituent steric and electronic effects, and ring flipping

Steric and the electronic effects caused by the substituents in the 1-, 4-, and 5-positions of substituted 2-oxo-1,2,3,4-tetrahydropyrimidines were investigated using density functional theory at the B3LYP/6-31++G(d,p) level. Results of this study show that the heterocyclic ring adopts a pseudo-boat conformation, in which the C4 and N1 atoms are deviated from ring planarity. The C4-substituent occupies a pseudo-axial position and the space orientation of the substituent depends on the type and position of the additional substituent in this aryl group. The heights of the C4 and N1 atoms from the boat plane and the orientation of 5-CO moiety toward the heterocyclic ring depend on the electronic and steric effects of the substituents in the various positions. Ring flip calculations for 4-phenyl substituent explain the extreme steric effect caused by the substituent in the 1-position. These calculations indicate the more favored pseudo-axial orientation of the phenyl group over the equatorial orientation.

Steric effect of substituents in haloarenes on the rate of cross-coupling reactions

The relative reactivity of ortho- and para-methyl-substituted iodoarenes in the Sonogashira reaction and palladium-catalyzed methoxycarbonylation, as well as of similarly substituted bromoarenes in the Suzuki reactions and cobalt-catalyzed methoxycarbonylation, was studied. Introduction of a methyl group into the para position of aryl halide slows down the cross-coupling. o-Methylhaloarenes are less reactive in palladiumcatalyzed reactions as compared to both unsubstituted haloarene and para-substituted analog. The presence of a methyl group in the ortho position with respect to the reaction center accelerates cobalt-catalyzed methoxycarbonylation.

ChemInform Abstract: Substituent Constants and Substituent Effects in Organoselenium and Organotellurium Compounds

AbstractReview: 99 refs.

ONIOM Study of the Mechanism of Olefin Hydrogenation by the Wilkinson’s Catalyst: Reaction Paths and Energy Surfaces of trans- and cis-Forms

Abstract The mechanism of the olefin hydrogenation by Wilkinson’s catalyst is examined by the ONIOM method taking account of the substituents of the phosphine ligands. We optimized the equilibrium and transition state structures involved in the catalytic cycles by both active species, trans-[RhCl(PPh3)2] (1Rt) and cis-[RhCl(PPh3)2] (1Rc), and determined the energy profiles of these catalytic cycles. The steric repulsion between the two phosphine ligands in the cis-form was very small throughout the reaction, and in fact 1Rc was more stable by 3.3 kcal mol−1 than 1Rt by the stacking interactions between Ph groups, indicating that not only 1Rt but also 1Rc is a possible catalytically active species. The entire energy profiles showed that the olefin insertion is a rate-determining step in the case of the trans active species 1Rt. Although the activation energy of the olefin insertion is reduced so much in the case of the cis active species 1Rc, the transition states TS2 and TS3 of olefin insertion and isomerization also lie at the highest point of the energy surface. However, the cis-form is more favorable than the trans-form, since the top of the energy surface is lower for the cis-form than for the trans-form. The calculations suggested that if we think a large-size substituent of olefin, the cis-form that provides a larger space for the reaction will have an energy advantage. The steric or electronic effect of the Ph substituents of the phosphine ligands did not affect the entire energy profile substantially in both cases of trans- and cis-forms, although the inclusion of the Ph groups is important to evaluate the stability of isomers and to determine the most favorable path.