Influence of steric factors on the direction of reactions

Russian Journal of Organic Chemistry -     

Electronic and steric control in regioselective addition reactions of organolithium reagents with enaldimines

A reaction mode of imines derived from naphthalene-1-carbaldehyde and acyclic alpha,beta-unsaturated aldehydes with organolitium reagents was dependent on the characteristic nature of a substituent on the imine nitrogen atom. An imine having an electron-withdrawing aryl group on the nitrogen atom behaves as a 1,2-directing imine toward organolithium reagents. In contrast, an imine bearing an alkyl or a bulky aryl group favors 1,4-addition of organolithium reagents. Electronic and steric tuning of a substituent on the imine nitrogen atom for a reaction mode was rationalized on the basis of molecular orbital calculations.     

Electronic absorption spectra of merocyanine forms of spiropyrans

A simple method is proposed for the measurement of the absorption spectra in the visible and UV regions of photocolored solutions of spiropyrans, formed by the action of constant lowintensity UV irradiation, and for the construction, on the basis of these data, of the absorption spectra of the merocyanine forms of spiropyrans. The absorption spectra of the merocyanine forms of two spiropyrans in various solvents are examined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1534–1538, November, 1972.     

Electronic factors in reactions of sulfinylamines with dienes

By means of the frontier MO approach, using the MNDO method, it has been shown that the (4+2)-cycloaddition of N-sulfinylalkylamines and N-sulfinylpolyfluoroalkylamines to 1,3-butadiene and isoprene proceeds under normal electronic conditions. The -electron stabilization energies of the transition state of the reactions have been estimated. An interpretation is given for the high reactivity of the N-sulfinylpolyfluoroalkylamines and the regioselectivity of their reactions with isoprene.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 3, pp. 258–261, May–June, 1992.The author wishes to thank participants in the Republic Seminar on Quantum Chemistry (May 19–21, 1992) for valuable discussion of this work.     

Azirines in reactions involving the formation of a pyrrole ring (review)

The literature data on the synthesis of pyrroles from azirines are correlated in this review. The following processes are examined: the thermolysis and photolysis of azirines with aromatic radicals and double bonds, the rearrangement of aryl- and vinylazirines under the influence of metal complexes, the cycloaddition of olefins, the cycloaddition of acetylenes, and reactions with CH acids.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1299–1312, October, 1986.     

Electronic and steric effects in binding of deep cavitands

A deep, self-folding cavitand responds to minor electronic differences between suitably sized adamantane guests. Binding constants range from <0.5 to 4000 M(-1) for guests as similar as 1-bromoadamantane and 1-cyanoadamantane. The barriers to guest exchange also vary up to 3 kcal mol(-1).     

Electronic and steric effects in thermal denitrosation of N-nitrosoamides

N-Alkyl-N-nitrosoamides undergo competitive reactions whose rates are dependent upon the interplay of a number of factors. There already exists a significant body of work delineating the effects of pH on the partitioning of the nitrosoamides along their deaminative (-N(2)) and denitrosative (-"NO(+)") pathways. In this paper, the issue of pH dependence is discussed with particular attention to nitrosoamide decompositions in nonaqueous media. The role of the acidity of the medium in the partitioning of the nitrosoamide between deamination and denitrosation and in the choice of deaminative pathways is revisited. In nonaqueous media under near-neutral conditions, the partitioning's pH dependence is evidently accompanied by a sensitivity to structural features in the nitrosoamide. Thus, diminution of steric crowding around the N-nitroso moiety as well as the presence of strongly electron-withdrawing acyl units (i.e., those derived from strong acids, e.g., tosyl and trifyl) increase the relative yield of amides by encouraging the denitrosative pathway. A mechanism for thermal denitrosation of nitrosoamides under near-neutral conditions is proposed in which rapid protonation at the acyl O rather than slow protonation at the amidic N is the first step in the reaction profile. A rate-limiting, bimolecular reaction between the O-conjugate acid and adventitious nucleophiles at the nitrosyl group then occurs followed by rapid tautomerization to amide.     

Electronic and steric effect manifestations in the structure of 9-Azidoacridine

The crystal structure of 9-azidoacridine (9AA) was determined by X-ray structure analysis (the compound crystallizes in the rhombic system). The crystallographically independent fragment of the structure of 9AA was found to contain two molecules. Both molecules were nonplanar, and the azido group was displaced out of the acridine nucleus plane by 34.6° (molecule A) and 28.6° (molecule B). The barriers to azido group rotations about the C-N bond were calculated by the semiempirical PM3 and nonempirical DFT B3LYP quantum-chemical methods. According to the B3LYP/6-31G^* calculations, the structures with the azido group situated in the acridine nucleus plane and perpendicularly to this plane are 0.21 and 1.66 kcal/mol, respectively, higher in energy than the completely optimized structure, in which the dihedral angle between the azido group and acridine nucleus planes is 32°. The PM3 method overestimates the steric strain energy of 9AA and underestimates the energy of azido group conjugation with the acridine nucleus compared with B3LYP calculations.     

Opacity analysis of steric requirements in elementary chemical reactions

The orientation dependence of the barrier to reaction is incorporated into the classical line-of-centers model to obtain the dependence of the “cone of acceptance” upon impact parameter and collision energy. Simple expressions for the orientation-averaged opacity function, the reaction cross section and the steric factor are derived. Comparison is made with recent classical trajectory calculations by Blais and Truhlar for the H + D₂ reaction.     

The effect of steric bulk in Sonogashira coupling reactions

The rates of Sonogashira coupling reactions using [Pd-PR(3)] complexes depend on a combination of the steric bulk of phosphines and substrates; however, below a critical cone angle of ca. 170 degrees the catalytic activity drops drastically.     

Electronic and steric effects in fragmentation reactions of Os3(CO)9(mu-C4Ph4)

P-Donor nucleophiles of cone angle >or=145 degrees are known to react with Os3(CO)9(mu-C4Ph4) quite differently from those with cone angles 相似文献   

Electronic and steric effects on the rate of the traceless Staudinger ligation

Interplay between electronic effects imparted by phosphinothiol substituents and steric effects imposed by amino-acid reactants affects the rate of the traceless Staudinger ligation of peptides in a predictable manner.     

Role of steric factors in formation of O—H...M hydrogen bonds with metallocenes

Molecular mechanics calculations of geometric parameters and energies of molecular complexes with a O-H...M hydrogen bond have been performed for osmocene and decamethylosmocene with three proton donors. The results of calculations demonstrated that when rings are methylated, steric hindrances to formation of this hydrogen bond increase. This is the reason for anomalously low formation constants of H-bonded Cp ₂ ^* M molecular associates compared to CP₂M associates.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1925–1927, October, 1995.The work was supported by the Russian Foundation for Basic Research (Project No. 93-03-4610) and the International Science Foundation (Grant MP5 000).     

Nitrile oxides of the thiophene series: influence of steric factors on stability and reactivity in reactions of 1,3-dipolar cycloaddition

The synthesis of a series of di- and trialkyl-substituted thiophenecarbonitrile oxides is described. Their addition to styrene is studied together with the quantitative evaluation of the influence of different alkyl groups on the rate of the 1,3-dipolar cycloaddition reaction by the method of competing reactions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1034–1040, August, 1988.     

电子和空间位阻效应促进铱配合物催化1-丁醇经Guerbet反应选择制备2-乙基己醇

2-乙基己醇是一种增塑剂醇,可用于合成增塑剂、表面活性剂和溶剂等诸多化工领域.目前工业上2-乙基己醇的合成是以化石资源丙烯为原料,与氢气和一氧化碳通过氢甲酰化反应制备1-丁醛,后者经羟醛缩合和催化加氢反应生成2-乙基己醇.由生物质发酵产生的1-丁醇为原料,只需要经Guerbet反应可一步直接生成2-乙基己醇.比较而言,...     

Electronic and steric effects of substituents on the coordinating properties of porphyrins

Kinetics of the formation of zinc complexes of porphyrin with various substituents was studied in acetic acid and pyridine. The introduction of strong electron-withdrawing substituents (nitro groups) to the meso-positions of the porphyrin macrocycle was shown to change the reaction rate by an order of magnitude or less compared to the phenyl-substituted analogs. However, the introduction of a large number of bulky substituents leads to the deformation of the porphyrin ligand and thus affects much stronger the coordination properties of porphyrins, decreasing or increasing the rate of the complexation reaction by several orders of magnitude.