Showing papers on "Steric effects published in 1983"
TL;DR: In this article, the rate of addition of alkyl radicals to alkenes is controlled by steric and polar effects, and the stabilities of the educts and products are of only limited importance, since the transition states for these exothermic reactions occur very early on the reaction coordinate.
Abstract: There are many reactions in which CC bonds are formed by addition of free radicals to alkenes. Information about the mechanism is important for the synthesis of specific target molecules. The rate of addition of alkyl radicals to alkenes is controlled by steric and polar effects. The stabilities of the educts and products are of only limited importance, since the transition states for these exothermic reactions occur very early on the reaction coordinate. Variations in reactivity and selectivity can be described using frontier orbital theory: for nucleophilic radicals the dominant interactions are those between SOMO's and LUMO's, and for electrophilic radicals those between SOMO's and HOMO's. The large differences in the steric effects of α - and β- substituents of alkenes can be explained by postulating an unsymmetrical transition state— the radical approaches one of the C atoms preferentially. Regioand stereoselectivities can be predicted and are determined, in general, by steric effects.
598 citations
209 citations
TL;DR: In this paper, organo-titanium reagents were added stereoselectively to (R)-2,3-isopropylidene glyceraldehyde to give the alcohols, which were converted into the optically active derivatives.
110 citations
TL;DR: Alkoxide ligands may act as four or two electron donor ligands and may readily change between terminal and bridging sites as mentioned in this paper, which allows for the facile interconnversion of saturated and unsaturated metal centers.
102 citations
01 Jan 1983
TL;DR: The definition of steric parameters initially based on Van der Waals raddi (rv) is described in this article, and a table of various sets of rv is given.
Abstract: The definition of steric parameters initially based on Van der Waals raddi (rv) is described. Tables of various sets of rv are given. These different types of rv are shown to be interrelated. The rv values of Bondi are chosen as the standard set. Substituents are classified according to the degree of conformational dependence of their steric effect. Values of the steric parameters v are obtained directly from rv whenever possible. For symmetric MZn groups they are obtained from calculated rv values. For planar π-bonded groups equations are derived which permit the calculation of both v and the corresponding delocalized electrical effect parameters. Values of v for other grups are based on kinetic data scaled by correlation with groups for which v is known. Over 300 values of v are reported.
96 citations
TL;DR: Using a pulsed high-pressure mass spectrometer, the hydrogen-bonded dimer ions BH/sup +/B and monohydrates BH /sup +/.H/sub 2/O of 2-alkylpyridines, 2,6-dialkylpyridine, and tertiary amines were investigated in the gas phase in the absence of solvent effects as discussed by the authors.
Abstract: Using a pulsed high-pressure mass spectrometer, the hydrogen-bonded dimer ions BH/sup +/.B and monohydrates BH/sup +/.H/sub 2/O of 2-alkylpyridines, 2,6-dialkylpyridines, and tertiary amines were investigated in the gas phase in the absence of solvent effects. Steric hindrance causes major entropy effects unfavorable to dimerization or hydration. As long as a single confirmation exists in which the hydrogen bond can obtain optimal geometry, the bond strength is not weakened by steric crowding. However, steric crowding may result in major entropy effects by the hindrance of internal rotors in dimers and monohydrates. 8 figures, 3 tables.
86 citations
TL;DR: Resonance Raman enhancement of the Fe-C-O bending mode upon Soret excitation may be correlated with the overlap between the porphyrin and CO orbitals when the CO ligand is tilted.
Abstract: We report resonance Raman studies of the Fe-C-O distortion in sterically hindered heme-CO complexes. The steric hindrance is provided by a hydrocarbon chain strapped across one face of the heme. Increasing the steric hindrance (by decreasing the chain length), which reduces the CO binding affinity, is found to increase the Fe-CO stretching frequencies: heme 5 (unstrapped), 495 cm-1; FeSP-15, 509 cm-1; FeSP-14, 512 cm-1; FeSP-13, 514 cm-1. This is interpreted in terms of a decrease in the CO effective mass and increased interactions between the C atom of CO and the N atom(s) of the pyrrole ring(s). Resonance Raman enhancement of the Fe-C-O bending mode upon Soret excitation may be correlated with the overlap between the porphyrin (pi*) and CO (pi*) orbitals when the CO ligand is tilted. Its intensity relative to that of the Fe-CO stretching mode increases with increasing steric hindrance in these "strapped hemes". In addition, we have estimated the Fe-C-O angles from isotope data in various heme-CO complexes. It is inferred that the angles are 167 +/- 5 degrees (FeSP-15) and 175 +/- 5 degrees (FeSP-14, FeSP-13, Mb X CO, and Hb X CO).
76 citations
76 citations
TL;DR: In this article, the crystal and molecular structures of dimeric cyclopalladated acetato(OAc)-bridged complexes of a 2-arylpyridine (1 ) and a 2, 6-diary pyridine ligand have been determined by X-ray diffraction.
68 citations
TL;DR: In this paper, the results of Lewis acid catalyzed addition reactions of alkyl halides with unsaturated hydrocarbons were studied and solvolysis constants of model compounds were used to predict the outcome of any such addition reactions if systems with a considerable steric hindrance are excluded.
Abstract: Lewis acid catalyzed addition reactions of alkyl halides 1 with unsaturated hydrocarbons 2 have been studied.
1:l addition products 3 are formed if the addends 1 dissociate faster than the corresponding products 3; otherwise,
polymerization of 2 takes place. For reaction conditions under which 1 and 3 exist mainly undissociated, solvolysis
constants of model compounds can be used to predict the outcome of any such addition reactions if systems with
considerable steric hindrance are excluded.
62 citations
TL;DR: In this article, the existence of steric interactions between the N-substituents of the B-thexyl group was indicated by the 1H and 11B NMR data.
TL;DR: In this article, the cycloaddition rate of cyclooctyne to a number of N-containing heterocyclic compounds can be correlated with the reduction potentials of these dienes provided steric substituent effects are approximately equal.
TL;DR: In this article, a triple quadrupole mass spectrometer was used to measure the proton affinity of 3-aminopentane, and the effect of precursors' internal energy on this measurement was investigated through MS/MS experiments covering excitation through collisions at various energies and target pressures.
TL;DR: In this paper, a systematic mechanistic study was performed for the formation of palladium(II) carbene complexes by nucleophilic attack of aromatic amines on isocyanide derivatives.
TL;DR: In this paper, a Michael-type reaction L-ascorbic acid (1) undergoes addition to acrolein to give the tricyclic hemiacetal lactone (3) was studied by a combination of NMR and IR spectroscopy.
TL;DR: The 1 H NMR spectra of 2,2′-bipyridine complexes have been reported in this article, where the relaxation behavior of the protons attached to the ligands has been studied, and T 1 measurements are shown to be a valuable aid in the assignment of the spectra.
Patent•
15 Apr 1983
TL;DR: In this paper, a process for the anionic polymerization of a conjugated 1,3-diene in a polar solvent is described, where the initiator is a difunctional initiator, especially an electron transfer reagent.
Abstract: A process for the anionic polymerization of a conjugated 1,3-diene in a polar solvent consists of contacting the 1,3-diene with an organo alkali or alkaline earth metal initiator and a sterically hindered Lewis acid, allowing the 1,3-diene to polymerize in the polar solvent and terminating the polymerization reaction. Preferably the Lewis acid is a substituted triaryl derivative of a Group III element, especially boron. However in certain cases (e.g. where the aryl derivative itself is large) the Lewis acid may be an unsubstituted triaryl derivative of a Group III element. Suitable Lewis acids include trimesityl boron and tri (2,6-dimethylphenyl) boron. In preferred embodiments of the present process the 1,3-diene is butadiene, the polar solvent is tetrahydrofuran and the initiator is a difunctional initiator, especially an electron transfer reagent. The main advantages of the present process are that polymers with enhanced 1,4-content, a narrow molecular weight distribution and, in the case of telechelic polymers, with an enhanced difunctional specificity are obtained.
TL;DR: Highly diastereoselective alkylations of propionates of chiral alcohols derived from (+)-camphor are described in this paper. But the latter effects are in part rationalized on the basis of preferential formation of (Z)- or (E)-lithium enolates.
TL;DR: In this article, a zwitterionic/ diradical intermediate arising out of the primary interaction of singlet oxygen with the thiocarbonyl chromophore is believed to be the common intermediate for the ketone and sulfine.
Abstract: Oxidation of diaryl, aryl alkyl, and dialkyl thioketones by singlet oxygen generated via self-sensitization and other independent methods yielded the corresponding ketone and sulfine in varying amounts. A zwitterionic/ diradical intermediate arising out of the primary interaction of singlet oxygen with the thiocarbonyl chromophore is believed to be the common intermediate for the ketone and sulfine. While closure of the zwitterion/diradical to give 1,2,3-dioxathietane would lead to the ketone, competing oxygen elimination is believed to lead to the sulfine. This partitioning is governed by steric and electronic factors operating on the zwitterionic/diradical intermediate.
TL;DR: In this article, the 13C NMR spectra of C-10 epimeric chlorophylls and pheophytins were measured and assigned by chemical shift comparison, by long-range selective 1H decoupling experiments and by an examination of the fully coupled spectra.
Abstract: The 13C NMR spectra of C-10 epimeric chlorophylls a and a′, pheophytins a and a′, pyrochlorophyll a and pyropheophytin a have been recorded and assigned by chemical shift comparison, by long-range selective 1H decoupling experiments and by the examination of the fully coupled spectra. Various factors influencing the 13C chemical shifts of the chlorophyll derivatives, e.g. the coordination of magnesium to the chlorin nucleus, the effect of solvent and the steric strain at the periphery of the macrocycle, have been examined. The 13C NMR spectra of chlorophyll a measured in acetone-d6 and tetrahydrofuran-d8 (THF) were compared, and remarkable solvent effects on the 13C chemical shifts were observed. These effects were interpreted mostly in terms of specific chlorophyll-solvent interactions. Different electron donor and steric properties of acetone and THF were considered to cause conformational alterations in the macrocycle, induced by the ligation of the solvent molecule(s) to the axial position(s) of the central magnesium atom of chlorophyll a. These results show that 13C NMR spectroscopy is a method of high information value for investigations of the unique electron donor acceptor (EDA) properties of the chlorophylls. The structural differences between the C-10 epimeric chlorophylls and pheophytins were examined in terms of the substituent chemical shift (SCS) parameters for the C-10 methoxycarbonyl group. The analysis showed that the change from the (10R) to the 10(S) configuration induces conformational alterations in the whole macrocycle which are, however, most prominent in rings IV and V. Owing to the increased steric interaction (repulsion) between the bulky substituents at C-7 and C-10, the peripheral strain is larger in the (10S) form, and is relieved by more pronounced deviations of rings IV and V from the macrocyclic plane compared with the (10R) form. The examination of the SCS parameters also showed that the peripheral steric strain is dissipated to a larger extent over the entire macrocycle in the Mg-free derivatives. These results confirm the previous conclusions based on 1H NMR and CD data. The possible function of chlorophyll a′ in photosynthesis is briefly discussed.
TL;DR: It appears that steric hindrance prevents the direct attack of Glu-143 on the carbonyl carbon of an extended substrate, therefore ruling out the anhydride pathway in thermolysin-catalyzed hydrolysis of polypeptide substrates and their ester analogues.
Abstract: The mode of binding of the irreversible thermolysin inhibitor ClCH2CO-DL-(N-OH)Leu-OCH3 [Rasnick, D., & Powers, J.C. (1978) Biochemistry 17, 4363-4369] has been determined by X-ray crystallography at a resolution of 2.3 A and the structure of the covalent complex refined to give a crystallographic residual of 17.0%. This is the first such structural study of an active-site-directed covalent complex of a zinc protease. As anticipated by Rasnick and Powers, the inhibitor alkylates Glu-143 in the thermolysin active site, and the hydroxamic acid moiety coordinates the zinc ion. The formation of the covalent complex is associated with a significant shift in a segment of the polypeptide backbone in the vicinity of the active site. This conformational adjustment appears to be necessary to relieve steric hindrance which would otherwise prevent alkylation of Glu-143. It is suggested that this steric hindrance, which occurs for thermolysin but would not be expected for carboxypeptidase A, accounts for the previously inexplicable difference in reactivity of these two metalloproteases toward N-haloacetyl amino acids. The relevance of this steric hindrance to the mechanism of catalysis is discussed. In agreement with previous results [Kester, W. R., & Matthews, B. W. (1977) Biochemistry 16, 2506-2516], it appears that steric hindrance prevents the direct attack of Glu-143 on the carbonyl carbon of an extended substrate, therefore ruling out the anhydride pathway in thermolysin-catalyzed hydrolysis of polypeptide substrates and their ester analogues.
Book•
01 Jan 1983
TL;DR: The upsilon steric parameter is defined and determined using molecular shape descriptors as mentioned in this paper, and applications of various steric constants to quantitative analysis of structure-activity relationships are discussed.
Abstract: Features and problems of practical drug design.- Topological indices for structure-activity correlations.- The upsilon steric parameter - definition and determination.- Molecular shape descriptors.- Volume and bulk parameters.- Applications of various steric constants to quantitative analysis of structure-activity relationships.
TL;DR: In this paper, the first relatively stable unsymmetrical diphosphenes (3e, f, g >) were characterized by 31 P NMR spectroscopy.
TL;DR: In this article, the phase equilibria in aqueous mixtures of alkylpyridines (methyl, dimethyl, and ethyl) are matched with the theoretical parameters describing the strength and orientational specificity of the intermolecular hydrogen bonding.
Abstract: The lattice‐gas models of reentrant phase transitions in binary mixtures with hydrogen bonding, introduced and studied by Walker, Vause, and Goldstein, are shown to be sensitive to subtle changes in the molecular properties of the mixture components. Specifically, the phase equilibria in aqueous mixtures of alkylpyridines (methyl, dimethyl, and ethyl) are matched with the theoretical parameters describing the strength and orientational specificity of the intermolecular hydrogen bonding. The parametric trends are found to agree with those expected from consideration of the electronic and steric effects of ortho‐, meta‐, and para‐alkyl substitution in aromatic compounds. These trends also correlate well with other thermodynamic and kinetic data on these alkylpyridines. Critical exponent renormalization in these systems is also discussed.
TL;DR: In this article, the X-ray crystal structures of Mn2R4(PMe3)2, Mn2(CH2SiMe3), CH2CMe3, and CH2Ph have been determined.
Abstract: The reactions of manganese(II) dialkyls with tertiary phosphines or of MnCl2 with magnesium dialkyls in the presence of phosphines leads to dimeric complexes of stoicheiometry Mn2R4(PMe3)2(R = CH2SiMe3, CH2CMe3, and CH2Ph) and Mn2(CH2SiMe3)4(PR3)2[R3= Et3, Me2Ph, McPh2, and (cyclo-C6H11)3]. The X-ray crystal structures of Mn2(CH2CMe3)4(PMe3)2, Mn2(CH2Ph)4(PMe3)2, and Mn2(CH2SiMe3)4(PMePh2)2 have been determined. In the dimers there are two asymmetrically bridging alkyl groups with one additional terminal alkyl and one phosphine per manganese. The structures show a variation in bond lengths probably due to steric effects, with Mn–Mn distances of 2.667(1)–2.828(1)A, and long Mn–P distances [2.562(1)–2.684(1)A]. There is a close contact between one hydrogen atom on each of the bridging methylene groups and the manganese atom (Mn ⋯ H–C 2.15–2.29 A). Infrared and e.s.r. spectra of the compounds are discussed.
TL;DR: In this paper, a crystal structure determination was carried out for 2,4,6-tripyrrolidino-2',4',6'-trinitrobiphenyl (3a): space group C2/c, a = 16.545, b = 14.071, c = 20.177, 3 = 91.361, Z = 8 (temperature ≈ 120 K), despite the four bulky substituents in the o,o'-positions.
Abstract: Reaction of the activated halonitrobenzenes 2a-f with the bis- and tris(dialkylamino)benzenes 1a-e affords, via a direct nucleophilic substitution, the highly substituted biphenyls 3a-o; the lesser substituted biphenyls 5 and 6 were prepared by an Ullmann reaction. All these biphenyls are deeply colored; the dark red color can be assigned to an intramolecular charge transfer. A crystal structure determination was carried out for 2,4,6-tripyrrolidino-2',4',6'-trinitrobiphenyl (3a): space group C2/c, a = 16.071 (2) A, b = 14.545 (1) A, c = 20.177 (2) A, {3 = 91.361 (9)°, Z = 8 (temperature ≈ 120 K). The dihedral angle between the two arene rings was found to be only 52.5°, despite the four bulky substituents in the o,o'-positions. With this far-from-orthogonal torsional angle about the biphenyl linkage, the strong intramolecular charge transfer from the π system of the donor into the π system of the acceptor arene becomes easily understandable. A PPP calculation with the torsional angles taken from the X-ray structure analysis satisfactorily reproduces the experimental absorption spectrum of 3a. The shift of the long-wavelength absorption between the individual biphenyls 3a-o, 5, and 6 likewise is accounted for satisfactorily by the calculation; it depends primarily on the intrinsic donor strength of the different NR2 moieties (pyrrolidino > dimethylamino > piperidino > morpholino). The IH NMR spectra (in dilute solution) likewise mirror this gradation in Nr2 donor capacity; they also show that steric hindrance at the biphenyl linkage is mainly the result of interaction between the o-dialkylamino groups and the C6 skeleton of the acceptor arene. This is borne out by the crystal structure analysis.
TL;DR: In this article, a single unified free energy relationship for all of these rather disparate chemical processes relates the activation free energy to the standard free energy change for electron transfer and the work term for ion-pair formation.
Abstract: The rates of arene and alkene brominations and mercurations, Diels-Alder cycloaddition as well as alkylmetal cleavages by electrophiles, electron acceptors and free atoms can be quantitatively analyzed in terms of the driving force for electron transfer. The single, unified free energy relationship for all of these rather disparate chemical processes relates the activation free energy to the standard free energy change for electron transfer and the work term for ion-pair formation. The latter is evaluated from charge transfer spectral data of the electron donoracceptor complexes formed as transient intermediates. The large variations in the apparent Bronsted slopes from unity to even negative values in these systems can be readily attributed to changes in the work terms arising from steric effects in ion-pair formation.
TL;DR: In this paper, the teric interaction between trimethylsilyl groups in the title compound is relieved by opening the SiCSi angle (123.2° (9)), by stretching the central CSi bonds slightly (1.889-(4) A) and by a torsional displacement of the methyl groups from staggered positions (25° (5)).