Showing papers on "Steric effects published in 1977"

Cleavage at Asn-Gly bonds with hydroxylamine.

Abstract: Publisher Summary This chapter presents a procedure for nonenzymic cleavage of proteins with hydroxylamine, which provides a relatively specific means of producing large peptide fragments suitable for further chemical analysis. Cleavage occurs at Asn-Gly bonds and results from the tendency of the asparaginyl side chain to cyclize, forming a substituted succinimide that is susceptible to nucleophilic attack by hydroxylamine. The increased susceptibility of Asn-Gly bonds in comparison with other asparaginyl bonds may result from the greater ease with which the asparaginyl side chain can cyclize in the absence of steric hindrance imposed by a side chain on the succeeding amino acid. The extent of cleavage achieved varies with the protein; cleavage is enhanced by complete denaturation of the protein and by the use of 6 M guanidine as a solvent during hydroxylaminolysis. A low level of cleavage at Asn-X bonds has been observed in some cases, but aspartyl bonds appear to be resistant under conditions used. The infrequency of Asn-Gly bonds in most proteins results in the production of very large fragments that may overlap CNBr-produced fragments and could serve as new start points for sequential Edman degradation.

The nonclassical ion problem

Abstract: 1. That Fascinating Nonclassical Ion Problem.- 1.1. Introduction.- 1.2. Origins.- 1.3. The Nonclassical Ion Era.- 1.4. Steric Assistance.- 1.5. An Alternative Interpretation.- 1.6. The Rococo Period of Carbonium Ion Structures.- 1.7. Difficulties in Challenging an Accepted Theory.- 1.8. Further Difficulties-A "Soft" Theory.- 1.9. Still Further Difficulties-Selective Reviews.- 1.10. Conclusion.- Comments.- 2. Steric Assistance in Solvolytic Processes.- 2.1. Introduction.- 2.2. Steric Assistance in the Solvolysis of Highly Branched Alkyl Derivatives.- 2.3. Steric Assistance in the Relative Effects of Methyl and tert-Butyl Groups.- 2.4. Steric Effects in Norbornyl Derivatives.- 2.5. Steric Effects in Ring Systems.- 2.6. Conclusion.- Comments.- 3. ?-Participation-A Factor in Fast Rates?.- 3.1. Introduction.- 3.2. n-, ?- and ?-Participation.- 3.3. ?-Participation and Fast Rates.- 3.4. ?-Participation vs. Steric Assistance.- 3.5. Steric Assistance-Not ?-Participation.- 3.6. The Fast Rates of Exo-Norbornyl and Cyclopropylcarbinyl.- Comments.- 4. Carbon-Bridged Cations.- 4.1. Introduction.- 4.2. What Is a Nonclassical Ion?.- 4.3. ?-Bridging vs. Hyperconjugation.- 4.4. Static Classical, Equilibrating Classical, or ?-Bridged Cations.- 4.5. Stereochemistry and ?-Bridged Cations.- 4.6. ?-and Aryl-Bridged Cations.- 4.7. Conclusion.- Comments.- 5. The Cyclopropylcarbinyl Cation.- 5.1. Introduction.- 5.2. Exceptionally Fast Rates of Solvolysis.- 5.3. Rearrangements in the Solvolysis of Cyclopropylcarbinyl Derivatives.- 5.4. ?-Bridged Structures for the Cation.- 5.5. Stereochemical Characteristics.- 5.5. Conclusion.- Comments.- 6.The 2-Norbornyl Cation.- 6.1. Introduction.- 6.2. Nonclassical "Structures" for the 2-Norbornyl Cation.- 6.3. Basic Facts.- 6.4. Transition State or Intermediate.- 6.5. Equilibrating Cations.- 6.6. Are Exo Rates Unusually Fast?.- 6.7. Do the High Exo:Endo Rate Ratios Require ?-Bridged Cations?.- 6.8. Do the High Exo:Endo Product Ratios Require ?-Bridged Cations?.- 6.9. Theoretical and Empirical Solutions.- 6.10. Conclusion.- Comments.- 7. Stabilized 2-Norbornyl Cations.- 7.1. Introduction.- 7.2. The Gassman-Fentiman Approach.- 7.3. Exo:Endo Rate Ratios in 2-p-Anisyl-2-norbornyl.- 7.4. Exo:Endo Rate Ratios in 2-p-Anisyl-2-camphenilyl.- 7.5. Exo:Endo Product Ratios.- 7.6. Goering-Schewene Diagrams.- 7.7. Other 2-p-Anisyl Systems.- 7.8. Other Stabilized Systems.- 7.9. The Selectivity Principle and the 2-Norbornyl Problem.- 7.10. Conclusion.- Comments.- 8. Exo:Endo Rate Ratios as a Steric Phenomenon.- 8.1. Introduction.- 8.2. Steric Characteristics of the Norbornyl System.- 8.3. Steric Characteristics of the 7,7-Dimethylnorbornyl System.- 8.4. Steric Hindrance to Ionization.- 8.5. Misconceptions.- 8.6. Steric Hindrance to Ionization and the Foote-Schleyer Correlation.- 8.7. Steric Effects in U-Shaped Systems.- 8.8. Exo:Endo Rate and Product Ratios as a Steric Phenomenon.- 8.9. Conclusion.- Comments.- 9. Equilibrating Tertiary 2-Norbornyl Cations.- 9.1. Introduction.- 9.2. Theoretical Considerations.- 9.3. The 1,2-Di-p-anisyl-2-norbornyl Cation.- 9.4. The 1,2-Diphenyl-2-norbornyl Cation.- 9.5. The 1,2-Dimethyl-2-norbornyl System.- 9.6. Conclusion.- Comments.- 10. Effect of Increasing Electron Demand.- 10.1. Introduction.- 10.2. Basic Considerations.- 10.3. Electron Demand in ?-Systems.- 10.4. Electron Demand in Cyclopropylcarbinyl Systems.- 10.5. Electron Demand in 2-Norbornyl Systems.- 10.6. Increasing Electron Demand in Secondary 2-Norbornyl.- 10.7. Extrapolation of Data from the Tertiary to the Secondary Systems.- 10.8. Conclusion.- Comments.- 11. Substituent and Structural Effects in 2-Norbornyl.- 11.1. Introduction.- 11.2. Secondary vs. Tertiary 2-Norbornyl Cations.- 11.3. Substituents as a Probe for Charge Delocalization.- 11.4. The Search for Charge Delocalization at C6.- 11.5. The Search for Charge Delocalization at C2.- 11.6. The Search for Charge Delocalization at C1.- 11.7. Deuterium as a Substituent.- 11.8. Deactivating Substituents.- 11.9. Structural Modifications.- 11.10. Conclusion.- Comments.- 12. Capture of Unsymmetrical 2-Norbornyl Cations.- 12.1. Introduction.- 12.2. Deamination of 2-Norbornylamine.- 12.3. Attempted Trapping of Unsymmetrical Cations in Solvolytic Processes.- 12.4. Capture of Unsymmetrical Cations in Solvolytic Processes.- 12.5. Additions to 2,3-Dideuterionorbornene.- 12.6. Addition of Deuterium Chloride to Norbornene.- 12.7. Addition of Deuteriotrifluoroacetic Acid to Norbornene.- 12.8. Addition of Perdeuterioacetic Acid to Norbornene.- 12.9. Resume.- 12.10. Conclusions.- Comments.- 13. Equilibrating Cations under Stable Ion Conditions.- 13.1. Introduction.- 13.2. Equilibrating Cations.- 13.3. Applicability of Results to Solvolysis.- 13.4. 13C NMR Shifts as a Measure of Charge Delocalizations.- 13.5. Nonadditivity of 13C Shifts as a Basis for Nonclassical Structures.- 13.6. PMR Spectra.- 13.7. ESCA Spectra.- 13.8. Cation Stabilities.- 13.9. Conclusion.- Comments.- 14. New Concepts-New Systems.- 14.1. Introduction.- 14.2. The Hyperconjugative Model.- 14.3. The Exo-6-H Participation Model.- 14.4. The Search for a Stereoelectronic Contribution.- 14.5. New Proposed ?-Bridged Systems.- 14.6. Conclusion.- Comments.- 15. Final Comments.- 15.1. Introduction.- 15.2. The Search for ?-Participation and ?-Bridging.- 15.3. Does ?-Delocalization Exist?.- 15.4. Are There ?-Bridged Cations?.- 15.5. Position on ?-Bridged Cations.- 15.6. Conclusion.- Comments.- Epilog.

Steric Interactions in Organic Chemistry: Spatial Requirements of Substituents

Abstract: From the very extensive factual material available on steric interactions in organic chemistry, the present paper reviews the investigations that provide information about the “size” (“spatial requirement”) of the substituents. After an introductory section on the results obtained with the aid of spectroscopic data and by analysis of chemical reactions, attention is turned to conformational processes, including hindered rotation in ethanes, in the biphenyl system, in butadienes, multiply substituted arenes, molecular propellers, and triptycenes. The main emphasis is placed on the explanation of sterically hindered ring inversions in bridged arenes, a class of compounds particularly suitable for the study of steric interactions. The results obtained by variation of the parameters in this system are also discussed with respect to the spatial requirements of organic substituents.

Energy-structure correlation in metalloporphyrins and the control of oxygen binding by hemoglobin.

Abstract: The contribution of the porphyrin skeleton to the potential energy surface metalloporphyrins is calculated by the semiempirical method of quantum mechanical extension of the consistent force field to eta electron molecules. This calculation makes it possible to correlate the observed structure of metalloporphyrins with the strain energy of the porphyrin skeleton. It is found that the out-of-plane metal displacement in pentacoordinate heme systems is due to both the restricted size of the porphyrin hole and the "1-3" steric interaction between the axial ligand and the heme nitrogens. The main components of the active site of hemoglobin are simulated by a histidine-heme-oxygen system. The energy surface of this system provides a quantitative explanation for the control of ligand binding by hemoglobin. It is shown that the heme acts as a diaphragm, designed to provide simultaneous binding to the histidine and the sixth ligand under the steric requirements of the 1-3 interactions. The dependence of the hemoglobin potential surface on the distance between the proximal histidine and the heme plane is evaluated for the R and T states, using the calculated heme potential and the observed energy of heme-heme interaction.

The 13C NMR spectra of poly(1‐pentenylene) and poly(1,3‐cyclopentylenevinylene)

Abstract: 13C NMR spectra were obtained for polymers made by ring-opening polymerization of cyclopentene and bicyclo[2.2.1]hept-2-ene respectively. The fraction of cis double bonds could be determined with much greater precision from 13C NMR spectra than from IR spectra and varied from 0,66 to 0,31 for the samples of poly(1-pentenylene), (2), and from 1,0 to 0,14 for the samples of poly(1,3-cyclopentylenevinylene), (4). This is the first time an all-cis polymer of 4 has been reported. The spectra of 2 showed a cis (upfield) and trans (downfield) peak for each of CH and α-CH2, but only one peak for β-CH2. The spectra of 4 showed multiple fine structure, the main splittings corresponding to a cis (upfield) and trans (downfield) peak for α-CH, and a reverse line order for the other three carbons; subsidiary splittings were observed for all but the olefinic carbons, interpreted in terms of sensitivity of the chemical shifts to the cis/trans structure at the next nearest double bond. A complete interpretation of the line orders in 4 is given in terms of steric compression effects. The possibility that ring tacticity accounts for some of the fine structure cannot be entirely discounted. The stereochemistry of 4 is discussed in relation to the four possible modes of addition of monomer to a carbene chain carrier during polymerization.

Electronic and steric effects on the rate of oxidative addition of methyl iodide to diaryl(2,2′-bipyridyl)platinum(II) complexes

Abstract: The rates of oxidative addition of methyl iodide to the comolexes [PtR2(bipy)][bipy = 2,2′-bipyridyl; R = Me, Ph, 4-XC6H4(X = F, Cl, Me, or OMe), and 3-XC6H4(X = F or OMe) have been studied.The second-order rate constants follow the series R = Me > 4-MeOC6H4 > 4-MeC6H4 > 3-MeOC6H4 > Ph > 4-FC6H4 > 4-ClC6H4 > 3-FC6H4, and can be correlated with the energy of a metal-to-ligand charge-transfer transition in the electronic spectra of the platinum(II) complexes. The complexes [Pt(4-CF3C6H4),(bipy)] and [Pt(2-MeC6H4)2(bipy)] fail to react with methyl iodide, in the former case due to electronic effects of the CF3 group and in the latter case due to steric hindrance by the methyl substituents.

13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho‐ and peri‐interactions involving the carbonyl substituent

Abstract: Carbon-13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives 13C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non-hindered molecules and molecules in which the carbonyl substituent is subject to ortho- or peri-interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range 13C,13C coupling constants involving the carbonyl carbon, in particular 2J and 3J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. 2J(s-t)>2J(s-c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s-t and s-c refer to the orientation of the CO group relative to the aromatic bond in question with respect to the connecting single bond). For ketones 3J(t,s-c) 3J(c) and 3J(t)>2J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.

The dimerization of 1‐alkynes by rhodium(I) catalysts. Effect of phosphorus ligands on regioselectivity

Abstract: The dimerization of 1-alkynes by rhodium(I) complexes in the presence of phosphorus ligands is described. The products are linear and branched dimers, the ratio of which is correlated with the electronic parameters, vCO of Ni(CO)3L, of the ligands L, but no simple correlation is apparent between their steric parameter and the selectivity. Electron-donating ligands promote the formation of the linear dimer. The substituents of the 1-alkynes also affect the distribution of linear and branched dimers. Electron-donating substituents prefer linear isomer to branched one. The reactivity of the substituted 1-alkynes (RCCH) increased with substituent R in the order

13C n.m.r. spectra of N,N‐dialkylamides

Abstract: Carbon-13 n.m.r. spectra of formic, acetic, propionic and butyric acid amides with N,N-di-n-alkyl substituents have been completely assigned with the aid of extensive double resonance experiments. The data obtained were used to study long range steric effects on chemical and solvent shifts.

Crystal and molecular structure of tetra-µ-benzoato-bisquinolinedi-cobalt(II), a binuclear cobalt(II) carboxylate

Abstract: The structure of the title compound has been determined by three-dimensional X-ray crystal structure analysis. The crystals are orthorhombic with unit cell dimensions a= 17.150(10), b= 19.703(11), c= 11.655(7)A, Z = 4, space group Pcab. Full-matrix least-squares refinement using 1 221 reflections reached R= 0.061. The centrosymmetric molecule comprises two cobalt ions bridged by four benzoate anions with the two oxygen atoms of each benzoate group bonded to different cobalt ions. Square-pyramidal five-co-ordination of cobalt is completed by a quinoline molecule, but steric effects cause significant deviations from regularity with N–Co–O angles from 92 to 104°, Co–O distances from 2.017 to 2.072 A, Co–O–C angles from 119.1 to 130.3°, and an included Co–Co–N angle of 168.3°. The Co–Co separation is 2.832 A.

Carbon‐13 n.m.r. studies of methylquinolines and methylisoquinolines

Abstract: Spectra are reported for all seven methylquinolines, nine dimethylquinolines, six methylisoquinolines and one dimethylisoquinoline. Substituent effects for the α, ortho, meta, para, vinylogous para and peri positions are reported for the monomethylcompounds. Additivity of these substituent effects is demonstrated for the dimethyl compounds which also exhibit a vicinal dimethyl steric effect.