Complete defluorination of 1,2,3,4-tetramethyl-5-(trifluoromethyl)cyclopentadiene by titanium tetrakis(dimethylamide)--selective formation of a cyclic hexanuclear titanium fluoroamide and 6,6-dimethylaminotetramethylfulvene

1,2,3,4-Tetramethyl-5-(trifluoromethyl)cyclopentadiene (Cp*CF3-H, 1) reacts with [Ti(NMe2)4] (2) under mild conditions to give [Ti(mu-NMe2)(NMe2)(mu-F)(F)]6 (3) in nearly quantitative yield. The molecular structure of 3 consists of a ring of six [TiF2(NMe2)2] edge-bridged octahedra. Titanium complexes containing the Cp*CF3 ligand, which was the primary intention of these investigations, were not observed. C5Me4=C(NMe2)2 (4) was isolated as a by-product. The complete defluorination of an aliphatic CF3 group occurs during the reaction. The reaction mechanism involves the primary formation of a difluorofulvene intermediate C5Me4=CF2 (5), which was monitored by NMR studies. Density functional theory calculations predict a highly charged C6 atom (+0.87) in 5, which is discussed as the driving force of the reaction.     

Quadricyclanes. Part I: Photoelectron spectra and electronic structure

The photoelectron spectra of quadricyclane ( 3 ) and 3-methylidene quadricyclane ( 4 ) have been measured. The results served as a basis for the elucidation of the electronic structure of these compounds, which agreed with theoretical calculations. It is found that the symmetry of the HOMO of 3 is different when compared to that of its valence isomer norbornadiene ( 1 ). The results also indicate that the structure of the highest occupied orbitals cannot be derived by only considering the Walsh-orbitals of the two three-membered rings. In addition one of the Walsh-components of the four-memebered ring has to be taken into account.     

Towards an Understanding of the Carbon-Carbon Bond

In this essay, the classical question of “the influence of the number and kind of substituents on the strength of the C? C bond”, is pursued with the modern tools of contemporary physical organic chemistry. Based on the work of Karl Ziegler, the products and kinetics of thermolysis of a large number of highly branched aliphatic hydrocarbons and phenyl- or cyano-substituted derivatives were investigated. For each class of compounds, a linear relationship was found between the free enthalpy of activation of the homolytic cleavage of the weakest C? C bond and the strain energy in the ground state. These relationships permitted a quantitative separation of steric and electronic effects on the cleavage of C? C bonds. The influence of the size of the substituent groups on bond angles, bond lengths, and the conformational behavior of model compounds was studied by means of experimental structure determinations and force field calculations. C? C bond lengths up to 164 pm, bond angles at tetracoordinate carbon as large as 126°, and unusual eclipsed and gauche preferred conformations were found.     

Modelluntersuchungen zur papainkatalysierten Peptidsynthese im wäßrig-organischen Zweiphasensystem

Several model peptides have been synthesized enzymatically using papain as a catalyst in biphasic aqueous-organic systems. The effect of different cosolvents,pH, buffer concentration, and reaction time on the papaincatalyzed synthesis was examined. A comparison of the results obtained indicates that water-immiscible organic solvents provide higher yields than methanol in papain mediated peptide synthesis with carboxyl components in the carboxyl free form. Furthermore, it could be established that papaincatalyzed peptide synthesis can be considerably speeded up by employing acyl peptide esters instead of acyl peptides. The former should promote the rapid formation of the acyl-enzyme intermediate.     

Model studies on protease-catalysed peptide synthesis using 9-fluorenylmethoxycarbonyl protected amino acid derivatives

Summary Fmoc-Phe-OMe,Fmoc-Ala-OMe andFmoc-Gly-OH were coupled with H-Leu-NH₂ under catalytic action of chymotrypsin, papain and thermolysin, respectively. The influence of different reaction media and several reaction parameters, such as reactants and enzyme concentrations as well as reaction time, on the peptide bond formation was investigated. Abbreviations: IUPAC-IUB rules for peptides are followed, see (1984) Eur. J. Biochem.138: 9. All amino acids except of Gly are ofL-configuration.DMF=N,N-dimethylformamide,DTE=dithioerythritol,EDTA=ethylenediaminetetraacetic acid, disodium salt,N/C=ratio nucleophile component/carboxyl component,-OMe=methyl ester,TLC=thin layer chromatography.     

Modeling a reaction path by molecular mechanics: Dimerization of carbon free radicals

A variable force field model for radical dimerization was developed. It uses MM2 force constants for most atoms and new parameters only for the core atoms involved in bonding changes. The change in hybridization from sp² to sp³ is modeled using distance-dependent switch functions. The validity of the model has been tested by calculating the minimum energy path of the dimerization of di-tert-butylmethyl radicals. The calculated and experimental values for the enthalpy of activation of both dimerization of the radicals and dissociation of the dimer are in excellent agreement. The model has also been successfully applied to the stereoselective dimerization of 1-phenylneopentyl radicals: The form of the potential energy surface yields an explanation for the observed stereoselectivity. Another common feature in radical dimerization seems to be the formation of adsorption complexes prior to dimerization that can lead to increased reactivity. The results suggest that it is important to analyze the whole reaction path and not only the transition state alone.     

Studies on enzymatic peptide synthesis in biphasic aqueous-organic systems with product extraction

The dipeptide derivativesZ-Tyr-Leu-NH₂ andMca-Tyr-Leu-NH₂ were synthesized by -chymotrypsin-catalyzed coupling reactions in solvent systems consisting of buffer and ethyl acetate. In comparison to a pure aqueous medium, in which only insignificant synthesis takes place, the product formation is greatly enhanced in a biphasic medium due to extraction of the dipeptide into the organic phase. The influence of several reaction parameters, such as buffer concentration, reaction time, volume ratio of organic and aqueous phase, and reagent concentration on the yield ofZ-Tyr-Leu-NH₂ was investigated. Replacement of the hydrophobicZ-group by the more hydrophilic chloroacetyl group resulted in better dipeptide yields at higher reaction rates. Abbreviations: IUPAC-IUB rules for peptides are followed, see Eur J Biochem 27: 201 (1972).Ac = acetyl,Glt = 4-carboxybutyryl (glutaryl),Mca = monochloroacetyl,Z = benzyloxycarbonyl, –OMe = methyl ester, –Nan = 4-nitroanilide, TLC = thin layer chromatography. All amino acids are ofL-configuration.     

Papainkatalysierte kinetisch kontrollierte Peptidsynthese. Verwendung von Alkylestern des Arginins als Aminokomponente

Summary The papain-catalyzed reaction of esters of Z-alanine with various arginine esters was investigated. Using H-Arg-OPr ⁱ as a nucleophile the expected dipeptide product results in high yield. Otherwise, reactions with arginine esters of primary alkohols provide products undergoing further reactions. This allows the synthesis of N-protected peptide esters containing two or more arginine residues in a one-step reaction. The influence of reaction conditions on the process was investigated.

AbkürzungenDie IUPAC/IUB-Regeln für Aminosäure- und Peptidderivate wurden befolgt; vgl. Eur. J. Biochem.53, 1 (1975). Die verwendeten Aminosäuren hatten (L)-Konfiguration.-Glo-Gly-OH=Glykolylglycinester,-OCam=Carboxamidomethylester, -OBu ⁱ=Isobutylester,-OMe=Methylester,-OPr ⁱ=Isopropylester,Z=Benzyloxycarbonyl.