Geltungsbereich und Mechamismus der durch Silberionen katalysierten Propargylester-Allenylester-Umlagerung nach Saucy und Marbet

The mechanism of the catalysis of the reversible (propargyl ester)/(allenyl ester) rearrangement ( 10 ? 11 ) by silver(I) ions was investigated, using optically active and diastereoisomeric esters as well as ¹⁴C- and ¹⁸O-labelling. In order to work with crystalline materials, mainly p-nitrobenzoates ( 10 , 11 : R⁴ = p? O₂N? C₆H₄) were used. In some cases the rearrangement 10 ? 11 was studied using acetates (R⁴ = CH₃). The alkyl substituents R¹, R², R³, were widely varied (cf. Tables 1, 2). The solvents in which the rearrangements were performed were in most cases dry chlorobenzene and 96% aqueous dioxane. Silver tetrafluoroborate, the benzene complex of the latter, and silver trifluoroacetate (in chlorobenzene) as well as silver nitrate (in aqueous dioxane) served as catalysts. The amounts of the silver catalysts used varied between 0,5 and 10 mol-%; reaction temperatures applied were in the range 35–95°, The results obtained are as follows:

• 1 The rate-determining step of the (propargyl ester)/(allenyl ester) rearrangement ( 10 ? 11 ) occurs in a silver(I) complex with the substrates ( 10 , 11 ), which is formed in a pre-equilibrium. This follows from kinetic experiments (cf. Fig. 6, 7, 8, 10) and the fact that the rate of rearrangement (of 10a ) is strongly decreased when cyclohexene is added (cf. Fig. 9). In solvents which are known to form complexes with silver(I) ions the rate of rearrangement (of 10a )is much slower than in solvents with similar dielectric constants but with small capacity for complex formation with silver(I) ions (cf. Table 4). Taking into account what is known about silver(I)-alkene and -alkyne complexes (cf. [18]), it is obvious that the (propargyl ester)/(allenyl ester) rearrangement occurs in a π-complex of the silver(I) ion with the triple bond in the propargyl ester or one of the two C,C double bonds in the allenyl ester, respectively.
• 2 The shift of the carboxyl moiety in the reversible rearrangement 10 ? 11 occurs intramolecularly. p-Nitrobenzoic acid-[carboxyl-¹⁴C] is not incorporated during the rearrangement, neither in the reactant 10 nor in the product 11 and vice versa. A crossing experiment gave no mixed products (cf. Scheme 2, p. 882).
• 3 An internal ion pair can be excluded for the rearrangement 10 ? 11 because the ¹⁸O-carbonyl label in the reactant is found exclusively in the alkoxy part of the product (cf. Scheme 3, p. 886, and Table 9). Thus, the rearrangement 10 ? 11 occurs with inversion of the carboxyl moiety.
• 4 The rearrangement of optically active propargyl esters ( 10g , 10i ) leads to completely racemic allenyl esters ( 11g , 11i ). However, rearrangement of erythro- and threo- 10j -[carbonyl-¹⁸O] (Scheme 3) shows that the stereospecifically formed allenyl esters erythro- and threo- 11j -[¹⁸O]-epimerize rapidly in the presence of silver(I) ions. This epimerization is twice and forty times, respectively, as fast as the rearrangement of the corresponding propargyl esters (cf. Fig. 1–5). During epimerization or racemization the ¹⁸O-label is not randomized (cf. also Scheme 4, p. 898).
• 5 The equilibrium of the rearrangement 10 ? 11 depends on the bulkiness of the substituents R¹, R², R³ and of the carboxyl moiety (cf. Table 2).

Taking into account these facts (points 1–5), the reversible (propargyl ester)/(allenyl ester) rearrangement promoted by silver(I) ions can be described as a [3s, 3s]-sigmatropic reaction occurring in a silver(I)-π-complex with the C,C triple bond in 10 and a C,C double bond in 11 . It is suggested that complex formation in 10 and 11 occurs with the π-bond which is not involved in the quasicyclic (containing six orbitals and six electrons) transition state of the rearrangement (Fig. 11). Thus, the rearrangement is of a type which has recently been called a charge-induced sigmatropic reaction (cf. [26]). Therefore, in our case, the catalysis by silver(I) ions is of a different type from that of transformations of strained cyclic molecules promoted by silver(I) ions (cf. [14] [16] [27]–[31]). Side reactions. Whereas the rearrangement of propargyl esters 10 in presence of silver tetra- fluoroborate in chlorobenzene or silver nitrate in aqueous dioxane leads to the corresponding allenyl esters 11 , the rearrangement of 10 with silver trifluoroacetate, especially in the presence of trifluoroacetic acid, results in the formation of the dienol esters 12 and 13 , which clearly are derived from 11 (see Scheme 1, p. 881). As shown by the rearrangement of 11 in the presence of p-nitrobenzoic acid-[carboxyl-¹⁴C], 12 and 13 arise in part from a not isolated di-p-nitrobenzoate (cf. Scheme 6, p. 905), since radioactivity is found in 12 and 13 .     

Self-Replication of Oligonucleotides in Reverse Micelles

The coupling between the tri(deoxynucleotides) d[(MeO)C-G-Ap] ( 1 ) and d[(NH₂)T_d5′-C-G-] ( 2 ) to yield the phosphoramidate-linked (hexadeoxy-nucleotide) d[(MeO)C-G-Anh⁵′T_d5′-C-G] ( 3 ) was investigated both in aqueous solution and in reverse micelles constituted of CTAB (cetyl(trimethyl)ammonium bromide) in hexane/pentan-1-ol 9:1. No siginificant difference was found concerning the yield and the kinetics of the reaction in the two systems. The coupling between 1 and 2 was also carried out in the presence of the template d[(MeO)C-G-A-T-C-G] ( 4 ), an analogue of 3 , so as to reproduce the conditions of template-directed self replication. It was shown that the trinucleotide coupling in the presence of a template obeys the so-called square-root law both in H₂O and in reverse micelles. No significant difference of the time course of the reaction in H₂O and in reverse micelles was observed. This shows that self-replication of oligonucleotides occurs within geometrically bounded structures, which represents a step forward in the mimicking of minimal life processes.     

Dynamics of chloromethanes in cryptophane-E inclusion complexes: a 2H solid-state NMR and X-ray diffraction study

In this paper, we present a variable temperature (2)H solid-state NMR investigation of cryptophane-E:chloroform and cryptophane-E:dichloromethane inclusion complexes. The (2)H line shapes and nuclear spin relaxation rates were analyzed in terms of the distribution of C-D bond orientations and the time scale of the guest dynamics. It was found that encaged chloroform produces broad (2)H spectra, and that its reorientation is relatively slow with a correlation time of approximately 0.17 mus at 292 K. In contrast, the (2)H line shapes of encaged dichloromethane are narrow and the motion of this guest molecule is fast with a correlation time of approximately 1.4 ps at 283 K. The (2)H NMR data were complemented by an X-ray diffraction study of the cryptophane-E:dichloromethane structure, which was utilized in the analysis of the NMR parameters.     

Photo-induced reduction of flavin mononucleotide in aqueous solutions

The photo-induced reduction of flavin mononucleotide (FMN) in aqueous solutions is studied by absorption spectra measurement under aerobic and anaerobic conditions. Samples without exogenous reducing agent and with the exogenous reducing agents ethylene-diamine-tetraacetic acid (EDTA) and dithiothreitol (DTT) are investigated. Under anaerobic conditions the photo-induced reduction with and without reducing agents is irreversible. Under aerobic conditions the photo-reduction without added reducing agent is small compared to the photo-degradation, and the photo-reduction of FMN by the reducing agents is reversible (re-oxidation in the dark). During photo-excitation of FMN the dissolved oxygen is consumed by singlet oxygen formation and subsequent chemical reaction. After light switch-off slow re-oxidation (slow absorption recovery) occurs due to air in-diffusion from surface. EDTA degradation by FMN excitation leads to oxygen scavenging. The quantum efficiencies of photo-reduction under aerobic and anaerobic conditions are determined. The re-oxidation of reduced FMN under aerobic conditions and due to air injection is investigated.     

Umcyclopropanisierung bei der Tetrabromierung eines tricyclischen Ketons zu 3exo, 4endo, 6exo-Tribrom-7-brommethyl-1,5-dimethyl-tricyclo[3.2.1.02,7]octan-8-on

Transcyclopropanation during the Tetrabromination of a Tricyclic Ketone to 3 exo, 4 endo, 6exo-Tribromo-7-bromomethyl-1,5-dimethyl-tricyclo[3.2.1.0^2,7]octan-8-one Bromination of the tricyclic ketone 1 with an excess of bromine at low temperature gives in approximately 30% yield the highly crystalline tricyclic tetrabromide 2 (Scheme 1). The structure of 2 was established by NMR.- and especially X-ray-analysis (Fig.1). Treatment of 1 with 1 mol-equ. of bromine gives an unstable dibromide, to which the structure 3 was assigned on the basis of its NMR.-spectrum and its further bromination to 2 (Scheme 1). In the course of the tetrabromination of 1 the original cyclopropane ring is opened in the first step ( 1 → 3 ) and another cyclopropane ring is formed in the second step ( 3 → 2 ) (cf. Scheme 3).     

Quantitative Bestimmung von Mono- und Dienen in Kohlenwasserstoffgemischen

Zusammenfassung Konjugierte Diene und Monoene werden colorimetrisch in Kohlenwasserstoffgemischen durch Kupplung mit diazotiertem p-Phenylendiamin bestimmt. Die Methode ermöglicht auch die Bestimmung beider Kohlenwasserstoffgruppen nebeneinander.

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Summary Conjugated dienes and monoenes may be determined in mixtures of hydrocarbons by colorimetry of the dye obtained by coupling with diazotized p-phenylenediamine. Both groups of hydrocarbons can also be determined in presence of each other.

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Der ESSO-AG Hamburg danken wir für weitreichende Unterstützung der vorliegenden Arbeit und für die Erlaubnis, die gefundenen Resultate zu veröffentlichen.

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Herrn Prof. Dr. C. Mahr zum 65. Geburtstag gewidmet.

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Marxmeier, H.: Diplom-Arbeit, Marburg 1960.

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Wolf, W.: Staatsexamensarbeit, Marburg 1962.     

Preparation and certification of solutions of perdeuterated polycyclic aromatic compounds intended for use as surrogate internal standards

Two standard solutions of deuterated polycyclic aromatic compounds (PACs) have been prepared for use as surrogate internal standards. Solution DPAC-1 contains 21 deuterated PACs, and is intended for use with mass spectrometric (MS) detection. Most of the difficulties in certifying concentrations in DPAC-1 arose from the fact that none of the individual compounds was 100% deuterated, so that effects of mass spectrometric fragmentation are convoluted with those of isotopic distributions. The best methods are discussed for using such internal standards so as to minimize these problems, together with those arising from kinetic isotope effects. Solution DPAC-2 contains 6 deuterated PACs, and is primarily intended for use with reversed-phase high-performance liquid chromatography (HPLC) with fluorescence detection (FLD, dual programmed wavelength mode), in which the signals for analyte and internal standard are separated chromatographically rather than via the detector. Full details of the preparation of these solutions are described. In addition, examples of their use in the analysis of a certified coal-tar extract (NIST SRM 1597) are described briefly. In one example a novel HPLC-MS technique was employed, and in the other the HPLC-FLD technique was used.NRCC No. 38030     

The electronic relaxation of benzophenone in the vapor phase

The time-resolved luminescence of benzophenone was studied for two excitation wavelengths as a function of pressure. In isolated molecule conditions a single microsecond emission was observed. At increasing pressure the lifetime of this emission becomes shorter, while simultaneously a millisecond emission shows up, its intensity increasing with pressure. The microsecond emission was identified as a combination of hot phosphorescence and diluted fluorescence of the singlet—triplet scrambled molecular eigenstates, the millisecond emission as the normal thermalized phophorescence. From the analysis that is possible after this identification it appears that both radiative and nonradiative rates increase with excitation energy. The rate of formation of thermalized triplet levels is about 20% of the molecular collision rate.     

A Semi-Empirical Model of the Energy Barrier of Proton Transfer Reactions

The energy barrier in proton transfer reactions is described by a Johnston-type equation (1) (n = order of bond to be broken). The barrier model is discussed in terms of free energies. The V_i values are free energies of ionic cleavage in aqueous solution of the X? H and Y? H bonds; they are computed from eqns. (4c) and (4d). The values of p₁ and p₂ affect curvature (absence or presence of maximum) and symmetry of the barrier. It is postulated that pi is a typical constant of the reacting bond and can be transferred from one transition state to another. With the aid of eqn. (1) and its first derivative, values of pi and n_m (bond order at maximum of barrier) can be based on quantities determined experimentally, Δ≠ and ΔG. For O? H bonds, pi ≈ 1.0. For C? H bonds pi is larger than 1.0 and depends on the structure of the carbanionic moiety (influence of resonance and inductive effects). As there cannot be a maximum if p₁ = p₂ = 1.0, the suggested model of the barrier leads to a better understanding why proton transfer must be ‘fast’ in some reactions and ‘slow’ in others. The computed values of n_m may be utilized to gain some insight into the nature of the transition states; they supply a basis for the discussion of primary hydrogen isotope effects.     

Two-photon excitation spectroscopy of phenanthrene singlet states below 50000 cm−1

The two-photon excitation spectrum of phenanthrene in liquid solution is reported in the energy range 29000–49000 cm^?1. Comparison with the one-photon spectrum and extended CNDO/S calculations allows assignment of eight singlet states. The strongest two-photon band is assigned to the B_b state not seen directly in the UV spectrum. This high intensity and other features of the spectrum are in sharp contrast to the pairing selection rules which forbid two-proton transitions to “plus” states in alternate hydrocarbons.