Reactivite des carbanions benzyliques : VII. Assistance electrophile et stereoselectivite d'alkylation de l'isopropyl-9 lithio-10 dihydro-9,10 anthracene

The stereoselectivity of alkylation of 9-isopropyl-10-lithio-9,10-dihydroanthracene is affected by the coordinating ability of the solvent and the leaving group, as well as by temperature. The results point to electrophilic assistance by the cation for the contact ion pairs.     

Reactivite des carbanions benzyliques : II. Stereochimie d'alcoylation des ethyl et isopropyl-9 lithio-10 dihydro-9,10 anthracenes

A detailed study of the stereochemistry of alkylation of 9-ethyl- and 9-isopropyl-10-lithio-9,10-dihydroanthracenes by alkyl halides (R′ = Me, Et, i-Pr) is reported. The reaction is under kinetic control. cis-9-Ethyl-10-isopropyl-9,10-dihydroanthracene has been found more stable than the trans isomer. If we take into account the stereoelectronic factor, which favors axial attack, and the main steric interactions in the transition state, stereochemistry seems to be mainly controlled by the competition between axial-axial and peri interactions.     

Reactivite des carbanions benzyliques : VIII. Etude de la structure d'alkyl-9 lithio-10 dihydro-9,10 anthracenes par rmn du proton et spectrometrie d'absorption electronique; influence du substituant,du solvant et de la temperature

The structure of the 9,10-dihydroanthracenyl anion and of a series of 9-alkyl-10-lithio-9-10-dihydroanthracenes (9-R-10-LiDHA, I–V where R = H, Me, Et, i-Pr, t-Bu) was studied in solution by electronic absortion spectrometry and proton magnetic resonance. Our electronic absorption results, in addition to those of other authors, show that the contact ion pairs (c.i.p.) have an absorption at λ_max}- 400 nm (I–III) and 415 nm (V) whereas the loose ion pairs (l.i.p.) absorb at λmax}- 450 nm (I–V). In the NMR the chemical shift of the proton para with respect to the carbanionic center was examined as a function of solvent (THF, THF/HMPA, and in some cases ether or pure HMPA) and temperature (+20 to ?40°C). The para proton is shielded significantly with regard to the aromatic protons of the hydrocarbon (Δδ(H_para) ca. 1–1.7 ppm). The weakest shielding was observed in ether, in agreement with the existence of c.i.p. The largest shielding (THF/HMPA or pure HMPA) is in connection with the presence of l.i.p. where the negative charge is less localised at position 10. Moreover, in the same solvent, and at the same temperature, Δδ(H_para) was observed to increase with the substituent bulk, up to the point that there are only l.i.p. present. As found previously (namely for the fluorenyl anion) the l.i.p./c.i.p. ratio increases when temperature decreases. The results of this structural study allow to rationalize the protonation stereochemistry of 9-alkyl-10-lithio-9,10-dihydroanthracenes in the above-mentioned solvents.     

Reactivite des carbanions benzyliques : VI. Structure et reactivite du trimethylsilyl-9 dihydro-9,10 anthracene; preparation des bis(trimethylsilyl)-9,9 et trimethylsilyl-9 alkyl-9 dihydro-9,10 anthracenes

The structure of 9-trimethylsilyl-9,10-dihydroanthracene (Me₃SiDHA) has been studied by NMR; the coupling constants of the three 9,10 protons do not change at between 25 and 90°C (ABM spectrum; J_AB }- 18.2; J_AM ? 1.2; J_BM ? 0.3 Hz in toluene-d₈), indicating that the Me₃Si group prefers the quasi axial position. In the presence of BuLi in THF, Me₃SiDHA reacts with Me₃SiCl to yield three new products: 9,9-bis(trimethylsilyl)-9,10-dihydroanthracene (IId) (major), (9,10-dihydroanthracen-9-yl)dimethylsilyl(trimethylsilyl)methane (V) and (9-trimethylsilyl-9,10-dihydroanthracen-9-yl)dimethylsilyl(trimethylsilyl)-methane (VI) together with the known 9,10-bis(trimethylsilyl)-9,10-dihydroanthracene (cis and trans). The carbanion of Me₃SiDHA can also be alkylated (alkyl  Me, Et, i-Pr) to the new 9,9-disubstituted derivatives: 9-Me₃Si-9-alkyl-DHA.The formation of the carbanion of Me₃SiDHA has been investigated and reveals a competition in the abstraction by the base of H₉ and H₁₀; the latter is less crowded but the 9-carbanion is stabilized by the vicinity of the silicon atom. The carbanion obtained from Me₃SiDHA reacts exclusively at C(9) with D₂O. A study of the alkylation of 9-trimethylsilyl-9-deuterio-9,10-dihydroanthracene shows that the generation of the 10-carbanion is followed by a 1,4 hydrogen rearrangement which yields the 9-carbanion. The mechanism is discussed and the roles of Me₃Si and t-Bu are compared in the DHA series.     

Reactivite dans la serie du methylenecyclopropane—I : Stereochimie de la reduction des dialkyl-1,1 dibromo-2,2 methylene-3 cyclopropanes par un hydrure organostannique

Reduction of dialkyl-gem-dibromomethylenecyclopropanes by organotin hydrides affords a mixture of two isomeric monobromo compounds, whose stereochemistry has been determined. The reaction goes by way of an intermediate methylenecyclopropyl radical and gives, as the major product, that isomer which is formed from the thermodynamically more stable radical.     

Reactions retrodieniques—v: Acces aux cumulenes par thermolyse eclair d'ethano-9,10 dihydro-9,10 anthracenes. Synthese du pentatetraene.

Allene, methylallene, 1,1-dimethylallene and butatriene have been obtained in excellent yield by flash thermolysis (involving retro-Diels-Alder decomposition) of various 9,10-ethano-9,10-dihydroanthracenes. Flash thermolysis of vinylallene 11 leads in 85% yield to a mixture of pentatetraene (1, 70%) and pentadiyne (17, 30%); pentatetraene is stable enough to be purified by GLC at 25° and its ¹H and ¹³C NMR, MS, IR and UV spectra have been recorded.     

Etude par RMN du Proton de la Stéréochimie de Composés Ethyléniques Trisubstitués. Calcul Empirique et Calcul par Incréments de la Constante d'Ecran

The Pascual and Tobey approaches for evaluating the chemical shift of an olefinic proton have been tentatively applied to the case of substituents which often cause difficulties in signal assignment. The empirically calculated values of the differential shielding between olefinic protons in both Z and E configurations are compared to the experimental values of the chemical shifts in a series of 2-arylpropene nitriles. The Z configuration is proved to correspond to the less shielded nucleus and the observed solvent effects are consistent with this assignment. When the Z configuration predominates its nature has been unambiguously established by studying also the corresponding E isomer of which a very small quantity could be isolated. The results offer a good way for the computation of new substituent additive parameters (NH₂, OH, ONa).     

Etude de la repartition de l'uranium dans les roches par la methode des traces de fission. Application a l'echantillonage en vue du dosage de l'uranium

The distribution of uranium in two different rock species: basalt from the Fangataufa atoll (located in Polynesia) and granit from the Hoggar mountains (located in Sahara) was studied by the fission track method. The two distributions observed appear quite different: uranium is uniformly distributed in the vitrous part of the basalt while in granit it is located on the intergranular joints. Applying the sampling model proposed by A. D. WILSON to the observed distributions of uranium, the weight of the analytical sample corresponding to a required sampling error can be estimated. Results show that representative sample weights of the bulk powdered material, as usually obtained by milling, are about some tenths of milligrams for basalt and more than the grams for granit (for uranium analysis). So, in trace element analysis, the sampling error could be more important than the analytical one. This fact could explain the relatively poor accuracy of trace determinations in rock samples (including the reference samples).      

Etude de la structure du cortège électronique par la méthode des états de valence

Résumé La méthode des états de valence décrite précédemment est appliquée à l'état fondamental de l'atome neutre de Carbone. On obtient les valeurs suivantes des énergies associées aux termes spectraux ³ P,¹ D et ¹ S: — 75,47900, — 75,36826 et — 75,30856 en unités Rydberg. L'erreur relative reste inférieure à 4 pour mille.

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The method of valence states previously described, is applied to the ground configuration of the neutral carbon atom. The term energies obtained are (in Rydbergs) ³ P: — 75.47900, ¹ D: — 75.36826, and ¹ S: — 75.30856, with errors below 4%.

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Zusammenfassung Die früher beschriebene Methode der Valenzzustände wird auf die Grundkonfiguration des neutralen C-Atoms angewandt. Man erhält folgende Termenergien (in Rydberg): ³ P: — 75,47900, ¹ D: — 75,36826 und ¹ S: — 75,30856, mit Fehlern unter 4%.

     

Analogues méso-hétérocycliques du dihydro-9,10 anthracène. XII — Sur quelques indoles dérivés de la dibenzo-p-dioxine

2-Aminodibenzo-p-dioxine undergoes facile condensation with different ω-bromoketones to give 2-aroylmethylaminodibenzo-p-dioxines, which undergo Möhlau-Bischler cyclisation under certain conditions. Some indolo[5,6-b] [1,4]benzodioxine were prepared by this method and their biochemical effect on the zoxazolamine hydroxylase in rats was studied.     

Réactions induites par ies halogénures de pyridinium. XXVII. Sur la formation d'ethers cycliques par action du chlorure ou du bromure de pyridinium sur l'hexanediol-2,5

It has been shown that cyclodehydration of 2,5-hexanediols into 2,5-dimethyltetrahydrofurans can be effected with pyridinium chloride or bromide as well as with any other reagent formerly used for the purpose. The various secondary reactions accompanying this cyclodehydration have been determined, and plausible mechanisms for the ring closure are outlined.