A carbon-13 NMR study of the ion pair structure of the dibenzo[b,f]pentalene dianion

The ¹H and ¹³C NMR chemical shifts of dibenzo[b, f]pentalene and its 5,10-dimethyl derivative are presented and compared with those of the corresponding dilithium dianions. As probed by the relative ¹³C NMR chemical shifts, the charge distribution within the dianion system is clearly dependent on the actual ion pair state. This condition is demonstrated by varying the solvent and temperature. The polarization of charge towards the pentalene carbons, i.e. the preferred cation positions, is observed on going to tight ion pair conditions. Further support for this model is gained from ⁷Li NMR. The limitations of the use of ¹H and ¹³C NMR chemical shifts to measure charge distributions within anion systems are discussed.     

Die Autokorrelationsfunktion der von einem Opalglas gestreuten Intensit?t

Ohne Zusammenfassung     

Six- and eightfold palladium-catalyzed cross-coupling reactions of hexa- and octabromoarenes

Palladium-catalyzed sixfold coupling of hexabromobenzene (20) with a variety of alkenylboronates and alkenylstannanes provided hexaalkenylbenzenes 1 in up to 73 % and 16 to 41 % yields, respectively. In some cases pentaalkenylbenzenes 21 were isolated as the main products (up to 75 %). Some functionally substituted hexaalkenylbenzene derivatives containing oxygen or sulfur atoms in each of their six arms have also been prepared (16 to 24 % yield). The sixfold coupling of the less sterically encumbered 2,3,6,7,10,11-hexabromotriphenylene (24) gave the desired hexakis(3,3-dimethyl-1-butenyl)triphenylene (25) in 93 % yield. The first successful cross-coupling reaction of octabromonaphthalene (26) gave octakis-(3,3-dimethyl-1-butenyl)naphthalene (27) in 21 % yield. Crystal structure analyses disclose that, depending on the nature of the substituents, the six arms are positioned either all on the same side of the central benzene ring as in 1 a and 1 i, making them nicely cup-shaped molecules, or alternatingly above and below the central plane as in 1 h and 23. In 27, the four arms at C-1,4,6,7 are down, while the others are up, or vice versa. Upon catalytic hydrogenation, 1 a yielded 89 % of hexakis(tert-butylethyl)benzene (23). Some efficient accesses to alkynes with sterically demanding substituents are also described. Elimination of phosphoric acid from the enol phosphate derived from the corresponding methyl ketones gave 1-ethynyladamantane (3 b, 62 % yield), 1-ethynyl-1-methylcyclohexane (3 c, 85 %) and 3,3-dimethylpentyne (3 e, 65 %). 1-(Trimethylsilyl)ethynylcyclopropane (7) was used to prepare 1-ethynyl-1-methylcyclopropane (3 d) (two steps, 64 % overall yield). The functionally substituted alkynes 3 f-h were synthesized in multistep sequences starting from the propargyl chloride 11, which was prepared in high yields from the dimethylpropargyl alcohol 10 (94 %). The alkenylstannanes 19 were prepared by hydrostannation of the corresponding alkynes in moderate to high yields (42-97 %), and the alkenylboronates 2 and 4 by hydroboration with catecholborane (27-96 % yield) or pinacolborane (26-69 % yield).     

Conformational analysis and kinetics of ring inversion for methylene- and dimethylsilyl-bridged dicyclooctatetraene

Dicyclooctatetraenylmethane (1) and dicyclooctatetraenyldimethylsilane (2) in THF-d(8) at 272 K exist as mixtures of diastereomers in ratios of 1:0.8 and 1:1, respectively. Nine energy minima (four meso and five racemic conformers) were located for each compound by geometry optimization at the HF/6-31G level of theory. The effects of torsional strain, steric interactions and dynamic electron correlation were analyzed. The diastereomeric ratios for 1 and 2 were reproduced reasonably well from the total energy calculated for each conformer corrected for its conformational enthalpy and entropy contributions. The ratio of rate constants for bond shift (BS) (k(BS)(1)/k(BS)(2)) is three times greater than the corresponding ratio for ring inversion. This suggests that additional substituent effects, such as pi interactions, are operative in the transition state for BS.     

Reaction-Induced Metal-Metal Oxide Interactions in Pd-In2O3/ZrO2 Catalysts Drive Selective and Stable CO2 Hydrogenation to Methanol

Ternary Pd-In₂O₃/ZrO₂ catalysts exhibit technological potential for CO₂-based methanol synthesis, but developing scalable systems and comprehending complex dynamic behaviors of the active phase, promoter, and carrier are key for achieving high productivity. Here, we show that the structure of Pd-In₂O₃/ZrO₂ systems prepared by wet impregnation evolves under CO₂ hydrogenation conditions into a selective and stable architecture, independent of the order of addition of Pd and In phases on the zirconia carrier. Detailed operando characterization and simulations reveal a rapid restructuring driven by the metal-metal oxide interaction energetics. The proximity of InPd_x alloy particles decorated by InO_x layers in the resulting architecture prevents performance losses associated with Pd sintering. The findings highlight the crucial role of reaction-induced restructuring in complex CO₂ hydrogenation catalysts and offer insights into the optimal integration of acid-base and redox functions for practical implementation.     

Non-invasive detection of cocaine dissolved in beverages using displaced Raman spectroscopy

We demonstrate the potential of Raman spectroscopy to detect cocaine concealed inside transparent glass bottles containing alcoholic beverages. A clear Raman signature of cocaine with good signal-to-noise was obtained from a ∼300 g solution of adulterated cocaine (purity 75%) in a 0.7 L authentic brown bottle of rum with 1 s acquisition time. The detection limit was estimated to be of the order of 9 g of pure cocaine per 0.7 L (∼0.04 moles L⁻¹) with 1 s acquisition time. The technique holds great promise for the fast, non-invasive, detection of concealed illicit compounds inside beverages using portable Raman instruments, thus permitting drug trafficking to be combated more effectively.     

Reactivity of 13,13-dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward organolithiums: remarkable resistance to the DMS rearrangement

Reactions of dibromocyclopropane 2a, containing two spiro-fused 1,3-dioxane rings, with MeLi gave only the methylation products 8 and 9 even at elevated temperatures. In contrast, the cyclohexane analogue 2b treated with MeLi underwent a smooth rearrangement to bicyclo[1.1.0]butane 11b at -78, -10, or +35 degrees C. Treatment of 2a with PhLi gave the alpha-Ph anion 13 as the only product, which underwent smooth methylation with MeI to give 14. Under the same conditions, 2b with PhLi gave bicyclo[1.1.0]butane 11b accompanied by bromophenyl derivative 8b. Treatment of either dibromide with t-BuLi gave a mixture of products including debrominated cyclopropanes 12. Experimental results were augmented with DFT calculations for salts 23 and MP2//DFT-level calculations for carbenes 22. They demonstrated a higher stability of the dioxane alpha-bromo anion with respect to alpha-elimination by 4.8 kcal/mol and also a lower tendency of the carbene 22a to undergo rearrangement by 4.0 kcal/mol than the cyclohexane analogues. These differences have been attributed to the inductive effect of the four oxygen atoms, which results in lower LUMO energy, the higher positive charge at the carbenic center, and the overall more electrophilic character of carbene 22a as compared to the cyclohexane derivative 22b. The rearrangement of carbenes 22 to the corresponding allenes 1, the thermodynamic products, requires a higher activation energy DeltaG(double dagger)(298) by 4.2 kcal/mol for dioxane and 6.4 kcal/mol for cyclohexane derivatives than for the formation of the bicyclo[1.1.0]butanes 11. The DeltaG(double dagger)(298) for intramolecular insertions to the CH bond is low and calculated as 6.0 kcal/mol for dioxane 22a and 2.0 kcal/mol for the formation of cyclohexane 22b.