Isolation, X-ray Structures, and Electronic Spectra of Reactive Intermediates in Friedel−Crafts Acylations

Reactive intermediates in the Friedel−Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV−vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediates with other cationic electrophiles that have been structurally established via X-ray studies

Isolation and X-ray Structures of Labile Benzoic- and Acetic-Acidium Carbocations

New carbocationic salts (via O-protonation of substituted benzoic acids) are prepared for the first time by controlled hydration of the corresponding benzoylium salts and isolated in pure crystalline form. Precise X-ray structural analyses reveal the rather unexpected (electronic) structure of the carboxylic-acidium functionality

Crystallographic Distinction between “Contact” and “Separated” Ion Pairs: Structural Effects on Electronic/ESR Spectra of Alkali-Metal Nitrobenzenides

The classic nitrobenzene anion-radical (NB-• or nitrobenzenide) is isolated for the first time as pure crystalline alkali-metal salts. The deliberate use of the supporting ligands 18-crown-6 and [2.2.2]cryptand allows the selective formation of contact ion pairs designated as (crown)M+NB-•, where M+ = K+, Rb+, and Cs+, as well as the separated ion pair K(cryptand)+NB-•both series of which are structurally characterized by precise low-temperature X-ray crystallography, ESR analysis, and UV−vis spectroscopy. The unusually delocalized structure of NB-• in the separated ion pair follows from the drastically shortened N−C bond and marked quinonoidal distortion of the benzenoid ring to signify complete (95%) electronic conjugation with the nitro substituent. On the other hand, the formation of contact ion pairs results in the substantial decrease of electronic conjugation in inverse order with cation size (K+ \u3e Rb+) owing to increased localization of negative charge from partial (NO2) bonding to the alkali-metal cation. Such a loss in electronic conjugation (or reverse charge transfer) may be counterintuitive, but it is in agreement with the distribution of odd-electron spin electron density from the ESR data and with the hypsochromic shift of the characteristic absorption band in the electronic spectra. Most importantly, this crystallographic study underscores the importance of ion-pair structure on the intrinsic property (and thus reactivity) of the component ions - as focused here on the nitrobenzenide anion

Structural Effects of Carbon Monoxide Coordination to Carbon Centers. π and σ Bindings in Aliphatic Acyl \u3cem\u3eversus\u3c/em\u3e Aromatic Aroyl Cations

The binding of carbon monoxide to carbon centers has been examined with two series of aromatic and aliphatic oxocarbonium ions that are successfully isolated as crystalline and highly reactive (hygroscopic) aroylium and acylium salts with poorly coordinating counteranions. X-Ray crystallographic analyses at −150 °C afford precise structural parameters for the characteristic linear carbonyl bond (rCO) and the bond to the carbon centers (rCα). The correlations of these structural parameters evaluated for alkyl (Me, Et and i-Pr) and aryl (p-Me, 2,4,6-trimethyl, p-MeO and p-fluorophenyl) oxocarbonium ions with the corresponding carbonyl stretching frequencies in the solid-state (reflectance) IR spectra yield valuable insight into the binding mode of carbon monoxide. Most noteworthy is the synergic (π–σ) bonding in aroylium structures in contrast to the mainly σ bonding in acylium structures that are organic mimics for carbon monoxide bonding in classical and nonclassical metal carbonyls, respectively

Biochemical and structural characterization of alanine racemase from Bacillus anthracis (Ames)

<p>Abstract</p> <p>Background</p> <p><it>Bacillus anthracis </it>is the causative agent of anthrax and a potential bioterrorism threat. Here we report the biochemical and structural characterization of <it>B. anthracis </it>(Ames) alanine racemase (Alr_<it>Bax</it>), an essential enzyme in prokaryotes and a target for antimicrobial drug development. We also compare the native Alr_{<it>Bax </it>}structure to a recently reported structure of the same enzyme obtained through reductive lysine methylation.</p> <p>Results</p> <p><it>B. anthracis </it>has two open reading frames encoding for putative alanine racemases. We show that only one, <it>dal1</it>, is able to complement a D-alanine auxotrophic strain of <it>E. coli</it>. Purified Dal1, which we term Alr_<it>Bax</it>, is shown to be a dimer in solution by dynamic light scattering and has a V_maxfor racemization (L- to D-alanine) of 101 U/mg. The crystal structure of unmodified Alr_{<it>Bax </it>}is reported here to 1.95 Å resolution. Despite the overall similarity of the fold to other alanine racemases, Alr_{<it>Bax </it>}makes use of a chloride ion to position key active site residues for catalysis, a feature not yet observed for this enzyme in other species. Crystal contacts are more extensive in the methylated structure compared to the unmethylated structure.</p> <p>Conclusion</p> <p>The chloride ion in Alr_{<it>Bax </it>}is functioning effectively as a carbamylated lysine making it an integral and unique part of this structure. Despite differences in space group and crystal form, the two Alr_{<it>Bax </it>}structures are very similar, supporting the case that reductive methylation is a valid rescue strategy for proteins recalcitrant to crystallization, and does not, in this case, result in artifacts in the tertiary structure.</p

A reactivity-selectivity study of the Friedel-Crafts acetylation of 3,3′-dimethylbiphenyl and the oxidation of the acetyl derivatives

<p>Abstract</p> <p>Background</p> <p>Friedel-Crafts acetylation is an important route to aromatic ketones, in research laboratories and in industry. The acetyl derivatives of 3,3′-dimethylbiphenyl (3,3′-dmbp) have applications in the field of liquid crystals and polymers and may be oxidized to the dicarboxylic acids and derivatives that are of interest in cancer treatment.</p> <p>Findings</p> <p>The effect of solvent and temperature on the selectivity of monoacetylation of 3,3’-dmbp by the Perrier addition procedure was studied using stoichiometric amounts of reagents. 4-Ac-3,3′-dmbp was formed almost quantitatively in boiling 1,2-dichloroethane and this is almost twice the yield hitherto reported. Using instead a molar ratio of substrate:AcCl:AlCl₃ equal to 1:4:4 or 1:6:6 in boiling 1,2-dichloroethane, acetylation afforded 4,4′- and 4,6′-diacetyl-3,3′-dmbp in a total yield close to 100%. The acetyl derivatives were subsequently converted to the carboxylic acids by hypochlorite oxidation. The relative stabilities of the isomeric products and the corresponding σ-complexes were studied by DFT calculations and the data indicated that mono- and diacetylation followed different mechanisms.</p> <p>Conclusions</p> <p>Friedel-Crafts acetylation of 3,3′-dmbp using the Perrier addition procedure in boiling 1,2-dichloroethane was found to be superior to other recipes. The discrimination against the 6-acetyl derivative during monoacetylation seems to reflect a mechanism including an AcCl:AlCl₃ complex or larger agglomerates as the electrophile, whereas the less selective diacetylations of the deactivated 4-Ac-3,3′-dmbp are suggested to include the acetyl cation as the electrophile. The DFT data also showed that complexation of intermediates and products with AlCl₃ does not seem to be important in determining the mechanism.</p

Biochemical And Structural Characterization Of LiaR From Vancomycin-Resistant E. Faecalis: The 'Master Regulator' of the Cell-Envelope Stress Response

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