Generation of the 4-biphenylyloxenium ion, 1a, from hydrolysis of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, is demonstrated by common ion rate depression and azide trapping. The ion is less selective with a shorter lifetime (12 ns at 30 degrees C) in aqueous solution than the corresponding nitrenium ion, 6a (ca 0.3 mus at 30 degrees C). Its lifetime is considerably longer than those of structurally related carbenium ions. Unlike 6a, 1a reacts with N3-, in part, at the distal ring to generate 4'-azido-4-hydroxybiphenyl, 5. These results are rationalized by calculations at the HF/6-31G* and pBP/DN*//HF/6-31G* levels that show that 1a is planar but is less stable than 6a relative to their hydration products by ca. 12 kcal/mol. The p-tolyloxenium ion, 1b, has not been unequivocally demonstrated to be formed in H2O, but kinetic data show that it must be at least 104-fold less stable than 1a relative to their respective 4-acetoxy derivatives. It is also calculated to be ca. 19 kcal/mol less stable than the corresponding nitrenium ion, 6b, relative to their hydration products.

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Azide trapping shows that the 4'-substituted-4-biphenylyloxenium ions 1b-d are generated during hydrolysis of 4-aryl-4-acetoxy-2,5-cyclohexadienones, 2c and 2d, and O-(4-aryl)phenyl-N-methanesulfonylhydroxylamines, 3b and 3c. In addition, the 4'-bromo-substituted ester, 2d, undergoes a kinetically second-order reaction with N3- that accounts for a fraction of the azide adduct, 5d. Since both first-order and second-order azide trapping occurs simultaneously in 2d, the second-order reaction is not enforced by the short lifetime of 1d, which has similar azide/solvent selectivity to the unsubstituted ion, 1a. In contrast the 4'-CN and 4'-NO2 ions 1e and 1f cannot be detected by azide trapping during the hydrolysis of the dichloroacetic acid esters 2e' and 2f' even though 18O labeling experiments show that a fraction of the hydrolysis of both esters occurs through C(alkyl)-O bond cleavage. These esters exhibit only second-order trapping by azide. Correlations of the azide/solvent selectivities of 1a-d with the calculated relative driving force for hydration of the ions (DeltaE of eq 4) determined at the pBP/DN//HF/6-31G and BP/6-31G//HF/6-31G levels of theory suggest that 1e and 1f have lifetimes in the 1-100 ps range. Ions with these short lifetimes are not in diffusional equilibrium with nonsolvent nucleophiles, and must be trapped by such nucleophiles via a preassociation mechanism. The second-order trapping that is observed in these two cases is enforced by the short lifetime of the cations, and may occur by a concerted S(N)2' mechanism or by internal azide trapping of an ion sandwich produced by azide-assisted ionization. Comparison of azide/solvent selectivities of the oxenium ions 1a-c with the corresponding biphenylylnitrenium ions 8a-c shows that 4'-substituent effects on reactivity in both sets of ions are similar in magnitude, although the nitrenium ions are ca. 30-fold more stable in an aqueous environment than the corresponding oxenium ions. The magnitude of the 4'-substituent effects for electron-donating substituents suggest that both sets of ions are more accurately described as 4-aryl-1-imino-2,5-cyclohexadienyl or 4-aryl-1-oxo-2,5-cyclohexadienyl carbocations. Calculated structures of the oxenium ions are also consistent with this interpretation.

The title compounds serve as potential precursors to aryloxenium ions, often proposed, but primarily uncharacterized intermediates in phenol oxidations. The uncatalyzed and acid-catalyzed decomposition of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, generates the quinol, 3a. (18)O-Labeling studies performed in (18)O-H(2)O, and monitored by LC/MS and (13)C NMR spectroscopy that can detect (18)O-induced chemical shifts on (13)C resonances, show that 3a was generated in both the uncatalyzed and acid-catalyzed reactions by C(alkyl)-O bond cleavage consistent with formation of an aryloxenium ion. Trapping with N(3)(-) and Br(-) confirms that both uncatalyzed and acid-catalyzed decompositions occur by rate-limiting ionization to form the 4-biphenylyloxenium ion, 1a. This ion has a shorter lifetime in H(2)O than the corresponding nitrenium ion, 7a (12 ns for 1a, 300 ns for 7a at 30 degrees C). Similar analyses of the product, 3b, of acid- and base-catalyzed decomposition of 4-acetoxy-4-methyl-2,5-cyclohexadienone, 2b, in (18)O-H(2)O show that these reactions are ester hydrolyses that proceed by C(acyl)-O bond cleavage processes not involving the p-tolyloxenium ion, 1b. Uncatalyzed decomposition of the more reactive 4-dichloroacetoxy-4-methyl-2,5-cyclohexadienone, 2b', is also an ester hydrolysis, but 2b' undergoes a kinetically second-order reaction with N(3)(-) that generates an oxenium ion-like substitution product by an apparent S(N)2'mechanism. Estimates based on the lifetimes of 1a, 7a, and the p-tolylnitrenium ion, 7b, and the calculated relative stabilities of these ions toward hydration indicate that the aqueous solution lifetime of 1b is ca. 3-5 ps. Simple 4-alkyl substituted aryloxenium ions are apparently not stable enough in aqueous solution to be competitively trapped by nonsolvent nucleophiles.

Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O(2) in aqueous solution leads to two reactive intermediates with lambda(max) 360 and 460 nm, respectively, while in pure CH(3)CN only one species with lambda(max) 350 nm is produced. The intermediate with lambda(max) 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N(3)(-), comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR(3)) spectra of 1b further confirms its identity. The second intermediate with lambda(max) 360 nm (350 nm in CH(3)CN) has been characterized as the radical 5b, based on its photolytic generation in the less polar CH(3)CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH(3)CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room temperature is 170 ns. This intermediate decays with first-order kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 mus. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.

Two independent computational methods have been used for determination of amide resonance stabilization and amidicities relative to N,N-dimethylacetamide for a wide range of acyclic and cyclic amides. The first method utilizes carbonyl substitution nitrogen atom replacement (COSNAR). The second, new approach involves determination of the difference in amide resonance between N,N-dimethylacetamide and the target amide using an isodesmic trans-amidation process and is calibrated relative to 1-aza-2-adamantanone with zero amidicity and N,N-dimethylacetamide with 100% amidicity. Results indicate excellent coherence between the methods, which must be regarded as more reliable than a recently reported approach to amidicities based upon enthalpies of hydrogenation. Data for acyclic planar and twisted amides are predictable on the basis of the degrees of pyramidalization at nitrogen and twisting about the C-N bonds. Monocyclic lactams are predicted to have amidicities at least as high as N,N-dimethylacetamide, and the β-lactam system is planar with greater amide resonance than that of N,N-dimethylacetamide. Bicyclic penam/em and cepham/em scaffolds lose some amidicity in line with the degree of strain-induced pyramidalization at the bridgehead nitrogen and twist about the amide bond, but the most puckered penem system still retains substantial amidicity equivalent to 73% that of N,N-dimethylacetamide.

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The 4-(benzothiazol-2-yl)phenylnitrenium ion 11 is generated from hydrolysis or photolysis of O-acetoxy-N-(4-(benzothiazol-2-yl)phenyl)hydroxylamine 8, a model metabolite of 2-(4-aminophenyl)benzothiazole 1 and its ring substituted derivatives that are being developed for a variety of medicinal applications including anti-tumor, anti-bacterial, anti-fungal, and imaging agents. Previously we showed that 11 had an aqueous solution lifetime of 530 ns, similar to the 560 ns lifetime of the 4-biphenylylnitrenium ion 12 derived from the well known chemical carcinogen 4-aminobiphenyl. We now show that the analogy between these two cations extends well beyond their lifetimes. The initial product of hydration of 11 is the quinolimine 16 which can be detected as a long-lived reactive intermediate that hydrolyzes in a pH-dependent manner into the final hydrolysis product, the quinol 15. This hydrolysis behavior is equivalent to that previously described for a large number of ester metabolites of carcinogenic arylamines including 4-aminobiphenyl. The major azide trapping product (90% of azide products) of 11, 20, is generated by substitution on the carbons ortho to the nitrenium ion center of 11. This product is a direct analogue of the major azide adducts, such as 22, generated from trapping of the nitrenium ions of carcinogenic arylamines. The azide/solvent selectivity for 11, kaz/ks, is also nearly equivalent to that of 12. A minor product of the reaction of 11 with N3-, 21, contains no azide functionality, but may be generated by a process in which N3- attacks 11 at the nitrenium ion center with loss of N2 to generate a diazene 25 that subsequently decomposes into 21 with loss of another N2. The adduct derived from attack of 2′-deoxyguanosine (d-G) on 11, 28, is a familiar C-8 adduct of the type generated from the reaction of d-G with a wide variety of arylnitrenium ions derived from carcinogenic arylamines. The rate constant for reaction of d-G with 11, kd-G, is very similar to that observed for the reaction of d-G with 12. The similar lifetimes and chemical reactivities of 11 and 12 can be rationalized by B3LYP/6-31G(d) calculations on the two ions that show that they are of nearly equivalent stability relative to their respective hydration products. The calculations also help to rationalize the different regiochemistry observed for the reaction of N3- with 11 and its oxenium ion analogue, 13. Since 8 is the likely active metabolite of 1 and a significant number of derivatives of 1 are being developed as pharmaceutical agents, the similarity of the chemistry of 11 to that of carcinogenic arylnitrenium ions is of considerable importance. Consideration should be given to this chemistry in continued development of pharmaceuticals containing the 2-(4-aminophenyl)benzothiazole moiety.