818. Aromatic substitution. Part II. Nitration of aromatic compounds with nitronium tetrafluoroborate and other stable nitronium salts

Lin

Proc. Natl. Acad. Sci. U.S.A.

et al. 1978

Electrophilic nitration of toluene and benzene was studied under various conditions with several nitrating systems. It was found that high ortlopara regioselectivity is prevalent in all reactions and is independent of the reactivity of the nitrating agent. The methyl group of toluene is predominantly ortho-para directing under all reaction conditions. Steric factors are considered to be important but not the sole reason for the variation in the ortho/para ratio. The results reinforce our earlier views that, in electrophilic aromatic nitrations with reactive nitrating agents, substrate and positional selectivities are determined in two separate steps. The first step involves a ir-aromatic-NO2 ion complex or encounter pair, whereas the subsequent step is of arenium ion nature (separate for the oftho, meta, and para positions). The former determines substrate selectivity, whereas the latter determines regioselectivity.Thermal free radical nitration of benzene and toluene with tetranitromethane in sharp contrast gave nearly statistical product distributions. Stable nitronium salts were introduced as new nitrating agents by Olah and coworkers (1) in 1956. In the course of these studies (2-5), the competitive nitration of benzene and toluene, as well as other aromatics, was carried out in organic solvents.Under usual conditions of electrophilic nitration, toluene reacts about 20 times more rapidly than benzene whereas, with nitronium salts, toluene was found to react only 1.7 times faster than benzene (2). The practical disappearance of intermolecular (substrate) selectivity was accompanied by no significant alteration of isomer distribution (regioselectivity). This observation led to the suggestion that the transition state of highest energy (which determines substrate selectivity) is of starting aromatic (i.e., 7r-complex) nature, which is then followed by separate a-complex formation (for the individual positions), determining positional selectivity.In a series of studies, we have found that, in electrophilic aromatic substitutions, the position of the transition state of NO2Y + R-Xhighest energy is not rigidly fixed (6) but can shift from "early" (r-complex-like) to "late" (u-complex-like) nature, depending upon the reactivity of the electrophiles and the basicity of the aromatic substrates.In order to further explore electrophilic nitration, we carried out a comprehensive study of nitration of benzene and toluene under various conditions. RESULTS AND DISCUSSION With Nitronium Salts. Although we had previously examined competitive nitration using high-speed mixing (7), it was considered of interest to extend the studies by using more advanced methods such as the mixing chamber of an efficient Durrum-Gibson stopped-flow apparatus. Competitive nitrations, with nitronium hexafluorophosphate in nitromethane, provided the data in Table 1. Whereas mixing still can be incomplete before reaction, with the nitration rates being very fast (or reaching the encounter-controlled limit), the data seem to indicate that, ...

mentioning

confidence: 99%

Electrophilic and free radical nitration of benzene and toluene with various nitrating agents

Lin

Proc. Natl. Acad. Sci. U.S.A.

et al. 1978

“…Olah and Kuhn reported the preparation of nitronium tetrafluoroborate and its application as a nitrating agent (8)(9)(10) Tables 1-4. In mono-or dinitration of aromatics, nitronium salts generally react under quite mild conditions, and yields are 80-100%.…”

mentioning

confidence: 99%

Recent aspects of nitration: New preparative methods and mechanistic studies (A Review)

Narang

Proc. Natl. Acad. Sci. U.S.A.

et al. 1982

New preparative methods of electrophilic nitration and transfer nitration are reviewed, including reactions relating to the ambident reactivity of the nitronium ion. Recent aspects of the mechanism of electrophilic aromatic substitution are discussed.Nitration is one of the most studied and best understood of organic reactions (for reviews, see refs. [1][2][3][4] Typical results of preparative nitrations of arenes, nitroarenes, ary1carboxylic acid esters and halides, and arylnitriles are summarized in Tables 1-4. In mono-or dinitration of aromatics, nitronium salts generally react under quite mild conditions, and yields are 80-100%.Trinitration of aromatics, particularly of benzene, represented a challenging problem. Although the preparation of 1,3,5-trinitrobenzene from rn-dinitrobenzene has been reported in low yield (11), 1,3,5-trinitrobenzene is usually prepared by indirect methods (12). The nitration of m-dinitrobenzene to 1,3,5-trinitrobenzene with nitronium tetrafluoroborate in fluorosulfuric acid solution in contrast could be carried out with ease (62% yield).Nitronium salts in strongly acidic solvents (such as FS03H or HF) are thus capable of effecting an efficient preparative nitration of benzene to 1,3,5-trinitrobenzene which, due to its increased stability relative to TNT, can be of significance.Nitronium salts also readily nitrate aliphatics. Alkanes and cycloalkanes, such as cyclohexane or adamantane, give high yields. The difficulty is not in lack of reactivity but due to the fact that tertiary (and reactive secondary) nitroalkanes themselves can react further under acidic conditions via cleavage of the C-,N bond. Thus, reaction conditions must be found to minimize further reaction; in contrast, this is not the case in aromatic nitrations. Primary nitroalkanes are less susceptible but nitration of their less reactive C-H bonds is also more difficult. The nitration of methane, for example, is feasible only in strongly acidic solution (such as FSO3H).Nitronium salts also add readily to oilefins in the presence of 4487

“…The nitronium tetrafluoroborate, however, suppressed the oxidation, in accord with the known properties of this mild nitration reagent. [28] Reaction of 1,2-di-ptolylethane with nitronium tetrafluoroborate (Scheme 2) afforded a mixture of three major products, which were isolated by repeated chromatographic purifications, and were identified as 7, 8, and 9.…”

Section: Methodsmentioning

confidence: 99%

Computational and Experimental Evidence of Through‐Space NMR Spectroscopic J Coupling of Hydrogen Atoms

Dračínský

Jansa

Bouř

2011

Chemistry A European J

J coupling in NMR spectroscopy is conventionally associated with covalent bonds. A noncovalent contribution often called through-space coupling (TSC) has been observed for heavy atoms. In this study, the TSC was detected and analyzed for the more common (1)H-(1)H coupling as well. In synthesized model molecules the hydrogen positions could be well controlled. For several coupling constants the through-space mechanism was even found to be the predominant factor. The nature and magnitude of the phenomenon were also analyzed by density functional computations. Calculated carbon- and hydrogen-coupling maps and perturbed electronic densities suggest that the aromatic system strongly participates in the noncovalent contribution. Unlike covalent coupling, which is usually governed by the Fermi contact, TSC is dominated by the diamagnetic term comprising interactions of nuclei with the electron orbital angular momentum. The computations further revealed a strong distance and conformational dependence of TSC. This suggests that the through-space coupling can be explored in molecular structural studies in the same way as the covalent one.