7r-electron systems

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The seven-membered CN3S3 ring, another ten 7r-electron system, was first obtained as the ester 12.19 (R = COaMe), which is a minor product of the reaction of S4N4 with dimethylacetylene dicarboxylate. It has a planar structure with bond lengths that indicate delocalization. The parent 1,3,5,2,4,6-trithiatriazepine 12.19 (R = H) is obtained as a colourless solid by carefully heating the ester with aqueous HCl followed by decarboxylation. ... 

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

The term aromatic will be used in a strict non-historical sense to mean possessing a cyclic 7r-electron system (6 and 10 electrons for the mono- and bi-cyclic rings discussed in this review). Heteroaromatic compounds, like carboaromatics, have widely different degrees and types of electronic dissymmetry and polarizabihty. Consequently, their reactivity varies tremendously with any one reagent and their relative reactivity changes drastically with the type of reagent. In this sense, aromatic compounds show differences in reactivity but not in aromaticity. The virtues of this qiuilitative concept of aromaticity and the pitfalls of trying to use it as a quantitative concept in modern context have been ably presented by Peters and by Balaban and Simon. ... 

The exclusion of nitrogen-bridged bicyclic azines is purely arbitrary since the work on such azinoazines with a 10 7r-electron system (and the necessarily accompanying quaternary nitrogen-bridgehead atom) shows the activation expected of other aromatic azinium... 

The reactivities of 4- and 2-halo-l-nitronaphthalenes can usefully be compared with the behavior of azine analogs to aid in delineating any specific effects of the naphthalene 7r-electron system on nucleophilic substitution. With hydroxide ion (75°) as nucleophile (Table XII, lines 1 and 8), the 4-chloro compound reacts four times as fast as the 2-isomer, which has the higher and, with ethoxide ion (65°) (Table XII, lines 2 and 11), it reacts about 10 times as fast. With piperidine (Table XII, lines 5 and 17) the reactivity relation at 80° is reversed, the 2-bromo derivative reacts about 10 times as rapidly as the 4-isomer, presumably due to hydrogen bonding or to electrostatic attraction in the transition state, as postulated for benzene derivatives. 4-Chloro-l-nitronaphthalene reacts 6 times as fast with methanolic methoxide (60°) as does 4-chloroquinoline due to a considerably higher entropy of activation and in spite of a higher Ea (by 2 kcal). ... 

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

A second theoretical index, and one for which there appears to be more justification in its application to free-radical reactions, is the atom localization energy. This index is a measure of the energy required to localize one electron of the 7r-electron system in the aromatic molecule at the point of attack of the radical. The formation of the intermediate adduct in a free-radical aromatic substitution may be regarded as the sum of two processes one, the localization of an electron at the point of attack and the other, the pairing of this... 

Molar absorptivity is a physical constant characteristic of the particular substance being observed and thus characteristic of the particular 7r electron system in the molecule. Typical values for conjugated dienes are in the range e = 10,000 to 25,000. Note that the units are usually dropped. 

Reduction should result in l,2-dihydro-l,2-diazocine (2) which formally contains a cyclic, conj ugatcd 10 7r-electron system and potentially possesses aromatic character. To date, however, only the synthesis of a dibenzo derivative has been described (see Section 1.2.1.3.). 

Zollinger and coworkers (Nakazumi et al., 1983) therefore supposed that the diazonium ion and the crown ether are in a rapid equilibrium with two complexes as in Scheme 11-2. One of these is the charge-transfer complex (CT), whose stability is based on the interaction between the acceptor (ArNj) and donor components (Crown). The acceptor center of the diazonium ion is either the (3-nitrogen atom or the combined 7r-electron system of the aryl part and the diazonio group, while the donor centers are one or more of the ether oxygen atoms. The other partner in the equilibrium is the insertion complex (IC), as shown in structure 11.5. Scheme 11-2 is intended to leave the question open as to whether the CT and IC complexes are formed competitively or consecutively from the components. ... 

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... 

The intermediates shown in reactions 1A and 2A are radicals in which the unpaired electron is in the a-position. Similarly, in IB and 2B, the unpaired electron is in the /3-position. The terms a- and (3-radicals (i.e., reactions 1A and IB) and o-radicals (i.e., reactions 2A and 2B) are used, respectively, to distinguish radicals in which the unpaired electron is or is not conjugated with the aromatic 7r-electron system. Formation of o-radical intermediates would require higher energy than the formation of the radicals in reactions 1A and IB. Using the same logic as we have for reactions 1A and IB, the a-incorporation of H into the spent naphthalene suggests that reaction 2A is more likely than reaction 2B. 

Only even functions of the linear case remain acceptable in the cyclic system and hence the quantum condition becomes L(= 27rr) = nX. The energy levels for a cyclic 7r-electron system follows immediately as... 

In general, linear 7r-electron systems with Z1r = 2N electrons at the lowest energy levels have closed-shell singlet states while cyclic systems reach closed shell structures only when ZT = 4N + 2. Cyclic 7r-electron systems with Zn 4N + 2 will therefore exhibit multiplet ground states according to Hund s rules, and should be chemically reactive because of the unpaired electrons. Hiickel s rule that predicts pronounced stability for so-called aromatic ring systems with 4jV + 2 7r-electrons is based on this shell structure. The comparison with cyclic systems further predicts that ring closure of linear 7r-electron systems should be exothermic by an amount... 

While main group metals and early transition metals are hard Lewis acids favoring coordination of carbonyl groups, gold, one of the late transition metals, is classified as a soft metal that activates 7r-electron systems like olefins and... 

There have been very few reports of polysilanes containing a carbon functional group such as unsaturated groups directly attached to the silicon main chain. This appears to be due to the difficulties in synthesis and characterization due to instability of these compounds. These polymers are expected to show significant differences to conventional polysilanes in their properties due to the interaction of main-chain cr-electron and side-chain 7r-electron systems. 

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