7t-electrons

In cases of more effective 7t-electron donor or tz-donor neighboring groups, as is the case in forming /3-phenylethyl (studied by Don Cram from UCLA Nobel Prize in chemistry, 1987) or /3-halogen bridged species, these have sufficient electrons to form 2e-2c bonds (with some intermediate delocalization). 

Indole is classified as a 7c-excessive aromatic compound. It is isoelectronic with naphthalene, with the heterocyclic nitrogen atom donating twm of the ten 7t-electrons. 

With its sextet of 7T electrons, thiophene possesses the typical aromatic character of benzene and other similarly related heterocycles. Decreasing orders of aromaticity have been suggested to reflect the strength of this aromatic character benzene > thiophene > pyrrole > furan (9) and benzene > thiophene > selenophene > teUurophene > fuian (10). 

Graphite is strongly diamagnetic because of its abundance of 7t electrons. Grinding the crystaUite smaller than 20 nm creates a deficiency in 7t electrons and thus destroys the diamagnetism. The value of the specific magnetic susceptibiUty for Sri Lanka graphite is ca — 6.5 x 10 at 20°C,... 

No simple electrophilic substitution, for example nitrosation, nitration, sulfonation or halogenation of a C—H bond, has so far been recorded in the pteridine series. The strong 7T-electron deficiency of this nitrogen heterocycle opposes such electrophilic attack, which would require a high-energy transition state of low stability. 

In the case of vinylfurans and vinylpyrroles there is the possibility of cycloaddition involving either the cyclic diene system or the diene system including the double bond. 2-Vinylfuran reacts in high yield with maleic anhydride in ether at room temperature to form the adduct involving the exocyclic double bond. Similarly, 2- and 3-vinylpyrroles react with 7T-electron-deficient alkenes and alkynes under relatively mild conditions to give the corresponding tetrahydro- and dihydro-indoles (Scheme 51) (80JOC4515). 

MO calculations at the 6-3IG level have been done on both acrolein and aminoethylene. The resulting MOs were used to calculate charge distributions. Figure 1.26 gives the 7t-electron densities calculated for butadiene, acrolein, and aminoethylene. Inclusion of the hydrogen and a orbitals leads to overall charges as shown. These charge distributions result from a polarization which is counter to the n polarization. 

The 7t-electron delocalization requires proper orbital alignment. As a result, there is a significant barrier to rotation about the carbon-carbon bonds in the allyl cation. The results of 6-31G/MP2 calculations show the perpendicular allyl cation to be 37.8 kcal/mol higher than the planar ion. Related calculations indicate that rotation does not occur but that... 

Simple Hiickel calculations on benzene, in contrast, place all the n electrons in bonding MOs. The 7t-electron energy of benzene is calculated by summing the energies of the six 71 electrons, which is 6a -F 8/S, lower by 2/S than the value of 6a -F 6/S for three isolated double bonds. Thus, the HMO method predicts a special stabilization for benzene. 

In this mechanism, a complexation of the electrophile with the 7t-electron system of the aromatic ring is the first step. This species, called the 7t-complex, m or ms not be involved directly in the substitution mechanism. 7t-Complex formation is, in general, rapidly reversible, and in many cases the equilibrium constant is small. The 7t-complex is a donor-acceptor type complex, with the n electrons of the aromatic ring donating electron density to the electrophile. No position selectivity is associated with the 7t-complex. 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

Flexible six- and eight-7t-electron systems would impose an entropic barrier to concerted 10-7i-electron concerted reactions. Most of the examples, as in the cases above, involve cyclic systems in which the two termini of the conjugated system are held close together. 

Cyclooctatetraene provides a significant contrast to the preference of aromatic hydrocarbons for one-electron reduction. It is converted to a diamagnetic dianion by addition of two electrons. It is easy to understand the ease with which the cyclooctatetraene radical accepts a second electron because of the aromaticity of the 10-7t-electron aromatic system which results (Section 9.3). 

Small chiral molecules. These CSPs were introduced by Pirkle about two decades ago [31, 32]. The original brush -phases included selectors that contained a chiral amino acid moiety carrying aromatic 7t-electron acceptor or tt-electron donor functionality attached to porous silica beads. In addition to the amino acids, a large variety of other chiral scaffolds such as 1,2-disubstituted cyclohexanes [33] and cinchona alkaloids [34] have also been used for the preparation of various brush CSPs. 

Aromaticity (Chapter 15 introduction) The special characteristics of cyclic conjugated molecules. These characteristics include unusual stability, the presence of a ring current in the 1H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated species that have An + 2 7T electrons. 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

The unsubstituted parent 1,5-diazocine (1) is hitherto unknown. Benzannulated systems, however, have been prepared, particularly dibenzo f>,/][l, 5]diazocine and its derivatives which have been intensively investigated due to their pharmacological properties. So far, all compounds synthesized show no evidence for delocalization of the 7t-electrons. One special review on the chemistry of 1,5-diazocines is available.1... 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

In the reaction of the strongly electrophilic 4-nitrobenzenediazonium ion with 2-naphthol-6,8-disulfonic acid, which yields a sterically hindered o-complex, Roller and Zollinger (1970) actually observed the rapid formation of a 7T-complex spec-trophotometrically at low pH. The concentration of the 7T-complex decreases slowly and at the same rate as that of the formation of the azo product. H NMR data indicate that the 7t-complex is not localized. All 7T-electrons of the benzene and the naphthalene system are involved in the complex formation to a similar degree, in... 

One possible explanation for the abnormal results noted above comes from considering the 7t-electron density distribution when + M substituents are present. This is indicated on the structure (XII) such that a solid constituent p orbital is a region of higher... 

Similar considerations apply to the thiirene oxide system (18), since in this case too the sulfur s 3d-orbitals have the potential of interacting with the 2p-orbitals of both the adjacent carbon and oxygen atoms. Such an interaction should facilitate some kind of conjugation of the carbon-carbon double-bond -electrons with the formally unoccupied 3d-orbitals, which in turn would give rise to Hiickel-type stabilization associated with cyclic array of 4n + 2 (n = 0) 7t-electrons. 

Conversely, since increasing Uwr and coverage of O8 stabilizes the Rh-C2H4 bond via enhanced 7t-electron donation to the metal, it follows that smaller pc2H4 values (Pc2h4) are required to reduce the surface Rh oxide as experimentally observed. 

The mechanism of substitution on an electron-rich benzene ring is electrophilic substitution, electrophilic attack on an atom and the replacement of one atom by another or by a group of atoms. The fact that substitution occurs rather than addition to the double bonds can be traced to the stability of the delocalized 7T-electrons in the ring. Delocalization gives the electrons such low energy—that is, they are bound so tightly—that they are unavailable for forming new cr-bonds (see Sections 2.7 and 3.12). 

In the second step, the hydrogen ion is pulled out of the ring by the HS04 ion acting as a Bronsted base. As in the bromination reaction, the restoration of the delocalization of the 7t-electrons facilitates the removal of the hydrogen ion. 

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