Tr-electrons

The discussion of the ir-bond orders is interesting because it gives a picture of the distribution of Tr-electrons along the cr-frame of the ring... 

A decrease of a- and tt-electronic density in both adjacent positions. For the a system this decrease is approximately the same at the 2- and 4-positions, which expresses an equivalent electron withdrawing from nitrogen in both positions. On the other hand, the decrease in tr-electronic density is twice as large at C-2 as at C-4. 

Making allowance for those effects gives a good correlation between the chemical shifts and the it- and/or tr-electron density of the carbon atom bearing the proton (133, 236,237). 

Compounds Positions Chemical shifts tr Electron densities... 

Figure 5 PPP calculations of tr-electron densities and bond orders...

The carbocation is stabilized by delocalization of the -tr electrons of the double bond, and the positive charge is shared by the two CH2 groups. Substituted analogs of allyl cation are called allylic carbocations. 

Conjugated diene (Section 10.5) System of the type C=C—C=C, in which two pairs of doubly bonded carbons are joined by a single bond. The -tr electrons are delocalized over the unit of four consecutive ip -hybridized carbons. 

Delocalization (Section 1.9) Association of an electron with more than one atom. The simplest example is the shared electron pair (covalent) bond. Delocalization is important in conjugated -tr electron systems, where an electron may be associated with several carbon atoms. 

Ring current (Section 13.5) Electric held associated with circulating system of -tr electrons. 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

Indole and carbazole are characterized by the enhanced Tr-electron density within the six-member cycles, which allows one to predict r] coordination as the preferential coordination route. In contrast to pyrrole, indole, in accord with experimental (62JA2534 64JA3796 71JA5102) and theoretical (82T3693) data, has... 

The most satisfactory route to the synthesis of the ri -borole complexes is the reaction of dihydroboroles (2-borolenes and 3-borolenes) with metal carbonyls. An alternative method of synthesis includes formation of the borole adducts with ammonia, 320 (R = Me, Ph) [87JOM(336)29]. Thermal reaction of 320 (R = Me, Ph) with M(C0)6 (M = Cr, Mo, W) gives 321 (M = Cr, R = Me, Ph M = Mo, W, R = Ph). There are data in favor of the Tr-electron delocalization over the borole... 

Ylidic four Tr-electron four-member X -phosphorus heterocycles as electronic isomers of heterocyclobutadienes 98AG(E)270. 

Both the cycioheptatrienyl radical and the anion are reactive and difficult to prepare. The six-Tr-electron cation, however, is extraordinarily stable. In fact, the cycioheptatrienyl cation was first prepared more than a century ago by reaction of Br2 with cycloheptatriene (Figure 15.7), although its structure was not recognized at the time. 

The Diels-Alder cycloaddition reaction (Section 14.4) is a pericvclic process that takes place between a diene (four tt electrons) and a dienophile (two tr electrons) to yield a cyclohexene product. Many thousands of examples of Diels-Alder reactions are known. They often take place easily at room temperature or slightly above, and they are stereospecific with respect to substituents. For example, room-temperature reaction between 1,3-butadiene and diethyl maleate (cis) yields exclusively the cis-disubstituted cyclohexene product. A similar reaction between 1,3-butadiene and diethyl fumarate (trans) yields exclusively the trans-disubstituted product. 

Multiple-bond participation in solvolysis may be looked upon as a competition in a nucleophilic displacement between solvent and the tr electrons of the multiple bond or as an electrophilic addition of a carbonium ion, or carbonium ion like species, to the multiple bond. 

It is clear from the above equations that numerous parameters (proton exchange rate, kcx = l/rm rotational correlation time, tr electronic relaxation times, 1 /rlj2e Gd proton distance, rGdH hydration number, q) all influence the inner-sphere proton relaxivity. Simulated proton relaxivity curves, like that in Figure 3, are often used to visualize better the effect of the... 

Carbonyl group of the aldehyde decreases the BDE of the adjacent C—H bond. This is due to the stabilization of the formed acyl radical, resulting from the interaction of the formed free valence with Tr-electrons of the carbonyl group. For example, DC—H = 422kJmol 1 in ethane and D( n 373.8 kJ mol 1 in acetaldehyde. The values of Dc H in aldehydes of different structures are presented in Table 8.1. In addition, the values of the enthalpies of acylperoxyl radical reactions with aldehydes were calculated (D0 H= 387.1 kJ mol-1 in RC(0)00 H). 

The low C=C barriers in push-pull ethylenes compared to the 6S.S kcal/ mol in ethylene show that die effects of delocalization on the tr-electron energy in the transition state must be much greater than the effects in the ground state— that is, the important substituent effects on the barriers must occur in the transition state. Besides, an effect that improves delocalization in the ground state would be barrier raising, if it were not accompanied by an at least equal stabilization of the transition state. 

The treatment of these -electron systems, which have two Tr-electrons and one skeleton C-atom fewer than anthracene, was carried out by Verrijn Stuart and Kruizinga (1958) by means of the HMO and SCF method. 

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