Tr electron delocalization

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... 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

Species of this kind were first proposed in connection with interpretation of results concerning the exchange of polymethylcyclopen-tanes over palladium films 12), but they may have a wider significance. Adsorbed species having Tr-electrons delocalized over four carbon atoms were also proposed 12). The fate of the hydrogen atom lost in the formation of Structure (C) from (B) (which is its most likely precursor) will be discussed in Section II, B, 1. 

It means that the increase of stabilization of chelate chain is associated with a decrease of aromaticity, i.e., Tr-electron delocalization in the ring. A similar situation was observed for the first X-ray determined ionic H-bonding in the crystal state for ortho-hydroxy Schiff base—5-nitro-A-salicylideneethylamine... 

If the energy of such H-bonding is computed as a difference between the open and closed structures [31] (see Scheme 5) of Cl— and F— substituted derivatives of malonaldehydes, it turns out that an increase of energy is associated with an increase of Tr-electron delocalization in the chelate chain (OCCCO), as shown in Fig. 8. 

The nature of substituents may affect significantly the H-bond characteristics and associated Tr-electron delocalization. Analysis of changes of C-O bond length in a set of / (2ra-substituted phenol derivatives interacting with fluoride and j9<2ra-substituted phenolate derivatives interacting with HF allows to show... 

H-bonds intermolecular H-bonds Tr-electron delocalization topological parameters critical points QTAM (Quantum Theory Atoms in Molecules method) covalent hydrogen bonds interaction energy decomposition scheme. 

The mechanism for acid-catalyzed hydrolysis of amides involves attack by water on the protonated amide. Amides are weak bases with pK values in the range from 0 to -2A An important feature of the chemistry of amides is that the most basic site is the carbonyl oxygen. Very little of the N-protonated form is present. The major factor that favors the O-protonated form is the retention of Tr-electron delocalization over the O-C-N system. No such delocalization is possible in the N-protonated form. 

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. 

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... 

We can understand the differences in properties between the carbon allotropes by comparing their structures. Graphite consists of planar sheets of sp2 hybridized carbon atoms in a hexagonal network (Fig. 14.29). Electrons are free to move from one carbon atom to another through a delocalized Tr-network formed by the overlap of unhybridized p-orbitals on each carbon atom. This network spreads across the entire plane. Because of the electron delocalization, graphite is a black, lustrous, electrically conducting solid indeed, graphite is used as an electrical conductor in industry and as electrodes in electrochemical cells and batteries. Its... 

There is also substantial stabilization of [4+] by electron delocalization from the cyclic a-vinyl group. This is shown by a comparison of the thermodynamic driving force (p Tr lies between —7.8 and —8.5) and absolute rate constant (ks = 1 -6 x 107 s 1) for the reaction of [4+] in 25% acetonitrile in water with the corresponding parameters for reaction of the resonance-stabilized l-(4-methoxyphenyl)ethyl carbocation in water (p Tr = — 9.4and s= 1 x 108 s Table 5). 

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 13-membered ring is available in a wide variety of unrestricted and restricted forms and undoubtedly represents one of the most thoroughly studied large-membered heterocycles. The size of the system s perimeter is sufficiently large to accommodate the presence of as many as three trans double bonds which, when strategically implanted, allow the molecular skeleton to adopt a rigidly flat or near-flat geometry in which to aromatically delocalize its 14 tr- electrons. 

The 1,2-dithiolium (158) and 1,3-dithiolium ions (159) are iso-Tr-electronic with the tropylium ion, from which they may be formally derived by replacing two pairs of double bonds by sulfur atoms. Structural data and calculations demonstrate that the rings are substantially stabilized by -ir-electron delocalization. 

The variable nature of the X3 species in the isoelectronic series, form a general model for electronic delocalization, enabling one to classify the species either as distortive or as stable ones. Using the isoelectronic analogy, one might naturally ask about the isoelectronic Tr-species in allyl radical does it behave by itself like H3 or like Li3 Moreover, the same two extreme VBSCDs for X3 in Fig. 6.19 can be shown for X3 +, X3, X4 and X6 species (64). Likewise one might wonder about the status of the corresponding isoelectronic -n-components in allyl cation, anion, cyclobutadiene, or benzene. These questions were answered in detail elsewhere and the reader is advised to consult a recent review (64). Below we briefly discuss the problem of one of our molecular icons, benzene. 

The size difference between carbon and sulfur atoms leads to relatively inefficient overlap of -tr-orbi-tals in the C=S bond. Consequently, thiocarbonyl compounds are in general highly reactive and have a tendency to di-, oligo- or poly-merize. This is particularly true for thioaldehydes, thioketones, and thio-ketenes. In contrast, thioamides (1) are usually perfectly stable and can be handled without problems. This stability can be understood in terms of a pronounced resonance interaction between the C =S TT-bond and the nonbonding electron pair on nitrogen. The analogous electron delocalization prevails in thiolactams. ... 

From the advent of organic chemistry, dienes (and polyenes) have played a very important role in both the theoretical and synthetic aspects. For example, 1,4-addition of bromine to 1,3-butadiene to form l,4-dibromo-2-butenc rather than 3,4-dibromo-l-butene as the major product was a challenging problem for theoretical chemists, who inteipreted the phenomenon in terms of resonance or delocalization of Tr-electrons. ... 

---