E Substituents

A -E substituent is a group isoconjugate with an odd AH anion and attached through an active atom to the parent molecule. In principle, car-banionoid substituents such as —CHj (derived from the simplest odd AH 

Next consider union of HR with an odd radical HS-. In this case the NBMOs of R and S again interact in RS but now there are three electrons to be accommodated in the resulting pair of MOs (Fig. 4.13b). While two of these can be accommodated in the lower, bonding MO of the pair, the third has to go into the upper, antibonding MO. While there is a decrease in n energy on union of HS and HR , it is much less than in the case of the cation HS+. 

Simple (carbanionoid) — E substituents at an active position therefore have a very large stabilizing effect on odd AH cations and a smaller one on odd AH radicals, while odd AH anions are somewhat destabilized. This order is exactly the opposite shown by + substituents. 

Let us next consider the effect of introducing heteroatoms. To get a neutral substituent rather than an ionic one, it is necessary that one of the active atoms in the carbanion HR should be replaced by a heteroatom. The 7r-electron density at that position will then be 1 + with or the corresponding NBMO coefficient. Consider now introduction of the substituent into an odd AH cation HS . The resulting derivative RS will be an even AH (identical with that produced by union of the odd AH radicals HR- and HS Fig. 4.15a) the 7r-electron density of the heteroatom will now be unity. The energy of union dEj, of the heteroatomic substituent (HR ) with HS will therefore be less than that (5Ej,) of the carbanion HR with HS , the difference between the two energies being given by 

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When the compound can be considered as derived from a base whose name does not end in -amine, its quaternary nature is denoted by adding ium to the name of that base (with elision of e), substituent groups are cited as prefixes, and the name of the anion is added separately at the end. Examples are... 

Jung M. E. Substituent and Solvent Effects in Intramolecular Diels-Alder Reactions Synlett 1990 186-190... 

Weinert, E. E. Dondi, R. Colloredo-Melz, S. Frankenheld, K. N. Mitchell, C. H. Freccero, M. Rokita, S. E. Substituents on quinone methides strongly modulate formation and stability of their nucleophilic adducts. J. Am. Chem. Soc. 2006, 128, 11940-11947. 

The transition-state model of this reaction has been proposed as (1), based on X-ray analyses of single crystals prepared from Ti(OPr )4, (R,R)-diethyl tartrate (DET), and PhCON(OH)Ph and from Ti(OPr )4, and (R,R) N,/V -dibenzyltartramide.30-32 The Z-substituent (R2), located close to the metal center, destabilizes the desired transition state and decreases enantioselectivity (vide supra). When the Z-substituent is chiral, face selection induced by the substituent strongly affects the stereochemistry of the epoxidation, and sometimes reversed face selectivity is observed.4 In contrast, the. E-substituent (R1) protrudes into an open space and E -allylic alcohols are generally good substrates for the epoxidation. 

A +E substituent R and a — E substituent T can conjugate mutually through an odd alternant hydrocarbon radical S if R, T are both attached to active atoms in S. 

An electron-donating (MeO) or an electron-withdrawing (E) substituent linked to the benzene ring is without effect in the reaction. 

The level of syn induction increases as the size of the E-substituent increases as well as the size of the allylic substituent. An interesting application of these reaction conditions was reported in the synthesis of an advanced intermediate in halicholactone synthesis (equation 60). It is important to note that the reaction proceeded not only with excellent diastereocontrol, but also with complete chemoselectivity in favor of the allylic alcohol double bond. However, the participation of the trimethylsilylethoxymethyl (SEM)... 

The relative stabilities of metalloporphyrins can be compared by transmetallation reactions.18 As expected from the Zjyi ratio, monovalent metals are easily displaced by other metal ions. The order of stability is indicated as follows Cu > Zn > Cd > Hg > Pb (> Ba) > Li > Na > K. In most cases the transmetallation rate is first order both in metal ion and in metalloporphyrin, and the intermediate is considered to be the dinuclear complex [M(Por)M ] For Cu/Zn(2,4-disubstituted DPIX DME) in boiling pyridine, the substitution rate increases as the porphyrin basicity decreases, i.e. substituent = Br > CH—CH2 > CH2CH3(> Etio). 

The studies of cyclohexane and its derivatives by Hassel and co-workers in the late thirties and early forties using mainly the electron diffraction method laid the foundation of conformational analysis. In 1943 Hassel295 summarized that cyclohexane exists mainly in the chair conformation as distinct from any other possible conformation. The chair conformation will have distinguishable axial, a, and equatorial, e, substituents. (See Fig. 13). The equatorial position is the energetically favored one. Furthermore, Hassel stated that there is a rapid inversion of the ring with an associated low barrier. This motion interchanges the a and e positions with the result that a and e conformers cannot be isolated. 

RNA modification database at http //medlib.med.utah.edu/RNAmods furnishes information concerning naturally modified nucleosides in RNA (Limbach et al., 1994). After clicking Search, select individual files (pick corresponding radios under categories base type, RNA source, and phylogenetic occurrence) to be accessed, and output options (common name, structures, and/or mass values). Enter partial name (i.e., substituent name such as methyl, thio, etc.) and choose the parent base(s) from which modified nucleosides are derived (Figure 5.8). Click Search button. The tabulated search results based on selected files are returned according to the requested output options. 

According to Dewar (l%9) a n donor is referred to as - E substituent and a n acceptor as + E substituent. This is the sign convention originally introduced by Lapworth and Robinson. Here, we use the opposite sign convention, which was proposed later by Ingold. 

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