Substitution orbital considerations

To explain tlie stereodieniistiy of tlie allylic substitution reaction, a simple stereoelectronic model based on frontier molecular orbital considerations bas been proposed fl55. Fig. G.2). Organocopper reagents, unlike C-nudeopbiles, possess filled d-orbitals fd - configuration), wbidi can interact botli witli tlie 7t -fC=C) orbital at tlie y-carbon and to a minor extent witli tlie cr -fC X) orbital, as depicted... 

When enantiomerically pure allyl p-tolyl sulfoxide is deprotonated and then treated with electrophilic 2-cyclopentenone, a conjugate addition occurs forming a new carbon-carbon bond with very high control of absolute stereochemistry (equation 25)65. See also Reference 48. Similarly, using more substituted enantiomerically pure allylic sulfoxides leads to virtually complete diastereocontrol, as exemplified by equations 26 and 27 the double bond geometry in the initial allylic sulfoxide governs the stereochemistry at the newly allylic carbon atom (compare equations 26 vs. 27)66. Haynes and associates67 rationalize this stereochemical result in terms of frontier molecular orbital considerations... 

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

In contrast with the behavior of the benzene derivatives, the reaction of a-cyanoalkyl (principally cyanomethyl) ions with other aromatic substrates, leads mainly to the expected substitution products, ArCH(R)CN (equation 39). This difference in behavior (not as pronounced in the thiophene system)132 has been rationalized by molecular orbital considerations.18,41,133 Examples of these successful syntheses are collected in Table 3. 

To react with the majority of dienes, the dienophilic double or triple bond needs to be strained (e.g. benzynes Section 4.1.7) or electron deficient. Recently, transient allyl cations have been introduced as highly reactive, electron-poor dienophiles (Section 4.1.4.1). However, dienophile activation is most often achieved by substitution with at least one electron-withdrawing substituent (e.g. 0=0, CO2R, CN, NO2 or S02Ar). Molecular orbital considerations provide the following rationalization. In a normal Diels-... 

In one study, various substituted allyl radicals were generated by sulfenate photolysis, and it was shown that coupling was controlled both by steric and by frontier molecular orbital considerations [153]. p-Scission is favored by a-substitution and the stability of the putative alkyl radical. A particularly clever device used in another study was the thermal equilibrium between allylic sulfoxides and sulfenates used to generate allyloxy and other C3H5O radicals [154]. 

If we consider more highly substituted ethanes such as GCH2CH2G (where G is a group or atom other than hydrogen), the barriers to rotation are somewhat larger, but they are still far too small to allow isolation of the different staggered conformations. The factors involved in this rotational barrier are together called torsional strain and include the orbital considerations discussed above as well as repulsive interactions called steric... 

Thiophene and its derivatives, however, have become the most-studied chalcogen-containing monomers. Doped (oxidised) PT is highly conducting, and 3-alkyl-substituted PTs are soluble, easily characterised polymers,83 85 see Figure 7. Self-assembly is an important consideration in these substituted PTs, allowing control of the degree of planarity, n-orbital overlap, and hence optical... 

Finally, we discuss the effects of chalcogen (S to Se) substitution. Although in (EDS-TTF)2FeBr4 remarkably close Se---Br contacts exist between donor and anion layers, little magnetic exchange interaction exists between the magnetic anions. The molecular orbital calculation reveals that the contribution of Se 4p orbital to the HOMO of the EDS-TTF molecule is considerably smaller than the contribution of... 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

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