Molecular orbital alkylation

The problems associated with predicting regioselectivity in quinone Diels-Alder chemistry have been studied, and a mechanistic model based on frontier molecular orbital theory proposed (85). In certain cases of poor regioselectivity, eg, 2-methoxy-5-methyl-l,4-ben2oquinone with alkyl-substituted dienes, the use of Lewis acid catalysts is effective (86). 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Abbreviations Aik, alkyl AN, acetonitrile Ar, aryl Bu, butyl cod, 1,5-cyclooctadiene Cp, cy-clopentadienyl Cp , pentamethylcyclopentadienyl Cy, cyclohexyl dppm, diphenylphosphinome-thane dpme, Ph2PC2H4PMe2 Et, ethyl fod, 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octane-dionate HOMO, highest occupied molecular orbital LUMO, lowest unoccupied molecular orbital Me, methyl MO, molecular orbital nbd, norbornadiene Nuc, nucleophile OTf, triflate Ph, phenyl Pr, propyl py, pyridine THE, tetrahydrofuran TMEDA V,V,M,M-tetramethylethylenediamine. 

Molecular Orbital Description of Arylaminyl Radicals. Aryl-aminyl radicals, as previously discussed, are intermediates in both the photolysis of alkyl N-arylcarbamates (7, 8) and the photolysis of arylamines (18). A simplified mechanism for photolysis of arylamines and alkyl N-arylcarbamates is illustrated in Scheme IV for the general case. An indication of the reactivity of the... 

An unusual observation was noted when ethanolic solutions of 2-alkyl-4(5)-aminoimidazoles (25 R = alkyl) were allowed to react with diethyl ethoxymethylenemalonate (62 R = H) [92JCS(P1)2789]. In addition to anticipated products (70), which were obtained in low yield ( 10%), the diimidazole derivatives (33 R = alkyl) were formed in ca.30% yield. The mechanism of formation of the diimidazole products (33) has been interpreted in terms of a reaction between the aminoimidazole (25) and its nitroimidazole precursor (27) during the reduction process. In particular, a soft-soft interaction between the highest occupied molecular orbital (HOMO) of the aminoimidazole (25) and the lowest unoccupied molecular orbital (LUMO) of the nitroimidazole (27) is favorable and probably leads to an intermediate, which on tautomerism, elimination of water, and further reduction, gives the observed products (33). The reactions of amino-imidazoles with hard and soft electrophiles is further discussed in Section VI,C. 

An alkyl radical and a nitroxide radical exist in an equlibrium with the corresponding alkoxyamine as their coupling product (Eq. 57). Moad and Rizzardo [213] and Kazmaier et al. [214] independently estimated the effects of the structure of the alkyl group and the nitroxide on the dissociation energy of various alkoxyamines into the radicals by semiempirical molecular orbital calculations. The bond dissociation energies determined are summarized in Table 5 ... 

Wong and coworkers37 studied a series of both symmetrical and unsymmetrical tetraalkyltin compounds with different alkyl substituents, focusing their attention on the effect of these on the ionization energy of the highest occupied MOs. It is useful to recall the type of molecular orbitals deriving from the triply degenerate HOMO in the symmetrical R4M (local symmetry Td) upon substitution of one or more R ... 

Figure 7. Molecular orbitals diagram of the mixing process involved in the insertion of ethene into a Mt-C(alkyl) bond for a generic d° neutral group 3 or cationic group 4 catalyst.

Fig. 2. Significant molecular orbitals of terminal transition metal-oxo units in a six-coordinate 4 ligand environment. The d° configuration is a formal triple bond. The highest occupied molecular orbital in the d configuration is formally nonbonding (8 symmetry) so the metal-oxo bond order remains 3.0. However, d-electron counts above d populate orbitals that are antibonding between the metal and the terminal multiply bonded ligand (0x0 in this case, but alternatively, alkyl-imido, nitrido, sulfido, etc.). Note that all the equatorial ligand orbitals and the metal dx2 y2 orbital (hi in 4 symmetry) are ignored for simplicity.

Several types of molecular orbital calculation have been carried out on the H- and the 2.ff-structures of w-triazole, and on simple alkyl derivatives from these, estimates of several physical properties, in-... 

The tetrahedral structure of these surface alkyl complexes on MCM-41(5oo) has been highlighted by XANES a sharp, intense pre-edge peak at 4969.6 0.3 eV is characteristic of an electronic transition of titanium, from the Is energetic level to molecular orbitals mixing 3d and 4p of Ti with the orbitals of the Ugands, in a complex where titanium is in a tetrahedral symmetry [28-31]. The same argument can be applied for species obtained from alcoholysis of 2a and 2b, especially using tert-butanol. 

Theoretically, the regioselectivity observed in photochemical [2 + 2] cycloaddition of 56 with 1,1-dimethoxyethene is in good agreement with experimental results and has been explained on the basis of pertubational molecular orbital theory." Hartke and co-workers" described an interesting contrast in the reactivity of tropolones in an intramolecular Diels-Alder reaction (Scheme 6.18). Thus, alkylation of 64a and 64b with 65 gave 66a and 66b, respectively, that were subjected to cyclization in refluxing toluene. Whereas 66a decomposed under the reaction conditions, 66b afforded 67b in high yield. 

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