Substituents, acceptor

Different behavior is expected for X = N-R Figs. 3 and 4) the equilibrium should be shifted toward 1 when an electron donor substituent (a = -l) is present (in any position) and toward form 2 when electron acceptor substituents are present. Ii has been shown chat a simultaneous... 

Both the reactivity data in Tables 11.3 and 11.4 and the regiochemical relationships in Scheme 11.3 ean be understood on the basis of frontier orbital theory. In reactions of types A and B illustrated in Seheme 11.3, the frontier orbitals will be the diene HOMO and the dienophile LUMO. This is illustrated in Fig. 11.12. This will be the strongest interaction because the donor substituent on the diene will raise the diene orbitals in energy whereas the acceptor substituent will lower the dienophile orbitals. The strongest interaction will be between j/2 and jc. In reactions of types C and D, the pairing of diene LUMO and dienophile HOMO will be expected to be the strongest interaction because of the substituent effects, as illustrated in Fig. 11.12. 

Electron-donor and electron-acceptor substituents selectively interact with different ring orbitals. Compare the HOMO and LUMO of azobenzene with the corresponding orbitals of the two substituted molecules. Which orbitals show signficant substituent contributions What are the nature of these contributions, bonding or antibonding Try to relate this to the effect which the substituents have on orbital energies and on the HOMO-LUMO gap in azobenzene. 

It should be noted that a considerable acceleration of the reaction for low-reactive 4-iodopyrazoles is observed for substrates in which acceptor substituents at the pyrazole nitrogen atom additionally play the role of protecting group. Thus, it has been shown (88M253) that iV-phenacyl- and iV-p-tosyl-4-iodopyrazoles interact with phenylacetylene, 2-methyl-3-butyn-2-ol, and trimethylsilylacetylene at room temperature for 3-24 h in 70-95% yields (Scheme 56). 

The interaction between a 4-iodopyrazole-3-carboxylic acid and copper ace-tylides having both donor and acceptor substituents at the triple bond generated six- rather than five-membered lactones, as in the aromatic series (Scheme 117). 

The reaction is of wide scope. Instead of ester groups as substituents at C-3 and C-5, other acceptor substituents—e.g. oxo, cyano, sulfonyl or nitro groups—can be employed in order to stabilize the 1,4-dihydropyridine system. 

Other substituted systems, however, might be planar due to conjugation effects with acceptor substituents, as has been found in an X-ray structural analysis of 1,4-dioxocin-6-carboxylic acid chloride the eight-membered ring is practically planar with a coplanar arrangement of the substituent.9... 

In the 1,4-dihelerocine scries 1 with different heteroatoms only the 1,4-oxazocines 2 are known. As in the case of the 1,4-diazocines, depending upon the substituent on the nitrogen, these systems exhibit either nonplanarity with a localized -electron system for acceptor substituents or planar delocalized structures for donor-substituted or the unsubstituted systems.10... 

The formation of porphyrins from 1,19-dideoxybi)enes-/r can be achieved starting either from the 1-methyl derivatives or from l,19-dideoxybilene-Z>-l,19-dicarboxylic acid esters. In the first case the desired methine bridge of the porphyrin stems from the 1-methyl group whereas in the latter case orthoformates have to be added in the condensation step as a precursor for the methine unit. The 1-methyl- and also 1,19-dimethyl-l,19-dideoxybilene- > salts can be cy-clized to the corresponding porphyrins with copper(II) acetate in methanol.56 However, when the bilenes contain /i-acceptor substituents, the yields of porphyrins obtained by this method are very low.57... 

Improvements can be achieved by variation of the solvent and the careful location of the acceptor substituents.58 59 Another problem of this synthetic approach is the formation of hexapyrroles as byproducts of the bilin preparation. In the case of the 1,19-dimethylbilenes one of the carbons of the terminal methyl groups has to be expelled, so most of the syntheses make use of the 1-methylbilenes. A mechanistic interpretation of the cyclization step is similar to that for 1,19-dideoxybiladienes-nc (vide infra). In a few cases60 64 bilcne-l-carbaldehydes are used. 

The cyclization of 1,19-dideoxybilene-i-dicarboxylic acid esters has been widely used for the synthesis of porphyrins. In this case, the use of tert-butyl esters which can be hydrolyzed with trifluoroacetic acid prior to the cyclization step is necessary. The cyclization step also requires trifluoroacetic acid and orthoformates. However, attempts to prepare porphyrins with /f-acceptor substituents can give rise to problems with side products and yields. 

The absorption and luminescence spectra of imidazo[ 1,2,4]triazines and related compounds were recorded. The phenyl groups on both the 6-and the 7-positions quenched the luminescence. An acceptor substituent such as CHO in position-7 sharply reduced the luminescence quantum yield (82MI4). A detailed study of the infrared spectra of imidazotriazines was carried out (75T433). 

Carbanions (counterions M = K +, Na +, Li+ or R4N+) bearing acceptor-substituents A, c.g., — M- or — I-substitucnts, vinyl, or phenyl groups, add to carbonyl compounds in uncatalyzed reactions preferentially at the x-position. 

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Those sets for which the resonance effect is predominant are the sets which are most likely to give rise to the free carbonium ion 5, as the substituents in these sets (sets 15-14 and 15-17 and possibly 15-18) are all donors by resonance, as is shown by their Or values. Those sets for which the localized effect is predominant may be accounted for in terms of intermediates 3 or 4. Sets 15-5, 15-7B2, and 15-12 gave significant values of jS. It is difficult to account for this fact in terms of intermediate 4. The results can be accounted for in terms of intermediate 3, however, if this species resembles other three-membered rings, such as cyclopropane, in its behavior. Sets 15-6, 15-8, 15-9, 15-12, and 15-15 include both donor and acceptor substituents. The successful correlation of... 

Stimulated by extensive research activities on donor/acceptor substituted stilbenes, Mullen and Klarner have reported a donor/acceptor substituted poly(4,4 -biphenyl-diylvinylene) derivative (85) in which the NR2 donor and CN acceptor substituents are located on the vinylene unit [111]. The synthesis is based on a C-C-coupling reaction of in situ generated carbanion functions with a (pseudo)cation function, followed by a subsequent elimination of MeSH with formation of the olefinic double bond. 

The photoadditions proceed through 1,4-diradical intermediates. Trapping experiments with hydrogen atom donors indicate that the initial bond formation can take place at either the a- or (3-carbon of the enone. The excited enone has its highest nucleophilic character at the (3-carbon. The initial bond formation occurs at the (3-carbon for electron-poor alkenes but at the a-carbon for electron-rich alkenes.191 Selectivity is low for alkenes without strong donor or acceptor substituents.192 The final product ratio also reflects the rate and efficiency of ring closure relative to fragmentation of the biradical.193... 

Diethyl arylphosphonates were synthesized by reactions of diethyl phosphonate with aryl iodides or bromides containing electron-donor or electron-acceptor substituents in the aromatic ring in aqueous MeCN or neat H2O in the presence of Pd complexes with water-soluble ligands.34 For example, MeCN/H20 (1 mL), PhBr (8.2 mmol), and Ph2P(C6H4S03Na-m) (approximately 0.4 mmol) were successively... 

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