Donor substituents

Donor substituents on the vinyl group further enhance reactivity towards electrophilic dienophiles. Equations 8.6 and 8.7 illustrate the use of such functionalized vinylpyrroles in indole synthesis[2,3]. In both of these examples, the use of acetyleneic dienophiles leads to fully aromatic products. Evidently this must occur as the result of oxidation by atmospheric oxygen. With vinylpyrrole 8.6A, adducts were also isolated from dienophiles such as methyl acrylate, dimethyl maleate, dimethyl fumarate, acrolein, acrylonitrile, maleic anhydride, W-methylmaleimide and naphthoquinone. These tetrahydroindole adducts could be aromatized with DDQ, although the overall yields were modest[3]. 

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... 

In azole chemistry the total effect of the several heteroatoms in one ring approximates the superposition of their separate effects. It is found that pyrazole, imidazole and isoxazole undergo nitration and sulfonation about as readily as nitrobenzene thiazole and isothiazole react less readily ica. equal to m-dinitrobenzene), and oxadiazoles, thiadiazoles, triazoles, etc. with great difficulty. In each case, halogenation is easier than the corresponding nitration or sulfonation. Strong electron-donor substituents help the reaction. 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

Given in Table 4.5 in addition to the Hammett equation are ct and substituent constant sets which reflect a recognition that the extent of resonance participation can vary for different reactions. The values are used for reactions in which there is direct resonance interaction between an electron-donor substituent and a cationic reaction center, hereas the a set pertains to reactions in which there is a direct resonance interaction between the substitutent and an electron-rich reaction site. These are cases in which the resonance conqionent of the... 

Enols are not as reactive as enolate anions, however. This lower reactivity simply reflects the presence of the additional proton in the enol, which decreases the electron density of the enol relative to the enolate. In MO terminology, the —OH and —0 donor substituents raise the energy of the tt-HOMO. 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

Among the reagents that are classified as weak electrophiles, the best studied are the aromatic diazonium ions, which reagents react only with aromatic substrates having strong electron-donor substituents. The products are azo compounds. The aryl diazonium ions are usually generated by diazotization of aromatic amines. The mechanism of diazonium ion formation is discussed more completely in Section 11.2.1 of Part B. 

Alkyl groups which are para to strong n-donor substituents such as hydroxy or ... 

Conjugated substituents at C-2, C-3, C-4, or C-5 accelerate the rearrangement. Donor substituents at C-2 and C-3 have an accelerating effect. The effect of substituents can be rationalized in terms of the stabilization of the transition state by depicting their effect on two interacting allyl systems. 

A donor substituent, e.g., triraethylsilyloxy, at C-2 has a strongly accelerating effect. 

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. 

Allylic and benzylic radicals are also stabilized by both acceptor and donor substituents. As shown in Table 12.5, theoretical calculations at the MP2 level indicate that substituents at the 2-position are only slightly less elfective than 1-substituents in the... 

Draw a Lewis structure (or a series of Lewis structures) for nitrobenzenium ion. Where is the positive charge Examine the electrostatic potential map for nitrobenzenium ion. Where would you expect electron-donor substituents to have the greatest stabilizing effect (consider meta and para positions only) Which is the more stable, meta or para-nitrotoluenium ion (intermediates in nitration of toluene) Compare electrostatic potential maps to that for nitrobenzenium ion. Does your result suggest that methyl acts as an electron donor ... 

Electron-donor substituents are known to accelerate the rate of electrophilic substitution on benzene, while electron-withdrawing groups are known to retard the reaction. One explanation is that electron donors stabilize the positive charge in the benzenium ion intermediate while electron-withdrawing substituents destabilize the positive charge. 

Boroles are the ri -complex-forming ligands. However, in the presence of the TT-donor substituents at the heteroatom, they tend to give V or species, especially for the low-valent early transition metals. 

This activation is more pronounced on the introduction of stronger electron-donor substituents, but this point is not yet sufficiently studied. The isoxazole ring is unsymmetrical, and the activating effect of the substituent depends on its position. The available evidence shows that a substituent at C-5 activates the nucleus (or rather the 4-position) more strongly than does a substituent at C-3. 

With certain donor substituents at C-3 the experimental findings may be rationalized rather by a diradical mechanism, where formation of the new carbon-carbon single bond leads to a diradical species 6, which further reacts by bond cleavage to give the diene 2 ... 

Donor-substituents D, e.g., bearing + M- or +I-substituents including alkyl groups, enhance the tendency of y-attack. 

Silylene complexes are not only stable with donor substituents but also with simple alkyl residues at silicon. These alkyl complexes still have a sufficient thermodynamic stability, but otherwise are reactive enough to allow a rich and diverse chemistry. Particularly the chlorocompounds 7 and 11 are valuable starting materials for further functionalization reactions the details of these reactions will be discussed in the forthcoming sections. The data for the known compounds are summarized in Table 1. 

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... 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

Ambroz and Kemp (1979 a, 1982 a, b) photolyzed benzenediazonium salts with donor substituents in the 4-position (2,5-di- -butoxy-4-morpholine-, 2,5-diethoxy-4-H-butylthio-, and 2,4,5-trimethoxy-benzenediazonium hexafluorophosphate in LiCl - H20 - acetone matrices at low temperatures, 77-130 K). Under these conditions the aryl cation formed is sufficiently stable to be identified spectroscopically. It is a triplet, a result that is consistent with earlier ab initio calculations by Dill et al. (1977), and with earlier observations by Lee et al. (1961), also at low temperature (77 K). 

Six-membered A-heteroaromatic compounds without electron-donor substituents (e. g., pyridine) were classified earlier by Albert (1968) as 7c-electron deficient. Therefore they are not reactive enough to be used as coupling components. 

After the compounds with one heteroatom in the ring, the second group of heteroaromatics are the heteroaromatic compounds with two or more heteroatoms in the same ring, but without electron donor substituents. 

The third group of heteroaromatic coupling components includes ring systems with electron-donor substituents. In principle they correspond to their (carbo)aro-matic analogues, the phenols and anilines. 

Other five-membered heteroaromatic compounds with electron-donor substituents, which have become interesting for industrial application during the last two decades, are the aminopyrazoles 12.62 and aminothiazoles 12.63 (see Schwander, 1982). 

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