Benzenes acceptor-substituted

According to polarity alternation the acceptor substituted benzenes should also undergo ipso substitutions [12] Indeed such reactions occur when A = SiMe3, SnRj, HgR. A useful application of the silyl group as site determinant is found in a synthesis of 8-methoxy-iV-methyl-l,2,3,4-tetrahydroisoquinoline [13]. The donor character at the ortho position of the methoxy function is accentuated and steric disfavor is overcome. 

Because of completely analogous considerations, every acceptor-substituted sigma complex E—C6H5—EWG is less stable than the reference compound E— C6H6+ (Figure 5.11). From this analysis, one derives the following expectations for Ar-SE reactions of acceptor-substituted benzenes ... 

Fig. 5.11. Ar-SE reactions with acceptor-substituted benzenes (EWG, electron-withdrawing group) comparing the regioselectivity and the reactivity with benzene. The thicknesses of the initial arrows again indicate qualitatively to what extent the reaction takes place via the respective transition states (but there is no relationship to the thicknesses of the arrows in Figure 5.10).

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

Which Wheland complexes are the most stable This is determined to a small extent by steric effects and to a considerably greater extent by electronic effects As a car-bocation, a substituted Wheland complex is considerably more stable than an unsubstituted one only when it carries one or more donor substituents, and unsubstituted Wheland complexes E—C6Hj are still considerably more stable than Wheland complexes that contain one or more acceptor substituents. Therefore, donor-substituted benzenes are attacked by electrophiles more rapidly than benzene, and acceptor-substituted benzenes are attacked more slowly. 

Threefold donor-acceptor-substituted benzene derivatives like [109] (Ledoux et al., 1990) or [110] (Verbiest et al., 1994 Wolff et al., 1996b Wortmann et al., 1997 Wolff and Wortmann, 1998) show better performance for [109], only powder data and computational results are available. Both are of the hexasubstituted type, but strong intra- and inter-molecular hydrogen bonds provide for planarity. The discrepancy (Bredas et al., 1992) between the observation of a moderate powder SHG efficiency of [109] and the published (Cady and Larson, 1965) centrosymmetric crystal structure (PT) has been resolved. The powder consists of two polymorphs, with the second one adopting the close to optimal space group P3i (Voigt-Martin et al., 1996,1997). 

The discovery of the dual fluorescence of the simple donor-acceptor substituted benzene derivative 4JV,iV-dimethylaminobenzonitrile (DMABN) by Lippert et al. [1] and the subsequent model compound studies by Grabowski et al. [2-6] including rigidized and pretwisted compounds such as MIN, TMABN and CBQ gave birth to the idea of Twisted Intramolecular Charge Transfer (TICT) states. 

Figure 7.47. Orbital ordering in donor- and acceptor-substituted benzenes.

According to polarity alternation the acceptor substituted benzenes should also undergo ipso substitutions [12] In ed such reactions occur when A = SiMe,... 

Electron acceptor-substituted benzenes such as benzonitrile and a,a,a-trifluorotoluene generally give ort/io-cycloadducts with olefins [18]. Some exceptions are also found. For example, photo addition of l,3-dioxol-2-one to benzonitrile gives only mera-adduct of emio-configuration having electron acceptor substituent at C-2 and C-4 carbon atoms. Possibly the electron acceptor ability of benzonitrile is higher at ortho- and para-positions [19]. The following examples are illustrative ... 

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