Benzenes donor-substituted

Here we are intentionally restricting from our discussion bond fixation of the type found in cross-conjugated acceptor-donor substituted benzenes. Fw a discussion of these effects, see Baldridge, K. K. Siegel, J. S. J. Am. Chem. Soc. 1992,114,9583. 

The o-adducts from such reactions are generally subjected to oxidative decom-plexation to generate the aromatic products. Protonation/deuteration of an adduct derived from a donor-substituted benzene could result in a net meta displacement of the donor [18]. Ipso substitution has also been revealed when thea-complex is wanned. 

Perhaps it should be mentioned also the orientation of the Birch reduction which is strongly dependent on the nature of the aromatic substituents. Donor-substituted benzenes furnish predominantly 1-substituted 1,4-cyclohexadienes while acceptor-substituted analogues give 3-substituted 1,4-cyclohexadienes. The regioselectivities can be explained by the destabilizing d-d pairing in the intermediates from d-substi-tuted cyclohexadienyl radical anions leading to the 3-substituted products, and the... 

Bis(trifluoromethyl)-l,2,4,5-tetrazine (426) cycloadds to benzyne giving a quantitative yield of l,4-bis(trifluoromethyl)phthalazine (427). Cycloaddition of (426) to benzene also forms (427) in good yield by oxidation of the initial dihydro derivative. Reasonable yields of l,4-bis(trifluoromethyl)-6-substituted phthalazines are obtained when electron donor-substituted benzenes are used in this reaction, and the regioselectivity is in agreement with calculations (87AG(E)332). 

Silabenzene 24 reveals a characteristic Si—H stretching vibration at 2217 cm-1, as expected for hydrogen attached to a sp2-hybridized silicon atom. Compound 24 shows a typical benzene-type UV spectrum with absorptions at X = 217, 272 and 320 nm, which fit into the series of the already known donor-substituted heterobenzenes30. An additional structural proof was the partially reversible photochemical conversion of 24 into Dewar... 

The diphenylmethyl and triphenylmethyl cations can be considered the basic chromophores of di- and triarylmethane dyes 11 —41. However, the electronic state of the donor-substituted di- and triarylmethine dyes can be described better by breaking down the chromophore of these dyes into a straight- or branched-chain polymethine subchromophore, respectively, and two ethylene units (from the formal cleavage of the benzene rings) [2], This model allows the similarities between the di- and triarylmethine dyes and the polymethine s to be recognized. 

Each H atom of a donor-substituted aromatic compound should be substituted faster by an electrophile than an H atom in benzene. 

Donor-substituted benzenes and electrophiles should produce mixtures of para- and orf/zo-disubstituted aromatic compounds, in which the /zara-disubstituted product is formed in a greater amount. Only traces of the /neto-disubstitution product are expected, even though a donor-substituted benzene is substituted faster at the meta-C atom than benzene itself at any of its C atoms. 

Fig. 5.10. Ar-SE reactions with donor-substituted benzenes (Do, donor substituent) comparing the regioselectivity and the reactivity with benzene. The thicknesses of the initial arrows show qualitatively to what extent the reaction takes place via the corresponding transition state.

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. 

Recent results revealed that the ortho photocycloaddition frequently occurs concomitantly with the meta photocycloaddition even in cases where only meta derivatives are obtained. Especially in the case or electron-donor substituted benzene derivatives, the competitive ortho cycloaddition is less stereo- or regioselective and the resulting products are less stable. Aside the meta adducts as main products, complex mixtures of ortho products or products resulting from rearrangements of these primary photoproducts have been obtained [15,16], Improved separation techniques recently enabled a better characterization of these products. Furthermore, using particular conditions like an acidic reaction medium, the intermediates resulting from ortho photocycloaddition could be transformed selectively in more stable final products [17]. 

The meta photocycloaddition is frequently observed in cases of alkyl or electron-donor substituted benzene derivatives. Two exo/endo isomeres 2a,b are obtained in high yields in the reaction of anisole 1 with cyclopentene (Sch. 2) [18]. The reaction was also efficiently carried out in an intramolecular way. Two efficient reactions of this type are depicted in Sch. 2. The azatriquinane 3 [19] and the silane derivative 4 [20] were obtained in high yields. The formation of different regioisomers can be controlled by the heteroatoms in the side chain. The longer C-Si bonds particularly favor the formation of compound 4. 

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. 

Benzene and substituted benzenes normally act as six-electron donors, although dihapto and tetrahapto complexes are also known. Dibenzenechromium was prepared eariy in this century but was not characterized until 1954. It was first synthesized via a Grignard synthesis. When PhMgBr reacts with CrClj in diethylelher solvent, a mono-hapto complex [CrPh lEt O) ] forms which rearranges. presumably by a free radical reaction to give, among other products, This ion can be reduced... 

In toluene, as well as with many other monosubstituted benzenes, the substitution group (methyl, in the case of toluene) acts as an electron donor. This counters the electron-withdrawing effect of the previously substituted nitro groups and allows higher local nitronium ion activity, thus allowing a much easier trinitration step. This is the key, then, to inexpensive synthesis of trinitro-aromatic explosives. Figure 3.5 is the TNT molecule, the first, and most important (as far as quantity of production goes) of the monosubstituted TNBs. 

Hydrolysis of phenoxymethyl penicillin and 14 derivatives with benzene ring substituents was studied by Panarin and Solovskii 40 at pH 2.0 and 30, 35 and 40°. Enhanced acid stability was shown by electron-accepting substituents while the hydrolysis rate was increased by electron-donor substitution. 

The [3 + 2] photocycloaddition (Scheme 6.79) usually involves the ground-state alkene and the Si excited state of an electron-donor substituted benzene derivative, often via an exciplex intermediate.807,809 811,816 The discrimination between the ortho- and metacycloaddition pathways is dependent on the electron donor acceptor properties of the reaction partners and the position and character of the reactants substituents.807 The reaction typically produces many regio- and stereoisomers however, a suitable structure modification can reduce their number. Intermolecular and intramolecular versions of the reaction are presented in Scheme 6.88 (a) photolysis of the mixture of anisole and 1,3-dioxole (226) leads to the formation of two stereoisomers, exo- and endo-221, in mediocre ( 50%) chemical yields 830 (b) four different isomers are obtained in the intramolecular photocycloaddition of an anisole derivative 228. 831... 

Some of these ideas could be generalized to a classification scheme for substituents in terms of their donor (D) and acceptor (A) properties. Following the usual partition procedure, four groups were specified (1) o acceptor and 7t acceptor, (2) o donor and 71 acceptor, (3) o donor and ti donor, and (4) o acceptor and 7t donor. Substituted benzenes were considered, and each system was partitioned in two subsystems. Subsystem A consisted of the benzene ring and subsystem B of the substituents. The change of electronic charge in the carbon framework of the ring due to substitution was defined as... 

Photochemical cycloaddition of cis-cyclo-octene to benzene gives a mixture of the 1,2-adduct (211) and the exo- and endo-1,3-adducts (212). A study of the photochemical cycloaddition of olefins, including medium-ring olefins, to benzene and substituted benzenes, shows that the 1,2-photocycloaddition differs from the 1,3-process in requiring an olefin of either donor or acceptor character the donors add endo, the acceptors add exo. ... 

Most aromatic chloro and bromo compounds have strong absorptions at 760-395cm" (13.10-25.32pm) and 650-395cm" (15.38-25.32pm) respectively, which is due to a combination of vibrational modes. Monosubstituted benzenes, dihalogen-substituted benzenes, and compounds with electron-donor or methyl substituents in the para position of halobenzenes all exhibit the former band. 

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