Anisole photochemically

The charge-transfer nitrations of the aromatic donors are generally carried out to rather low actinic conversions to avoid complications from light absorption by the nitroarene products, and in duplicate sets (with a dark control) to monitor simultaneously any competition from thermal processes. For example, the yellow solution of anisole and Me2PyN02 in acetonitrile at — 40°C is irradiated with the aid of the cut-off filter that effectively removes all excitation light with Aexc<400nm. After reasonable photochemical conversions are attained, the H NMR spectrum is found to be virtually identical to that of the reaction mixture obtained by electrophilic (thermal) nitration (60). 

In order to examine the effect of the nitrating agent, anisole can be also treated with Me02CPyN02, PyN02 and MeOPyN02. Control experiments carried out simultaneously in the dark establish the absence of any electrophilic (thermal) component to anisole nitrations under all charge-transfer conditions (Kim et al., 1993 ). The photochemical nitrations of... 

In the photochemical one-electron oxidation of aromatic sulfides, dimer radical cations were formed in rapid equilibrium with monomeric radical cation (59). The complex formation of a- and tt-types has been shown to be sensitive to the steric and electronic influence of substituent. For the case of jo-(methylthio)anisole the formation of TT-type dimer was shown to be reduced due to steric hindrance of two methyl groups. No formation of dimer radical cation was observed for jo-(methoxy)thioanisole and diphenyl disulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. Density-functional calculations supported the experimental results. The intramolecular formation of similar radical... 

Only very few examples of alkene-]2-i-2] cycloadditions are known ]345, 347, 348]. By using a large excess of the moderate electron-rich alkene p-propenyl-anisol ]348] or even less electron-rich alkyl-subshtuted 1,3-butadienes [347] no thermal [2-1-2] cycloaddition occurs, but a photochemical cycloaddition can be enforced. The mechanism is proven to be stepwise via a biradical or dipolar intermediate ]347-351], comparable to the addition of the alkynes. During the addihon of cis- and trons-alkenes the existence of this relahvely long lived intermediate leads to a loss of stereochemical integrity. Addihon of ds-4-propenylanisol or trans-4-propenylanisol results in both cases exclusively in the trans-adduct (Scheme 4.61). 

Although hole-catalyzed (cycloaddilions involving radical cation intermediates) and PET (photochemical electron transfer) mixed [2 -I- 2] cycloadditions have been reported from electron-rich alkenes, the only report of a cyclodimerization is that of (E)-4-(prop-l-enyl)anisole which gives stereoisomeric mixtures of the head-to-head dimers 1 and 2.12... 

Recently, it has been reported that photochemical fluorodediazoniation is improved when a solution of the diazonium tetrafluoroborate in hydrogen fluoride/pyridine (70 30 w/w) mixture is irradiated at 10°C.248 For example, 4-fluoro-3-(trifluoromethyl)anisole is obtained by this method in 95 % yield from the corresponding diazonium tetrafluoroborate. Concerning the synthesis of 4-fluorophenol under similar conditions, the yield is very dependent on the... 

The products formed in these reactions are very sensitive to the functionality on the carbenoid. A study of Schechter and coworkers132 using 2-diazo-1,3-indandione (152) nicely illustrates this point. The resulting carbenoid would be expected to be more electrophilic than the one generated from alkyl diazoacetate and consequently ihodium(II) acetate could be used as catalyst. The alkylation products (153) were formed in high yields without any evidence of cycloheptatrienes (Scheme 33). As can be seen in the case for anisole, the reaction was much more selective than the rhodium(II)-catalyzed decomposition of ethyl diazoacetate (Scheme 31), resulting in the exclusive formation of the para product. Application of this alkylation process to the synthesis of a novel p-quinodimethane has been reported.133 Similar alkylation products were formed when dimethyl diazomalonate was decomposed in the presence of aromatic systems, but as these earlier studies134 were carried out either photochemically or by copper catalysis, side reactions also occurred, as can be seen in the reaction with toluene (equation 36). 

Similar results were obtained [139] with the three dimethoxybenzenes and acrylonitrile, methacrylonitrile, and crotonitrile. The amounts of substitution products decrease in the order acrylonitrile (49%) > methacrylonitrile (45%) > crotonitrile (6%), which agrees with the electron affinities of these compounds. Simultaneously, the amount of addition product increases acrylonitrile, 0% methacrylonitrile, 38% crotonitrile, 67%. In the series of anisole and the dimethoxybenzenes with crotonitrile, the amount of substitution products decrease in the order ortho- and para-dim ethoxy benzene > meta-dimethoxyben-zene > anisole, which is just the reverse of the order of their oxidation potentials. Ohashi et al. [139] have attempted to relate the photochemical behavior of these systems to the free enthalpy of electron transfer in the excited state as calculated with the Rehm-Weller equation, AG = E(D/D+) - E(A /A) - el/eR - AE00. 

An unusual process that has attracted much interest recently is the photochemical reaction between chloroaromatic and an alkene, with the aryl ring and chloride adding across the double bond. This offers an alternative approach for the synthesis of alkylaromatics, as illustrated in Scheme 3.29. The irradiation of 4-chloroanisole caused the photoheterolysis of the Ar—Cl bond, thus forming a reactive phenyl cation and Cl. The cation added regiospecifically to the double bond of 1-hexene, and the resulting adduct cation was trapped by the chloride anion to yield 4-(2-chlorohexyl) anisole (46) in moderate yield [76]. 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Photochemical reaction in acetonitrile of 2 -deoxyuridine 5 -phosphate with the halo-heteroarenes 2-iodothiophene, 2-iodofuran, l-methyl-2-iodopyrrole and 3-iodothiophene affords the C-5 heteroaryl substituted nucleotides518.6-Aryluridines have been prepared by irradiation of 6-iodouridines in benzene, anisole, thiophene, Af-methylpyrrole or 2-methyl-furan in the presence of triethylamine519. 

Such a mechanism has been recognized to operate in some anomalous photochemical nucleophilic substitutions observed e.g. with anisoles, where... 

A nearly complementary pattern of reactivity has been found for photochemical electrophilic substitution. Proton exchange in the photolysis of toluene 8.4 takes place most rapidly at the meta position. In anisole 8.5, the corresponding reaction is predominantly ortho and meta. Nitrobenzene 8.6, however, exchanges protons most rapidly at the para position. 

Mattay, J., Runsink, J., Piccirilli, J. A., Jans, A. W. H., Cornelisse, J., Selectivity and Charge Transfer in Photoreactions of Donor Acceptor Systems. 8. Photochemical Cycloadditions of 1,3 Dioxoles to Anisole, J. Chem. Soc., Perkin Trans. 1 1987, 15 20. 

The reactive intermediate Fe(dmpe)2 cleaves the C—H bonds of other aromatic substrates, but the reactions are complex. The final products with anisole or methyl benzoate are methyl derivatives derived from cleavage of their respective O—CH3 bonds. Chlorobenzene gives a transient aryl hydride, but the ultimate product is the phenyliron chloro complex. Both Ru(dmpe)2H(CioH7) and Os(dmpe)2H(CioH7) are also in equilibrium with their respective M(dmpe)2 intermediates, but the equilibria are very sluggish and of little synthetic utility. An alternative approach to these species is photochemical activation of Fe(dmpe)H2 the products can be obtained more cleanly, and the intermediate Fe(dmpe)2 cleaves even aliphatic C—H bonds. 

Solvent effects in the photochemical reaction of A -(2-phenyl-4-penten-l-oxy)-pyridine-2(l//)-thione and Bu3SnH have been studied [36]. Neither the use of tert-butyl benzene, chlorobenzene, bromobenzene, anisole, cyclohexane, tetrahydrofuran, nor ethanol leads to a significant change in yields and selectivities for the formation of 2-methyl-4-phenyl tetrahydrofuran (16) (50-70%, cis/trans 88 12),... 

By contrast, and although an aspect of the reactbns of anisoles rather of their synthesis it is noteworthy that in a photochemical transformation (with the use of a Vycor filter) of a 2-substituted anisole in pentane at ambient temperature, two isomeric cyclopentacycloheptanes were obtained in 63% yield, a useful step from component B for a synthesis of rudmollin, a substance with anti-leukaemic properties (ref.82). 

Scheme 8. In fact, when placed in the reaction medium in the dark, (23) gives a product distribution similar to that observed in the photochemical reaction. In contrast with reduction in alcohols, the quantum yield in aqueous HC1 drops rapidly with decreasing HC1 concentration ( = 0.11 in 12moll-1 HC1, but = 0.012 in 6 mol l-1 HC1 48 whereas in 50% aqueous propan-2-ol, is constant at >0.1 mol l-1 HQ 47). Both this dependence of quantum yield on acidity and some radical scavenging observed when phenol or anisole (chlorine atom traps) are present are in accord with the mechanism outlined in Scheme 8. Cu and Testa49 have found that the 313 nm irradiation of protonated 5-nitroquinoline...

A methoxy substituent on a benzene ring has an effect that is opposite that of a nitro substituent, so the benzyl anion is a model for the MOs of anisole. Excitation removes an electron from and places it in ij/s, which produces a charge difference suggested by the resonance representation shown in Figure 12.52. These simple models help us understand the photochemically induced solvolysis in which m-nitrophenyl trityl ether (103) does not react in the dark but does readily undergo photosolvolysis, as shown in Figure 12.53. ° ... 

The problems endemic to the thermal and photochemical Buchner reactions were solved comprehensively in 1980 when rhodium(II) catalysts were introduced. The measurement of improvement using Rh(II) catalysts can be appreciated by comparing the thermal reaction of ethyl diazoacetate with anisole (35% yield, seven products) with its rhodium trifluoroacetate-catalyzed counterpart (83% yield, three products 19-21). The methoxy substituent clearly exerts a directive effect in favor of the 4-methoxy isomer 19, and all the products are kinetically controlled unconjugated esters. In general, the rhodium(II)-catalyzed decomposition of alkyl diazoacetates in the presence of a large excess of aromatic substrates at room temperature affords kinetically controlled cycloheptatrienyl esters in excellent yield. 

Chloroacetamides of aminoalkyl derivatives of electron-rich aromatics 17, such as phenols, anisoles, or electron-rich heterocycles, easily dechlorinate and offer a useful entry to new medium-sized heterocycles (e.g., apogeissoschizine 18), according to Scheme 8.6a [6]. This is another example of the photochemical access to complex structures, that has been conveniently extended to macrocycle 19 (Scheme 8.6b), as well as to the reaction of W-alkylphthalimides [6]. 

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