Arene Additions

Arene Additions.—Evidence has been provided for the production of cyclobutadiene in the photolysis of (330) in solution the products isolated are dimethyl phthalate and syn-tricyclo[4,2,0,0 ]octa-3,7-diene. Thus, irradiation of(330) in the presence of either cis- or trans-penta-l,3-diene gave the above products and the Diels-Alder 

It has been found that enc(o-dicyclopentadiene, of all the systems studied, is the best acceptor of energy transfer from the T2 state of naphthalene. The photochemical cycloaddition reaction of octafluoronaphthalene with aliphatic 1,3-dienes can be explained in terms of exciplex formation in non-polar solvents and solvated 

In this respect, 1,4-naphthoquinodimethane is much more stable than 1,4-benzo-quinodimethane. Excimer fluorescence and photodimerization of anthracenophanes and 1,2-dianthrylethanes have also been reported.  

In benzene solution 9,10-dicyanoanthracene and methyl 1,2-diphenylcyclopropene-3-carboxylate form an emitting exciplex leading to the formation of the cycloadduct (341). In polar solvents the epimeric endo- dd ict (342) and the olefin dimer were formed by an electron-transfer reaction. The formation of a staggered sandwich 

Lapouyade, H. Bouas-Laurent, and B. Clin, Tetrahedron Letters, 1976, 2277. 

Arene Additions.—Irradiation of perfluoro(pentaethylmethylbenzene) affords the corresponding prismane which is converted by heat into the isomeric Dewarbenz-enes. Discrepancies in previous reports on the products of photo-addition of furan to benzene have been clarified product ratios are sensitive to the experimental conditions. Mechanistic studies, including the use of added dienophile, have 

Shigemitsu, S. Yamamoto, T. Miyamoto, and Y. Odaira, Tetrahedron Letters, 1975, 2819. 

Bryce-Smith, R. R. Deshpande, and A. Gilbert, Tetrahedron Letters, 1975, 1627. 

The photocycloaddition of cis-1,2-dihydrophthalic anhydride to benzene, naphthalene, or anthracene affords in each case products arising from ( 4j + 4J reaction, a type of addition unknown for cyclohexa-1,3-diene itself. Energy transfer from benzene to the diene anhydride still provided the main reaction pathway, namely electrocyclic closure to give the cis-dihydroDewarbenzene anhydride. The naphthalene photo-adduct (393) on xanthone-photosensitized irradition gave the cage compound (394) similar reaction also occurs with the benzene photo-adduct. 

Conformationally rigid cis-2,3-dihydroindan-l-ones have been prepared and intramolecular energy transfer in these systems has been studied. For (395), for example, irradiation in the Lj, band and the n-n transition showed neither fluores-ence from naphthalene nor phosphoresence from indanone, but only phosphoresence from naphthalene. Energy-transfer and energy-wasting processes are discussed in relation to the configurations of the molecules. 

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Electron-donating substituents direct the incoming nucleophile predominantly to the meta-position and electron-withdrawing substituents to the ortho-position. Oxidative demetallation (DDQ, iodine) is applied to reoxidize the cyclohexadienyl ligand, releasing a substituted arene. Addition of nucleophiles to halobenzene-FeCp complexes leads to nucleophilic substitution of the halo substituent (Scheme 1.34). Demetallation of the product complexes is achieved by irradiation with sunlight or UV light in acetone or acetonitrile. 

Michael additions to conjugated carbonyls can be catalyzed by gold species. Among them, arene additions are the most studied area but other nucleophiles can attack the gold-coordinated enones as well. In fact, the intermolecular aza-Michael additions of carbamates to enones was reported in 2002 with both Au(I) and Au(III) salts, and in 2007 an intramolecular aUcoxide and amide conjugate addition has been developed and applied to the synthesis of (+)-andrachcinidine (equation 132). In the latter case, the enones are formed as intermediates in a previous gold-catalyzed step that is the hydration of an alkyne and methanol loss. Then the cyclization takes place to give piperidines. 

Using TAfS° (68) rs 14 kJ mol, A,G (68) rs —49 kJ mol is finally obtained, that is, the energetics of reactions (66) and (68) are comparable. A possible reason for the different reactivity of arenes and alkanes is that an arene may have a kinetic advantage over an alkane by forming a strong T/ -bond with the metal. The electron backdonation from the metal to the antibonding tt orbitals of the arene weakens the C(sp )-H bond and favors the formation of the aryl hydride. While this kinetic explanation may account, by itself, for the preference of arene addition, it is observed in Table 1 that for late transition metal complexes the differences DH° (M-Ph) — DH°(M-Me) are substantially higher than Z)//°(Ph-H) — Z)/7 (Me-H). This trend will, of course, imply that benzene activation is thermodynamically favorable, relative to methane activation. 

In work recently reported [20] it was found that reduction of aryldiazonium tetra-fluoroborates in nonpolar solution by aqueous hypophosphorous acid and catalytic cuprous oxide, good to excellent yields of arenes were obtained under very mild conditions. The single exception in the group of salts studied was 4-methoxybenzene-diazonium tetrafluoroborate which yielded only 67% of arene. Addition of 7.5 mole-9 of 18-crown-6 increased the yield to 88%. The effect of crown in this system is not understood but believed to involve crown enhanced solubility of the salt (see Eq. 15.6) [20]. 

Chemler et al. reported a copper-catalyzed doubly intramolecular alkene car-boetherification of unactivated alkenols to form bridged-ring tetrahydrofurans using the 2,2 -(propane-2,2-diyl)bis(4,5-dihydrooxazole) as hgand and MnOj as oxidant (Scheme 8.76). The formation of C-C bond is thought to occur via carbon radical arene addition, and this step constitutes an efficient C-H functionahzation [146]. 

Tsuchimoto and Shirakawa et al. again used the In(OTf)3 catalyst to develop the first double addition of heterocyclic arenes to alkynes [101]. The addition occurred geminally and unlike the previous arene addition to alkynes studied by Shirakawa et al., aliphatic alkynes were observed to react. The authors thus proposed that this reaction occurs via an In(OTf)3-alkyne complex, rather than through an alkenyl carbocation intermediate (Figure 8.50). 

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