Arenes addition

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. 

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... 

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... 

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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