2- Arylvinyl ketones

Metal compounds possessing Lewis acid character are often used in the Michael addition reaction, and the methodology is reasonably applied to the asymmetric reaction in the presence of chiral ligands. The mechanism could involve either purely Lewis acidic activation of the Michael acceptor or generation of new orga-nometallic species by the transmetalation or C-H activation, although they were not clear in many cases. The system of Sn(OTf)2 and chiral (S)-diamine developed by Mukaiyama promoted the asymmetric addition of trimethylsilyl enethi-olate 68 to P-arylvinyl ketones (Scheme 13) [70, 71]. The diamine-coordinated tin enolate was considered to be involved, and slow addition of 68 was essential to inhibit the racemate formation process. 

Treatment of a-(benzotriazol-l-yl)alkyl thioethers 831 with ZnBr2 weakens the bond with benzotriazole, and the obtained complex 832 may partially dissociate to thionium cation 835 that can be trapped by even mild nucleophiles. Thus, trimethylsilyl cyanide added to the reaction mixture causes substitution of the benzotriazole moiety by the CN group to give a-(phenylthio)carbonitrile 834. In a similar manner, treatment with allylsilane leads to y,S-unsaturated thioether 833. Addition of species 835 to the double bond of a trimethylsilyl ot-arylvinyl ether followed by hydrolysis of the silyloxy group furnishes (i-(phenylthio)alkyl aryl ketones 836 (Scheme 132) <1996TL6631>. 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

A simpler large scale method to obtain pyrantel, morantel (10a,b) and oxantel (11) involves condensation of 23 with an aryl aldehyde in presence of a base. Water is removed by azeotropic distillation or by using a chemical scavenger like methyl/ethyl formate, which reacts with water to push the reaction in the forward direction (Scheme 3). Other methods to prepare pyrantel and its derivatives are also reported [5,6,13-17]. The l-(2-arylvinyl)pyridium salts (18,19), structural congeners of pyrantel/moratel, are prepared by quaternisation of pyridine with the appropriate bromomethylaryl ketones (27) to afford 28. The latter is reduced with sodium boro-hydride to give the carbinol 29, which on dehydration leads to the formation of l-(2-arylvinyl)pyridinium bromides (18,19) [8]. 

Tandem coupling and cyclization. Functionalized allenes are converted to a-arylvinylated cyclopropanes and heterocycles including epoxides, tetrahydrofurans, and oxazolidinones. Furan derivatives are formed from allenyl ketones." ... 

The [(Ti-allyl)PdCl]2-catalyzed reactions of allene-aldehydes or -ketones 153 with Me3SiSnBu3 led to the cyclic 2-vinylic alcohols 155 via the formation of an aldehyde- or ketone-containing 2-trimethysilyl-2-alkenyl tin intermediate 154 (Scheme 63) [38]. In the presence of an aryl iodide, Pd2(dba)3 could catalyze the coupling cyclization of allene-aldehydes or -ketones 153 with Bu3SnSnBu3 leading to the cyclic 2-(r-arylvinyl) cyclic alcohols 156 stereoselectively with the cis isomer being the major product (Scheme 64) [39]. 

In 1991, a new C-N-C bond formation reaction using a nitrogenation-transmetalation process was described. Ketones and aryl or vinyl halides couple to give divinyl or arylvinyl amines in the presence of the titanium isocyanate complex [3THF MgaClaO TiNCO] and a palladium catalyst, via transmetallation of the titano imine complex with aryl or vinyl palladium bromide. Moderate to good yields of the desired products were observed (Scheme 2.176). 

Finally, and as pointed out in Scheme 8.101 (Chapter 8), there is an aromatic version of the Claisen rearrangement. As the ketone intermediate is conjugated (prior to its tautomerization to the corresponding phenol), the process may be considered an addition to an unsaturated carbonyl compound. Equation 9.35 represents the aromatic Claisen rearrangement, which, as shown from left to right, allows the conversion of an arylvinyl ether into a substituted phenol. 

Reactions of various activated alkenes with Mg/TMSCl in DMF system have been explored as well. When treated with this reduction mixture, a-substituted arylvinyl sulfones affords ( )-/3-substituted st)renes in high stereoselectivity and reasonable yields, through the desulfonation reaction (eq 83). Under the reaction conditions, Q ,/3-unsaturated ketones undergo facile and regioselective reductive dimerization to afford the corresponding bis(silyl enol ethers), that is, l,6-bis(trimethylsilyloxy)-1,5-dienes (eq 84) In contrast, Q ,/3-unsaturated esters or a,/3-epoxy... 

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