Vinylthionium ions

Conjugate addition to vinylthionium ions is observed in the reaction of allylic sulfoxide (120) with enol silyl ethers in the presence of trimethylsilyl triflate and diisopropylethylamine (Scheme 27). However, in certain cases moderate yields were obtained. This has been traced to a competing reaction of the intermediate (121) with the tertiary amine base to give the quaternary ammonium salt (122). 

P,P] The addition of (triphenyl)crotylstannane to vinylthionium ion 33.1 (Scheme 33) has been reported (70).f Only low levels of selectivity were observed. 

P,P] Vinylthionium ions such as 40.1 are potent Michael acceptors. Additions of silyl and stannyl enol ethers to vinylthionium ions such as 40.1 have been reported by Mukaiyama and co-workers (84-86). Two methods were employed for generation of the Michael acceptors elimination of ethyl sulfide from 40.2 under the action of a cationic trityl species (method A) and hydride abstraction from 40.3 and 40.4 by a trityl salt (method B) to give 40.1. Although both procedures use a strong Lewis acid to promote reaction, they differ substantially from the prior examples because an acceptor-acid... 

Vinylthionium ions are at the same oxidation state as a,/ -unsaturated esters. They are much more reactive than the corresponding enoates, however, reacting readily with silyl and stannyl enol ethers. 

Addition of Enol Silanes and Stannanes to Vinylthionium Ions (Scheme 40)... 

P,P] Only one example of the addition of a silyl ketene acetal to a vinylthionium ion has been reported (84). The addition of ketene acetal 45.1 to vinylthionium ion 45.2 provides a 94 6 (anti/syn) mixture of diastereomers in good yield. 

For the addition of enol silanes and silyi ketene acetals to vinylthionium ions see Hashimoto, Y. Mukaiyama, T. Chem. Lett. 1986, 1623-1626. 

The diastereoselective [4 + 3] cycloaddition between 2,5-disubstituted furans and vinylthionium ions was achieved (140L4476). The vinylthio-nium ion was generated either from a ort/io-phenylethynylbenzoyl allylic ester in the presence of a gold catalyst or from the allylic alcohol with a Bronsted acid. However, using 2-substituted furans led predominantly to the formation of Friedel-Crafts alkylated products. 

The quaternary allylammonium salt, formed by the nucleophihc addition of Hiinig s base to the Pununerer vinylthionium ion intermediate, undergoes Pd-catalyzed reaction with dimethyl sodiomalonate highly regioselectively (Scheme 6)... 

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