Thienium cations

Carbon - heteroatom bonds can be cleaved by an appropriate combination of a hard acid and a soft nucleophile. Synthetically useful selective C-0 bond cleavage in the presence of other C-0 bond(s) is described. Reductive dehalogenation of a-haloketones is presented as an example that illustrates the concept of hard-soft affinity inversion. Finally, regio- and stereoselective functionalization of 1,3-di-enes is demonstrated by the thienium cation Diels-Alder cyclization involving the C-S bond cleavage. 

Carbon-Sulfur Bond Cleavage The Thienium Cation Diels-Alder Reaction... 

The carbon-carbon bond may be formed when a carbon nucleophile is used in the combination system. The most representative examples include the Friedel-Crafts-type alkylation of aromatics (Scheme XI, equation 1) (18) and the acid-catalyzed Diels-Alder reaction (Scheme XI, equation 2). The reaction of a combination system consisting of aluminum chloride and 1,3-dienes leading to regio- and stereoselective functionalization of 1,3-dienes via the thienium cation Diels-Alder reaction (19) (Scheme XI, equation 3) is described here. 

Though thioketones have been used as dienophiles in hetero Diels-Alder reactions (20), only one example has been reported for the use of a thienium cation as a dienophile (21). 

Addition of aluminium chloride to B-ethylthio nitroalkenes resulted in formation of thienium cations, e,g. (64) which underwent [4+2]-type cycloadditions to 1,3-dienes. Cleavage of the cycloadducts yielded 1,4-substituted olefins (Scheme 41). 

Thioketones reacting as dienophiles are well documented and follow the course of equation (93). Table 14 illustrates the variety of substrates and some typical yields for the reaction. Related to the simple Diels-Alder cyclizations are the [4 + 2+] cationic polar cycloadditions of equation (94) (72JA8932) and the [4+ + 2] version shown in equations (95) and (96) (81TL3773). The use of an alkyne in equation (94) to trap the thienium species leads to a benzothiopyran. It may be seen that choice of the enophile/dienophile combination leads to clean access to 5,6-dihydro- or 3,4-dihydro-thiopyrans with synthetically attractive substitution patterns. 

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