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Synthesis of ferrocenyl-substituted 1,3-dithiolanes via [3 + 2]-cycloadditions of ferrocenyl hetaryl thioketones with thiocarbonyl S -methanides

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ABSTRACT

Ferrocenyl hetaryl thioketones react smoothly with in situ generated thiocarbonyl S-methanides to give 1,3-dithiolanes. In the case of aromatic S-methanides, the sterically more crowded 4,4,5,5-tetrasubstituted 1,3-dithiolanes (2-CH2 isomers) were formed as sole products. The reactions with cycloaliphatic S-methanides led to mixtures of 2-CH2 and 5-CH2 isomers with the major component being the sterically more crowded 2-CH2 isomers. The preferred formation of the latter products is explained by the assumption that the formal [3 + 2]-cycloadducts were formed via a stepwise reaction mechanism with a stabilized 1,5-diradical as a key intermediate. The complete change of the reaction mechanism toward the concerted [3 + 2]-cycloaddition was observed in the reaction of a sterically crowded cycloaliphatic thiocarbonyl ylide with ferrocenyl methyl thioketone.

No MeSH data available.


Mesomeric stabilization of the 1,5-diradical intermediate in the reaction of aromatic thiocarbonyl S-methanides with ferrocenyl hetaryl thioketones.
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C4: Mesomeric stabilization of the 1,5-diradical intermediate in the reaction of aromatic thiocarbonyl S-methanides with ferrocenyl hetaryl thioketones.

Mentions: Based on our earlier interpretation of the reaction mechanism leading to 1,3-dithiolanes via [3 + 2]-cycloaddition of thiocarbonyl S-methanides with aryl and hetaryl thioketones, we propose that the reactions with ferrocenyl thioketones 1 occur predominantly via an intermediate 1,5-diradical. The formation of the sterically more crowded 1,3-dithiolanes 5a–g confirms that the stabilized 1,5-diradicals of type 7 (Scheme 4) are key-intermediates in the reaction. On the other hand, the formation of 1,3-dithiolanes of type 5 in reactions with cycloaliphatic thiocarbonyl S-methanides competes with the concerted [3 + 2]-cycloaddition leading to the sterically less crowded 1,3-dithiolanes 6 (Scheme 2). These results demonstrate that the ferrocenyl moiety shows a comparable effect to that found for aryl and hetaryl groups. This interpretation of the reaction mechanism got additional support from the reaction of 1e with 3c. Apparently, the replacement of a radical-stabilizing aryl or hetaryl group by a methyl substituent results in the preferred concerted [3 + 2]-cycloaddition. It is well known that this mechanism, controlled by frontier-orbital (FMO) interactions, is strongly influenced by steric effects in the case of C=S dipolarophiles [8,17].


Synthesis of ferrocenyl-substituted 1,3-dithiolanes via [3 + 2]-cycloadditions of ferrocenyl hetaryl thioketones with thiocarbonyl S -methanides
Mesomeric stabilization of the 1,5-diradical intermediate in the reaction of aromatic thiocarbonyl S-methanides with ferrocenyl hetaryl thioketones.
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Related In: Results  -  Collection

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C4: Mesomeric stabilization of the 1,5-diradical intermediate in the reaction of aromatic thiocarbonyl S-methanides with ferrocenyl hetaryl thioketones.
Mentions: Based on our earlier interpretation of the reaction mechanism leading to 1,3-dithiolanes via [3 + 2]-cycloaddition of thiocarbonyl S-methanides with aryl and hetaryl thioketones, we propose that the reactions with ferrocenyl thioketones 1 occur predominantly via an intermediate 1,5-diradical. The formation of the sterically more crowded 1,3-dithiolanes 5a–g confirms that the stabilized 1,5-diradicals of type 7 (Scheme 4) are key-intermediates in the reaction. On the other hand, the formation of 1,3-dithiolanes of type 5 in reactions with cycloaliphatic thiocarbonyl S-methanides competes with the concerted [3 + 2]-cycloaddition leading to the sterically less crowded 1,3-dithiolanes 6 (Scheme 2). These results demonstrate that the ferrocenyl moiety shows a comparable effect to that found for aryl and hetaryl groups. This interpretation of the reaction mechanism got additional support from the reaction of 1e with 3c. Apparently, the replacement of a radical-stabilizing aryl or hetaryl group by a methyl substituent results in the preferred concerted [3 + 2]-cycloaddition. It is well known that this mechanism, controlled by frontier-orbital (FMO) interactions, is strongly influenced by steric effects in the case of C=S dipolarophiles [8,17].

View Article: PubMed Central - HTML - PubMed

ABSTRACT

Ferrocenyl hetaryl thioketones react smoothly with in situ generated thiocarbonyl S-methanides to give 1,3-dithiolanes. In the case of aromatic S-methanides, the sterically more crowded 4,4,5,5-tetrasubstituted 1,3-dithiolanes (2-CH2 isomers) were formed as sole products. The reactions with cycloaliphatic S-methanides led to mixtures of 2-CH2 and 5-CH2 isomers with the major component being the sterically more crowded 2-CH2 isomers. The preferred formation of the latter products is explained by the assumption that the formal [3 + 2]-cycloadducts were formed via a stepwise reaction mechanism with a stabilized 1,5-diradical as a key intermediate. The complete change of the reaction mechanism toward the concerted [3 + 2]-cycloaddition was observed in the reaction of a sterically crowded cycloaliphatic thiocarbonyl ylide with ferrocenyl methyl thioketone.

No MeSH data available.