Ylide compounds 1.5- dipolar electrocyclization

Cyclization of a thiocarbonyl ylide with the C=C-bond of an aromatic ring was observed in the reaction of aryl biphenyl-2-yl ketones with di(tosyl)diazomethane in the presence of Rh2(OAc)4 (189). In the case where the aryl ring contains a 4-methoxy group, benzo[c]thiophene (164) was the only product formed. In contrast, when the aryl ring consists of a 2,4,6-trimethylphenyl group, compounds 165 and 166 were produced. It would seem that after 1,5-dipolar electrocyclization of the intermediate thiocarbonyl ylide occurs, aromatization then takes place by elimination of toluenesulfinic acid or methyl toluenesulfinate. 

As mentioned on pages 317 and 324, the 1,3-dipolar electrocyclization of thiocarbonyl ylides leads to thiirane derivatives, which represents an excellent method for the preparation of those three-membered rings. Typically, thiiranes are isolated as the final products, but in some instances they are produced as intermediate compounds which spontaneously desulfurize to give alkenes [twofold extrusion (47,48)]. 

The reaction of a thiocarbonyl and a-oxodiazo compound that leads to 1,3-oxathioles has been rationalized by a 1,5-dipolar electrocyclization reaction (178). It was suggested that an intermediate thiocarbonyl ylide bearing a C=0 function at the a-position (extended dipole) was first formed. Due to the low reactivity of a-oxodiazo compounds, these reactions were carried out at elevated temperatures or in the presence of rhodium acetate as the catalyst. In some cases, catalysis by LiC104 was also reported (77-80). 

The metal-catalyzed formation of 2,3-dihydrothiophene derivatives via a 1,5-dipolar electrocyclization has been reported by Hamaguchi et al. (124). For example, the Rh2(OAc)4-catalyzed reaction of vinyldiazo compound 159 (R = Ph) with xanthione (160) produced the spirocyclic dihydrothiophene 161. In contrast, when 159 containing a methyl group (R = Me) was used, thiirane 162 was the sole product (Scheme 5.48). This result was rationalized by the selective formation of an intermediate thiocarbonyl ylide 163 with (Z)- and (E)-configuration, respectively. 

The generation and reactivity of conjugated azomethine ylides has been reviewed.148 1,3-Dipolar cycloaddition and 1,5- and 1,7-electrocyclizations have been described as powerful strategies for the synthesis of monocyclic and annulated five- and seven-membered nitrogen heterocyclic compounds. 

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