1,5-Dipolar electrocyclization, thiocarbonyl

Thiocarbonyl ylides without push-pull stabilization, in the absence of intercepting reagents, undergo 1,3-dipolar electrocyclization to give thiiranes. In accordance... 

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 yhde 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 spirocychc drhydrothiophene 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 yhde 163 with (Z)- and ( )-configuration, respectively. 

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)]. 

Geometrical trans to cis isomerization of 3,3 -, 4,4 -, and 5,5 -disubstituted 2,2 -diphenoquinones has been studied by computational methods.The double bond isomerization of butene-catalysed l-ethyl-3-methyl-imidazolium chloride ionic liquid has been similarly examined and stepwise isomerization is suggested.The reaction of l,2-di(l-adamantyl)-2-thioxoethanone with diazomethane and 2-diazopropane gave 2-acylthiiranes via 2 - - 3-cycloaddition, elimination of nitrogen, and 1,3-dipolar electrocyclization of the intermediate acyl-substituted thiocarbonyl ylides. Rearrangement of pyrimidine-5-carboxylic acids esters to 5-acylpyrimidones does not occur in the examples studied and a [l,4]-phospho-Fries rearrangement has been reported. ... 

Reactions with Carbenes and Carbenoids. Photochemical decomposition of diazomethane in the presence of 1 in an argon matrix at 10 K yields thiocarbonyl 5-methylide (8) via addition of methylene to the sulfur atom. While increasing the temperature, 8 undergoes a 1,3-dipolar electrocyclization to give thiirane 9 (eq 5). 

The decomposition of dimethyl diazomalonate with Rh2(OAc)4 in toluene at 50 °C in the presence of 1 leads to two products 15 and 16, which are formed via an intermediate thiocarbonyl yUde (eq 8). Whereas 1,3-dipolar electrocyclization affords a thiirane which spontaneously extrudes sulfur to give 15, the competitive 1,5-dipolar electrocyclization yields 1,3-oxathiole 16. ... 

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