Thiocarbonyl ylides electrocyclizations

Over the past two decades, important contributions to the chemistry of thiocarbonyl ylides were made by Huisgen et al. (27). By carrying out the reaction of thiobenzophenone with diazomethane at low temperature, formation of 2,5-dihydro-l,3,4-thiadiazole (15) with subsequent elimination of N2 was established as the route to the reactive thiobenzophenone (S)-methylide (16) (17,28). In the absence of intercepting reagents, 16 undergoes electrocyclization to give 17 or head-to-head dimerization to yield 1,4-dithiane 18 (Scheme 5.3). 

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

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. 

Thiocarbonyl ylides are both nucleophilic and basic compounds (40,41,86). For example, adamantanethione (5)-methylide (52) is able to deprotonate its precursor, the corresponding 2,5-dihydro-1,3,4-thiadiazole, and a 1 1 adduct is formed in a multistep reaction (40,86). Thioxonium ion (56) (Scheme 5.22) was proposed as a reactive intermediate. On the other hand, thiofenchone (S)-methylide (48) is not able to deprotonate its precursor but instead undergoes electrocyclization to give a mixture of diastereoisomeric thiiranes (41,87,88). The addition of a trace of acetic acid changes the reaction course remarkably, and instead of an electrocyclization product, the new isomer 51 was isolated (41,87) (Scheme 5.18). The formation of 51 is the result of a Wagner-Meerwein rearrangement of thioxonium ion 49. 

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. 

In equation 128 it is shown that thiocarbonyl ylide 117 may undergo a conrotatory electrocyclic reaction leading to thiirane 118. Thiirane is the smallest sulfur heterocycle and the Munich group has thoroughly studied not only the construction of this system, but also its destruction455, since the elimination of sulfur converts thiiranes into olefins 121 providing an important synthetic application for these molecules (equation 130). 

A 47r-electrocyclization route to (323) is shown in Scheme 17. The initial adduct (321) between (319 NR = morpholino, pyrrolidino, or piperidino) and a-bromoacetophenone (320) yields a transient carbonyl-thiocarbonyl ylide (322), which undergoes electrocyclization to the thiiran product (323). 

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. ... 

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