Mesoionic ylide

Thiadiazolines were also obtained by a 1,5-dipolar ring reconstruction of mesoionic ylides. The bromine adduct of 2,3-diphenyltetrazolium-5-thiolate (181) reacts with the sodium salt of diethyl malonate to yield as the only product 5,5-bis(ethoxycarbonyl)-4-phenyl-2-phenylazo-2,3-dihydro-1,3,4-thiadiazole (182) (Scheme 33). The same type of product was obtained from reactions with ethyl cyanoacetate and ethyl acetoacetate <88BCJ2979>. 

The decomposition of suitably crafted diazoimides 181, in the presence of a transition metal catalyst, affords the metallo-carbenoids 182 that undergo intramolecular cycUzation onto the neighboring amide carbonyl oxygen to form the five-membered ring carbonyl yUdes (isomiinchnones) 183 (Scheme 58). Early examples of inter- and intramolecular 1,3-dipolar cycloaddition of the mesoionic ylides 183 have mainly emanated from the research groups of Ibata [149], Maier [150] and Padwa [151]. These reactive species (isomimchnones) can be trapped by various electron-rich and electron-deficient dipolarophiles [152] to furnish the cycloadducts in high yield. Much work has been reported in this area and for clarity of presentation is described here under various subheadings. 

Tetrazolium ylides are quite reactive and are easily alkylated.168 The mesoionic tetrazolium thiolate 117 readily adds bromine to yield 174 which can then react with a number of active methylene compounds to give mesoionic compounds, e.g., 175.293,294 They also undergo 1,3-dipolar cycloaddition with olefins and acetylenes to yield bicyclic tetrazolo-thiazolines... 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

The N NMR shifts for the tetrazolium ylides, 2,3-diphenyl-tetrazolium-5-olate and -thiolate are also shown in Table 4. In CF3CO2H the 5-thiolate compound showed shifts very close to those of the 5-methyltetrazolium iodide salt suggesting protonation at the exocyclic sulfur atom <92MI 417-03>. A multinuclear NMR study of mesoionic 1,3-dimethyltetrazoles including N and N spectra has been reported <94JCS(P2)1327>. 

Rhodium-mediated decomposition (60) of diazoamide (158) led to formation of the mesoionic oxazolium ylide 159, which was efficiently trapped by the pendant alkene to produce the oxo-bridged tricyclic amide 160. 

Some years later, the first stable thiocarbonyl ylides 9 and 10 were prepared by the reaction of thiourea with cyano-substituted oxiranes (19,20) or by addition of Rh-di(tosyl)carbenoid to benzo-l,2-dithiole-3-thione (21), respectively. Enhanced stability and the low reactivity of 9 and 10, which enables their isolation in crystalline form, results from the push-pull substitution at the two termini [cf. also (22)]. Another class of stable thiocarbonyl ylides that are also able to afford [3 + 2]-cycloaddition products are the mesoionic 1,3-dithiole-4-ones of type 11 (23,24). 

Mesoionic 1,3-thiazole -ones of type 34 are known as thioisomiinchnones. As one of the mesomeric structures demonstrates, these species contain the structural fragment characteristic of thiocarbonyl ylides (61). A convenient access to thioisomiinclinones involves the reaction of A(-arylthiobenzamides with a-bromo-phenylacetyl chloride (62). 

An attractive approach toward the preparation of polycyclic systems containing a thiophene ring involves the intramolecular [3 - - 2] cycloaddition of thiocarbonyl ylides. A number of representative examples were reported using mesoionic compounds. Gotthardt et al. (151) used l,3-dithiolium-4-olates such as 89 bearing an olefinic side chain. Upon heating to 120 °C in xylene, the polycyclic tetrahy-drothiophene 90 was formed (Scheme 5.33). 

Intramolecular [3- -2]-cycloadditions of thiocarbonyl ylides with nonactivated acetylenes have also been described. Most representative examples involved the use of mesoionic substrates. The initially formed polycyclic adducts of type 110 undergo spontaneous elimination of phenyl isocyanate (24,62,151). A typical example leading to compound 111 is shown in Scheme 5.40. 

Kato et al. (123) also found that isomtinchnone 51a reacts with tropone (251) to afford 252, which is apparently the hrst example of a [47i+6ti] cycloadduct involving both a mesoionic heterocycle and a carbonyl ylide (Scheme 10.34). The one-pot reaction of 253 gave 252 in somewhat higher yield. Whereas heating the latter in bromobenzene affords o-benzoylmandelanilide (69%), heating 252 in refluxing toluene in the presence of DMAD leads to furan 254. 

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