Mesoionic species

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]. 

Two types of nuclei with an oxygen and three nitrogen atoms are possible, namely 1,2,3,4-oxatriazoles (1) and 1,2,3,5-oxatriazoles (7). The neutral aromatic species have not yet been reported but 1,2,3,4-oxatriazolium salts (2) and mesoionic species (3)-(6) are known. 1,2,3,5-Oxatriazolium salts (8) and mesoionic compounds (9)-(12) are not yet known but 1,2,3,5-oxatriazolines (13) and (14) have been reported. The two types of oxatriazoles will be discussed in two separate sections. 

The mechanism of these rearrangements is not known but it may be similar to that observed for other mesoionic species and follow the course shown in Scheme 4 <79JCS(Pl)732>. However, intermediates other than (20) and (22) are conceivable. 

A variety of cyclic carbonyl ylides and related mesoionic species have been found to undergo intra molecular cycloaddition. 

Deuterium-labeling studies support the involvement of tricyclic sulfonium cations (152 and 153) in the photorearrangement of phenylthiazoles (154)13 3 the proposed general mechanism is shown in Scheme 12. Highly strained thiirans (155) have been similarly obtained on irradiation of the mesoionic 2-alkylthiothiazol-4-ones (156),134 and an analogous intermediate (157) has been proposed to account for the conversion of the mesoionic species (158)... 

Dehydration of (V-acylamino-acids generates azlactones these are in equilibrium with mesoionic species, which can be trapped by reaction with alkynes, final loss of carbon dioxide giving the aromatic pyrrole. 

The cycloaddition behavior of phosphaalkynes toward 1,3-dipoles is particularly pronounced. Thus, reactions with diazo compounds give rise to the 37/-l,2,4-diazaphospholes 12 or their l//-isomers, respectively [16,18]. Azides [18a, 19], nitrilium betaines [18a, 20] mesoionic species [21], and sextet dipoles such as selenoxocarbenes [22] react analogously to form heteroatom-substituted phospholes. 

Dipolar addition of DM AD to the mesoionic species (342) (Equation (183)) <88H(27)227> or to the quaternary salt (343) (Equation (184)) give a pyrrolo-thiazine and -pyridazine, respectively. Nucleophilic attack on salt (344) by active methylene anions gives pyrrolopyridazines (Equation (185)) <77YZ422>. 

Cycloaddition of 1050 with DMAD gave the tetrasubstituted bispyrrole 1058. In this case, loss of 2 mol HCIO4 from the bis-oxazolium salt was presumed to generate a mesoionic species (not shown) that underwent cycloaddition with DMAD, followed by a retro Diels-Alder reaction and loss of 2 mol of HNCO to yield 1058. 

Aminothioisomunchnones (104) are mesoionic species they cannot be represented by Lewis structures lacking charge separation. Two such dipoles (Ar = Ph, P-O2NC6H4) have been found to react with benzaldehydes to produce j8-lactams, apparently via 3 -I- 2-cycloaddition. 

The various fonns of betaines are very important for their charge control functions in diverse applications and include alkylbetaines, amidoalkylbetaines and heterocyclic betaines such as imidazolium betaines. Some surfactants can only be represented as resonance fonns having fonnal charge separation, although the actual atoms bearing the fonnal charge are not ftmctionally ionizable. Such species are mesoionic and an example of a trizaolium thiolate is illustrated in table C2.3.3. 

The third compound of this protomeric equilibrium corresponds to the mesoionic 4-hydroxy thiazo e. Its existence has been suggested recently from reactivity experiments (416). When R in 174 is also a protomeriza-ble group, other stable protomeric species have been observed (Scheme 91) (417. 418). They are out of the scope of this review. 

The presentation of the rearranging intermediates as mesoionic ground-state species (196) and (199) has proved an adequate tool for the prediction of structural changes in this field. The steric factors which direct the partly selective rearrangement paths of the bicyclohexenone and dienone photoisomers of (174) and their corresponding alkylated homologs have been extensively discussed in a recent review by this author. ... 

Tire and NMR parameters of some 1-alkyl-4-benzimidazolyl-2-idene- (type 72) and l-alkyl-4-(5-methylpyrazolyl-3-idene)-l,4-dihydro pyridines (type 73) were discussed in 89CC1086 and 91JOC4223. Comparison of the shifts for DMSO-dg and CDCI3 solutions with data reported for quaternary pyridinium compounds as well as anionic species in the azole series and data obtained for mesoionic betaines of the azinium azolate class left no doubt that these heterofulvalenes have a betaine character and, therefore, the NMR signals correspond to their dipolar resonance form. 

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). 

Two types of mesoionic 1,3-oxathiolium species have been reported. The oxathiolium-5-olate (45) is stable and isolable (75CC417), and the presence of the trifluoroacetyl group... 

In view of the extreme instability of these mesoionic structures it appears unlikely that the corresponding 1,3-dioxoIium species will be isolable. 

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