Heterocyclic cationic

Two general types of heterocyclic cations may be recognized those in which the charge results from an increase in the bonding number of an atom by the addition of a proton (or equivalent), and those in which the charge results from a decrease in the bonding number by removal of hydride. These two possibilities are shown for pyrrole in (167) and (168). 

Partially hydrogenated heterocyclic cations may be named in a manner analogous to that used for the corresponding radicals, as in (173) and (174). 

Heterocyclic cations in which the bonding number is increased by a process other than the addition of a proton (or equivalent) present a special situation. Structures of this type usually result from the participation of an originally nonbonding electron pair or a heteroatom in the formation of a ring or a multiple bond. 

The [S2N]+ cation is an important reagent in S-N chemistry,63 especially in thermally allowed cycloaddition reactions with organic nitriles and alkynes, which give quantitative yields of heterocyclic cations (Scheme 3). It is conveniently prepared by reaction of S3N2C12 with AsF5 and S8 in liquid S02.63b The [SNS]+ cation is linear with S-N bond distances in the range indicating a bond order of two, i.e., S=N+=S. 

Transition state pKn values may also be estimated for reactions which involve nucleophilic attack by water and by hydroxide ion (Kurz, 1963, 1972). Such may be the case in the formation of pseudobases from quaternary heterocyclic cations (32a,b), a number of which have rates of... 

Apparently, the aromatization of the heterocyclic cation serves as a driving force of the Cope rearrangement in the transformation of the 3-formyl-4-allyl-4//-pyrane (481) into poly-substituted pyrylium salt 483 which presumably proceeds via 482 (equation 183)242. 

Among the 1,3-oxazinium derivatives, the 3-azapyrylium salts are the most interesting [they are also called 1 -1,3-oxazinium (72S333), 1,3-oxazin-1-ium (83MI1), 1,3-oxazinium (78AHC1 84MI1), and 1,3-oxazinylium salts (81BCJ2387)]. The absence of a universally accepted name for these compounds indicates the novelty of this field of heterocyclic cation chemistry. 

However, it is most surprising that alkyl-substituted 3-azapyrylium salts can undergo formylation and acylation at the side chain. These reactions are examples of the interaction of heterocyclic cations with cationoid electrophiles. Some of these processes are presented in Scheme 12 (90KGS134 91KGS265 94TH1). It is assumed that such transformations are possible due to the intramolecular deprotonation of the methyl group in 79 to form charged anhydro base 80 (see structure 39). The 4-acylmethyl-3-... 

Table 5 13C Chemical Shifts (S, p.p.m. from TMS) and One-bond l3C-1H Coupling Constants (Hz) of some Simple Heterocyclic Cations (cf. pyridine, column 1)...

The reactions of heterocyclic cationic substrates such as quinolinium ions have been treated in an authoritative article by Bunting.21 In the present chapter we shall be concerned with the formation of ff-adducts as obtained from neutral heteroaromatic substrates and nucleophilic reagents of all types. This matter has never been reviewed before but is included in part in a comprehensive review article by Terrier.22... 

Heterocyclic Cation Radicals A. S. Morkovnik and O. Yu. Okhlobystin, Khim. Geterotsikl. Soedin., 1980, 1011-1029. 

Equilibrium Constants for Heterocyclic Cation-Pseudobase Equilibration J. W. Bunting, Heterocycles, 1980, 14, 2015-2045. 

Predictably, the five-membered heterocyclic rings are readily reduced by metal-acid systems via the heterocyclic cations (B-77MI30507, 72HC(25-l)i, 8lAHC(29)34l). Indoles are reduced, for example, by zinc dust and phosphoric acid or by tin and hydrochloric acid, to give indolines. Under these conditions 1,2,3,4-tetrahydrocarbazole is reduced to the c/s-fused l,2,3,4,4a,9a-hexahydrocarbazole whilst the corresponding reduction of 2,3-dimethylindole yields a mixture of the cis- and trans- 2,3-dimethylindolines (72HC(25-i)l). 

IUPAC Rules B-6 and B-10 give examples of heterocyclic cations named by replacement principles. The normal replacement prefixes... 

Among the numerous classes of heterocyclic cations that are now actively studied, pyrylium salts are distinguished by the extraordinary variety of their transformations, making them good synthons and widely applicable. An exhaustive review on this subject has been published by an international group of chemists dealing with pyrylium chemistry [82AHC(Suppl.)]. 

The reaction is known also for the oxygen, sulfur, and selenium analogs of compound 54 (R = t-Bu). Interestingly, the yields of hexafluorophos-phates of the heterocyclic cations gradually decrease in this reaction in the order Te (85%) > Se (64%) > S (59%) > O (22%). 

In this chapter the dithiadiazole family is reported for the very first time. Since these fascinating five-membered heterocyclic cations, radicals, and dimers were not covered in CHEC-I(1984) and CHEC-II(1996), the discussion encompasses all available literature rather than focusing solely on publications post-1995. Before any further discussion, an appreciation of the two isomeric forms and the nomenclature associated with each is essential. The 671 cyclic systems of 1,2,3,5-dithiadiazolium chloride 1 and 1,3,2,4-dithiadiazolium chloride 2 are shown below. As this chapter demonstrates, the 1,2,3,5-isomeric form dominates the literature in this field, reflecting the wider synthetic opportunity and stability associated with it. 

PlCN may be used to introduce this group by nucleophilic substitution. This reaction has been used e.g. to prepare the di-cyanophosphino derivatives of heterocyclic cations, which in turn may be reduced by phosphite again. The resulting compounds are benzimidazolones and quinolones in which the PCN group as a novel pseudochalcogen takes the place of oxygen. 

From 2-thiocyanobenzoic Adds. For the synthesis of 2-chlorobenzo-TA 83, a method based on the cyclization of chloroanhydrides 80 was developed. The reaction may include the formation of heterocyclic cations 81 or 82, whose deprotonation results in 2-chloro products 83 (66AGE663 70CB413) (Scheme 25). Following this development, the one-pot syntheses of 2-chlorobenzo-TA, without isolation of chloroanhydrides 80, were elaborated (73S189 89MI1). 

---