Thiinium salts

H-Dibenzo[b,e]thiin-9-carboxylic acids as fungicides, 3, 941 10H-Dibenzo[b,e]thiinium salts reactions... 

Pyrylium and thiinium salts are very easily attacked by nucleophiles, particularly if there is an unsubstituted a- or y-position, and 1,3,5-oxadiazinium and -thiadiazinium salts are still more susceptible (see Scheme 14). 

Amines also react with thiinium salts to give pyridinium salts, but reaction goes less easily than with the pyrylium analogues. 1,3-Oxazinium and 1,3-thiazinium cations react with ammonia and primary amines to give pyrimidines and pyrimidinium cations, respectively. 

Dithiins react readily at sulfur with peracids, alkyl halides and hydroxylamine 0-sulfonic acid to give sulfoxides, thiinium salts and sulfilimines, respectively. Similar reactions are known for... 

Thiabenzenes should be considered as sulfonium betaines. They react readily with acids to give mixtures of 2H- and 4/f-thiinium salts, behave as dienes with dienophiles (Equation 9), and can be oxidized to sulfoxides. The sulfimide (450) is an aza analogue of a thiabenzene and it is oxidized by KMn04 to the corresponding sulfoximide. 

H-(e.g. 477) and 4//-thiins on treatment with acid disproportionate into thiinium salts (478) and the corresponding tetrahydrothiin. 

Pyrans and thiins are also easily aromatized, e.g. (483) + S2Cl2 — 1-benzothiinium ion. 2H-Thiins are aromatized by hydride acceptors such as triphenylmethyl cations to give thiinium salts, and similar conversions produce pyrylium salts from pyrans. 

Nucleophilic addition at the 2-position of pyrylium salts (223) occurs readily under mild conditions and when ammonia or primary amines are used the subsequent ring-opening/ring-closure sequences give pyridines (224) and pyridinium salts (222), respectively (Section 3.2.1.6.4.iii). The process is most useful for the synthesis of 2,4,6-trisubstituted pyridine derivatives. Thiinium salts (226) are conveniently prepared from pyrylium salts (225) by treatment with sodium sulfide (Section 3.2.1.6.5), Thiinium salts (226) react with ammonia and amines similarly to their pyrylium analogues. 

Ring expansion of thiinium salts has been used for the preparation of various thiepins intermediate (234) is prepared using N2C(Li)C02Et, and decomposes via the carbene (235) to afford (236) (see CHEC 5.17.3.4.2.ii). 

Reactions of hydrazine and methylhydrazine with pyrylium or thiinium salts, e.g. (137), provide major routes to H-1,2-diazepines (138) and l//-l,2-diazepines (139) (76H(4)1509, 80CJC494). 

There are no known de novo preparations of unfunctionalized thiabenzenes, and in fact of the two methods claimed.only one has any real scope. The earliest approaches used the addition of an aryllithium species to a thiopyrylium salt in the expectation of achieving reaction at sulfur. There is evidence for the transient formation of some sulfur-arylated materials, but as addition at positions 2, 4 and 6 also takes place, interpretation of results is hazardous. With the recognition of the ylidic nature of thiabenzenes , the preferred route of synthesis becomes more obviously deprotonation of a thiinium salt (equation 117) (75JA2718). Non-nucleophilic bases may be used to avoid competing addition reactions. The lability of the products is ameliorated if electron-withdrawing substituents are present on the a- and y-carbons, which also permits the use of milder bases for the proton removal. This method has been used in the preparation of both the monocyclic materials and their benzannelated counterparts, which may in certain cases be isolated, e.g. 9-cyano-10-methyl-10-thiaanthracene (80JOC2468). As yet there are no reports of substituents other than aryl and alkyl having been introduced into simple thiabenzenes . 

The effect of benzo substitution on thiinium tetrafluoroborate is shown in Figure 17. In a study of thiinium salts with different counterions (BF/ BPI14, I-, TfO") and solvents (CD3CN, DMSO-r/6) it was evident that these changes have the least effect on the C(2) chemical shift . 

There has been relatively little detailed study of the mass spectrometry of pyrylium and thiinium salts, due partly no doubt to the involatility of the compounds elimination of CO or CS is the major fragmentation pathway. A -C Jxides generally show an abundant 37-16 ion that is sometimes the base peak and di-iV-oxides show successive elimination of two atoms of oxygen. The intensity of the M-16 ion is often very substantially reduced in compounds containing a... 

By disproportionation. Dihydro compounds often disproportionate (Scheme 73), i.e., form a mole equivalent of aromatic and a mole equivalent of tetrahydro derivative for example, the dihydrocinnoline 498 on treatment with hydrochloric acid gives 4-phenylcinnoline and 4-phenyl-l,2,3,4-tetrahydrocinnoline 2H- (e.g., 499) and 4/7-thiins on treatment with acid form thiinium salts and the corresponding tetrahydrothiin. 

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