Pyrylium thiopyrylium

S, or NR. In the transition state to C=C rotation, pyrylium, thiopyrylium, or pyridinium ions are formed, which should cause a considerable lowering of the C=C barriers compared to those in systems lacking this delocalization possibility. This is clearly demonstrated by the fulvenes 64 and (5 (57). The barrier... 

J Pyrylium Thiopyrylium Selenopyrylium Pyridine Pyridinium Pyridine- A-oxide... 

The heterocyclic alkyl quaternary salts used for preparing spiropyrans are usually obtained by N-alkylating the heterocyclic bases by standard methods. The common bases having an active methyl group of the indolenine, benzothiazole, benzoxazole, and quinoline series some of their substitution products, and a few of their quaternary salts are commercially available. The pyrylium, thiopyrylium, 2-azaazulenium and bcnz[c,d]indolenium salts needed for potentially infrared-absorbing spiropyrans are not (as of the time of writing). 

Pseudobase formation by the pyrylium, thiopyrylium, and selenopyrylium cations is complicated by ring-opening reactions (Section V,A) which preclude a simple direct measurement of pKR + for these cations. In a kinetic study, Williams80 has found the ring opening of 2,4,6-trisubstituted pyrylium cations 23 to be controlled by pKa values in the range of 5.0-6.7 for R2,... 

UV Spectral Data of Pyrylium, Thiopyrylium, and Selenopyrylium, Perchlorates in Acetonitrile" ... 

The chemical shift of the methyl group in methyl-substituted pyrylium, thiopyrylium, and selenopyrylium cations is reported in Table V. Whereas for pyrylium and thiopyrylium the order of chemical shift of methyl group is a > y > /3, in the case of selenopyrylium the order is a > 13 > y. The presence of additional methyl groups causes only small... 

C Chemical Shifts" (ppm). C, H Coupling Constants (Hz), One- and Three-Bond C, C Coupling Constants (Hz) of Pyrylium, Thiopyrylium, and Selenopyrylium Fluoroborates in CDiCN ... 

The attack of the acid (154) on the readily polarizable 1,2-dithiafulvene (155) corresponds to the extremely ready addition of electrophilic reagents to the simple and vinylogous heptafulvene derivatives, which are iso-n-electronic with 155. The opening of the dithiole rings in 156 and 158 under the pressure of the carbanionoid electron pair liberated by the proton abstraction and of the free electron pair on the sulfur, as well as the elimination of elementary sulfur and the intramolecular electrophilic attack of the mercaptide ion (157) on the 5-position to form 158, are simply the typical reactions of 1,2-dithioles that have already been discussed (Section II, B, 3). The reactivity of the 3-methyl group in 154 finds many parallels in the ease of condensation of the methyl-substituted pyridinium, pyrylium, thiopyrylium, and tropylium salts, and particularly... 

Triphenylpyrylium tetrafluoroborate is a versatile and useful stable starting material. Its reaction with nitromethane under basic conditions has made 2,4,6-triphenylnitrobenzene easily available. In addition, pyrylium salts are readily converted to a variety of pyridine derivatives i i . 20 including alkyl- and arylpyridinium salts, to thiopyrylium salts," and to substituted azulenes. ... 

In contrast to the ease of reaction of ring nitrogen atoms in 77-deficient six-membered heterocycles with electrophiles, electrophilic heteroaromatic substitution at carbon of the unsubstituted compounds proceeds only under very drastic conditions and yields of products are usually very poor. This is also true with pyridinium, pyrylium and thiopyrylium salts,... 

Nucleophiles react particularly easily with quaternized azines and with pyrylium and thiopyrylium slats (cf. equation 23) typical examples, including the well-known reaction of pyridinium salts with hydroxide in the presence of potassium ferricyanide to give 2-pyridones, are summarized in equations (34)-(36) (note the rather unusual orientation of addition in the last case reaction normally occurs essentially exclusively a to the heteroatom if the position is free or occupied by a leaving group). 

The oxygen and sulfur heterocycles, including pyrylium and thiopyrylium salts, are readily reduced catalytically. Moreover, it is usually possible to control the degree of reduction both pyran-2- and -4-ones can readily be reduced to dihydro derivatives, in the former case the 5,6-dihydro compound. Examples of catalytic reduction of the oxo substituents to either the corresponding alcohol or to the methylene group are known, but these reactions normally occur only under pressure. 

Disproportionation has been observed frequently with thiopyrans and rarely with 4//-pyrans, and all cases involve a tetragonal carbon center (position 2 or 4) bearing at least one C—H bond. Some molecules of the substrate are aromatized to corresponding thiopyrylium or pyrylium ions and others reduced to dihydro or tetrahydro products. The relative abilities of pyrans and thiopyrans to disproportionate were interpreted within a proposed hydride transfer mechanism by a CNDO/2 method.45... 

Pyrylium salts, especially the 2,4,6-triaryl substituted, react with a wide variety of nucleophiles some of which lead to the formation of rings other than benzene or pyridine. Sodium sulphide, for instance, attacks the pyrylium ring at C-2 to produce the blue-coloured dienone (78) which then cyclizes on treatment with add to the thiopyrylium salt (79) (56HCA207). 

---