Thermal decomposition, heterocycles

Compounds of type (6), (7), (8) and (9), although not strictly derivatives of a saturated heterocyclic system, will be discussed in this chapter. Our discussion of (7) begins and ends here, since oxiranethiones or a-thiolactones are apparently unknown (80AG(E)276). Little is known of (8) and its derivatives, oxiranimines or a-iminolactams. They have been postulated as intermediates in the thermal decomposition of aziridinones (a-lactams) (Scheme 1) but there is no well-established case of the isolation of an oxiranimine (80AG(E)276). 

Phenyl radicals can be generated by the thermal decomposition of lead tctrabcnzoate, phenyl iodosobenzoate, and diphenyliodonium hydroxide,- - and by the electrolysis of benzoic acid.- These methods have been employed in the arylation of aromatic compounds, including heterocycles. A method of promise which has not been applied to the arylation of heterocycles is the formation of aryl radicals by the photolysis of aromatic iodides at 2537... 

Goldschmidt and Beer have examined the products formed during the thermal decomposition of diacyl peroxides of the type [COgMe —(CHziw—CHz—COO] 2, where n = 1 and 3, in the presence of a series of organic compounds including pyridine and acridine. The products and yields of the reaction with some aromatic and heterocyclic compounds are shown in Table VI. As expected, acridine and... 

Dihydro-2/7- 74 and -4//-l,2-oxazines and thiazines 75 are interrelated by prototropy, being enamines and imines, respectively. In the case of oxazines, the imine form 75 is favored, and there are several well established examples of this system, including the parent heterocycle 75 (X = O) [84MI2]. No tautomeric equilibrium between the 2H and 4H forms has been observed under normal conditions in solution or in the solid state. However, the formation of intermediate 2H isomers 77 was proposed both for the conversion of 3-phenyl-5,6-dihydro-4//-l,2-oxazine 76 (R = Ph, r = R = H) into 2-phenylpyrrole(89TL3471) under strong basic conditions and for thermal decomposition of cyclopentene-fused 1,2-oxazine 76... 

Pyridine (and quinoline) which in so many respects are aromatic and comparable to benzene, lose this character completely on hydrogenation to piperidine (and hydroquinoline), which are entirely of the same nature as secondary aliphatic amines. The completely hydrogenated heterocyclic bases undergo degradation reactions which have become important particularly in the investigation of the constitution of alkaloids. A. W. Hofmann s method of opening rings by means of exhaustive methylation may be illustrated with piperidine. By thermal decomposition of the quaternary ammonium base a C—N-link-age is broken and at the same time water is eliminated. 

Thermal Decomposition of 4-, 5-, and 6-Membered Ring Heterocycles with a... 

Classical organic chemistry provides a wide variety of potential analytes for electron ionization, the only limitation being that the analyte should be accessible to evaporation or sublimation without significant thermal decomposition. These requirements are usually met by saturated and unsaturated aliphatic and aromatic hydrocarbons and their derivatives such as halides, ethers, acids, esters, amines, amides etc. Heterocycles generally yield useful El spectra, and flavones, steroids, terpenes and comparable compounds can successfully be analyzed by El, too. Therefore, El represents the standard method for such kind of samples. 

There are few examples of the preparations of heterocyclic compounds containing two or more heteroatoms which involve cyclization with formation of a bond between two heteroatoms. The best known instances of this type of reaction, all of which are [6 + 0] reactions, are the preparations of benzocinnolines as outlined in equations (l)-(4). A similar type of approach to that outlined in equation (4) has been used for the direct preparation of the di-N-oxide (2) from the dioxime (1 equation 5). The naphthotriazine betaine (4) is obtained as one of the products of the thermal decomposition of the azidoazo compound (3 equation 6). 1,2-Dithiins and their dibenzo derivatives have been prepared by oxidation of appropriate dithiols and related starting materials as outlined in equation (7). All of these reactions are, however, somewhat specialized and there has been essentially no systematic study of the preparation of six-membered heterocycles via formation of a bond between two heteroatoms. 

The published research on the photochemical decomposition of di-azonium salts suggests that the two processes, a heterocyclic and a homolytic process, analogous to those of the thermal decomposition may occur. Various workers 36 187 have reported that phenols are formed when diazonium salts are photolyzed in water and aryl ethers result when an alcohol replaces water as the solvent. Homer and Stohr122 report that a process analogous to reductive deamination occurs in preference to ether formation results in alcohols. The importance of free radical intermediates in the photodecomposition, based on magnetic susceptibility measurements, has been stressed.25 Lee and his co-workers171 have recently suggested that in ethanol the photodecomposition of a diazonium salt occurs via a radical intermediate while in water an ionic process predominates. Thus, photodecomposition of a nitrobenzene diazonium chloride in water yielded both a nitrophenol and a chloronitrobenzene in ethanol, on the other hand, the major product of photolysis was the reduction product, nitrobenzene. 

One of the most striking observations during the present investigation is the existence of a well-defined maximum in the total yields of O-heterocycles formed from n-pentane (Table I). Although control experiments on the thermal decomposition and oxidation of O-heterocycles... 

Photoinduced ring cleavage also occurs readily in heterocyclic systems containing both oxygen and nitrogen. A series of dihydro-oxadiazinones, for example, undergo decomposition, and the results obtained parallel those observed on thermal decomposition cis- and Jraras-stilbene are obtained from the diphenyl derivative (71), whereas cis-cyclooctene is the major product of photolysis of the fused cyclooctane (72).62... 

The reactions of thiadiazolopyrimidines 58 (R = Me, allyl) with ethoxycarbonyl isothiocyanate and CS2 gave heterocycle 59 via thermal decomposition of 1 1 cycloadducts, which have a hypervalent sulfur (Equation 10). The mechanistic and reactivity features of these reactions were described <2004JHC99>. 

A range of heterocyclic azides were studied by differential scanning calorimetry, the enthalpies of thermal decomposition were lower than might have been expected, from 0.3—1 kJ/g. If ortho substituents onto which the azide could cyclise were present, decomposition enthalpies were, as one would expect, lower still [5], as well as the individually indexed compounds ... 

The mechanism of the thermal decomposition of NMMO and Lyocell solutions is extremely complex since the reaction, initiated by the action of carbenium-iminium ions, quickly enters an uncontrollable course. A central question was whether the heterocyclic ring of NMMO was cleaved during the reaction and whether products of this cleavage, having either vinyl ether or enamine structures, could be detected. Employing the trapping agents... 

---