Thermolytic reactions

The Application of Thermolytic Reactions for the Syntheses of Fluoroorganic Compounds Platonov, V C Yakobson, G G Svnthesis 374-384 88... 

Since the yield of hexabromobenzene from bromoether 1 is in the range of 60 %, this thermolytic reaction can be used for preparative purposes. 

The Kumada/Ishikawa group also investigated thermolytic reactions of alkynyl-polysilanes and silacyclopropenes in the presence of nickel catalysts and implicated a 1-silaallene-nickel complex as an intermediate in the reaction pathway... 

The preparation and investigation of the thietane oxide system (5a) is largely associated with stereochemical and conformational studies . The investigation of the thietane dioxides (5b) is substantially related to the chemistry of sulfenes , the [2 -I- 2] cycloaddition of which with enamines is probably the method of choice for the synthesis of 5b . The study of the thiete dioxide system (6) evolved, at least in part, from the recognition that the unstable thiete system 183 can be uniquely stabilized when the sulfur in the system is transformed into the corresponding sulfone , and that the thiete dioxide system is very useful in cycloadditions and thermolytic reactions. The main interest in the dithietane oxides and dioxides (7) appears to lie in the synthetic challenge associated with their preparation, as well as in their unique structural features and chemical behavior under thermolytic conditions . ... 

The formation of cyclic sulfinic esters (sultines) from vinyl sulfenes is known , and the trapping of the expected intermediate vinyl sulfene in the thermolysis of thiete dioxide (6fc and 194) has been convincingly achieved . Specifically, thermolysis of thiete dioxide 6b in the presence of norbornenes gave cycloadducts of the Diels-Alder type (i.e. 252b), resulting from the trapping of the vinyl sulfene formed. The accumulated evidence thus supports the proposed mechanism for these thermolytic reactions. 

These selection rules have been found to be remarkably successful in thermolytic reactions (symmetry allowed ones) photolytic reactions have been found to be more complex. 

For conjugated structures thermolytic reactions with loss of sulfur have been studied since 1980 and afford various types of cyclic and acyclic products (Section 4.11.5.1). Nucleophilic attack at ring carbon is very characteristic of 1,2,3-dithiazoles (Section 4.11.5.4). This type of reaction was especially prolific with Appel s salt, studied mostly by Appel, Rees and their co-workers. The preferential site of attack is C(5) but nucleophilic substitution may occur at C(4) when the C(5) site is blocked by poorly leaving substituents, see the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) for examples of 1,2,3-dithiazoles <84CHEC-I(6)924> and 1,2,3-oxathiazoles <84CHEC-l(6)930>. Nucleophilic attack at ring sulfur in 1,2,3-dithiazoles occurs on S(2) (Section 4.11.5.5), see... 

Thermolytic reactions are known for all azolone structures (8)-(ll). Thermolysis may be regarded as a six-electron thermally allowed process and follows two major pathways, both extruding YCX or ZCX fragments (Scheme 2). 

Scheme 1 summarizes some of the common electron-impact induced fragmentation pathways available to thietanes, thietes and their 5-oxides. Major processes include retro-2 + 2 fragmentation for thietanes, thiolactones and iminothietanes, loss of sulfur oxides for thietane 5-oxides, and loss of an a-hydrogen atom from thietes and benzothietes. Certain of these processes, considered to involve electron-impact induced pathways, may actually involve thermolytic reactions occurring in the source region of the mass spectrometer (78JHC421,71JA676). 

The amino carbinol pyridines 264, prepared from 263 by DoM reaction and aliphatic or aromatic carboxaldehyde quench, are useful intermediates for condensed naphthyridines 266-268. The thermolytic reaction proceeds via intramolecular Diels-Alder reactions of the aza-orf/io-quinodimethane species 265 (Scheme 79) (84CC1304). 

The Application of Thermolytic Reactions for the Syntheses ol Fluoroorganic Compounds... 

A study of the solvolytic and thermolytic reactions of a series of heterocyclic pentacoordinate silicon complexes was described241. 

All azolones 8-11 are susceptible to thermolytic reactions. There are two major pathways of thermolysis both involving extrusion of YCZ or XCZ fragments via a thermally allowed [a2s + + ff2s] process (Scheme 3). A general... 

The second pathway, followed by 1,4,2-dioxazolones, proceeds by extrusion of C02 and rearrangement to the corresponding isocyanates 54. The question of a nitrene intermediate or a concerted C02 extrusion and relevant mechanistic aspects have been discussed in CHEC-II(1996). Also, various thermolytic reactions of other fully conjugated l,4-(oxa/thia)-2-azole derivatives, in particular azonylenes 11 (X = S Y = 0 Z = CC12), azonylimines 10 (X = 0 Y = S Z = NR), and imino dioxazolidine derivatives 12 (X = Y = 0 W = Z = NR) have been discussed in CHEC-II(1996) <1996CHEC-II(4)506>. 

As it is well known, acyloxy, alkoxy, or phenoxy groups connected to sp2-hybridized carbon atoms in alkenes or aromatics are unreactive to nucleophilic substitution. However, after alkene ozonolysis such groups become attached to sp3-hybridized carbon atoms and become reactive. It was shown <1989TL1511> that such substitutions have to be carried out at 40 °C when they compete with thermolytic reactions of the ozonides, lowering the yields. However, if 2,3-dichloropropene and as- or /ra/rt-1,2,4-trichloro-2-butene are ozonized, one obtains stable ozonides 68a-70... 

Thermolytic Reactions. At the higher temperatures, nonoxidative reactions also occur as for example the formation of tetrasubstituted cyclohexenes via Diels-Alder reaction, or the formation of dehydrodimers and mono- or polycyclic dimers via combination of alkyl free radicals or free radical attack on double bonds. 

BASF has obtained European patent 1,110,913, which describes HCN production by the thermolytic reaction of gaseous formamide in the presence of a fine-particle, solid catalyst in a fluidized bed. They claim that the process is more economical and results in less equipment damage. They also received patent DE 10,144,891 for production of HCN from formamide in a flow-type reactor with electric resistance heating. 

The anionic tetrahedral clusters [Fe3(CO)9(/u,3-E)]2 (E = S (52), Se (53), Te (48)) can be obtained from the thermolytic reaction of K2E with [Fe(CO)5] in DMF. Although a lone pair of electrons is present on E, investigations of the reactivity of electrophiles and oxidants toward the lone pair of electrons on the apical Te atom in 48 suggest strongly that the apical Te atom is unreactive toward electrophiles and that reaction occurs at metal centers (Scheme 3).50... 

Not surprisingly, heat treatment, such as commercial and household frying, accelerates autoxidation. In addition to undergoing autoxidation, when fats are heated in the presence of moisture, as often is the case in food applications, fatty acids are released via hydrolysis of the ester linkages (233). The free fatty acids can accelerate oxidation of the oil. During heat treatment, the formation of dimeric and cyclic compounds seems to be the predominant thermolytic reaction of unsaturated fatty acids. In the presence of oxygen during heat treatment, however, oxidative... 

A second methcd utilized in the approach to dibenzometallole systems involves thermal ring closure of an appropriately substituted aromatic derivative. The method requires the synthesis of an unsymmetrically substituted organometallic derivative prior to the ring closure. Often, these syntheses involve several steps. The thermolytic reactions are summarized in Eq. (86). 

At the same time, the radical formation through thermolytic reactions is rather common. For example, the thermodynamic factors may allow prediction of the structure of the most likely radical to be formed in a pyrolytic process. This calculation can be based on the strength of a bond that breaks. The enthalpy (energy) for the dissociation of a bond A-B is not always available. However, for a homolytic dissociation with formation of free radicals, the enthalpy can be derived [4] from tabulated heats of formations AHf using the relation ... 

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