Thermal elimination process

The base-mediated conversion of arylsulfonylhydrazones to alkenes was first observed by Bamford and Stevens in 1952.6 In a representative transformation, tosylhydrazone 3 was converted to cyclohexene (4) in quantitative yield after refluxing for 90 minutes in an alkoxide solution derived from dissolution of sodium in ethylene glycol. These reactions are believed to proceed via intermediate diazo compounds, which are transformed to alkenes by thermal elimination processes. 

In contrast to the above thermal elimination processes, in which there is good reason to invoke sulfene formation, the chlorination of 26 is perhaps more appropriately labelled a... 

The thermal elimination process can be applied to most substituted groups in vinyl polymers by controlled pyrolysis at 600-700°C, producing polyvinylene compounds, for example, by the splitting off of acetic acid from poly(vinyl... 

The thermal elimination process can be applied to most substituted groups in vinyl polymers by controlled pyrolysis at 600 to TOO C, producing poly vinylene compoimds, e.g., by the splitting off of acetic acid from poly(vinyl acetate). By careful temperature control, one can achieve bifunctional reactions and/or intramolecular cyclizations. This has been developed commercially at relatively high temperatures, in the case of the polymerization of methacrylamide above 65°C, to yield a polymer with a substantial proportion of imide groups ... 

Hydrocarbyl Complexes. Stable homoleptic and heteroleptic uranium hydrocarbyl complexes have been synthesized. Unlike the thorium analogues, uranium alkyl complexes are generally thermally unstable due to P-hydride elimination or reductive elimination processes. A rare example of a homoleptic uranium complex is U(CH(Si(CH2)3)2)3, the first stable U(I11) homoleptic complex to have been isolated. A stmctural study indicated a triganol... 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

Sulfur-stabilized ylides underwent photodriven reaction with chromium alkoxy-carbenes to produce 2-acyl vinyl ethers as E/Z mixtures with the E isomer predominating (Table 22) [ 121-123]. The reaction is thought to proceed by nucleophilic attack of the ylide carbon at the chromium carbene carbon followed by elimination of (CO)5CrSMe2. The same reaction occurred thermally, but at a reduced rate. Sulfilimines underwent a similar addition/elimination process to produce imidates or their hydrolysis products (Table 23) [ 124,125]. Again the reaction also proceeded thermally but much more slowly. Less basic sulfilimines having acyl or sulfonyl groups on nitrogen failed to react. 

This section describes reactions in which elimination to form a double bond or a new ring occurs as a result of thermal activation. There are several such thermal elimination reactions that are used syntheses, some of which are concerted processes. The... 

Condensation polymers such as polyesters or polyamides undergo more complex thermal degradation processes where the resulting pathway is a combination of different reactions including scission, elimination and cyclization [75]. 

Thermal elimination of trimethylsilanol from A,-trimethylsiloxy isoxazolidines was also documented (175, 185). Presumably, this process is accelerated by the resulting trimethylsilanol. 

As mentioned above, one of the thermal decomposition pathways of alkylcopper compounds involves a j8-hydrogen elimination process, and so it is not surprising that the first well characterized alkylcopper compounds lacked such yS-hydrogens. Treatment of LiCH2SiMe3 with Cul afforded a tetrameric aggregate, the structure of which was unambiguously proven by an X-ray crystal structure determination (see Fig. I.IB in the previous section). This represented the first example of a well characterized alkylcopper compound [17]. 

This report describes a process to produce vinyl acetate with high selectivity from exclusively methanol, carbon monoxide, and hydrogen. The simplest scheme for this process involves esterifying acetic acid with methanol, converting the methyl acetate with syn gas directly to ethylidene diacetate and acetic acid, and finally, thermal elimination of acetic acid. Produced acetic acid is recycled. Each step proceeds in high conversion and selectivity. 

This unexpected result may be related to the increase in TOC on fraction G3 and may be further evidence of the polymerization phenomenon discussed earlier. However, this hypothesis must be carefully considered because of our limited knowledge of pyrolysis mechanisms. The possibility of phenol formation during the thermal fragmentation process from elimination reactions followed by cycliza-tion of poly conjugated chains has been suggested by Bracewell (22) and should be investigated. 

In the case of the gem diester (121), the major pathway results in aromatization of the dihydropyran ring with production of the pyrylium species (121a). This is derived from the [M - C02Et]+ ion with subsequent loss of carbon monoxide and elimination of ethanol. It was shown that RDA fragmentation of (121), a minor pathway only, is a thermally induced process (72CJC1539). 

The mechanisms of thermal decarboxylation probably are not the same for all cases, but when the acid has a double-bonded function such as 0=C, N=C, 0=N, or C=C attached to the a carbon then a cyclic elimination process appears to occur. For propanedioic acid the process is... 

The main gaseous product of thermal decomposition at rather low temperatures (100-200 °C) of Cs[Mo2 (S2)6] hH2 O is the S2 molecule, which results from a reductive elimination process. This has been proven by mass spectroscopy and matrix isolation Raman, UV/VIS and IR spectroscopy. 

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