1- Chloride eliminations

Preparation from Amines. The most common method of preparing isocyanates, even on a commercial scale, involves the reaction of phosgene [75-44-5] and aromatic or aUphatic amine precursors. The initial reaction step, the formation of N-substituted carbamoyl chloride (1), is highly exothermic and is succeeded by hydrogen chloride elimination which takes place at elevated temperatures. 

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

Because of its importance in biological areas, special efforts have been made with the synthesis of the thiazolo[2,3-t]-[l,2,4]thiadiazole derivative 308 <2001BML1805>. The pathway started from the benzothiazole derivative 305 which was treated with chlorosulfonylacetyl chloride to form an intermediate 306, which underwent cyclization to a second intermediate 307 with hydrogen chloride elimination. The last step is the attack of the first intermediate 306 at the thiadiazine carbon atom to form the final product 308. 

On the other hand, treatment of 494 with lithium and a catalytic amount of biphenyl gave the corresponding macrocycles of 501 in 10-20% yield by lithium chloride elimination . ... 

Chloramine-T has also been employed, both as a halogenating reagent and base the reaction proceeds in good yield with aromatic as well as with aliphatic aldoximes (81). The role of chloramine-T probably involves an initial chlorination of the aldoxime to give the hydroximoyl chloride, followed by base-catalyzed hydrogen chloride elimination to afford the nitrile oxide (81). 

Scheme 9.15 Syntheses of saturated group 13-pnictogen rings by (a) salt elimination and (b) trimethylsilyl chloride elimination.

Paudler observed alcohol addition to the N( 1)—C(6) bond of 1,2,4-triazine 2-oxides unsubstituted at the 6-position (96) in the presence of hydrogen chloride. Elimination of water followed, leading to 6-alkoxy-1,2,4-triazines (97) (77JOC3489). 

The in situ generated disubstituted ureas (26) also react with phosgene to yield thermally unstable allophanoyl chlorides (27) and chloroformamides (28) (75). As shown in Figure 5, the allophanoyl chlorides eliminate hydrogen chloride to form the isocyanate. The chloroformamides, however, yield chloroformamidine-A/-carbonyl chloride (29), which decomposes to yield both carbodiimides (30) and isocyanide dichlorides (31). The carbodiimides simply contribute to yield loss. The isocyanide dichlorides, although present in small amounts, are a contributor to chlorine-containing impurities which detrimentally affect product performance. 

Compound 2 (80 mmol) was mixed with an excess of amine 3 (1.5 mL) in the microwave reactor ( =4 cm). Then, the mixture was immediately submitted to focused microwave irradiation at the suitable temperature during 30 min. Extraction of the reaction mixture from the microwave reactor with 15 mL of methylene chloride, elimination of solvent and excess of 3 in vacuo followed by the analysis of the crude reaction mixture by 1H NMR spectroscopy indicate the formation of the desired compound 4. The compounds 4 were purified by recrystallization. 

The compounds resulting from the reaction of 748 were characterized by HRMS directly coupled to the reactor. The stable products 750 and 751 were analysed by GC and 111 NMR spectroscopy. The formation of the cyclodisilazane 750 is explained by dimerization of the unstable silanimine 749 only in the cold trap, as the reaction is carried out under high dilution conditions (equation 247). It was also shown that the hydrogen chloride elimination did not occur in the ion source of the mass spectrometer. 

VII. Palladium Acetate Addition with Chloride Elimination... 

The active-methyne compounds, which derive from the acylation of the enolates of active-methylene compounds with carboxylic acid chlorides, eliminate the extra acceptor(s) in an additional step or immediately in situ. The defunctionalizations involved include one or two decaboxylations depending on the nature of the reactants and subsequent processing steps (Figures 13.66 and 13.67)... 

The head-to-tail structure of poly (vinyl chloride) permits the continuous regeneration of an allylic chloride moiety as hydrogen chloride elimination proceeds along a chain. Thus, once initiated, loss of hydrogen chloride may proceed along a polymer chain without abatement. 

The first term represents the unimolecular decomposition of allylic chloride. The first and fourth terms are responsible for hydrogen chloride elimination and do not result in stabilization. The second and third terms are responsible for stabilization. This work has provided an estimate of the rate constants for dibutyltin dilaurate and dibutyltin bis (monobutyl maleate). 

Table II includes the reaction half-time (6410 minutes) calculated for the unimolecular hydrogen chloride elimination for 4-chloro-2-hexene...

The behavior of dibutyltin bis(dodecylmercaptide) on reaction with 4-chloro-2-pentene proved interesting (Figure 3). With only the two reactants in chlorobenzene, virtually no reaction took place up to 5 hours. However, the addition of dibutyltin dichloride resulted in a rapid reaction. Furthermore, the addition of a few milligrams of azobisisobutyro-nitrile eliminated any induction period. This latter consequence is not interpreted to result from a free radical stabilization mechanism, but it is presumed to be caused by free radical-catalyzed hydrogen chloride elimination, resulting (by neutralization with the stabilizer) in the formation... 

Coating the vessel with potassium chloride eliminated the chain reactions and simplified the kinetics. It was found that the quantum yields of hydrogen, carbon monoxide, and formic acid decreased with an increase in oxygen pressure in both coated and clean vessels. The quantum yield of carbon dioxide was large, ca. 3.0, variable (and, therefore, presumably heterogeneous) in the coated vessel, but in the clean vessel it increased with the oxygen pressure (Fig. 7). 

Addition of tellurium tetrachloride to 2,2-diphenylpent-4-en-1-oic acid followed by hydrogen chloride elimination produced bis[3,3-diphenyl-2-oxotetrahydrofuran-5-yl-methyl tellurium, dichloride2. 

Amides being formally hydrated ynamines would constitute cheap starting materials for the latter and many stratagems have been devised to achieve this goal. It must be kept in mind, however, that direct dehydration of amides is not feasible. One indirect approach has already been exemplified, namely the prior conversion of amides into thioamides. Another classical method involves halogen chloride elimination from amide chlorides. These versatile salts have received only scant attention prior to 1960. 

The organozinc intermediate thus formed reacts with aldehydes as Grig-nard reagents do to form alcohols. In the presence of aluminum chloride, elimination of chlorine and fluorine from the vicinal carbons of the dichlorotrifluoroethyl group generates halogenated allylic alkoxides that are protonated to allylic alcohols, in the present case J, 2-chloro-3,3-difluoro-1 -phenylpropen-2-ol [114]. 

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