Halides, aliphatic base effect

Inorganic bases have been also employed in this system. When Butcher first used alkali carbonates [50], it was reported that, in DMF and at ambient temperature, the carbamation of primary and secondary aliphatic amines (or also arylamines) with alkyl halides under a C02 atmosphere (0.1 MPa) was effectively promoted by Cs2C03 [50, 51]. The Cs+ cations in the solvent used (DMF) did not form ion pairs with counterions, and favored the formation of naked carbamate anions that were more reactive at the O-ends with alkyl halides. Jung further found that the addition of tetrabutylammonium iodide (TBAI) to the system RR NH/ C02/RX/Cs2C03/DMF promoted the carbamation process with a higher yield and selectivity with respect to N-alkylation [51]. The process has been successfully extended to the synthesis of carbamate functionalities on solid phases. In this case, resin-bound carbamates are readily released from the resin by treatment with LiAlH4 in THF, yielding the respective N-methyl secondary amines [51]. 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

Aryl chlorides are more reluctant to participate in amination than most other aryl halides/pseudohalides. To tackle this problem, Caddick et al. examined the effect of palladium-N-heterocyclic carbenes as catalysts in rapid microwave-promoted reactions [87]. Para-tolyl and -anisyl chloride were reacted with aromatic and aliphatic amines in mostly good yields within 6 minutes of heating at 160 °C. Reactions using anisyl, tolyl or phenyl chlorides and aliphatic amines have also been reported by Maes et al. using a phosphine ligand and a strong base, which creates the desired products after 10 minutes of heating at 110-200 °C [88]. 

The reaction of ylides with saturated aliphatic alkyl halides (like methyl iodide, ethyl iodide etc.) usually stops at the stage of the alkylated salt because the +/ effect of the aliphatic substituent causes the resulting salt to be a weaker acid than the conjugated salt of the original ylide (which would result in the course of a transylidation reaction). However since partial transylidation also occurs between al-kylidenephosphoranes and phosphonium salts with equal or not very different base and acid strength, mixtures may result from Ae reaction with saturated aliphatic alkyl halides. At this point it should be mentioned that the synthesis of dialkylated ylides via the salt method is also difficult since the preparation of the necessary phosphonium salt is accompanied by -elimination. The successful synthesis of dialkylated ylides may be achieved by fluoride ion induced desilylation of a-trimethylsilylphosphonium salts (see equation 18). There is no doubt about the course of ylide alkylation in cases where the inductive effect of the new substituent leads to complete transylidation (e.g. equation 54). ... 

More recent approaches which avoid acidic conditions are based on the application of trimethylsilyl bromide or iodide (equation 12). With aliphatic acid halides a high reactivity (often the reaction is exothermic) is observed. The isolation of the products can be effected immediately after the addition of trimethylsilyl halide is complete. With aromatic halides heating is sometimes required. Easy woilc-up is possible, because only the low boiling trimethylsilyl chloride has to be removed before the product is isolated by distillation. Table 5 gives some examples. 

Bromine-containing compounds on a weights basis are at least twice as effective as chlorine-containing ones. Because of the smaller quantities of bromine compounds needed, their used hardly influences the mechanical properties of the base resins and reduces markedly the hydrogen halide content of the combustion gases. Aliphatic, cycloaliphtic, as well as aromatic and aromatic-aliphatic bromine compounds are used as flame retardants. 

Despite the advantage of sulphate-based anions in ionic liquids over fluorinated or halide-containing anions in terms of greenness , the former ([bmim][0S04] and [moim][MDEGS04]) have in general not proven to be effective in the separation of aromatic-aliphatic mixtures, with the exception of [emim][ES04]. This indicates that a small sulphate-based anion is more effective for the separation of the separation problems presented in... 

Further invention of pyridine-enhanced precatalyst preparation stabilization and initiation (PEPPSI) ligands has led to a remarkable improvement in palladium-catalyzed C—S coupling [159, 160]. This catalyst effectively couples the less reactive aryl chlorides even at room tempauture [161]. Recently, NHC-based catalyst, [Pd(IPr )(cin)Cl] 18, has been developed for the coupling of aryl halides with aliphatic or aromatic thiols (Scheme 20.53) [162]. 

A facile conversion of several aliphatic and aromatic acyl peroxides, halides, and anhydrides to the "per" acids has been effected by treatment with H2O2 and a threefold excess of base in the presence of a soluble magnesium saltl > The magnesium salt Is proposed to stabilize the "per" acid as it is formed, and to prevent its decomposition. 

Abstract This review covers the recent recyclable protocols for the C-N bond forming reactions between aromatic, heterocyclic and aliphatic amines such as imidazoles, benzimidazoles, benzylamines, piperidine, pyrrole, imides, anilines, hexyl, cyclohexyl amines, and amides as coupling partners with aryl iodides, bromides, chlorides, and arylboronic acids employing copper-mediated systems. The physical properties and characterization of the catalysts and their use in organic synthesis will be outlined. Most importantly, these recyclable versions developed by many groups in the recent years are potential candidates for commercial exploitation. The effect of additives, solvents, temperature, base, the nature of aryl halides on reactivity, and recycle studies of the heterogeneous catalysts are included in this... 

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