Low-temperature Solvents

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

Competing side reactions in cationic polymerization of carbonyl monomers include cyclotrimerization and acetal interchange. Cyclotrimerization is minimized by low-polarity solvents, low temperatures, and initiators of low acidity. Acetal interchange reactions among different polymer chains do not occur except at higher temperatures. Acetaldehyde and higher aldehydes are reasonably reactive in cationic polymerization compared to formaldehyde. Haloaldehydes are lower in reactivity compared to their nonhalogen counterparts. 

Recent interest in the use of N-unsubstituted 2-quinolones stems from the fact, that they coordinate effectively to chiral lactam-based templates via two hydrogen bonds. The prototypical template to be used in photochemical reactions is compound 115, which can be readily prepared from Kemp s triacid [108]. The template is transparent at a wavelength X > 290 nm, and can be nicely used in stoichiometric amounts for enantioselective photochemical and radical reactions [109]. Conditions which favor hydrogen bonding (nonpolar solvent, low temperature) are required to achieve an efficient association of a given substrate. The intramolecular [2 + 2]-photocycloaddition of 4-alkylquinolone 114 proceeded in the presence of 115 with excellent enantioselectivity, and delivered product 116 as the exclusive stereoisomer (Scheme 6.41) [110]. Application of the enantiomer ent-115 ofcomplexing agent 115 to the reaction 111 —> 112 depicted in Scheme 6.40 enabled enantioselective access to (+ )-meloscine [111]. 

Solvates Same as true Same as true Unique solvent Unique solvent Low-temperature Same as true... 

Favored by strong base, polar aprotic solvent, low temperature LDA, THF, -78 °C... 

Addition of LOO" to Double Bonds Peroxyl radicals are quite specific in their addition preferences, and competition with hydrogen abstraction is generally unfavorable except under select conditions. The ROO addition becomes competitive when abstractable hydrogens are limited (aprotic solvents, low temperature) and when there is a double bond that is conjugated, terminal, or 1,1-disubstituted... 

As with ketone enolate anions (see 16-34), the use of amide bases under kinetic control conditions (strong base with a weak conjugate acid, aprotic solvents, low temperatures), allows the mixed Claisen condensation to proceed. Self-condensation of the lithium enolate with the parent ester is a problem when LDA is used as a base, ° but this is minimized with LICA (lithium isopropylcyclohexyl amide).Note that solvent-free Claisen condensation reactions have been reported. ° ... 

Use Plasticizer for PVC, intermediate for pharmaceutical and agricultural chemicals, high-boiling solvents, low-temperature lubricants. 

Notes and discussion. This procedure is one of the first cited examples of the use of an a-trichloroacetimidate donor undergoing SN2-like displacement during a carefully controlled glycosylation reaction. The lack of a participating group on 0-2 of the donor and the use of a non-participatory solvent, low temperature and mild activator are essential for the promotion of this process. 

Certain aldehydes and ketones, when used as solvents, intercept and reduce a labile intermediate in the ozonolysis of olefins. The intermediate, which can be considered the progenitor of many other ozonolysis products, is formulated as the Staudinger molozonide, e.g. (577), and its reduction generates the corresponding dioxetan (578) with a Baeyer-Villiger oxidation of the aldehyde or ketone solvent. The dioxetan intermediate, normally cleaved to the carbonyl components, has now been isolated and characterized by using pinacolone as a solvent. Low-temperature infrared studies of simple alkene-ozone reactions have been made. ... 

The side reactions lead to loss of initiator, termination of chain growth and formation of polymers with broad molar mass distributions. For methyl methacrylate (R = Ri = CH3) the side reactions essentially can be eliminated by using polar solvents, low temperatures, bulky initiators (for which reaction with the C=0 group is sterically-hindered) and large counter-ions, e.g. by polymerization in tetrahydrofuran at —75°C using cumylcaesium as initiator. 

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