Electrophilic addition temperature effects

Alkylation of the -lactam ester 8 displayed high yields and excellent diastereoselectivi-ties 132 133. In this case, an interesting temperature effect has been observed. When the enolate was generated at — 78 °C. the diastereoselectivity was low, however, when generated at 0°C, a diastereoselectivity of >95% was observed. The temperature at the addition of the electrophile is not crucial and can be — 78 °C or 0°C134. 

Transfer processes can be caused by monomer, counterion, and other components of the reaction mixture (additives, solvents, impurities). The latter reactions are sometimes called spontaneous because they are zero order in monomer. However, the spontaneous elimination of /3-protons is very unlikely, and proton elimination must be assisted by some basic reagent. The ratio of the rate constants of /8-proton elimination to that of electrophilic addition depends on several factors. The relative rate of transfer decreases with temperature, and therefore polymers with higher molecular weights are formed at sufficiently low temperatures. The effect of solvent and counterion is not yet sufficiently understood. 

Electrophilic addition of PhSeCl to alkene 319 is highly stereo- and regioselective (steric effects) and provides adduct 338 if carried out in the presence of an acetate nucleophile. Methanolysis followed by treatment with formalin and protection of the endo alcohol gives ketone 339. Low-temperature oxidation of the selenide 339 generates a selenoxide, which does not... 

Reaction of 2,4-dinitrobenzenesulfenyl chloride with norbornadiene, used to increase the effective electrophilicity of weak electrophiles in electrophilic addition to 1,3-dienes, gave a mixture of chloronorbornenyl and chlorotricyclo[2.2.1.0 ]heptyl sulfides 20 and 21 with regioselec-tivity and stereochemistry depending on the solvent and temperature of the addition.Thus in acetic acid at 25°C two acetoxy sulfides trans- and cis-22 were also formed. 

Although C4 addition occurs with phenol complexes even for cases where C4 is substituted, in many cases, ortho addition is thermodynamically favored. In this scenario, the regiochemistry can be effectively controlled by adjusting reaction variables such as temperature, time, and catalyst [44]. Under basic conditions, the active form of the phenol complex is the phenoxide species, which can undergo reversible Michael reactions at C4 and C2, provided that the resulting enolate is not protonated. For instance, the addition of MVK to the osmium complexes of para-cresol (31) or estradiol (27, Fig. 8) occurs at C4 to give the 4H-phenol product (28,32) at -40 °C with an amine base. However, if the reaction is carried out at 20 °C or is run in the presence of a Zn " co-catalyst, the initially formed enolate may undergo retroaddition, and ultimately, the reaction yields the orthoalkylated product (30,33 see Fig. 8). Electrophilic addition at the ortho... 

Anomeric alkylation has been extensively studied by Schmidt and co-workers,62 and the main alkylating agents used were dialkyl sulphates, benzyl bromide, allyl bromide63 or various O-triflates, allowing the synthesis of disaccharides.64 The stereochemical outcome of anomeric alkylation is known to depend on many parameters such as the base, the solvent and its effect on solubility and concentration, the temperature, chelation effects, presence of additives, and nature of the electrophilic species.62 65... 

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