Intramolecular addition reactions Magnesium

Alkyl alkanoates are reduced only at very negative potentials so that preparative scale experiments at mercury or lead cathodes are not successful. Phenyl alkanoates afford 30-36% yields of the alkan-l-ol under acid conditions [148]. Preparative scale reduction of methyl alkanoates is best achieved at a magnesium cathode in tetrahydrofuran containing tm-butanol as proton donor. The reaction is carried out in an undivided cell with a sacrificial magnesium anode and affords the alkan-l-ol in good yields [151]. In the absence of a proton donor and in the presence of chlorotrimethylsilane, acyloin derivatives 30 arc formed in a process related to the acyloin condensation of esters using sodium in xylene [152], Radical-anions formed initially can be trapped by intramolecular addition to an alkene function in substrates such as 31 to give aiicyclic products [151]. 

A comparative study demonstrated the moderate efficiency of a metallation/cyclization/hydrolysis reaction sequence of enynyl bromides, (95) + (96) -> (97), which obviously implies intramolecular zinc-and magnesium-ene additions to a terminal alkyne unit (Scheme 22 Table 7). ... 

C.ii.b. Stille Couplings. A polymer-bound tin hydride has been used to hydrostannylate alkynes under palladium catalysis hydrostaimylation to give polymer-bound alkenyl-stannanes.t Alternatively, the latter could be prepared from a polymer-bound tin chloride and an alkenyllithium or -magnesium halide reagent. These alkenylstannanes were employed in intermolecular as well intramolecular Stille reactions. The inter-molecular reactions provided the coupling products in good yields. In addition, the... 

Oppolzer and co-workers have reported that the magnesium-ene reaction is a versatile method for intramolecular addition of allylic magnesium reagents to alkenesP A zinc-600 ° reaction can be initiated by the addition of -BuLi to the tert-butyl ketone 331 followed by the addition of zinc chloride. The resulting zincated spiro-derivative 332 could be quenched with an acid chloride leading to the ketone 333 in 60% and >98% syn-diastereoselectivity (Scheme 2-114). ° ... 

The substitution pattern in the enolate is crucial for the ring size of the cyclization product. Upon reaction with carbene complex 58 /3-substituted lithium enolates 59a H) lead to densely substituted cyclopentanols 60 suggesting a [2-i-2-i-l]cycloaddition pathway. /3-Unsubstituted lithium enolates 59b (R =H), however, form 1,3,3,5-tetrasubstituted cyclohexane-l,4-diols 61 that indicates a [2-I-2-I-1-I-1] sequence [41]. The branching point in the mechanism seems to be intermediate B formed upon addition of the allyl magnesium bromide to penta-carbonylchromate intermediate A. Intermediate B formed from /3-substituted enolates 59a is supposed to undergo an intramolecular carbometallation reaction to give cyclopentanol derivative 60. In contrast, intermediate B originating from... 

The overall outcome of this reaction looks somewhat puzzling, but the reaction course, which includes successive alkylation with the Grignard reagent and elimination of magnesium thiolate, was proposed as in Scheme 3 [228]. The first oxidative addition of Ni° species to the Csp -S bond was promoted through coordination by an additional intramolecular oxygen or sulfur functional group. 

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