Anionic quenching

The reaction is probably initiated by fluoride attack on the carbonyl group, as shown in Figure Si6.12. A Brook rearrangement, anion quenching, and fluoride-induced collapse of the tetrahedral intermediate then complete a plausible pathway for this transformation. 

The process now known as reductive alkylation of rc-conjugated anions (quenching of anions) is as old as the preparation of the ions themselves5). The highly colored solutions obtained by the addition of alkali metals to solutions of aromatic hydrocarbons in ether were reacted with electrohpiles such as protons or alkyl halides (Scheme 2). The products of such a process are reduced hydrocarbons. The Birch reduction is one example oT such a process, reaction of an anion with an alkyl halide leading to an alkylated reduced hydrocarbon is another example 165). The complexity of the quenching experiments is demonstrated by the naphthalene radical anion 150-1581... 

A series of 5 -thienyl derivatives were prepared using metalation chemistry. Thiazole 6 (Equation 4) was treated with 1.0 equivalent of n-butyllithium in THF at -78 C for 2 h and the resulting anion quenched with 1.1 equivalents of an electrophile. Extractive workup and chromatography afforded the 5 -substituted derivatives in 40-80% yield. In the course of these experiments, we discovered that it was necessary to avoid excess alkyl lithium in this reaction sequence (Equation 5). Under these reaction conditions, excess base metalated both the thiophene and thiazole rings, resulting in a mixture of mono- and bis-alkylated thienyl-thiazoles. 

Several alternative methods followed this early work. In one, aromati2ation is effected by treating the ketal of androstadienedione with the radical anion obtained from lithium and diphenyl in refluxing tetrahydrofuran. Diphenylmethane is added to quench the methyllithium produced from the... 

Extensions of this concept have utilized enamine hydrolysis (171, X = R N) and the quenching of the enolate anion (171, X = O ) e.g. ref. 353). a,(i-Un-saturated ketones are usually more stable than their p,y counterparts, but there are notable exceptions to this, and in such cases the deconjugated ketone may be isolated from the equilibrated system. For example, retro steroids (9, 10a) have a large proportion of A -3-ketone at equilibrium, and 17-ketones yield the more stable A -system on treatment with acid. ... 

Unlike the parent system, 5-methyl-5//-dibenz[c,e]azepine (1, R1 = Me R2 = H) on treatment with lithium diisopropyl amide fails to yield the tautomeric phenanthridine-imine (see Section 3.2.1.5.4.2.), but forms the 5-carbanion, which on quenching with deuterium oxide furnishes 5-methyl-[5-2H,]-5//-dibenz[e,e]azepine (l).83 5,7-Diphenyl-5//-dibenz[r,e]azepine (1. R1 = R2 = Ph) behaves similarly. In contrast, however, 5,7-dimethyl-5//-dibcnz[c,e]azepine (1, R1 = R2 = Me) yields theazaallyl anion 3, which on addition of deuterium oxide deuterates regiospecifically at the 7-methyl group to give derivative 4. 

When H2O2 is a necessary component of a luminescence system, it can be removed by catalase. If a luminescence system involves superoxide anion, the light emission can be quenched by destroying O2 with superoxide dismutase (SOD). The ATP cofactor usually present in the fresh extracts of the fireflies and the millipede Luminodesmus can be used up by their spontaneous luminescence reactions, eventually resulting in dark (nonluminous) extracts containing a luciferase or photoprotein. The process is, however, accompanied by a corresponding loss in the amount of luciferin or photoprotein. The use of ATPase and the elimination of Mg2+ in the extract may prevent such a loss. 

A comparison of the configuration of the substrates and reaction products shows that the oxiranyl anions arc configurationally stable under the reaction conditions. Only one example is known in which isomerization was observed. When the ci.v-tm-butyl-substituted epoxysilane27 was metalated and quenched with 2-cyclohexenone, addition product 27 was obtained under inversion of the anionic center. Presumably the strain created in forcing the ter/-butyl and the trimethylsilyl group cis on the oxirane ring facilitates the isomerization process13. 

Treatment of the phenylsulfide 34 with butyllithium, followed by quenching of the anion 35 A/35 B with chlorotrimcthylsilane afforded a mixture of the diastereomers 36 A and 36B in a ratio of 98 2. 

An analogous study concerned with the synthesis of prostaglandin analogs was also highly diastereoselective. The initial anionic adducts were quenched with various aldehydes, however, the diastereoselectivity at the carbinol stereocenter was not reported21. 

---