Reduction by dissolving metal

A3-Pyrrolines, e.g. (176), are formed on the reduction of pyrroles and simple alkylpyrroles with zinc and acid. These are derived from the corresponding a-protonated species (66JA1335). 

Indole-2-carboxylic esters undergo reduction with magnesium in methanol to give the related indoline (86TL2409). 

Dissolving metals reduce the heterocyclic ring of isoindoles. 

Reduction of thiophene and 2-ethylthiophene to the corresponding 2,5-dihydrothiophenes can be carried out with zinc and trifluoroacetic acid. The mechanism is again thought to involve protonation 

Reactivity of Five-membered Rings with One Heteroatom 

---

The SR substituent can be displaced nucleophilically by amines and hydroxide (867 — 868) and removed reductively by dissolving metals, e.g. from (869) with Zn/H+. Oxidation gives the corresponding sulfoxide and sulfone, in which nucleophilic displacement is easier. Thus, 2- and 4-(phenylsulfonyl)pyrimidines give the corresponding replacement products with various nitrogen and... 

Other reactions at surfaces (catalytic hydrogenation and reduction by dissolving metals)... 

Although it was once believed that reductions by dissolving metals in alcohols or liquid NH3 would invariably provide the thermodynamically more stable alcohol as the major product, it has been known for many years that the thermodynamically less stable epimeric alcohol is sometimes the major product on either metal-alcohol or metal-NHs-proton donor reduction." " A detailed explanation for the stereochemistry of these reductions based on a combination of steric and frontier molecular orbital interactions has been presented. ... 

In terms of mechanism and stereochemical consequences, reductions by dissolving metals in liquid NH3 are very similar to reductions by the same metals in alcoholic media. However, reductions carried out in liquid ammonia do not suffer from the same inherent problems as those by metals in alcohols. There is no evidence for equilibration of the product alcohols, and ketones which undergo epimerization prior to reduction with metals in alcohols are reduced cleanly by metals in NH3. For example, menthone (4) on reduction with Li-NHs-ethanol gives a mixture of alcohols (6) and (7), with no trace of alcohols (8) and (9).22... 

Reductions by dissolving metals in liquid NH3 in the absence of proton donors, are, in general, inferior in terms of general synthetic utility to reductions carried out under other conditions. In many cases, particularly when Li is used as a reducing agent, bimolecular reduction is difficult to predict and the ratios of epimeric alcohols may vary as a function of the metal used as the reducing agent. ... 

C—S bonds can be cleaved reductively by dissolving metals, by processes which depend on the ability of sulfur to accept electrons.1,3-Dithiolanes tend strongly to suffer cleavage of both C—S bonds with sodium in liquid ammonia when C-2 is unsubstituted or carries a phenyl group isopropylidene derivatives (70), however, give single cleavage (equation 39). Bond rupture requires formation of a relatively stable carbanion. 

Free radical attack at ring carbon atoms. 6.3 Electrochemical reactions and reactions with free electrons. 6.4 Catalytic hydrogenation and reduction by dissolving metals Reactions with Cyclic Transition States. 7.1 Diels-Alder reactions and 1,3-dipolar additions. 7.2 Photochemical cycloadditions... 

Reduction by dissolving metals Partial hydrogenation of the nucleus Hydrocarbons from alcohols... 

Usually, PET processes are preferred to other fragmentation processes in HMPA, but only dialkylthiocarbamates give satisfactory yields of R -H. Again a very good parallel can be drawn between the PET reaction of carbonic acid derivatives and their reduction by dissolving metals in primary amines or HMPA. ... 

Epoxides can also be reduced to saturated alcohols. Lithium aluminum hydride acts as a nucleophilic reducing agent, and the hydride is added at the less substituted carbon atom of the epoxide ring. Lithium triethylborohydride is more reactive than LiAlH4 and is superior for epoxides that are resistant to reduction. Reduction by dissolving metals, such as lithium in ethylenediamine, also gives good yields. 

---