Stereochemistry dissolving metal reductions

With regard to stereochemistry, dissolving metal reductions of cyclic alkanones frequently lead to the thermodynamically more stable diastereomeric alcohol3, which is the equatorial alcohol for most cyclohexanols. 

The stereochemistry of the photocycloadducts can be predicted from the result of a dissolving metal reduction of the same ,/)-unsaturated ketone. For example, sodium/ammonia reduction of 3,4-dimethylcydohexenone yielded trans- and cis-dimethylcyclohexanone 37 in a ratio of 84 16, which was similar to the ratio of the two photocycloadducts 38 (80 20) [33b, 42],... 

Electrochemical reduction of camphor-and norcamphoroxime at a Hg cathode proceeds with a high degree of stereoselectivity to give products of opposite stereochemistry to those formed in the dissolving metal (Na-alcohol) reduction of the oximes. The electrolyses are proposed to proceed by a kinetically controlled attack by the electrode on each oxime from the less hindered side (Fig. 62) [348]. In contrast, the corresponding N-phenyl imines yield products of the same stereochemistry as those isolated from a dissolving metal reduction. Cyclic voltammetry and polarographic data point to RH and intermediates in this case that are proto-nated from the least hindered side [349]. 

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

Diphenylmethylene acetals are used to protect 1.2-diols including catechols. They can be cleaved by a variety of methods including add hydrolysis, dissolving metal reduction and hydroge nolysis. The steric bulk of the phenyl rings has been used to control the stereochemistry of reactions at proximate centres.1 181... 

The alkyne was then reduced to an E alkene by a dissolving metal reduction, a step which also hydrolysed the five-membered heterocycle. The next step, an epoxidation, is needed to install the third of the chiral centres at the left-hand end of penarisidlne. However, hydrogen-bond directed epoxidation of this allylic alcohol would be expected to give the syn product shown, which has the wrong relative stereochemistry between the brown OH group and the epoxide. 

Alkynes can be reduced to either a cis or trans alkene. Catalytic hydrogenation of an alkyne using a poisoned catalyst (H2, Lindlar catalyst) results in the syn addition of one equivalent of H2 to give a cis alkene product. Dissolving metal reduction (Li, NH3) of an alkyne produces the corresponding trans alkene. This strategy is well suited for synthesizing monosubstituted alkenes and disubstituted alkenes with a specific stereochemistry. 

The syntheses of Schemes 13.33 and 13.34 employ the same starting material, an available racemic decalin derivative. Both syntheses use similar methods to establish the required trans junction and the stereochemistry at C-3 and C-4. The stereochemistry of the ring fusion is established by a dissolving-metal reduction. The protonation generates the trans ring junction because the proton is delivered... 

Electrochemical reductions of camphor oxime (R = Me) and norcamphor oxime (R = H) at a mercury cathode proceed with a high degree of diastereoselectivity (equation 3) . The products are in fact of opposite stereochemistry to those formed in dissolving metal (sodium-ethanol) reductions of the oximes. 

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. ... 

Originally the phenylsulfone 147 Ar=Ph was used and the adduct was acylated then treated with dissolving metal (e.g. sodium amalgam) to bring about a reductive elimination by electron transfer. Addition of two electrons to 151 gives the dianion 152 that breaks down to the carbanion 153 or something like it - at any rate without stereochemistry - and so to the alkene. 

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