Dissolving metal reduction, alkenes

This method is a very interesting counterpoint to the Lindlar reduction discussed in Section 19.3.3. Whereas Lindlar reduction of an alkyne gives a Z-alkene, dissolving metal reduction of an alkyne gives an -alkene. The reduction process can therefore be controlled, and either a Z- or an -alkene can be prepared. Assume that alkenes are not reduced to the alkane under the conditions described using Na or Li in ammonia or ethanolic ammonia. 

When hydrogenation is performed in the presence of a poisoned catalyst (such as Lindlar s catalyst), the alkyne is reduced to a cis alkene. When R is used as the catalyst, the alkyne is reduced aU the way to an alkane. Treatment of the alkyne with sodium in liquid ammonia affords a trans alkene (dissolving metal reduction), as shown here ... 

Phosphate groups can also be removed by dissolving-metal reduction. Reductive removal of vinyl phosphate groups is one method for conversion of a carbonyl compound to an alkene.224 (See Section 5.7.2 for other methods.) The required vinyl phosphate esters are obtained by phosphorylation of the enolate with diethyl phospho-rochloridate or /V A /V -tetramethyldiamidophosphorochloridate.225... 

This reaction, called a dissolving metal reduction, produces an (E)- or r[Pg.310]

Reductive elimination from 1,2-dibromides generates the alkene in excellent yields. Conformationally rigid, periplanar tro 5-diaxial, also staggered trans-diequatorial, cyclohexane dibromides all afford the alkene at a mercury cathode [110]. In the bicyclo[2,2,2]octane series, the rra s-2,3-dibromide forms the alkene on dissolving metal reduction [111]. The rigid cis-periplanar 1,2 dibromobicy-clo[2,2,l]heptane, at a mercury cathode, also gives the strained alkene which can be trapped as a furan adduct [112]. 

Fairly selective reduction of certain alkenes can be achieved using the sodium-hexamethylphosphoramide (HMPA)-tm-butyI alcohol system,193 despite the general trend, that nonconjugated alkenes are usually quite resistant to the dissolving metal reduction method (see Section 11.3.2). In the case of 9(10)-octalin, the transformation leads to a nearly equilibrium product distribution ... 

It was observed in 1941 that with sodium in liquid ammonia, called dissolving-metal reduction, different dialkylacetylenes were converted to the corresponding trans alkenes in good yields and with high selectivity 195... 

The observation was a significant finding since at the time, when the only synthetic method to reduce alkynes selectively was their conversion by heterogeneous catalytic hydrogenation (Raney nickel) to cis alkenes. The dissolving-metal reduction provided easy access to high-purity trans alkenes since the latter do not readily react further under the conditions used. The efficient reduction of 1-alkynes in this system requires the presence of ammonium ion.196... 

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

Dihalocydopropanes readily undergo reductive dehalogenation under a variety of conditions. Suitable choice of reagents and reaction conditions will allow the synthesis of monohalocyclopropanes or the parent cyclopropanes.19 " The ease of reduction follows the expected order I > Br > Cl > F. In general, complete reduction of dibromo and dichloro compounds is accomplished by alkali metal in alcohol,99-102 liquid ammonia103 or tetrahydrofuran (equations 28 and 29).104 The dihalocydopropanes can be reduced conveniently with LAH (equation 30).105 LAH reduction is particularly suited for difluoro compounds which are resistant to dissolving metal reductions.19 106 It is noteworthy that the sequence of dihalocar-bene addition to an alkene followed by the reduction of the dihalocyclopropyl compounds (equation 31) provides a convenient and powerful alternative to Simmons-Smith cyclopropanation, which is not always reliable. 

Reduction of an alkyne to an (E)-alkene can be achieved by treating the alkyne with lithium or sodium metal in ammonia at low temperatures (Following fig.). This is called dissolving metal reduction. 

The partial reduction of alkynes provides methods that are both regio-and stereospecific. Dissolving metal reductions tend to give trans alkenes, whereas catalytic methods of reduction generate the cis alkenes (Scheme 3.3). A Lindlar catalyst (Pd/CaCOj + PbO, partially poisoned with quinoline) has been recommended for use in this context. 

Although catalytic hydrogenation is a convenient method for preparing cis alkenes from alkynes, it cannot be used to prepare trans alkenes. With a dissolving metal reduction (such as Na in NH3), however, the elements of H2 are added in an anti fashion to the triple bond, thus forming a trans alkene. For example, 2-butyne reacts with Na in NH3 to form rram-2-butene. 

The trans alkene is formed during dissolving metal reduction because the vinyl carbanion formed in Step [3] is more stable it has the larger R groups farther away from each other to avoid steric interactions. Protonation of this anion leads to the more stable trans product. 

Dissolving metal reduction of an alkyne with Na, NH3 to form a trans alkene (12.5C)... 

Dissolving Metal Reduction of an Alkyne to a Trans Alkene 435... 

Both disubstituted alkynes (Chapter 3.3, this volume) and isolated terminal double bonds may be reduced by alkali metals in NH3, but isolated double bonds are usually stable to these conditions. However, 16,17-secopregnanes (10 equation 8) afford mixtures of cyclization products (11) and (12) in 61% to 80% yield with Na naphthalenide-THF, Na-NHs-THF, Na-THF or Li-NHs-THF. With Na-NHa-THF-r-butyl alcohol, a 91% yield of a 72 28 mixture of (11) (12) (R = Me) is obtained. This type of radical cyclization of alkenes and alkynes under dissolving metal reduction conditions to form cyclopentanols in the absence of added proton donors is a general reaction, and in other cases it competes with reduction of the carbonyl group. Under the conditions of these reactions which involve brief reaction times, neither competitive reduction of a terminal double bond nor an alkyne was observed. However, al-lenic aldehydes and ketones (13) with Li-NHs-r-butyl alcohol afford no reduction products in which the diene system survives. ... 

Treatment of enolates with (R0)2P(0)C1 also results in 0-trapping to yield the corresponding enol phosphates. Dissolving metal reduction of enol phosphates is a useful procedure for the deoxygenation of ketones with concomitant, regiospecific formation of the alkene. ... 

Semireduction of internal alkynes in the presence of a transition metal catalyst (e.g., Ni2B, Pd/C) provides disubstituted cw-alkenes. On the other hand, dissolving metal reduction of alkynes or reduction of propargylic alcohols with LiAlH4 or with Red-Al [sodium bis(2-methoxyethoxy)aluminum hydride] furnishes tran -disubstituted alkenes. ... 

The two hydrogen atoms add to the opposite faces of the alkene (i.e. anti-addition) using sodium in liquid ammonia. This dissolving metal reduction produces a solvated electron, which adds to the alkyne to produce a radical anion (bearing a negative charge and an unpaired... 

In Section 6.8, you saw the mechanism for the dissolving-metal reduction that converts an alkyne to a trans alkene. 

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