Protonation of anion intermediates in metal-ammonia reduction: 1,2- vs. 1,4-dihydro aromatic products

Rabideau, Peter W.; Huser, Diane L.

doi:10.1021/jo00171a022

Cited by 24 publications

(9 citation statements)

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“…protonation occurs predominantly at the central carbon atom [69] to form the nonconjugate dienes (XXIX) and (XXXIV) , which do not undergo further reduction. If protonation takes place at a terminal atom of the pentadienyl anions, overreduction of the resulting conjugate 1,3 -diene may occur, giving cyclohexenes.…”

Section: Partial Reduction Of Aromatic and Heteroaromatic Rings 431mentioning

confidence: 99%

Dissolving Metals

Yus

Foubelo

2008

Modern Reduction Methods

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17 IntroductionDissolving metals have been used extensively as reducing agents for more than a century but today they have been partially displaced from the central fi eld of organic synthesis by the use of other more selective methodologies, such as metal hydrides [1] and catalytic hydrogenations [2] . However, dissolving metals are still of interest for the selective reduction of specifi c polar functional groups (such as hindered cyclic ketones) and the reductive cleavage of some activated bonds [3] .Regarding the mode of action of electropositive metals in reduction processes, it is generally assumed that an initial single -electron transfer ( SET ) from the metal to the organic substrate takes place to give a radical anion [4] . Depending on the organic substrate and on the reaction conditions (mostly on the solvent), the highly reactive resulting radical anion can then decompose via a number of different routes. For instance, for compounds (I) with multiple C = X bonds (X = CR 2 , O, NR, S), the radical anion (II) could be protonated in the presence of a proton source (the solvent) to give a new radical (III) , which after a further SET from the metal, and further protonation of the resulting anion (IV) , would lead to the reduced product (V) . In the absence of a proton source, coupling of two radical anions (II) can occur to give fi rst the dianionic species (VI) and, after hydrolysis, compounds of type (VII) . Organic substrates (VIII) with activated bonds X − Y (X = CR 3 , OR, SR, NR 2 ; Y = CR 3 , OR, SR, NR 2 , Hal (F, Cl, Br, I)) also undergo reductive cleavage by means of electropositive metals through a SET mechanism. In this case, the initially formed radical anion (IX) suffers the activated X − Y bond scission to generate a radical (X) and an anion (XI) . Radical (X) would be converted to an anion after a new electron transfer from the metal, and fi nal protonation would yield the corresponding reduced products (XII) and (XIII) , respectively (Scheme 17.1 ).Most of the reductions by means of dissolving metals have been performed using highly electropositive metals, such as Li, Na and K, in liquid NH 3 as solvent. Less electropositive metals (Mg, Ca, Sr, Ba, Ni, Cu, Zn, In, Sn, Pb) have also been used, so by choosing the appropriate metal, solvent and cosolvent it is possible to

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Section: Partial Reduction Of Aromatic and Heteroaromatic Rings 431mentioning

confidence: 99%

Dissolving Metals

Yus

Foubelo

2008

Modern Reduction Methods

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“…The resulting solutions of solvated electrons are powerful reducing agents that may be used to perform highly selective reactions. 12 For the more reactive substrate (10), the radical anion (11) may be protonated to afford the pentadienyl radical (12), or reduced further to a dianion (13) 13 which is then protonated to afford the same pentadienyl anion (14) that would be fonned by reduction of (12). 11 In the first case electrons are added in a reversible step to fonn a radical anion (2), which is protonated and reduced further to a pentadienyl anion (4).…”

Section: The Birch Reductionmentioning

confidence: 99%

“…Our (13) (11)ẽ _ (19) (EWG = CO 2 -, C0 2 R, COR, CONR 2 , CN, Ar) understanding of the details of the reduction process as applied to benzenoid substrates (1) and (10) is summarized in Schemes 1 and 2, respectively. Protonation of anions (4) and (14) occurs predominantly at the central carbon atom14 to fonn the unconjugated 1,4-dienes (9) and (15), respectively, which are resistant to further reduction. 12 For the more reactive substrate (10), the radical anion (11) may be protonated to afford the pentadienyl radical (12), or reduced further to a dianion (13) 13 which is then protonated to afford the same pentadienyl anion (14) that would be fonned by reduction of (12).…”

Section: The Birch Reductionmentioning

confidence: 99%

Partial Reduction of Aromatic Rings by Dissolving Metals and by Other Methods

Mander

1991

Comprehensive Organic Synthesis

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“…Reduction of the parent hydrocarbon (12) or the methoxyderivative (13) proceeds very slowly (tt z 10 h) to give the expected dihydro-compounds (1 5 ) and (1 6), respectively. The alcohol (14), however, is reduced within minutes to the tetrahydro-product (18), which arises through protonation at substituted positions on the more hindered face of the aromatic ring; the reaction is therefore clearly intramolecular. It may be assumed that the diene ( 17) is an intermediate, and it was found that a sample that had been prepared independently was reduced, under the same conditions, to (1 8).-34…”

Section: Intramolecular Protonation Of Birch Intermediatesmentioning

confidence: 99%

Recent developments in the Birch reduction of aromatic compounds: applications to the synthesis of natural products

Hook¹,

Mander²

1986

Nat. Prod. Rep.

107

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Reviewing the literature published to May 1985 L 2.1 2.2 I-Naphthoic Acids 7.6 Di hydrophenan threnones 8.1 Arylsilanes 8.2 Benzylsilanes 9.1 Hydrogenolysis of Nuclear Substituents 8 Reduction of Aryl-and Benzyl-silanes 9 Reductive Fission of Hetero-substituents 9.1.1 Simple Ethers 9.1.2 Deoxygenation of Phenols 9.1.3 Alkoxy-substituted Aromatic Acids and Ketones 9.2.1 Benzylic Alcohols 9.2.2 Benzylic Acetals 9.2.3 Benzylic Amines 9.2 Hydrogenolysis of Benzylic Substituents 10 Conclusion 1 1 References * This review was prepared to mark the occasion of the 70th birthday of Professor 2-Acetylnaphthalenes Birch on 3rd August 1985.

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Protonation of anion intermediates in metal-ammonia reduction: 1,2- vs. 1,4-dihydro aromatic products

Cited by 24 publications

References 0 publications

Dissolving Metals

Dissolving Metals

Partial Reduction of Aromatic Rings by Dissolving Metals and by Other Methods

Recent developments in the Birch reduction of aromatic compounds: applications to the synthesis of natural products

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