Reduction Benkeser

The preeminence of lithium and sodium in metal reductions has been challenged by the use of calcium in amines, developed by Benkeser. In the initial work, naphthalene was reduced to an 80 20 mixture of A -octalin (522) and A ( )-octalin (523) with lithium in diethylamine-dimethylamine. Replacing lithium with calcium gave a 77 23 mixture of 522 and 523 in 92% yield. This method has come to be called the Benkeser reduction. This has become an important modification because Birch reductions with sodium are 

Just as lithium and sodium can be used in low molecular weight amines for reduction of alkynes, calcium is an effective reducing agent in amine solvents. Reaction of 2-nonyne with a calcium-methylamine-ethylenediamine mixture gave 87% yield of an 86 8 4 mixture of trans-2-nonene/traMs-3-nonene/tra 5 -4-nonene, the latter two products arising by bond isomerization. 98 

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BIRCH hOCKEL - BENKESER Reduction Reduction ol aromatics, unsaturated ketones coniugated dienes by alkali metals in liquid ammonia or amines... 

Reduction of aromatic rings with lithium or calcium " in amines (instead of ammonia—called Benkeser reduction) proceeds further and cyclohexenes are obtained. It is thus possible to reduce a benzene ring, by proper choice of reagent, so that one, two, or all three double bonds are reduced. Lithium triethylborohy-dride (LiBEtsH) has also been used, to reduce pyridine derivatives to piperidine derivatives." ... 

Polyfullerenes C qH with n reaching from 18 to 44 were observed imder Birch conditions (for n > 44 see Section 5.3.4). Isomers with more than 36 hydrogens could not be obtained with the usual Birch procedure. Much milder conditions were necessary and were found with the Benkeser reduction [43]. CgoHjg, obtained with Birch reduction, was subjected to a reduction with li in refluxing ethylene-diamine and yielded four new polyhydrofullerenes, C qH with n = 38, 40, 42 and 44 [43]. These derivatives could be separated by preparative HPLC and characterized by mass spectrometry. 

By proper choice of reaction conditions (metal, solvent, the order of addition of reagent and reactant), fused polycyclic aromatics can be converted to different partially reduced derivatives with high selectivity. When the red complex of naphthalene, for example, formed in the Na-NH3 solution, is quenched with aqueous ammonium chloride solution, 1,4-dihydronaphthalene is formed209 [Eq. (11.58)]. Reaction in the presence of an alcohol yields 1,4,5,8-tetrahydronaphthalene210 [Eq. (11.59)]. The Benkeser reduction affords further saturation to the isomeric octalins211 [Eq. (11.60)] or even to decalin212 [Eq. (11.61)] ... 

Birch and Benkeser reductions of [60]fullerene leading primarily to multiple isomers... 

Kaiser has published a review which discusses the similarities and differences between reductions by the Birch procedure and by lithium in low molecular weight amines (Benkeser reduction). He concludes that the Benkeser reduction is more powerful but less selective than the Birch reduction. 

BERNTHSEN Acndine synthesis 32 BIGINELLI Pynmidone synthesis 33 BIRCH - HUCKEL - BENKESER Reduction 34 BISCHLER Benzomarine synthesis 35 BISCHLER - MOHLAU Indole synthesis 35 BISCHLER - NAPIERALSKI Isoquinoline synthesis 36... 

Extensive investigations have been made into further methods for the reduction of aromatic rings based on the use of dissolving metals in other solvents, especially the lower molecular weight amines (the Benkeser reduction), electrochemical methods (cathodic reductions), photochemical methods and the reaction of radical anions with silylating reagents rather than proton sources. The aim of much of this work has been to produce the normal Birch products more conveniently or cheaply, but very often the outcome has been quite distinct. The alternative method may then provide access to products which are not so easily obtained by the standard metal-liquid ammonia methodology. 

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