Metal reduction polycyclic

For reviews on alkali metal reduction of polycyclic hydrocarbons see ... 

The metal reduction of the polycyclic system is usually carried out in an ether solvent and by an alkali metal at low temperature (—78 °C). When potassium metal is applied it is best to prepare a metal mirror. Sodium and lithium react, either directly in the form of a metal wire, or after treatment in an induction furnace. Cesium is prepared by thermolysis of cesium azide. It has recently been found that the application of an ultrasonic bath facilitates the reaction and avoids side reactions. The reaction can be carried out in a modified NMR tube or in an ESR cavity. Diamagnetic ions are prepared in extended NMR tubes to which the metal is extruded and sealed under vacuum. Reaction rates are difficult to compare as the electron-transfer process depends on various experimental conditions such as concentration, temperature, the presence of impurities, the solvent and the nature of the metal surface. It may take from minutes to days to form the first radical-anion the second step then follows and can sometimes be rather slow 10 13). 

The intensive electrochemical studies of polycyclic systems, especially cyclic volta-metry (CV) are now at a stage which justifies naming cyclic voltametry an electrochemical spectroscopy as was suggested by Heinze 65). Early electrochemical studies referred only to the thermodynamic parameters while CV studies provide direct insight into the kinetics of electrode reactions. These include both heterogeneous and homogeneous electron-transfer steps, as well as chemical reactions which are coupled with the electrochemical process. The kinetic analysis enables the determination of reactive intermediates in the same sense as spectroscopic methods do. As already mentioned, electron transfer processes occur in both the electrochemical and metal reduction reactions. 

Dibenzo[b,fjpentalene (indeno[2,la]indene dianion) (32 ) has attracted much interest82). Its preparation via lithium metal reduction of 3 and from 32 by deprotonation with BuLi is very easy. NMR studies ( H, 13C and 7Li) of this system yield information on the electron distribution and the ion pair structure 60,81,82). The intermediate step, i.e. the radical anion 3 affords the estimation of charge densities. The starting material, viz. 3, is a 4nn system and is an excellent example of the Randic approach of polycyclic system s8). The H NMR spectrum of 32 shows the balance... 

The polycyclic anions were first prepared by metal reduction in 1914 by Schlenk et al.5 a) and studied later by Schlenk and Bergmann 5 b). This class of conjugated anions opened a new era in carbanion chemistry by pointing out the electron transfer process as a source for charged species. The mechanism of the metal reduction of polycyclic hydrocarbons has been investigated and is well established 1,215 18-68>. The addition of two electrons to the fully conjugated (4n + 2)n-molecules yields 4mt paratropic systems 2°. 137 139>. The chemistry of this reaction is simple, with electrons initially on the alkali metal going to 7t-molecular orbitals associated with the aromatic hydrocarbon molecule (Eq. 13). 

The dissolving metal reduction of mono- or polycyclic 4-piperidinones affords the corresponding pipcridinols generally with high selectivity. A series of monocyclic 4-piperidinones has been reduced stereoselectively with lithium/ammonia, e.g., l,3-dimethyl-4-piperidinone affords the corresponding 4-piperidinol with high equatorioselectivity (98 2). 

Studies of metal reduction of polycyclic aromatic hydrocarbons (PAHs) [1] were reported as early as 1867 by Berthelot [2], where he described the fusion of metallic potassium with naphthalene 1. Studies in solution of electron transfer from Na and K to PAHs started as early as 1913 with Schlenk et al. [3] who reported that when anthracene reacted with the alkali metal in ether, 1 1 and 2 1 metal anthracene adducts were obtained. Not understanding what the 1 1 adduct really was, it was termed radical and later on it was suggested by Hvickel that the radical is a singly charged species [4],... 

Reduction of aromatic compounds to dihydro derivatives by dissolved metals in liquid ammonia (Birch reduction) is one of the fundamental reactions in organic chemistry308. When benzene derivatives are subjected to this reduction, cyclohexa-1,4-dienes are formed. The 1,4-dienes obtained from the reduction isomerize to more useful 1,3-dienes under protic conditions. A number of syntheses of natural products have been devised where the Birch reduction of a benzenoid compound to a cyclohex-1,3-diene and converting this intermediate in Diels-Alder fasion to polycyclic products is involved (equation 186)308f h. 

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)] ... 

Benzo[ >]thiophene dianion (173) has been prepared by reduction of benzo[ >J thiophene with sodium metal at - 78°C in [2H8]THF. The H and 13C NMR spectra of the purple solution obtained prove that it is the dianion and not a radical anion. This is the first example of a sulfur-containing (4n)Tr polycyclic dianion. The dianion undergoes oxidation to benzo[ ]thiophene with oxygen (85CC1033). 

Reduction of the cyclic allene 72 by sodium metal affords the polycyclic allyl anion 37, which was crystallized and characterized. Reaction with methyl iodide resulted in the formation of the methylated compounds 38 and 39 (Scheme 2) <1996OM3480>. 

The reduction process of polycycles by lithium metal converts the neutral atoms to anions. The electron transfer is best achieved in ethereal solvents. This enables the stabilization of the lithium cation by coordination to the oxygen atoms of the solvent. The hydrocarbon anion and the cation are linked together by electrostatic forces in which the solvent molecules are also involved, therefore the ion-solvation equilibrium should be considered8. The limiting cases in this equilibrium are free ions and contact ion-pairs (CIP), and in between there are several forms of solvent separated ion-pairs (SSIP)9. In reality, anionic species of aromatic hydrocarbons in ethereal solvents exist between CIP and SSIP. Four major factors influence the ion-solvation equilibrium of lithium-reduced 7T-conjugated hydrocarbons, as observed by H and 7Li NMR spectroscopies8,10. 

Addition of alkyllithium compounds at the ort/zo-position of oxazolines is possible with het-eroarenes as well as naphthalenes 24 (benzene derivatives usually tend to ort/zo-metallations) [12]. After reductive cleavage of the auxiliary, enantiopure aldehydes 26 are obtained, which have found wide application as versatile chiral precursors for complex polycyclic natural products. 

The first chapter discusses the concept of aromaticity, after which there is a description of aromatic substitution reactions. Chapters covering the chemistry of the major functionalized derivatives of benzene follow. A chapter on the use of metals in aromatic chemistry discusses not only the chemistry of Grignard reagents and aryllithium compounds but also the more recent uses of transition metals in the synthesis of aromatic compounds. The penultimate chapter discusses the oxidation and reduction of the benzene ring and the text concludes with the chemistry of some polycyclic compounds. 

The metal-NH3 and metal-amine reductions of acetophenone and acetyl derivatives of polycyclic aromatics are complex and afford primarily mixtures of Birch reduction products. In some cases a ketonic carbonyl survives the reduction, and in some cases it is reduced to the corresponding alcohol. ... 

More work with other aromatic compounds will be necessary to verify this conclusion. However, the results for anthracene certainly suggest that the results obtained in the reaction are strongly dependent upon the aromatic substrate. The results obtained thus far also suggest that the reduction and subsequent alkylation of large polycyclic molecules, e.g., compounds such as pyrene, are not the most critical factors for the formation of soluble alkylation products. Studies of the molecular weight distributions and spectroscopic properties of the alkylation products obtained with different metals and different electron transfer agents are underway to resolve this issue. 

The reduction of an aryl-substituted alkene or alkyne by Li metal to give a dilithium dianion is similar to the reduction of a polycyclic aromatic compound. The products are represented by localized structures in which a C—C bond is reduced, e.g., in stilbene ... 

In the late 20 s, Schlenck and his group demonstrated that the reduction of rc-conjugat-ed systems can be carried out by alkali metals and that the reaction affords dianions 5>. It was also realized that a one electron transfer process occurs in an alternating fashion 1,5) to form radical anions and polyanions. Ever since, this reaction has become the main process of preparation of polycyclic anions 10 13), (Eq. 1) ... 

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