Alkali Metal Reduction

Isobacteriochlorins, since they are tetrahydroporphyrins, can be obtained by tetrahydrogena-tion of porphyrins and dihydrogenation of chlorins. However, alkali-metal reduction of porphyrins and metalloporphyrins always gives a mixture of chlorins, bacteriochlorins or isobacteriochlorins.14 The method of choice for the preparation of pure isobacteriochlorins, e.g. 2, is the diimide reduction of zinc(II) chlorins, e.g. l.15a,b... 

Alkali metal reduction is a widely employed method for the preparation of radicals derived from various classes of conjugated compounds such as hydrocarbons, heterocycles, nitro compounds, quinones, and nitriles. For... 

In an attempt to approach a measure of tt-bonding in the Al-Al bond Uhl et al. examined the alkali metal reduction of tetrakis[bis(trimethylsilyl)methyl]dialane. Sodium or potassium reduction of [(Me3Si)2HC]2Al—Al[CH(SiMe3)2]2 in di-methoxyethane (DME) produced dark blue radical monoanions of [(Me3Si)2 HC]2A1—Al[CH(SiMe3)2]2 (Eq. 2).6... 

Due to the rapid development of material science in recent years, much interest has been focused on the investigation of silicon-silicon bond formation. We discovered the electrochemical polymerization of organohalosilanes in the mid seventies as a possible alternative to alkali metal reduction [1,2]. Since then, several papers have been published on this subject [3,4,5 and ref. therein]. 

In contrast to the conformationally mobile dianthrylethanes, the rigid cyclophane [11], with a face-to-face arrangement of the rc-layers, is electrostatically less favourable for reduction and only gives rise to a dianion upon alkali-metal reduction (Huber et al., 1983). 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

Reductive cleavage of carbon-carbon bonds upon alkali metal reduction of... 

Alkali metal reductions of metallic salts using an arene as electron carrier, lithium being the most used metallic component (Rieke method) . Although not belonging to this group of metals, magnesium-anthracene has found some apphcations in the activation of other metals . 

Acyloin-type reactions of esters provide the simplest route to 1-siloxy-l-alkoxycyclopropane [21,22] Eq. (6). The reaction of commercial 3-halopropionate with sodium (or lithium) in refluxing ether in the presence of Me3SiCl can easily be carried out on a one mole scale [21]. Cyclization of optically pure methyl 3-bromo-2-methylpropionate [23], available in both R and S form, gives a cyclopropane, which is enantiomerically pure at C-2, yet is a 1 1 diastereo-meric mixture with respect to its relative configuration at C-l Eq. (7). Reductive silylation of allyl 3-iodopropionate with zinc/copper couple provides a milder alternative to the alkali metal reduction [24] Eq. (8). 

Other methods have appeared, but have been less frequently used, e.g. chromium chloride in acetone 140- 143 (y/yfe infra) and dissolving alkali-metal reductions.144,145 Hydrogenolysis with palladium/hydrogen has also given the monoreduced cyclobutanone.99,100... 

A further complication in the reduction of aryl Group IV derivatives is the formation of biphenyl radical anion (135, 100, 97, 20) or its derivatives (32). However, this reaction is not peculiar to organometallic radical anions but appears to be quite general for alkali metal reduction of phenylated compounds (135). 

Compound I may be prepared by the reduction of carbon disulfide either electrochemically6 or by use of an alkali metal.7 In this report we present the synthesis of BEDT-TTF, and derivatives, via the procedure just described beginning with the alkali metal reduction of carbon disulfide. In our hands we find this procedure easier to set up and use than that involving the electrolytic reduction of carbon disulfide. 

Alkali metal reduction of ketones may involve the following steps ... 

Although not catalytic, this system is cyclic in the sense that the W(CO)6 may be converted back to the dianion by photolysis in the presence of NMe3 followed by alkali metal reduction. Labeling studies have shown that the carbon of the sixth carbonyl is obtained from carbon dioxide and not from decomposition of the transition metal carbonyl. 

Alkali metal reduction of dihalide precursors is shown to be a valuable route to various silole and germole dianions in solution. Crystal structure determinations for [K(18-crown-6)+h(C4Me4Si2 ] and the dimer [K4(18-crown-6)3][C4Me4Ge]2 are consistent with the presence of delocalized jr-systems and with -bonding modes for all of the potassium cations. 

These studies of reduction of benzenoid aromatics reveal that the solvent, the electrolyte cation, the current density and the water content are all important variables. In general it is important to have a rather negative potential (large TAA+) and a proton source (water) present under conditions where hydrogen evolution or attack on the solvent does not occur. Under such conditions difunctional molecules can be selectively reduced by control over the number of Faradays/mole which are passed. This kind of predictable selectivity should give the electrochemical method real advantage over alkali metal reductions and the possibility to use materials other than liquid ammonia and alkali metal is quite attractive. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

Mechanism and Stereochemistry of Alkali Metal Reductions of Cyclic and Unsaturated Ketones in Protic Solvents"... 

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