Lithium metal reduction reductive dimerization

This disilyloctadiene is obtained as a mixture of (Z,Z)- and (Z,E)-isomers by reductive dimerization of a mixture of butadiene and ClSi(CH3)3 with lithium metal (55% yield). The main by-product is l,4-bis(trimethylsilyl)-2-butene. 

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],... 

The first evidence for the self-assembly of corannulene tetraanions into a supramolecu-lar dimer, 44, was provided by studies on derivatives of 4371. Owing to their lower symmetry, dimers of monosubstituted corannulene tetraanions are expected to exhibit supramolec-ular stereochemistry, and thus exist in meso and/or as dl dimeric forms (Figure 14a, and b respectively). Reduction of tert-butylcorannulene (45) with excess lithium metal in THF-d i leads to two sets of alkyl groups in almost equal abundance, thus pointing to the presence of tightly bound dimers. Compelling evidence for dimerization comes from the successful detection of a mixed dimer between 434 and 454. In addition, diffusion... 

Further reduction of the dianions, 482- and 492-, with lithium metal gave the corresponding trianion radicals, but in contrast to the reduction of 43, the reduction could not be made to proceed further. This behavior can be explained by the tendency of 434 to dimerize, unlike the tetraanions of 48 and 49. When the reduction is performed with potassium metal, both 48 and 49 yield tetraanions (484 /4K+, 494 /4K+). The tetraanion 484 /4K+... 

The benzophenonedilithium compound 50, formed by reduction of benzophenone with lithium metal, crystallizes as a dimer (69). The four lithium atoms in the structure are divided into two different sets. The two benzophenone moieties are bridged, through the carbonyl oxygen atoms, by two symmetry-equivalent lithium atoms. Each of the two other lithiums is bonded to one phenyl ring and the ketone functionality reminiscent of that observed in benzyllithium (29), dilithiodibenzyl ketone (42), and dilithiodibenzylacetylene (49). The two different types of lithium atoms are complexed further to THE and TMEDA. 

Summary Vinylsilanes are known to react with lithium metal either to 1,2-dilithioethanes by reduction or to 1,4-dilithiobutanes by reductive dimerization. The reaction of substituted vinylsilanes with lithium metal is employed in the approach to vicinal and geminal dilithiated vinylsilanes by two consecutive additions of lithium metal and subsequent eliminations of lithium hydride. A mechanistic investigation in the reactivity of a- and (3-substituted vinylsilanes towards lithium metal discloses several new reaction pathways, whereby the choice of solvent plays an important role in apolar solvents like toluene vinyllithium compounds are obtained. Compound 14, R = Ph, which is not stable under the reaction conditions, finally affords the 1,4-dilithium compound 27. Compound 18, R = SiMes, on the other hand either adds to the starting vinylsilane (forming the monolithium compound 39) or shows an unusual dimerization to 47, which is studied in detail. 

Vinylsilanes as mono- and geminal disilyl-substituted C=C-double bonds like 1 [2] or 3 [3] afford, when brought to reaction with lithium metal in THE, the products of reductive dimerization, i. e., the 1,4-dilithiobutanes 2 and 4. This type of reaction is known as Schlenk dimerization [4]. Symmetrically tetrasilyl-substituted C=C-double bonds as in 5 on the other hand add lithium metal with formation of 1,2-dilithioethanes (Scheme 1) as stable intermediates in these reactions radical anions, like 7, can be observed, which are then reduced once again, here to the dianion 6 [5]. These two types of reaction are analogous to the reductions of the corresponding styrene derivatives... 

It has to be mentioned that 2,2-diphenylvinyllithium, which corresponds to structure 18, does not afford l,l-dilithio-2,2-diphenylethene upon treatment with lithium [18], Furthermore, two additional results are of interest when discussing the introduction of substituents into the / position of the vinylsilane (Scheme 4) Seyferth et al. reported that (Z)-propenyltrimethylsilane, (Z)-13, R = Me, was isomerized quantitatively to the corresponding ( )-isomer through the intermediate radical anion 20 when brought into contact with a catalytic amount of lithium metal in THF [19], the alkyl substituent deactivates the double bond, no further reduction was observed. Eisch and Gupta, on the other hand, showed that ( )-styryltrimethylsilane, (A )-13, R = Ph, afforded the expected products of reduction and of reductive dimerization, 21 and 22 meso compound), respectively [20],... 

Alkenes can be obtained from aldehydes or ketones on reductive dimerization by treatment with a reagent prepared from titanium(III) chloride and zinc-copper couple (or L1A1H4), or with a species of active titanium metal formed by reduction of titanium(III) chloride with potassium or lithium metal. This McMurry coupling reaction is of wide application, but in intermolecular reactions generally affords a mixture of the E- and Z-alkenes (2.99). 

Step 1 A one-electron reduction of the diene by lithium metal gives a radical anion. Step 2 One-electron reduction of this radical anion gives a monomer dianion. Step 3 Alternatively, radical coupling gives a dimer dianion. 

A wide variety of a. -unsaturated ketones have been reduced to saturated ketones, usually in good yield, by metal solutions, mainly in liquid ammonia.The reduction is applicable to compounds with any degree of substitution on the double bond. Although only 2 equiv. of these metals are required for the conversion of an enone to a saturated ketone, it is often convenient to employ the metal in excess. A suspension of lithium bronze (Li4NH3) in ether allows the employment of the metal in stoichiometric amounts. Proton donors are often employed to reduce competing side reactions, such as dimerization. The presence of proton donors in the medium may lead to the conversion of an a,p-unsaturated ketone to the saturated alcohol, but at least 4 equiv. of metal must obviously be present for this type of reduction to take place. 

Alternatively, the radical anions of pyridine and other azines are obtained on reduction by metals or other reducing agents, e.g. lithium diethylamide. Under suitable conditions the salts of the radical anions can be obtained as crystalline materials (Section 6.2.3). Alternatively, dimerization follows as an example, treatment of pyridine and other azines with 1 equiv. sodium in HMPA gives the well characterized radical anion, whereas in tetrahydrofuran the dimeric dianion is formed [125]. A later study with pyridine showed that treatment with sodium leads to a tetrahydro-4,4-bipyridine dianion (57), which is rearomatized to yield 58 in the presence of excess sodium (Scheme 40) [126], Treatment with LiNEt2 gives 2,2 -bipyridine, however, possibly because of stronger coordination with the lithium cation [127]. 2,2 -Biquinoline and 1,1 -biisoquinoline are similarly obtained [128]. 

Keywords Dilithiovinylsilanes / Lithium Hydride Elimination / Reductive Metalation / Solvent Effect / Vinyllithium Dimerization... 

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