Radical anions lithium

This technique is especially useful for the preparation of a-lithio ethers, sulfides, and silanes.36 The lithium radical anions of naphthalene, 4, 4/-di-f-butyldiphenyl (DTBB) or dimethylaminonaphthalene (LDMAN) are used as the reducing agent. 

Figure 1. Lithium radical anions see Table 1 for refs. Lithium radical anions of (A) biphenylene, (B) fluorene, (C) perylene, (D) heptalene and (E) 4,4 -bipyridine.

A powerful access to lithiiun carbanions is from phenylthio ethers and acetals using lithium radical anions such as lithium naphthalenide (42a, LN), S- N,N-dimethylamino)-naphthalenide (42b, LDMAN) or 4,4 -di(ferf-butyl)biphenyle-nide (42c, LDBB) [Eq. (15)] [30-32]. The reaction is not usually stereospecific since a configurationally labile radical 43 is involved [Eq. (15)] [33]. 

The lithium radical anion of naphthalene or dimethylaminonaphthalene (LDMAN) is used as the reducing agent. This method can also be applied to prepare unsub-... 

Scheme 1-51. Biphenyl formation by accidental oxidation of phenyllithium and of the biphenyl/lithium "radical anion" (74).

Several alternative methods followed this early work. In one, aromati2ation is effected by treating the ketal of androstadienedione with the radical anion obtained from lithium and diphenyl in refluxing tetrahydrofuran. Diphenylmethane is added to quench the methyllithium produced from the... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

Interest in the synthesis of 19-norsteroids as orally active progestins prompted efforts to remove the C19 angular methyl substituent of readily available steroid precursors. Industrial applications include the direct conversion of androsta-l,4-diene-3,17-dione [897-06-3] (92) to estrone [53-16-7] (26) by thermolysis in mineral oil at about 500°C (136), and reductive elimination of the angular methyl group of the 17-ketal of the dione [2398-63-2] (93) with lithium biphenyl radical anion to form the 17-ketal of estrone [900-83-4] (94) (137). 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

Reductive dunenzation to form fluorinated benzopinacols proceeds m the partly fluormated case either with zinc or by photolysis but is not observed with perfluorobenzophenone [651 (equation 53). Trifluoroacetophenone is reduced electrochemically in dimethylformamide to a stable radical anion, which, m the presence ot lithium ion, rapidly dunerizes to pinacol in higher yield than that available by photoreduction [66] (equation 54)... 

Sulfur is known to be easily reducible in nonaqueous solvents and its reduction products exist at various levels of reduction of polysulfide radical anions (S . ) and dianions (Sm2 ) 173], Recently Be-senhard and co-workers [74] have examined the effect of the addition of polysulfide to LiC104-PC. Lithium is cycled on an Ni substrate with Qc=2.7 C cm 2 and cycling currents of 1 mA cm. The cycling efficiency in PC with polysulfide is higher than that without an additive. The lithium deposition morphology is compact and smooth in PC with added polysulfide, whereas it is dendritic in PC alone. 

To a flame-dried, argon-purged flask, equipped with a glass-coated jstirring bar, was added THF (10 ml) and lithium ribbon (5.8 mg atom). The mixture was cooled to between -45 and -55 C (hexan-l-ol/dry ice), and l-(dimethylamino)naphthalene (5.1 mmol) was added slowly. The Idark-green colour of the radical anion appeared within 10 min, and Iformation of LDMAN was complete after 3.5 h of rapid stirring. 

More recently, 84 may have been identified by ESR spectroscopy of solutions of Li2S ( >6) in DMF at 303 K. The lithium polysulfide was prepared from the elements in liquid ammonia. These polysulfide solutions also contain the trisulfide radical anion ( 2.0290) but at high sulfur contents a second radical at g=2.031 (Lorentzian lineshape) was formed which was assumed to be 84 generated by dissociation of octasulfide dianions see Eq. (32) [137],... 

For some halides, it is advantageous to use finely powdered lithium and a catalytic amount of an aromatic hydrocarbon, usually naphthalene or 4,4 -di- -bu(ylbiphcnyl (DTBB).28 These reaction conditions involve either radical anions or dianions generated by reduction of the aromatic ring (see Section 5.6.1.2), which then convert the halide to a radical anion. Several useful functionalized lithium reagents have been prepared by this method. In the third example below, the reagent is trapped in situ by reaction with benzaldehyde. 

Coincidentally, another research group was working on the same problem. This team, lead by Porschke, used lithium in combination with TMEDA to reduce the Al-Al bond in [(Me3Si)2HC]2Al—Al[CH(SiMe3)2]2 and afforded an Al-Al one-electron ir-bond in the radical anion [(Me3Si)2HC]2Al—Al[CH(SiMe3)2]2 (Eq. 3).7... 

As the final example in this section, a Li-mediated carboaddition/carbocycliza-tion process will be described. Thus, Cohen and coworkers observed a 5-e%o-trig-cy-clization by reaction of the lithium compound 2-349 and a-methyl styrene 2-350 to give 2-352 via 2-351 (Scheme 2.82). Quenching of 2-352 with methanol then led to the final product 2-353 [189]. In this process, 2-349 is obtained by a reductive lithia-tion of the corresponding phenyl thioether 2-348 with the radical anion lithium 1-(dimethylamino)naphthalenide (LDMAN) (2-354). Instead of the homoallylic substance 2-348, bishomoallylthioesters can also be used to provide substituted six-membered ring compounds. 

A lithium atom donates an electron to The radical anion acts as the 7i bond of the alkyne. An electron a base and removes a... 

Therefore, it has been considered that the formation of the dimer involves a mechanism different to the simple head-to-head radical coupling of the parent monomer. As suggested by the authors, it is likely that the overall mechanistic sequence is initiated by the radical-anion 472 of compound 469 formed by a single electron transfer (SET) process, which is the first stage of the bromine-lithium exchange (Scheme 68) [128],... 

Figure 6. Comparison of the TR3 spectrum of Ru(bpy)s2 (top) to the c.w. resonance Raman spectrum of bipyridine radical anion (lithium reduction) (bottom), (Reproduced from Ref. 19c. Copyright 1981, American Chemical Society.)...

Whitlock et al.14 discovered a reductive cyclization of enediynes promoted by lithium naphthalenide that provides substituted fulvenes and suggested a dianionic mechanism (Scheme 6). However, even now it is still unclear whether the enediyne dianion is indeed the cyclizing species or whether the initially formed acyclic radical-anion cyclizes first to give a fulvene radical-anion which is further reduced by lithium to give the cyclic dianion. 

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