Organolithium compounds, electron-transfer

Organomagnesium and organolithium compounds are strongly basic and nucleophilic. Despite their potential to react as nucleophiles in SN2 substitution reactions, this reaction is of limited utility in synthesis. One limitation on alkylation reactions is competition from electron transfer processes, which can lead to radical reactions. Methyl and other primary iodides usually give the best results in alkylation reactions. 

The most important application of organolithium reagents is their nucleophilic addition to carbonyl compounds. One of the simplest cases would be the reaction with the molecule CO itself, whose products are stable at room temperature. Recently, it was shown that a variety of RLi species are able to react with CO or f-BuNC in a newly developed liquid xenon (LXe) cell . LXe was used as reaction medium because it suppresses electron-transfer reactions, which are known to complicate the reaction . In this way the carbonyllithium and acyllithium compounds, as well as the corresponding isolobal isonitrile products, could be characterised by IR spectroscopy for the first time. 

The presence of organolithium compounds in etheric solvents at temperatures above 0°C may lead to extensive decomposition of the solvent and solute a slow electron transfer side reaction of lithium naphthalene or sodium naphthalene with the THE solvent (equation 5) has been reported . The three isomeric forms of BuLi were shown to induce extensive decomposition of THE. The main path for this process is metallation at position 2 of THE, leading to ring opening and elimination of ethylene. An alternative path is proton abstraction at position 3, followed by ring opening. The presence of additives such as (—)-sparteine (24), DMPU (25), TMEDA and especially HMPA does not prevent decomposition but strongly affects the reaction path. ... 

Retention in the reactions of 15 is established both from presumed retention in the Sn-Li exchange step of a stannylation-destannylation sequence and by evidence that the s-BuLi-(-)-sparteine complex used to make the organolithium reliably removes the pro-/ proton adjacent to a carbamate (see below for crystallographic evidence involving a similar compound).11 The stereochemistry of the products 16, almost all formed essentially in enantiomerically pure form, was proved for the C02 adduct and the Mel adduct by comparison with known compounds. The only electrophiles for which incomplete retention of stereochemistry has been observed are the benzylic or allylic halides. These probably react in part by single electron transfer SE1 mechanisms, rather than by partial SE2inv.15 For example, 15 reacts with allyl bromide to give 16 (E = allyl) with only 42% ee. 

In view of the trend to more controlled and stereoselective reactions with readily available, less expensive and environmentally non-problematic reagents, the light-induced inner-sphere electron transfer between M-C bonds of less polar co-ordinating organometallics (Zn, Al) and the organic substrate seems to be a particularly attractive alternative to thermal reactions from organolithium or -magnesium compounds. 

Epoxides can be reduced under Birch conditions by solvated electrons [34] and by arene radical anions [35] without the presence of low-valent metal complexes. In both cases )5-lithiumoxy organolithium compounds are formed after further reduction with a second equivalent of the electron transfer reagent. These species are stable enough to be trapped by electrophiles at low temperatures. They do not show the typical reactivity patterns of radicals. Thus, these transformations will not be dealt with here in detail. 

ORGANOLITHIUM COMPOUNDS AS ELECTRON-TRANSFER AGENTS IN CARRON-CARRON ROND FORMATION. An instructive example of how RLi, rather than lithium metal, can act as an electron source (cf. Figure 2) is the cyclization of 2-(2-biphenylyl)-l,l-diphenylethene (34) by n-butyllithium. To enhance electron transfer, the chelating strong-donor TMEDA was employed. The observed cyclization to 9-(diphenylmethyl)fluorene (35) can best be explained by electron transfer (Scheme XIII).2... 

Reactions (1) and (2) involve single electron transfers from the metal to either the ketone or the halide. In the latter case this leads to the formation of what the authors named the precursors of the organolithium compound or transitory species on the metal surface. 

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