Lithium alkyls, exchange reactions

Pentafluorophenyllithium can be readily prepared by direct reaction of penta-fluorophenyl halides with lithium amalgam [193,194] or lithium-hydrogen and lithium-halogen exchange reactions of pentafluorophenyl halides with alkyl-lithiums [195-205] (Scheme 70). 

Reductive Metalation. The powerful reductive nature of this reagent makes it an important tool for lithium-heteroatom exchange reactions. Thus, it was established early on that (phenylthio)alkanes can be converted into their requisite alkyl-lithium species. This has become the method of choice over generation by lithium metal alone. The resultant alkyllithium species can either be quenched with a proton source (eq 1), or intercepted with an electrophile. This has subsequently evolved into a powerful technique, since the reaction is general for all chalcogens (eq 2f and halides (eq 3). ... 

In a further effort to identify the active intermediate that initiates the reaction, they tested the effect of a few possible ingredients on the production of EMC based on the knowledge about the chemical composition of the SEI on carbonaceous anodes. These model compounds included Li2C03, LiOCHs, and LiOH, while lithium alkyl carbonate was not tested due to its instability and therefore rare avail-ability. The results unequivocally showed that LiOCHs effectively catalyzes the ester exchange. 

As pointed out earlier (Sect. 7.1), alkyl lithium reagents induce metal-hydrogen exchange reactions. This possibility was investigated first by Paddock and coworkers [266,227]. They have found that the anion generated by the reaction of methylphosphazene with n-butyl lithium interacts with electrophiles such... 

Clayden and Julia reported the 1,3-C,H insertion reaction of lithium carbenoid (69) derived from a primary alkyl chloride (68) by H-Li exchange reaction (eqnation 19). Treatment of 68 with a mixture of n-BuLi and tert-BnOK gave three prodncts. These... 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

Compared to the lithium alkyls, the carbon-metal bond in the corresponding sodium and potassium compounds is more polar and thus the lower alkyl derivatives are no longer soluble in hydrocarbons nor are they volatile (262). Therefore, little has been done to elucidate any exchange reactions in which they might participate. 

Seitz, L. M., and Brown, T. L., Organometallic exchange reactions. VII. Distribution of phenyl and alkyl groups on lithium and lithium-magnesium species in ether, J. Am. Chem. Soc., 89, 1607 (1967). 

Bailey, W. F. Brubaker, J. D. Jordan, K. P. Effect of solvent and temperature on the lithium-iodine exchange of primary alkyl iodides reaction of tBuLi with 1-iodooctane in heptane-ether mixtures./. Organomet. Chem. 

Drandarov, K. Hesse, M. Lithium and proton templated co-polyazamacrolactamization, new general routes to macrocyclic polyamines. Tetrahedron Lett. 2002, 43, 7213-7216. Murahashi, S.-L Yoshimura, N. Tsumiyama, T. Kojima, T. Catalytic alkyl group exchange reaction of primary and secondary amines. 

Studies on other exchange reactions of the tetraalkyl anions are fairly limited and are summarized in Table V. Williams and Brown have provided the only quantitative data and have proposed mechanisms for both lithium and alkyl exchange (153). They have suggested that the lithium exchange between (LiR)4 and LiMR4 proceeds with the rate-determining step... 

Lithium-halogen exchange provides a versatile and often more facile alternative to lithium-hydrogen exchange. In particular, 1-bromocyclopropenes such as (77, R = Me) react very readily with lithium alkyls at 0 °C and below. The bromocyclo-propene may in principle be obtained by dehydrobromination of a dibromocyclo-propane, but an attractive alternative is the reaction of a trihalocyelopropane with two equivalents of methyl lithium, followed by trapping with an electrophile 39) ... 

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