Lithium-hydrogen exchange, and

In the case of bromides, lithium-bromine exchange in normally preferred to lithium-hydrogen exchange and alternative reactions ensue ... 

In the same way methyllithium in diethyl ether and butyllithium in hexane give predominantly lithium-hydrogen exchange and very little addition. This contrasts strongly with the reaction of Grignard reagents with cyclopropanes, which gives efficient addition vide supra). 

The reaction was also achieved using phenyllilhium, and in one case an excess of this was used leading to an intermediate lithioeyclopropene by lithium-hydrogen exchange and this was quenched in situ with iodomethane giving, for example 1,2-dimethylcyclopropene from 2-iTiethylallyl chloride. ... 

Pentafluoropropenyllithium could be prepared by lithium-hydrogen exchange of pentafluoropropene with -butyllithium in ether at -78°C [130], but a bulky base such as ferf-butyllithium or LDA had to be employed for the preparation of 2-pentafluoropropenyllithium from the corresponding 2-hydro-pentafluoropropene in ether and pentane solution at -78 °C [131] (Scheme 50). 

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) ... 

In pyrimidine, the most likely metallation site for lithiation is C-5 (cf. base-catalyzed hydrogen exchange, Section 2), but even two methoxy groups in the 4- and 6-positions failed to allow lithium-hydrogen exchange at -40°C in this system (65JCS6695). At lower temperature (<-70°C), however, 5-pyrimidinyllithium derivatives can be prepared by... 

Lithium-hydrogen exchange between n-BujSnH and an aminolithium gives tri-n-butyltin lithium in high yield ... 

Organolithium reagents undergo addition to the 7t-bond of cyclopropenes, but the reaction competes with lithium-hydrogen exchange at the vinylic position and rearrangement to a methylenecyclopropane by removal of an allylic exocyclic hydrogen. 

The treatment of both thiazole and benzothiazole with an appropriate base such as butyl- or phenyllithium leads to the exclusive deprotonation at C-2. The progress of the lithiation can be followed by deuteration of the reaction mixture. These studies indicate that the lithium-hydrogen exchange occurs instantaneously <88BCJ3637>. When the 2-position is blocked, deprotonation occurs selectively at C-5. However if the substituent at C-2 is an alkyl group, the kinetic acidities of the protons at the a-position and at the 5-position are similar (see Section 3.06.7.3). The reaction of... 

Benzyne formation and their reactions are not limited to simple benzene derivatives. Heterocyclic mole-ules can also be used for benzene reactions. One important use of benzyne derivatives is in the Diels-Alder caction (see sec. 11.4.A). When 3-bromo-2-methoxypyridine (394) was treated with EDA at -78°C, for. ample, lithium-hydrogen exchange occurred to give 395. Loss of LiBr led to benzyne (396), which reacted with furan to give the Diels-Alder product (397) in 74% yield. ... 

Similar considerations apply to the degenerate lithium-hydrogen exchange reactions across the entire first row of the periodic table, including the reaction of methane with methyllithium. The latter reaction is a model for the metalations of hydrocarbons by alkyllithiums. This type of reaction was modeled for substituted benzenes and toluenes by reaction with lithium hydride ... 

Kinetic studies of base-catalysed hydrogen exchange have been carried out by Roberts, by Shatenshtein, and by Streitweiser and their coworkers. In earlier work, potassium amide was used as base in liquid ammonia as solvent, whereas later workers used lithium and caesium cyclohexylamides in cyclohexylamine. The reaction can be represented by equilibria (239) and (240)... 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

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... 

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