Organolithium reagents, addition structure

The asymmetric addition of organolithium reagents to arylox azolines has been used to construct highly complex polycyclic terpene structures found in natural products. For example, the asymmetric addition of vinyllithium to chiral naphthyloxazoline 3 followed by treatment of the resulting anionic intermediate with iodoethyl dioxolane 61... 

The first stable silaallene, 56, was synthesized in 1993 " " by the intramolecular attack of an organolithium reagent at the /f-carbon of a fluoroalkynylsilane (Scheme 16). Addition of two equivalents of r-butyllithium in toluene at O C to compound 54 gave intermediate 55. The a-lithiofluorosilane then eliminated lithium fluoride at room temperature to form the 1-silaallene 56, which was so sterically hindered that it did not react with ethanol even at reflux temperatures. 1-Silaallene 56 was the first, and so far the only, multiply bonded silicon species to be unreactive toward air and water. The X-ray crystal structure and NMR spectra of 56 is discussed in Sect. IVA. 

Several asymmetric 1,2-additions of various organolithium reagents (methyllithium, n-butyllithium, phenyllithium, lithioacetonitrile, lithium n-propylacetylide, and lithium (g) phenylacetylide) to aldehydes result in decent to excellent ee% (65-98%) when performed in the presence of a chiral lithium amido sulfide [e.g. (14)], 75 The chiral lithium amido sulfides invariably have exhibited higher levels of enantioselectivity compared to the structurally similar chiral lithium amido ethers and the chiral lithium amide without a chelating group. 

Some imidoyllithiums 52 derived from triphenylmethyl isocyanide (51) dissociate to produce nitriles, which react with an organolithium reagent to give the corresponding imines and, after their hydrolysis, ketones. The intermediate nitrile can be isolated working at —78°C, whereas for the isolation of imines or ketones, after addition of the organolithium, the reaction was allowed to warm to room temperature (Scheme 14)74. The structure of the imidoyllithium intermediate has been assigned by IR spectroscopy... 

The products obtained from the reaction of (chloromethyl)trimethylsilane with organolithium reagents depend very much on the structure of the lithium compound. While lithium 2,2,6,6-tetramethylpiperidide initiates an a-elimination as described above, the treatment with sec-butyllithium leads to the formation of chloro(trimethylsilyl)methyllithium (11). This reagent cyclopropanates an electron-deficient alkene through sequential Michael addition and intramolecular ring closure (MIRC reaction), for example, the formation of cyclopropane 12. 

Also, addition of small quantities of Lewis bases such as amines to alkyllithium reagents in hydrocarbons markedly affects reactivity, especially in connection with various anionic polymerization reactions. Findings such as these prompted a number of research groups in the early 1950 s to study in detail the role of Lewis bases in the structures of organolithium compounds (4, 5). In each case it was concluded that coordination complexes form when amines are added to organolithium reagents in hydrocarbons. 

Organolithium reagents are oligomers (i.e., dimers, trimers, and higher species) in nondonor solvents such as alkanes LiMe is a tetramer with a cubane structure 14.1, for example. RLi forms solvates with THE Addition of the chelating ligand Me2NCH2CH2NMe2 (TMEDA) leads to formation of a monomer, and this increases the reactivity. n-BuLi can deprotonate toluene... 

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