Of benzyllithium

In regard to this latter point, Cubbon and Margerison noted (31) that adding n-butyllithium to toluene led to the formation of solutions which "developed a yellow-orange color.") If the spectrum of benzyllithium in toluene in the presence of anisole resembles that in benzene where the X max is reported (32,33) to be 292 nm, the decay in absorbance with time noted (13) at 330 nm may be attributable to transmetallation involving toluene rather them the foregoing aromatic ethers. 

The structures of benzylic carbanions have been extensively studied and a summary of the more important recent results is presented in Table 15. The H-NMR spectrum of benzyl potassium 202) in THF is upfield of that of benzyllithium 203) as expected on the basis of the relative polarizing powers of the two cations. The spectrum of oligomeric poly(styryl)lithium which is qualitatively similar, is shifted upfield on... 

According to the principle of least nuclear motion [45] aromatic deprotonation should be faster than benzylic metalation, because the benzylic carbanion is expected to rehybridize slightly toward sp2 to achieve stabilization by conjugation with the aromatic n system. This is, in fact, often observed [217, 401, 423-425], but with some substrates benzylic metalation can effectively compete with aromatic metalation[181, 425, 426] (Scheme 5.47). Thus, treatment of toluene with BuLi/TMEDA or BuLi/DABCO at 80 °C for 0.5 h or with BuLi/KOtBu in Et20 at -20 °C for 4 h leads to clean formation of benzyllithium [85, 427, 428], The kinetic preference for aromatic deprotonation, because of the principle of least nuclear motion, thus seems to be too weak to control the regioselectivity of deprotonations in all instances. 

Krief and coworkers have also shown that vinyl sulphides are useful traps for the intramolecular carbolithiation reactions. Interestingly, as the sulphide substituent can be reductively removed, the cyclization reactions of benzyllithiums derived from 73 and 75 are synthetically equivalents to cyclizations onto disubstituted double bonds, giving rise by complementary routes to compound 74 (Scheme 20)46. Like the corresponding carbolithiation reactions onto monosubstituted alkenes they are highly stereoselective but dependent on the solvent used, i.e. the derivatives in which the methyl- and the phenylthio groups are cis (THF) or trans (pentane) one to the other are selectively formed. 

In recent years a number of dilithio compounds has been synthesized which give rise to more complicated spin systems of the AA XX - and ABXY-type [94,95]. Here the simple rule derived for the (RLi) -type systems breaks down. Another important observation in this context is the peculiar coupling of 3-7 Hz found for a number of benzyllithium... 

The halogen-metal exchange of secondary alkyl iodides leads mainly to hydrocarbons containing Wurtz-type coupling or elimination products [44], except in the case of benzyllithium derivatives which are easily generated from benzyl selenides [45]. 

The preparation of benzyllithium from benzyl halides and alkyllithiums is not feasible because the benzyllithium initially formed reacts with the starting benzyl halides, producing 1,2-diphenylethane. Metalation of toluene with n-BuLi in the presence of TMEDA at 30 °C results in a 92 8 ratio of benzyllithium and ring metalated products. Metalation of toluene with n-BuLi in the presence of potassium rert-butoxide, and treatment of the resultant organopotassium compound with lithium bromide, affords pure benzyllithium in 89% yield. Alternatively, benzyllithiums are accessible by cleavage of alkyl benzyl ethers with lithium metal. " ... 

A series of benzyllithiums bearing a phenylthio (Scheme 14, entries a and d, Scheme IS see Scheme 43) 14.126.151-155 g thiothiazolino (Scheme 14, entry or a dithiocarbamato group (Scheme... 

Compounds of Group 1. - (6Li, 15N) and (6Li, 13C) couplings were observed for mixed complexes formed between LiCH2CN and chiral lithium amides (1H, 6Li, 13C, 15N data).1 7Li and 31P H) HMQC experiments were used to assign the structures of benzyllithium complexes of /V-methyl-/V-ben-zylphosphinamide, e.g. (I).2 111 and 13C NMR and 13C-111 correlation spectra were used to confirm the presence of a C-Si-Ni-Li 4-membered heterocycle in [benzylbis(dimethylamino)-methylsilyl-K2-C,7V](7V, N, N, N -tetramethylenedia-mine-K2-iV,/V)lithium(I).3... 

Optimum Conditions for Preparing Benzyllithium from Toluene. Both the TMEDA and TED complexes of benzyllithium were investigated. Toluene metalation proceeds much faster than does benzene metalation under similar conditions. The benzyllithium complexes were more soluble in hydrocarbon solvents than were the corresponding phenyllithium complexes. This method of preparation of benzyllithium is the most convenient of the few literature procedures available. Other procedures described are the cleavage of benzyl methyl ether with lithium... 

The toluene-metalation system in this study differs from that of Broaddus in that pure toluene was used as the solvent a RLi to TMEDA ratio of two was used. Analyses were usually run after complete reaction of the n-butyllithium unless otherwise noted. Higher reaction temperatures were used, and the final concentration of benzyllithium in solution was higher. 

The equilibration of the ring anions to the benzyl anion is the probable explanation, especially considering the recent work of Gau and our observation of an equilibration between m-xylene and toluene in the presence of (benzyllithium)2 TMEDA (20). (In fact, this type of system might be the basis of another route for a hydrocarbon acidity scale.) The more rapid disappearance of the ortho isomer, compared with the meta, may be the result of a possible intramolecular route for conversion to the benzyl anion. The meta and para isomers probably change to the benzyl anion by an intermolecular route that would be slower and agree with what was observed. Although complete resolution by GLC of the para and meta isomers was not done in this study, the para disappeared faster than the meta, but much slower than the ortho. In other time studies on the disappearance of the meta isomer at room temperature, about half the initial amount of this isomer was gone in three days and all of it in two to three months. 

Synthetic Application of (Benzyllithium —TMED A and Benzyl-lithium—TED. All the synthetic application studies were run with an aged (benzyllithium)2-TMEDA toluene solution that had 0.8% of the total organolithium compounds present as the tolyllithium isomers (only meta present) or with benzyllithium-TED crystalline complex free of ring isomers. 

The solutions of (benzyllithium )2-TMEDA were much more convenient to use than the solid, relatively insoluble benzyllithium-TED complex. The reaction yields vary between these two complexes. Whether this is a function of the different solvent systems used for each, lack of optimization, or the actual tertiary diamine present was not examined, however. 

The reaction of toluene and biphenyllithium in tetrahydrofuran gives about a 1% yield of benzyllithium (23). By contrast, yields of 80% or greater are easily obtained via the metalation reaction... 

The discovery of the powerful metalating agent, n-BuLi-N,N,N, N -tetramethylethylenediamine, opened a new chapter in anionic grafting. This complex has been reported to metalate toluene and benzene within a few minutes to give quantitative yields of benzyllithium and phenyl-lithium, respectively (4). It also has been reported to polylithiate aromatic compounds (24, 25). 

The principal monotrimethylsilyltoluene is the meta isomer. This agrees with the results of earlier experiments which find mainly m-lithio-toluene in the ring-lithiated byproducts in the synthesis of benzyllithium by lithiation of toluene with n-butyllithium-TMEDA (11, 12). On the basis of the current view that metalation of aromatic compounds proceeds... 

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