Butyllithium tetramers

Figure 7 Spin multiplets of the lithiated carbon in the static O fi) and fluxional (right) rert-butyllithium tetramer showing the change in C. Li coupling and multiplicity the coupling constants are 5.4 and 4.1 Hz, respectively. (Reprinted with permission from ref. [75]. Copyright (1986) American Chemical Society)...

Figure 13 Typical spin multiplets of the lithiated carbon from simple alkyllithium aggregates showing C, Li coupling (a) phenyllithium monomer (—100°C) (b) n-butyllithium dimer (—100°C) (c) /ert-butyllithium tetramer (—88°C) the coupling constants are 14.8, 7.9, and S.4Hz, respectively...

Methoxyallene (21) is stirred together with TMEDA and w-BuLi. Afterwards (li )-(+)-camphor (20) is added to this mixture. TMEDA is used as an additive to form more-reactive butyllithium dimers instead of butyllithium tetramers. 

The data reported for a number of organolithium compounds in various states are given in Tables VI (13, 30, 32, 33, 34) and VII. The mass effect observed upon substitution of Li for Li is of particular interest, in that its magnitude is a rough measure of the extent to which lithium motion is involved in the normal mode. If only lithium motion were involved, the ratio of frequencies would be vu>lvu- = (7/6) , or 1.08. It is of interest that this ratio is observed for the aforementioned symmetric mode of the tert-butyllithium tetramer. This vibration can therefore be visualized as a symmetric motion of the lithium atoms along the local threefold axes of the tetramer. ... 

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

The size of the ligand is crucial in deciding the degree of association, as shown by the very similar structural features of methyl- and "butyllithium derivatives. With hexameric structures in cyclohexane, the addition of diethyl ether affords tetramers in both cases. Further demonstrating the importance of the donor, the two-dimensional phenyl ligand also adopts a tetrameric structure in diethyl ether [Li(OEt2)Ph]4, which upon addition of the bidentate... 

For the anionic polymerization of methacrylonitrile (MAN), many initiators have been developed, which include alkali-metal alkyls such as butyllithium [42], triphenylmethylsodium [43], phenylisopropylpotassium [43], the disodium salt of living a-methylstyrene tetramer [44], alkali-metal amides [45], alkoxides [46], and hydroxide [47], alkali metal in liquid NH3 [48], quaternary ammonium hydroxide [49], and a silyl ketene acetal coupled with nucleophilic or Lewis acidic catalysts [50]. However, only a single example of the synthesis of PMAN with narrow molecular-weight distribution can be cited, and the reported number-average molecular weights were much higher than those calculated from the stoichiometry of the butyllithium initiator [42]. 

In solution lithium alkyls are extensively associated especially in non-polar solvents. Ethyllithium in benzene solution exists largely as a hexamer (9, 43) in the concentration range down to 0.1 molar and there is no evidence for a trend with concentration so presumably the hexamers persist to even lower concentrations. Indeed even in the gas phase at high dilution it exists as hexamer and tetramer in almost equal amounts (3). In a similar way n-butyllithium in benzene or cyclohexane is predominantly hexameric (62, 122). t-Butyl-lithium however is mostly tetrameric in benzene or hexane (115). In ether solution both lithium phenyl and lithium benzyl exist as dimers (122) and it has been suggested that butyllithium behaves similarly in ether (15) although this does not agree with earlier cryoscopic measurements (122). It is however certain that more strongly basic ethers cause extensive breakdown of the structure. 

This has been studied much less frequently and appears to be a rather more complex reaction. The first results obtained, for the butyl-lithium, styrene reaction in benzene have already been described. In a similar way the addition of butyllithium to 1,1-diphenylethylene shows identical kinetic behaviour in benzene (26). Even the proton extraction reaction with fluorene shows the typical one-sixth order in butyllithium (27). It appears therefore that in benzene solution at least, lithium alkyls react via a small equilibrium concentration of unassociated alkyl. This will of course not be true for reactions with polar molecules for reasons which will be apparent later. No definite information can be obtained on the dissociation process. It is possible that the hexamer dissociates completely on removal of one molecule or that a whole series of penta-mers, tetramers etc. exist in equilibrium. As long as equilibrium is maintained, the hexamer is the major species present and only monomeric butyllithium is reactive, the reaction order will be one-sixth. A plausible... 

A change from an aliphatic or aromatic hydrocarbon solvent (cyclohexane, benzene) to a polar solvent (THF) leads to a large increase in trans-1,4 and 3,4 microstructure (58). Organolithium compounds are highly associated sec-butyllithium in benzene or cyclohexane exists as a tetramer, and -butyllithium as a hexamer (64,65). This association in hydrocarbon solvents results pardy in the slow initiation observed between some organolitbiirms and isoprene (66). At low initiator concentrations, the polymerization rate of isoprene in alkyUithium polymerization is proportional to monomer and alkyUithium concentrations (67). 3,4-Polyisoprenes are obtained by modification of the lithium polymerization with ethers, such as the dialky] ethers of ethylene glycol or tertiary amines (68,69). 

Seebach and coworkers18 have reported that a combination of integration and NMR line shape fitting establishes that in THF-d8, w-butyllithium-6Li, 12, consists of an equilibrium between interconverting dimers D and tetramers T (equation 74) with K = (D)2 / T) being 2.6 x IQ-2 M at 185 K. 

With increasing temperature above 185 K and with increasing concentration of the n-butyllithium, the authors reported progressive averaging of the 13C—6Li coupling constant of dimers as well as of the resonances of dimer with tetramer. A line shape analysis of the 13C NMR of lithium bound carbon, using our PI method, best took account of the interconversion of tetramers with dimers via a degenerate process (equation 75),... 

Thomas and coworkers showed that f-butyllithium in pentane consists exclusively of cubic tetramers. Below 251 K, the 13C NMR of 6Li bound carbon consists of a 1 3 6 7 6 3 1 multiplet with 7(13C,6 Li) = 5.1 Hz, the familiar signature of a cubic tetramer24. On increasing the temperature above 251 K, this resonance broadens and resolves again by 268 K into a nonet with splitting of 4.1 Hz due to fast intraaggregate C—Li exchange. Carbon-13 NMR line shape analysis established AH = 25 1 kcalmol-1 and AS1 = 44 eu. 

Synthesis of unsubstituted mercuracarborands, as illustrated in Scheme 1, is a two-step process involving deprotonation of the acidic CH vertices of 1,2-carborane 7 followed by treatment with the appropriate mercury salt <1994JA7142, 1997ACR267>. Thus, -butyllithium efficiently deprotonates 1,2-carborane 7 giving doso-X.Z-VXz-1,2-C2BioHio 8, the pivotal intermediate. If treated with mercury acetate, neutral trimer 5 is formed. Using mercury chloride or bromide, however, leads to the formation of tetramer 9 and 10, respectively, as 1 1 anion-host complexes. 

Organolithium reagents (RLi) are tremendously important reagents in organic chemistry. In recent years, a great deal has been learned about their structure in both the solid state and in solution. X-ray analysis of complexes of n-butyllithium with A,A,A, A -tetramethylethylenediamine (TMEDA), THF, and 1,2-dimethoxyethane (DME) shows them to be dimers and tetramers [e.g., (BuLi DME)4]. X-ray analysis of isopropyllithium shows it to be a hexamer. 

Among the series of the parent systems 1-4, [5]radialene (3) is still unknown. The simplest derivative described so far is decamethyl[5]radialene (135) which has been obtained from l,l-dibromo-2-methyIpropene (22) by low temperature metalation with n-butyllithium followed by a metal exchange reaction with nickel or (better) copper salts and the thermal decomposition of the carbenoid thus formed (equation 10). The yield of 135 varies it is only 14% with CuBr SMe2, but it more than doubles (32%) when Cul PBu3 is employed . The formation of 135 is accompanied by di-, tri- and tetramer-ization of the dimethylvinylidene unit derived from 22 leading to tetramethylbutatriene and the respective permethylated [3]- and [4]radialenes. It is unlikely, though, that this... 

Many organolithium compounds are soluble in hydrocarbons exceptions are methyllithium and phenyllithium w hich are associated in these solvents. Butyllithium is mostly hexameric and /c/Y-butyllithium is tetrameric in cyclohexane. A Lewis basic solvent can interact with an organolithium oligomer, thereby decreasing the degree of association. Thus, methyllithium. which is tetrameric in the solid phase, becomes a solvated tetramer in ether, and BuLi. hexameric in hydrocarbons, becomes tetrameric in ether. In the more basic THE BuLi has a degree of association between dimeric and tetrameric at -108 °C, and phenyllithium is between monomeric and dimeric [3]. 

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