Butyllithium dimers

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. 

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

Treatment of diallenyl sulfone 354 with n-butyllithium resulted in a cyclodimerization to afford 2,6-dithiaadamantane derivative 356. This dimerization is considered to be initiated by formation of the a-sulfonyl carbanion 355 and to proceed through a carbanion walk or carbanion tour process426. 

As the simultaneous creation of Ge = C and P = C double bonds are unlikely, the P = C double bond, much less reactive than the Ge = C double bond, was formed first.173196 was prepared by debromofluorination of 197 [obtained by reaction of ArP = C(Br)Li181 with dimesityldifluorogermane] with ft-butyllithium at low temperature (Scheme 42). The reaction, followed by 31P NMR between -90°C and room temperature, showed the immediate formation of the lithio compound 198, which lost LiF at -60°C to give the germaphosphaallene 196 in 65-70% yield. 196 was stable at -50°C and dimerized slowly above this temperature. It was the first allenic compound of germanium to be characterized by physicochemical methods. 

In 1985, Fluck et al. reported that treatment of the P-fluoromethylene-phosphorane 3f with 2 equiv of butyllithium at -95°C gave the 1,1,3,3-tetrakis(dimethylamino)-lA5,3A5-diphosphete 5f in 34% yield.30 The formation of this four-membered ring could result from a [2 + 2] head-to-tail dimerization of the transient phosphinocarbene 2f. However, an alternative... 

Scheme 64. Dimerization of coppercyclopropylidenoids generated from oligospirocyclopro-panated dibromocyclopropanes and butyllithium in the presence of CuCb [159]...

In the group of Izod, the tris(phosphane oxide) 19 was 1,2-dilithiated by the reaction with two equivalents of w-butyllithium in THF at room temperature (Scheme 7). The similarity of the structural formula of compound 20 (Lewis formula) to 1,2-dilithium compounds found by Sekiguchi and coworkers (see Section n. E), where two lithium centres are bridging a C2 unit, is not maintained in the solid state. The X-ray structural analysis reveals a centrosymmetric dimer containing no carbon-lithium contacts (Figure 8). 

Surprisingly, reaction of l,l-dichloto-2,5-diphenylbenzocyclopropene(22) with n-butyllithium affords the dimeric triafulvene245. Exposure of the unsymmetrical benzocyclopropene 246 to the same reaction conditions leads by analogy to a 1 1 mixture of stereoisomeric triafulvenes 247 and 248. ... 

The metallation of 3,1-enynes and 1,3-diynes with butyllithium requires careful experimentation, because dilithiation (and in the case of the enynes subsequent dimerization) occurs when the base is present in excess [2,41] ... 

The AH- 1,2-diazepine (99) reacted with butyllithium at -78 °C via addition to the 2,3-imine bond, but LDA gave the anion (100) which dimerized in the presence of oxygen. Reaction of (99) with Na/K at -20 °C in THF however led to the formation of (101) via ring contraction of (100) (72TL4891). Interestingly, however, when (100) was generated with LDA at 0 °C in TMEDA as solvent, no ring contraction was observed and the anion... 

The trimethylsilyl radical produced either rapidly dimerizes or reacts with solvent so that very clean ESR spectra of the radical anion, with minimum interactions with the counterion, can be obtained (116). Further reduction to dianions is very slow, and exhaustive reduction to anion radicals minimizes problems associated with exchange between anion radicals and unreduced substrate (115). It now appears that the solvent HMPA greatly facilitates the one-electron reduction, not only for trimethylsilylsodium, but also for organolithium and magnesium reagents (110). It was found that 0.1F solutions of methyl-, n-butyl-, or f-butyllithium or benzylmagnesium chloride will quantitatively reduce biphenyl to its radical anion in less than 10 minutes (110). 

Pyrans 151a (R = Ph) and 151c gave with excess butyllithium at -30 to - 120°C 515-like anions, which isomerized to substituted cyclopentadiene anions and then, either dimerized or reacted with the reagent (Scheme 33).423... 

Silaphosphetanes can be synthesized. They result from the intramolecular cyclization of /3-chlorosilylphosphine using n-butyllithium (Scheme 100). Like the germoxetanes they exist as a monomer-dimer equilibrium, insert sulfur and benzaldehyde, and on heating extrude the silaphosphimine intermediate which then either dimerizes or inserts the original silaphosphetane (Scheme 101) (79JOM(182)9). 

Perchlorosilylalkanes will cyclize on catalyzed pyrolysis (Scheme 106) (64CB1111) or in the presence of methyllithium (Scheme 107) (75MI12001). f-Butyllithium will add to vinylchlorodimethylsilane, and the intermediate then cyclizes through what is believed to be a silene intermediate by a head-to-tail dimerization, as is typical of the less hindered silenes (Scheme 108) (77JA2013). 

Doubly labeled (13C —6Li) butyllithium, for example, displays a quintet splitting with JCtT.j = 7.8 Hz at — 90CC in tetrahydrofuran [478], Following the (2nl + 1) multiplicity rule, the number of lithium nuclei eLi with 1 = 1 coupling to each carbon is 2. Thus, butyllithium occurs as a dimer, similarly to cyclopropyl-, vinyl- and phenyllithium [478]. 

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. 

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