Dilithium compounds

If one of the groups R is Ph or SR, the abstraction of the second proton is much faster. Allenes give the same dilithium compounds as their acetylenic isomers . 

Disodium phthalocyanine (PcNa2) can be prepared in the same way as the dilithium compound by reacting sodium pentoxide in pentan-l-ol with phthalonitrile. It is more sensitive towards moisture and even alcohol than the dilithium phthalocyanine and is readily demeta-lated.58... 

Dipotassium phthalocyanine (PcK2) can be prepared analogously to the dilithium compound by refluxing phthalonitrile and potassium pentoxide in pentan-l-ol.58 With additional oxygen-donor ligands (e.g., crown ethers) it forms crystals with the potassium bulging outside of the phthalocyanine ring.133134... 

The LPDE system is applied to several reactions in which the metal ions coordinate to the lone pairs of heteroatoms, thereby activating the substrate. Initially, the effectiveness was shown in Diels Alder reactions (Scheme 1). In a highly concentrated (5.0 M) LPDE solution, Diels- Alder reactions proceeded smoothly.6-7 Generally, a catalytic amount of LiC104 is not effective in this reaction. In some cases, a catalytic amount of an additional Bronsted acid, such as camphorsulphonic acid (CSA), gives better results.8 An interesting double activation of carbonyl moieties by using dilithium compounds has been reported (compound... 

Better yields are obtained when polar solvents are utilized and an amine such as tetramethylethyl-enediamine is present, which also facilitates the formation of the dilithium compound. The lithium derivatives undergo a large number of reactions that can be used to produce the enormous number of ferrocene derivatives. Rather than trying to show a great number of reactions, a few of the common reactants and the substituents that they introduce on the cyclopentadienyl rings are shown in Table 21.3. 

The synthesis of crystalline disodium derivatives of primary phos-phanes and arsanes turned out to be more difficult than that of dilithium compounds. The reaction of NaN(SiMe3)2 with 2c led, as in its lithiation with BuLi, under redox reaction (H2 elimination, As-As bond formation) to the Na2As6 dimer 12 (Eq. 5). The latter has been... 

The proposed mechanism of this reaction is based on the nucleophilic attack of the alkyllithium compound at the carbenoid carbon atom or at the a-lithiooxy carbene. The dilithium compound 102 gives the alkene 103 by the loss of lithium oxide (equation 56). When an alkoxy residue, which is a better leaving group than U2O, is offered in the a-position of the corresponding dilithium compound, the elimination of lithium alkoxide takes place instead of lithium oxide. This is illustrated by the reaction of epoxide 104 that delivers the allylic alcohol 105 upon treatment with n-butyllithium (equation The... 

When the bromoalkenes 146, which are obtained in 88 to over 98% de, are submitted to another bromine-lithium exchange, the dilithium compound 148 is generated. This reaction takes place under complete retention of the configuration, as proven by the protonation that yields the Z-alkenes 149 . A debromination protocol that is based on a free... 

Dilithiated diamine 2 was synthesized by Karsch by a two-fold metalation of N,N,N, N tetramethylmethylenediamine (TMMDA) (1). The reaction was effected in n-pentane at low temperatures, yielding the poorly soluble Af,Af -bis(fithiomethyl)-Af,Af -dimethyl-methylenediamine (2) (Scheme 1). Due to its low solubility in toluene or THE, the highly pyrophoric compound was characterized by derivatization with several electrophiles, mainly chlorosilanes. Obviously, the addition of coordinating additives, such as TMEDA, DME (dimethoxyethane) or THE, does not enhance the solubility of the dilithium compound. Interestingly, as the author comments, TMEDA is only monolithiated in modest yields by alkyllithium bases. 

The related geminal dilithiated compound 11 was generated by the same group by reaction of a substituted sulphoximine with 2.5 equivalents of n-butyllithium in THF (Figure 4) Also in this case, the crystal structure of the dilithium compound could be cleared up by X-ray structural analysis (Figure 5). 

Substituted dilithiated 2-butenes of type 13 were generated by a two-fold deprotonation, starting from disubstituted 2-butenes 12 using n-butyllithium in the presence of TMEDA (Scheme 5). Both systems described by Raston and coworkers carry trimethylsilyl substituents in 1,4-positions in order to increase the acidity of the starting compounds on the one hand as well as the stability of the dilithium compounds on the other (polarizability effects). Intensive NMR studies on the structure of the dilithiated butenes in solution were performed by the authors. 

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). 

Korneev and Kaufmann successfully lithiated 2-bromo-l,l-diphenylethylene (46) by bromide-lithium exchange to form 2-lithio-l,l-diphenylethylene (47). A second lithia-tion could be effected in four hours at room temperature by deprotonation of the aromatic ring with w-butyllithium in the presence of TMEDA (Scheme 17). Like in the synthesis of compound 23, the first lithiation activates the ortho-hydrogen atom of the Z-phenyl substituent to give 1,4-dilithium compound 48. In total, three equivalents of the alkyl-lithium base are required the third equivalent is consumed in the trapping reaction of w-bromobutane with generation of octane. 

By a two-fold bromine-lithium exchange on l,2-bis(2-bromo-3,5-di-f-butylphenyl)-ethane (73), Yoshifuji and coworkers were able to generate 1,6-dilithium compound 74 . The reaction was carried out using n-butyllithium in THF at — 78°C (Scheme 26). 

The geminal dihalogenated cyclopropane derivatives 83a and 83b were lithiated by Vlaar and Klumpp . 7,7-Dichloro- (83a) and 7,7-dibromonorcarane (83b) were reacted with four equivalents of LiDBB in diethyl ether and several reaction conditions were examined by the authors such as reaction temperatures, the influence of different coordinating additives as well as various methods (Scheme 31). The achieved maximum yield for the geminal dilithium compound 84 was 55% (from 83b). Side-products, like the 1,2-dilithioethane derivative 85, the dilithiated dicyclohexylacetylene 86 or 1,3-dilithium compound 87, were observed in different quantities, sensitively depending on the reaction conditions. Also, carbenoid intermediates were formed as verified by trapping reactions (deuteriolysis). 

The bromide-lithium exchange of l,l-dibromo-2,2-diphenylethylene (88) was thoroughly examined by Maercker and coworkers. It could be shown that the number of side-products drastically decreases when LiDBB instead of metallic lithium is used as lithiation agent. The reaction was performed in THF at low temperatures by addition of the solution of the geminal dibrominated aUcene to the solution of LiDBB (Scheme 32). By this method, l,l-dilithio-2,2-diphenylethylene (89) could be obtained in 36% yield together with the 1,4-dilithium compound 48 and monolithiated 47 (51 and 2%, respectively). The yields were determined after trapping the reaction mixture with dimethyl sulphate. 

The aggregation behaviour and the structure in solution of two closely related dilithium compounds 90 and 91 (Figure 16) was studied by Gunther, Maercker and coworkers, using one- and two-dimensional NMR techniques ( H, C, Li) . While in diethyl ether, both compounds exist as a dimer, the dimeric structure is partly broken up in THF or by the addition of TMEDA. Also, activation barriers and thermodynamic parameters of aggregate exchanges were determined by temperature-dependent NMR studies. 

De Meijere, StaUce and coworkers were able to generate dilithiated 111, where the tricyclic non-metalated form can be considered as a subunit of the smallest possible fullerene By a two-fold tin-lithium exchange of the bis(trimethylstanno) derivative 110 with methyllithium, the dilithium compound 111 was cleanly obtained (Scheme 39) its solid-state structure could be clarified by means of X-ray crystallography. 

Variously substituted siloles of type 129 could be synthesized by Tamao and coworkers by the reaction of l,4-diaryl-l,4-dilithio-l,3-butadienes of type 128 with chlorosilanes . The l,l -spirobisilole 130 was accessible by reaction of the diphenyl snbstitnted dilithium compound 128a with tetramethoxysilane (Scheme 47). All of the dilithium compounds 128a-e were obtained by reaction of 2,5-diaryltellurophenes 127a-e with i-butyllithium in diethyl ether. 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

Dilithium compound 210, obtained from the lithiation of hexasilylfulvene 209 in THF at room temperature (Scheme 72), represents the first X-ray structural analysis of a dUithi-ated fulvene derivative. In the solid state, one lithium centre is capping the central five-membered ring, while the second lithium centre is located at the exocyclic carbon atom of the fulvene unit (Figure 27) . 

Various diboriodilithiomethanes of type 225 were synthesized by Bemdt and coworkers, adding different aryllithium compounds to the boron-carbon bonds of compounds 224a-e (Scheme 77) . The dilithium compounds have been characterized in the solid state by X-ray structural analysis. 225a-e adopt the structure of a 1,3-diborataallene system, where lithium-diethyl ether units are bridging the twisted B-C-B axis from both sides. 

Unexpectedly, the molecular structures of 227 and 229 are not similar. While tetrasi-lylated dilithium compound 227 is capped by the two lithium centres from both sides of the ring plane (Figure 29), hexasilylated dilithium compound 229 is capped by its two lithium centres from one side of the ring (Figure 30) ° . Moreover, the phenyl ring of... 

The solution structure of the dilithium compound 237, generated by treating frawi-1,2-diphenyl-l,2-bis(trimethylsilyl)ethylene (236) with excess lithium metal in THF (Scheme 83), was studied by ID and 2D NMR techniques in the group of FUrstner". As a conclusion, the dilithium compound 237 adopts a C2-symmetric twisted frawi-arrangement with two bridging lithium centres in solution, underlining the various X-ray structural studies in the solid state by Sekiguchi and coworkers. 

The iwfennolecular coupling of two lithium diphenylacetylide radical anions to give l,4-dilithio-l,2,3,4-tetraphenylbutadiene (247) has been reported by Braye and coworkers (Scheme 87)". Pauer and Power were able to clear up the solid-state structure of this well-known dilithium compound, confirming an S-cis conformation of 247 in the crystal" . Two lithium-DME units are capping the C4 moiety, which is not fully planar, from both sides (Figure 31). A similar structure of the corresponding TMEDA adduct has been reported by Schleyer" . 

The reduction of l,l-bis(diphenylphosphanyl) ethylene (248) with an excess of metallic lithium, activated by ultrasonic irradiation, leads to C—C coupling under the formation of a l,l,4,4-tetrakis(diphenylphosphanyl)butane (249) (Scheme 88)". Surprisingly, the lithium centres in the resulting dilithium compound do not form any lithium-carbon contacts, being coordinated by two diphenylphosphanyl groups and two TFIF molecules each. With this strucmral motif, the molecular structure is similar to the one of tris(phosphaneoxide) 20 (Section n. A), also obtained by Izod and coworkers upon deprotonation. ... 

The ideal dilithium compound for use in the synthesis of ABA diene-olefin copolymers should be highly difunctional, hydrocaibon-soluble, and stable to storage, as well as being conveniently prepared (preferably in the absence of polar ligands) from available starting materials. Unfortunately, many known dilithium compounds fail to conform to some or all of these criteria. 

DiLi-3 (18) and DiLi-4 (19) are oligomeric aromatic, dilithium compounds in hexane-triethylamine solvent. 

Larger ring systems with carbon are formed by a cyclization reaction of dilithium compounds with dichloro compoundsl99 ... 

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