1.4- Dilithio- 1,3-butadiene

Isolated triple bonds, on the other hand, react rather slowly with lithium dust except in strained rings. Thus we succeeded in adding lithium to the triple bond of cyclooctyne 134 even at —35 °C in diethyl ether to yield cis-1,2-dilithiocyclooctene 136 as a yellow solution containing up to 20% of the 1,4-dilithio-1,3-butadiene derivative 137, the product of dimerizing addition... 

Another novel and useful method for the synthesis of pyridine derivatives was reported using lithiated butadienes [22]. 1-Lithio-1,3-butadienes 50 treated with nitriles in the presence of hexamethylphosphoric triamide (HMPA) at room temperature for 1 h gave the substituted pyridines in excellent yields (Scheme 11.20). The butadienylketimine 51 is the key intermediate of the reaction. The same pyridine formation occurred in the reaction of nitriles with the corresponding 1,4-dilithio 1,3-butadienes 35, but the existence of a different mechanism was indicated by nuclear magnetic resonance (NMR) observation of the reaction mixture. 

Scheme 1 Selected examples of structures of 1,4-dilithio-1,3-butadienes, which favor a double dilithium bridged structure (s-cis configuration) in both solution and solid states...

Scheme 4 Intramolecular cyclization of silyl-substituted 1,4-dilithio-1,3-butadienes to afford a-lithio siloles via E/Z isomerization and nucleophilic attack...

Scheme 37 Coordination modes of pentalene dianion and formation of barium dibenzopentalenides via transmetallation of 1,4-dilithio-1,3-butadienes with Ba[N(SiMea)2]2...

Li H, Wei B, Xu L, Zhang WX, Xi Z (2013) Barium dibenzopentalenide as a main-group metal T] complex facile synthesis fiom 1,4-dilithio-1,3-butadienes and Ba[N(SiMe3)2]2, structural characterization and reactitui chemistry. Angew Chem Int Ed 52 10822-10825... 

When cyclic 1,4-dilithio butadiene and tertiary aliphatic nitriles are used, the 1,8-dilithio bis-ketimine intermediate undergoes two sequential 1,5-cycUzatiOTi steps with a lower energy barrier, generating tricyclic A -bipyrrolines 44. The calculation results clearly show the mechanism details and are in good agreement with the experimental observations [65]. 

Experimental evidence for the six electron systems has been described in Sect. 2.1.4. Skancke reproduced the relative stabihty of the cross conjugated systems relative to the linear isomers by calculating the trimethylenemethane and buta-l,4-diyl dianions [27] and their dilithio salts [28]. For the four electron systems butadiene is more stable than trimethylenemethane. Experimental examination of the relative stabihties of two electron systems using the trimethylenemethane and buta-14-diyl dications needs to overcome the intrinsic instabihties of dications dissatisfying the octet rule. 

However, the reaction of (lZ,3Z)-l,4-dilithio-l,3-butadiene (36) with phenylantimony dichloride (Scheme 5) affords 1-phenylstibole (37),14 which has been converted to l.l -distibaferrocene (9).15 Unfortunately, there is no totally satisfactory synthetic route to dilithio compound 36. Sn/Li exchange of (lZ,3Z)-l,4-bis(trimethylstannyl)-l,3-butadiene (38) with methyllithium gives a good yield of 36, but the preparation of 38 is... 

The THF adduct of dilithio-l,4-bis(trimethylsilyl)-2-butene (14) (Figure 6), which was prepared by the same method as the compounds of type 13 (a reduction of the corresponding 1,3-butadiene with metallic lithium, as described in Section II. E, can be an... 

When hexachloro-1,3-butadiene (81) is reacted with four equivalents of n-butyllithium in diethyl ether, a series of lithiation and elimination reactions gives l,4-dilithio-l,3-butadiyne (82) in good yields (Scheme 30). Ijadi-Maghsoodi and Barton verified this by derivatization with various chlorosilanes in yields up to 94%". ... 

The formation of doubly lithiated bis(lithiomethyl)acetylene (107) is accompanied by the formation of 2,3-dilithio-l,3-butadiene (108), when starting from bis(trimethylstanno-methyl)acetylene (106). As reported by Reich and coworkers, the molar ratio X of the two compounds depends on the applied temperature protocol (ratio determined by trapping reactions with chlorotrimethylsilane) (Scheme 37). 

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. 

In 1988, Ashe and Mahmoud prepared the first C-unsubstituted stannole, 1,1-dibutylstannole (19), by cyclization of 1,4-dilithio-l,3-butadiene with dibutyltin dichloride28 (equation 5). This stannole, stable in the monomeric state, has been isolated by distillation (28% yield). 

In 1967, an original synthesis of 1,4-dilithio-1,4 diphenyl-1,3-butadiene (DDB) from 1,1-dimethyl-2,5-diphenylsilole (see equation 17 below) was proposed by Gilman and coworkers34. This dilithium reagent reacts with polyhalides R EX4 to yield 2,5-diphenylmetalloles (26, E = Si34,35, Ge36, Sn34) (Scheme 6). 

The Zr-based route to main group heteroles is a convenient alternative to the method using l,4-dilithio-2,3-butadienes (see Section II.B.l.a), the more so as these dilithium reagents are easily available from zirconacyclopentadienes. Thus, the synthesis of 1,1-dichloro-2,3,4,5-tetramethylsilole (27) was readily performed from 70 by use of the I2In-BuLi/SiCLt sequence of reagents40 (see Section II.B.l, equation 8)... 

In the stannole series, the reactions could be explained by the extraordinary reactivity of the exocyclic tin-carbon bonds with regard to lithium reagents. Thus, the reaction of 1,4-dilithio-1,2,3,4-tetraphenyl-1,3-butadiene with 1,1 -dialkyl-2,3,4,5-tetraphenylstannole leads to alkyl-tin bond cleavage-cyclization by the dilithium reagent33 (equation 61). 

Tellurophene is decomposed by strong mineral acids5. Tetrachlorotellurophene Te,Te-dichloride is rapidly converted by aqueous base to 1,2,3,4-tetrachloro-l,3-butadiene and tellurous acid4. 2,5-Diphenyltellurophene reacted with butyl lithium in hexane in the presence of tetramethylethylenediamine at 20 to give l,4-dilithio-l,4-diphenyl-l,3-butadiene, a compound that can be used as a starting material for the synthesis of 1,4-disubstituted butadienes6. 

On the other hand, the isomeric, 4-dilithio-l, 3-butadiene in its symmetrically... 

The mechanism proceeds via radicals and is obviously rather complicated passing even through dimers as intermediates. Thus during synthesis of 1,1-dilithio-l-alkenes 81 from 80 by direct mercury-lithium exchange, we often obtained up to 7% of the 2,3-dilithio-l,3-butadienes 83 which might have been formed from the corresponding mercury compounds 82. That 82 is in fact an intermediate in the... 

In contrast we found that the addition of lithium to the central double bond of buta-trienes 722 takes place very easily The resulting 2,3-dilithio-l,3-butadienes 123 which are stable towards excess lithium interestingly are cleaved to 1,1-dilithio-1-alkenes 81 and 725 in the presence of mercury(II) chloride (see Sect. 3.3). 

Another fragmentation reaction especially useful for the synthesis of vinyllithium compounds is the well-known Shapiro reaction We were successful in preparing 2,3-dilithio-l,3-butadiene 166 by a double Shapiro reaction although in 12% yield only The main reaction product was 2-butyne. 

We obtained similar butadiene derivatives by a different route starting with 2,3-dilithio-l,3-butadienes 123 The last reaction yielding an interesting thioketale of a 2,3-diisopropylidenecyclopropanone derivative 181 is related to the aforementioned synthesis of cyclic ketones with gaseous carbon dioxide. 

The 2,3-dilithio-l,3-butadiene derivative 123a is also tetrameric involving two crystallographically different diene molecules in different environments Four lithium atoms form a tetralithiocyclobutane skeleton two bonds of which simultane-... 

Alkenyllithium compounds can also be prepared by metallation of alkenes, particularly when alkenyl hydrogens are rendered acidic by an a-substituent (equation 22). Transmetallation of alkenyl stannanes with organolithium reagents gives alkenyllithium compounds with retention of alkene stereochemistry (equation 23). Tin-lithium transmetallation has been used to prepare 1,4-dilithio-l,3-butadiene. Monosubstituted alkenyllithium compounds RHC=CHLi, can also be prepared from the corresponding diorganotel-luride, RHC=CHTeBu, by reaction with butyllithium in... 

Recent studies on the allylation of alkynes with bis (7r-allyl) nickel have revealed that the Ni(0) generated in this process causes the trimeri-zation and, more importantly, the reductive dimerization of a portion of the alkyne (8). A deuterolytic work-up led to the terminally di-deuter-ated diene (5), supporting the presence of a nickelole precursor (4) (Scheme 1). The further interaction of 4 with 1, either in a Diels-Alder fashion (6) or by alkyne insertion in a C-Ni bond (7), could lead to the cyclic trimer 8 after extrusion of Ni(0), thereby accounting for the trimerizing action of Ni(0) on alkynes. This detection of dimer 5 then provided impetus for the synthesis of the unknown nickelole system to learn if its properties would accord with this proposed reaction scheme. Therefore, E,E-l,4-dilithio-l,2,3,4-tetraphenyl-l,3-butadiene (9) was treated with bis (triphenylphosphine) nickel (II) chloride or l,2-bis(di-phenylphosphino ethane)nickel(II) chloride to form the nickelole 10 (9) (Scheme 2). The nickelole reacted with dimethyl acetylenedicarboxylate to yield 11 and with CO to produce 12. Finally, in keeping with the hypothesis offered in Scheme 1, 10a did act as a trimerizing catalyst toward diphenylacetylene (13) to yield 14. 

The convenience of this technique has led to the development of many commercial products, including thermoplastic elastomers based on triblocks of styrene, butadiene, and isoprene. The initiator used in these systems is based on hydrocarbon-soluble organolithium initiators. In some cases, a hydrocarbon-soluble dilithio initiator has been employed in the preparation of multiblock copolymers. Several techniques are used to prepare thermoplastic elastomers of the ABA type. All these are discussed in detail in Chapter 2. A short summary of these techniques is given here. 

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