Dilithio-initiators

A variety of dilithio initiators have been homogeneously prepared in polar solvents [e.g., THF, (C2H5)20, etc.] (82, 83) yet, polymerizations in such solvents result in higher vinyl (lower 1,4) content for the B blocks and poorer performance for the ABA copolymers. 

That kind of a problem seems to arise in the addition of sec-butyl lithium to di-isopropenyl benzene. The reaction proceeding in benzene was investigated by Rempp et al.407. The investigators intended to develop a dilithio initiator soluble in hydrocarbon. Therefore, they prepared a stoichiometric mixture of the reagents (2 1). Protonation of such a mixture, presumably after cessation of the reaction, yielded butane and the monoadduct,... 

They react with sec- or t-BuLi and are converted into insoluble dilithiated derivatives which become solubilized on addition of monomers and yield bifunctional polymers and A.B.A. block polymers. Useful dilithio-initiators were reported by Tung475. ... 

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. 

The first proton to be removed from iV-methylpyrrole by w-butyllithium is from an a-position a second deprotonation occurs to give a mixture of 2,4- and 2,5-dilithiated derivatives. The formation of a 2,4-dilithio derivative is noteworthy since in the case of both furan and thiophene initial abstraction of a proton at C-2 is followed by proton abstraction from C-5 (77JCS(P1)887). iV-Methylindole, benzo[6]furan and benzo[6]thiophene are also deprotonated at C-2. Selenophene and benzo[6]selenophene and tellurophene and benzo[6]tellurophene similarly yield 2-lithio derivatives (77AHC(21)119). 

H NMR can be used for assessment of the purity of organolithium RLi initiators. The presence of ROLi can also be determined. The dimetallated initiator l,4-dilithio-l,l,4,4-tetraphenylbutane cannot be determined by this method due to its low solubility. ... 

High 1,4 B-block ABA copolymers having good physical properties have been reported from l,4-dilithio-l,l,4,4-tetraphenylbutane (84) and 1,4-dilithio-l,4-dimethyl-1,4-tetraphenyIbutane (84) and 1,4-dilithio-1,4-dimethyl-1,4 diphenylbutane (85) initiation. These initiators have been prepared with a minimum of ether (preferably anisole). However, both of these compounds have shown a gradual loss of solubility as a result of association of the alkyllithiums. Attachment of solubilizing oligomeric polydienes apparently alleviated this problem (84, 85). 

Sequential iodine lithium exchange from the 1,5-diiodonaphthalene 63 generates either the monoiodo-monolithio species 64 or the dilithio species 65.70 Clean monolithiation is obtained by allowing the initially formed mixture of mono- and dilithiated products to equilibrate with unreacted starting material over a period of 6 h at 5 °C. No equilibration is necessary on the second lithiation, which is complete in 1 h at -78 °C. 1,8-Diiodonaphthalene 66 can similarly be converted to the dilithio compound 67.71... 

Attempts to synthesize by analogous procedures bis-2,2 -biphenylylenemethylstibo-rane (157) from 2,2 -biphenylylenemethylstibine dibromide 154 and 2,2 -dilithio-biphenyl failed, because of the obvious instability of the initially formed stibonium structure 155 which readily loses methyl bromide. The carbanionic stibine 156 so... 

The central ring of both dibenzo[l,4]- dithiins and -oxathiins is cleaved on treatment with Li and a catalytic amount of 4,4"-di-rert-butylbiphenyl (DTBB) to afford thiols after reaction of the dilithio intermediate with electrophiles. In certain instances, the initial product can be cyclised to the dibenzo- dithiepine and -oxathiepine <02CL726>. The dilithio salt from thianthrene reacts sequentially with two different carbonyl compounds to give a l,2-di(hydroxyalkyl)benzene. When C02 is used as the second electrophile, a phthalan results <02TL7205>. 

Different amino-, alkyl-, and aryl-substituted phosphepines 7-12 and 90 were also obtained in two steps directly from binaphthyl 84 and dichloro-Ar,Ar-dialkylphosphinamines, dichloro(alkyl)phosphines, or dichloro(aryl)phosphines in 60-80% yields. This method requires initial metallation of 2,2 -dimethylbinaphthyl 84 with -BuLi and isolation of dilithio salt in the crystalline form <2004TA2621, 2003JOM(675)91, 2002TL4977>. 

The same year, Gerlach described a synthesis of optically active 1 from (/ )- ,3-butanediol (7) (Scheme 1.2). The diastereomeric esters produced from (-) camphorsulfonyl chloride and racemic 1,3-butanediol were fractionally recrystallized and then hydrolized to afford enantiomerically pure 7. Tosylation of the primary alcohol, displacement with sodium iodide, and conversion to the phosphonium salt 8 proceeded in 58% yield. Methyl-8-oxo-octanoate (10), the ozonolysis product of the enol ether of cyclooctanone (9), was subjected to Wittig condensation with the dilithio anion of 8 to give 11 as a mixture of olefin isomers in 32% yield. The ratio, initially 68 32 (E-.Z), was easily enriched further to 83 17 (E Z) by photolysis in the presence of diphenyl disulfide. The synthesis was then completed by hydrolysis of the ester to the seco acid, conversion to the 2-thiopyridyl ester, and silver-mediated ring closure to afford 1 (70%). Gerlach s synthesis, while producing the optically active natural product, still did not address the problem posed by the olefin geometry. 

Acetylenes. Propyne can be converted into the 1,3-dilithio derivative by reaction with 2 eq. of -butyllithium in the presence of 1 eq. of TMEDA or DABCO (-60°, hexane-ether). This derivative reacts with electrophiles initially at the propargylic carbon, then at the terminal acetylide site. The reactions can be carried out in sequence in one pot. ... 

An especially simple way to prepare triblock polymers having identical end blocks involves the employment of difunctional initiators, such as l,4-dilithio-l,l,4,4-tetraphenylbutane. The growing chain has the structure... 

The variety of products isolated from such reactions can be explained by the ready reaction of octaphenylcyclotetrasilane with silyllithium compounds. Thus, 1,4-dilithiooctaphenyltetrasilane formed by the initial lithium cleavage of octaphenylcyclotetrasilane is able to cleave more octaphenylcyclotetrasilane. Also, cleavage of the a,oo-dilithio compounds by lithium can increase the variety of disilanyllithium compounds, which can in turn cleave octaphenylcyclotetrasilane, leading to a wide distribution of... 

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