Association alkyllithium compounds

In their papers Rodionov and coworkers described the polymerization of organolithium compounds in terms of the formation of lithium bonds (Scheme 1), analogous to hydrogen bonds, which brought about cyclic or linear association of these compounds in solution . However, the strong association of alkyllithium compounds, persisting even in the vapour phase, indicates that their association takes place through the formation of... 

Alkyllithium compounds as well as polymer-lithium associate not only with themselves but also with other alkalimetal alkyls and alkoxides. In a polymerization initiated with combinations of alkyllithiums and alkalimetal alkoxides, dynamic tautomeric equilibria between carbon-metal bonds and oxygen-metal bonds exist and lead to propagation centers having the characteristics of both metals, usually somewhere in between. This way, one can prepare copolymers of various randomness and various vinyl unsaturation. This reaction is quite general as one can also use sodium, rubidium or cesium compounds to get different effects. 

Alkyllithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. -Butyllithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization ofbutadiene, isoprene, and styrene with linear and branched structures. Because of the high degree of association (hexameric), w-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

Alkyllithium compounds have solubility and stability because of their ability to associate to form aggregates of definite structure. Such aggregation confers stability but is not extensive enough to cause insolubility. Methyllithium and n-butyllithium, for instance, exist in a highly associated form, typical of electron-deficient bonding, e.g., (MeLi) and (BuLi)4. 

The addition of amines and ethers to alkyllithium compounds profoundly affects polymerization of such species. Amines and ethers alter the association of RLi compounds and change the course of the polymerization and its kinetics. Also, the presence of small amounts of such impurities as water, alcohols, or a-acetylenes, influences the kinetic chain length. The chain-termination reaction with such acidic protons is almost instantaneous. However, there are certain types of protons, such as a-aromatic, secondary amine, and /3-acetylenic, that are not acidic enough to react immediately but will undergo transmetalation during the course of a polymerization reaction. This results in termination or chain transfer of the polymer chain, and limits the realization of polymers of... 

In THF, the alkyllithium compounds are aggregated [157] and the situation is reminiscent of the conditions in hydrocarbon solutions. At high concentrations, the association number (i. e. the number of molecules in the aggregate) decreases. This anomaly is explained by the existence of aggregate—solvent complexes, for example (MeLi)4 8THF Benzyllithium and its polymeric analogue polystyryllithium are not associated. Phenyllithium is mostly present as a dimer or monomer. Both forms are in equilibrium and are solvated. Only the monomeric form of the initiator is active. In practice, benzyllithium reacts only in the form of an ion pair. The fraction of the free benzyl anion must be very small [151c]. 

Alkyllithium compounds are generally associated into dimers, tetramers, or hexamers in hydrocarbon solution [3, 44], The degree of association is related to the steric requirements of the alkyl group, that is, the degree of association decreases as the steric requirements of the alkyl group increase. 

The relative reactivities of alkyllithiums as polymerization initiators are intimately linked to their degree of association as shown below with the average degree of association in hydrocarbon solution, where known, indicated in parentheses after the alkyllithium compound [44, 55, 56] ... 

Initiation The mechanism of initiation of anionic polymerization of vinyl monomers with alkyllithium compounds and other organometallic compounds is complicated by association and cross-association phenomena in hydrocarbon solvents and by the presence of a variety of ionic species in polar media [3, 4, 45, 48, 55, 56]. The kinetics of initiation is complicated by competing propagation and the occurrence of cross-association of the alkyllithium initiator with the propagating organolithium [55]. Thus, only the initial rates provide reliable kinetic data. 

A good example of this kinetic behavior was found in the study of the n-butyllithium-styrene system in benzene, in which a kinetic order dependency on n-butyllithium concentration was observed, consistent with the predominantly hexameric degree of association of n-butyllithium (Worsfold and By water, 1960). However, this expected correspondence between the degree of association of the alkyllithium compound and the fractional kinetic order dependence of the initiation reaction on alkyllithium concentration was not always observed (Young et al., 1984). One source of this discrepancy is the assumption that only the unassociated alkyllithium molecule can initiate polymerization. With certain reactive initiators, such as 5 c-butyllithium in hexane solution, the initial rate of initiation exhibits approximately a first-order dependence on alkyllithium concentration, suggesting that the aggregate can react directly with monomer to initiate polymerization (Bywater and Worsfold, 1967a). A further... 

The functionalization of PSLi with GPTMS was also performed in the presence of lithium alkoxides, specifically, lithium 2,3-dimethyl-3-pentoxide. It is known that lithium alkoxides cross-associate with alkyllithium compounds in solution. The cross-association would be expected to... 

The most direct method of preparing telechelic polydienes utilizes a dilithium initiator which is soluble in hydrocarbon solution [220, 221]. The most expedient method of preparing such a dilithium initiator is to react two moles of an alkyllithium compound with a divinyl compound which will not homopolymerize. Unfortunately, because of the association behavior of organolithium compounds in hydrocarbon media [176-178], many potential systems fail because they associate to form an insoluble network-like structure [221]. Expediencies such as addition of Lewis bases can overcome solubility problems of dilithium initiators, however, such additives tend to produce high amounds of 1,2- and 3,4-microstructures (see Table IV). One exception is the adduct formed from the addition of two equivalents of sec-butyllithium to l,3- i5 (l-phenylethenyl)benzene as shown in Eq. (79) [222,223]. Although this is a hydrocarbon-soluble, dilithium initiator, it was found that biomodal molecular weight distributions are obtained monomodal distributions can be obtained in the presence of lithium alkoxides or by addition of Lewis base additives [224,225].This initiator has also been used to prepare telechelic polymers in high yields [226]. 

A further consequence of the association of alkyllithium compounds and the corresponding fractional kinetic orders for addition to DPE is the observation that the relative reactivities of alkyllithium compounds with 1,1-DPE in THF are concentration dependent [25]. Thus, structure-reactivity correlations do not necessarily follow from consideration of the expected acidities of the corresponding hydrocarbons [3]. At low concentrations, n-butyllithium is more reactive than benzyllithium, while at higher concentrations it was predicted by extrapolation that benzyllithium would be more reactive than n-bu-tyllithium. 

Solvation of the organolithium polymeric species, with possibly some disruption of the polymers to less-associated species, leads to adducts which may function as a source of carbanions in this sense the adducts behave as bases in furnishing a nucleophilic moiety. It is important to realize, however, that such basic behavior can occur only in the presence of a basic solvent, or it requires a strong Lewis add. Examples of the latter situation are found in the reaction of alkyllithium compounds vrith other organometals, as described in the following section. 

In general, simple alkyllithiums exist predominantly as either hexamers (for sterically unhindered RLi) or tetramers (for sterically hindered RLi) in hydrocarbon solvents and as tetramers in basic solvents (9-12). Polymeric organolithium compounds such as poly(styryl)lithium exist as dimers in hydrocarbon solution and are unassociated in basic solvents such as tetrahydrofuran (13-15). The state of association of poly-(dienyl)lithiums in hydrocarbon solution is a subject of current... 

Quantitative Analysis of Alkyllithium Initiator Solutions. Solutions of alkyllithiiim compounds frequendy show turbidity associated with the formation of lithium alkoxides by oxidation reactions or lithium hydroxide by reaction with moisture. Although these species contribute to the total basicity of the solution as determined by simple acid titration, they do not react with allylic and benzylic chlorides or ethylene dibromide rapidly in ether solvents. This difference is the basis for the double titration method of determining the amount of active carbon-bound lithium reagent in a given sample (55,56). Thus the amount of carbon-bound lithium is calculated from the difference between the total amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, allyl chloride, or ethylene dibromide. 

Lewis bases exert dramatic effects on the rate, stereochemistry, and reaction pathway in organolithium chemistry 4). A partial explanation for these observations can be deduced from the effects of Lewis bases on the degree of association of organolithium compounds as shown in Table 3. In general, the presence of basic molecules tends to decrease the average degree of association of organolithium compounds. Thus, simple alkyllithiums which are hexameric in hydrocarbon solution... 

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). 

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

The simplest compounds of this type are the tetraalkylaurates(III), prepared by the reaction of trialkyl(triphenylphosphine)gold(III) complexes with alkyllithiums, which proceeds by phosphine displacement as illustrated in equation 84. Recent studies have shown the reaction to be stereoselective, with cis- (or tram-) dimethylalkylgold(III) complexes giving square planar cis- (or trans-) tetraalkylaurates and an associative mechanism involving a pentacoordinate gold(III) intermediate has been postulated363. 

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