Organolithium catalysts

Conjugated Dienes and Other Monomers. Alkyllithiums such as n-butyllithium—and even the growing polyethylene carbon-lithium bond complexed with chelating diamines such as TMEDA—are effective initiators for the polymerization of conjugated dienes such as 1,3-butadiene and isoprene. A polybutadiene of high 1,2-content can be produced from butadiene in hydrocarbon solvents using these N-chelated organolithium catalysts. 

This striking effect of a strong donor base on the catalytic activity of an organolithium compound contrasts with the reverse effect of donor bases on the growth reaction of ethylene with trialkylaluminum compounds. The catalytic activity of the latter is connected with the electron deficient nature of the uncoordinated, monomeric, trialkylaluminum species (10). These facts point to a difference in the mechanism of ethylene addition between the amine-coordinated organolithium catalyst and the trialkylaluminum compounds. 

Chelated Organolithium Catalyst. To 1.6 ml of 1.6N n-butyllithium in hexane were added 50 ml toluene and 0.6 ml PMDT, generating PMDT-BzLi in situ. The reaction was kept at 40°C while butadiene was introduced into the atmosphere above the reaction (22 cc/min) for 2 hrs. The reaction was then quenched with 5 ml of water. The organic layer was separated, dried (K2C03), and the solvent was removed under vacuum to give 5 grams of product having Mn — 2549. 

Telomerization of Benzene and Butadiene. To 0.26 gram (2.63 mmoles) of phenylsodium were added 50 ml of benzene and 0.7 ml of iso-HMTT. The reaction was kept at 70°C while butadiene was introduced into the atmosphere above the reaction for 2 hrs at 22 cc/min. A work-up similar to that given under chelated organolithium catalyst gave 7 grams of benzenebutadiene telomer with Mn = 998. 

The varions faetors influencing the stereoregnlarity, when the propagating chain end is a carbanion, are conveniently highlighted in a study of the polymerization of methyl methacrylate by organolithium catalysts. 

It has been postulated that the syn TT-ahyl stmcture yields the trans-1 4 polymer, and the anti TT-ahyl stmcture yields the cis-1 4 polymer. Both the syn and anti TT-ahyl stmctures yield 1,2 units. In the formation of 1,2-polybutadiene, it is beheved that the syn TT-ahyl form yields the syndiotactic stmcture, while the anti TT-ahyl form yields the isotactic stmcture. The equihbtium mixture of syn and anti TT-ahyl stmctures yields heterotactic polybutadiene. It has been shown (20—26) that the syndiotactic stereoisomers of 1,2-polybutadiene units can be made with transition-metal catalysts, and the pure 99.99% 1,2-polybutadiene (heterotactic polybutadiene) [26160-98-5] can be made by using organolithium compounds modified with bis-pipetidinoethane (27). At present, the two stereoisomers of 1,2-polybutadiene that are most used commercially are the syndiotactic and the heterotactic stmctures. 

While the synthesis of fnnctionalised secondary alcohols and amines can be achieved withont catalyst by the addition of organolithium and organomagnesium reagents to C=N and C=0 gronps, these methods lack a significant functional group tolerance. In order to overcome this limitation and access to more functionalised compounds, the catalytic arylation of aldehydes and imines has been extensively studied [2]. 

Organolithium compounds can add to a, (3-unsaturated ketones by either 1,2- or 1,4-addition. The most synthetically important version of the 1,4-addition involves organocopper intermediates, and is discussed in Chap 8. However, 1,4-addition is observed under some conditions even in the absence of copper catalysts. Highly reactive organolithium reagents usually react by 1,2-addition, but the addition of small amounts of HMPA has been found to favor 1,4-addition. This is attributed to solvation of the lithium ion, which attenuates its Lewis acid character toward the carbonyl oxygen.111... 

Preparation and Properties of Barium Salt. The catalyst used to prepare this new class of crystallizing polybutadienes consists of a barium t-butoxide-hydroxide salt in combination with an organolithium ( 8, 9, 10). Rather specific preparative techniques must be used in forming this barium salt, as shown in Figure 1. The use of an amine solvent provided quantitative conversion of the metal to barium salts. 

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