Dienes anionic

There are a number of examples of chiral ferrocene derivatives prepared by reaction of substituted cyclopenta-diene anions with FeCl2 or a monocyclopentadienyl iron species rather than using the strategy of introduction and modification of a side chain on ferrocene itself. Addition... 

The hyperfine splitting arising from the four protons at the terminal carbon atoms is found to be -0.76 mT, while that of the two other protons is —0.28 mT, indicating that, as theory predicts, the SOMO of this diene anion radical is more heavily concentrated on Cl and C4, as opposed to C2 and C3. In contrast, the cation radical of 1,3-butadiene is apparently too short-lived m fluid solution to be observed by ESR spectroscopy. 

The reaction of 1,3-butadiene with the solvated electrons of liquid anunonia results in homogeneous electron transfer to yield the diene anion radical, which is ultimately converted to a mixture of cis- and /ran5-2-butene [117]. The mechanism of this reduction (Scheme 70) involves protonation of the anion radical at Cl or C4 of the diene, where most of the negative charge resides, followed by homogeneous reduction of the resulting allylic radical by the solvated electrons and finally protonation of the allylic carbanion at the primary carbanion site. 

An interesting aspect of this reaction is the formation of substantial amounts of cw-2-butene, which would appear to require the intermediacy of the j-cis-1,3-butadiene anion radical, even though butadiene exists almost exclusively in the s-trans conformation (98 %). At —33°C, 13 % of the 2-butene mixture is the cis alkene, and at -78 °C 50 % of the mixture is cw-2-butene. In the case of 1,3-pentadiene, 68 % of the 2-pentene is the cis isomer. The most plausible explanation for these stereochemical results appears to be the reversible reduction of the diene to the diene anion radical at -78 °C by the pool of solvated electrons, which yields an equilibrium mixture of the s-cis and j-tran5-anion radicals (ca. 50 50), which are... 

It is important to mention the pivotal role of the alcohol component in the typical Birch reduction. On one hand, the relatively acidic alcohol rapidly protonates the intermediate IV (Scheme 13.2) to give the 1,4 diene in the absence of alcohol, an isomerization of this intermediate to the most stable 1,3-diene anion XIII takes place, and its final protonation affords the corresponding 1,3-diene (17), which is susceptible to reduction by the reaction media to give the final olefin cyclohexene (18) (Scheme 13.8). It is well known that 1,3-dienes are reduced by dissolving metals to the corresponding olefins in quantitative yields [11]. 

The preferred mode of ring-opening in the anion radicals (527) has been shown to be conrotatory, although the HOMO in the product is symmetrical and would predict disrotation. INDO MO calculations on the parent cyclobutene and buta-1,3-diene anion radicals, however, do predict the conrotatory mode. 

This chapter describes the general aspects of anionic polymerization of nonpolar vinyl monomers such as styrenes and 1,3-dienes. Anionic polymerization is defined as a chain polymerization in which the active centers are anions, in the form of... 

The Diels-Alder reaction of dienophiles 5.1a-e, containing neutral, cationic or anionic substituents, with diene 5.2 in the absence of Lewis acids is retarded by micelles of CTAB, SDS and C12E7. In the situation where the dienophile does not bind to the micelle, the reaction is inhibited because uptake of... 

Asymmetric Heck reaction of the conjugated diene 184 and subsequent acetate anion capture of the rr-allylpalladium intermediate afforded 185 in 80% ee. which was converted into the key intermediate 186 for the capnelle-... 

The intramolecular insertion of a conjugated diene into 7r-allylpalladium, initially formed in 789, generates another rr-allyl complex 790, which is trapped with acetate anion to give a new allylic acetate 791. No further reaction of the allylic acetate with alkene takes place[489]. 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Isobutyl group (Section 2 13) The group (CH3)2CHCH2— Isoelectric point (Section 27 3) pH at which the concentration of the zwittenonic form of an amino acid is a maximum At a pH below the isoelectric point the dominant species is a cation At higher pH an anion predominates At the isoelec tnc point the ammo acid has no net charge Isolated diene (Section 10 5) Diene of the type... 

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

The mechanism of the anionic polymerization of styrenes and 1,3-dienes initiated by alkaU metals has been described in detail (3,20) as shown in equations 3—5 where Mt represents an alkaU metal and M is a monomer molecule. Initiation is a heterogeneous process occurring on the metal surface. The... 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-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). 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

Interest in the synthesis of 19-norsteroids as orally active progestins prompted efforts to remove the C19 angular methyl substituent of readily available steroid precursors. Industrial applications include the direct conversion of androsta-l,4-diene-3,17-dione [897-06-3] (92) to estrone [53-16-7] (26) by thermolysis in mineral oil at about 500°C (136), and reductive elimination of the angular methyl group of the 17-ketal of the dione [2398-63-2] (93) with lithium biphenyl radical anion to form the 17-ketal of estrone [900-83-4] (94) (137). 

In solution-based polymerisation, use of the initiating anionic species allows control over the trans /cis microstmcture of the diene portion of the copolymer. In solution SBR, the alkyUithium catalyst allows the 1,2 content to be changed with certain modifying agents such as ethers or amines. The use of anionic initiators to control the molecular weight, molecular weight distribution, and the microstmcture of the copolymer has been reviewed (15). 

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