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Monomer enolates

Similar information is available for other bases. Lithium phenoxide (LiOPh) is a tetramer in THF. Lithium 3,5-dimethylphenoxide is a tetramer in ether, but addition of HMPA leads to dissociation to a monomer. Enolate anions are nucleophiles in reactions with alkyl halides (reaction 10-68), with aldehydes and ketones (reactions 16-34, 16-36) and with acid derivatives (reaction 16-85). Enolate anions are also bases, reacting with water, alcohols and other protic solvents, and even the carbonyl precursor to the enolate anion. Enolate anions exist as aggregates, and the effect of solvent on aggregation and reactivity of lithium enolate anions has been studied. The influence of alkyl substitution on the energetics of enolate anions has been studied. ... [Pg.390]

The work of Myers et al. [6] illustrates the synthetic potential of the use of metal salts (instead of HMPA ) in alkylation reactions of enolates, employing easily accessible amide, enolates of the chiral auxiliary pseudoephed-rine. It is not surprising that the mechanism of chiral induction is not yet fully understood further investigations are necessary. Nonetheless, unanswered questions in enolate chemistry remain even for tailor-made, well-established auxiliaries, whose asymmetric induction can be explained convincingly by working models on monomer enolate structures, considering chelation control and steric factors. [Pg.20]

In an attempt to generate convenient routes to organofunctional phosphazenes which could undergo homopolymerization reactions, we explored the reactions of lithium enolates, particularly that of acetaldehyde, with halophosphazenes which leads to the series of organofunctional monomers N3P3ci6 n(0CH=CH2)n15 16... [Pg.293]

The lanthanocene initiators also polymerize EtMA, PrMA and BuMA in a well-controlled manner, although syndiotacticity decreases as the bulk of alkyl substituent increases. Reactivity also decreases in the order MMA EtMA > PrMA > BuMA. Chain transfer to provide shorter polymer chains is accomplished by addition of ketones and thiols.460 The alkyl complexes (190) and (191) also rapidly polymerize acrylate monomers at 0°C.461,462 Both initiators deliver monodisperse poly(acrylic esters) (Mw/Mn 1.07). An enolate is again believed to be the active propagating species since the model complex (195) was also shown to initiate the polymerization of MA. [Pg.26]

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. [Pg.26]

The (TPP)A1X family of initiators has been used to initiate the polymerization of a range of other monomer classes including epoxides, episulfides, and methacrylates.776 In the latter case the propagating species is an aluminum enolate and this too may initiate the ROP of lactones, such as 6-VL, albeit slowly. In this way a block copolymer P(6-VL)-b-PM M A of narrow molecular weight distribution (Mw/Mn= 1.11) has been prepared.787... [Pg.40]

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. [Pg.35]

The crystal structure of the 1 2 adduct 145 obtained by the reaction of [Cp SmH]2 with MMA was determined by X-ray analysis. One of the two monomer units is in the O-enolate form and the other unit coordinates to the samarium atom by the carbonyl group. A comparison between 145 and the... [Pg.35]

These observations showed that the reaction can be simplified by preformation of the indanone enolate in toluene/50% NaOH and subsequent addition of catalyst and CH3CI (Figure 12). This eliminates the "induction period and most importantly the high sensitivity of rate and ee to the catalyst/indanone ratio. Detailed kinetic measurements on this preformed enolate methylation in toluene/50% NaOH determined that the reaction is 0.55 order in catalyst. This is consistent with our finding that the catalyst goes into solution as a dimer which must dissociate prior to com-plexation with the indanone anion. If the rate has a first order dependence on the monomer, the amount of monomer is very small, and the equilibration between dimer and monomer is fast, then the order in catalyst is expected to be 0.5. The 0.5 order in catalyst is not due to the preformation of solid sodium indanone enolate but is a peculiarity of this type of chiral catalyst. Vlhen Aliquat 336 is used as catalyst in this identical system the order in catalyst is 1. Finally, in the absence of a phase transfer catalyst less than 2% methylation was observed in 95 hours. [Pg.77]

Vlhen the chiral methylation is carried out with 30% aqueous NaOH the indanone is deprotonated at the interface but does not precipitate as the sodium enolate (Figure 11). In this system there are 3 to 4 molecules of H2O per molecule of catalyst available while in the 50% NaOH reactions the toluene is very dry with only 1 molecule of H2O available per catalyst molecule thus forcing the formation of tight ion pairs. Solvation of the ion pairs in the toluene/30% NaOH system should decrease the ee which we indeed observe with an optimum 78% versus 94% in the 50% NaOH reaction. In the 30% NaOH reactions the ee decreases from 78% to 55% as the catalyst concentration increases from 1 mM to 16 mM (80 mM 5, 560 mM CH3CI, 20 C). Based on these ee s rates of formation of (-h)-enantiomer and racemic product can be calculated. When the log of these rates are plotted versus the log of catalyst concentrations (Figure 13) we find an order of about 0.5 in the catalyst for the chiral process similar to that found using 50% NaOH consistent with a dimer-monomer pre-equilibrium. The order in catalyst for the... [Pg.77]

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... [Pg.299]

Lithium enolate of p-phenylisobutyrophenone exists as a mixture of monomer and tetramer in dilute THF with = 5.0 x 10 On the other hand, it was shown to... [Pg.909]

Streitwieser and coworkers recently reported that the lithium enolate of p-(phenylsul-fonyl)isobutyrophenone exists as a monomer-dimer mixture iu THE, with the equilibrium constant = (5.0 0.1) x 10 M . The rate of the reaction of the enolate with p-tert-butylbenzyl bromide was measured spectrophotometricaUy at the enolate concentration range of 7 x 10 to 5 x 10 M, where the percentage of the monomer was 4.5 to 11%. The logarithmic plot of the rate vs the enolate concentration was linear with a slope of 0.50 0.04, indicating that the reacting species is the monomer that exists as a minor component in the equilibrium with the dimer. [Pg.909]

Lithiation of 2-biphenylcyclohexanone gave two enolate species in THF one an unconjugated secondary enolate and the other a conjugated tertiary enolate. The former exists dominantly as the tetramer and the latter as a monomer-dimer mixture. In both cases the monomer reacts faster than the aggregates with alkylating substrates. ... [Pg.909]

Polar monomers, such as methyl (meth)acrylate, methyl vinyl ketone, and acrylonitrile, are more reactive than styrene and 1,3-dienes because the polar substituent stabilizes the carba-nion propagating center by resonance interaction to form the enolate anion. However, the polymerizations are more complicated than those of the nonpolar monomers because the polar... [Pg.418]

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... [Pg.553]

Propagation proceeds through an enolate species in the bimetallic mechanism described by Eq. 8-73 in which two metallocene species are involved—one is a neutral enolate, and the other is the corresponding metallocenium cation [Collins et al., 1994 Li et al., 1997]. In LV, the propagating chain is coordinated at the neutral transition metal center (Zr3) and monomer... [Pg.701]

Polymerization of methyl methacrylate (MMA) was initiated with methylalumi-num tetraphenylporphyrin (1, X=Me) under irradiation with visible light. At a monomer conversion less than 10%, Lewis acids such as methylaluminum diphenolates 3a-i and methylaluminum dialcoholates 4a,b were added to the system in order to examine whether they could accelerate the polymerization (Scheme 2) (Table 1). For example, a CH2CI2 solution (10 ml) of a mixture of MMA and 1 (X=Me) with a mole ratio of 217 was irradiated at 35 °C with xenon arc light (k>420 nm) to initiate the polymerization. During 2.5-h irradiation, all the molecules of the initiator 1 (X=Me) were transformed into the growing enolate species 2 (R=Me), where the conversionof the monomer was 6.1%, as estimated by NMR. [Pg.43]


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