Equilibration, monomer-dimer

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. 

Figure 3 Equilibrium association of CAB as a function of final protein concentration ([CAB]f ). Unfolded CAB in 5 M GuHCl was rapidly diluted to a final GuHCl concentration of 2.0 M and a range of final protein concentrations. Each solution was allowed to equilibrate for three to eight hours prior to QLS analysis. The equilibrium monomer ( ), dimer ( ), and trimer ( ) concentration is shown for each final protein concentration.

An attempt to determine the sequential equilibrium constants, Ki, K2, etc., was reported by Vrancken et al.183). They initiated polymerization of a-methyl styrene by a stepwise addition of the monomer to a dilute solution of its dimeric dianions in tetrahyd-rofuran. After each addition the concentration of the residual, equilibrated monomer was determined, and its value was plotted vs. the total concentration of the supplied monomer. The resulting curve is shown in Fig. 4, and its shape allows, in principle, the determination of Ki, K2 and Kp = K3 = K4, etc. Although the approach is sound, this method gives only a reasonable estimate of Kt because the deviation of K2 from the Kp s value is too small to be determined. The maximum seen in Fig. 4 arises from an increase in the activity coefficient of the monomer caused by increasing volume fraction of the polymer - an effect discussed earlier. 

Figure 7-5 The ionic strength dependence of the monomer/dimer equilibrium of P. denitrificans cytochrome C550- Cytochrome csso (2.3 nmol) dissolved in 10 mM Hepes, pH 8.0, containing the appropriate NaCl concentration, were passed down a Superdex 75 column equilibrated in the same buffer. The elution volume was compared with a set of standards (serum albumin, ovalbumin, carbonic anhy-drase, myoglobin and cytochrome c) in order to obtain a value of Mr. O, oxidized cytochrome C550 , cytochrome C550 reduced with 1 mM ascorbate.

Crystallization leads only to the dinuclear titanium(IV) helicates. However, the monomer-dimer equilibrium in solution can be investigated easily by using different NMR techniques. In the dimer, the protons of R near the carbonyl unit of one complex moiety are neighbored by an aromatic catecholate of the second and thus experience an anisotropic shift to high field. In the monomers, the enantiomeric A and A complexes equilibrate very quickly, generating only one signal of CH2 groups at the substituent R. In the dimer, the stereochemistry is locked and no fast racemization... 

Y.-X. Chen, T. J. Marks, Constrained geometry dialkyl catalysts. Efficient syntheses, C-H bond activation chemistry, monomer-dimer equilibration, and a-olefin polymerization catalysis. OrganometalUcs 16, 1997 3649. 

In the infrared, 2-hydroxycyclobutanone has a carbonyl band at 1780 cm in chloroform solution. Kept in nitrogen-filled screw-capped vials in the freezing compartment of a refrigerator, 2-hydroxyoyolo-butanone slowly but completely solidifies as its dimer. The infrared spectrum of the solid in a KBr disk shows no carbonyl. However, a chloroform solution of the solid does show the characteristic 1780 em band, indicating rapid equilibration with the monomer. 

By means of this expression, the values of Yt yield [A]f, and Eq. (3-28) provides the means for data analysis. Or, with additional algebra, one can express Y, directly, and float both k] and Ye in the calculation. As an example of the application of Eq. (3-28), consider the dimer-monomer equilibration of triphenyl methyl radical 2... 

Many selenoketones18 20 and telluroketones21 23 can be obtained in the solid state as stable 1,3-diselenetanes and 1,3-ditelluretanes and their formation can be assumed as proof of the existence of unstable selenoketone and telluroke-tone. In solution dimers of aromatic selones equilibrate with monomeric species, according to Scheme 5, where equilibrium is shifted by dilution towards the monomer.18,24... 

Further addition of acetate caused another series of spectral changes which could best be interpreted as conversion of dimer to monomer. This reaction was also slow. Studies on fully equilibrated solutions in the... 

Equilibration of carboxylic acid dimers and monomers in the sample solution depends on the extent to which the carboxy group is involved in hydrogen bonding with the solvent ... 

The three kinetic ranges of high, moderate, and low values of kf/a are all observed in the equilibration and reduction of the 17-electron complex CpCr(CO)3 (Cp = r 5-C5H5). In this system (Eq. 23.18 and drawing), the monomer CpCr(CO)3 corresponds to A and the metal-metal bonded dimer Cp2Cr2(CO)6 to Y in Equation 23.17. The monomer is reduced to the 18-electron monoanion (E0 = -0.82 V vs. Fc), and the dimer is electroactive only at much more negative potentials. 

Zinc increases the rate of dimerization 21, 65, 66) however, Apple-bury and Coleman 48) showed that zinc is not necessary for dimerization to occur. Starting at a high pH and slowly lowering the pH they found that all of the zinc is lost by the time the pH reaches 4.0, yet the molecule though inactive is still dimeric. However, upon increasing the pH of a solution of monomers, the dimer reforms by pH 5.0, yet the zinc does not bind completely until pH 6.0. Also, the optical rotatory dispersion (ORD) spectrum is the same for dimer at pH 8.0 and monomer at pH 4.0, but a spectral change occurs for the monomer at pH 2.0 48, 67). Applebury and Coleman 48) reasoned that there must be a kinetic barrier which prevents any rapid equilibration of the dimer monomer system at intermediate pH values and that the same barrier exists in the hysteresis loop involved in the titration of carboxyl groups on the enzyme 21, 67). 

In the reservoir model , which was confirmed by Noyori and coworkers as the prototype case of catalytic asymmetric organozinc additions to carbonyl compounds in the presence of (-)-3-exo-(dimethylamino)isoborncol (DAIB) yielding chiral benzyl alkanols with a higher ee than that of the added DAIB [17,18], there is a reversible equilibration between monomers and dimers. It is assumed that the monomers r and s are the catalysts and that the heterochiral dimers [r s] are of higher thermodynamic stability than their homochiral ([rr] and [s s]) analogues ... 

Electron transfer from oxygen to copper gives a phe-noxyl radical, which couples with another copper-bound radical to form the C—O—C dimer and Cu(I). The reaction behaves as a step reaction rather than a chain reaction. A quinol ether rearrangement occurs to equilibrate polymer and monomer. High-molecular-weight polymer is formed only in the late stages of reaction. Indeed, other phenols are incorporated into the polymer if they are added at the end of the reaction because of the quinol ether rearrangement. 

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