Model reactions between

During the hardening of PMF-resins no co-condensation occurs in the hardened state two independent interpenetrating networks exist [58]. Indications for a co-condensation via methylene bridges between the phenolic nucleus and the amido group of the melamine had been found by H-NMR only in model reactions between phenolmethylols and melamine. 

At this point it is appropriate to discuss the mechanism for ADMET, because ADMET polymerization is more involved than its chain polymerization counterpart— ROMP. Figure 8.6 illustrates the accepted mechanistic pathway which leads to productive metathesis polymerization, as first described by Wagener et al.14a A general model reaction between an a,o>-diene with a metal alkylidene... 

Lattanzi screened various amino alcoUiol derived thioureas in the enantioselec-tive Morita-Baylis-HiUman model reaction between cyclohexenecarbaldehyde and 2-cyclohexene-l-one (MBH adduct 1 Scheme 6.151) under solvent-free... 

Following this grafting-from approach, monoliths containing 7p,mol (0.07%) Pd were prepared. In a model reaction between styrene and iodoben-zene, TOFs of 1.2-1.6 s were found. These figures clearly exceed those... 

A number of situations may be visualized. Electron transfer may take place between a pair of redox proteins in solution. Certain reactions in the cytoplasm of the red blood cell fall into this category, such as that between hemoglobin and cytochrome b reductase. These reactions will probably occur by an outer-sphere mechanism, as was described earlier for model reactions between isolated electron-transfer proteins and also between these proteins and simple complexes. Interaction between such proteins probably utilizes specific charged areas on their surfaces. The possibility of inner-sphere reactions may have to be considered in a few cases. 

These deviations from linearity indicate the existence of an oligomeric distribution of chiral ligands. Noyori proposed a rationale as follows Due to the different dissociability (stability) of homochiral and heterochiral dimer, the enantiopurity of the remaining reactive catalyst (monomer) is improved as compared with that of the submitted chiral ligand 6 (Scheme 9.5) [11]. Heterochiral dimer is thermodynamically more stable than homochiral dimer, which is consistent with Noyori s rationale mentioned above [12a]. An ab initio molecular orbital study was also reported in a simplified model reaction between formaldehyde and dimethylzinc catalyzed by achiral 2-aminoethanol [12b]. 

Gonzalez et al. [63] investigated the model reaction between ketene (48) and imine (R)-(39) to yield (3-lactam (3R,4S)-(49) (Scheme 11). These authors found that, at the B3LYP/6-31 + G level of theory, only cycloadducts (3R,4S)-(49) should be observed. 

Scheme 27 Model reactions between ketene and IV-MH3 containing imines (108a-c) to form P-lactams (llla-c)...

Scheme 30 Model reaction between ketene and A-metalated imine (117)...

In the meantime, Covey and colleagues (85) have identified the products of model reactions between various nucleophiles and Compound XVII. They obtained two types of product, depending on the nature of the nucleophile. Adducts of the type of Compound XXI were formed when nucleophile = imidazole, thiol, phenol, and carboxylate adducts of the structural type XXII were obtained when nucleophile = pyrrolidine, water, or methanol. 

If the diene fragment of the 2-pyrone and the acetylene dienophile are unsymmetrically substituted, the formation of two isomeric benzene products is possible. The model reaction between 4,5,6-triphenyl-2-pyrone (20) (XXIII) and phenylacetylene yields approximately equal amounts of 1, 2, 3, 4-(XXV) and 1,2,3,5-tetraphenylbenzene (XXVI). Therefore, this position isomerism would be expected to materialize during polymer formation when similar bispyrone monomers are polymerized with diethylnylbenzene. 

Now let us make a short survey of quite different approaches to the problem of identifying the kinetic copolymerization models based on the investigations of the model reactions between the low-molecular compounds only. In this very promising direction it is hard to overestimate the paramount contribution made by Bevington, Tirrell et al. [286-295], whose publications could be divided into two groups. 

U sing TR ESR, a model reaction between the free radicals of Pis and stable nitroxyl radicals of 2,2,6,6-tetramethylpiperidine-A-oxyl (TEMPO) family was studied. We will abbreviate the TEMPO fragment further as N. Nitroxyl biradicals (N-O-N), had radical termini in proximity to each other (see Scheme 12.12). 

The equilibrium constant of polyesterification is typically equal to that of the analogous model reaction between monofunctional compounds. This can be explained by the proposition [3] that the reactivity of a functional end-group in any growing polymer chain is independent of the degree of polymerization or chain-length. There have been several experimental verifications of this theory. Thus the equilibrium constant for the reaction between dimethyl terephthalate (DMT) and ethylene glycol at 280°C was found [31] to be 4.9 under certain conditions. Under identical conditions, the equilibrium constant for the model reaction was 5.0. 

On the other hand, the formation of C—Y bonds was observed in the copper-catalyzed model reactions between the isolated polymers or model compounds such as 1-13 and TEMPO or hydroxy-TEMPO.163 242... 

This supported triamide was utilized in the model reaction between the trichlorosilyl enol ether of cyclohexanone and benzaldehyde (Scheme 3.2), carried out at — 78 °C after 3 h the aldol product was obtained in 80% yield and 1 1 synjanti diastereoselectivity when the temperature was increased to — 23 °C,... 

The different reactivity was ascribed to the different activation of the electron-poor carbon-carbon double bond by the electron-withdrawing group EWG. The catalyst could be utilized with similar results for at least two further cycles in the model reaction between 1-nitropropane and methyl vinyl ketone (E cycle 80% yield, 2 cycle 78% yield, 3 cycle 79% yield). 

The Michael 1,4-addition of methylene activated compounds to various a,p-unsaturated carbonyl compounds in water was promoted by amines grafted on silica gel by a post-modification method. Since the reaction was carried out in a triphasic system, due to low solubility of organic substrates, internal heating by microwave irradiation (100 W) was exploited. In the model reaction between ethyl 2-oxocyclopentanecarboxylate (52) and 3-buten-2-one (53) (Scheme 3.16), the temperature was ramped for 2 min from room temperature to 80 °C. 

The one-portion addition of the reagents in the model reaction between benzaldehyde, ethyl cyanoacetate and dimedone resulted in exclusive formation of the unwanted dimeric product 78 (Figure 3.3), as a consequence of the preferential condensation of benzaldehyde and dimedone due to the higher acidity of dimedone (p/y, 12.0) than that of ethyl cyanoacetate (pKa 13.1). 

The reusability of the catalyst was checked and a consistent activity was observed after several cycles in the model reaction between p-chlorobenzal-dehyde and McaSiCN. 

In the model reaction between 3-phenylpropanal and allyltrichlorosilane (106, Scheme 3.30, R = PhCH2CH2), catalyst 103 was more reactive than 105 polymers with higher loadings showed higher activity and even a catalytic amount of 103 (10 mol.%) was shown to be elfective at a longer reaction time. 

UV spectra also supported complex formation in the binary system. In a model reaction between ZC and benzalacetophenone, it was concluded that the monomer was activated by ZC, while the... 

To demonstrate the feasibility of the quinoxaline activated poly(arylene ether) synthesis, a model reaction between 2,3-diphenyl-6-fluoroquinoxaline, 2, and sodium cresolate was carried out in NMP (160°C) (Scheme II). Quantitative... 

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