Reaction realkylation

Radiochemical experiments show that the number of polymer chains terminated by allyl groups are a minor fraction of the total, and the majority of chains derive from the realkylated hydrides. In the presence of hydrogen it is evident from Table III that chain transfer reactions dominate and some saturated polymer is formed ... 

In these polymerizations the initiation steps (17) and the realkylation of the hydride are rapid. The majority of the polymer is derived from the hydride (XX). It therefore follows that after the initiation reaction the dominant processes are (18)-(21). 

The catalysts described in Table XII cannot be used to make tailored-block copolymers because of reaction (19). The latter continues in the absence of monomer resulting in detachment of chains from the transition metal centers forming hydride (XX). Introducing a second monomer would lead to realkylation of the chain centers giving a homopolymef of the second monomer. Hence mixtures of homopolymers would be obtained with little block-copolymer formation. 

DIPN was the initial product, and the selectivity was up to 40%, but it decreased by the isomerization to 1,3-DIPN. These compounds converted to approximately equal amounts of 2,6- and 2,7-DIPN after prolonged periods. However, 1,4-DIPN was obtained in high yield at the low temperature of 160 °C. These changes reflect the differences in reactivity and thermodynamic stability of the -positions of naphthalene. The product distribution for HY was governed kinetically at low temperatures and/or at a short reaction period, but thermodynamically at high temperatures. The isopropylation occurs at active a-positions to form a,a-DIPN in the first step. In the second step, a,a-DIPN converted to thermodynamically stable a,P- and (3, 3-DIPN by transalkylation and dealkyla-tion-realkylation at acid sites in the supercages of HY. 

The acid 350 was demethylated with pyridine hydrochloride, then realkylated with benzyl bromide in aqueous potassium hydroxide to give 351. The latter was converted to the diazoketone 352 by the sequential treatment of 351 with oxalyl chloride and etheral diazomethane. Reaction of 352 with concentrated hydrobromic acid gave the bromoketone 353. The latter was reduced with sodium borohydride at pH 8 -9 to yield a mixture of diastere-omeric bromohydrins 354. Protection of the free hydroxyl as a tetrahydro-pyranyl ether and hydrogenolysis of the benzyl residue afforded 355. The phenol 355 was heated under reflux with potassium m/V-butoxide in tert-butyl alcohol for 5 hr to give a 3 1 epimeric mixture of dienone ethers 356 and 357 in about 50% yield. Treatment of this mixture with dilute acid gave the epimeric alcohols 358 and 359. This mixture was oxidized with Jones reagent to afford the diketone 349. 

The thioalkyl-containing macrobicyclic complexes have been dealkylated and realkylated easily under the action of potassium thiolates in aprotonic media (Scheme 14). The products of de- and realkylation reactions were detected by PD and FAB mass spectrometry. In this respect the thioalkyl-containing clathrochelates are close to the aryl alkyl sulfides. In the course of thioalkyl derivative synthesis with an excess of potassium thiolate, a mixture of dealkylated products was obtained in addition to the desired hexathioalkyl clathrochelates. The addition of corresponding alkyl iodide and potassium carbonate to the reaction mixture in the final stage of reaction led to an increase in yields by alkylation of HS groups, resulting in the side dealkylation process [65]. 

After reaction with the monomer to form a new propagating chain the position is formally the same as transfer with monomer. However, the two mechanisms can be distinguished kinetically if realkylation of the catalyst is slow compared with propagation. There is no direct evidence for this reaction although it is well established that the relatively stable alkyls of ms nesium and aluminium form metal hydride bonds on decomposition at elevated temperatures [83]. The existence of spontaneous termination has been deduced from a consideration of the kinetics, and by analogy with the effects of hydrogen on the polymerization. 

The rate was increased by the presence of hydrogen. A rather complex polymerization mechanism is proposed in which the propagation step is regarded as an alkyl transfer reaction, followed by realkylation with monomer of the metal hydride so formed. 

These results confirm that, because of their structure and physico-chemical properties, the HM and HZSM-5 zeolites are not suitable catalysts to produce the main reaction products with high selectivity values, because the side reactions of oligomerization, cracking, realkylation and coking are prevailing. 

Alkylation and realkylation reactions have been reported for derivatives of five different ring systems. These are summarized in Table 4. 

The role of methanol is quite important ih this reaction it acts as the acceptor of the trityl group. In a similar fashion ethylmercaptane, used in considerable excess, facilitates the acidolytic cleavage of the S-trityl group and prevents realkylation of the regenerated sulfhydryl group ... 

Konig et al. [274] investigated the long-term stability of two selected ILs over several months under process-hke conditions with a subsequent IC-MS analysis to identify the resulting decomposition products. As an example, samples of the imidazohum-based ILs l-ethyl-3-methylimidazolium chloride (EMIM Cl) and 1-ethyl-3-methyhmidazohum acetate (EMIM Ac) were thermally and catalytically stressed. The most obvious decomposition of a dialk)dimidazolium cation is the reverse reaction of the quaternization reaction. The typical products are imidazole derivatives and alkylated anions (reaction (1) in Figure 8.120). Further considerable decomposition products are cations with scrambled alkyl chains. These cations can be formed either due to a realkylation of a previously dealkylated... 

The reason for the appearance of the dichlorospecies and the oligopropene is a very fast fi-chloride elimination reaction, which itself is much faster than the coordination and insertion of a further vinylchloride monomer. In the polymerization experiment with MAO, the chloro species can be realkylated by MAO either to produce further propene from vinylchloride or to oligomerize free propene to atactic oligopropene, respectively (Scheme 8). 

---