Group transfer reactions benzene system

Hydrocarbon analogues with p = 0, q = 2 provide another interesting example of a group transfer reaction in which the driving force of the reaction is aromatization of cyclohexadiene to benzene system (Scheme 6.3). 

Sangalov et al., prepared arenonium ions in the form of Gustavson complexes starting from various substituted benzenes, hydrogen chloride and aluminium chloride. These complexes were drown to be active initiators for the polymerisation of styrene and isobutene at —30 and —78°C in methylene chloride. No kinetic investigation was carried out on these systems. The authors claimed that initiation took place by protonation of the olefin and that incorporation of aromatic groups from the cataylst in the products was due to transfer reactions. 

BzA was polymerized in a higher yield than BA, in spite of the greater bulkiness of the benzyl group. This seems to be the result of a high affinity of the aromatic group to poly(NMAAm), which causes BzA monomer to proceed easily to an active center. Poly(NMAAm) swells considerably in aromatic solvents such as benzene, acetophenone and methyl benzoate, but not in the aliphatics such as acetone, acetonitrile and AcOEt. However, the block copolymer yield in the BzA system was lower than with other acrylates. This is attributed to the participation of labile benzylic hydrogen of BzA in chain transfer reactions. 

Organic azides, which are now easy to prepare and compatible with many functional groups, are a useful reagent for N-atom transfer reactions. The initial example of catalytic aziridination reactions of alkenes using aryl azides was reported by Cenini and co workers in 1999 [ 13]. In the presence of (TPP)Ru(CO) (TPP = tetraphenylporphyrin) complex as a catalyst, the reaction of cyclooctene withp-nitrophenylazidein benzene afforded the corresponding aziridine (Scheme 2.8). This reaction system can be also applied to aziridination of various styraie derivatives with aryl azides. 

Nafion-H appears to be a very useful catalyst for transalkylation reaction as indicated in these studies. Transalkylation of benzene with diethylbenzenes, as well as with diisopropylbenzene, is efficiently catalyzed by Nafion-H in a flow system. The efficiency of the catalyst is, however, more limited when the transferring group is a methyl group.268 Beltrame and co-workers have also carried out269 detailed mechanistic studies on the isomerization of xylenes over Nafion-H. 

Having optimized the catalytic enantioselective phase-transfer alkylation system, the group explored the scope and limitations. A variety of electrophiles were reacted with the benzophenone imine glycine tert-butyl ester 1 catalyzed by 5 mol% of the selected chiral dimeric PTCs, benzene-linked-l,3-dimeric PTC 37, 2 -F-benzene-linked-1,3-dimeric PTC 41, and naphthalene-linked-2,7-dimeric PTC 39, at reaction temperatures of 0°C or — 20 °C (Scheme 4.8). 

In general, carbonyl groups do not function as good initiators in vinylsilane-mediated cyclization reactions and relatively few examples exist. The cyclization of vinylsilanes with ketones or aldehydes as initiators is a highly underdeveloped reaction that holds considerable potential. As reported by Tius and coworkers, treatment of aldehyde (13) with a catalytic amount of p-toluenesulfonic acid gave a mixture of the tetralins (14) and (15) in a combined yield of 53%, with lesser amounts of the enone (16 Scheme 8). The enone system presumably arises from the intramolecular 1,3-hydride transfer of the intermediate a-silyl carbocation (17). Similarly, the vinylsilane (18) undergoes cyclization to produce the disub-stituted benzene derivative (19), although yields are low. 

Oxidation of the amino acid moieties in irradiated aqueous systems by reaction with OH is well established for fluid systems, but it is not likely to be encountered in frozen systems. Being a strong oxidant, the OH reacts by electron transfer. It also adds readily to double bonds and abstracts H from C—H, N—H, and S—H bonds, but with lower reaction rate constants. A compendium of rate constants for aqueous solution has been published (52) and a few representative values for amino acids are shown in Table I. As discussed by Simic (53), the predominant sites for reaction in amino acids and peptides can be inferred from these values, which indicate that the ring groups are favored, while abstraction from the peptide backbone is less likely. Hydroxylation of the phenylalanine ring also occurs as was found for the prototype reaction with benzene (54). Formation of phenoxyl radical following OH addition to tyrosine should be similar to the mechanism established for phenol (55) in which elimination of water occurs as is shown in reaction 12 ... 

Phospholipase D (EC 3.1.4.4) is a lipolytic enzyme that hydrolyzes the terminal phosphodiester bond on PLs. Due to its ability to transfer the phosphatidyl moiety of glycerophospholipids to various alcohols (transphosphatidylation), PLD is also used to synthesize PLs with desired head groups that are poorly accessible via the chemical route (Figure 23.4). This ability has been utilized for the synthesis of natural PLs that are rare in nature, such as PG and PS. Novel types of PLs (phosphatidyl-X) have also been synthesized via PLD-mediated transphosphatidylation to add the amphiphilic properties of PLs to the acceptor compounds. These reactions are typically carried out in biphasic systems with water (containing PLD or a hydrophilic alcohol acceptor) and an organic solvent such as chloroform, ether, ethyl acetate, benzene, or toluene. 

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