Cation scavengers

CF3CO2H, PhSCH3, 25°, 3 h. ° The use of dimethyl sulfide or anisole as a cation scavenger was not as effective because of side reactions. Benzyl ethers of serine and threonine were slowly cleaved (30% in 3 h complete cleavage in 30 h). The use of pentamethylbenzene had been shown to increase the rate of deprotection of 0-Bn-tyrosine. ... 

CF3COOH, 2.5% phenol, 30°, 2 h, 65% yield. Zervas and co-workers tried many conditions for the acid-catalyzed formation and removal of the 5-diphenyl methyl, 5-4,4 -dimethoxydiphenylmethyl, and 5-tripheny I methyl thioethers. The best conditions for the 5-diphenylmethyl thioether are shown above. Phenol or anisole act as cation scavengers. 

BF3-Et20, NaCNBHs, THF, reflux 4-24 h, 65-98% yield. Functional groups such aryl ketones and nitro compounds are reduced and electron-rich phenols tend to be alkylated with the released benzyl carbenium ion. The use of BF3 Et20 and triethylsilane as a cation scavenger is also effective." ... 

CF3COOH, CH2CI2, 25°, 1 h. The addition of Et3SiH to the deprotection step improves the yields over that obtained with the normal cation scavengers. ... 

The lower yield for olefin than for S02 was explained by scavenging of the formed free olefin on the cationic sites of the irradiated polymer in a homopolymerization reaction, thus reducing G(olefin). Adding cation scavengers it was found that the overall product yield was reduced with concurrent reduction of the S02/olefm ratio towards unity73. Thus it can be concluded that the homopolymerization of the olefin is occurring by a cationic mechanism. 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

As part of the same study, the capacity of this novel resin to act as an allyl cation scavenger was demonstrated in a palladium-catalyzed O-alloc deprotection of O-alloc benzyl alcohol (Scheme 7.107) [125], Benzyl alcohol was obtained in high yield with only trace amounts of by-product, thereby eliminating the need for further purification. The resulting C-allylation of the resin was evident from the presence of C-allyl signals in the relevant MAS-probe 1H NMR spectrum. 

The parasitic formation of polymers of high DP and/or broad DPD in the same reaction mixture as the living polymers is due to cationic polymerisations initiated by adventitious impurities it can be prevented by cation scavengers such as halide ions and other bases. 

Since 1965, Ueno and the present author (K. H.) have extensively studied the radiation-induced polymerization of rigorously dried styrene in bulk (39, 40, 41). They found that the rate of polymerisation was increased remarkably by drying the monomer. This was attributed to the radiation-induced cationic polymerization which was enhanced by the removal of water, a cation scavenger, from the polymerization system. Potter et al. reported the same findings independently (42). 

The cationic polymerization was further evidenced by studying the effect of added proton scavengers, such as ammonia and trimethylamine, and the copolymerization with a-methylstyrene and isobutylvinylether. It is now believed that both radical and cationic polymerization of styrene are able to proceed by ionizing radiations even in bulk at room temperature, and the latter polymerization is much more predominant in the absence of cation scavengers, though it is effectively suppressed in their presence. 

However, this mechanism does not explain the chain reaction. Tabata and coworkers measured the optical spectrum of the dimer cation radical, by pulse radiolysis of benzonitrile solution of the dimer immediately after the pulse. They found only a peak at 770 nm without other peaks, except for a possible small shoulder at 740 nm (which is within the limit of experimental error). Addition of cation scavengers leads to elimination of this spectrum, while oxygen does not remove it, suggesting that the spectrum is due to a cation. This 770-nm peak of the cation of the cyclodimer of VC reminds one of the 770-nm peak found 1.6 jus after the pulse in the case of 1 M VC solution. It should be noticed that while in this second paper the authors also mentioned this shift from 790 nm to 770 nm, the data in their figure show a peak at 790 nm both immediately and 1.6 jus after the pulse. Consequently, Tabata and coworkers suggested that the observed spectrum in pulse radiolysis of aerated solution of VC in benzonitrile is a composite of the spectrum of VC cation together with that of the cation of the cyclodimer of VC. The contribution of each intermediate to the observed spectrum depends on the concentration of VC and how long after the pulse the spectrum was taken. In a dilute solution, the dimer cation will be produced as time proceeds, but it is absent immediately after the pulse. In concentrated solutions, both cations coexist even immediately after a pulse. 

The surprising factor of the reaction, which shows how intriguing and constantly innovative synthetic chemistry can be, is the fact that the annulation reaction reported in equation 40 occurs in satisfactory yields in the absence of an alcohol additive (the cation scavenger), which is normally used in these reactions"". As a matter of fact, when the reaction was performed at a carbon anode and current density of 0.5 mAcm, the hydrolysed ketone 56 was formed as the main product and the yield of 55 was only 5% when using a 4/1 CHsCN/i-PrOH mixture as solvent. The 55/56 molar ratio turned 6.5/1 (67% yield) in the absence of alcohol, the other experimental condition being essentially the same . 

The initially recommended conditions for Bpoc cleavage (Table 9) have been refined for use in solid-phase synthesis with 0.5% TFA in CH2Cl2.f l Although such conditions are compatible with most of the Boc, tBu, and even Trt side-chain protections, repetitive treatment of growing peptide chains under these acidic conditions leads to partial deprotection of Lys(Boc), Tyr(tBu), and His(Trt) derivatives whilst the residues Asn(Trt) and Gln(Trt) are sufficiently stable.P l The 2-(biphenyl-4-yl)propene formed in the acidolytic deprotection step as well as its dimer represent a source for the formation of related cations which may react with nucleophilic side-chain functionalities as present in tryptophan and methionine residues, thus requiring efficient cation scavengers, e.g. indoles or Trp derivatives. 

Use Chemical reagent, cation scavenger, coagulant, carrier for radioactive phosphorus. 

PhSH, BF3-Et20, 98% yield. With dimethylsulfide as the cation scavenger an adjacent PMB ether is stable. ... 

The addition of pyrrole as a cation scavenging agent has been recommended for use in deprotection during solid-phase DNA and RNA synthesis. The Px or Tx groups have been recommended as a better alternative to DMTr group in DNA and RNA synthesis because of their faster cleavage rates. 

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