Benzyl linkage

The prototype of a functional group with an appropriate breakable bond is the Wang resin 3, which contains a 4-hydroxybenzyl alcohol linker moiety. The benzyl alcohol hydroxyl group can be functionalized using either electrophilic or nucleophilic substrates (Scheme 6.1.1) to give a benzylic linkage. It is very stable in many reactions, but can be cleaved by acids such as trifluoroacetic acid or HF. Acids, alcohols, esters, and amides can be obtained as products after cleavage. 

Though the emission stability of 129 is much better than for PDAFs, long-wavelength emission is seen with time, possibly due to the susceptibility of the benzyl linkages to oxidation. Accordingly, a new class of polyfluorenes with aryl substituents at the 9-position has been developed (Scheme 61) [228]. Aryl substituents are much less susceptible to oxidation to lluorenone than alkyl groups and their bulk reduces interchain interactions, and hence exciton diffusion to any defect sites that may be formed. 

In the presence of sulfide or sulfhydryl anions, the quinonemethide is attacked and a benzyl thiol formed. The P-aryl ether linkage to the next phenylpropane unit is broken down as a result of neighboring-group attack by the sulfur, eliminating the aryloxy group which becomes reactive phenolate ion (eq. 2). If sulfide is not present, a principal reaction is the formation of the stable aryl enol ether, ArCH=CHOAr. A smaller amount of this product also forms in the presence of sulfhydryl anion. 

Scheme 4a shows the condensation of a benzyl alcohol group with a phenolic ring position occupied by hydrogen to produce a methylene linkage between two phenolic rings and producing one mole of water as a by-product. This type of condensation occurs at both high and low pH. It is the type most commonly seen in both resoles and novolacs. 

In a further development on this theme, the thiol, 153, is first alkylated to the corresponding benzyl ether (158). Treatment with sodium methoxide removes the proton on the amide nitrogen to afford the ambient anion (159). This undergoes alkylation with methyl bromide on the ring nitrogen thus it locks amide into the imine form (160). Chlorolysis serves both to oxidize the sulfur to the sulfone stage and to cleave the benzyl ether linkage there is thus obtained the sulfonyl chloride, 161. 

The authors have also elaborated a microwave-enhanced one-pot procedure [90] for the Huisgen 1,3-dipolar cycloaddition reaction. In a typical procedure, a pyrazinone with a triple bond connected to the core via C - O linkage, was reacted with a suitable benzylic bromide and NaNs in presence of the Cu(I) catalyst in a t Bu0H/H20 system under microwave irradiation (Scheme 26). The cycloaddition was found to proceed cleanly and with full regioselectivity. As the azide is generated in situ, this procedure avoids the isolation and purification of hazardous azides, which is especially important when handling the ahphatic ones, which are known to be toxic and explosive in nature. 

Fig.1. Two poly(benzyl ether) dendron subunits connected by an azo linkage...

The selective and facile cleavage of the benzylic ether linkages of 1,2,3 or 4 is accomplished by treatment with iodotrimethylsilane to form the corresponding benzylic iodide. Further addition to these iodide derivatives of 1 affords dendrimers of generation 1 with phosphonium ion sites at the periphery. Such a strategy is conducted up to generation 3 with a phosphine or a phosphonium core (Scheme 3). 

These results indicate that the aliphatic portion of the coal is very important in the initial phases of coal conversion. Weak linkages must be associated with the aliphatics in coal though they have not as yet been completely identified. Both of the above methods show an increase in the aromatic methyl content of SRCs at short times which indicates that cleavage at a benzylic carbon is important in dissolving the coal. 

Scheme 2 Drug-PEG conjugate via a benzyl carbamate linkage and its thiolytic cleavage...

The most convincing evidence in favor of a uniform 3,5-diester linkage between nucleotides has been obtained by the action of various enzymes on synthetic diesters of known constitution.218 217 Ribonuclease and spleen extracts were found to act only on nucleoside 3-(benzyl hydrogen phosphates), but not on other isomers, to give nucleoside cyclic phosphates which are broken down further to give nucleoside 3-phosphates. It is concluded, by analogy, that polynucleotides, which are substrates for these enzymes, also possess ester groupings at the 3-positions, rather than at the... 

The Lipari-Szabo approach was used to understand the cause of the pH-dependent relaxivities of PAMAM-type dendrimeric Gdm complexes (74). Three different generations (5,7,9) of PAMAM dendrimers loaded with the [Gd(EPTPA)(H20)]2 chelate via a benzyl-thiourea linkage have been investigated (Scheme 8). The relaxivities show a strong and reversible pH dependency for all three dendrimer complexes,... 

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