TEMPO groups

A planar BLM cannot be investigated by means of the molecular spectroscopical methods because of the small amount of substance in an individual BLM. This disadvantage is removed for liposomes as they can form quite concentrated suspensions. For example, in the application of electron spin resonance (ESR) a spin-labelled phospholipid is incorporated into the liposome membrane this substance can be a phospholipid with, for example, a 2,2,6,6-tetramethylpiperidyl-A-oxide (TEMPO) group ... 

The hydroxy-tempo derivative 11 is first reacted with p-chloromethyl styrene to give a Tempo capped polymerizable styrenic compound 12. Copolymerization of 12 with styrene gives the multifunctional initiator 13, which has a PS backbone with attached Tempo groups. Reaction of 13 with styrene at 130 °C gives the grafted copolymer 14. After cleavage of the benzyl ether bonds, a Mn of 23000 is determined with a Mw/Mn value of 1.20. 

Alternative bimolecular methods have been reported that involve mixing appropriate ratios of monomer with free-radical initiators (such as benzoyl peroxide) and an excess of the nitroxide stable free-radical moiety. Such bimolecular methods do not afford the same degree of control of molecular weight and polydispersity since the stoichiometry of the mediating system cannot be accurately dehned, which is a crucial factor in these controlled polymerization systems. A wide variety of unimolecular nitroxide based initiator systems have been described in the literature with those based upon the 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) group proving to be the most commonly used. 

The remaining signals are assigned to the bulk of the copolymer. The peaks at 7.0-7.3 ppm arise from the aromatic protons of the styrene units. The vinylic protons of the various isoprenyl stereoisomers give a complex pattern in the 4.6-5.9 ppm range. Signals between 0.9 and 2.8 ppm are due to the aliphatic protons of styrene and isoprene units as well as the methyl and methylene protons of the terminal TEMPO group. 

Hbbartner and coworkers have recently employed spin label 11 (Fig. 1) to detect simultaneously two competing structures of the incompletely self-complementary RNAs, the hairpin and duplex [30]. In the hairpin conformation the two spin labels are 6 bp apart, resulting in a distance of 1.8 nm (Fig. 15). Upon addition of complementary RNA strand the hairpin structure becomes disrupted and a continuous 20-bp duplex is formed. In the newly formed helical structure the two TEMPO groups are 11 bp apart, yielding a distance distribution centered at 3.1 nm. By increasing the amounts of complementary RNA the ratio between the two coexisting structures shifts completely towards the RNA duplex. 

When chloromethylstyrene was utilized in the ATRP of 31 (Scheme 6.17), the polymer 33 was obtained. The benzylic chlorides do not interfere with the ATRP reaction, su esting that the TEMPO group is a considerably more active initiator than the benzylic chloride [70,71]. Nucleophilic ring-opening polymerization with an oxazoline furnishes doubly grafted, water-soluble, fluorescent PPEs 34. 

Polypropylene-g-PS copolymers were synthesized by combination of metallocene and TEMPO living free-radical polymerization techniques (41). The backbone was synthesized by copolymerization of propylene and a TEMPO-fimctionalized derivative containing a a-double bond. The TEMPO groups were then used for the pol5mierization of styrene by hving free-radical pol5onerization (Fig. 7). 

Right-handed peptoids 141 with the TEMPO group attached at the N-terminus showed catalytic activity and enantioselectivity (the overall conversion after 2 h was 84% with 60% enantioselectivity for (5)-l-phenylethanol, resulting in 99% enantiomeric excess (ee) of the less-reactive -enantiomer). Left-handed peptoid 142 exhibited... 

Figure 6 A homopolymer formed in the presence of the radical TEMPO group. Reproduced with permission from Katsumata, T. Qu, J. Shiotsuki, M. etal. Macromolecules 2m, 41,1175. ...

Figure B2.4.8. Relaxation of two of tlie exchanging methyl groups in the TEMPO derivative in figure B2.4.7. The dotted lines show the relaxation of the two methyl signals after a non-selective inversion pulse (a typical experunent). The heavy solid line shows the recovery after the selective inversion of one of the methyl signals. The inverted signal (circles) recovers more quickly, under the combined influence of relaxation and exchange with the non-inverted peak. The signal that was not inverted (squares) shows a characteristic transient. The lines represent a non-linear least-squares fit to the data.

The identification of both phenylethyl and 1-phenyl-1,2,3,4-lelrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMP provides the most compelling evidence for the Mayo mechanism.316 The 1-phenyl-1.2,3,4-tetrahydronaphthalenvl end group is also seen amongst other products in the TEMPO mediated polymerization of styrene,317318 However, the mechanism of formation of radicals 96 in this case involves reaction of the nitroxide with the Diels-AIder dimer (Scheme 3.63). The mechanism of nitroxide mediated polymerization is discussed further in Section 9.3.6. 

The majority of polymers formed by living radical polymerization (NMP, ATRP, RAFT) will possess labile functionality at chain ends. Recent studies have examined the thermal stability of polystyrene produced by NMP with TEMPO (Scheme 8.3),2021 ATRP and RAFT (Scheme 8.4).22 In each case, the end groups... 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

The same group recently disclosed a related free radical process, namely an efficient one-pot sequence comprising a homolytic aromatic substitution followed by an ionic Homer-Wadsworth-Emmons olefination, for the production of a small library of a,/3-unsaturated oxindoles (Scheme 6.164) [311]. Suitable TEMPO-derived alkoxy-amine precursors were exposed to microwave irradiation in N,N-dimethylformam-ide for 2 min to generate an oxindole intermediate via a radical reaction pathway (intramolecular homolytic aromatic substitution). After the addition of potassium tert-butoxide base (1.2 equivalents) and a suitable aromatic aldehyde (10-20 equivalents), the mixture was further exposed to microwave irradiation at 180 °C for 6 min to provide the a,jS-unsaturated oxindoles in moderate to high overall yields. A number of related oxindoles were also prepared via the same one-pot radical/ionic pathway (Scheme 6.164). 

The addition of an acceptor decreases the rate of POOH decomposition. The increase of added [InH] creates a tendency for k-% to decrease to the kA value, i.e., Ax —> A d at [InH] —> DC. Acceptors, which do not react with hydroperoxide groups, were used sterically hindered phenols and stable nitroxyl radicals (TEMPO) were found to be efficient acceptors. The ratio kinA(2kt)m can be calculated from the values Ax and A d according to the formula ... 

Treatment of D-glucose with nitromethane and sodium methoxide afforded nitrosugar 46 which, on reduction of the nitro group, protection of the resulting amine with Fmoc-Cl and TEMPO oxidation afforded 47 in a three-step sequence. 

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