2,2, 6,6-Tetramethylpiperidine synthesis

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

The second synthesis of crystalline 43 was reported by Mori as summarized in Scheme 62 [93]. The building block (4.R,5S)-A was prepared by an enzymatic process, while another building block C was synthesized via Sharpless asymmetric epoxidation. Coupling of A with C gave D, which was cyclized under Op-polzer s conditions to give crystalline E. When E was oxidized with Dess-Martin periodinane or tetra(n-propyl)ammonium perruthenate or Jones chromic acid, crystalline 43 was obtained. Swern oxidation or oxidation with 2,2,6,6-tetramethylpiperidin-1 -oxyl of E afforded only oily materials. Accordingly, oxidation of E to 43 must be executed extremely carefully. A synthesis of oily 43 was reported by Gil [94]. 

Normal-phase TLC has been employed for the control of the synthesis of some new reactive azo dyes containing the tetramethylpiperidine fragment. The chemical structure of the basic molecule and the substituents of the new derivatives are shown in Fig. 3.16. The new derivatives were characterized by their RF values determined in different mobile phases. Compositions of mobile phases were n-propanol-ammonia (1 1, v/v) for dye 1.2 (Rp = 0.84) n-propanol-ammonia (2 1, v/v) for dyes 1.3 (RF = 0.50) and 1.4 (RF = 0.80) and n-heptane-diethyl ether (1 1, v/v) for dyes 1.5 (RF = 0.80) and 1.6 (RF = 0.76). The results indicated that together with other physicochemical methods such as IR and H NMR, normal-phase TLC is a valuable tool for the purity control and identification of new synthetic dyes [96],... 

Some new benzanthrone dyes were synthesized and applied for the one-step colouration and stabilization of polystyrene. The chemical structures of monomeric benzanthrone dye (formula 1), the stabilizer TTMP 2,2-(2,2,6,6-tetramethylpiperidine-l-yl)-4,6-dichloro-l,3,5-triazine (formula 2) and the new synthetic product showing both colouration and stabillizer capacity (formula 3) are shown in Fig. 3.135. The synthesis process was controlled by TLC using a silica stationary phase and an n-heptane-acetone (1 1, v/v)... 

P Petrova-Miladinova and T.N. Konstantinova, On the synthesis of some reactive azo dyes containing tetramethylpiperidine fragment. Dyes Pigm., 67 (2005) 63-69. 

Figure 13.15 Synthesis of 2,2,6,6-tetramethylpiperidine A-oxide (TEMPO) functionalized dendrimers.

In 1994, Osa and co-workers reported a very interesting means of obtaining chiral bi-naphthol based on electrocatalytic synthesis [139]. This work required the coating of the electrode with a poly(acrylic acid) (PAA) layer bonded via an amide connection to 4-amino-2,2,6,6-tetramethylpiperidin-l-yloxyl [91, 92]. (-)-Sparteine (208) was used as both the mediator and as a chiral agent (present in a stoichiometric amount) in the anolyte compartment. The yields and enantiomeric excesses obtained were excellent (Table 34). The complexity of... 

A polyadduct formed from aniline, phenothiazine, oxirane and 2,3-epoxypropane [145] was proposed as a component for the synthesis of a light stable PUR. A polyadduct of 4,4 -isopropylidenebisphenol with 2-hydroxy-4-(2,3-epoxypropyl)-benzophenone [131], polymeric HALS 115 formed from bis(2,3-epoxypropyl)ani-line and 4-amino-2,2,6,6-tetramethylpiperidine [146] or a brominated oligomer 116 used in combination with antimony trioxide as FR for PET [147] are other examples of polyaddition stabilizers. 

Gallc er and co-workers devised a formal enantioselective synthesis of ( — )-3 in wduch the stereogenic center at C-6 was derived fiom Cbz-protected (S)-2-amino-4-pentenoic acid (36) (44). Acylation of 3,3-dimethoxy-pyrrolidine (37) with this acid yielded amide 38, which was converted into aldehyde 39 by cleavage of the terminal alkene vnth osmium tetroxide and sodium periodate (Scheme 5). The indolizidine nucleus was constructed from 39 by a problematic intramolecular aldol condensation, which was eventually optimized by using 2,2,6,6-tetramethylpiperidine as base followed by adsorption onto, and elution from, silica gel (45). Diastereoselective reduction of the ketone group of the aldol product 40 was accomplished in better than 95% enantiomeric excess (ee) with the Corey... 

Actually, triacetonamine still remains the only starting compound for the synthesis of 2,2,6,6-tetramethylpiperidine derivatives. The main methods of its preparation are given in Scheme 2. A summary of methods of synthesis for triacetonamine and other hindered piperidines are summarized in (15). 

Formyl-2,2,6,6-tetramethylpiperidine-l-oxyl obtained by the reaction given below could have been of great importance for the synthesis chemistry of 2,2,6,6-tetramethylpiperidine derivatives. But, after its synthesis had been reported, there is no evidence of its utilization (28). ... 

The use of TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) as an oxidant for the oxidation of primary and secondary alcohols to the corresponding aldehyde or ketone in combination with primary oxidants has also received significant attention in the past few years [18]. It is, therefore, not surprising to see the amount of effort that has been devoted to the synthesis of immobilized versions of this family of reagents (Figure 4.3) [19], on a wide variety of supports (e.g. silica-supported... 

Great attention has been paid to HAS and their safety application in plastics and coatings. The 4-unsubstituted 2,2,6,6-tetramethylpiperidine is considered as relatively toxic, the acute oral toxicity being about 1 g/kg. The substitution in position 4 (i.e. the general mode in the synthesis of HAS for polymer purposes) dramatically improves the situation. Therefore, commercial HAS like 28 (R = H), 34,35a or 35b were approved for stabilization of packaging materials in contact with food [307]. Some data are available on properties of TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxyl) and its 4-amino or 4-hydroxy derivatives. They were found to act as weak intrinsic direct mutagens in Salmonella typhimurium. TEMPO increases intracellular hydroperoxide concentration. This may indicate its pro-oxidative effect which does not result, however, in cellular toxicity [314]. 

TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) as an important reagent in alcohol oxidation and its application in synthesis of natural products between 2000 and 2004 06MRO155. 

The oxidation of alcohols with (diacetoxyiodo)benzene in the presence of catalytic amounts of TEMPO (2,2,6,6-tetramethylpiperidin-l-oxyl) in aqueous solutions is a common synthetic procedure (Section 3.1.5) [17-23]. An optimized protocol, published in Organic Synthesis for the oxidation of nerol 5 to nepal 6 (Scheme 6.4), consists of the treatment of the alcohol 5 solution in buffered (pH 7) aqueous acetonitrile with (diacetoxyiodo)benzene and TEMPO (0.1 equivalent) at 0 °C for 20 min [17]. 

Carroll and coworkers have published a series of papers on the use of aryliodonium salts in the synthesis of fiuorine-containing aromatic and heterocyclic products [63, 74, 75]. It has been found that the addition of radical scavengers such as TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) during the fluoridation of diaryliodonium salts leads to a significant improvement of both the reproducibility of the process and the... 

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