2,2, 6,6-Tetramethylpiperidine reactions

Primary and secondary aliphatic amines, morpholine and 2-methylaziridine and aniline and even the sterically hindered 2.2,6,6-tetramethylpiperidine readily react with 6-bromo-trithiadiazepine 7, in certain cases in the presence of /V./V-diisopropylethylamine, at room temperature by substitution of the bromine atom ammonia, for example, yields trithiadiazepin-6-amine 22 (R1 = R2 = H). There is compelling evidence that these reactions proceed by an elimination-addition mechanism via the heteroaryne, trithiadiazepyne 21.391... 

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). 

Oxidations Using Oxoammonium Ions. Another oxidation procedure uses an oxoammonium ion, usually derived from the stable nitroxide tetramethylpiperidine nitroxide, TEMPO, as the active reagent.31 It is regenerated in a catalytic cycle using hypochlorite ion32 or NCS33 as the stoichiometric oxidant. These reactions involve an intermediate adduct of the alcohol and the oxoammonium ion. 

Another chemically more interesting spin labeled B12 derivative involves coordinate attachment of the nitroxyl function to the cobalt atom of a cobinamide. Fig. 22 shows a reaction in which an alkyl cobin-amide is mixed with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl. The nitroxide displaces water from the 6th coordination position very slowly and therefore this reaction is usually allowed to proceed for a few days with a large excess of nitroxide. From the properties of the coordinated nitroxide derivative discussed below, it is certain that the cobalt is coordinated by the N—O functional group. An analogous compound to that shown in Fig. 22 can be made with a similar nitroxide in which the 4-hydroxyl-group is missing. In this case the N—O-function is the only basic site on the molecule and therefore must be the position of attachment to the cobalt 119). 

The hindered secondary amines can be highly effective photostabilizers for various polymers (]+.,5.,.6) Various hindered amines have been shown to retard oxidation, but most share the common feature of being secondary or tertiary amines with the a-carbons fully substituted. The most widely exploited representatives of this class are based on 2,2,6,6-tetramethylpiperidine either in the form of relatively simple low molecular weight compounds, or more recently as backbone or pendant groups on quite high molecular weight additives ( i.,5.,6). The more successful commercial hindered amines contain two or more piperidine groups per molecule. Photo-protection by tetra-methylpiperidines (near UV transparent) must result from the interruption of one or more of the reactions 1 to 3. Relatively recent results from our own laboratories, and in the open literature will be outlined in this context. 

M. x 108M 1s 1 at 25°C, but may be appreciably lower in the solid state. In comparison k2 for oxygen competition for the alkyl radical is 2 x 109M-1s 1. Thus for air-saturated PPH ([02] 8 x 10-1,M)reaction7 will be protection against xenon irradiation was improved as compared to the parent piperidine by about 25, but the nitroxide itself was reduced to the 1 x 10 M level within the first lOOh and persisted at this level until brittle failure (7,) In contrast the parent amine is completely destroyed in the first lOOh of xenon exposure. 

For this reason we have concentrated on the study of ketone photolysis in the presence of oxygen and the interaction of the oxygen-centered radicals arising in this reaction with certain tetramethylpiperidine derivatives. ... 

In the case of DiPK our experimental findings have not enabled us to decide between these two possibilities. However, according to the work of Rosantsev Q and IngoldS-i reactions between peroxy radicals and nitro-xides derived from tetramethylpiperidine are not very probable, so reaction (15) should not be important. 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

Reaction of the Ru macrocyclic complex [RuLCy (L= 1,5,9,13-tetramethyl-l,5,9,13-tetraaza-cyclohexadecane) with N02 results in a disproportionation of the initial [Ru °LCl(N02)], the final products being traTO-[Ru L(0)Cl]" " and [Ru L(OH)(NO)] " "." The reaction between [Ru(OEP)Me] (H2OEP = octaethylporphyrin) and 2,2,6,6,-tetramethylpiperidine-l-oxyl (TEMPO) produces [Ru(OEP)CO]. There is clear evidence that the CO ligand is derived from the axially bound CH3 group, making this reaction an important example of CH3 to CO transformation." ... 

Methyl to CO transformation has been observed in the reaction of [Ru(OEP)Me] with 2,2,6, 6-tetramethylpiperidine-l-oxyl (TEMPO). There is clear evidence that the CO ligand in the... 

We now turn to the remaining two hydroxylamines that are N-hydroxylated derivatives of 2,2,6,6-tetramethyl-4-piperidone and -4-piperidinol. The enthalpies of formation of some simple 4-piperidones and their corresponding 4-piperidinols have recently been determined. The values of gaseous N-methyl-4-piperidone and Af-methyl-4-piperidinol are —160.7 1.7 and —226.8 1.8 kJmol (also see Reference 18). The difference between these contemporary values is — 66.1 2.5 kJmol while for the hydroxylated and methylated counterpart species the difference is —47.0 4.8 kJmoH. For comparison, the formal enthalpy of reduction of 3-hexanone to 3-hexanol is ca —54 kJmoH. As has been discussed earlier, reduction enthalpies are not necessarily constant . Relatedly, reaction 8 that exchanges N-methyl and N-hydroxy and parent and tetramethylpiperidines is endothermic by 19.1 5.4 kJmol . The deviation from thermoneutrality is more... 

During the induction periods caused by adding antioxidants, a small contraction in volume occurred because of the formation of dimers of chloroprene (14). This reaction occurs during the oxidation but was most easily studied by dilatometry in the absence of oxygen. A few values of the initial rate of dimerization of chloroprene, inhibited against polymerization with 2,2,6,6-tetramethylpiperidine-l-oxyl, are given in Table III. Their dependence on temperature is given by... 

A convenient procedure for the oxidation of primary and secondary alcohols was reported by Anelli and co-workers (8,9). The oxidation was carried out in CH2CI2 with an aqueous buffer at pH 8.5-9.5 utilizing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 1) as the catalyst and KBr as a co-catalyst. The terminal oxidant in this system was NaOCl. The major disadvantage of using sodium hypochlorite or any other hypohalite as a stoichiometric oxidant is that for each mole of alcohol oxidized during the reaction one mole of halogenated salt is formed. Furthermore,... 

According to NMR data, reaction of lithium tetramethylpiperidine (TMP) and t-Bu2Zn, in a 1 1 molar ratio (Scheme 6) affords the heteroleptic zincate [f-Bu2Zn(TMP)]Li (36) . An interesting feature of this compound is that it is capable of metalate arenes that contain a directing metalating group (DMG) in ortho-posilion and thus allow further derivatization by reaction of the zincate intermediate with electrophiles (E+). 

Since the reaction between methyllithium and 2,2,6,6-tetramethylpiperidine is relatively slow at lower temperatures, lithium 2,2,6,6-tetramethylpiperidide is best prepared at room temperature. The reagent may, however, be used over a wide range of temperatures. 

Amines react with perchloryl fluoride. Thus, l-fluoro-2,2,6,6-tetramethylpiperidine (22) is form ed from the piperidine derivative 35 on the other hand, 1-fluoro salts 23 can he prepared by reactions with l-alkylpiperidines.36... 

Alkylation of isoprene at the methyl group was achieved through the reaction of the corresponding metal derivative (fert-BuOK-lithium tetramethylpiperidine, THF) with 1-bromoheptane (60% yield).257 Regioselective monoalkylation of isoprope-nylacetylene can be performed in high yields through the dilithium derivative 258... 

In recent attempts to formylate a 2,2,6,6-tetramethylpiperidin-4-one (161) with dichlorocarbene, Lind and Winkler254 and also Lai and Westfahl255 reported independently, and almost simultaneously, an unexpected formation of a pyrrolidin-2-one the crude yield was quantitative. A reaction mechanism has been formulated. 

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