Tetramethylpiperidine formation

On the other hand, the predominant formation of the diastereomeric aldols 3 b results from the titanium enolate 1 b of (S )-5,5-dimethyl-4-tert-butyldimethylsilyloxy-3-hexanone. For this purpose, the ketone is first deprotonated with A-(bromomagnesio)-2,2,6,6-tetramethylpiperidine and the magnesium enolate, presumably (E) configurated, formed is thereby treated with hexamethylphosphoric triamide and triisopropyloxytitanium chloride. After sonification, the aldehyde is added to give predominantly aldol adducts 3b the diastereomeric ratio of 3b/2b surpasses 95 5 and the chemical yields range from 85 to 88%53b. 

Tetramethylpiperidine, dibromomethane (99%) and 1,1,1,3,3,3-hexamethyldisilazane (98%) were purchased from Aldrich Chemical Company, Inc., and used without further purification. Use of less hindered secondary amines (such as diisopropylamine) in place of tetramethylpiperidine results in lower yields because of the formation of carboxamide by-products. 

It is known that the nitrosonium cation is a strong oxidant (54). In (55) it was found by multinuclear NMR ( H, 13C, 19F and 14N) that the interaction of nitrosonium tetrafluoroborate with 2,2,6,6-tetramethyl-4-R-piperidine-1 -oxyl radicals 22a-e resulted in formation of 4-R-2,2,6,6-tetramethylpiperidine-l-oxoammonium tetrafluoroborates (Scheme 16). Cations 23a-e could be classified as nitrosonium complexes of biradicals 24a-e. 

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... 

We close this section with a statement of surprise and disappointment. For no apparent reason there are no published enthalpy of formation measurements for 1-hydroxy-2,2,6,6-tetramethylpiperidine, the parent species for the above 1-hydroxylated piperidines and the hydroxylamine counterpart to probably the most famous nitroxide radical, TEMPO . 

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... 

All the aforementioned evidence for cluster formation is indirect. In this laboratory we have recently observed a special type of cluster formation that supports the notion that such clusters may exist. Rey and McConnell41 have described the preparation of two spin labels, N,N -dipalmitoyl-N,N -bis-(l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-l, 10-diaminodecane (VII) and N,N -dimethyl-N,N -dihexadecyl-N,N -bis-(l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1,10-diammoniumdecane diiodide (VIII). 

The use of aqueous citric acid avoids the formation of insoluble gelatinous precipitates, which result when aqueous hydrochloric acid is employed. Sulfuric acid is a suitable alternative to citric acid but must be used in substantial excess to prevent precipitation. 2,2,6,6-Tetramethylpiperidine may be recovered from the citric acid extract by making the aqueous solution basic and extracting with ethyl ether. 

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. 

The following Swern oxidation is an inexpensive, mild and fast transformation. It provides aldehydes starting from primary alcohols in the absence of water, exclusively. Other mild oxidation methods for the formation of aldehydes are known Dess-Martin periodinane (DMP), o-iodoxybenzoic acid (IBX), chromium(III) reagents, tetramethylpiperidine 7V-oxide and sodium hypochlorite (TEMPO/NaOCl), tetrapropylammonium perruthenate and N-methylmorpholine 7V-oxide (TPAP/NMO), " and palladium(II)-catalyzed oxidations are reported. ... 

The solvent MeCN may be incorporated in the product in an altogether different type of process, which leads to formation of substituted acetonitriles. For instance, the oxidation of 2,2,6,6-tetramethylpiperidine, or the corresponding morpholine, in MeCN-NaC104 under oxygen-free conditions leads to the formation of an aminoace-tonitrile derivative [187]. The reaction was suggested to follow the mechanism given in Eq. (56). 

Tetramethylpiperidine derivatives are capable of acting in both ways according to our findings, these additives react very efficiently with peracid radicals. In addition, they are expected to accumulate at hydroperoxide sites by complex formation ( 1, 2). This means they are partly located at the sites where photooxidation is initiated. The respective complex formation constants are... 

The formation of alternating copolymers through the polymerization of pairs of monomers, one of which is the donor and the other the acceptor of an electron, is well known. We shall mention only a few studies out of a great number of those recently published. First, those dealing with the nature of active centers in such systems will be examined. When radical initiators are used, e.g., benzoyl peroxide as in17), and the reaction is inhibited with different radical polymerization inhibitors, such as stable radicals like 2,2,6,6-tetramethylpiperidine 1-oxide, quinones, fluorene etc., questions concerning the nature of active centers can be regarded as solved. 

The reaction involves the formation of carbanions that are stabilized by boryl groups . Similar carbanions are prepared from 9-methyl-9-borabicyclo[3.3.1]nonane (B-methyl-9-BBN), 9-alkenyl-9-borabicyclo[3.3. l]nonanes(B-alkenyl-9-BBN) , or alkenyldisiamylboranes with 2,2,6,6-tetramethylpiperidine (Li-TMP) in tetrahydro-furan (THF) ... 

Westheimer has reported on the results of a more deta d study of the formation of monomeric metaphosphate anion from (1,2-dibromo-l-phenyl-propyl)phosphonic acid, and its reaction with acetophenone. The yields of phenylethenyl phosphate can be raised to 40—80 % by careful choice of the nature and concentration of base. The effect of the pJ Ta of the added base appears to be complex some hindered bases, e.g., 2,2,4,4-tetramethylpiperidine and diisopropylamine, raise yields (to the maximum observed), but others, e.g. diisopropyl-methylamine, have the opposite effect. ... 

Regioselective metalation of a wide variety of trifluoromethylated pyridines and quinolines was exhaustively studied by the Schlosser research group." " It was demonstrated that regioselectivity of direct lithiation of fluorinated heterocycles depends signihcantly on reagents, reaction conditions and type of solvent. For example, the lithiation of 2-CF3-pyridine using lithium 2,2,6,6-tetramethylpiperidine (LiTMP) in THF at —75°C results in exclusive formation of acid 109, while the reaction with BuLi in the presence of Et2NCH2CH20Li in less polar ether leads to selective formation of isomeric acid 110 (Fig. 7.39). ... 

In THF, (53) with tetraphenylcyclopentadienone and with tetramethylpiperidine, as base, gave the imexpected dithiin (86) in low yield (Equation (2)) together with the product of reaction of the aryne with the very hindered amine used. The dithiin structure (86) is interesting since the aryne appears to have been incorporated twice. A possible pathway for its formation is proposed in <89PS(43)261,91JCS(P1)317>. 

The selection of the lithiation reagent also has a strong influence over the regioselectivity of the electrophilic substitution the bulkier the base, the more favorable the reaction at the C5 position. For example, lithium 2,2,6,6-tetramethylpiperidine (LiTMP) directs the formation of the C5-substituted product at a 79 1 ratio to the C2-substituted thiophene. A variety of electrophiles can be substituted after lithiation by LiTMP. ... 

The modifications of the Gilman-Speeter reaction include the activation of zinc by tri-methylsilyl chloride (TMSCl) and the application of lithium ester enolate" or lithium thioester enolate as the substitute for the traditional Reformatsky reagent. In these modifications, it was found that TMSCl-activated zinc is much more effective in promoting the reaction between ethyl bromoacetate and Schiff bases. In addition, in the presence of a chiral ether ligand, the reaction between lithium ester enolate and imines affords 0-lactams of high enantiomeric excess, probably due to the formation of a ternary complex reagent. " The enantioselectivity and reactivity of the ternary complex depend on the size and nature of the lithium amide used. For example, the lithium amide from 2,2,6,6-tetramethylpiperidine (LTMP) is unfavorable for this reaction." ... 

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