Amines hindered secondary

Milch of our present understanding of the mechanisms of NCA polymerization Is summarized In several recent reviews ( ) In addition, several papers ( - ) and references cited therein provide more current Information. Although there Is agreement about the mechanism of NCA polymerizations that are initiated by primary amines (weak base mechanism), there Is considerable controvert concerning the mechanisms of NCA polymerizations that are Initiated by tertiary amines, hindered secondary amines or basic salts. Three mechanisms have been invoked to explain these reactions. These are the mechanism of Wleland (10-11),... 

Monofunctional, cyclohexylamine is used as a polyamide polymerization chain terminator to control polymer molecular weight. 3,3,5-Trimethylcyclohexylamines ate usehil fuel additives, corrosion inhibitors, and biocides (50). Dicyclohexylamine has direct uses as a solvent for cephalosporin antibiotic production, as a corrosion inhibitor, and as a fuel oil additive, in addition to serving as an organic intermediate. Cycloahphatic tertiary amines are used as urethane catalysts (72). Dimethylcyclohexylarnine (DMCHA) is marketed by Air Products as POLYCAT 8 for pour-in-place rigid insulating foam. Methyldicyclohexylamine is POLYCAT 12 used for flexible slabstock and molded foam. DM CHA is also sold as a fuel oil additive, which acts as an antioxidant. StericaHy hindered secondary cycloahphatic amines, specifically dicyclohexylamine, effectively catalyze polycarbonate polymerization (73). 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

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. 

With the exception of intramolecular amination reactions, all of the early aryl halide aminations were catalyzed by palladium complexes containing the sterically hindered P(o-tol)3. In papers published back-to-back in 1996, amination chemistry catalyzed by palladium complexes of DPPF and BINAP was reported.36,37 These catalysts allowed for the coupling of aryl bromides and iodides with primary alkyl amines, cyclic secondary amines, and anilines. 

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. 

Similarly, a solid-supported imide has been reported to serve as an acylating reagent under microwave conditions by Nicewonger and coworkers [130], The starting imide was immobilized on aminomethyl polystyrene and in this case benzoyl chloride was chosen to prepare the acylating reagent (Scheme 7.111). Primary amines and piperazines were smoothly acylated at room temperature, but more hindered secondary amines required more time and higher temperatures, and anilines... 

Several syntheses of l,3-dioxoperhydropyrrolo[l,2-c]imidazoles have been developed using different strategies. a-Substituted bicyclic proline hydantoins were prepared by alkylation of aldimines 135 of resin-bound amino acids with a,tu-dihaloalkanes and intramolecular displacement of the halide to generate cr-substituted prolines 136 and homologs (Scheme 18). After formation of resin-bound ureas 137 by reaction of these sterically hindered secondary amines with isocyanates, base-catalyzed cyclization/cleavage yielded the desired hydantoin products <2005TL3131>. 

The first move in this direction was to improve the weatherability of impact-resistant polystyrene. Because polybutadiene, the most widely used rubber in impact-resistant polystyrene, is unsaturated, it is sensitive to photooxidation, and impact-resistant polystyrene is therefore not suitable for outdoor applications. A saturated rubber might be able to help here. In the ABS sector this has been successfully tried out with acrylate rubber (77) and EPDM (78, 79), and the latter has also been used in impact-resistant polystyrene (80, 81) This development has elicited satisfactory responses only in certain areas and more work still has to be done. For instance, attempts have been made to improve resistance to weathering by using silicone rubber (82 ). This approach is effective, but economic factors still stand in its way. Further impetus may also be expected from stabilizer research. Hindered secondary amines (83), to which considerable attention has recently been paid, are a first step in this direction. 

Treatment of hydroxylamines 4 (R1 = cyclohexyl, Ph or 3,4-(MeO)2CgH3) with acetone gives nitrones 5, which are transformed by Grignard reagents R2MgBr (R2 = Me, Et, Bu, Ph or allyl) into the hydroxylamines 6 the latter are converted into the hindered secondary amines 7 by means of carbon disulphide10. 

Ordinarily, alkyl nitrate esters will not nitrate amines under neutral conditions. However, Schmitt, Bedford and Bottaro have reported the use of some novel electron-deficient nitrate esters for the direct At-nitration of secondary amines. The most useful of these is 2-(trifluoromethyl)-2-propyl nitrate, which nitrates a range of aliphatic secondary amines to the corresponding nitramines in good to excellent yields. Nitrosamine formation is insignificant in these reactions. 2-(Trifluoromethyl)-2-propyl nitrate cannot be used for the nitration of primary amines, or secondary amines containing ethylenediamine functionality like that in piperazine. Its use is limited with highly hindered amines or amines of diminished nucleophilicity due to inductive or steric effects. 

Emmons and co-workers prepared a series of aliphatic secondary nitramines by treating amines with a solution of dinitrogen pentoxide in carbon tetrachloride at —30 C (Equation 5.9). The amine component needs to be in excess of two equivalents relative to the dinitrogen pentoxide if high yields of nitramine are to be attained. This is wasteful because at least half the amine remains unreacted. However, yields are high and there is no reason why the amine cannot be recovered as the nitrate salt. The method is particularly useful for the nitration of hindered secondary amines substrates such as those with branching on the a carbon. 

Direct oxidation of primary amines with peroxide oxidants does not provide appreciable yield of hydroxylamines. As was mentioned above, oxidation of secondary amines usually proceeds smoothly giving moderate to good yields of iV,iV-disubstituted hydroxylamines. Oxidation of sterically hindered secondary amines such as 125 (equation 88) can also be done with peracids . Further oxidation of the resulting Af,A-disubstituted hydroxylamines 126 with an excess of m-chloroperbenzoic acid is known to end up with the corresponding nitroxyl radicals of type 127 (equation 88) ° although the reaction can be stopped at the hydroxylamine stage. 

NCA polymerization by secondary amines may involve the amine or activated monomer mechanisms or both mechanisms simultaneously. Unhindered secondary amines such as dimethylamine and piperidine react like primary amines, and polymerization occurs by the amine mechanism. Polymerization by slightly hindered amines such as diethylamine, N-methylbenzylamine, and di-n-propylamine involves a combination of the amine and activated monomer mechanisms. More hindered secondary amines, such as di-n-isopropylamine and dicyclohexylamine, react almost exclusively via the activated monomer mechanism. 

Exxon s Flexsorb SE solvents achieve high hydrogen sulfide selectivity by virtue of their molecular structure. These solvents are sterically hindered secondary amines. A bulky molecule is used to shield the available hydrogen radical on the nitrogen atom and prevent the insertion of carbon dioxide. The reaction with hydrogen sulfide is not sensitive to the amine s structure, so the steric hinderance affords higher hydrogen sulfide selectivity. 

These results illustrate the practicality of preparing trialkylamines by the reductive alkylation of dialkylamines with aliphatic ketones. Excellent yields are obtained, particularly with the more reactive and less hindered ketones, such as cyclohexanone and acetone, and with the less hindered secondary amines. Platinum sulfide, or other platinum metal sulfides, are the catalysts of choice when more hindered reagents require more severe operating conditions. 

Although primary and secondary amines are destroyed by PDC, hindered secondary amines can resist the action of PDC long enough to allow selective oxidation of alcohols.146... 

Some amines react under Pfitzner-Moffatt conditions, yielding an adduct with the carbodiimide or a S, S -dimethy lsulfilim i ne, resulting from attack of the amine on activated DMSO. The reactivity of different amines is very diverse, and observed in amines, which are not substantially protonated under the reaction conditions, while they still posses enough nucleophilicity. Thus, tertiary amines do not interfere, while hindered secondary ones seldom do it. 

During the 70 s, Celia et al. treated the hindered secondary amine 52 with / -chloroperbenzoic acid, with the intention of transforming it into the nitroxide 53.1 Unexpectedly, the oxidation of the amine functionality was accompanied by the transformation of the alcohol moiety into a ketone, resulting in the formation of compound 54. 

A sterically less hindered N atom is found, for example, in the a-aminated a-chiral aldehyde B in Figure 10.42. This is because this amine is secondary and bears only two nonhydrogen substituents. The N atom of aldehyde B is so unhindered that it can be bound firmly in a chelate by the magnesium. Consequently, the addition of a Grignard reagent to aldehyde B exclusively affords the chelation-controlled product. 

Amines react with primary alkyl halides to give alkylated ammonium halides. Alkylation proceeds by the SN2 mechanism, so it is not feasible with tertiary halides because they are too hindered. Secondary halides often give poor yields, with elimination predominating over substitution. 

The relative reactivity of the alcohol and amine in the example just given could be overturned by conducting a reaction under thermodynamic control. In kinetically controlled reactions, the idea that you can conduct chemoselective reactions on the more reactive of a pair of functional groups— carbonyl-based ones, for example—is straightforward. But what if you want to react the less reactive of the pair There are two commonly used solutions. The first is illustrated by a compound needed by chemists at Cambridge to study an epoxidation reaction. They were able to make the following diol, but wanted to acetylate only the more hindered secondary hydroxyl group. 

Dealkylation of amines. Most amines (primary, secondary, and tertiary) form crystalline salts with benzeneselenol. When heated (150°) some of these salts decompose to form the dealkylated amine and an alkyl phenyl selenide. Salts of primary alkylamines for unknown reasons undergo dealkylation very slowly hindered, tertiary amines are dealkylated most rapidly. 

The vast utilization of hindered secondary amines like diisopropylamine or dicyclohexylamine in carbanion chemistry is also based on the difference in their behavior toward protons and other electrophiles. Thus quite a number of the methods depend upon the use of the lithium or magnesium salts of these amines for the generation of carbanionic species. These salts are very strong kinetic bases and therefore are able to abstract a proton from a variety of C-H acids. At... 

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