With sterically hindered amines

Although complete replacement of chlorine atoms can be accomplished with most kinds of amines, this becomes difficult in the reactions with sterically hindered amines. Thus in the reactions of N3P3C16 with cyclohexylamine (71) and adamantyl amine (72), the yields of fully substituted products are low. In the reactions of N3P3C16 with further sterically encumbered amines such as... 

Reaction of 9-benzyloxy-3,4-dihydro-277-pyrido[2,l- ][l,3]oxazinium mesylate with C5H9NH2 and BnNH2 occurred at position 9a, while with sterically hindered amines (e.g., Bu NH2, secondary amines), PhNH2 and NaN3 reacted at position 2 to give l-(3-hydroxypropyl)-2-(substituted imino)-3-benzyloxy-l,2-dihydropyridines and... 

Reaction of aLkyknagnesium reagents with sterically hindered amines leads to the formation of magnesium amides 110-112 °, reacting much faster than the parent alkyl-magnesium derivatives with C—H acidic substrates (Scheme 9). 

The anions of these mercaptans, successfully generated with steric hindered amines underwent smooth addition to the a,/3-unsaturated five membered ring ketones 201. Under defined reaction conditions we obtained 203 as a major product when... 

The dynamics of reversible onium ion formation has been studied by generating carbenium ions in the presence of nucleophiles using pulse radiolysis or flash photolysis, and following the rate of disappearance of the carbenium ions by UV. As discussed in Chapter 2, the kinetics of reaction of various electrophiles with nucleophiles obey a general reactiv-ity/selectivity relationship. The rates of reaction of various nucleophiles with carbenium ions are summarized in Table 9. These rates often approach diffusion controlled limits (k 10 ° mol-,-L-sec l). The rates are slower for less nucleophilic and less electrophilic compounds, and are particularly slow with sterically hindered amines such as lutidine (2,6-dimethylpyridine) [63]. Solvent effects are minimal when the reactions are diffusion controlled, although tributyl amines react slower with carbenium ions in more nucleophilic dichloroethane than in methylene chloride. 

Figure 25 Illustration of the distorted tetrahedral Cu species with sterically hindered amines ...

Synthesis.—Diarylthioureas have been made from arylamines and either bis-(alkoxythiocarbonyl) disulphides or CSj and an alumina catalyst. The standard reaction of amines with isothiocyanates was used in the preparation of adamantyl- and glycosyl- thioureas. Benzoyl isothiocyanate, when treated with sterically hindered amines, e.g. Pr jNH or dicyclohexylamine, gives... 

Hydroxylamines are multifunctional. They can act both as C-radical scavengers and as hydroperoxide decomposers. They reach full effect even in low concentrations without negative effects on color. That is why hydroxylamines are mainly used as processing stabilizers for color-critical applications. Generally, hydroxylamines are combined with sterically hindered amines [538]. 

C Andrei, I Dragutan, AT Balaban. Photostabilization of Polymers with Sterically Hindered Amines. Bucharest Ed Academiei Romane, 1990, pp 9, 203. 

The chemistry of acid gas reactions with sterically hindered amines is discussed in some detail by Sartori and Savage (1983) and by Weinberg et al. (1983). A sterically hindered amine is defined stracturally as a primary amine in which the amino group is attached to a tertiary carbon atom or a secondary amine in which the amino group is attached to a secondary or tertiary carbon atom. Typical sterically hindered amines are shown in Fig iire 2-7 (Sartori and Savage, 1983). 

The reactions of alkylmagnesium reagents with sterically hindered amines in THF lead to the formation of magnesium amides 56-58,which react with C-H acidic substrates much faster than alkylmagnesium reagents (Scheme 2-25). 

Sterically hindered amines often tend to form ureas with (BOC)20, because of isocyanate formation. The problem can be avoided by reacting the amine with NaHMDS and then with (BOC)20. The isocyanates can also be converted to the BOC group by heating with r-BuOH. When other alcohols are used, the corresponding carbamate is produced. ... 

In 1992, Bergman et al. reported that zirconium bisamides Cp2Zr(NHR)2 catalyze the intermolecular hydroamination of alkynes with sterically hindered primary amines to give enamines or their tautomeric imines (e.g., Eq. 4.77) [126]. 

The reductive alkylation of DAP with acetone led to high conversions and selectivity to the dialkylated product over Al, Bl, and BS2 catalysts. The ASl catalyst, which typically has lower activity than the Al or Pt-based catalysts showed greater formation of heterocycles. These results indicate that a more active catalyst, a shorter reaction time, a higher operating temperature, or sterically hindered amines/ketones will help minimize the formation of the heterocycles. Similar high selectivities were obtained with DAP-MIBK reaction over BSl and BS2 catalysts with no heterocycles being formed. However, over Al, the undesired heterocyclic compound was over 15%. This indicates that the reaction between diamines and ketones has a significant potential to form heterocyclic compounds unless the interaction between these is kept to a minimum by the use of a continuous flow reactor as proposed by Speranza et al. (16) or by other methods. 

Even HALS compounds which absorb weakly at 337 nm can be analysed directly without matrix assistance, with the exception of the high-MW Hostavin N 30 (ca. 1500 Da), which fragments by direct laser desorption ionisation of intact molecules occurs only in the presence of a (dithranol) matrix. Direct laser desorption leads only to noncharacteristic, low-MW fragments. Hostavin N 20 leads to [M + H]+, [M + Na]+, [M + K]+ and some fragmentation peaks. MALDI-ToFMS of Tinuvin 765, which consists of a mono- and bifunctional sterically hindered amine, only shows the adduct peaks of the bifunctional amine apparently, the monofunctional amine is not ionisable. 

Sterically hindered amines like 2,6-dimethylaniline can be formylated with AT-for-mylimidazole in excellent yield starting from formic acid and N,Nf-oxalyldiimidazole [411... 

The group of Caddick has described the synthesis of functionalized sulfonamides by microwave-assisted displacement of pentafluorophenyl (PFP) sulfonate esters with amines (Scheme 6.159) [306]. Their ease of handling due to their higher crystallinity, along with their long shelf-life and their ability to react under aqueous reaction conditions, makes these an attractive alternative to sulfonyl chlorides. The microwave-assisted reaction of alkyl PFP esters with amines is a facile process which proceeds cleanly and in good yields with a number of different amines, including primary, secondary, and sterically hindered amines, and anilines. Optimum condi-... 

A variety of amides has been formed in moderate to high yields with aryl bromides or iodides as aryl precursors and single-mode microwave heating for 15 min at 150 °C (Eq. 11.15) [29]. Under these conditions, aliphatic, unhindered primary, and secondary amines reacted readily, whereas sterically hindered amines or amines of low nucleophilicity, e.g. anilines, afforded low yields and incomplete conversions. Among the homogeneous catalytic systems tested the most suitable for the use with aryl bromides was a 2 1 mixture of BINAP and Herrmann s palladacycle. 

Finally, Persson et al. (1995) measured the UC/14C KIEs for the SN2 reactions between several amine nucleophiles and labelled methyl iodide in dimethoxyethane or acetonitrile at 15°C and 30°C, respectively, to determine how sterically hindered nucleophiles affects the transition state of a Menshut-kin reaction. The results in Table 25 show that all the fc11// 14-values for these reactions are large. In fact, they are all near the theoretical maximum value for these KIEs. Secondly, the KIE for the reaction with the more sterically hindered amine, 2,6-lutidine, is larger than that for the less sterically hindered... 

DR. STEPHEN NEUMANN (Eastman Kodak Co.) In your observation of the proton transfer rates between the metal base and the amine base, do you have any feeling for whether the slower transfer is unique to the metal bases or whether it has to do more with the bulk of the overall base participating in the proton transfer For instance, would a sterically hindered amine show rates similar to the metal bases. 

In another approach, a polymer-bound TEMPO-species, referred to as PIPO (polyamine immobilized piperidinyl oxyl), was derived by oxidation of commercially available Chimasorb 944, an oligomeric sterically hindered amine. Together with bleach as co-oxidant PIPO (12) afforded aldehydes and ketones in short reaction times and with yields from 80 to > 99% (Scheme 4.6) [70]. 

This equation seems to be a key reaction point in the antioxidant action of these amines—>N radicals in the presence of oxygen are transformed via peroxy radical intermediates into nitroxyl radicals. The nitroxyl radicals are very persistent and react efficiently with radicals produced on polyolefine degradation. Such radical interception blocks the chain in radical oxidation and therefore causes the antioxidant activity of sterically hindered amines (Brede et al. 1998). 

More recently, Polish chemists have reported a synthesis of both aryl and aliphatic secondary nitramines by treating amine substrates with ethyl magnesium bromide followed by reaction with n-butyl nitrate (Equation 5.8). This method, which uses nonpolar solvents like hexane or benzene, has been used to synthesize aliphatic secondary nitramines, and At-nitro-A-methylanilines which otherwise undergo facile Bamberger rearrangement in the presence of acid. The direct nitration of At-unsubstituted arylamines usually requires the presence of an electron-withdrawing group. Reactions are retarded and yields are low for sterically hindered amines. 

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. 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

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