Amines, comparative reactivity

TABLE III. Comparative Reactivity of HM(C0)4 with Amines Under High Pressures of C0 H2 (1 1)... 

The umpolung of dominant stereoelectronic interactions between amine and carbonyl imposed by the sterically demanding substituents leads to experimental observations that differ dramatically from the usual trends observed in acyl substitutions. Usually, thioesters can serve as acylating agents for amines, but the reaction proceeds in the opposite direction for the unusual amides from Figure 5.36. In these systems, PhSH readily displaces the amine in a nearly quantitative, room-temperature, non-catalyzed reaction (Figure 5.37). The authors compare reactivity of such amides with the reactivity of acyl chlorides. 

The values of the activation parameters, together with the influence of the solvent on these parameters, has been interpreted in terms of a highly linear, polar transition state (Fig. 2) (Halpern, 1970 and references therein), or a transition state containing unusually stringent stereochemical restrictions. These conclusions are further substantiated by the comparable reactivities of substituted benzyl bromides towards both /ra/i5-[IrCl(CO)(PPh3)2] and tertiary amines. 

Although fluorinated alkoxy and aryloxy polyphosphazenes have been investigated in some detail, fluorinated aminophosphazene polymers have received less attention. A possible reason is the lower reactivity of these amines compared to the fluo-roalkoxides. However, an example of this class has been investigated recently with the synthesis of 2,2,2-trifluoroethylamino derivatives (Figure 1.7) [33]. 

The majority of hydroxy groups in the polyesters used for foam making are primary groups and are comparatively reactive towards isocyanates. It is therefore sufficient to include only simple tertiary amine catalysts in the formulation to obtain a satisfactory foam. The reactions which occur during the foaming process are the same as those described for polyether-based foams. 

Less studied than secondary amines in the past decade, primary amines took a growing importance more recently. Indeed the higher reactivity and the lower steric hindrance of primary amines compared to secondary amines can be helpful to promote some Michael addition. The reduction of steric demand makes possible the pathway through a (Z)-enamine intermediate leading to anti-adducts. 

The N-basicity of the commonly used amines (pyrrolidine > piperidine > morpholine) drops by 2-3 orders of magnitude as a consequence of electron pair delocalization in the corresponding enamines. This effect is most pronounced in morpholino enamines (see table below). Furthermore there is a tendency of the five-membered ring to form an energetically favorable exocyclic double bond. This causes a much higher reactivity of pyrroUdino enamines as compared to the piperidino analogues towards electrophiles (G.A. Cook, 1969). 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

The hterature suggests that more than one mechanism may be operative for a given antiozonant, and that different mechanisms may be appHcable to different types of antiozonants. All of the evidence, however, indicates that the scavenger mechanism is the most important. All antiozonants react with ozone at a much higher rate than does the mbber which they protect. The extremely high reactivity with ozone of/)-phenylenediamines, compared to other amines, is best explained by their unique abiUty to react ftee-tadicaHy. The chemistry of ozone—/)-PDA reactions is known in some detail (30,31). The first step is beheved to be the formation of an ozone—/)-PDA adduct (32), or in some cases a radical ion. Pour competing fates for dissociation of the initial adduct have been described amine oxide formation, side-chain oxidation, nitroxide radical formation, and amino radical formation. 

The rate of amination and of alkoxylation increases 1.5-3-fold for a 10° rise in the temperature of reaction for naphthalenes (Table X, lines 1, 2, 7 and 8), quinolines, isoquinolines, l-halo-2-nitro-naphthalenes, and diazanaphthalenes. The relation of reactivity can vary or be reversed, depending on the temperature at which rates are mathematically or experimentally compared (cf. naphthalene discussion above and Section III,A, 1). For example, the rate ratio of piperidination of 4-chloroquinazoline to that of 1-chloroisoquino-line varies 100-fold over a relatively small temperature range 10 at 20°, and 10 at 100°. The ratio of rates of ethoxylation of 2-chloro-pyridine and 3-chloroisoquinoline is 9 at 140° and 180 at 20°. Comparison of 2-chloro-with 4-chloro-quinoline gives a ratio of 2.1 at 90° and 0.97 at 20° the ratio for 4-chloro-quinoline and -cinnoline is 3200 at 60° and 7300 at 20° and piperidination of 2-chloroquinoline vs. 1-chloroisoquinoline has a rate ratio of 1.0 at 110° and 1.7 at 20°. The change in the rate ratio with temperature will depend on the difference in the heats of activation of the two reactions (Section III,A,1). 

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