Amines, tertiary reaction with arenes

The initial step in the addition reactions of arenes and aryl olefins with tertiary amines is electron transfer quenching of the singlet arene or aryl olefin acceptor by a ground state amine donor. The free energy of electron transfer can be calculated using Weller s equation (Eq. I) ... 

The failure of tertiary (AI,IV-dimethylaminoalkyl)arenes and stilbenes to undergo intramolecular addition may reflect structural differences between inter- vs. intramolecular exciplexes. Polar solvents are generally required for the observation of in-termolecular addition reactions of tertiary amine exciplexes. Equilibration between solvent-separated and radical ion pairs may be necessary in order to achieve an appropriate reaction trajectory for a-C-H proton transfer. In the case of intramolecular exciplexes with short chain linkers, electron transfer in polar solvents may occur in extended geometries which are inappropriate for proton transfer and chain folding may not compete effectively with exciplex decay. The exceptions to these generalizations, benzene and styrene, form more localized anion radicals which undergo both inter- and intramolecular reactions with tertiary amine cation radicals in nonpolar solvents. 

Intramolecular addition reactions of arenes and aryl olefins with secondary and primary amines have proven to be of broader scope than the analogous reactions with tertiary amines. The intramolecular addition of nonconjugated o-allylanilines 51 to yield the 2-methylindolines 52 was reported by Koch-Pomeranz et al. in 1977. Intramolecular electron transfer from the singlet aniline to the ground state alkene followed by N-H proton transfer to the alkene terminal carbon was proposed to account for the regioselective formation of indolines. Proton transfer to the internal carbon would yield tetrahydroquinolines, which were... 

The reactions of HN3 with cyclic alcohols to yield mixtures of ketones, amines, and products with an enlarged ring are catalyzed by H2SO4 [1]. Tertiary alcohols are converted to azides in the presence of acid [12] or TiCU [13]. Aldehydes and ketones with HN3 undergo a Schmidt-type reaction by liberating N2 and inserting NH In the presence of H or Lewis acids [14]. Ketones yield secondary amides and, in the case of cyclic ketones, lactames. Aldehydes are converted to nitriles or N-formylamines. Tetrazole derivatives result with excess HN3 [1, 15]. However, a-azido ethers are obtained from aldehydes and HN3 in the presence of alcohols by catalysis of TiC [16]. Carboxylic acids and anhydrides form amines, N2, and CO2 in Schmidt reactions with HN3. Intermediates are carbamic acids which form by insertion of NH into the R-COOH bond [1, 14]. High yields result for acids of arenes [17]. 

As discussed in more detail in Section 4.7, which reviews studies on the reaction mechanism, the major product that competes with the arylamine is the arene resulting from hy-drodehalogenation of the aryl halide. Hartwig showed that steric hindrance was crucial to minimize formation of this side product when monophosphines are used [135]. A second side product from reaction of a primary amine is diaryl alkyl tertiary amine, resulting from... 

The studies related to the interactions of electronically excited arene molecules with tertiary amines have provided a basis for the present understanding of exciplexes and radical ion-pair phenomena [41,82], PET reactions of amines yield planar amine radical cations (Eq.20) which are deprotonated to give a-amino radicals (Eq.21) and usually cross-coupling (Eq.22) between radical pairs of donor-acceptor terminates the photoreaction [32a, 83]. Mechanistic studies revealed contact ion pair (CIP) intermediate for these reactions [84, 85]. 

The reactions of excited states of arenes and arenealkenes with primary and secondary amines result in products arising out of aminyl radical addition, whereas oc-aminoalkyl radical addition occurs with tertiary amines [11, 278, 279]. 

Whereas addition reactions of singlet aryl olefins and arenes with secondary and tertiary amines are common, addition of ammonia has not been reported and addition of primary amines... 

The photochemical reactions of aryl olefins and arenes with secondary and primary amines are assumed to occur via a sequential eleetron transfer-proton transfer meehanism similar to that for tertiary amines (Scheme I). Exciplex fluorescence is not observed in solution however, weak exciplex fluorescence has been reported for the slyrene-diethylamine system in a su-personie jet. Rate constants for quenching of strong acceptors such as stilbene and styrene by EtjNH are diffusion controlled. 

Workentin et al. have recently reported the results of an extensive laser flash photolysis investigation of the reactions of the cation radicals of 9-phenyl- and 9,10-diphenylanthracene (PA and DPA, respectively) with amines. Primary amines react with both cation radicals via nucleophilic addition with rate constants which reflect both the amine basicity and a steric requirement for bond formation. Steric effects are more pronounced for addition of DPA " vs. PA ", presumably due to the presence of substituents at both the 9- and 10-position. Tertiary amines and anilines react with PA " and DPA " via electron transfer with rate constants which correlate with amine ionization potentials. Rate constants for nucleophilic addition of primary amines are faster in acetonitrile than in acetonitrile/water solution. The rate-retarding effect of water is attributed to an equilibrium between the fiee amine (reactive) and hydrated amine (unreactive). The beneficial effect of water on preparative ET-sen itized photoamination may reflect its role as a catalyst for the proton transfer processes which follow C-N bond formation (Scheme 2). Hydration of the amine also should render it less reactive in primary and secondary electron transfer processes which can compete with the formation of the arene cation radical. 

On the other hand, even the recently prepared Herrmann-Beller catalystf still requires higher temperatures for efficient coupling rates of the Heck reaction. Interestingly, the complexation of chloroarenes with the Cr(CO)3 fragment activates the arene-chlorine bond considerably toward the oxidative addition. Thus, Cr(CO)3 complexed chloroarenes react about 15 times faster than iodoarenes in Pd-catalyzed cross-couphng reactions under mild conditions, in particular in Pd/Cu-catalyzed cross-couplings with terminal acetylenes in refluxing THF and/or tertiary amines (Scheme 36). ... 

As noted in Chapters 2 and 11, a series of -q -arene complexes of osmium have been prepared, and the reactivity of these species has been studied extensively by Harman. The reactions of iq -arene complexes of Os(II) illustrate how strong backbonding can cause the uncoordinated portion of an aromatic system to be more susceptible to electrophilic attack than the corresponding free arene. ° Osmium(II) pentamine complexes of phenols, anilines, acetanilides, and anisoles react with electrophiles at the uncoordinated portion of the ring. For example, the simple phenol complex in Equation 12.78 reacts with Michael acceptors at the 4-position of the coordinated phenol in the presence of a mild tertiary amine base. This reactivity and selectivity for reaction at the 4-position is greater than the reactivity of free phenol. The reactions of electrophiles with aniline derivatives occur in a similar fashion and lead to products from alkylation of the aromatic ring predominantaly at the 4-position (Equation 12.79). Related reactions occur with complexes of electron-rich five-membered pyrrole and furan heterocycles. Examples of electrophilic attack on -q -pyrrole complexes of Os(II) are shown in Equation 12.80. ... 

---