DFT insights into superelectrophilic activation of α,ß-unsaturated nitriles and ketones in superacids.

Superelectrophilic activation of α,ß-unsaturated carbonyl compounds and their isoelectronic analogs, proceeding normally under superacidic conditions, have led to a great variety of beneficial synthetic transformations. However, the essence of such activation is not yet fully recognized, while a number of alternative views on the subject have been discussed at length in the literature. Here, taking the example of virtual reactions of cinnamonitrile and benzalacetone with benzene, their feasible mechanistic variants, including multiple protonation (coordination to AlCl3) of the reactants, were analyzed based on density functional theory (DFT). It is revealed that the most plausible reaction pathways involve the initial N- or O-protonation (coordination to AlCl3) of the activated compounds followed by subsequent protonation on the α-C-atom. Dicationic superelectrophiles thus formed ensure practically barrier-free reactions with benzene in addition to a more favorable energetic profile of their generating, which is in marked contrast to other potential reaction pathways.

Unusual temperature-sensitive protonation behaviour of 4-(dimethylamino)pyridine.

Stimuli-responsive and, in particular, temperature-responsive smart materials have recently gained much attention in a variety of applications. On the other hand, 4-(dimethylamino)pyridine (DMAP) and related structures are widely used as nucleophilic catalysts and also as specific parts of rationally designed molecules, where reversible reactions of the pyridinic nitrogen with electrophiles are involved. In our study, we have found an unexpectedly significant impact of temperature on the protonation degree of DMAP derivatives, especially in the case of protonation of the 4-(dimethylamino)-1-(2,3,5,6-tetrafluoropyridin-4-yl)pyridinium cation, derived from the reaction of DMAP with pentafluoropyridine. Thus, when dissolved in the TfOH-SO2ClF-CD2Cl2 acid system at 30 °C, this cation underwent a slight (<7%) protonation on the dimethylamino group, while the temperature decrease to -70 °C resulted in its complete protonation. Notably, such a scale of this phenomenon has never been observed before for other weak nucleophiles, being many times lower at the same change of temperature. The mechanistic aspects of these intriguing results are discussed.

Acid-Catalyzed Versus Thermally Induced C1-C1' Bond Cleavage in 1,1'-Bi-2-naphthol: An Experimental and Theoretical Study.

Experiments show that 1,1'-bi-2-naphthol (BINOL) undergoes facile C1-C1' bond cleavage under action of triflic acid at temperatures above 0 °C to give mainly 2-naphthol along with oligomeric material. CASSCF and MRMP//CASSCF computations have demonstrated unambiguously that this unusual mode of scission of the biaryl bond can occur in the C1,C1'-diprotonated form of BINOL via a mechanism involving homolytic cleavage prompted by the intramolecular electrostatic repulsion. These findings also provide insights into the mechanism of a comparatively easy thermal cleavage of BINOL, implying the intermediacy of its neutral diketo form.

Mechanistic investigation of superelectrophilic activation of 1,1'-bi-2-naphthols in the presence of aluminum halides.

7,7'-Dihydroxy-1,1'-bi-2-naphthol, as a result of superelectrophilic activation in the presence of an excess of aluminum halides, reacts with cyclohexane and benzene to yield 5,6,7,8,5',6',7',8'-octahydro-7,7'-dioxo-bi-2-naphthol and its 5,5'-diphenyl derivative, respectively. In contrast, isomeric 6,6'-dihydroxy-1,1'-bi-2-naphthol does not react at all under the same reaction conditions, while the parent 1,1'-bi-2-naphthol (BINOL) reveals an alternative mode of behavior. The mechanistic aspects of these intriguing results are discussed on the basis of experimental and theoretical (DFT) study of the protonation and complexation properties of the starting BINOLs.

Superelectrophilic activation of 1-nitronaphthalene in the presence of aluminum chloride. Reactions with benzene and cyclohexane.

1-Nitronaphthalene smoothly reacts with benzene and undergoes selective reduction with cyclohexane in the presence of aluminum chloride to give 2,4,4-triphenyl-3,4-dihydronaphthalen-1(2H)-one oxime and 5,6,7,8-tetrahydro-1-naphthylamine, respectively. The mechanistic aspects of these and related reactions are discussed on the basis of DFT, providing insight into the protonation behavior of 1-nitronaphthalene coordinated to AlCl3.

Protonation Behavior of 1,1'-Bi-2-naphthol and Insights into Its Acid-Catalyzed Atropisomerization.

The behavior of 1,1'-bi-2-naphthol (BINOL) in variety of (super)acid media has been studied by NMR. The results are combined with the theoretical (DFT) study of the role of mono- and diprotonated forms of BINOL in the acid-catalyzed atropisomerization of this compound. It is demonstrated that the process of enantiomeric configuration exchange proceeds mainly via internal rotation around the C1(sp3)-C1'(sp3) bond in intermediates such as C1-monoprotonated keto or C1,C1'-diprotonated forms of BINOL, depending on the acidity level.

Superacidic activation of quinoline and isoquinoline; their reactions with cyclohexane and benzene(1).

Quinoline (1) and isoquinoline (2), upon activation by strong acids, lead to intermediate N,C-diprotonated dications, which are involved in reactions with weak nucleophiles. Thus, 1 and 2 undergo selective ionic hydrogenation with cyclohexane in CF3SO3H-SbF5, HBr-AlBr3-CH2Br2, or HCl-AlCl3-CH2Cl2 acid systems to give their 5,6,7,8-tetrahydro derivatives. They also readily condense with benzene in the presence of HBr-AlBr3 or HCl-AlCl3 to provide 5,6,7,8-tetrahydro-5,7-diphenylquinoline (10) and 5,6,7,8-tetrahydro-6,8-diphenylisoquinoline (12), respectively.

Superelectrophilic activation of polyfunctional organic compounds using zeolites and other solid acids.

Zeolites and other available solid acids have been successfully applied to initiate reactions, which were earlier recognised to involve superelectrophilic intermediates and thus required excess of superacids to be carried out.

Superacidic activation of 1- and 3-isoquinolinols and their electrophilic reactions.

Isomeric 1- and 3-isoquinolinols (11 and 12) when activated in CF(3)SO(3)H-SbF(5) acid system undergo selective ionic hydrogenation with cyclohexane to give 5,6,7,8-tetrahydro-1(2H)- and 5,6,7,8-tetrahydro-3(2H)-isoquinolinones (22 and 27). Under the influence of aluminum chloride similar products were also obtained along with 3,4-dihydro-1(2H)- and 1,4-dihydro-3(2H)-isoquinolinones (23 and 28), respectively. Compounds 11 and 12 also condense with benzene in the presence of aluminum halides, under mild conditions, to give 3,4-dihydro-3-phenyl-1(2H)- and 1,4-dihydro-1-phenyl-3(2H)-isoquinolinones (24 and 29), respectively. Prolonged reaction time or catalysis under strongly acidic HBr-AlBr(3) provides an alternative reaction pathway to yield 5,6-dihydro-6,8-diphenyl-1(2H)- and 5,6,7,8-tetrahydro-6,8-diphenyl-3(2H)-isoquinolinones (25 and 30), respectively. Products 24 and 29 were also found to revert back to 11 and 12 in the presence of aluminum halides in o-dichlorobenzene. The mechanism of these intriguing reactions, which involves superelectrophilic dicationic intermediates, is discussed.

Reactions of 5-, 6-, 7-, 8-hydroxyquinolines and 5-hydroxyisoquinoline with benzene and cyclohexane in superacids.

Isomeric 5-, 6-, 7-hydroxyquinolines (11-13) and 5-hydroxyisoquinoline (14) gave N,C-diprotonated dications in CF(3)SO(3)H-SbF(5) superacid medium. Compounds 11, 13, 14, and 8-hydroxyquinoline (5) underwent selective ionic hydrogenation with cyclohexane in the presence of aluminum chloride. Compounds 11 and 14 condense with benzene in the presence of aluminum halides. The detailed mechanism of reactions, which involves superelectrophilic dicationic intermediates, is discussed.