Proton transfer intermolecular

A mechanism for the piperazine-catalyzed formation of 4//-chromenes is complex cascade of reactions, starting with piperazine acting as a base which activates malononitrile, promoting Knoevenagel condensation, and also formation of an enamine, followed by Michael condensation, proton transfer, intermolecular cycliza-tion via a nucleophilic addition of the enolate oxygen to the nitrile group (hetero-Thorpe-Ziegler), and finally hydrolysis and tautomerization. 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

The first observation of the proton transfer in pyrazoles in the solid state was made for the intermolecular tautomerism in 3,5-dimethylpyrazole 10b (85JA5290). The degenerate rearrangement was recorded using the... 

CT) complex with absorption maxima at 470 and 550nm, was produced. These species were formed only in polar solvents with relatively high proton affinity. The data suggested an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion... 

However, the aminoazo product is formed via two pathways. The first is through the 1 1 addition complex (HAArNj )n as side-equilibrium and an intermolecular rearrangement involving redissociation of this complex into the reagents followed by formation of another 1 1 addition complex (HAArNJ )c and the classical C-o-complex (oc in Scheme 13-13). The second pathway starts from the first mentioned 1 1 complex (HAArNJ )N to which a second molecule of amine is added. This complex forms the aminoazo product by proton transfer to a base. The base may be the second amine molecule of the 1 2 complex. 

The ability of water to ionize, while shght, is of central importance for life. Since water can act both as an acid and as a base, its ionization may be represented as an intermolecular proton transfer that forms a hydronium ion (HjO ) and a hydroxide ion (OH ) ... 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

Sakurai et al. have provided what is probably the most important mechanistic finding in the area of intermolecular additions of silenes in recent years, namely a detailed proposal for the mechanism of alcohol addition to the silicon-carbon double bond.68 A cyclic silene 116 was synthesized in the presence of various amounts of methanol and other alcohols, and varying proportions of methanol adducts 117 and 118 were obtained. It was concluded that the methanolysis involved two steps, the first being the association of the oxygen lone pairs with the sp 2-hybridized silicon atom of the silene. The second step, proton transfer, could occur in two ways. If the proton was transferred from the complexed methanol molecule (path a) its delivery would result in syn addition. However, if a second molecule of methanol participated (path b), it would deliver its proton... 

Amaut LG, Formosinho SJ (1993) Excited-state proton-transfer reactions. 1. Fundamentals and intermolecular reactions. J Photochem Photobiol A Chem 75 1-20... 

Klymchenko AS, Demchenko AP (2003) Multiparametric probing of intermolecular interactions with fluorescent dye exhibiting excited state intramolecular proton transfer. Phys Chem Chem Phys 5 461 -68... 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Waluk J (2000) Conformational aspects of intra and intermolecular excited state proton transfer. In Waluk J (ed) Conformational analysis of molecules in excited State. Willey-VCH, Weinheim, pp 57-112... 

Hynes JT, Tran-Thi TH, Grunucci G (2002) Intermolecular photochemical proton transfer in solution new insights and perspectives. J Photochem Photobiol A Chem 154 3-11... 

Morillo M, Cukier RI (1990) On the effects of solvent and intermolecular fluctuations in proton transfer reactions. J Chem Phys 92 4833-4838... 

Chalcogenic acids, R E(0)0H, are also tricoordinate and considered to have pyramidal structures. However, no studies on their optical activity have been reported since facile racemization of chalcogenic acids may occur via achiral chalcogenate anions, which are formed by the extrusion of a proton and/or by an intra- or intermolecular proton transfer reaction. 

Equilibrium processes of intermolecular proton transfer between the thiolate center and SH group are observed in a solution along with the isomerization of betaine 201 to 701. These processes result in salts 71 (Scheme 32) similar to salts 69 in the carbon series, which was proved by 29Si NMR spectroscopy. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

The mechanism for the addition of singlet carbenes to alcohols has been studied in some detail (Bethell et al, 1971 Kirmse et al, 1981). By and large, the evidence supports two routes. The first, more common, sequence features initial formation of an ylid. Under some circumstances this reaction is reversible (Zupancic et al., 1985 Liu and Subramanian, 1984 Warner and Chu, 1984). Next, proton transfer, either intramolecularly, which may be slowed by symmetry constraints, or by a pair of intermolecular protonation and deprotonation steps, gives the ether. These reactions are outlined in (7). 

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