Intermolecular reactions Proton transfer

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]. 

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

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... 

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. 

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). 

The computational results show that transition structures derived from hydroperoxo Re complexes lie slightly higher in energy than those obtained for the corresponding peroxo complexes, nevertheless their involvement in the epoxidation reaction cannot be excluded. However, for neither MoVI nor Revn evidence Get alone preference) for hydroperoxo reaction pathways is as clear as for TiIV complexes. Of course, more complex mechanisms involving intermolecular proton transfer and/or hydrogen bonded intermediates may change this picture to some extent. 

Mechanistically speaking there have been no recent advances. What is known is that, at least for 4-OH formation, the reaction is intermolecular, requires two proton transfers at some stage and that a symmetrical intermediate is involved (often described as... 

The usual method for establishing partially rate-limiting proton transfer, determination of the rate constants in D2O, would give ambiguous results (Bruice and Piszkiewicz, 1967). However, intramolecular general acid catalysis [equation (48)] is the preferred mechanism in view of the intermolecular buffer acid catalysis observed with the unsubstituted compounds. General acid catalysis [75] should therefore be favoured in the intramolecular reaction. 

Since the addition of the ethylene and transfer of the proton may be concerted reactions, the energy barrier for the alkylation may be much lower than for an addition followed by an intermolecular proton transfer. 

DIHYDROGEN BONDS AS INTERMEDIATES IN INTERMOLECULAR PROTON TRANSFER REACTIONS... 

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