Intramolecular, addition proton-transfer

In conclusion, a typical time of 300 fs has been found for the excited-state intramolecular double proton transfer in TAB and DAC. The proton transfer dynamics is not influenced by aggregation. In addition, a vibronic cooling time of 20 ps has been measured for the probe molecules in the molecular and stacked configurations. Finally, aggregation is found to almost completely hamper the rotational diffusion motions of the molecules during the fluorescence state lifetime of 4 ns. 

The mechanism of the indolization of aniline 5 with methylthio-2-propanone 6 is illustrated below. Aniline 5 reacts with f-BuOCl to provide A-chloroaniline 9. This chloroaniline 9 reacts with sulfide 6 to yield azasulfonium salt 10. Deprotonation of the carbon atom adjacent to the sulfur provides the ylide 11. Intramolecular attack of the nucleophilic portion of the ylide 11 in a Sommelet-Hauser type rearrangement produces 12. Proton transfer and re-aromatization leads to 13 after which intramolecular addition of the amine to the carbonyl function generates the carbinolamine 14. Dehydration of 14 by prototropic rearrangement eventually furnishes the indole 8. 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

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). 

Intramolecular photoaddition of tertiary amine and styrene moieties has been extensively studied by Aoyama29 and Lewis group28,30,31 (equations 4-8). Equations 4 and 5 show that if the intramolecular additions result in the formation of a five- or six-membered ring, the product yields are excellent. Highly regioselective intramolecular proton transfer is proposed to occur via least motion pathways from the lowest energy... 

Ah initio calculations to map out the gas-phase activation free energy profiles of the reactions of trimethyl phosphate (TMP) (246) with three nucleophiles, HO, MeO and F have been carried out. The calculations revealed, inter alia, a novel activation free-energy pathway for HO attack on TMP in the gas phase in which initial addition at phosphorus is followed by pseudorotation and subsequent elimination with simultaneous intramolecular proton transfer. Ah initio calculations and continuum dielectric methods have been employed to map out the lowest activation free-energy profiles for the alkaline hydrolysis of a five-membered cyclic phosphate, methyl ethylene phosphate (247), its acyclic analogue, trimethyl phosphate (246), and its six-membered ring counterpart, methyl propylene phosphate (248). The rate-limiting step for the three reactions was found to be hydroxyl ion attack at the phosphorus atom of the triester. ... 

The addition of substituted anilines to frani -jS-nitrostyrene has been reported to involve the formation of a zwitterionic intermediate in the rate-determining step, followed by a rapid intramolecular proton transfer. " ... 

The effect of ring substituents on the rate constants, deuterium kinetic isotope effects and Arrhenius parameters for ene-additions of acetone to 1,1-diphenylsilane have been explained in terms of a mechanism involving fast, reversible formation of a zwitterionic silene-ketone complex, followed by a rate-limiting proton transfer between the a-carbonyl and silenic carbon. A study of the thermal and Lewis acid-catalysed intramolecular ene reactions of allenylsilanes with a variety of... 

Upon fonnation of intermediate LI, conjugate addition to a chalcone and subsequent proton transfer is proposed to lead to enolate LIII (Scheme 37). An intramolecular aldol addition provides activated carboxylate LIV in which alkoxide acylation regenerates the catalyst and delivers p-lactone LVI which, upon decarboxylation, gives rise to a trisubstituted cyclopentene. 

The radical-anions from from alkenes with electron withdrawing substituents will add to carbon dioxide [28]. This process involves the alkene radical-anion, which transfers an electron to carbon dioxide for which E° = -2.21 V vs. see [29]. Further reaction then occurs by combination of carbon dioxide and alkene radcal-anions [30]. The carbanion centre formed in this union may either be protonated or react with another molecule of carbon dioxide. If there is a suitable Michael acceptor group present, this carbanion undergoes an intramolecular addition reaction... 

Mechanism. The first step is the typical acid-catalysed addition to the carbonyl group. Then the alcohol nucleophile attacks the carbonyl carbon, and forms a tetrahedral intermediate. Intramolecular proton transfer from nitrogen and oxygen yields a hemiacetal tetrahedral intermediate. The hydroxyl group is protonated, followed by its leaving as water to form hemi-acetal, which reacts further to produce the more stable acetal. 

Ab initio calculations have been earned out on the gas-phase acid-catalysed hydrolysis reactions of sulfinamide (319) using the 3-21G sets.290 The first step in the acid-catalysed hydrolysis of A-methylmethanesulfinamide (319 R1 = R2 = Me) is O-protonation and this form is then transformed by addition of water to the sulfiirane intermediate (320). Intramolecular proton transfer from 0 to N follows and then slow N—S bond cleavage to give products.290 Studies with (319 R1 = Me, R2 =aryl) also... 

A mechanism involving coordination of the alcoholic oxygen to a silicon atom, followed by a fast intramolecular proton transfer explains the predominant syn addition of alcohol to disilene. However, intermolecular proton transfer leading to the awft-product competes with intramolecular proton transfer at high alcohol concentrations. The proposed mechanism is presented in Scheme 9. 

More recently, Apeloig and Nakash have studied diastereoselectivity in the reaction of (E)-5 with p-methoxyphenol53. In both benzene and THF, the stereochemistry of the products was independent of the phenol concentration. The syn/anti ratios of the addition products were 90 10 in benzene and 20 80 in THF. They have suggested that intramolecular proton transfer after rotation of the Si—Si bond of the phenol-coordinated intermediate is responsible for the formation of the anti-addition rather than intermolecular proton transfer. This must be a special case due to much slower (by a factor of 109-1012) rates of addition of phenol to (E)-5. Since phenolic oxygen is definitely less basic than alkyl alcoholic oxygen, coordination of oxygen in the zwitterionic intermediate in the reaction of (E)-5 with phenol must be loose and hence the intermediates should have much chance of rotation around the Si—Si bond. 

Based on these results, Sakurai and coworkers proposed for the addition of alcohols to the silene 44 the mechanism shown in Scheme 14. In the first stage the silene forms an alcohol-silene complex 48 as suggested by Wiberg61, and this stage is followed by an intramolecular proton migration in 48 (the first-order rate constant, k ) which competes with the intermolecular proton transfer from an additional external alcohol (the second-order rate constant, ko). These two processes give the syn and anti addition products, respectively. 

In the photolysis of pentamethyldisilane 51a three methanol addition products are obtained. Compound 53a, which has not been observed by previous workers85, is attributed to be the product of intramolecular proton transfer. Photochemical reactions of aryldisi-lanes have been recently reviewed86. 

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