Zwitterionic primary adduct

Zwitterionic borane adducts were reacted with trityl cation salts to provide a series of primary borenium cation derivatives (Scheme 12). For example, the A-heterocyclic carbene (NHC) borane (45) was prepared from the corresponding NHC (44). With the addition of the trityl salt, the dicationic dimer (46) was isolated. Product (46) was characterized by X-ray crystallography and trapped by nucleophiles. Its observation by NMR resonance (5 + 10.6) is in accordance with the GIAO calculated value (5 + 0.8). Similar borenium dications were generated from amine nucleophiles, such as compound (47). The authors considered the question why some borenium ion salts... 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

A kinetic smdy of the formation of zwitterionic adducts (28) from 1,3,5-trinitrobenzene and diazabicyclo derivatives indicates that reactions are surprisingly slow, with rate constants many orders of magnitude lower than those for related reactions with primary or secondary amines. The use of rapid-scan spectrophotometry was necessary to study the kinetics of reaction of 4-substimted-2,6-dinitro-A -n-butylanilines (29) with n-butylamine in DMSO the two processes observed were identified as rapid deprotonation to give the conjugate base and competitive a-adduct formation at the 3-position. The reactions of MAf-di-n-propyl-2,6-dinitro-4-trifluoromethylaniline (30), the herbicide trifluralin, and its A -ethyl-A -n-butyl analogue with deuteroxide ions and with sulfite ions in [ H6]DMS0-D20 have been investigated by H NMR spectroscopy. With deuteroxide a-adduct formation at the 3-position is followed by... 

When a-substituted JV,iV-dimethylacetamidines are used as the bidentate nucleophiles, the reaction proceeds according to Scheme 6. The primary attack occurs at an a position by the nucleophilic nitrogen to yield the zwitterionic adduct 77 (Amax = 506 nm) and is followed by intramolecular ring closure at the y position leading to a bicyclic adduct (78). In contrast, with the N-oxide of 3,5-dinitropyridine the points of attachment of the reagent are both a to the aza group. 

Yasuda et al. have recently reported that irradiation of o-alkenylphenols and naphthols in the presence of primary or secondary amines results in Markovnikov addition of the amine. For example, irradiation of the alkenylphenol 12 with methyl-amine affords the adduct 13 (Scheme 4), The initial step in these reactions is proton transfer from the acidic phenol singlet state to the amine, which leads to the formation of the singlet phenolate anion and ammonium ion. Proton transfer from the ammonium ion to the phenolate anion yields a zwitterion, which undergoes nucleophilic addition of the amine. Thus the amine serves as a base in the initial step of this reaction and then as a nucleophile. 

The reaction of TNB with primary or secondary aliphatic amines in DMSO [127] gives the anionic adducts (53) as shown in Scheme 6.24. Kinetic studies in DMSO [71] or water [151] show that proton transfo- from the initially formed zwitterion (52) to a second amine molecule may be rate determining. 

There is considerable evidence for the formation of the bicycHc oxyaUyl zwitterion prior to the epoxy-cyclopentenone. Pavlik and Barltrop showed that irradiation in trifluoroethanol (TFE) or methanol furnished solvent adducts as primary photoproducts, and Barltrop and co-workers excluded the possible formation of the TFE adduct 6e from epoxycyclopentenone 3e. Barltrop demonstrated that irradiation of 3,5-dimethyl-4-pyrone le in furan/trifluoroethanol mixtures furnished the [4-l-3]-cycloadduct 9e as one of the major photoproducts (Scheme 3). A similar result was reported by West and co-workers using tetramethylpyrone Id. Interestingly, in both cases, a single diastereomer was observed, corresponding to approach of furan from the face opposite the epoxide oxygen and reaction via an endo... 

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