Zwitterion intermediates, and

The presence of an substituent, found in the pyrrolidine enamine of 2-methylcyclohexanone, blocks the possibility of an intramolecular proton transfer in the zwitterionic intermediate and thus only the benzocyclobutane... 

The reactions of enamines as 1,3-dipolarophiles provide the most extensive examples of applications to heterocyclic syntheses. Thus the addition of aryl azides to a large number of cyclic (596-598) and acyclic (599-602) enamines has led to aminotriazolines which could be converted to triazoles with acid. Particular attention has been given to the direction of azide addition (601,603). While the observed products suggest a transition state in which the development of charges gives greater directional control than steric factors, kinetic data and solvent effects (604-606) speak against zwitterionic intermediates and support the usual 1,3-dipolar addition mechanism. 

The reactions and compound presented in this chapter support the notion that the formation of o-QMs from the parent phenols is a quite complex process. In the case of the oxidation by Ag20 but also likely in other oxidations, a zwitterionic intermediate is involved that can be stabilized intermolecularly, for example, by electrostatic interaction with other suitable zwitterions, or intramolecularly by neighboring groups or inductive/mesomeric effects. By stabilizing the zwitterionic intermediate and destabilizing the o-QM, the energetic gap between these two intermediates is lowered and... 

There is some evidence that the cycloaddition reaction of benzynes with 1-substituted pyrroles may proceed in two steps via a zwitterionic intermediate and that the reaction is thermally reversible, for although 1-methylpyrrole yields the [w4 + w2] adduct (258) with... 

To clarify the effects of linking chains, Gilbert et al. investigated the photocycloaddition of 163-180, which produce various types of adducts [231,234-237] (Scheme 53). In some cases, not only the meta addition but also ortho and para addition and ene reaction took place. The observed regiochemistry was rationalized in terms of the stabilization of zwitterionic intermediates and the steric constraints. 

Kochi et al. [111-113] have utilized time-resolved laser spectroscopy to examine the excited complexes. For the indene (IN)/TCNE system, excitation of the CT complex (532 nm) afforded not only absorption bands assigned to TCNE anion radical and IN cation radical, but also an absorption band due to an unidentified intermediate. Kochi proposed a 1,4-diradical or 1,4-zwitterionic intermediate, and postulated bond-formation as follows [111] ... 

In conclusion, it now appears that the cause of base catalysis in the aminolysis of aromatic substrates may not be of common origin. Further investigations of the mode of decomposition of these zwitterionic intermediates and the analogous o-complexes are clearly desirable. DMSO will remain central to these studies (Fendler et al., 1975b), due to the possibility of isolation of such complexes in this medium. It now appears that DMSO may also be valuable for studies of electrophilic catalysis, since its use tends to avoid ion pairing effects which complicate kinetic studies in aprotic media of lower dielectric constant. The problem of ion pairing is considered further in the following section. 

The authors assume this reflects the rotamer population of the zwitterionic intermediate and that 51 arises by abstraction of an equatorial proton (Scheme 36). However, this would be more likely to occur via a twist conformation which could arise, at least in part, from equatorial attack on the enamine. Interestingly the tetrasubstituted enamine (51) crystallized from the reaction mixture and was shown on heating in benzene, without added acid catalyst, to revert to the same mixture of 51 and 52. This presumably reflects the greater carbon acidity of a methylene alpha to a sulphone group which can therefore catalyse the interconversion or cause reversion to enamine followed by re-alkylation100. 

Decomposition of a-diazo ketoamides 208 in the presence of substituted propiolic esters gives spirocyclic oxiranes 209. The reaction involves intramolecular addition of a rhodium carbenoid onto the oxygen atom of the amide group to yield the carbonyl ylide, which, after 1,4-H-migration, produces a cyclic ketene N,0-acetal 210. The latter further reacts with the activated triple bond of the dipolarophile to form a zwitterionic intermediate and, finally, a spirocycloadduct (Scheme 26) (90JA2037). 

Seml-empiricai and ab initio calculations have been used to study the l,2 cycloaddition of 02( Ag) to aminoethylene and ethylene. Electron-rich olefins react via a zwitterionic intermediate and the total reaction paths were determined using Fakul s IRC theory.The same group has examined... 

Carbene-like nitrile ylides have been further implicated in the photorearrangements of substituted 2/ir-azirines (78) and (79) which proceed by a 1,1-cycloaddition reaction (Scheme 7). The process is believed to involve a biradical or zwitterionic intermediate and, in all cases examined, the product is the bicyclic system (80), although at... 

A simple and environmentally friendly route for the preparation of 1,2,3-diazaphospholes and -hydrazonophosphonates has been developed. The reaction of 1,2-diaza-1,3-butadienes with dialkylphosphonites under solvent-free conditions proceeded via a zwitterionic intermediate and gave, by precipitation, the stable ylidic ot-phosphanylidene hydrazones which, in turn were transformed into the corresponding 3-phenyl-2/f-l,2,33, -diaza-phospholes and -hydrazonophosphonates (Scheme 5). 

Reaction of chlorotriaryl derivatives with potassium diisopropylamide have been reported to give alkyl-shifted phenazine derivatives by either intramolecular nucleophilic attack on an aryne intermediate leading to a zwitterionic intermediate and alkyl transfer via a 5-endo-tet process or via a Smiles rearrangement (Scheme 193). ... 

Applying the steady-state approximation to the zwitterion intermediate and assuming that the terms containing the rate constant k i, are small by comparison with the other terms, the rate of carbamate formation can be expressed as... 

The authors gave the following rationalization to explain the ylide annulation (Scheme 20.25). Triphenylphosphine reacts with bromide 26 to form a phospho-nium salt, which is deprotonated by K2CO3 to generate the corresponding phos-phonium ylide in situ. A Michael addition of the ylide to the electron-deficient olefins, followed by another intramolecular Michael addition of the zwitterionic intermediates, and then P-eHmination of triphenylphosphine completes the catalytic cycle. The formation of the two isomers can be explained by a- or y-attack of yHde to dually activated olefins 28. Of course, the possibility of migration of the double bond in cyclopentene products 30 under the reaction conditions cannot be excluded. 

In the presence of a Lewis acid, / ,y-unsaturated ketones and oximes or imines undergo nucleophilic addition to give zwitterionic intermediates, and subsequent oxy-2-azonia-Cope rearrangements give homoallylic amides (Scheme 23). ... 

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