Type A zwitterion

Application of Bond Order Considerations to the Type A Zwitterion Rearrangement... 

Fig. 6. Bond Order Effects in the Type A Zwitterion Rearrangement. Note Basis orbitals are shown with arbitrary orientation, hence plus-minus overlaps do not imply antibonding between any particular orbital pair. However, with an odd number of such overlaps, the system is Mobius...

Our research began with a simpler 2,5-cyclohexadiene as shown in Scheme 1.2. Using the n-x configuration as outlined in Scheme 1.1, but utilizing the shorthand notation here for simplicity, we can understand the rearrangement of 4,4-diphe-nyl-2,5-cyclohexadiene to afford the bicyclic photoproduct actually obtained. The final step involves a type A zwitterion whose rearrangement is known to proceed as shown. 

Scheme 1.4 (a) (left) Excitation processes for 2,5-dienones. (b) (right) Bonding correlations with conversion of the n-7t state to the Type A zwitterion (far right). 

FIGURE 75.2 The Type A Zwitterion (i.e., 1SS<, with the aryl groups shown as different due to substitution or to cavity asymmetry). 

Samuel, C.J., Type A zwitterion in cyclohexadienone photochemistry chemical trapping and stereochemistry, /. Chem. Soc., Perkin II, 736-740, 1981. 

The first column corresponds to six-membered transition states where two types can be differentiated situation A, where the proton transfer leads to a neutral tautomer, and situation B (often Tinuvin P or TIN 75), where the proton transfer leads to a zwitterionic tautomer (Scheme 25). 

In the case of the epoxypolyamide varnish, however, as the pH increased the resistance of the film at first rose, then at about pH 8.8 it started to fall until at pH 11 the change-over in the type of conduction occurred. This suggests that the resin was acting as a zwitterion with an isoelectric point at about pH 8.8. Thus before the isoelectric point the membrane would be positively charged and an increasing concentration of hydroxyl ions would... 

Finally, Schevchenko and Roschenthaler reported a new type of zwitterionic compound through the reaction of a methylenediphosphine with an electron-poor enone giving rise to 58 in the presence of a small amount of Et2NH2p [71]. 

In the first step, catalyst 64c attacks ketene 66 to form a zwitterionic enolate 71, followed by Mannich-type reaction with imine 76 (Fig. 40). A subsequent intramolecular acylation expels the catalyst under formation of the four-membered ring. Utilizing 10 mol% of 64c, N-Ts substituted (3-lactams 77 were prepared from symmetrically as well as unsymmetrically substituted ketenes 66, mainly, but not exclusively, with nonenolizable imines 76 as reaction partners [96]. Diastereos-electivities ranged from 8 1 to 15 1, yields from 76 to 97%, and enantioselectivities from 81 to 94% ee in the case of aliphatic ketenes 66 or 89 to 98% ee for ketenes bearing an aromatic substituent. Applying complexes 65 or the more bulky and less electron-rich 64b, ee values below 5% were obtained. 

When the acid and basic pftjS of a zwitterion differ by only 3 or 4 units the compound may exist at isoelectric pH as an equilibrium between the true zwitterion and the formally neutral species. Oral absorption is much improved if this type of equilibrium exits. Charge proximate zwitterions are better absorbed than charge separated zwitterions. When the posihve and negative charge centers are close in space there is an overlap in the polarized aqueous salvahon shells so that the compound is less polarized than if the charges were far apart. 

Doubts have recently been expressed regarding the validity of the metaphosphate pathway for hydrolysis of the monoanion of 2,4-dinitrophenyl phosphate (111) 70,71,72) since the basicity of the 2,4-dinitrophenolate group is insufficient to produce a zwitterion corresponding to 106 or even a proton transfer via intermediates of type 103 or 105 (pKa values in water 4.07 for 2,4-dinitrophenol, 1.0 and 4.6 for 2,4-dinitrophenyl phosphate). Instead, hydrolysis and phosphorylation reactions of the anion 111 are formulated via oxyphosphorane intermediates according to 114. 

Later, a zwitterionic monophosphino-[l,2,4]diazaphospholide 42 was obtained from the reaction of 3,5-bis(triphenylphosphonio)-[l,2,4]diazaphospholide chloride (38) with complex boron hydrides [43], In the initial step, selective extrusion of one PPh3 group takes place to yield the boron adduct of type 41, which on further reaction with triethylamine liberates zwitterionic diazaphospholide 42 (Scheme 12). 

Carbohydrate detection is important for applications such as glucose monitors these are arguably one of the most successful and relevant biosensors. An interesting fluorescence recovery-type saccharide sensor based on the reactivity of carbohydrates with boronic acids was reported in 2002 [36]. Specifically, modification of the cationic viologen-linked boronic acid derivative 40 to a zwitterionic species 41 upon covalent and reversible reaction of boronic acid with monosaccharides (Scheme 1) can cause the dissociation of the ion-pair in-... 

The reaction of diazoazoles and isocyanates leading to azolo-tetra-zinones of type 258 (Scheme 75) can be regarded as a [7 -l- 2]cycloaddition of the diazoazoles to the electron-deficient hetero double-bond of the isocyanates (pathway a) or, alternatively, as a two-step reaction involving [3 -I- 2]cycloaddition of the diazoazoles to the isocyanates, leading to the spirostructure 259 and subsequent [l,5]acyl shift (pathway b). An additional two-step mechanism (pathway c) could involve nucleophilic attack by the azole ring nitrogen on the carbonyl isocyanate to give a zwitterionic intermediate that collapses to the [7 + 2]cycloadduct 258. 

In contrast to the sulfinylimines, the sulfonylimines are used only at a low temperature. Nagai et have studied the reaction of a sulfonylimine with enamines that do not have any reactive hydrogen atoms in the molecule ( Type C enamine). The cyclic product may be produced via a two-step mechanism in which a zwitterionic intermediate (168) occurs (Eq. 34). The Me... 

X-Ray analysis confirms that l-ethoxycarbonyl-l//-azepine and nitrosobenzene form a [6+2]ir adduct. Subsequently, however, careful analysis of the reaction mixture by H NMR has shown that the [4+2]ir adduct is also formed albeit in low yield (17%). No interconversion of the two adducts could be detected even after eight weeks, from which fact the authors conclude that both adducts derive directly from a zwitterionic intermediate of type (142) rather than in a concerted manner (80TL319). 

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