Zwitterionic species

However, as can also be seen in Fig. 11, primary and secondary amines do not perform very effectively as primers, compared to tertiary amines, even though they also contain long alkyl chains. It has been demonstrated that, instead of directly initiating ECA polymerization, primary and secondary amines first form aminocyanopropionate esters, 12, because proton transfer occurs after formation of the initial zwitterionic species, as shown in Eq. 7 [8,9]. 

The intermediacy of zwitterionic species was proposed for the addition of phenylsulfenyl chloride to dimethylaminomethyl-substituted enamines (465,466). They receive support from the observed lack of stereospecific addition of methylsulfonyl chloride to cw-morpholinopropene (although the trans enamine gives only one adduct) (466). 

Oxidation yields the cross-conjugated mesomeric betaine 217, but no zwitterionic species, which would result by deprotonation of the indole NH group (88LA1111). The corresponding 5,6-unsaturated indole, however, gave the zwitterion. The same publication describes a total synthesis of Flavoserpentine. 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

The preparation and characterization of diastereomeric A-iminodiacetatoplatinum(II) complexes of (R,S)- and (R.R)-, 2-chxn have been reported,373 where the substituted iminodi-carboxylato ligand is bonded to the metal center in a tridentate manner (159). One arm of the ligand forms a stable five-membered A,O-chelate, and the remaining carboxylate group interacts with the positively charged metal center to form a zwitterionic species. Unequal proportions of the diastereomers are formed in solution. 

Schuster and co-workers<11 16) have provided one of the best examples where both diradicals and zwitterionic species are probably involved. The dienone (14), when photolyzed in 1,1,2-trimethylethylene, forms only the oxetane (15) all other photoreactions are eliminated. This finding confirms that n w and not states are involved in the dienone photorearrangements. 

Ogura and Matsuura<30) argued that which mechanism (the slither or pivot) takes place depends on whether the rearrangement takes place from the ground state zwitterionic species or from the excited state diradical species. They claim that the following transformation occurs via the diradical pivot mechanism ... 

Kitagawa, S., J. Takeda, and S. Sato. pH-dependent inhibitory effects of angiotensin-converting enzyme inhibitors on cefroxadine uptake by rabbit small intestinal brush-border membrane vesicles and their relationship with hydrophobicity and the ratio of zwitterionic species. Biol. Pharm. Bull. 1999, 22, 721-724. 

It has since become increasingly clear that zwitterionic zirconates may be generated in many other reactions and may lead to unexpected and interesting chemical consequences, as suggested by the results and interpretations shown in Scheme 1.73. It should be noted that the empty orbital and electrophilicity of Zr must lead to zwitterionic species containing zirconates and carbocationic centers. Further systematic investigations in this area appear to be desirable. 

Upon treatment with dimethyldioxirane (DMDO), benzannulated dihydropyrrolizine 84 afforded two dimers 86 and 86, each as a mixture of two diastereomers < 1997JA1159>. The zwitterionic species 85 is postulated as intermediate in these dimerizations (Scheme 10). 

The quaternary bismuth(v) complex BiR PFg- (R = />-C6H4NMe2) was obtained by the reaction of R3BiCl2 with AgPF6. 4 A zwitterionic species with a stibonium center is Me3Sb+CH2COO, the antimony analog of betaine.127... 

O Connor proposed a mechanism involving deinsertion of carbon monoxide from the vinylketene complex 106 to form the new cobaltacyclobutene 109. The cobalt may then undergo a 1,3-shift to the carbonyl of the ester group to create the oxycobaltacycle 110, before deinsertion of the cobalt moiety forms the furan 108. Alternatively, 109 may rearrange to the vinyl-carbene 111, which then undergoes ester-carbonyl attack on the carbene carbon to form the zwitterionic species 112, which finally aromatizes to yield the furan 108. Notice that this latter postulate is identical to the final steps of the mechanism formulated by Wulff (see Section V,B) for the reaction between a cobalt carbene and an alkyne, in which a cobaltacyclobutene is a key intermediate.51... 

Scheme 8 Alternative pathway to the tricyclic elisabethane skeleton via a zwitterionic species and further cyclizations...

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-... 

HCHO and PH3 proceeds in the presence of K2PtCl4 at room temperature and affords the crystalline product in an essentially quantitative yield in 2.5 h [4]. Palladium compounds are also active in the catalysis [5]. In these reactions the active species is believed to be zero valent. Two mechanistic possibilities have been proposed as illustrated in Scheme 2. The first elemental process involved in the catalytic cycle is oxidative addition of a P-H bond, which is well precedented [6]. In one of the mechanistic possibilities the processes that follow the oxidative addition are the insertion of the C=0 bond into H-M species and P-C reductive elimination, the latter of which is also precedented [7]. In the other, the coordinating phosphide ligand makes a nucleophilic attack [8] at the formaldehyde carbon forming zwitterionic species. 

Several unique synthetic strategies for bidentate(amino)(oxy)- and (amino)(aryl) carbenes have been described (Scheme 10). For the former, the reaction of the amino(phosphino)carbene with an ort/io-quinone leads to the transient formation of a zwitterionic species featuring both a phosphonio nucleofuge and an aryloxide nucleophile that allow for a subsequent intramolecular substitution process. The... 

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