Zwitterions Reaction Type

Another remarkable example of reaction type (c) in Table 5, somewhat related to the Menschutkin reaction, is the Sn2 reaction between tris(n-butyl)phosphane and carbon disulfide, to give zwitterionic tris( -butyl)phosphonium dithiocarboxylate via a corresponding dipolar activated complex [790, 791],... 

Reaction Type Diradical States Zwitterionic States... 

In the last 20 years, several mechanisms have been postulated for this reaction with concerted or concerted two-stage mechanisms, as weU as 1,4-biradicals, 1,4-zwitterions, or perepoxides as intermediates. A series of semiempirical and ah initio calculations has been pubHshed, but the exact mechanism of this reaction type is still doubtful.The suprafacial reaction course (see Section 8.4), the lack of... 

Bielectrophiles have found appreciable applications in the synthesis of ring-fused systems, especially those involving [5,6] fused systems. The following serve to illustrate these applications. Reaction of pyrazole with (chlorocarbonyl)phenyl ketene (214) (Type 1, Scheme 6) readily formed the zwitterionic pyrazolo[l,2-a]pyrazole derivative (215) (80JA3971). With l-methylimidazole-2-thione (216), anhydro-2-hydroxy-8-methyl-4-oxo-3-phenyl-4//-imidazo[2,l-6][l,3]thiazinium hydroxide (217) was obtained (80JOC2474). 

D. Further Reactions of the Initial Michael-Type Zwitterions... 

Theoretical calculations explain the photochemical behavior of phenylthiazoles (Fig. 14) (99MI233). The RCRE mechanism cannot be invoked because the radical intermediates have higher energies than the corresponding triplet states. Furthermore, the formation of the Dewar isomer is favored in comparison with the formation of the zwitterionic intermediate. Nevertheless, the reaction conditions used by Kojima and Maeda could allow for an endothermic reaction giving this type of intermediate. The same results were obtained using 2,5-diphenylthiazole. 

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

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]. 

The reactions of the butadiynediyldimetal(Fe, Ru) complexes with Fe2(CO)ci at room temperature afforded mixtures of products, from which three types of products, viz. the ps-acetylide cluster compound 4, the pj-ti -propargylidene-ketene compound 5 and zwitterionic cluster compound 6, were isolated. While the reaction with an excess amount of Co2(CO)g results in addition to the sterically congested Fp -C=C part [6]. The distributions of the products were dependent on the metal fragments situated at the other end of the conjugated carbon rod. The cluster compounds so obtained were characterized by spectroscopic and... 

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. 

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). 

An efficient pathway to 3,8-dihydro derivatives of the same ring system 496 was published by Lee et al. < 1994S1057, 1997S1461 > the ureido ethylidenehydrazones of type 494 were treated with triphenylphosphine in the presence of triethylamine in a carbon tetrachloride solution under refluxing conditions to yield the product 496 in acceptable yields. The transformation was rationalized to proceed via the zwitterionic intermediate 495. It is important to note that, in the cases of R = aryl group, the reaction proceeded in a totally different direction to furnish... 

Two types of derivatives of 1,2-cyclohexadiene with two heteroatoms were proposed as reactive intermediates more than 20 years ago. Lloyd and McNab [168] observed the reaction of the 5-bromo-l,2-dihydropyrimidinium ions 411 with thiourea in refluxing ethanol to give the bromine-free cations 413. Suspected as intermediates, the 5d2-dihydropyrimidines 412 were initially considered as zwitterions of the type 414-Zj. However, quantum-chemical calculations on the parent systems suggested an unambiguous preference of the allene structure 414 over the zwitterion 414-Za [169]. 

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