Zwitterion initiator

In the context of the anionic polymerization of CA derivatives, as considered in Section 10.3.2.2.1, it is notable that the polymerization of cyanoacrylates is also photoinitiated by substituted pyridine pentacarbonyl complexes of tungsten or chromium, i.e. M(CO)5L with M = Cr or W, and L = 2- or 4-vinylpyridine [60]. Photo-released pyridine adds to CA, and the resulting zwitterion initiates the anionic chain polymerization (see Scheme 10.16). 

For polymerization of TOX in media of low polarity, it has been shown that in a rigorously dried system BF3 alone is not able to initiate polymerization. There are, however, some indirect indications that in polar solvents such as nitrobenzene, which facilitate charge separation and formation of zwitterions, initiation may proceed even in the absence of a coinitiator. ... 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Unsaturated compounds undergo ozonization to initially produce highly unstable primary ozonides (15), ie, 1,2,3-trioxolanes, also known as molozonides, which rapidly spHt into carbonyl compounds (aldehydes and ketones) and 1,3-zwitterion (16) intermediates. The carbonyl compound-zwitterion pair then recombines to produce a thermally stable secondary ozonide (17), also known as a 1,2,4-trioxolane (44,64,125,161,162). 

The initiation mechanism is well defined because of the recent isolation and spectroscopic characterization of the initial zwitterion from ethyl cyanoacrylate (ECA) and a phosphine [8,9]. Specifically, zwitterion 4 was prepared from the reaction of equimolar amounts of dimethylphenyl phosphine, 5, and ECA, 6,... 

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 initial product formed when methyl vinyl ketone is mixed with an enamine [such as N,N-dimethylisobutenylamine (10)] is the dihydropyran (11) from a 1,4 cycloaddition (ll,20a,20b). The chemical reactions that the dihydropyran undergoes indicate that it is readily equilibrated with the cyclobutane isomer 12a and zwitterion 12 (11). Treatment of adduct 11 with phenyllithium gives cyclobutane 13, possibly via intermediate 12a (11). 

The initial reaction between a ketene and an enamine is apparently a 1,2 cycloaddition to form an aminocyclobutanone adduct (58) (68-76a). This reaction probably occurs by way of an ionic zwitterion intermediate (75). The thermal stability of this adduct depends upon the nature of substituents Rj, R2, R3, and R. The enolic forms of 58 can exist only if Rj and/or R4 are hydrogens. If the enamine involved in the reaction is an aldehydic enamine with no 3 hydrogens and the ketene involved is di-substituted (i.e., R, R2, R3, and R4 are not hydrogens), then the cyclo-butanone adduct is thermally stable. For example, the reaction of dimethyl-ketene (61) with N,N-dimethylaminoisobutene (10) in isopropyl acetate... 

The mechanism of the cycloaddition of sulfenes to enamines does not involve a concerted process in many if not all cases, but rather a two-step process in which a zwitterion is the initially formed intermediate (158,159). 

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

Conjugated dienamines were found to give predominantly double four-membered-ring adducts as well as a small amount of the six-membered-ring adduct (466,468). This important result indicates preferred attack at the terminal carbon of the dienamine system (in contrast to alkylation, for instance) in the generation of an initial zwitterionic intermediate. Addition of sulfonyl chloride and triethylamine to a homocyclic dienamine gave only the bridged product (446). 

Reactions of vinylogous amides with methanesulfonyl chloride also led to the formation of six-membered rings. Here the initial attack on oxygen produces a zwitterionic intermediate which can collapse to an enol sulfonic acid lactone (383,469). 

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

Addition o/ a Second Mole of Dimethyl Acetylenedicarboxylate to the Initial Zwitterion... 

The ultraviolet spectra of the pyridinecarboxylic acids (334) were initially interpreted assuming that the proportion of the zwitterion structure 335 was not appreciable,and the early pK work was inconclusive. However, Jaffe s calculations based on the Hammett equation indicated that about 95% of nicotinic and isonicotinic acids existed in the zwitterion form, and ultraviolet spectral data showed that the actual percentages of picolinic, nicotinic, and isonieotinie acids existing in the zwitterion form in aqueous solution are 94, 91, and 96%, respectively.This was later confirmed by Stephenson and Sponer, who further demonstrated that the proportion of the zwitterion form decreases in solvents of low dielectric constant, becoming very low in ethanol. Dipole moment data indicate that isonicotinic acid exists as such in dioxane, and 6-hydroxypyridine-3-carboxyiic acid has been shown to exist in form 336 u.sing pK data. ... 

In an initial step triphenylphosphine adds to diethyl azodicarboxylate 5 to give the zwitterionic adduct 6, which is protonated by the carboxylic acid 2 to give intermediate salt 7. The alcohol reacts with 7 to the alkoxyphosphonium salt 8 and the hydrazine derivative 9, and is thus activated for a SN2-reaction ... 

Stabilised sulphur ylides react with alkenylcarbene complexes to form a mixture of different products depending on the reaction conditions. However, at -40 °C the reaction results in the formation of almost equimolecular amounts of vinyl ethers and diastereomeric cyclopropane derivatives. These cyclopropane products are derived from a formal [2C+1S] cycloaddition reaction and the mechanism that explains its formation implies an initial 1,4-addition to form a zwitterionic intermediate followed by cyclisation. Oxidation of the formed complex renders the final products [30] (Scheme 8). 

The mechanism for aldehyde-derived enamines involves a Michael-type 1,4-addition of the enamine to the alkenylcarbene complex to generate a zwit-terionic intermediate which evolves to the final product by cyclisation. On the other hand, ketone-derived enamines react through an initial 1,2-addition to the carbene carbon to generate a different zwitterionic intermediate. Then, a [l,2]-W(CO)5 shift-promoted ring closure produces a new intermediate which, after elimination of the metal moiety, furnishes the corresponding cyclopen-tene derivatives (Scheme 30). 

The a-substitution in the alkenylcarbene complex seems to be crucial to direct the reaction to the five-membered rings. The mechanism proposed for this transformation supposes an initial 1,2-addition of the enolate to the carbene carbon atom to generate a zwitterionic intermediate. Cyclisation promoted by... 

CBz-protected benzimidazole gave primarily oxazinone [31], while 3/f-indoles incorporated two equivalents of imine (Eq. 4) [32]. In these cases it appears that the initially formed zwitterionic ketene-imine adduct could not close, and reacted with additional photo activated carbene or substrate. 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

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