Initiators zwitterions

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]. 

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). 

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

Kricheldorf HR, Garaleh M, Schwarz G (2005) Tertiary amine-initiated zwitterionic polymerization of pivalolactone - a reinvestigation by means of MALDI-TOF mass spectrometry. J Macromol Sci, Part A Pure Appl Chem 42 139-148... 

Isomerization of siloxycydopropane 3 to the allylic silyl ether in the presence of Znl2 has close mechanistic similarity to the homoenolate formation, Eq. (11) [26]. The initial zwitterionic intermediate after ring cleavage undergoes a hydride shift rather than elimination of the silyl group. 

Silylaminophosphines 302 reacted smoothly with carbonyl compounds to give O-silylphosphinimine 304 via a 1,4-silyl migration from N to O in the initial zwitterionic adduct 303 (equations 189 and 190)449-450. Upon heating, the cyclic phosphinimines 306 in equation 190 decomposed cleanly to the starting materials 305450. 

Fischer s base (55) reacts with tetracyanoethylene to yield a mixture of spiro and fused pyrrolines, which are believed to arise from an initial zwitterionic adduct, as shown in Scheme 351. 

Macrocycles were not detected among any of the products. However, cyclization of the initial zwitterion did occur with the two dioxaphospholane nucleophiles and several electrophiles63. ... 

The initial zwitterionic intermediate after ring cleavage undergoes a hydride shift rather than elimination of the silyl group. 

Reactions of enamines with activated olefins can give several possible products (Scheme 3). The exact composition of the adducts formed depends critically on the substrates employed and the reaction conditions utilized. An initial zwitterionic adduct (3.1) is believed to be the first intermediate formed. Intermediate 3.1 is usually unstable under the reaction conditions and undergoes one of four possible transformations. Cyclization of 3.1 can occur in two ways. Intramolecular closure can occur by addition of the acceptor-stabilizing substituent (Y) to the carbon of the immonium ion to provide 3.2... 

In the TNB-aniline system, a detailed kinetic analysis showed that the lack of formation of a stable a complex in the absence of a tertiary amine catalyst was the result of a thermodynamic, rather than a kinetic, factor (34). That is, the equilibrium constant for formation of the initial zwitterionic adduct (TNB NH2C6H5) is very small, such that in the absence of tertiary amine the overall equilibrium for complex formation is unfavorable. Addition of a tertiary amine, however, causes the equilibrium to be shifted to the right, due to the much more favorable deprotonation of the zwitterionic adduct to the anionic adduct, corresponding to the increased basicity of (CH3CH2)3N or DABCO as compared to C6H5NH2. 

Interestingly, the diphenylketene derived 1 1 adduct is stable to hydrolysis, but if the reaction forming it is carried out in the presence of traces of water, an iV-acyl derivative analogous to that from the dichloroketene reaction is obtained, apparently via trapping of an initial zwitterionic adduct (Scheme 5). Treatment of the carbodiimide with oxalyl chloride apparently affords the unusual spirocyclic 2 1 adduct (10) (Scheme 5) larger ring carbodiimides, in contrast, afforded only 1 1 adducts <86JOC417>. 

The initial zwitterion that forms upon combination of a nucleophilic with an electrophilic monomer is called a genetic zwitterion Intramolecular reactions can produce macrocycles ... 

Anionic polymerizations are initiated in polar systems by bases and Lewis bases. For example, alkali metals, alcoholates, metal ketyls, metal alkyls, amines, phosphines, and Grignard compounds act as initiators. However, the polymerization mechanism does not depend on the nature of the initiator alone. For example, tertiary amines and phosphines do not only initiate anionic polymerizations under certain conditions, they can also initiate zwitterion polymerizations. In addition, polyinsertions can proceed in less polar systems. Thus, anionic polymerizations are often carried out in polar solvents. Ethers and nitrogen compounds, such as tetrahydrofuran, ethylene glycol dimethyl ether (glyme), diethylene glycol dimethyl ether (diglyme), pyridine, and ammonia are most commonly used. 

A Pd-mediated reaction of a vinyl epoxide with an isocyanate offers synthetically useful transformation (Scheme 4). A diastereoselective vicinal hydroxyamination is established, which involves trapping an initial zwitterion with an isocyanate and subsequent nucleophilic attack of a nitrogen anion on a rr-allylpalladium intermediate (Scheme 4)... 

The initial zwitterion A generated from imidazole and cyanophenylacetylene is thought to be transformed into carbene B, which further inserts into the C=0 bond of N-vinylpyrrolecarbaldehyde (shown on the example of N-vinylpyrrole-2-carbaldehyde). Two subsequent rearrangements of adduct C furnish intermediate D, which in its enol form adds to the second molecule of cyanophenylacetylene (Scheme 2.156). 

---