Tetramethylene zwitterion

Hall112,329 has proposed a unifying concept based on tetramethylen.es (resonance hybrids of 1,4-diradical and zwitterionic limiting structures - Scheme 3.67) to rationalize all donor-acceptor polymerizations. The predominant character of the tetramethylenes (zwitterionic or diradical) depends on the nature of the substituents. 12" 40 However, more evidence is required to prove the more global application of the mechanism. 

Electron-rich bifunctional vinyl ethers (e.g. ethylene glycol divinyl ether) react with electron-poor alkenes (e.g. TCNE) to produce cyclobutanes in good yields via tetramethylene zwitterion intermediates. In some cases, cyclobutanes reacted with the solvent (MeCN) to yield tetrahydropyridines.9 Trifluoromethanesulfonimide is an... 

One way around this difficulty is to generate the tetramethylenes from the cyclobutane adducts. The cyclobutane adduct of NVCz and tetracyanoethylene placed in a solution of excess /V-vinylcarbazole causes cationic homopolymerization of the latter [136]. However a cyclobutane whose substitution pattern will lead on cleavage to a tetramethylene diradical at reasonable temperatures has not yet been found. A possible explanation is that a tetramethylene diradical has one less bond than a tetramethylene zwitterion, and so is less stable [137]. Another explanation may be that tetramethylene zwitterions prefer to exist in the cis form for coulombic reasons, but tetramethylene diradicals appear to prefer a trans, extended conformation and are difficult to generate from cyclic precursors. 

This cautionary attitude toward the stand d enumeration of mechanistic criteria seems fully justified when one reflects on the sorts of evidence advanced in support of 1,4-tetramethylene zwitterionic intermediates in thermal [2 + 2] cycloadditions. 

Photolysis of the zwitterionic pyrido[l,2-h]pyridazines (89) and their 5,6-benzo and 2,3-tetramethylene derivatives afforded 6,7,8,9-tetrahydro-4//-pyrido[l,2-fl]pyrimidin-4-ones (90) and their 8,9-benzo [77H(8)377] and 2,3-tetramethylene derivatives (75JOC2201). Photoinduced ring transformation of 2-phenylpyrido[l,2-fe]pyridazinium-4-olate (73) was investigated in methanol (94T4699). 

From organic chemistry it is known that cycloaddition reactions leading to cyclobutanes are required to be stepwise reactions, according to the Woodward-Hoffmann rules [131]. A bond is formed between the two olefins, leading to a tetramethylene intermediate (T). In a subsequent step, the second bond is formed, yielding the cycloadduct. Depending on the reactants, either zwitterionic or diradical tetramethylenes can be proposed as intermediates [132, 133]. 

A unifying hypothesis for the observed organic chemistry was advanced by Huisgen [132], who suggested that all tetramethylenes lie on a continuous scale between zwitterionic and diradical structures and may be regarded as resonance hybrids of the two extreme forms. The predominant nature of the tetramethylene intermediate is determined by the terminal substituents, and the termini can interact with each other by through-bond interaction [132, 134]. 

As an illustration of initiation of a cationic polymerization by a zwitterionic tetramethylene, the polymerization of JV-vinylcarbazole (NVCz) in the presence of dimethyl 2,2-dicyanoethylene-l,l-dicarboxylate was studied in great detail [136] (Scheme 3). The cationic homopolymerization of NVCz could be initiated by adding either the electrophilic olefin or the cyclobutane adduct. The proposed mechanism involves bond formation to the zwitterionic tetramethylene, which closes reversibly to the cyclobutane adduct, and can be trapped with methanol. 

These homodimerizations of the donor olefins are well accounted for by the ion-radical chain mechanism, and provide powerful evidence for the occurrence of single electron transfer. Were it not for these cyclodimers, ionic homopolymerizations of D or A could be interpreted as initiation by zwitterionic tetramethylenes formed from D and A. 

The tetramethylene is a resonance hybrid of 1,4-diradical (, = y in Eq. (22)) and zwitterionic (, = +,—) limiting structures. The character of the tetramethylene is determined by the nature of the terminal substituents. A very strong donor substituent at one of the terminal carbons and a very strong acceptor substituent at the other leads to zwitterionic intermediates. Otherwise, for instance, phenyl or vinyl group at the donor terminal and diester, cyano-ester or anhydride at the acceptor terminal, will favor the diradical form. 

P-Substituents have no influence on the diradical or zwitterionic character of tetramethylene. However, their presence favors the gauche conformation over the irons (extended) form [89] ... 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

The most important competing process to the bond-formation is the complete electron transfer to form ion-radicals, which occurs where no bond formation is possible, for example, for aromatic donor-acceptor pairs. For vinyl copolymerizable pairs, the bond will form between the components to give a diradical tetramethylene. For the ionic homopolymerization system, on the other hand, it is difficult to distinguish the ion-radicals from zwitterionic tetramethylenes by the kinetic analysis. In this case, the accompanying cycloaddition reaction offers powerful evidence for the zwitterion formation, i.e., the bond-formation. 

A clear case of cationic homopolymerization initiated by zwitterionic tetramethylene was thoroughly investigated in the reaction of VCZ with tetrasubstituted... 

Similarly, zwitterionic tetramethylenes as initiators of anionic polymerization were also observed. For example, methyl a-cyanoacrylate polymerizes via an anionic mechanism in the presence of n-butyl vinyl ether [90]. A Diels-Alder adduct is also formed. In another example, the reaction of isobutyl vinyl ether and nitroethylene leads to an unstable adduct [91], which is capable of initiating the anionic polymerization of excess nitroethylene, and also the cationic polymerization of added VCZ. 

Lewis examined two systems, tra/w-a-phenylcinnamonitrile with a 2,4-hexadiene [109] and 9-cyanoanthracene with a butene [39], in an attempt to find evidence to support a proposed 1,4-zwitterionic tetramethylene intermediate. His results, including the lack of a pronounced solvent effect when the polarity was varied, failed to support such an intermediate. Curiously, he did not propose a 1,4-diradical intermediate which would not be susceptible to solvent stabilization. 

Other theoretical studies have been concerned with diradical or zwitterionic tetramethylenes approached from [2 + 2] cycloreversions. For l,l-dicyano-2-methoxycyclobutane, cleavage of C-1—C-2 gives a gauche intermediate (98) which may isomerize to the trans intermediate (99). They are nearly isoenergetic, and both have predominant dir ical character. ... 

Hall has introduced an empirical test to estimate the relative importance of diradical and zwitterionic forms in tetramethylene intermediates rrans-1,4-tetramethylene diradical intermediates may initiate alternating radical copolymerizations if they add to another alkene faster than they undergo conformational isomerization to the gauche form and give a cyclobutane product through carbon-carbon bond formation, while zwitterionic 1,4-tetramethylene intermediates may initiate ionic homopolymerizations. 

This empirical yardstick draws a boundary between [2 + 2] cycloadditions that occur through essentially diradical or essentially zwitterionic tetramethylenes the resonance hybrid view which encompasses a continuous range of more or less dipolar intermediates is neglected in favor of a more decisive either/or discrimination. 

For a series of 10 donor-acceptor substituted /ranj-tetramethylenes, calculations have shown a trend paralleling the empirical diradical/zwitterionic test proposed by Hall. Three of the ten cases, (104), (105) and (106), were predicted to have zwitterionic ground states. 

Numerous [2 + 2] cycloadditions which may proceed through dipolar intermediates (tetramethylenes which may be represented as resonance hybrids of singlet diradicaJ and zwitterionic forms) react with partial or even complete loss of the stereochemical relationships present in starting materials. Reactions between the cis and trans isomers of l,2-bis(trifluoromethyl)-l,2-dicyanoethylene and cis- and trans-propenyl n-propyl ether are not completely stereoselective. The trans electrophilic alkene (107) and the cis isomer of the enol ether (108), for instance, give rise to products (109), (110) and (111), the last having trifluoromethyl groups in cis relationship one to the other. ... 

The case for 1,4-zwitterionic intermediates based on chemical trapping experiments depends on obtaining such products using reagents reactive toward the tetramethylene and unreactive toward both addends and the [2 + 2] cycloadduct. 

In summary, then, the diradical versus zwitterionic issue seems to have shifted in recent years, away from an either/or dichotomy and toward a more integrated view, one seeing tetramethylene intermediates as more or less dipolar diradicals. A small amount of zwitterionic character does not obviate the essentially diradical nature of these intermediates, any more than monoradicals forego radical character if they are more or less electrophilic or nucleophilic. 

A little bit of zwitterionic character goes a long way energetically, especially in transition states for cyclobutane formation. 1,4-Zwitterionic tetramethylene intermediates in [2 -i- 2] cycloadditions may well be important in some reactions, where donor and acceptor substituents are so strong that diradical character is overshadowed and yet one-electron transfer does not take over, but they do not at this point seem common. 

The question whether the intermediates 6.24 and 6.26 are zwitterions or biradicals has not yet been answered. As known from the tri- and tetramethylene species (see Hoffmann, 1968 Hoffmann et al., 1970 Hiberty, 1983 Harcourt and Little, 1984 and Ejiri et al., 1992), zwitterions and biradicals are probably extremes of a structure-dependent degree of charge separation. ... 

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