Tetramethylene intermediates

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]. 

In recent years, more and more photochemists consider that many photocycloaddition reactions take place stepwise, that is, one bond is first formed. The formed tetramethylene intermediate may close to a cyclobutane or initiate the polymerization. We will leave this discussion to Sect. 4. 

Electron donor-acceptor monomer pairs form charge-transfer complexes (CTC), which collapse to the tetramethylene intermediates through the bond-formation between the p-carbons ... 

Thus, Hall has proven that tetramethylene intermediates, arising from bond-formation between the P-carbons of reacting olefins, are the key to small molecule formation and the thermal copolymerization. This unifying concept of bondforming initiation has been also extended to spontaneous addition and polymerization reaction of heteroatom acceptor molecules and 7,7,8,8-tetrasubstituted quinomethanes. 

For a proposed cycloaddition mechanism, Caldwell [107] postulated two possibilities collapse of the exciplex to a 1,4-diradical tetramethylene intermediate or direct reaction to cycloadducts ... 

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. 

In the study of photochemical cycloadditions of singlet or triplet (sensitized) diphenylvinylene carbonate with vinyl ethers [110], however, Lewis clearly proposed cycloaddition either directly from the exciplex, or via collapse of the exciplex to a diradical tetramethylene intermediate. 

In contrast, many photochemists have proposed 1,4-diradical tetramethylene intermediates, so (1) and (2) are also very important possible routes. 

We propose an alternative mechanism bond formation to tetramethylene intermediate. The results which support this proposal come from not only our own experiments, but also from Raetzsch s and Shirota s data. 

Zewail has shown that the reaction mechanism involves the breaking of one of the carbon-carbon bonds in cyclobutane to produce a tetramethylene intermediate ... 

When the tetramethylene intermediate involved in [2 + 2] cycloadditions lacks stabilizing functionality at both termini, and the diene is free to assume, or is locked into, the (S)-cis conformation, the Diels-Alder process dominates ethylene and butadiene give vinylcyclobutane and cyclohexene in 1 5000 pro-... 

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. 

For the reaction of (122) with TCNE to form (123) the rate increase in going from carbon tetrachloride as solvent to acetonitrile is about 49(X), while for the reaction of (124) with (125) to produce (126) there is only about a factor of six increase in rate for reaction in acetonitrile relative to reaction in toluene there is no spectacular solvent effect. Does the latter reaction have a fundamentally different mechanism than is operative in [2 + 2] cycloadditions of enol ethers with TCNE Are the tetramethylene intermediates of quite different dipolar character ... 

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. 

Finally, similar results were obtained using In bulk, 5 is a very slow initiator even at 1 1 ratio at room temperature. This is due to very slow tetramethylene formation. It is expected that 5 will form a tetramethylene intermediate with more biradical character owing to the ester substituents, but it seems that this is not the case. No copolymer was formed, because of the presence of the chloride ion which would be eliminated relatively rapidly from the tetramethylene. 

The experimental evidence cited above indicates that this does not occur. As suggested at the right of Fig. 6.6, the in-plane glide (6i) begins well before the intended HOMO-LUMO crossing, which is avoided because all four MOs have the same irrep (o ) in The HOMO and LUMO of the extended tran-soid zwitterion are qualitatively similar to those in the biradical illustrated in Fig. 6.4, but are more widely separated in energy As a result, the orbital approximation - and the symmetry analysis based upon it - is no less reliable than for the biradical mechanism. The zwitterionic mechanism can be accomodated by Fig. 6.5, with self-evident modifications arising from the polar nature of the tetramethylene intermediate. ... 

The zwitterionic tetramethylene intermediate has been trapped with methanol as a linear 1-methoxybutane, indicating that the addition of tetracyanoethylene to NVK occurs in a stepwise manner. Once the zwitterions are formed, the course of the reaction is determined largely by experimental conditions. With excess NVK, cationic polymerization occurs. If the concentrations of NVK and tetracyanoethylene are comparable, collapse of the intermediate to the corresponding cyclobutane is favored. The cyclobutane formation is reversible, and the isolated cyclobutane alone is capable of initiating the cationic polymerization of NVK. The polymerization of NVK by the stable radical cation salts phenothiazin hexafluoroantimonate [464] and tris-p-bromophenylamminium hexachloro-antimonate [465] were shown to occur via charge transfer initiation too. 

Returning to the initiation process H K Hall and his collaborators have been investigating the spontaneous copolymerization of vinyl monomers containing donor and acceptor groups They have presented strong evidence for the existence of a tetramethylene intermediate 30 (Eq 7) which is a resonance hybrid of a zwitterion and a spin paired biradical, the terminal carbon atoms of which sense each other by through bond coupling. ... 

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