Tetramethylene diradical

It is the purpose of this paper to review the implications of this unusual stabilization for the radical polymerization of captodatively substituted olefins. Also, the possibility of producing 1,4-tetramethylene diradicals from these olefins opens an important new area for the spontaneous polymerization of olefins. The latter will be discussed in the context of the Bond-Forming Initiation Theory [9-10],... 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

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. 

An interesting potential energy surface that has been studied by EHT is the one for this reaction (19), in which [2], a tetramethylene diradical, is invoked as an intermediate to account for experimental facts ... 

The postulation of trimethylene and tetramethylene diradicals as reactive intermediates involved in many thermal isomerization and fragmentation reactions has a long history,but not until 1994 had they ever been detected in real time. The validity of the diradical hypothesis was tested through femtosecond studies, and the tests provided dramatic evidence confirming that these short-lives species are indeed real, directly experimentally accessible chemical entities. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

It is essential to note that the mechanism which involves diradical intermediates is based purely on analogy and as such remains to be proved. If tetramethylene diradicals are formed, it is surprising that, unlike monoradicals, they do not recombine with each other or abstract hydrogen atoms from other molecules. It has been claimed that photolysis of cyclopentanone in the presence of ethylene leads to cyclohexane and hexenes as products presumably through the reactions 5 and 6 (17). 

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. 

The Bond-Forming Initiation Theory gives a good interpretation of the observed spontaneous polymerizations of captodative monomers. The tetramethylene diradicals already implicated as initiators in the thermal (spontaneous) polymerizations of vinyl monomers can be particularly stabilized by captodative substituents. For comparison, and to initiate the polymerization of third monomers, captodative cyclobutanes and cyclopropanes are particularly appropriate precursors for generating tetra- and trimethylene diradicals. In particular the extensive work of Viehe [3,45,46] showed that thermolysis of captodative substituted cyclopropanes leads to trimethylene captodative diradicals at reasonable temperatures. Their initiating abilities for polymerization have not yet been determined. 

We shall utilize the above insights about the behavior of tetramethylene diradicals in our discussion of photoreactions of donor/acceptor olefins. 

Tetramethylene Diradicals as Key Intermediates in Photoreactions of Donor/Acceptor Olefins... 

The tetramethylene diradical or 1,4-diradical has long been an attractive subject in photochemistry. During the past twenty years it has become possible to obtain direct experimental information bearing on these species, and with the accumulation of these data we can now begin to understand the factors which influence diradical behavior. 

The tetramethylene diradical was also proposed as the intermediate for dimerization of olefins and [2 + 2] mixed additions between olefins. Hospool [105] suggested that if a triplet mechanism was operative, the produced diradical intermediate would undergo free rotation before the closure of the final bond and... 

---