Cyclobutane tetramethylene diradicals

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. 

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. 

In our simplified scheme, there are four possibilites to form cycloadduct from donor olefins and acceptor olefins (1) concerted reaction of the partners, (2) the partners form an exciplex, which collapses to cyclobutanes directly, (3) the exciplex collapses to tetramethylene diradical, which then close to cyclobutanes, and (4) direct formation of diradical from donor/acceptor pair and then cyclization. 

Fig. The diradical reaction shown shcmaticallyin an energy diagram. After elimination of carbon ntortoxidefrom the parent cyclopentanonephtochemicaUy the tetramethylene diradical is created this can undergo ring closure to yield cyclobutane or can produce two ethylenes.

Styrene, butadiene and divinylacetylene similarly give [2 + 2] cycloadducts of head-to-head regiochemistry as mixtures of cis and trans 1,2-disubstituted cyclobutanes. The tetramethylene diradical... 

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. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

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 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. 

As noted in the introduction, nonconjugated diradicals are those in which the partially filled orbitals reside on two different carbons that are connected by one or more saturated carbons or by unsaturated carbons in which the it bonds do not overlap the two radical centers. Two types are discussed here 1,3-diradicals, in which the radical centers are separated by a single carbon, and 1.4-diradicals, in which two carbons separate the radical centers. The archetypal 1,3-diradical is trimethylene (TM), the diradical formed by cleavage of a C-C bond in cyclopropane and tetramethylene, the diradical formed by breaUng a C-C bond in cyclobutane, is the archetypal 1,4-diradical. 

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