Tetramethyleneethane diradical

Scheme 6.25 shows the formulas of the compounds 88 [68], 89 [70], 90 [78], 91 and 92 [70], which emerge from the dimerization of 74, 76, 77 and 79, respectively. The structure is of the same type as that of the dimer 38 of 6 (Scheme 6.10). The configuration has not been determined, but it is assumed to be trans as in the case of 38. Hence the cyclization of the tetramethyleneethane diradicals of type 37 (Scheme 6.10), the immediate precursors of the isolated dimers, should proceed as a least-motion process at the unsubstituted radical centers. Only the trimethylsilyl group causes a predominantly alternative course, since the dimers 91 and 92 were obtained in a ratio of 1 2.

The dimer 352 of 351 was isolated from the product mixtures of two experiments conducted to trap 351 by alkenes, one with 350 and the other with 354 as substrate. Although no cycloadduct with the alkene was observed in one case, the yield of 352 amounted to only 0.8%. Nevertheless, the structure of 352 is interesting, since it suggests that the tetramethyleneethane diradical assumed to be the intermediate undergoes ring closure preferentially between two different allyl-radical termini. 

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. 

Pittner, J. Nachtigall, R Carsky, R Hubac, I. State-specific BriUouin-Wigner multireference coupled cluster study of the singlet-triplet separation in the tetramethyleneethane diradical, 7. Phys. Chem. A 2001,105,1354-1356. 

Figure 2. Potential energy curves for twisting of the tetramethyleneethane diradical (TME), computed by the CCSD, two-reference Hilbert space MR-CCSD (TDCCSD), and MR-BWCCSD methods.

Among the non-Kekule diradicals, tetramethyleneethane (TME, 7) has evoked lasting attention during the last two decades due to the controversy over its spin preference in the ground state between experiments and theoretical predictions [48-59], Now TME is known to be a slightly favored singlet diradical with a negligible... 

Fig. 13 Selected cyclic It-conjugated diradicals on the basis of tetramethyleneethane (TME) unit...

Nash, J. J. Dowd, P Jordan, K. D. Theoretical study of the low-lying triplet and singlet states of diradicals prediction of ground-state multiplicities in cyclic analogs of tetramethyleneethane, 7. Am. Chem. Soc. 1992,114, 10071-10072. 

Filatov, M. Shaik, S. Tetramethyleneethane (TME) diradical expmment and density functional theory reach an agreement, 7. Phys. Chem. A 1999,103, 8885-8889. 

In the preface to Diradicals, Borden writes It seems almost as hard to define what diradicals are as it is to study these reactive intermediates [1]. Salem and Roland described a diradical as an atom or molecule in which two electrons occupy two degenerate or nearly degenerate molecular orbitals [2]. A few examples befitting this description include the conjugated non-Kekule hydrocarbons trimethylene-methane (TMM, 1), tetramethyleneethane (TME, 2), and wc/a-quinodimethane 3, as well as the nonconjugated 1,3- and 1,4-diradicals (diyls) trimethylene (4) and tetramethylene (5), and the now very familiar benzene 1,4-diyl 6 (Fig. 1). 

In some diradicals the two partially filled MOs are both n orbitals, and the electrons in them are, consequently, delocalized. Diradicals for which Kekul6 structures cannot be written without leaving two n electrons nonbonded are called nonKekul6 diradicals. Trimethylenemethane [C(CH2)3] and tetramethyleneethane [(H2C)2CC(CH2>2] are both diradicals of this type, and they and other nonKekul6 diradicals are discussed in Section 5. 

Conjugated hydrocarbons for which all Kekul structures leave two n electrons nonbonded are called non-Kekule diradicals. Longuet-Higgins proved that, unless such molecules have rings with 4/i atoms, these molecules do indeed contain two NBMOs that are occupied by a total of two electrons. As shown in Figure 10. trimethylenemethane (TMM), tetramethyleneethane (TME), m-benzoquinodimethane (MBQDM) and tetramethyleneben-zene (TMB) are all non-Kekule diradicals. 

Figure 10 Four of the most important non-Kekul6 hydrocarbon diradicals trimethylenemethane (TMM), tetramethyleneethane (TME), m-benzoquinodimethane (MBQDM), and tetramethyleneben-zene (TMB)...

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