Energy Well of Diradicals, VII. – Conjugative‐Stabilized Trimethylenemethane Derivatives. Geometry Dependence of the Singlet‐Triplet Splitting
For the three 2,1′‐bis‐allyl diradicals 3–5 the singlet‐triplet splitting has been determined by the oxygen‐trapping technique. In agreement with theory the value for the planar diradical is large (>14 kcal · mol−1) whereas for the orthogonal geometry the energy gap is small (6.3 kcal · mol−1). In all cases a triplet groundstate is observed. From the rotational barrier of the exo methylene groups in 6 it is shown that the interconversion of the planar and orthogonal singlet states have activation barriers (6–9 kcal mol−1) which are responsible for their kinetic stability. In contrast to 6, where the formation of the orthogonal diradical proceeds by way of the planar diradical 3, the formation of the analogous orthogonal diradical 29 from homofulvene 17 is a concerted process. This difference is an important observation with respect to the fundamental understanding of concerted and non‐concerted reactions.
The concerted formation of the orthogonal trimethylene-type diradicals 3, 6, and 12 has been demonstrated for the three homofulvenes 2, 4, and 7 and shown to proceed preferentially in a disrotatory manner. The same type of orthogonal diradicals are formed, but in a nonconcerted fashion, from the bicyclic methylenecyclopropanes 26, 30, 33, and 40, through the planar diradicals 28, 31, 35, and 50, respectively.
The cover picture shows a phenylacetylene dendrimer having 12 diazo units, from which a dodecakiscarbene is generated. Diazo functional groups are usually labile and hence cannot be used as building blocks to prepare more complicated poly(diazo) compounds. We have shown that a diphenyldiazomethane prepared to generate a persistent triplet diphenylcarbene is also stable and can be used as a building block to construct poly(diazo) compounds, which can generate high‐spin polycarbenes. Details are discussed in the article by H. Tomioka et al. on page 2991 ff.
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