Continuous diradical

Gilbert Higley Tetrahedron Lett. 1973, 2075 Caramella Huisgen Schmolke /. Am. Chem. Soc. 1974, 96, 2997, 2999 Mazzocchi Tamburin /. Am. Chem. Soc. 1975, 97, 555 Zimmerman Fleming /. Am. Chem. Soc. 1983, 105, 622 Klumpp Schakel Tetrahedron Lett. 1983, 24, 4595 McGafhn de Meijere Walsh Chem. Ber. 1991, 124, 939. A "continuous diradical transition state has also been proposed Doering Sachdev J. Am. Chem. Soc. 

Doering, W. v. E. Sachdev, K. Continuous diradical as transition state. Internal rotational preference in the thermal enantiomerization and diastereoisomerization of cis- and trans-l-cyano-2-isopropenylcyclopropane, 7. Am. Chem. Soc. 1974, 96, 1168-1187. 

A homolytic cyclopropane C-C bond rupture that would furnish in turn a 1,3-diradical is also conceivable. However, it is always difficult to establish whether a purely diradical or ionic mechanism is in operation. Between these two extremes there exists a graded continuum of polarized diradicals of which the zwitterion represents the end of the spectrum. In addition, the continuous development of radical character during the formation of the transition state of a homolytic bond scission, called the continuous diradical, has been postulated to explain the behavior of some reactions. Alternatively, the contribution of a truly concerted transformation cannot be overlooked. ... 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

The electrolysis of adamantylideneadamantane solutions affords the radical cation, which can add molecular (triplet) oxygen to give the peroxide radical anion, which can react with adamantylideneadamantane to give the 1,4-diradical and another molecule of adamantylideneadamantane radical cation. The latter reacts with oxygen, to continue the chain of the reaction, while the former cyclizes to the corresponding 1,2-dioxetane (Scheme 18) (81JA2098). 

Pyrolyses of Nl- or N3-substituted derivatives of compounds 4 and 5 have continued to find use as routes to azacarbazoles, although the yields are often indifferent and there are no recent examples. The photochemical reactions are dealt with in Section IV.G. Pyrolysis media are paraffin (P) or PPA, and examples of products are compounds 247 (P, cytostatic) (83MI2), 248 (P) (84MI1), and 249 (from a 1-substituted derivative) (86MI2). Indications of diradical intermediates are provided by the thermolysis of compound 250 (P) (83MI2) where one product is a dimer. 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

Keywords Chemical orbital theory, Cw-stability, Cyclic conjugation. Disposition isomers. Diradicals, Donor-acceptor, Electron delocalization, Geminal bond participation, Inorganic heterocycles. Ring strain. Orbital phase. Orbital phase continuity. Polarization, Preferential branching. Reactivity, Selectivity, Stability, Tautomerism, Z-selectivity... 

The orbital phase theory has been developed for the triplet states [19]. The orbital phase continuity conditions (Scheme 4) were shown to be applicable. We describe here, for example, the triplet states of the TMM and BD diradicals, with three a spin electrons and one 3 spin electron. The a and 3 spins are considered separately (Scheme 8). 

There is a degree in the continuity and discontinuity of the orbital phase [20]. 2-Oxopropane-l,3-diyl (Scheme 10) is a hetero analog of trimethylenemethane (TMM) where the orbital phase is continuous in the triplet diradical (Sect. 2.1.5) and discontinuous in the singlet diradical (Sect. 2.1.6). The n and orbitals of carbonyl bonds are lower in energy than those of C=C bonds. The lowering strengthens the interaction of the radical orbitals (a, b) with and weakens that... 

Fig. 5a-c Through-bond interactions in the triplet state of 1,3-diradical, a Mechanism of electron delocalization and polarization of a-spin electrons, b Cyclic orbital interaction, c Orbital phase continuity... 

It is interesting that the sign of the left side of inequality (11) is opposite to that of inequality (6) for the triplet state. That means the phase continuity properties of the singlet and triplet states of a given diradical are opposite to each other. It should be... 

Orbital phase continuity in triplet state. The orbital phase properties are depicted in Fig. 5c. For the triplet, the radical orbitals, p and q, and bonding n (a) orbital are donating orbitals (labeled by D in Fig. 5c) for a-spin electrons, while the antibonding jt (a ) orbital (marked by A) is electron-accepting. It can be seen from Fig. 5c that the electron-donating (D) radical orbitals, p and q, can be in phase with the accepting (a ) orbital (A), and out of phase with the donating orbital, Jt/a (D) at the same time for the triplet state. So the orbital phase is continuous, and the triplet state of 1,3-diradical (e.g., TMM and TM) is stabilized by the effective cyclic orbital interactions [29, 31]. 

Orbital phase discontinuity in singlet state. In contrast to the triplet state, orbital phase continuity conditions cannot be satisfied simultaneously (denoted by the dashed line in Fig. 6c) in the singlet. Thus, the singlet 1,3-diradical suffers from the orbital phase discontinuity. According to the orbital phase properties, the triplet states of TMM (1) and TM (2) were predicted to be more stable than their singlet states by the orbital phase theory [29, 31]. 

Singlet o-type diradical. Figure 8 shows the phase relationship between the electron donating and accepting orbitals in a o-type diradical (Scheme 4b). It can be seen that the cyclic -p-0j -02 -q-02-0j- orbital interaction satisfies the continuity requirements in the singlet state (Fig. 8) the neighboring orbitals in p(D)-Oj (A)-02 (A)-q(A)-02(D) are all in phase while those in the sequence p(D)-Oj(D)-02(p) are all out of phase. The phase is continuous for the cyclic interaction. 

Fig. 8a, b The cyclic orbital interaction (a) and orbital phase continuity (b) in the singlet state of O-type 1,3-diradical... 

In the singlet state of Jt-type 1,3-diradical (e.g., TM, 2), there may also exist the through-space interaction between radical centers, i.e., p...q interaction (Fig. 9), in addition to the previously addressed cyclic -p-o -q-o- orbital interactions (Fig. 6). The through-space interaction is indispensable for the bond formation between the radical centers. The corresponding delocalization of the a-spin electron is shown in Fig. 9a. Clearly, the involvement of the through-space p... q interaction gives rise to two cyclic orbital interactions, -p-o -q- and -p-o-q-. From Fig. 9, one can find that the cyclic -p-o -q- orbital interaction can satisfy the phase continuity requirements for the a-spin electron the electron-donating radical orbital, p (D) can... 

The cyclic orbital interaction of p and q with o or with o can significantly occur at the transition state of the ring closure of 1,3-diradicals. The continuous orbital phase for the cyclic orbital interaction with o implies effective stabilization of the transition states when the o bonds are electron acceptors. 

To summarize, the properties of triplet and singlet diradicals are closely related to the effectiveness of through-bond and through-space interactions, which are governed by the orbital phase continuity/discontinuity properties. In the next two sections, we will utilize this simple model to predict the spin preference and intramolecular reactivity for a broad range of diradicals. 

The preceding orbital phase predictions of some topological units (like 1, 4-6) can be easily extended to more complex cychc diradicals [29], as shown in Fig. 12. On the basis of TMM sub-structure (1), diradicals 8-11 are predicted to be phase continuous in their triplet states. Such a triplet preference in their ground states is in agreement with calculation results and available experiments, as listed in Table... 

The introduction of heteroatoms into the hydrocarbon diradicals is a frequently applied strategy to tune the spin preference and relative stabilities of diradicals. The heteroatoms may change the energies of donor or acceptor orbitals, and consequently affect the donor-acceptor interaction involved in the cyclic orbital interaction. Take 2-oxopropane-l,3-diyl, or so-called oxyallyl (OXA, 18) as an example [29]. It is a hetero analog of TMM, as shown in Fig. 14. The replacement of CH with oxygen in the central fl unit leads to a decrease in energies of Jt and k orbitals. This may enhance the orbital interaction through one path (denoted by bold lines) and weaken that via the other (denoted by wavy lines) relative to the continuous cyclic orbital interaction in the parent species 1 (Fig. 14). As a result, the p-Jt -q... 

Most of the triplet diradicals have disrotatory conformations, in which the cyclic orbital interactions are favored by the phase continuity. 1,3-Diradicals with the second-row substituents, 25-28, prefer the conformer a with the central C-H bond in conjugation (except for the conrotatory conformation b in 27), whereas those substituted by the third-row groups, 29-32 favor the disrotatory conformations c... 

On the basis of the orbital phase continuity/discontinuity in the involved cyclic orbital interactions, some general rules were drawn for the Jt-conjugated and localized diradicals ... 

The singlet Kekule diradicals, i.e., the excited state of Kekule molecules, are destabilized by the orbital phase continuity while the triplet Kekule diradicals and the singlet and triplet non-Kekule diradicals are stabilized by the orbital phase continuity. 

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

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