Singlet biradicals

Grafenstein, J., Kraka, E., Cremer, D., 1998, Density Functional Theory for Open-Shell Singlet Biradicals , Chem. Phys. Lett., 288, 593. 

In view of the results obtained for attack of acetone singlets on 1-methoxy-butene to yield singlet biradicals (partial loss of stereochemistry due to bond rotation in the biradical), acetone attack on dicyanoethylene to yield oxetane stereospecifically via a similar biradical intermediate is difficult to envision. Thus a new mechanism must be developed to account for these results. 

The formation of the A-vinylaziridine 70 in the photoreaction of 68 deserves additional comment. Depending on the multiplicity, the intermediate 72 formed by path b could be a triplet 1,3-biradical. However, if intersystem crossing occurs along the reaction coordinate, the singlet biradical must be considered as a dipolar azomethine ylide. According to literature precedents, both intermediates, the 1,3-biradical and the ylide, will cyclize to form the observed aziridine. This is the first case in a DPM process where a zwitterion can be postulated as a possible intermediate. 

The IR spectra of 43, 47, and 53 cannot be used alone for predicting the ground-state multiplicity, because of the similarity between the A and A states. However, the computational results in conjunction with the low temperature used in the experiments and the lack of ESR signals when 47 was generated were interpreted in favor of singlet biradical ground states [108]. 

Kubo T, Shimizu A, Sakamoto M, Uruichi M, Yakushi K, Nakano M, Shiomi D, Sato K, Takui T, Morita Y, Nakasuji K (2005) Synthesis, intermolecular interaction, and semiconductive behavior of a delocalized singlet biradical hydrocarbon. Angew Chem Int Ed 44 6564-6568... 

K. It too gives the characteristic adducts obtained from the singlet, but the addition does not occur directly between the bicyclic hydrocarbon (28) and the alkene. Instead, the reaction occurs in two steps, first the reversible unimolecular ring opening of 28 to singlet biradical 14b, followed by a bimolecular capture of the latter (Scheme 5.5). Another hydrocarbon isomer 29 can be prepared as a transient intermediate. Its thermal conversion to the biradical 14b apparently occurs at even lower temperature. 

Interest in the aza-Bergman reaction and the influence of protonation on 2,5-pyridyne and its derivatives was (once more) directly stimulated by the search for less toxic antimmor drugs. One approach to increase the selectivity is to decrease the reactivity of the biradical intermediate in hydrogen-abstraction reactions.In this regard, Chen has established a simple model that correlates the reactivity of singlet biradicals with their singlet-triplet gap This model is based on the assumption that the reactivity of the triplet biradical is... 

There is computational and/or experimental evidence that a structurally diverse range of singlet biradicals occupy plateau-like regions of their PESs. A selection of them, including the reactions that they mediate, are shown in Scheme 21.2. The occurrence of a plateau at a mechanistically crucial region of the PE hypersurface guarantees that there can be no clear distinction between concerted and stepwise mechanisms, and also strongly indicates that the reaction may be one whose course will not be describable by any simple kinetic model. 

SCHEME 18. A singlet-biradical forming process. When the Mg—O bond formation proceeds, the left-side t-Bu group is pushed away as a i-Bu via homolytic C—Mg cleavage... 

SCHEME 19. The singlet biradical 43 is formed in the reaction between the two parent benzophe-nones and (t-BuMgCl)2. The radical 44 is formed from the two 2,4,6-trimethylbenzophenones and (t-BuMgCl)2. A, B, C and D denote for the respective phenyl and mesityl rings... 

A fascinating feature of the chemistry of boron-phosphorus heterocycles is the existence of stable singlet biradicals, e.g. fBuBP Pr2)2 (for a discussion of the electronic structure, see Section 5.4.2.2). The synthesis of this four-membered ring is illustrated in Scheme 5.2. Related derivatives of the type (RBPR 2)2 are also prepared by a salt-elimination reaction of the appropriate lithium phosphide with a 1,2-dichlorodiborane or, in the case of the perphenylated derivative (PhBPPh2)2, by reduction of the cyclic dimer [Ph(Cl)BPPh2]2 with lithium naphthalenide (Scheme 9.11). ... 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The reason for the difference in stereochemistry is that, as quenching studies show, the reactive state of the aromatic carbonyls (which undergo ISC rapidly) is T1 but in aliphatic carbonyls it is The intermediate formed from acetaldehyde and m-2-butene, then, is the singlet biradical 34, whereas the intermediate from benzaldehyde and w-2-butene is the triplet biradical 35. That the stereo-... 

Since ozone is a strong 1,3-dipole,635 or at least has a strong polarizability (even if a singlet biradical structure is also feasible), it is expected to be readily protonated in superacids, in manner analogous to its alkylation by alkylcarbenium ions. Protonated ozone HC>3+, once formed, should have a much higher affinity (i.e., be a more powerful electrophile) for cr-donor single bonds in alkanes than neutral ozone. 

---