Intramolecular radical recombination

Irradiation of solutions of concentration higher than 3% results in the formation of an insoluble gel because of intermolecular radical recombination similar to (4). In dilute solutions, however, intramolecular radical recombination occurs leading to cyclization. Decrease in the hydro-dynamic volume of the molecule is responsible for the decrease in the intrinsic viscosity of the solution. 

Propargylic radicals are generated on reduction of metal-complex-stabilized Nicholas cations with Zn. This principle, which has been found very recently [21], can be used for ring-closure reactions via intramolecular radical recombination [Eq. (12)]. 

Although, consequently, a dimer diradical was assumed instead to be the relevant initiating species, the constitution of this species remained undetermined. The above-described NMR analysis of the [2.2]paracyclophane side products proved to be highly valuable in answering this question, with l,2-dihalo-[2.2]paracyclophanes 13a appearing exclusively, and systems such as 13c and 13d never being observed [62]. Since it is reasonable to assume that these side products are the result of an intramolecular radical recombination of the same dimer diradicals, which alternatively induce chain propagation, it was concluded that a dimerization of 11 must lead to either 14 or 15, but never to systems such as 16 (Scheme 29.4). When, in addition, the relative stabilities of the radical centers of 14 and 15 are taken into account, it was concluded that 14 with CHX-localized radicals would be the relevant species. These conclusions have been fuUy corroborated by computational studies, which have confirmed the process 11 14 within this... 

The putative dimerization products 15 and 16 lack one or two types of this latter radical-stabilizing effect ( 8-10 kj moP per radical), which is directly reflected in the higher reaction barriers leading to their formation. Thus, it can be concluded that the Giich reaction is a free radical chain polymerization of the p-quinodimethane monomers 11, initiated by the dimer diradicals 14. If unsuccessful as initiators, the diradicals 14 can undergo an intramolecular radical recombination, leading to the [2.2]paracyclophane side products. 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

Abstraction/recombination reactions are not limited to radical- radical recombination. An intriguing alternative has been demonstrated19, zo, as illustrated by the intramolecular addition of the free radical 2, generated by remote abstraction, to an alkene. As previously observed for other radical cyclizations, there is a significant selectivity for the thermodynamically less preferred entfo-product21 from this cyclization. 

Formal intramolecular hydrogen abstraction (proceeding through coupled electron and proton transfer) occurs in the titi triplet excited state of furanone derivatives, upon acetone photosensitization. After hydrogen transfer from the tetrahydropyran to the (I position of the furanone moiety, radical recombination leads to the final products (18). The results of computational studies on model structures are in accordance with experimental observations and reveal that the reactivity and selectivity are mainly determined by the hydrogen-abstraction step. ... 

TV,TVs] representation for radical reactions, where the eleetron transfer and the spintransfer can be assumed to be concerted. The [iV,iVs] representation is more appropriate for processes implying excitation to the lowest-energy state of a specified multiplicity, " or intramolecular electrocyhzation processes occuring by radical recombination. ... 

The intramolecular photoinduced hydrogen abstraction of carbonyl compounds, known as the Norrish type 11 reaction, is one of the most important primary photochemical processes, usually giving the corresponding cyclic alcohols via hydrogen abstraction of the excited triplet carbonyl oxygen followed by radical recombination. 

In contrast to the bimoleciilar recombination of polyatomic radicals ( equation (A3.4.34)1 there is no long-lived intennediate AB smce there are no extra intramolecular vibrational degrees of freedom to accommodate the excess energy. Therefore, the fonnation of the bond and the deactivation tlirough collision with the inert collision partner M have to occur simultaneously (within 10-100 fs). The rate law for trimoleciilar recombination reactions of the type in equation (A3.4.47) is given by... 

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