Biradical conjugated

This highly conjugated molecule was stabilised with nitroxyl biradical side chains. The resulting material had sufficient ferromagnetism that a usable compass needle could be made from it. Despite the success of this demonstration, organic ferromagnetism remains a curiosity. Such polymers are not likely to replace conventional ferromagnetic metals in any application within the foreseeable future. 

The ability of (Z)-l,2,4-heptatrien-6-ynes (enyne-allenes) and the benzannulated derivatives to undergo cyclization reactions under mild thermal conditions to produce biradicals has been the main focus of their chemical reactivities [1-5]. With the development of many synthetic methods for these highly conjugated allenes, a variety of biradicals are readily accessible for subsequent chemical transformations. Cyclization of the enyne-allene 1 could occur either via the C2-C7 pathway (Myers-Saito cyclization) leading to the a,3-didehydrotoluene/naphthalene biradical 2 [6-10] or via the C2-C6 pathway (Schmittel cyclization) producing the fulvene/benzofulvene biradical 3 [11] (Scheme 20.1). 

SCHEME E The biradical model for the direct ,Z-photoisomerization of (a) alkenes and (b) conjugated dienes... 

The ODPM reactivity of (B.y-unsaturated aldehydes is not restricted to y-phenyl-substituted compounds but can also be extended to systems in which the intermediate biradicals are stabilized by conjugation with a vinyl group. Thus, m-methoxyacetophenone-sensitized irradiation of 40 (Structures 40-49) for 20 min, affords the cyclopropane derivative 41 (47%) as a 1 8 mixture of cisitrans isomers. Similarly, irradiation of 42, for 15 min, under the same conditions, yields 43 (52%) as the trans isomer exclusively [51]. 

The relative rates of reaction of the singlet TMM derivative 14b with a series of alkenes (32) parallel those of a conjugated diene with the same alkenes in Diels-Alder reactions. These relative rates also are well correlated by the frontier orbital model for a concerted reaction. The absolute rates of the biradical cycloadditions are many orders of magnitude greater than those of the model dienes. The relative rates of the alkenes in the cycloadditions of the triplet biradical 14b, on the other hand, follow the reactivity order of their addition reactions with monoradicals. 

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

The prototype of this reaction is the Myers-Saito reaction, the rearrangement of eneyneallene (Z)-hepta-l,2,4-triene-6-yne (70) to a,3-didehydrotoluene (71). This C2—C7 cyclization yields a benzylic 7i-conjugated a,7t-biradical and is therefore... 

B. Valence isomerization. The conjugated polyenes in their excited biradical state can lead to intramolecular cyclization in a number of ways giving rise to a number of products. In such cyclization, valence bonds are reorganized without migration of atoms or groups but by migration of c or it electrons only. For example, on excitation of pentadiene, the following products are observed ... 

Conjugated dienes take part readily in triplet-sensitized photodimerization. and the products obtained from buta-1.3-diene (2.691 include a (4 + 2) adduct as well as slereoisomeric (2 + 2) adducts. The reaction is non-concerted. and a rationalization for the products is provided on the basisof the formation of a biradical intermediate as shown (which is the most stable of the three possible biradicals that might be formed in the first step), by the attack of triplet diene on ground-state diene. Cross-addition takes place in some systems, such as myrcene (2.70) where a triplet diene group attacks the alkene within the same molecule direct irradiation of myrcene gives mainly... 

The reactions with a.p-unsaturated carbonyl compounds also lead to cyclobutenes (2.102), and there is evidence that, in some cases at least, the mechanism is non-concerted and goes through biradical intermediates that can be trapped by a second molecule of the conjugated alkene (2.103). Intramolecular photocydoadditions offer routes to polycyclic structures, and the cyclobutene unit in the product provides a basis for subsequent chemical transformations such as oxidation 12.104). 

A large group of paramagnetic organic molecules containing even numbers of electrons are known. Compounds for which a conjugated, biradical structure can be formulated actually exist predominantly as quinoidal singlets.78... 

Meta placement of the radical sites or substituents which impose steric interference to conjugation result in stabilization of the biradical forms.79... 

When diene triplets are formed in the presence of high concentrations of conjugated diene, they add to form biradical intermediates which then close to cyclobutane and cyclohexene structures.281... 

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