Diradical Subject

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). 

The question as to whether these pathways involve an intermediate or are concerted has major implications for the stereochemistry of the product expected. This subject will be discussed more fully in the next section. The diradical obviously does not exist long enough to react with another molecule of olefin since no tetrahydropyrans have ever been detected from these reactions. It should also be pointed out that if the formation of the intermediate is reversible, olefin (or diene) isomerization may, in some cases, involve this pathway rather than triplet-triplet transfer. 

Dimerization of lff-azepines is an extensively studied phenomenon and involves a temperature dependent cycloaddition process. At low (0°C for 1 R = Me) or moderate (130 °C for 1 R = C02R or CN) temperatures a kinetically controlled, thermally allowed [6 + 4] dimerization to the exo -adduct (73) takes place, accompanied by a small amount (<10%) of symmetrical dimer (74). The latter are thermodynamically favored and become the major products (83%) when the Iff-azepines are heated briefly at 200 °C. The symmetrical dimers probably arise by a non-concerted diradical pathway since their formation from the parent azepines by a concerted [6+6]tt cycloaddition, or from dimer (73) by a 1,3-sigmatropic C-2, C-10 shift are forbidden on orbital symmetry grounds. Dimerization is subject to steric restraint and is inhibited by 2-, 4- and 7-substituents. In such cases thermolysis of the lif-azepine brings about aromatization to the correspondingly substituted JV-arylurethane (69JA3616). 

The comments made about the diradical hypothesis with respect to the photochemistry of cyclopentanone are equally applicable to cyclohexanone. Since the formation of none of the products listed in reactions 15-18, and (15,29) is quenched by even 10-20 mm. of oxygen the existence of diradical intermediates in this system is subject to question. The alternative mechanism would be one that causes a concerted split of the ketone molecule in the excited state into two (in the cases of reactions 15 and 16) or three (reaction 17) molecular fragments. Both 16 and 17 are analogous to reactions 3 and 2 in the photochemistry of cyclopentanone and do not involve a shift of hydrogen atom from one... 

If alkenes or alkynes are subjected to strain, their jt bonds are weakened, and such compounds often behave chemically as diradicals. Their tendency to dimerize or polymerize will be significantly enhanced, and quick reaction with oxygen will occur in air [18, 19]. Reactions of strained alkenes which lead to a decline of strain, for example Michael additions or cycloadditions, can proceed significantly faster than with related, unstrained alkenes (Scheme 3.6). 

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

The tetramethylene diradical or 1,4-diradical has long been an attractive subject in photochemistry. During the past twenty years it has become possible to obtain direct experimental information bearing on these species, and with the accumulation of these data we can now begin to understand the factors which influence diradical behavior. 

In another example, the hexaacetylene 109 - after deprotection with potassium carbonate in methanol - is subjected to typical Bergman trapping conditions, resulting in the formation of the anthracene derivative 110 [61]. As a third, more complex illustration, the aroma-tization of the triacetylene 111 may be considered. Here, the 1,4-diradical intermediate faces another triple bond as an internal trap, and, after hydrogen transfer from 1,4-cyclohexadiene, the tricyclic allylic alcohol 112 is produced [61]. 

The most likely multistep mechanism of this type is shown in the lower part of Figure 12.17. It is a two-step mechanism where the diastereomeric diradicals F and G are the two intermediates that allow for rotation about the configuration-determining C—C bond. Each of the two radical centers is part of a well-stabilized allyl radical (cf. Section 1.2.1). It is unknown whether the formation of biradical F is subject to simple diastereoselectivity in comparison to G (for the occurrence of simple diastereoselec-tivity in one-step Diels-Alder reactions, see Section 12.3.4). Biradicals F and G cyclize without diastereocontrol to deliver the [4+2]-cycloadducts biradical F forms a mixture of1 2trans,cis-[D]2-C and 1,2trans, trans [D]2-C, since a rotation about the C2—C3 bond is possible but not necessary. For the same reason, biradical G forms a mixture of 1 2cis,cis- I) 2-C and 1,2cis,trans [D]2-C. 

The mechanism of 1,3-cycloaddition of nitrile oxides has been a subject of sometimes bitter controversy. While these reactions displayed the regioselec-tivity and stereoselectivity appropriate to concerted pericyclic mechanisms,62 it was suggested that the results were equally in accord with a diradical mechanism.86 However, no direct evidence for a diradical intermediate has been produced, and the balance of the theoretical argument now favors the... 

We are far from exhausting the subject of regioselectivity in dipolar cycloadditions with these few examples. Frontier orbital theory, for all its success in accounting for most of the otherwise bewildering trends in regioselectivity, is still fundamentally defective. We should keep in mind that the frontier orbitals used here must reflect some deeper forces than those that we are taking into account in this essentially superficial approach. Nevertheless, no other easily assimilated theory, whether based on polar or steric factors, or on the possibility of diradical intermediates, has had anything like such success. 

---