1,4-Diradicals, formation photochemical

Photolysis of the 1,2,3,5-oxathiadiazine (132) in the presence of ethanol leads to the quinazolone (133). The product is not formed in the absence of ethanol, and the proposed mechanism involves electrocyclic ring opening, addition of ethanol, photochemical diradical formation and closure on to the C-6 phenyl ring followed by aromatization as shown in Scheme 8 (79JOC4435). 

In accord with these theoretical predictions most simple endoper-oxides are experimentally found to be thermally unstable with respect to diradical formation (15). Furthermore, photochemical studies indicate that upon excitation to low lying excited singlet states, simple peroxides dissociate into radicals, whereas higher energy excitation apparently leads to the formation of molecular oxygen ( 2/ ) 4),... 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

This chapter has to do with reactions wherein the photochemical event is the breaking of a bond in a molecule. For a single bond this results in the formation of a pair of radicals or a diradical. For a double bond as in diazo compounds or in azides a carbene or a nitrene and nitrogen are formed. All these intermediates will then undergo further mono- or bi-molecular dark reactions or eventually recombine to ground state starting materials. 

Cycloaddition reactions (see Table I) involving unsaturated carbohydrates are regio- and stereo-selective. These selectivities can be understood by assuming that the photochemical interaction between the two 7r-systems results in formation of the more stable 1,4-diradical. The reaction between 3,4,6-tri-O-acetyl-D-glucal (1) and acetone pro-... 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

Photochemical reaction with disiliranes leads to a [3-i-2]-cycloaddition (Section 4.3.9). Disilylcyclobutanes [67, 68] and cyclotetrasilanes [68, 69] react in a similar fashion. In both cases the four-membered ring is photolytically cleaved and a diradical is formed. This diradical adds in a formal cycloaddition to the [6,6] double bond of CgQ. The cycloadduct 20 (R = 4-MeCgH4) can be obtained from 19 in 13% yield, but it is not the major product (Scheme 6.12). Rearrangement of an H atom leads to 21 in 46% yield. The product distribution depends strongly on R. Changing R from 4-MeCgH4 to phenyl leads to an exclusive formation of 21. 

Thermal or photochemical extrusion of nitrogen from 1-arylbenzotriazoles (114) leads to the formation of carbazoles (Scheme 14). The mechanism is believed to involve cyclization of a diradical (115) or an iminocarbene (116) to the 4a/f-carbazole (117) followed by an aromatizing hydrogen... 

Photolysis of l-methylnaphtho[l,8-de]triazine (32, R = Me) also results in extrusion of nitrogen and formation of a diradical intermediate (167). Thus, reaction in cyclohexane as solvent gives, among other products, bicyclohexyl and 1-methylaminonaphthalene, while 8-phenyl-1-methylaminonaphthalene is the only product formed when benzene is used as solvent. Photochemical decomposition of 32, R = Me, in the presence of olefins results in an unusual ring transformation, and with a-methylstyrene, for example, the triazine is converted into the dihydroazaphenalene derivative (168). When vinyl bromide and trans-... 

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

A tricky formation of a cyclobutane by transannular participation of a double bond was observed in the photochemical dcnitrogenation of 4,5-diazatetracyclo[6.2.1.1 3,6.02-7]dodeca-4,9-diene (20). The formation of the tricyclic diene 22 in addition to the expected cyclopropane 21 can be rationalized as a formal homo Cope rearrangement in the intermediate diradical.42... 

Figure 15.7. Dauben-Salem-Turro analysis of the photochemical step of the Norrish Type I reaction for (a) saturated carbonyls and (b) conjugated carbonyls. The reaction is most efficient on the 3( 7 ) surface to yield triplet diradical products. It is also efficient on the 3(nn ) surface since IC permits formation of products in their ground state.

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