Group 13 systems biradicals

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

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 lack of Markovnikov directing effects in this system has been interpreted as evidence against a biradical or ionic intermediate. Since product la results from H-abstraction from either methyl group (a) or (b) of 1 (equation 1), the relative reactivity of these groups was not known. 

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 intramolecular hydrogen abstraction has been largely developed for the case of the nitro group [22,37], and is well known for some derivatives of o-nitrotoluene, for o-nitrobenzyl alcohols, ethers and esters [38-40] as well as on the acetals of o-nitrobenzaldehyde [41], With these compounds, the nn (singlet or triplet) state abstracts a hydrogen and the biradical relaxes to the aci-nitro form (the first intermediate is actually observed in bicyclic systems where formation of the nitronic acid is sterically hindered) [41], which under basic conditions dissociates to the corresponding anion [42],... 

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