Triplet methylene, cycloaddition reactions

The enone (92) is reactive in its triplet state and when irradiated in methylene chloride solution is converted into the tetracyclic compound (93). The reaction involves a step-wise process in which the biradical (94) is involved. This process is reminiscent of (2-i-2)-cycloaddition reactions where bonding occurs at the P-atom of the enone, and rather than completing the cyclization, hydrogen (or deuterium) abstraction occurs. A detailed stereochemical analysis of the system was carried out and proof of the stereochemistry of the final product (93) has been presented. 

The simplest cycloaddition, if the dimerization of two methylenes to form ethylene (Section 4.3) can be excluded, is the (l-f-2)-addition to ethylene of a singlet carbene CX2, where X is a halogen or another electronegative substituent. The reactions of methylene itself, and other carbenes that have triplet ground-states, will be deferred until the symmetry properties of electron spin are taken up. An incidental practical advantage of setting up the correlation digrams in Fig. 6.1... 

Herzberg and collaborators provided the first experimental evidence that a meta-stable, bent singlet was initially produced from diazoalkane and decayed to a ground-state triplet. - The hquid-phase chemistry of methylene, for example, is simple in comparison with its gas-phase chemistry. However, hot molecules produced by extremely exothermic reactions of methylene play a minor role in the gas phase. Carbenes have been used on countless occasions to make three-membered rings via [1+2]-cycloaddition. 

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