Triplet ground state double bond additions

The results of experimental observations and molecular orbital calculations indicate that many carbenes have a nonlinear triplet ground state. Exceptions are the dihalocarbenes and carbenes with oxygen, nitrogen or sulfiar atoms attached to the bivalent carbon, all of which are singlets. The singlet and triplet states of a carbene do not necessarily show the same chemical behaviour. For example, addition of singlet carbenes to olelinic double bonds to form cyclopropane derivatives is more stereoselective than addition of triplet carbenes. 

Nitrenes,239 R—N, are the nitrogen analogs of carbenes, and most of what we have said about carbenes also applies to them. Nitrenes are too reactive for isolation under ordinary conditions. Alkyl nitrenes have been isolated by trapping in matrices at 4 K,24n while aryl nitrenes, which are less reactive, can be trapped at 77 K.241 The ground state of NH, and probably of most nitrenes,242 is a triplet, though nitrenes can be generated in both triplet and singlet states. In additions of EtOOC—N to C=C double bonds two species are involved,... 

A chemical test of a quite different nature is used for the identification of excited methylene, CH2. Methylene in the ground state is a triplet, but the photolysis of diazomethane or ketene (the standard methods for CH2 production) may lead to the formation of appreciable amounts of the excited singlet methylene. The fraction of singlet methylene has been determined by a study of the stereospecific addition of CH2 to double bonds eig.. the addition of methylene to tronj-butene-2 to yield dimethylcyclopropane ). It is assumed that spin will be conserved in the addition thus triplet methylene will form initially a triplet adduct (i.e. a biradical), and rotation may occur around the 2 3 C-C bond in the radical before ring closure. On the other hand, singlet methylene may form the cyclopropane directly, so... 

The mechanisms of two particular classes of photochemical (2 + 2)-cycloaddition reactions have been studied extensively, viz. (i) of reactions in which either the heterocyclic or the substrate contains an enone moiety,1231,24 which are believed to occur via the excited triplet state of the enone, and (ii) of oxetane formation, which is thought to proceed via n — 7T excitation of the carbonyl group, followed by intersystem crossing to the triplet state and addition to the carbon-carbon double bond in its ground state6 in some cases, however, an oxetane has been reported to be formed from a ground-state ketone and an excited-state olefin (Section III,C,2). 

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate). They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward-Hoffinann s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO(HOMO) of the ground state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the concerted cycloadditions occur favorably in the lowest triplet state Ti and in the second excited singlet state S2. 

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