Triplet carbene, reactions

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

When monitoring the transient due to triplet carbenes is difficult because of the inherent weak nature of the bands and/or severe overlapping with the absorption bands of the parent diazo compounds, it is more convenient to follow the dynamics of the triplet carbene by measuring the rate of the products formed by reaction of triplet carbenes with quenchers such as radicals (Section 5.3) and carbonyl oxides (Section 6.5). In this case, note that the observed rate constant (feobs) of a triplet carbene reaction is the sum of the decay rate constants of the triplet. These may include decay via an associated but invisible singlet with which the triplet is in rapid equilibrium. Thus in general. 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

The observed rate constant (fcobs) of a triplet carbene reaction is the sum of all decay rate constants of the triplet. These may include decay via an associated but... 

It is not easy to explain why the triplet reactions that are energetically much less favored than those of the singlets become dominant at low temperature. Based on Ea and log A measured for triplet carbene abstraction (see Section 5.3), one can estimate the rate constant at 77 K to be <10 M s, suggesting that triplet carbene reactions in matrices at 77 K should not occur. Obviously, reactions of carbenes within matrices are controlled by factors that are not operating in solution phase, as one might expect from dramatic changes in reaction conditions. 

Reactivity of Cycloalkenecarbenes Table 12. Triplet carbene reactions resulting from energy transfer... 

On the other hand, cycloaddition of cyclobutene by bis(methoxycarbonyl)carbene yielded dimethyl bicyclo[2.1.0]pentane-5,5-dicarboxylate (10) as the sole product (55% yield) in the direct photolysis, whilst the triplet carbene reaction in addition to 10 produced two vinylcy-clopropanes 11 and 12 which probably arise from the same intermediate 1,3-diradical. 

We discuss qualitative and quantitative aspects of singlet and triplet carbene reactions in Section 8.3 and 8.4. 

Sometimes the barriers to triplet reactions are not effectively higher than those for singlets. In these cases, triplet carbene reactions can be observed. Like their singlet counterparts, triplet carbenes add to alkenes to give cyclopropanes (Fig. 10.45). 

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