Carbenes spin states

Sander and co-workers have also previously examined the effects of carbene spin state and philicity on the carboxylation reaction in low-temperature matrices. ... 

They concluded that the reactivity of carbenes toward CO2 is determined by their philicity (more nucleophilic carbenes are more reactive) and that carbene spin state interestingly has little effect. Kovacs and Jackson have suggested that this reactivity pattern may be explained by a nonequilibrium surface crossing mechanism. ... 

A word is needed about the assignment of rate constants to specific carbene spin states. Where a measured rate constant can be attributed with some confidence to a particular spin multiplicity, that multiplicity is indicated (i.e. XA and 3BA). Where the multiplicity is uncertain, the experimentally determined rate constant is reported and no spin state is indicated (i.e. FL). In the latter cases, the reported rate constant can often be viewed as the product of the actual bimolecular rate constant and the equilibrium constant (Table 8) connecting the carbene spin states 6 Griller el al., 1984c. This conclusion is reached solely from the analysis of products in C6H12... 

Shape selectivity 7 Steering reaction outcomes 8 Control carbene spin state 9 Control intramolecular reactions 10 Inhibit intermolecular reactions 10... 

The isocyanide ligand, CNR, is formally isolobal with a carbene ligand. Several studies have investigated iron(Ill) porphyrin isocyanide complexes which have the general formula [FeCPorKCNR) ]. However, these studies have mostly been concerned with spin state and spectroscopic properties rather than chemical transformations and will not be discussed in detail here. Crystallographic details are given for two of the complexes. " ... 

The chemistry of carbenes is inextricably linked with issues of spin state. This is equally true for tunneling as for classical reactions. The factors that determine the... 

Direct irradiation or thermolysis of a diazo-compound (2) is believed to generate the carbene initially in its singlet spin state. Triplet sensitization (5) is presumed to give the triplet carbene directly without first forming its singlet state via the triplet diazo-compound. In some cases, careful comparison of the results of direct irradiation experiments with those from triplet sensitization can provide useful information to identify the spin state initiating a reaction. 

The reaction of BA with isopropyl alcohol is particularly revealing. Direct irradiation of DABA in neat isopropyl alcohol gives the ether (17%) expected from the reaction of the singlet carbene, and the radical coupling products characteristic of the triplet spin state (21). The ratio of the yields of radical-derived products to ether depends inversely on the concentration of isopropyl alcohol in a benzene solution. This behavior is particularly meaningful when compared with that of FL, XA and DPM. For these... 

The chemical behavior of PM is characteristic of a carbene for which equilibration of spin-states is faster than irreversible reaction. In particular, direct and triplet-sensitized irradiations of 2-n-butylphenyldiazomethane give exactly the same products (Baer and Gutsche, 1971). A similar conclusion was reached in the study of ANM (Hadel et al., 1983). 

Although interconversions of the two spin states under matrix-immobilized spectroscopic conditions have not yet been observed in the cases of the pyrrole derivative 46f and the bis(phenoxyl) 53, reversible thermal isomerizations of the singlet and triplet states have been reported in the cases of the bis(aminoxyl) 54 and the carbene 55. 

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

Product Studios. Alkenes are known as diagnostic reagents for spin state of reacting carbenes. Thus, the reaction of a singlet carbene with an olefin usually results in the formation of a cyclopropane through stereospecific addition to the C—C double bond, while a triplet carbene gives rise to a nonstereospecific addition product (see Section 7). 

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