Carbene spin state effects

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. ... 

Diazirine 63 was photolyzed in various solvents and within CyDs. The solution results are summarized in Table 8. The conventional solvents were used to gauge whatever effects the CyD hosts had on carbene 64. Hydrocarbon solvents, like pentane ( -C5Hi2) and cyclohexane (c-C6H12), were used to mimic the inner cavities of CyDs, which are also nonpolar, hydrophobic environments. Tetrahydrofuran (THF) was employed because the cyclic ether resembles the D-Glcp monomer units of the CyDs. Moreover, since CyDs also possess many hydroxyl (O-H) groups, it seemed appropriate to perform control experiments in alcoholic solutions of diazirine 63. Finally, chloroform (CHC13) was used to assess the spin-state of carbene 64. 

If the reaction rates of a specific carbene with various quenchers are studied in the same solvent, and with small concentrations of Q, K will be constant. Relative reactivities for the singlet state of a spin-equilibrated carbene can thus be derived. However, few researchers have varied the acidity of ROH, estimated kinetic isotope effects, and compared alcohols with ethers (Table 4). The data indicate proton transfer to diarylcarbenes (139d, 139k, 205, 206)112-117 and diadamantylcarbene (207).118... 

On the other hand, the cyanomethylene H—C—C=N triplet as well as a number of odd alternant methylene compounds, propargylene and its homologues, H— —C=CR (R = H, CH3, C6H5), and H—C— C=CC=CR [R = CH3, C(CH3)3, C6Hs] have been predicted to be linear on the basis of the zero-field splitting parameters from electron spin resonance (ESR) experiments (15). The linear nature of the first named H—C—C=N triplet carbene has been confirmed by a micro-wave study in the gas phase. However, if the potential well in which the linear configuration lies is shallow, the ground state may not show the effects of nonlinearity (142). 

If the E parameters corresponded to the internuclear angle at C-1, the order of decrease we would expect to be 4 > 5 > 2 however, the actual sequence is 4 > 2 > 5. This has been explained by the different effects of the x andy components of the spin densities on the a orbital at C-1 > 12). From the E / D value it was concluded that all cycloalkene-carbenes have angles which are large compared to the internuclear angle at C-1 thus all the bonds at C-1 should be bent, i. e. C-1 should not be in the ring plane > i > 12). This suggests that 4 is almost planar i >, allowing delocalization of the electron. The electronic state of 1 has not been studied yet. 

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