Electronic effects, triplet carbenes

This effect was first observed for the pairs of 1- and 2-naphthylcarbenes (12). ° Since then, reports of geometric isomerism in triplet carbenes have appeared with increasing frequency two sets of triplet signals having similar but nonidentical ZFS parameters are observed. The spectra are assigned to the two conformations of the carbene in which the ct orbital at the divalent carbon and the aromatic moiety are coplanar. When the distribution of the spin in the 7t orbital is unsymmetric, the dipole spin-spin interaction of the p electron with the electron localized in the CT orbital is different for the two conformations. Consequently, the ZFS parameters will be different and in cases in which the differences are sufficiently large, it is possible to observe the spectra of the two isomers. 

The stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

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). 

The final carbene in this group is dimethylsila-anthrylidene, SA. Low temperature spectroscopy identifies the triplet as the ground state (Sekiguchi et al., 1982). This carbene appears to react rapidly with methanol and with cyclohexane (Sugawara et al., 1983b). This pattern of reactivity and the reported rate constants fits quite well within the equilibrium model. However, the products of these reactions have not yet been identified. Nonetheless, it is possible to conclude tentatively that ACsj is smaller for SA than for DPM. If true, this means that the electronic effects of the silicon substituent outweigh any increase in the carbene-centre bond angle. 

Kinetic protectors we first investigated were alkyl groups since it was our initial hope to stabilize and hopefully isolate triplet carbene consisting of only hydrocarbons. However, the use of alkyl groups as effective kinetic protectors was supposed to be very difficult in the light of the strong affinity of carbenes for electrons since they react even with very weak sources of electrons, e. g., C-H bond a electrons. 

It is easy to see how difficult it is to stabilize triplet carbene. It is not stabilized by a polar effect, either internally or externally, since it is electronically neutral... 

The effect of multiplicity of carbenes on their reactivity is most vividly marked in the following features rationalized by Skell et al. from experimental data [37-39]. First, the reaction of carbenes occurs in the singlet electron state at a much faster rate than in the triplet, with the absolute rates of typical reactions of addition to multiple bonds and of insertion into the C—H bonds exceeding, under normal conditions, the rate of intercombination conversion. Secondly, the singlet carbenes are characterized by one-step stereospecific addition to double bonds, as, for instance, in the cyclopropanation reaction, while the triplet carbenes react in a nonstereospecific way to form first an intermediate biradical through addition to one of the atoms of the double bond. The formation of a trimethylene radical, in the course of reaction of triplet methylene ( B ) with ethylene, has been confirmed by semiempirical [40, 41] and ab initio [42, 43] quantum chemical calculations. 

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