Carbenes transition state symmetry

The singlet carbene, on the other hand, can add through a less symmetrical transition state in a slightly bent -approach, thus circumventing the barrier imposed by orbital symmetry ... 

In cartoon form, what is needed is a ligand that will extend sterically to set up the local C2-symmetry around the apical position of the rhodium, where the carbene binds and where the C-H insertion reaction is taking place. The resulting chiral environment would then favor transition state 52, leading to one enantiomer, over transition state 53, leading to the competing enantiomer. 

Fig. 9.2 A species with the symmetry of an oxirene scrambles the label in an oxo carbene. But this does not tell us whether the oxirene is an intermediate or merely a transition state...

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sg has a flat biradicaloid region of high energy with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyerand Klessinger, 1995). 

We have studied the dimerisation of several diaminoearbenes by density functional calculations the transition states for dimerisation of diaminocarbene itself and for the tetramethyl derivative 9 have been reported. [59] Results for saturated five and six-membered ring carbenes are included in Table 1, [60] and the transition states for these (which have C2 symmetry) are shown in Figure 2. The differences in the energy required for the reverse reaction are quite striking Thus although there is undoubtedly steric hindrance in tetrakis(dimethylamino)ethylene, it is more stable relative to the earbene than the non-alkylated derivative. This may be due to destabilisation of... 

The philicity of a carbene directly depends on the structure of the transition state of an addition reaction. The rules of orbital symmetry conservation forbid the least-motion C2v-symmetry reaction path [41]. For electrophilic carbenes, characterized by predominance of the n — p interaction, preferable is the so-called 7r-approach (Fig. 8.3). In the case of nucleophilic carbenes, optimum conditions for the overlap between the (Tcxy 7r -orbitals are provided by the asymmetrical cr-approach (Fig. 8.3b). By making use of certain assumptions, Rondan, Houk, and Moss [44, 45] calculated the overlap integrals Sjj between the corresponding frontier orbitals of carbene and alkene for the n- and the (7-approaches. Then, having computed the energies of those orbitals, they obtained the energies of stabilization of the composite system arising in two... 

Conventional EPR techniques have been successfully used to measure the D and E values of matrix-isolated carbenes in the ground triplet state because the steady-state concentration of triplet species is sufficiently high in the system. The technique cannot be used, however, for excited species having triplet hfetimes of the order of 10-100 ns, since their steady-state concentration is too low. The D parameters are estimated from the external magnetic field effect on the T—T fluorescence decay in a hydrocarbon matrix at low temperamre. The method is based on the effect of the Zeeman mixing on the radiative and nonradiative decay rates of the T -Tq transition in the presence of a weak field. The D values are estimated by fitting the decay curve with that calculated for different D values. The D T ) values estimated for nonplanar DPC (ci symmetry) is 0.20... 

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