Carbenes frontier orbital interactions

Fig. 4.14 Frontier orbital interactions for carbenes with electrophilic and nucleophilic...

Frontier Orbital Interaction in Nucleophilic Attack on Fischer Carbene Complexes... 

Fig. 6. 57 Frontier orbital interactions in the 1,4-addition of a carbene to an s-cis diene...

Self-consistent field molecular orbital calculations by Fenske and coworkers have confirmed that nucleophilic additions to Fischer and related complexes [e.g., (CO)sCr=CXY, (T)5-C5H5)(CO)2Mn=CXY], are frontier orbital-controlled rather than charge-controlled reactions (7-9). Interaction of the HOMO of the nucleophile with the carbene complex LUMO (localized on Ca) destroys the metal-carbon w-interaction and converts the bond to a single one. 

Substituents have a profound effect on the reactivity of carbenes. Donor substituents lower the energy if they are conjugated to the empty p orbital 4.103, and electron-withdrawing substituents lower the energy if they are conjugated to the filled p orbital 4.104. Since these interactions leave the other frontier orbital more... 

When the metal is niobium, the two metal frontier orbitals, dy and dz2, are closer in energy and match the corresponding orbitals of triplet free carbene. Figure 10-4b provides a picture of the interactions involved (only the important orbital interactions are shown). The elections in the M-C bond are much more equally distributed between the metal and Ccarbene (because the energy levels of the metal and carbon orbitals are comparable) than with an electrophilic carbene complex. In resonance terminology, this means that structure 10 becomes an important contributor to the overall structure of this particular Schrock carbene complex.11... 

The frontier orbital theory also agrees with the observed experimental reactivity of Fischer and Schrock carbene complexes. Calculations show that the linear Cr=C —R group is more stable than the bent one. " Thus, the deviations of M = C —R fragments from linearity probably result from intermolecular interactions within the crystal lattice. 

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... 

Clearly, the above-mentioned values of the philicity relate each time to a particular alkene, and the orders of philicity may be different with different alkenes [45]. Figure 8.3 visualizes this behavior the stabilization or destabilization of the 7r7i -levels of alkene, induced by substituents, may alter relative values of the contributions from the n — p and a — n interactions. In practice, their values are often estimated without calculating AE confining oneself to finding the difference between the energies of frontier orbitals. Thus, the relative rate constants of addition of dichlorocarbene with substituted styrenes correlate well with the difference between the energies of the HOMO of styrene and the LUMO of carbene [47]. 

We may redraw 6 as 7a and 7b, in terms of frontier MOs. Here we emphasize the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interactions that operate in the transition state 7a depicts the LUMO(carbene)/HOMO(alkene) or p-n interaction 7b shows the HOMO (carbene)/LUMO(alkene) or a-71 interaction. These formulations are especially... 

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