HOMO-LUMO interactions carbene

An HSAB analysis of singlet carbene reactivity based on B3LYP/6-31G computations has calculated the extent of charge transfer for substituted alkenes,122 and the results are summarized in Figure 10.3 The trends are as anticipated for changes in structure of both the carbene and alkene. The charge transfer interactions are consistent with HOMO-LUMO interactions between the carbene and alkene. Similarly, a correlation was found for the global electrophilicity parameter, co, and the ANmax parameters (see Topic 1.5, Part A for definition of these DFT-based parameters).123... 

HOMO-LUMO interactions in carbene alkene addition... 

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... 

Figure 14.10. Non-least-motion approach of a singlet carbene to an olefin showing the most favorable HOMO-LUMO interaction a) primary interaction (b) secondary interaction.

This result can be understood on the basis of HOMO-LUMO interactions in these cyclopropanations (Moss et al., 1979 Rondau et al., 1980 for an earlier hypothesis see Hoffmann, 1968). As shown in Figure 8-3, the carbene is inherently both an electrophile and a nucleophile. Depending on the character of substituents, it is, in the transition state, the [LUMOcarbene HOMOaikene] (p/ ) electrophilic (E) orbital interaction or the [HOMOcarbene LUMOaikeneK< / ) nucleophilic (N) interaction that is dominant. ... 

Fig. 8-3. HOMO-LUMO interactions in carbene-alkene cycloadditions (after Moss, 1989).

Figure 3. HOMO-LUMO interaction between a (phophino)(silyl) carbene and styrene...

FIGURE 5.2 HOMO—LUMO interactions in the linear approach of singlet carbene suprafacial to the 7u-system. 

However, in the nonlinear approach of carbene, the plane of bonds of two substituents on carbene is parallel to that of the C—C bond of the 7r-system. As shown in Figure 5.3, in this approach the HOMO—LUMO interactions are oriented suprafacially to the (4n + 2) rr-system and are bonding. As the electrons reorganize themselves into new bonds, the plane of bonds of the two substituents on carbene and that of the C—C bond of the olefins become at right angles to each other. However, there is no way to prove that carbene approaches linearly or nonlinearly. 

These reactions can be considered in terms of HOMO-LUMO interactions of reactants. Carbenes can approach alkenes in two ways ... 

As a cycloaddition, singlet carbcne addition to an aikene must obey the rules of orbital symmetry discussed in Chapters 35 and 36. We might consider the empty p orbital of the carbene (LUMO) interacting with the k bond (HOMO) of the aikene or the lone pair of the carbene in its filled sp2 orbital (HOMO) interacting with the 7U antibonding orbital of the aikene (LUMO). 

The insertion of a carbene into an alkene is a result of the simultaneous interaction of the HOMO of the alkene with the LUMO of the carbene or the LUMO of the alkene with the HOMO of the carbene. It is the HOMO of a nucleophilic carbene that interacts predominantly with the LUMO of the alkene and, likewise, the LUMO of an electrophilic carbene that interacts predominantly with the HOMO of the alkene. In the case of the highly nucleophilic dimethoxycarbene, the interaction of HOMO of the carbene with the LUMO of the alkene is so very strong that it gives zwitterionic intermediates such as 151, which results in the loss of stereochemistry in going from a c/.v-alkene to a -cyclopropane. With the less nucleophilic carbenes, the geometrical integrity of the alkene is retained in the product. Additionally, nucleophilic carbenes do not insert Oc-h bonds. 

Nevertheless, the use of relative reactivities to characterize carbenic philicity is restrictive the apparent philicity is related to the alkenes selected for the relative reactivity measurements. What if the set of alkenes were expanded by the addition of an even more electron-deficient alkene Such a test was applied in 1987 [65], using a-chloroacrylonitrile, 26, which is more 7t-electron deficient than acrylonitrile, 27. We found that PhCF or PhCCl added 15 or 13 times, respectively, more rapidly to 26 than to 27. In preferring the more electron-deficient olefin, the carbenes exhibited nucleophilic character. However, because they also behave as electrophiles toward other alkenes (Table 4), they must in reality be ambiphiles. In fact, we now realize that all carbenes have the potential for nucleophilic reactions with olefins the crucial factor is whether the carbene s filled a orbital (HOMO)/alkene vacant Ji orbital (LUMO) interaction is stronger than the carbene s vacant p orbital (LUMO)/aIkene filled k orbital (HOMO) interaction in the transition state of the addition reaction. [63]... 

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]. 

The transition states of the cycloadditions leading to the isomeric cyclopropanes have been calculated (semiempiric program AMI). At the beginning of the reaction, the interaction of the HOMO of the carbene with the LUMO of the enol ether dominates, but in the transition state the interaction between the LUMO of the carbene and the HOMO of the enol ether is more important. The cyclopropanation yields of the pivaloylated diazirine 15 are higher because of the more efficient competing reaction of the O-benzylated (as compared to the 0-pivaloylated) carbene with the starting diazirine. Azines are isolated as the main by-products. Calculations show that a pseudoaxial attack of the carbene is favored. 

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. 

Interaction 7a features net electron donation from the alkene n orbital (HOMO) to the vacant carbene p orbital (LUMO), and tracks the electrophilic character of the carbene. Interaction 7b represents electron donation from the filled carbene a orbital (HOMO) to the vacant alkene ti orbital (LUMO) and reflects the carbene s nucleophilic character. Both interactions operate simultaneously in the addition transition state, but which one is dominant ... 

Answer to 4(d). We can consider the orbitals of cycloheptatrienylidene to arise from the interaction of the n orbitals of hexatriene and the valence orbitals of a di-coordinated carbon atom (a 2p orbital and an spn hybrid orbital). The n orbitals of hexatriene may be obtained from an SHMO calculation. The interaction diagram is shown in Figure B7.2. The p orbital of the carbene site is raised as a result of the dominant interaction with ns of hexatriene. The orbital 714, which is closest in energy to the carbene s p orbital, does not interact because of symmetry, and n5 interacts less strongly because the coefficients at the terminal positions of the hexatriene are smaller. The larger HOMO (sp")-LUMO (n or n 5) gap permits the ground state to be singlet. 

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