Electronic effects nucleophilic carbenes

Tlie nucleophilic character of these carbenes has been demonstrated by Hammett -Taft correlations (p = +2 in competition experiments, with Ar- N=C=S/Ar-N =C=0 as substrates) (20). The syn-anti distribution of isomers in cycle pro pa nation reactions with carbenes is the result of steric and electronic effects. The dissipation of the charge by secondary interactions of the carbene substituents and the substrate are determining factors. As an example. CCIF reacts according to the orientation of the Cl group syn to the... 

The enthalpies of reaction for nucleophilic carbencs depend on the stereoelec-tronic properties of the ligands affecting the availability of the carbene lone pair. An example of electronic influence is the 3.5 kcal/mol enthalpy difference between the isosteric pair IMes and IMesCI that shows the electron-withdrawing nature of Cl compared to H. This trend again is in line with electron donor/withdrawing ability of arene substituents. The effect in this la.st case is a long range electronic... 

By studying the effect of various a-substituents, it has been shown that the bond to the most highly substituted a carbon is preferentially cleaved and that the more nucleophilic alkyl carbon migrates to the relatively electron-poor free radical to form the carbene intermediate,... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

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. 

Carbene complexes of transition metals [2,21,225-236] are typical representatives of compounds with a double metal-carbon bond. They are seen as derivatives of a two-covalent carbon in their singlet state [226,232,236]. As a rule, the carbene ligand is an effective a-donor and a comparatively weak n-acceptor. Formation of a cr-bond M — C takes place via transference of a nonbonding electronic pair with a nucleophilic a-orbital of the carbenic carbon to the metal atom. Simultaneously, it is also possible to form a 7t-bond as a result of the interaction of symmetrically appropriate metallic d-AO with a vacant electrophilic /7-orbital of the carbene [236,237], This situation is a key factor that determines the polarization of most of the carbene complexes according to type 145 (Fig. 2.6). 

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