Single-Electron Activation

The isolation of XIIc was of importance since it demonstrated for the first time that monoheteroatom-substituted carbenes could be stable, indefinitely, at room temperature. The next step was to investigate whether a single electronically active substituent is sufficient to isolate a carbene. 

Up to 2000, the number and variety of stable carbenes have been limited by the perceived necessity for two strongly interacting substituents. The preparation of stable or persistent (aryl)- or (alkyl)-(phosphino)carbenes as well as (aryl)(amino)-carbenes demonstrates that a single electron-active substituent allows the spectroscopic characterization of singlet carbenes under standard laboratory conditions. It has been shown that an amino substituent is more efficient than a phosphino substituent to stabilize a carbene center and that the steric bulk of the spectator substituent can be decreased even to the size of a methyl group in the phosphino series, so that a broad range of observable carbenes will be readily available. 

When we started our carbene project, it was a common statement that carbenes only occured as reactive intermediates this is no longer valid. Up to the last months, the number and variety of stable carbenes have been limited by the perceived necessity for two strongly interacting substituents our most recent work establishes that only a single electron-active substituent is necessary to isolate a carbene. Therefore a broad range of these species will soon be readily available, which will open the way for new synthetic developments and applications in various fields. Note that despite the limited number of stable carbenes known, these species have already found applications even on a large scale. [67]... 

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