Amino groups stable singlet carbenes

If the resonance stabilization of singlet carbenes is increased even more, for example, by replacing the CH3O substituents of (CH30)2C by amino groups [as in (R2N)2C] or phosphorus substiments (13), stable, isolable, nucleophilic carbenes can be obtained. These species are considered in Chapter 8 in this volume. 

The stereospecificity observed with (Z)- and ( )-2-deuteriostyrene presents convincing evidence for the concerted nature of the cyclopropanation reaction, and therefore the genuine singlet carbene nature of stable (phosphino)(silyl)carbenes (I). The diastereoselectivity is at first glance surprising. Indeed, it is clear that steric factors cannot govern the observed selectivity since the bis(amino)phosphino group... 

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. 

In some cases donation of nonbonded electrons by atoms )3 to the carbene carbon can stabilize singlet carbenes dramatically. Arduengo and co-workers reported that treating 1,3-di-l-adamantylimidazolium chloride (34) with the anion CH3SOCH2 in THF produced the singlet carbene 1,3-di-l-adamantyl-inudazol-2-ylidene (35, equation 5.29). This carbene is stabilized thermodynamically by electron donation from the two amino groups, and it is stabilized kinetically by bulky adamantyl substituents that make it less likely to react intermolecularly. As a result, 35 is stable in the presence of air and moisture, and the crystals of 35 melt at 240-241°C. 

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