Phosphino carbenes cyclopropanation

In contrast to silylcarbenes, the analogous stannylcarbenes 2p are not stable, which explains why they have attracted little interest. Their instability is probably due to the long carbon-tin bond, which does not allow sufficient steric protection of the carbene center. Their reactivity seems to be quite similar to that of stable (phosphino)(silyl)carbenes Cyclopropanation reactions have been reported with methyl acrylate as well as coupling reactions with tert-butyl isonitrile.73... 

In marked contrast, we used the cyclopropanation reaction of (phosphino)(silyl)-carbene (la) with methyl acrylate to demonstrate the carbene nature of our compound. More recently, we studied in detail the stereochemistry of this type of reaction.The (phosphino)(silyl)carbene (la) reacted efficiently with 3,3,4,4,5, 5,6,6,6-nonafluorohex-l-ene and (Z)- and ( )-2-deuteriostyrene giving the corresponding cyclopropanes in good yields. The stereochemical outcome was such that all monosubstituted alkenes gave exclusively the syn isomer (with respect to the phosphino group), and the addition of disubstituted alkenes was totally stereospecific (Scheme 8.16). 

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... 

Reactions related to cyclopropanation can also be carried out with (phosphino) (silyl)carbenes (I). For example, benzaldehyde reacts with la at 0 °C leading to the corresponding epoxide, again as only one diastereomer. Even more striking are the reactions with benzonitrile and tert-butylphosphaalkyne that lead initially to azirine and phosphirene. Both three-membered heterocycles subsequently undergo ring expansion reactions affording azaphosphete and diphosphete, respectively (Scheme 8.18). This reaction is a new route for the synthesis of heterocyclobutadienes, and this demontrates the usefulness of (phosphino)(silyl)carbenes (I) for the synthesis of novel species. 

In the following months, we read a lot on carbene chemistry, and in 1989 we published a paper entitled [Bis(diisopropylamino)phosphino]trimethylsilyl-carbene A stable nucleophilic carbene . [23] We found that 17 readily underwent at room temperature the classical cyclopropanation reaction with electron-poor olefins such as methyl acrylate leading to 23 in high yield. Moreover, it appeared that 17 also cleanly reacted with tert-hvfifi isocyanide to give the ketene imine 24 in 90% yield this was an example of carbene-carbene-like coupling reaction. 

Reactions related to cyclopropanation can also be carried out with (phosphino)(silyl)carbenes. For example, benzaldehyde reacts with 17 at 0°C leading to the corresponding epoxide, again as only one diastereomer. [23] Even more striking are the reactions with benzonitrile and tert-butylphosphaalkyne... 

Phosphino)(trifluomethyl)carbene 41 does not react with electron-rich alkenes but cleanly undergoes cyclopropanation reactions (toluene, -78°C) with methyl... 

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