Push-pull carbenes

To date, the (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes are the only stable carbenes that feature a 77-donor and a 77-type-withdrawing substituent. They have a singlet ground state with a planar environment at phosphorus and a short phosphorus-carbon bond distance. However, because of the reluctance of phosphorus to keep this planar geometry, these push-pull carbenes behave in a manner very close to that of most of the transient carbenes. 

In summary, the phosphino group clearly acts as 7i-donor substiment, while to some extent the sUyl acts as n attractor due to the aptitude of silicon for hypervalency (phosphino)(silyl)carbenes are push-pull carbenes. Importantly, when... 

Figure 8.8. Molecular structure of push-pull carbene XIIc.

The first isolated singlet carbenes were push-pull systems featuring both an electron-donating and an electron-withdrawing group that interact directly with the carbene center (Scheme 2). Phosphinocarbenes 6-8 are perfect in this category,... 

Carbon acids activated by strong ji-acceptors that also contain a jt-donor capable of interacting with the ji-acceptor in a push-pull fashion pose an interesting problem. Fischer carbene complexes such as 20,94 23,97 36,96 37,96 38,98 and 3998 fall into this category. Thermodynamic... 

Treatment of the conjugated telluropyran aldehyde 82 with the carbanion of the tungsten carbene 83 afforded the conjugated push-pull Fischer-type carbene complexes with extended conjugation 84 (Equation 34) <2005JOM4982> (cf. Equation 10). These complexes exhibit interesting nonlinear optical properties (see Section 7.11.8.2). 

A number of push-pull Fischer-type carbene complexes 84 have been prepared and exhibit interesting nonlinear optical properties <2005JOM4982> (see Equations 10 and 34). The influence of changing the chalcogen atom and the effect of varying the chain length on these optical properties have also been investigated. 

It is rather difficult to generate donor-substituted carbenes (path b), and methylena-tion of push-pull-olefins (path c) is not very efficient due to the low reactivity of these alkenes. Therefore these two alternative [2 + l]-cycloadditions have been of relatively low importance so far. This is also true for the addition of suitable nucleophiles to cyclopropenes activated by electronwithdrawing substituents (path e). 

In contrast, a stable carbene has been synthesised having a push-pull combination of electron donation and withdrawal [194] (Figure 9.81). 

Other examples of [2+1] cycloaddition reactions with trifluoromethyl-substituted carbenes are known with carbenes bearing a halogen or a methoxy substituent, which seems to result in a push-pull stabilization.179-185 Yields depend on the mode of generation of the carbenes best yields of eyclopropanes tire obtained with carbenes generated from (polyfluorocthyl)mercury compounds.180- 84... 

Thus, the measurement of the relative activity of organo(organooxy)carbenes toward alkenes shows that methoxy(phenyl)carbene is ambiphilic with an extraordinarily strong nucleophilic component,while methoxy(methyl)carbene shows nucleophilic properties.Therefore, these two carbenes react readily with electrophilic alkenes. On the other hand, methoxy(trifluoromethyl)carbene, in spite of possible push-pull stabilization, is highly reactive and electronically indiscriminant in its reactions with alkenes.The absolute rate constants of its addition to alkenes confirm such a reactivity pattern. 

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