Nucleophilic additions singlet carbenes

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. 

A second characteristic reaction of carbenes is addition to olefins to yield cyclopropanes. Singlet carbenes might react as either nucleophiles or electrophiles triplets may be expected to behave like free radicals- The data in Table 5.9, showing the increase in rate of addition on substitution of electron-donating... 

A singlet carbene is inherently both an electrophile and a nucleophile, what is behaviorally decisive is whether, in the carbene/alkene addition transition state, it is the LUMO(carbene)/HOMO(alkene) or HOMO(carbene)/LUMO(alkene) interaction (cf., Fig. 5) which dominates and determines the electronic distribution. If the former interaction dominates, the carbene will exhibit electrophilic selectivity if the latter interaction is more important, nucleophilic selectivity will be observed. If both interactions are comparable, the carbene will display an ambiphilic selectivity pattern, in which it acts as an electrophile toward electron-rich alkenes, but as a nucleophile toward electron-poor alkenes. [8,69]... 

Carbene 42 adds to a variety of electron deficient olefins, and even to styrene " with -substituted styrenes, the nucleophilicity of these additions can be observed (for 42 with cyclohexyl replacing isopropyl). [117] Moreover, additions of 42 to Z- and ii-2-deuteriostyrene are stereospecific with respect to the styrene substituents, as anticipated for singlet carbene additions. [117] Thus, the isolable phosphinosilylcarbenes are nucleophiles, but retain the essential reactivity expected of such carbenes. 

When additions and insertions are not possible, singlet carbenes react as either carban-ions or carbocations. In protic solvents, for example, singlet carbenes give ethers. Singlet di-phenylcarbene first abstracts a proton from the alcohol to give a carbenium ion, and then a nucleophile adds (Eq. 10.57). The alternative mechanism, in which the C-O bond is formed first, is seen for other carbenes. Either way, the overall transformation re.sults in the insertion of the carbene into the O-H bond of the alcohol. 

The radical versus electrophilic character of triplet and singlet carbenes, respectively, also shows up in relative reactivity patterns. The relative reactivity of singlet dibromocarbene toward selected olefins (Table 9.1) can be seen to be more in accord with the electrophilic models (bromination, epoxidation) than with the radical model (addition of CCls). Carbene reactivity is strongly affected by substituent groups. Various singlet carbenes have been characterized as nucleophilic. 

Finally, addition reactions of the isolable phosphasilylcarbenes (13) to such electron-poor substrates as methyl acrylate, C4F9CH=CH2, and styrene afford cyclopropanes. The additions of 13a to (E)- or (Z)-p-deuteriostyrene are stereospecific, and the competitive additions of 13b to ring-substituted styrenes exhibit nucleophilic selectivity, consistent with singlet, nucleophilic carbene addition (Fig. 7.8). ... 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

---