Ambiphiles, singlet carbenes

Carbene reactivity is strongly affected by substituents.117 Various singlet carbenes have been characterized as nucleophilic, ambiphilic, and electrophilic as shown in Table 10.2 This classification is based on relative reactivity toward a series of both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituent to act as an electron donor. For example, dimethoxycarbene is devoid of electrophilicity toward alkenes because of electron donation by the methoxy groups.118... 

Carbenes as divalent carbon species and their analogue carbenoids can form two bonds on a single carbon atom center introducing two different groups at one time or consecutively.1 This is due to the ambiphilic character of singlet carbenes or triplet biradical character, which can be controlled by changing the... 

The philicity of singlet carbenes is an important concept to classify carbenes that was systematically studied by Moss. [9-11] The relative reactivity (selectivity) of a series of singlet carbenes in cyclopropanation reactions with electron rich and electron poor carbenes was used to quantify the carbene philicity. An empirical carbene philicity scale with a parameter niQ- (where X and Y are the substituents at the carbene center) was defined (Figure 1). Electrophilic carbenes show Wqxy values below 1, nucleophilic carbenes above 2, and ambiphiles are between. [10] Ambiphilic carbenes act as an electrophile towards electron-rich alkenes and as a nucleophile towards electron-poor alkenes. The niQ- Y values obey an empirical linear free energy relationship with the Taft substituent parameters and Oj. This allows to estimate the niQ- Y values of unknown carbenes. 

We should also note in passing that triplet carbenes also seem to react with substituted styrenes with ambiphilic selectivity. Thus, both triplet 9-xanthylidene, 16, [53] and diphenylcarbene, 17, [54] add to X-C( H4CH=CH2 (X = p-MeO, p-M.e, H, p-C, p-Br, and w-Br), and afford broken Hammett patterns with k i minima at X = H (a = 0.0). The ambiphilicity of these triplet species is distinct from that of the singlet carbenes, MeOCCl or PhOCF, discussed above. The philicity of the latter can be understood by molecular orbital... 

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]... 

The electrophilic, ambiphilic and nucleophilic characters of a range of singlet carbenes have been compared to their intrinsic reactivity index This quantification is supposed to evaluate the nucleophilic and electrophilic properties of compounds and has been previously assessed for a range of organic compounds but appears unadapted to singlet carbene and presents no advantages compared to frontier molecular orbital and empirical evaluations. 

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