Carbenes ambiphilic

A carbene carrying both a donor and an electron-withdrawing substituent presents a new pattern of reactivity, often called ambiphilic, since such species can show both nucleophilic and electrophilic properties. Thus chloro(methoxy)carbene 4.222 has a low enough energy LUMO, making it electrophilic towards simple alkenes, and yet a high enough HOMO to make it able to react with electrophilic alkenes like methyl acrylate.443 None of the carbenes discussed above is capable of both of these reactions. 

The account given so far leaves no room for anomalies, and yet they abound. Some of the nucleophilic carbenes do not react with the common electrophilic probes, and some of the electrophilic carbenes do not react with the common nucleophilic probes. Furthermore, there is quite frequently only a poor correlation between the calculated frontier orbital energies and the patterns of reactivity. The usual qualifications have to be invoked—that the frontier orbital theory is not a complete account of all the forces at work. One of the more obvious of the other forces is steric hindrance, of course, and another is that some carbenes are unselective, because they are so reactive that they are diffusion controlled.444 

Triplet carbenes have a similar set of molecular orbitals to those shown in Fig. 4.19, but with one electron in each of the orbitals n and pz. The shape of a triplet carbene may be anywhere between tetrahedral, if the singly occupied orbitals are localised, and linear, if they are well delocalised by substituents. This is especially noticeable when the carbene has two C-substituents like phenyl groups, which can overlap one with each of the unpaired electrons.446 The reactions triplet carbenes undergo follow the patterns of radical chemistry (Chapter 7). 

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Nucleophilic carbenes, which might show a different site selectivity, rarely undergo cycloadditions, but methoxychlorocarbene, an ambiphilic carbene, adds to the exocyclic double bond of 6,6-dimethylfulvene 6.278, to give the cyclopropane 6.277, whereas dichlorocarbene adds to one of the ring double... 

The problem of ambiphilic carbenes has also been considered a carbene. substituted both by an electron donating and an electron-attracting substituents, such... 

In the path b process, the addition of a heterosubstituted carbene to an olefin is governed by the reactivity of each component. Bis-thiomethyl- and bis-thiobenzylcarbenes add only to electron-rich olefins, whereas dimethoxycarbene adds primarily to electron-poor systems. Ambiphilic carbenes such as phenoxychlorocarbene and methoxychlorocarbene add to a wide range of double bonds. Table 16 lists the cyclopropanone equivalents which have been prepared in this manner. 

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. 

PhOCCl) and phenoxyfluoroearbene (PhOCF). [37] PhOCCl has w xy = 1.49 as ealeulated from Eq. 4, [36] nearly identieal to that of CF2, w xy = 1.48 (experimental) or 1.47 (calculated). [8] Despite the well expressed electrophilicity of CF2, [9] PhOCCl behaves as an ambiphile cf, Table 3. [38] Thus, W(2xy (calcd) = 1.49 already marks an empirical boundary for ambiphilic carbenes, at least as viewed through the lens defined by the aUcenes in Tables 2 and 3. PhOCF has w xy (calcd) = 1.74, considerably higher than that of PhOCCl, and 0.15 higher than that of the first ambiphile, MeOCCl (Table 2). The data of Table 3 indicate that PhOCF also behaves as an ambiphile. 

Finally, consider the addition of an experimentally ambiphilic carbene (MeOCCl) to a simple alkene (e.g., propene). Now the carbene and aUcene FMO s are roughly in balance both differential FMO energies will be comparable. This is depicted in the A Case of Fig. 6. Now changing the alkene s Me substituent to BuO will raise both the K and 7t orbital energies of the alkene, and convert the A Case to the N Case i.e., the addition reaction will become electrophilic. Alternatively, changing propene s Me substituent to CN will convert the A Case to the N Case the addition reaction will become nucleophilic. Of course, these imaginary manipulations describe the behavior of an ambiphilic carbene, and the expression of these thought experiments is represented by the relative reactivities of MeOCCl collected in Table 2. 

Fig. 8.3. Principal variants of relative positions of the energy levels of substituted carbenes CXY and ethylene depending on the substituents X, Y a electrophilic carbenes b nucleophilic carbenes c ambiphilic carbenes. The order of energy level is adhered to for CCI2 (a) and C(NH2)2 (b)...

It is less than unity for electrophilic carbenes, more than unity for nucleophilic carbenes and is approximately equal to unity in the case of ambiphilic carbenes. 

A characteristic of transition states of the carbene-to-alkene addition reactions, particularly sensitive to the philicity of a carbene, is the angle of slope of the carbene plane relative to the double-bond plane. According to calculations [44, 45] one may hold that the carbenes for which in the transition states of addition to alkenes the angle a < 45° are electrophilic. The angle a > 50° is typical of nucleophilic carbenes, while the 45° < a < 50° region relates to the ambiphilic carbenes. Ab initio [44, 52, 53] and semiempirical (MNDO) [54] calculations of pathways of addition reactions of various carbenes have verified this dependence. 

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