Ambiphilic character

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 particular substitution pattern of lithium carbenoids, the fact that both an electropositive metal and an electronegative substituent X are bound to the same carbon atom, causes the ambiphilic character of this species. The chameleon-like reactivity becomes evident from the resonance formulas of the carbenoid lb (equation 1) Whereas the carbanionic character is expressed by the resonance formula la, the electrophilic character is represented by Ic. In an analogous way, the reactivity of vinylidene carbenoids 2b is expressed by the mesomeric structures 2a and 2c. 

The chemistry of lithium carbenoids, organohthium compounds with an electronegative heteroatom in the a-position, has been developed in two respects during the last two decades. First, the ambiphilic character of these partly short-lived and mostly thermally sensitive compounds is well understood today, due to structure determinations, spectroscopic investigations and theoretical calculations. Second, many of the lithium carbenoids, formerly considered as exotic species, have developed into useful reagents that proved to be particularly fruitful in stereoselective syntheses. 

With the ort/io-phenylene linker, the formation of intramolecular P-B interaction is quasi-thermoneutral. As a consequence, the corresponding PBs retain their ambiphilic character whatever their ground-state structure (open, closed, or equilibrium between the two forms), and thus coordinate to transition metals with participation of both antagonist sites (see Section IV). 

The ambiphilic character of JT-allylmthenium complexes is in remarkable contrast to palladium chemistry [29]. A series of (jt-C3H5)RuX(CO)3 (X = Br, OAc or OTf) complexes prefer the attack of electrophiles such as aldehydes as well as the attack of nucleophiles such as NaCH(C02Me)2, while Jt-allylpalladium complexes react exclusively with nucleophiles. Thus, stoichiometric reactions of Jt-allylmthenium complex with benzaldehyde and the sodium salt of diethyl malonate afford the corresponding homoallyl alcohol and allylmalonate, respectively (Scheme 5.1). The carbonyl ligand plays a very important role, and ambiphilic reactivity is realized only in ruthenium complexes bearing a carbon monoxide ligand. 

Fig. 10.13. Hammett plots of cycloaddition rates of substituted aryl azides with nucleophiUc, electrophilic, and unsubstituted alkenes showing the ambiphilic character of the azide cycloaddition. Reaction with maleic anhydride (electrophilic) is favored by donor substituents. Reaction with pyrroUdinocy-clohexene (nucleophilic) is favored by acceptor substituents.

A subsequent study by Goddard and Periana [31] in the intermolecular C—H activation of benzene and methane by [Ir(acac )2(X)] complexes (X = CH3COO and OH) also facilitates an easy bond activation process and demonstrates a similar ambiphilic bonding character. An acetate-assisted deprotonation via a 6-membered transition state was computed as the lowest-energy process in both cases. This underlines again the high potential of such ambiphilic character for an easier C—H activation process. 

The bisindole alkaloids of Catharanthus exhibit ambiphilic behavior in solution, owing, in large part, to their gross lipophilic character combined with the presence of four nitrogen atoms of varying basicity. The upper... 

Data is also available for addition reactions of electrophilic radicals, CH(CN)2, and so-called ambiphilic radicals, CH(C02Et)2 and CH2C02-/-Bu, which derive their electrophilic character from n-delocalization of the carbon-centered radical onto electron-attracting substituents [108-111], and for which the enthalpy of the addition process rather than the polar nature of the radicals may be the primary rate-determining factor [112,113]. 

An (alkoxysilyl)lithium, ( -BuO)Ph2SiLi, can be prepared from (t-BuO)Ph2SiSnMe3 by treatment with n-BuLi in THF (Scheme 10). This species is stable at -78°C for a few hours and behaves as an ordinary nucleophilic silyl anion 54d). (f-BuO)Ph2SiLi also behaves as an electrophile under certain conditions It undergoes self-condensation smoothly or butylation in the presence of an excess amount of n-BuLi and TMEDA (Scheme 10). (t-BuO)Ph2SiLi thus exhibits an ambiphilic nature, that is, silylenoid character. The ambiphilic reactivity of (t-BuO)Ph2SiLi, how-... 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

An interesting series of compounds for which a fairly broad range of data exists is diazomethane, methyl diazoacetate, and diethyl diazomalonate, in which each additional ester group should make the 1,3-dipole successively more electrophilic. The data are given in Table 10.4. We see that diazomethane is primarily nucleophilic in character, dropping sharply in reactivity from electrophilic to nucleophilic dipolarophiles. The other two reactants clearly show an ambiphilic reactivity. These reagents show increased reactivity with both EWG and ERG dipolarophiles, with the diazomalonate shifted somewhat more toward electrophilic character. 

By about 1980, a reasonable empirical understanding of carbenic philicity seemed to be at hand electrophiles, ambiphiles, and nucleophiles had been identified and could be assigned m xy or p xv values that reflected the magnitude and character of their selectivity during their additions to alkenes. And yet, our rationalization of these reactions only in terms of relative reactivities and linear free energy relations was surely incomplete. 

Nevertheless, the use of relative reactivities to characterize carbenic philicity is restrictive the apparent philicity is related to the alkenes selected for the relative reactivity measurements. What if the set of alkenes were expanded by the addition of an even more electron-deficient alkene Such a test was applied in 1987 [65], using a-chloroacrylonitrile, 26, which is more 7t-electron deficient than acrylonitrile, 27. We found that PhCF or PhCCl added 15 or 13 times, respectively, more rapidly to 26 than to 27. In preferring the more electron-deficient olefin, the carbenes exhibited nucleophilic character. However, because they also behave as electrophiles toward other alkenes (Table 4), they must in reality be ambiphiles. In fact, we now realize that all carbenes have the potential for nucleophilic reactions with olefins the crucial factor is whether the carbene s filled a orbital (HOMO)/alkene vacant Ji orbital (LUMO) interaction is stronger than the carbene s vacant p orbital (LUMO)/aIkene filled k orbital (HOMO) interaction in the transition state of the addition reaction. [63]... 

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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