Ambiphilic selectivity

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]... 

In the present volume, our intention was to cover several modern approaches to phosphorus chemistry which were not, or at least not completely, covered in the previous volumes. The selected topics are expected to have broader relevance and to be interesting to a more general readership, since key aspects of phosphorus chemistry are pointed out. Indeed, several fields are investigated coordination chemistry, catalysis, supramolecular chemistry, biochemistry, hybrid organic-inorganic materials, new ambiphilic ligands, and biology. 

A few years later, Landis drew similar conclusions for the l,l -ferro-cenyldiphosphines 53a-c featuring pendant benzoxaborolidine moieties.63 Combined with rhodium precursors, these ambiphilic ligands lead to catalytically active species in hydrogenation and hydroformyla-tion reactions (Tables 7 and 8), but the presence of the Lewis acid moiety has no significant effect on activity and selectivity (compared to ligands 51 and 52). 

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... 

Alkoxy(or -aryloxy)halocarbenes are ambiphilic (for selectivity indices, see ref 9), they react at a high rate with electron-rich and electron-poor alkenes, however, they react at a slow rate, with alkenes of intermediate reactivity. Steric hindrance decreases the rate addition of fluoro- and chloro(phenoxy)carbene to 2,3-dimethylbut-2-ene. ° For discussion of structure-activity relationship of carbenes, see ref 9, and Houben-Weyl, Vol. El9b, pp 14-34. 

Ambiphilic ligands were investigated early on in Si—Si and C—C coupling with the aim to substantiate the beneficial influence the introduction of boranes or alanes may have. Although the exact role of the Lewis acid moiety during catalysis remains generally unclear, comparison with Lewis acid-free systems has often revealed significant improvments in terms of activity and/or selectivity. 

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. 

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. 

PhCF and PhCCl appear to be electrophilic their selectivities resemble that of MeCCl in that all three carbenes are less reactive toward methyl acrylate and acrylonitrile than trans-butene. Moreover, PhCF and PhCCl certainly differ from ambiphilic MeOCCl, which is much more reactive toward the electron-... 

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]... 

What about the classic ambiphile, MeOCCl In Table 8, we summarize values for MeOCCl, determined by a combination of absolute and relative rate measurements. [101] Also included are analogous data for PhCOMe, [102] MeCOMe, [73] and MeCOCH2CF3. [103] For MeOCCl, we note that the ambi-philic reactivity pattern emerges from the absolute rate constants of Table 8 as clearly as it does from the relative rate constants of Table 4 high reactivity toward electron-rich or electron-poor alkenes, but low reactivity toward aUcenes of intermediate electron density. However, whereas the relative rate data can only inform us about the carbene s selectivity pattern, the absolute rate data reveals the carbene s true reactivity. In fact, for the addition of MeOCCl to trans-butene (3.3 x 10 M s ) is the lowest bimolecular rate constant yet measured for a carbene/aUcene addition in solution. [101] And with 1-hexene, only 5% of MeOCCl addition was observed this reaction is so slow that other competitive processes prevail. [101]... 

From the data of Table 9, we can see that although both earbenes are ambiphilic, the nucleophilic properties of PhCOMe (as revealed by its reactivity toward the electron-poor alkenes) are much more pronounced than those of PhCOAc. However, from the values, we observe that PhCOAe is somewhat more reactive than PhCOMe. Thus, PhCOAc is both more reaetive and less (nucleophilically) selective than PhCOMe. A simple explanation takes note of the resonance representation for PhCOAe, 35. [105] Here, the typieal methoxy group resonanee donation (ef, 4) also operates for PhCOAe, as represented by... 

Similar results were reported by Platz et al. [75] Interestingly, positive p values were observed not only for reaetions of ArCCl with (e.g.) 1-pentene (-1-1.3), and 1-hexene (+1.3), but also with eleetron defieient alkenes, (e.g.) diethyl fumarate (+ 0.45) and ethyl aerylate (+0.75). The Hammett rho values thus suggest that these latter earbene additions are eleetrophilie, even though the selectivity and reactivity of PhCCl toward the set of alkenes in Table 7 is clearly parabolic and ambiphilic. One does note, however, that the electrophilicity of the additions, expressed by p, deereases as the alkenes become less electron-rieh. Thus p deereases from +2.49 for butyl vinyl ether, to +1.3 for 1-pentene, to +0.45 for diethyl fumarate. [75] A more detailed explanation, based on the Houk enthalpy/entropy analysis, [97] has been offered. [75]... 

Nitrocyclopropanation of a,p-unsaturated ketones 16usingbromonitromethaneas an ambiphilic substrate in the presence of organocatalysts E-I allows the preparation of several interesting trisubstituted cyclopropanes 17 in high levels of both diastereo and enantioselectivities. A general scheme compiling selected recent achievements on this purpose is depicted in Scheme 5.7. Chiral primary [25] and secondary [26, 27] amines as well as thiourea [28] and squaramide [29] derivatives E-I (all of them as bifunctional catalysts) were capable of catalyzing the transformation. 

The use of [IrH5(PPr 3)2] 987 as catalyst for the selective C-C bond cleavage of mono- and dinitriles has been reported. This compound behaves as an ambiphilic catalyst for bringing about the one-pot three-component assemblage of alkenes, nitriles, and water resulting in the formation of glutarimides. ... 

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