Ambiphilicity

Photochemically produced chloromethoxycarbene revealed ambiphilicity. Relative rates of cyclopropanation showed that electron donating as well as electron attracting substituents favor the reaction (Scheme 2) (79JA4736). 

However, consideration of polar factors in the traditional sense does not provide a ready explanation for the regiospecificity shown by the r butoxy radicals (which arc electrophilic, Tabic 1.3) in their reactions with the tluoro-olcfins (Tabic 1.2).22,23 Apparent ambiphilicity has been reported21 for other not very electrophilic radicals in their reactions with olefins and has been attributed to the polarizability of die radical. 

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... 

Gudat D (2003) Zwitterionic Phospholide Derivatives - New Ambiphilic Ligands. 232 175-212... 

This scheme can be extended by using mixtures of dienes with electron-deficient alkenes such as acrylonitrile. Due to its nucleophilic nature, addition of radical 68 to acrylonitrile is faster than addition to butadiene. The resulting ambiphilic adduct radical then adds to butadiene to form a relatively unreactive allyl radical. Oxidation and trapping of the allyl cation by methanol lead, as before, to products such as 72 and 73, which are composed of four components the radical precursor 67, acrylonitrile, butadiene and methanol (equation 30)17,94. 

Although we are not specifically concerned here with kpp and the kinedcs of carbene-pyridine ylide formation, we note that the magnitude of is directly related to the structure and reactivity of the carbene. fcpyr ranges from 105 M s-1 for ambiphilic alkoxycarbenes to 109-10I° M-1 s 1 for electrophilic halocarbenes or alkylcarbenes. Very nucleophilic carbenes (MeOCOMe) do not react with pyridine.13... 

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... 

Kondo and Watanabe developed allylations of various types of aldehydes and oximes by using nucleophilic (7r-allyl)ruthenium(ll) complexes of type 154 bearing carbon monoxide ligands (Equation (29)).345 These 73-allyl-ruthenium complexes 154 are ambiphilic reagents and the presence of the carbon monoxide ligands proved to be essential to achieve catalytic allylation reactions. Interestingly, these transformations occur with complete regioselectivity only the more substituted allylic terminus adds to the aldehyde. 

The ambiphilic reactivity of aromatic cation radicals, as described in Schemes 12 and 13, is particularly subtle in the charge-transfer nitration of toluene and anisole, which afford uniformly high (>95%) yields of only isomeric nitrotoluenes and nitroanisoles, respectively, without the admixture of other types of aromatic byproducts. Accordingly, let us consider how the variations in the isomeric (ortho meta para) product distributions with... 

The 772-phosphinocarbene complexes of tungsten show ambiphilic behavior. With Lewis acids such as MeS+, electrophilic attack occurs at the metal-carbon double bond affording the dicationic tunstaphosphathiabi-cyclo[1.1.0]butane complexes 102.96,97 On the other hand, nucleophiles such as trialkyl phosphines or the cyclopentadienyl anion C5H5 add at the carbe-nic center, affording phosphoranylidene complexes 10397 or tungstaphos-... 

An Improved Preparation of 3-Bromo-2(H)-pyran-2-one An Ambiphilic Diene for Diels-Alder Cycloadditions. 

In spite of the general ambiphilicity of phosphonio-substituted phosphoHde derivatives, the aromaticity of the phosphoHde ring [10, 11] tends to reduce their electrophilicity while the intramolecular compensation of the negative charge by the phosphonio-substituents lowers at the same time their nucle-ophilicity [15, 16]. Bis-phosphonio-benzophospholides and -1,2,4-diaza-phospholides are therefore less reactive towards electrophiles and nucleophiles than other types of phosphorus containing multiple-bond systems and lack the notorious hydrolytic instabihty of many of these species [15, 16, 24]. Reactions are observed, however, with sufficiently strong electrophiles such as triflic acid or methyl triflate, or nucleophiles such as OH" or lithium alkyls, respectively. 

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. 

Chapter 6, written by D. Gudat, is concerned with zwitterionic phospholide derivatives which can be considered as useful ambiphilic ligands. This review gives an account of the current knowledge on the synthesis, physical properties, and - in particular - chemical behavior of phosphino-substituted phospholide derivatives. 

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... 

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. 

If CCI2 and CH3OCCH3 represent electrophilic and nucleophilic carbenes, respectively, in which the transition states predominantly resemble 7a or 7b, can we identify carbenes for which both p-n and a-7i electronic interactions are comparably important Such carbenes would react as electrophiles with electron-rich alkenes, but as nucleophiles with electron-poor alkenes that is, they would be ambiphiles Indeed, Table 7.3 shows that CH30CC1 and CgHsOCCl behave in just this manner. Their relative reactivities are high toward both electron-rich and electron-poor alkenes, but lower toward alkenes that are electronically moderate (trawi-butene or 1-hexene ). 

Most strikingly, the Ae values for CH3OCCI in Table 7.4 cross-over. For the electron-rich alkenes, Aee < As, whereas As < Aee for the electron-poor alkenes. Here is a clear prediction that CH3OCCI should behave as an ambiphile, reacting rapidly with both electron-rich and electron-poor alkenes, but reacting more slowly with alkenes of modest electronic properties. Again, the relative reactivities of Table 7.3 are consonant with these ideas. 

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