Nucleophiles properties

In a sophisticated variation of the Knoevenagel condensation ("Panizzi ) methyl 3,3-dimethoxypropanoate (from ketene and dimethoxymethenium tetrafluoroborate D.J. Crosby, 1962) is used as a d -reagent. Because only one carbonyl group activates the methylene group, a strong base with no nucleophilic properties (p. 10) has to be used. A sodium-sand mixture, which presumably reacts to form silicate anions in the heat, was chosen... 

The nucleophilic properties of the 2-iminonitrogen, however, may be masked under certain conditions as shown by Scheme 225 (710). 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

The nucleophilic properties of phosphorus in phosphaferrocene were demonstrated by reaction with n-butyllithium occurring at the phosphorus atom (81IC3252 820M312). 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

The carbonyl group also possesses electrophilic properties at the carbon atom and nucleophilic properties at the oxygen atom. Nucleophilic attack of the carbonyl group is favored if this is attached to an aromatic ring (inductive effect) and there is also a methoxy or phenolic OH group present in the 4-position. Changing a neutral reaction medium by proton addition has the same effect. 

C-H acidic compounds do not possess any basic properties. But they can form anions in the presence of strong bases, and these possess sufficiently strong nucleophilic properties to be able to add to a polarized carbonyl group. Examples are listed in Table 6. 

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

The following chapter concerns another kind of low-valent organophosphorus compounds, namely phosphinidenes. Little is known about free phos-phinidenes in contrast to the corresponding transition metal complexes. Many new reagents have been generated exhibiting either electrophilic or nucleophilic properties. The reactivity of these carbene-like reagents is evaluated (K. hammer tsma). 

With the iron atom in its most negative oxidation state of —2 this complex possesses nucleophilic properties and thus can be used in nucleophilic substitution reactions. As the iron atom in this complex formally has ten valence electrons, it is isoelectronic with Pd(0), which is a well-known catalyst in allylic substitution reactions [49]. 

The nucleophilic property of water seems sufficient to explain the following... 

Structure B is of most interest. It is responsible for the activity of nitronates as 1,3-dipoles in [3+ 2]-cycloaddition reactions. This is the most important aspect of the reactivity of nitronates determining the significance of these compounds in organic synthesis (see e.g., Ref. 267). In addition, this structure suggests that nitronates can show both, O -nucleophilic properties, that is, react at the oxygen atom with electrophiles, and a-C-electrophilic properties, that is, add nucleophiles at the a-carbon atom. 

The resonance structures of nitronates are most similar to those of nitrones, but nitronates have the additional structure D. Strange as it may seem, the contribution of this structure more likely slightly diminishes a-C-electrophilic activity of nitronates, move than is favorable for the appearance of the nucleophilic properties. In any case, no transformations, in which nitronates unambiguously act as C-nucleophiles, have been rigorously established. 

The foregoing sections have dealt with the nucleophilic properties of anionic species whose salts were solubilized in apolar solvents with the aid of cation-complexing agents. Apart from the nucleophilic character of the anion, however, its basic strength is also expected to increase with decreasing extent of solvation. This section will deal with hydrogen-abstraction reactions of crown ether-solubilized bases. 

It should be kept in mind that the terms acidity and basicity of the solvent have to be intended not only according to the Lewis concept (electrophilic vs. nucleophilic properties), but also according to the Bronsted concept (proton donor vs. proton acceptors), or to the hydrogen bonding capacity (hydrogen bond donor vs. hydrogen bond acceptor). 

The significant, often decisive, influence of the solvent in chemical reactions similarly is valid for electrochemical reactions too, for example, due to protic or aprotic and electrophilic or nucleophilic properties. If an excess of reactants can be used as solvent, a particularly uncomplicated operation will be possible. An additional solvent should be inert. The requirements for the solvent in dissolving power for reactants and products and the criteria regarding an easy separation of the products from the reaction mixture, for example, the boiling point, are comparable for chemical and electrochemical conversions. Generally, there is an interest to use, as far as possible, inexpensive, nontoxic, and easy to handle solvents. 

The nucleophilic properties of the media are crucial for the nature of the methylthio products formed. With pyridine, a product of vicinal methylthiation-pyridination was obtained. In pure acetonitrile the solvent itself plays the role of a nucleophile [127], whereas in a low-nucleophilic liquid SO2 a product of... 

The nucleophilic property of the encapsulated anion was dramatically changed compared to the free anion. Thus, [2-Ag4(/U-Cl)4(/U4-Cl)] acts as a nucleophile to convert alkyl-iodide to alkyl-chloride in high yield. Puddephatt proposed a mechanism where the [2-Ag4(/U-Cl)4(/U4-Cl)] anion reacts with alkyl-iodide to give [2-Ag4(/z-Cl)4(/U4-I)] and alkyl-chloride. The reaction occurs in high yield with the reactivity sequence tertiary>second-ary>primary alkyl. 

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