Anion chemistry

Much of the chemistry of oxygen can be rationalized in terms of its electronic structure (2s 2p ), high electronegativity (3.5) and small size. Thus, oxygen shows many similarities to nitrogen (p. 412) in its covalent chemistry, and its propensity to form H bonds (p. 52) and p double bonds (p. 416), though the anionic chemistry of 0 and OH is much more extensive than for the isoelectronic ions N , and NH2. Simi-... 

The polynuclear hydroxo-anions isolated as solids will also be present in solution. Their existence points to a chemistry that is formally comparable to polynuclear oxy-anion chemistry—compare [Be2(OH)7]3 to Cr20 . In both cases the protonation of a divalent mononuclear anion results in the formation of a binuclear bridged structure. 

The aldol reactions introduced thus far have been performed under basic conditions where enolate species are involved as the reactive intermediate. In contrast to the commonly accepted carbon-anion chemistry, Mukaiyama developed another practical method in which enol species can be used as the key intermediates. He is the first chemist to successfully demonstrate that acid-catalyzed aldol reactions using Lewis acid (such as TiCU) and silyl enol ether as a stable enol equivalent can work as well.17 Furthermore, he developed the boron tri-fluoromethane sulfonate (triflate)-mediated aldol reactions via the formation of formyl enol ethers. 

In the presence of catalytic amounts of sodium methoxide, dimethylketene /3-lactone dimer is polymerized at moderate temperature to a polyester.3 At higher temperatures (above 100°), disproportionation to the cyclic trimer, hexamethyl-1.3,5-cyclohexanetrione, takes place.9 Addition of a stoichiometric amount of sodium methoxide to the lactone dimer generates the sodium enolate of methyl 2,2,4-trimethyl-3-oxovalerate. This reaction provides a convenient entry into certain ester anion chemistry that formerly required the use of a strong base like tritylsodium.10... 

Transfer of the acyl group from the acylzirconocene chloride to aluminum (transmetala-tion) by treatment with aluminum chloride has been reported to give an acylaluminum species in situ, and the possibility of the acylaluminum acting as an acyl anion donor has been suggested (Scheme 5.5) [7]. However, the acyl anion chemistry through this trans-metalation procedure appears to be limited since only protonolysis to the aldehyde proceeds in good yield, which could be achieved by direct hydrolysis of the acylzirconocene chloride. 

The acyl anion chemistry of acylzirconocene chlorides has also been applied to the stereoselective preparation of ( )-a,(3-unsaturated selenoesters and telluroesters (Scheme 5.35) [38]. Although no carbon—carbon bond was formed, this reaction reflects the synthetic interest in ( )-a,(3-unsaturated selenoesters and telluroesters, which are well-known precursors of acyl radicals and acyl anions, respectively. 

A rather nice example of enolate anion chemistry involving the Michael reaction and the aldol reaction is provided by the Robinson annulation, a ring-forming sequence used in the synthesis of steroidal systems (Latin annulus, ring). 

Hydroxycoumarin can be considered as an enol tautomer of a 1,3-dicarbonyl compound conjugation with the aromatic ring favours the enol tautomer. This now exposes its potential as a nucleophile. Whilst we may begin to consider enolate anion chemistry, no strong base is required and we may formulate a mechanism in which the enol acts as the nucleophile, in a simple aldol reaction with formaldehyde. Dehydration follows and produces an unsaturated ketone, which then becomes the electrophile in a Michael reaction (see Section 10.10). The nucleophile is a second molecule of 4-hydroxycoumarin. 

The normal Hantzsch synthesis leads to a symmetrical product. The diesters formed may be hydrolysed and decarboxylated using base to give pyridines with less substitution. Note that we are using the ester groups as activating species to facilitate enolate anion chemistry (see Section 10.9)... 

Let us now look at an example of how nature exploits the equivalent of enol and enolate anion chemistry. Enolization provides another application... 

Mechanistically, we can consider it as attack of an enolate anion equivalent from acetyl-CoA on to the ketone group of oxaloacetate. However, if we think carefully, we come to the conclusion that this is not what we would really predict. Of the two substrates, oxaloacetate is the more acidic reagent, in that two carbonyl groups flank a methylene. According to the enolate anion chemistry we studied in Chapter 10, we would predict that oxaloacetate should provide the enolate anion, and that this might then attack acetyl-CoA in a Claisen reaction (see Box 10.4). The product expected in a typical base-catalysed reaction would, therefore, be an acetyl derivative of oxaloacetate. 

The reaction of an amino group with an aldehyde or ketone leads to an imine, which, as we have just seen with aldolase, provides a splendid example of how to bond a carbonyl substrate to an enzyme, and yet maintain its chemical reactivity in terms of enolate anion chemistry. Another type of covalent interaction is quite commonly encountered, and this exploits the thiol group of cysteine. Thiols are more acidic than... 

These provide examples of enolate anion chemistry. This can be surmised from the variety of bond-forming reactions on ketone or ester substrates all achieved under basic conditions. 

The first reaction involves a ketone reaction with an aldehyde under basic conditions, so enolate anion chemistry is likely. This is a mixed aldol reaction the acetone has acidic a-hydrogens to form an enolate anion, and the aldehyde is the more reactive electrophile. The reaction is then driven by the ability of the intermediate alcohol to dehydrate to a conjugated ketone. 

Ozone, 17 63 depletion, 46 109-110 fluoride, see Trioxygen difluoride Ozonide radical anion, chemistry, 33 76... 

The development of anionic chemistry over the past 30 years has led to the emergence of new processes and products of Industrial Importance, the most significant being a family of thermoplastic elastomers. These unique elastomers are presently commercialized by Shell Chemical Company as Kratons and by Phillips Chemical Company as Solprenes. Their uniqueness is the result of deliberate design of the polymeric structure and composition. 

MACROMER (10) is a trademark by CPC International of a new family of monomers. Because they are synthesized via anionic chemistry, their molecular weight is controlled by the ratio of monomer to initiator and they also have very narrow molecular weight distributions. The typical polymeric portions of MACROMEHf that have been investigated are polystyrene, polydiene, and blocks of the two (5, lCi). Some of the typical MACROMER functional groups that were examined are shown in Figure 8. These are shown to indicate the wide variety of functional groups that are useful for various polymerization mechanisms (4). 

The side chains are of a predetermined size and composition via anionic chemistry and based on the reactivity placed at the terminus of the MACROMER and the comonomer with which it will be reacted. The number of side chains per backbone and a distance apart can be reasonably estimated. This distance between side chains must be sufficient that the backbone can manifest its Tg. Considering the earlier comments of molar concentration of the MACROMER in a typical copolymerization recipe, there will not be many MACROMERS per backbone on a statistical basis. 

The development of anionic chemistry has placed a number of powerful tools in the hands of the polymer chemist. Polymer molecules of predetermined molecular weight, molecular weight distribution, composition, and configuration can now be synthesized nearing the purity of simple organic molecules. Controlled polymeric structures have been realized that are highly desirable as models to advance theoretical studies and indeed have vast economic values to industry as profitable consumer items. 

In recent times many advances in isopoly anion chemistry have been made by shifting reaction chemistry from aqueous to aprolic solution. This can often be done by employing a solubilizing cation such as tetrabutyfammonium ion. For example, when [(n-Bu)4N]0H and [(/i-Bu)4N][H7V ,OjK] are mixed in acetonitrile a new isopolyvanadate forms 53... 

In keeping witii its 5d 6s2 electron configuration, tungsten forms many compounds in which its oxidation state is 6+, just as molybdenum does. It forms divalent and tetravalent compounds to about the same extent as molybdenum but its bivalent and pentavalent compounds are somewhat fewer. Its anion chemistry is closely akin to that of molybdenum. 

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