Other Electrophiles

Other electrophiles include [CPh3] to replace H, as in Mn(=C=CHPh)(CO)(PPh3) 

Reactions with dioxygen generally afford the corresponding metal carbonyl derivatives, with loss of organic aldehyde or acid. This reaction can be expressed as an analog of multiple bond metathesis and corresponds to oxidation of vinylidene to CO [46]. Oxidation of OsHCl(=C=CHPh)(L)2 affords the styryl complex 0sC102( -CH=CHPh)(L)2 [243]. 

A previous report demonstrated that esters, alkyl halides, vinyl ethers, acetic anhydride and epoxides did not react under MBH reaction conditions or give intractable mixtures however, many kinds of electrophiles have been 

A three-component solution phase synthesis of substituted sulfonamides by reaction of an acrylic ester, an aldehyde and toluenesulfonamide under MBH reaction conditions has been reported. The reaction is catalyzed by 

Ar = Ph, 3-CIPh, 3-N02Ph, 4-N02Ph, 4-MeOPh, 2-naphthyl, 2-furanyl, 2-pyiidyl  

Lamaty et al. further presented the first examples of the A-supported aza-MBH reaction between PEG-SES amine 220, prepared from a novel SES-type linker attached to an appropriate PEG polymer, acrylate and aldehydes 

8 equiv TiCl4, 12 equiv EtsN, 0.1 equiv PPhs, CH2CI2  

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In superacidic systems, water is completely protonated and no equilibrium containing free water is indicated. However, the nonbonded electron pair of H30 is still a potential electron donor and at very high acidities can be further protonated (however limited the equilibrium with H30 may be). Thus the acidity of such superacidic systems can exceed that of H30 and the leveling ont is by that of H40 . We found that similar situations exist with other electrophiles, raising their electrophilic nature (electrophilicity) substantially. 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Nitration can be effected under a wide variety of conditions, as already indicated. The characteristics and kinetics exhibited by the reactions depend on the reagents used, but, as the mechanisms have been elucidated, the surprising fact has emerged that the nitronium ion is preeminently effective as the electrophilic species. The evidence for the operation of other electrophiles will be discussed, but it can be said that the supremacy of one electrophile is uncharacteristic of electrophilic substitutions, and bestows on nitration great utility as a model reaction. 

The results in table 2.6 show that the rates of reaction of compounds such as phenol and i-napthol are equal to the encounter rate. This observation is noteworthy because it shows that despite their potentially very high reactivity these compounds do not draw into reaction other electrophiles, and the nitronium ion remains solely effective. These particular instances illustrate an important general principle if by increasing the reactivity of the aromatic reactant in a substitution reaction, a plateau in rate constant for the reaction is achieved which can be identified as the rate constant for encounter of the reacting species, and if further structural modifications of the aromatic in the direction of further increasing its potential reactivity ultimately raise the rate constant above this plateau, then the incursion of a new electrophile must be admitted. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

Ten years ago we became interested in the possibility of using nitration as a process with which to study the reactivity of hetero-aromatic compounds towards electrophilic substitution. The choice of nitration was determined by the consideration that its mechanism was probably better imderstood than that of any other electrophilic substitution. Others also were pursuing the same objective, and a considerable amount of information has now been compiled. 

The electrophilic site of an acyl cation is its acyl carbon An electrostatic poten tial map of the acyl cation from propanoyl chloride (Figure 12 8) illustrates nicely the concentration of positive charge at the acyl carbon as shown by the blue color The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Figure 12 9)... 

Other Electrophilic Reactants. ReversibHity of the electrophilic reactions enables substituted dye derivatives to be obtained. Thus, the halogenation of cyanines, oxonoles, and merocyanines has been studied (3,65,66). Halogen atoms are mobHe in the polymethine chain, and the derivatives themselves can function as halogenation reagents. 

As with addition of other electrophiles, halogenation of conjugated dienes can give 1,2- or 1,4-addition products. When molecular bromine is used as the brominating agent in chlorinated hydrocarbon solvent, the 1,4-addition product dominates by 7 1 in the case of butadiene. ... 

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Because nitration has been studied for a wide variety of aromatic compounds, it is a useful reaction with which to illustrate the directing effect of substituent groups. Table 10.3 presents some of the data. A variety of reaction conditions are represented, so direct comparison is not always valid, but the trends are nevertheless clear. It is important to remember that other electrophiles, while following the same qualitative trends, show large quantitative differences in position selectivity. 

Because of the limited range of aromatic compounds that react with diazonium ions, selectivity data comparable to those discussed for other electrophilic substitutions are not available. Because diazotization involves a weak electrophile, it would be expected to reveal high substrate and position selectivity. 

Reactions of Heterocyclic Enamines with Other Electrophilic Reagents... 

Other electrophilic substitutions proceed with difficulty, or not at all. Nitrosation and diazo coupling require the presence of the strongly activating dimethylamino group (see Section VIII). Bromine adds, in the presence of sunlight, to give tetrabromotetrahydrobenzofuroxan (48) the initial attack is probably free-radical in nature. The product can be dehydrobrominated to form 4,7-, or a mixture of 4,5- and 4,6-dibromobenzofuroxan, depending upon the conditions. More conventional electrophilic bromination conditions have been tried in an attempt to obtain a monosubstituted product, but without success. 

A somewhat similar method to that mentioned above involves alkylation of 4,4-dimethyl-1,3-oxathiolane 5,5-dioxide 32 at the 2 position followed by FVP at 400°C, which results in fragmentation with loss of SO2 and isobutene to give the aldehydes 33. Other electrophiles that may be used include aldehydes, ketones, and McsSiCl, making this a convenient formyl anion equivalent (79TL3375). 

As with other electrophilic substitution reactions, there is practically no work available on the halogenation of isoxazoles with functional substituents. The only instance that indicates that the general pattern holds true here is the extremely rapid bromination of 3-anilino-5-phenylisoxazole (65), in which the isoxazole ring is the first to react with 1 mole of bromine, yielding... 

Many7 other electrophiles besides HBr add to conjugated dienes, and mixtures of products are usually formed. For example, Br2 adds to 1,3-butadiene to give a mixture of l,4-dibromo-2-butene and 3,4-dibromo-l-butene. 

Reports of the generation and subsequent electrophile trapping of nonstabilized metalated aziridines appeared before those for metalated epoxides. Desulfinylation of sulfinylaziridine 250 with EtMgBr gave metalated aziridine 251, which, remarkably, could be kept at 0 °C for 1 h before quenching with D2O (Scheme 5.64). The deuterated aziridine 252 (E = D) was obtained in excellent yield, but acetaldehyde was the only other electrophile found to be trapped efficiently [90],... 

It should be noted that epoxidation of a dienone with mCPBA or other electrophilic epoxidation reagents proceeds with complementary regioselectivity, yielding y,8-epoxy enones instead of the ot,P-epoxy ketones discussed above. This feature has been utilized in several natural product syntheses Scheme 9.8 demonstrates... 

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