Electrophiles carbon

The next example uses another carbon electrophile how can you use the relationship of the two functional groups in TM 249 to design a synthesis of the molecule ... 

It will be easy to put in bond a because it is para to the MeO group, but bond b might be difficult Tire strategy is to put in bond a first and use an intramolecular reaction to force bond b to go in the right place. Succinic anhydride is a convenient four carbon electrophile ... 

The dianions of methylated thiophenecarboxylic acids e.g. 155) are also readily generated by reaction with LDA they undergo preparatively useful reactions with a range of carbon electrophiles (80JOC4528). 

Other carbon electrophiles which are frequently employed include aldehydes, ketones, esters, nitriles and amides of the type RCONMei. An indirect method of acylation involves the initial reaction of a lithio compound with an aldehyde followed by oxidation of the resulting secondary alcohol to the corresponding acyl derivative. 

Another useful way to think about carbon electrophilicity is to compare the properties of the carbonyls lowest-unoccupied molecular orbital (LUMO). This is the orbital into which the nucleophile s pair of electrons will go. Examine each compound s LUMO. Which is most localized on the carbonyl group Most delocalized Next, examine the LUMOs while displaying the compounds as space-filling models. This allows you to judge the extent to which the LUMO is actually accessible to an approaching nucleophile. Which LUMO is most available Least available ... 

Enolate reactivity depends on the electrophile. Enolates generally form CC bonds with carbon electrophiles, and OSi bonds with silicon electrophiles. 

Coupling reaction of organotin compounds with carbon electrophiles... 

Together with reactions named after Heck and Suzuki, the Stille reac-tion belongs to a class of modern, palladium-catalyzed carbon-carbon bond forming reactions. The palladium-catalyzed reaction of an organotin compound 2 with a carbon electrophile 1 is called Stille coupling. 

Oxidative addition—Reaction of the carbon electrophile with palladium-(0) complex 5 to give a palladium-(II) complex 6. 

The palladium component may be added to the reaction mixture as Pd(0)- as well as Pd(II)- compound in the latter case the Pd(II)- first has to be reduced to Pd(0)- by excess stannane. Since the first publication on this coupling method by Stille in 1978, this reaction has gained increased importance in synthetic organic chemistry. This is due to the fact that many different types of substrates can be used in this reaction. The following table lists possible carbon electrophiles and stannanes that can be coupled in any combination. 

The reactions in this chapter are arranged in order of leaving group hydrogen, metals, halogen, and carbon. Electrophilic substitutions at a nitrogen atom are treated last. 

Removal of the one- carbon electrophilic formaldehyde (cf p T 10) leaves a chlorophenol (7) made by direct chlorination of (8). The long allyl chain of (8) cannot be put in by Friedel-Crafts alkylation or rearrangement will occur Cp T 9) so acylation and reduction are preferred. 

Ester (9) can easily be made from acid (H)- You might consider two approaches to this a one-carbon electrophile addition via chloromethylation (Table T 2.2) and oxidation or FGl (Table 2,3) back to p-chlorotoluene (12). The latter is easier on a large scale. The p-chlorotoluene (12) can be made either by direct chlorination of toluene or by the diazotisation route (p T 12) again from toluene. 

But how do we convert (41) to (40) Is there an order of events which will ensure the right isomer Adding a one-carbon electrophile, e.g. to give (42), is easy, but brominatlon could now occur anywhere. 

As can be seen from the above examples, the decarboxylation reaction can be said to generate carbanion-equivalent, which is capable of undergoing the enantioselective reactions not only with a proton but also with a carbon electrophile in an aqueous medium. In the future extension of this field, this characteristic point should be utilized for the design of the unique reactions. 

Most reactions of alkenyl and allylic silanes require strong carbon electrophiles and Lewis acid catalysts are often involved. The most useful electrophiles from a synthetic standpoint are carbonyl compounds, iminium ions, and electrophilic alkenes. 

Bowman has surveyed the reactions of cx-substituted aliphatic nitro compounds with nucleophiles, which undergo either S l substitution or polar reaction (Scheme 5.16).118 The reactions between a wide variety of nucleophiles and BrCH2N02 are shown in Scheme 5.17.119a b All the thiolates, PhS02 and I attack Br to liberate the anion of nitromethane. The hard nucleophiles, MeO , OH, and BH4 attack the hard H+ electrophilic center. Phosphorous nucleophiles attackthe oxygen electrophilic center, and only Me2S attacks the carbon electrophilic center. 

Double bonds, carbon-carbon, electrophilic bromination of structure, solvent and mechanism, 28, 171... 

Treatment of 1-pyridinium sulphonate with sodium or potassium hydroxide generates sodium or potassium salts of 5-hydroxy-2,4-pentadienal (glutaconaldehyde), which are starting materials for a variety of transformations (equation 178)171b 301. For example, the reaction of the potassium salt with a carbon electrophile has been used for the preparation of a dienol aldehyde (equation 179)mb which was an intermediate in the total synthesis of a mutagen, (S)-3-(dodeca-l,3,5,7,9-pentaenyloxy)propane-l,2-diol. 

An example in which cleavage from the resin was achieved using a carbon electrophile demonstrated the ability of these polymer-bound metathesis products to act as substrates for carbon-carbon bond formation (Eq. 19). 

Scheme 1.46. Reactions of alkenylalanes and alkenylaluminates obtained via Zr-catalyzed carboalumination with carbon electrophiles.

To replace the aforementioned acyl-main group and acyl-transition metal complexes, the natural course of events was to search for a stable and easy-to-handle acyl-metal complex that reacts as an unmasked acyl anion donor. Thus, the salient features of acylzirconocene chlorides as unmasked acyl anion donors remained to be explored. In the following, mostly carbon—carbon bond-forming reactions with carbon electrophiles using acylzirconocene chlorides as acyl group donors are described. 

The reactivity of acylzirconocene chlorides towards carbon electrophiles is very low, and no reaction takes place with aldehydes at ambient temperature. In the reaction described in Scheme 5.12, addition of a silver salt gave the expected product, albeit in low yield (22—34%). The yield was improved to 79% by the use of a stoichiometric amount of boron trifluoride etherate (BF3OEt2) (1 equivalent with respect to the acylzirconocene chloride) at 0 °C. Other Lewis acids, such as chlorotitanium derivatives, zinc chloride, aluminum trichloride, etc., are less efficient. Neither ketones nor acid chlorides react with acylzirconocene chlorides. In Table 5.1, BF3 OEt2-mediated reactions of acylzirconocene chlorides with aldehydes in CH2C12 are listed. 

More recently, reports by Quayle and co-workers, have shown that benzylated 2-chloroglucal derivatives can be lithiated, with. vec-butyllithium in THF at -78 °C, and the ensuing lithiated species trapped with a variety of carbon electrophiles that include enolizable carbonyl derivatives.25 4-Cholesten-2-one underwent 1,2-addition to yield a 5 1 mixture... 

Scheme 4 Tin-lithium exchange, on 1-stannyl glycals, followed by reaction with carbon electrophiles.

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