Acidic C-H bonds

The opposite case—reaction of an arenediazonium species with an aliphatic substrate —is possible if a sufficiently acidic C—H bond is present e.g. with /3-keto esters and malonic esters. The reaction mechanism is likely to be of the Sel-type an electrophilic substitution at aliphatic carbon ... 

In general, the reaction can be performed only with organometallics of active metals such as lithium, sodium, and potassium, but Grignard reagents abstract protons from a sufficiently acidic C—H bond, as in R—C=C—H —> R—C=C—MgX. This method is best for the preparation of alkynyl Grignard reagents. ... 

This review has been concerned largely with interactions and reactions of unsaturated hydrocarbons with zinc oxide. The picture of the active site as a metal oxide pair capable of heterolytic fission of an acidic C—H bond provides a consistent framework for discussion of these results. We believe this view may be generally applicable. In its application, however, we must keep in mind that zinc oxide may be much more effective for heterolytic cleavage (i.e., more basic) than oxides such as, say, alumina.4... 

TABLE 12. Acidities and homolytic bond dissociation energies of the acidic C—H bonds in 9-(dialkylamino)fluorenes 19-R1 R2NF1H)... 

If further acidic C—H bonds in the molecule cause problems, the tin-trick can be applied. The asymmetric deprotonation of a bifunctional carbamate (39a) is accomplished at an early stage and the masked carbanionic centre carried through the synthesis as a stan-nyl group. For instance, the (S)-5-silyloxy-l-tributylstannyl-pentyl carbamate 39b (> 95% ee) was produced by the usual means and converted by standard steps via the aldehyde 78 into the allyl chloride 79 (equation 17) . Lithiodestannylation of 79 by n-BuLi proceeds faster than reductive lithiation in the allylic position to form the lithiocarbamate 80,... 

INTRAMOLECULAR DIHYDROGEN BONDS IN SOLID AMINO ACIDS C-H BONDS AS WEAK PROTON ACCEPTORS... 

C-H bond unreactive to insertion, 1160 dioxirane oxidation, 1156 Sulfonic acids, C-H bond unreactive to insertion, 1160 Sulfonyl endoperoxides parasiticidal activity, 1309 synthesis, 1306-9, 1332 Sulfonyl peroxides, 1001-2, 1004-7 Sulfonylperoxy radical, superoxide reactions, 1035-9 Sulfoxidation... 

Among the oxidants that have been used to generate radicals, manganese (HI) acetate has emerged as a powerful reagent to mediate radical cyclizations.147 The manganese(III) acetate-mediated oxidation of enolizable carbonyl compounds is one of the best methods available for the cyclization of electrophilic radicals. The substrates are vety easily prepared by standard alkylation and acylation reactions. Radicals are formed with high selectivity by oxidation of acidic C—H bonds, and, because the reaction is an oxi-... 

To circumvent problems of nucleophilicity, lithium diisopropylamide (LDA), potassium hexamethyldisilylamide (KHMDS), and KH are often employed for proton removal since they are very strong bases (pKa > 35) but relatively poor nucleophiles. Hence they remove protons from acidic C-H bonds but normally do not attack carbonyl groups or other electrophilic centers. 

The present study was initiated to provide a direct comparison of IETS and IR spectra for an identical molecule adsorbed on an aluminum oxide covered, evaporated aluminum substrate. Further, it was of interest to see if a weakly acidic C-H bond, such as that present in 1,3-dialkanediones, would show dissociative chemisorption similar to the well-known chemisorptions of Bronsted acids containing acidic O-H bonds (see above). The molecules chosen for this study were acetic acid and 2,4-pentanedione. Both oxide covered copper and aluminum were used as substrates in order to see the effects of substrate oxide on the chemisorption spectra. 

In its reactions with metal carbonyl derivatives having ligands bearing acidic C—H bonds, NaN(SiMe3)2 behaves as a proton acceptor as the silyl amide is a strong base (161). 

The variety of suitable nucleophiles is far greater water, alcohols, phenols, amines and carboxylic acids are commonly used, but others including silanes or compounds with acidic C-H bonds such as malonates have been reported as well. The resulting products can, in some cases after further conversion, be used in many different applications, such as nonionic surfactants, emulsifiers, fragrances, flavouring agents, cosmetics and polymer components. 

LDA is too hindered to attack C=0, so it attacks C-H instead. And, if there is a choice of C-H bonds, it will attack the least hindered possible. It will also prefer to attack more acidic C-H bonds, and C-H bonds on less substituted carbons are indeed more acidic. Furthermore, statistics helps, since a less substituted C atom has more protons to be removed (three versus two in this example) so, even if the rates were the same, the less substituted enolate would predominate. 

C—H 7t Bonds. Even weaker hydrogen bonds, never more than 4 kJ mol 1 (1 kcal mol-1), can be detected between C—H bonds and C=C n bonds. These interactions are seen in solid-state structures like that of benzene with its edge-to-face arrangement 2.94, and in some noticeable upfield shifts in the -NMR spectra on changing solvents from carbon tetrachloride to benzene. Larger shifts in the NMR spectra are seen with the more acidic C—H bonds, with chloroform showing an upfield shift of 1.35 ppm at infinite dilution, because of the formation of a bond to the centre of the n system 2.95. 

HFA has been found to insert into one of the acidic C—H bonds of malodinitrile (186). 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

Halohydrocarbon solvents containing an acidic C—H bond shift this equilibrium in favour of free or less associated species, thus perturbing the ion channels which determine the permeability of neuron membranes to K /Na ions in the nervous system. Hydrogen bonds play a decisive role in determining the structure and dimension of these ion channels, on which this permeability depends [300],... 

In general, 2-alkenes will form the more-substituted n-allylic complex, and 1-methylcycloalkanes react to give the r-allyl complex from abstraction of one of the -CH3 hydrogens. If a given aUcene is umeactive, use of the very electrophilic Pd(02CCp3)2 may result in reaction. When the alkene contains a particularly acidic C-H bond, such as one 0 to a carbonyl group, that C-H bond is normally the one abstracted to give the allylic complex (equation 54). 

S.2.3.2. by Proton-Metal Exchange with Acidic C—H Bonds. 

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