Benzoyl cation intermediate

The cr-substituent effects and solvent effects (extended Grunwald-Winstein equation) for solvolyses of / -X-substituted benzoyl chlorides (X = OMe, Me, H, and Cl) in 97% wt/wt hexafluoroisopropanol/water was explained by two competing pathways. Plots of log k versus a were consistent with a cationic path and explained by an 5 n2-5n1 spectrum of mechanism. Electron-donating groups favoured the reaction and values of p increased in the order AcOH < HCO2H < TEE < HFIP. A benzoyl cation intermediate was trapped in hexafluoroisopropanol. Ab initio calculations of heterolytic bond dissociation energies of various chloro- and fluoro-substituted and other benzoyl chlorides were correlated with log k for solvolyses... 

In addition to cation intermediates, radical intermediates can be used to introduce bromine or chlorine into a molecule. Both allylic and benzylic moieties form resonance stabilized free radicals that react with bromine or chlorine to give the corresponding halide. Allylic radicals are easily accessible from the corresponding allylic halides, particularly allyl iodides (secs. 13.3-13.5). Benzylic radicals are available from benzylic halides and also directly from the hydrocarbon, if it bears a benzylic hydrogen. Addition of bromine to 167 (in the presence benzoyl peroxide and photochemical initiation) gave benzylic bromide 168 in high... 

The example of a silyl—Hilbert—Johnson nucleosidation reaction in Section 25.3 is presumed to involve an intermediate ribosyl cation that is stabilized by intramolecular interactions involving the C2 benzoyl group. This intermediate blocks attack by the heterocyclic base from the a face of the ribose ring but allows attack on the j8 face, as required for formation of the desired product. Propose a structure for the ribosyl cation intermediate that explains the stereoselective bonding of the base. 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

The Vilsmeier-Haack formylation procedure (Scheme 24) provides the most effective synthesis of formylpyrroles and indoles. Reaction of the heterocycles with the immonium cation (72), derived from DMF or (V-methylformanilide with an acid chloride, such as phosphorus oxychloride, thionyl chloride, phosgene, oxalyl chloride, benzoyl chloride or bromotriphenylphosphonium bromide, yields the intermediate heteroarylimmonium salt (73). Under suitable reaction conditions, this salt may be isolated from the reaction involving phosphorus oxychloride as an impure chlorophosphate (78TH30500) or precipitated from the reaction system as the thermally unstable perchlorate by the addition of sodium... 

The thiadiazolium iodide (92) hydrolyzes to N-benzoyl-jV-phenyldithiocarbamate (93). Above pH 4, the reaction is of first order and almost independent of pH, in accord with the rate determining formation of the neutral intermediate (94), via a cationic transition State (77SST(4)431, p. 435). 

We found that the stereoisomeric lithium (94) or sodium (95) enolates showed precisely the same stereoisomerism. Benzoylation of the enolates (XI) and (XII) gave the appropriate stereoisomeric benzoates. Hence, the enolates are not subject to steric isomerization under these conditions. Consequently, no C-metallic derivative of the ketone species is involved, otherwise it would function as an intermediate, allowing the stereoisomeric enolates to isomerize to an equilibrated mixture. Indeed, if a tetramethyl-ammonium salt whose cation cannot form any covalent bond with oxygen is... 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured." (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate," and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-... 

We have already encountered selective derivatizations, for example with chloro-triphenylmethane and f-butylchlorodiphenylsilane which etherify selectively the primary alcohols. Pivaloyl chloride, McjCCOCl, also displays a preference for this position. Selective reactions can be observed with the mildly benzoylating reagent W-benzoylimidazole 5.4. When it is a matter of differentiating between secondary positions, it can be advantageous to go through intermediates which are formally alkoxides of the dibutyltin cation, Bu2Sn2. ... 

Pyrimidine reacts with trimethylsilyl cyanide and benzoyl chloride with aluminum chloride as catalyst, to give the Reissert-type compound (366) <81JHC443>. The intermediate acylpyrimidinium cation (367) adds a soft nucleophile such as the silyl ether of acetophenone, and the adduct rapidly undergoes a second A-quaternization to (368) with a subsequent nucleophilic attack to form (369) (Scheme 61). The reactive intermediate can also be trapped by reactions with heteronucleophiles <88JCS(P1)725>. Intramolecular nucleophilic addition is shown in the formation of the spiran derivative (370) <92JOC2526>. 

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