Ketones cationic

For example, 2-hexanone in MeCN gave 5-acetamino-2-hexanone (44) in 40% yield (Scheme 17) [82], and 2-pentanone was oxidized in trifluoroacetic acid to give a mixture of 4-trifluoroacetoxy-2-pentanone (24%) and 3- and 4-penten-2-one (6%) [83]. A mechanism involving intramolecular hydrogen abstraction by a ketone cation radical that forms a car-benium ion via a [l,5]-hydrogen shift was proposed. 

Poly(methyl vinyl ketone) Cationic+dehydratation 63-65... 

Isopropylmagnesium bromide Tert. alcohols from ketones Cation specificity... 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

The attack of OH obeys the Markovnikov rule. Higher alkenes are oxidized to ketones and this unique oxidation of alkenes has extensive synthetic appli-cations[23]. The oxidation of propylene affords acetone. Propionaldehyde is... 

Acyl cations (acylium ions) generated by treating an acyl chloride or acid anhydride with aluminum chloride attack aromatic rings to yield ketones The arene must be at least as reactive as a halobenzene Acyl cations are relatively stable and do not re arrange... 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular ion peak m their mass spectra Aldehydes also exhibit an M— 1 peak A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acylium ions) by cleavage of an alkyl group from the carbonyl The most intense peak m the mass spectrum of diethyl ketone for example is m z 57 corresponding to loss of ethyl radi cal from the molecular ion... 

The cationic intermediate is simply the protonated form (conjugate acid) of the a halo ketone Deprotonation of the cationic intermediate gives the products... 

Acylation of acetylenic compounds provides /ra/ j -P-chlorovinyl ketones (181). Vinyl cations were proposed to be the intermediates in these reactions. 

When aiomatics aie present, they can capture the intermediate vinyl cation to give P-aryl-a,P-unsatutated ketones (182). Thus acylation of alkyl or aryl acetylenes with acyhum salts in the presence of aromatics gives a,P-unsaturated ketones with a trisubstituted double bond. The mild reaction conditions employed do not cause direct acylation of aromatics. 

Methyl Isopropenyl Ketone. Methyl isopropenyl ketone [814-78-8] (3-methyl-3-buten-2-one) is a colorless, lachrymatory Hquid, which like methyl vinyl ketone readily polymerizes on exposure to heat and light. Methyl isopropenyl ketone is produced by the condensation of methyl ethyl ketone and formaldehyde over an acid cation-exchange resin at 130°C and 1.5 MPa (218 psi) (274). Other methods are possible (275—280). Methyl isopropenyl ketone can be used as a comonomer which promotes photochemical degradation in polymeric materials. It is commercially available in North America (281). 

These association reactions can be controlled. Acetone or acetonylacetone added to the solution of the polymeric electron acceptor prevents insolubilization, which takes place immediately upon the removal of the ketone. A second method of insolubiUzation control consists of blocking the carboxyl groups with inorganic cations, ie, the formation of the sodium or ammonium salt of poly(acryhc acid). Mixtures of poly(ethylene oxide) solutions with solutions of such salts can be precipitated by acidification. 

Unsaturated ketones react with phenyUiydrazines to form hydrazones, which under acidic conditions cyclize to pyrazolines (35). Oxidation, instead of acid treatment, of the hydrazone with thianthrene radical cation (TH " ) perchlorate yields pyrazoles this oxidative cyclization does not proceed via the pyrazoline (eq. 4). 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

With l,3-dimethyl-2,l-benzisoxazolium salts, however, considerable reactivity has been reported. Condensation occurs readily with aldehydes, ketones, orthoesters and diazonium salts to yield styryl, cyanine and azo compounds, respectively (78JOC1233). In the presence of triethylamine, dimerization was observed, and the reactions of the cation were considered to involve the intermediacy of the anhydro base (77JOC3929). 

Various methods have been employed to overcome this difficulty. For example, a method used by Chemical Abstracts involves naming the dihydro derivative of the heterocyclic ketone (or imine or exocyclic alkene) form, and adding the words mesoionic didehydro derivative (example 131). A similar approach, favoured by Ollis (76AHQ19)1), involves naming the corresponding cation hydroxide, with the prefix anhydro (indicating removal of the elements of water) (example 132). 

While most synthetic examples of this cyclization have involved protonation of divinyl ketones to give 3-hydroxy-1,4-pentadienyl cations, theoretical studies suggest that the cyclization would occur even more readily with alternative substituents at C-3. °... 

Enamines of A" -3-ketones (45) are stable to lithium aluminum hydride, but lithium borohydride reduces the 3,4-double bond of the enamine system." In the presence of acetic acid the enamine (45) is reduced by sodium borohydride to the A -3-amine (47) via the iminium cation (46). ... 

The major difference in reactivity between CF3OF and FCIO3 lies in the capacity of the former to react with olefins without the benefit of an electron releasing group and even with electron deficient olefins such as a,y5-un-saturated ketones. Reactions with nonactivated double bonds indicate the presence of an oc-fluoro cationic intermediate [e.g., (64)] as exemplified by the reaction with the -3-ketone (63), which yields the fluorophenol (65). 

---