A-cleavage of acetone

The assumed conservation of a plane of s)unmetry in the cleavage of acetone may allow us to rationalize the theoretical results in terms of a state correlation diagram. Relative to this plane of symmetry one electron has either o or w symmetry, and the correlation between states involves a simple count of the electrons of each t3q>e. In the a cleavage of acetone the acetyl radical has two available low-lying states. The first... 

FIGURE 48.2 Orbital correlation diagram illustrating the a-cleavage of acetone. A linear acyl radical (left) is predicted to be photochemically generated from the nit excited state of acetone. The orbital symmetry is represented by the letters a (asymmetric) and s (symmetric), with respect to the o mirror plane. 

Unlike acetone, diethyl ketone cleaves well even in cumene 22>. 2-Pentanone triplet undergoes considerable a-cleavage in competition with rapid intramolecular hydrogen abstraction, as judged by the buildup of an efficient triplet quencher 23>. Biacetyl is the only likely candidate for that quencher and is the major product of a-cleavage of methyl ketones at temperatures low enough that decarbonylation of the acetyl radical is slow. Isopropyl, tert-butyl, and benzyl ketones all cleave quite efficiently and various reports have appeared on the CIDNP spectra of products derived from the radicals 24>. 

The L-ascorbic acid that we use as a supplement in tablets or capsules and food industry uses as an antioxidant is synthesized by the classic Reichstein synthesis from D-glucose to D-sorbitol followed by oxidation of D-sorbitol to L-sorbose by Acetobacter suboxydans in submersion culture the production of diacetone L-sorbose and its oxidation with permanganate in alkaline solution to the carbonic acid hydrolytic cleavage of acetone to form 2-oxo-L-gulonic acid and enolization to ascorbic acid. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

This cleavage is a retro aldol reaction It is the reverse of the process by which d fruc tose 1 6 diphosphate would be formed by aldol addition of the enolate of dihydroxy acetone phosphate to d glyceraldehyde 3 phosphate The enzyme aldolase catalyzes both the aldol addition of the two components and m glycolysis the retro aldol cleavage of D fructose 1 6 diphosphate... 

There are many variations of the basic process and the patent Hterature is extensive. Several key patents describe the technology (16). The process steps are oxidation of cumene to a concentrated hydroperoxide, cleavage of the hydroperoxide, neutralization of the cleaved products, and distillation to recover acetone. 

After cleavage the reaction mass is a mixture of phenol, acetone, and a variety of other products such as cumylphenols, acetophenone, dimethyl-phenylcarbinol, a-methylstyrene, and hydroxyacetone. It may be neutralised with a sodium phenoxide solution (20) or other suitable base or ion-exchange resins. Process water may be added to facilitate removal of any inorganic salts. The product may then go through a separation and a wash stage, or go direcdy to a distillation tower. 

The neutralized cleavage product, consisting of acetone, phenol, water, hydrocarbons, and trace organic impurities, is separated in a series of distillation columns. Also in this section alpha-methylstyrene is either recovered as a product or hydrogenated to cumene. 

Production of a-methylstyrene (AMS) from cumene by dehydrogenation was practiced commercially by Dow until 1977. It is now produced as a by-product in the production of phenol and acetone from cumene. Cumene is manufactured by alkylation of benzene with propylene. In the phenol—acetone process, cumene is oxidized in the Hquid phase thermally to cumene hydroperoxide. The hydroperoxide is spHt into phenol and acetone by a cleavage reaction catalyzed by sulfur dioxide. Up to 2% of the cumene is converted to a-methylstyrene. Phenol and acetone are large-volume chemicals and the supply of the by-product a-methylstyrene is weU in excess of its demand. Producers are forced to hydrogenate it back to cumene for recycle to the phenol—acetone plant. Estimated plant capacities of the U.S. producers of a-methylstyrene are Hsted in Table 13 (80). 

The acetone supply is strongly influenced by the production of phenol, and so the small difference between total demand and the acetone suppHed by the cumene oxidation process is made up from other sources. The largest use for acetone is in solvents although increasing amounts ate used to make bisphenol A [80-05-7] and methyl methacrylate [80-62-6]. a-Methylstyrene [98-83-9] is produced in controlled quantities from the cleavage of cumene hydroperoxide, or it can be made directly by the dehydrogenation of cumene. About 2% of the cumene produced in 1987 went to a-methylstyrene manufacture for use in poly (a-methylstyrene) and as an ingredient that imparts heat-resistant quaUties to polystyrene plastics. 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

Hg(C104)2, 2,4,6-collidine, acetone, H2O (9 1), 5 h NH3, dioxane, H2O (1 1). In this case Hg(II) is used to cleave the MTM group, liberating a hydroxyl group, which assists in the cleavage of the carbonate on treatment with ammonia. Cleavage by ammonia is 500 times faster for this hydroxy derivative than for the initial MTM derivative. 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

It is sometimes impossible to isolate the resultant jS-ketonitrile derivatives as such as they are unstable and readily polymerize, e.g. cyano-acetone. They are therefore identified as their corresponding arylhydrazones. The simpler alkyl-w-cyanomethyl ketones are, however, much more stable and their production through the cleavage of 5-alkylisoxazoles has been suggested as a preparative synthetic method. ... 

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