A-cleavage reactions

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). 

Release by Silver-Assisted Cleavag e. A soluble silver complex formed imagewise in the undeveloped areas of the silver haUde layer may be used to effect a cleavage reaction that releases a dye or a dye precursor. The process yields positive dye transfer images directiy with negative-working emulsions (46). An example is the silver-assisted cleavage of a dye-substituted thiazolidine compound, as shown in equation 7. 

This reaction is referred to as the type-I or a-cleavage reaction of carbonyl omnpnunds. This type of reaction is not so common in solution, although some cyclic ketooes do undergo decarbonylation ... 

A cleavage reaction reminiscent of that already noted with chloranil " has been observed at room temperature with DDQ. This reaction is remarkable in that it stops at the dihydrophenanthrene stage (78, 79) and takes place... 

In addition, there is a cleavage reaction whereby a perfluoroorgamc group is cleaved from a metal by a base, for example, phenyllithium [4], ethylmagnesium bromide [5], or a fluoride ion [6] (equations 3-5)... 

Methyl esters (RCO2CH3) undergo a cleavage reaction to yield carboxylate ions plus iodomethane on heating with Lil in dimethylformamide ... 

Electrochemical reductions of sulphones have been reviewed212, and have been discussed at intervals213-215. There is evidence that the cathodic reduction reaction proceeds via a radical anion, followed by a cleavage reaction, as outlined in equation (91)212,213. 

The structural requirements of sulphones to react cathodically and to possess specific electrochemical properties are summarized in Scheme 1. In other words, condition (a) means that aromatic sulphones and a unsaturated sulphones are electroactive, i.e., electron transfer to the LUMO leads to the anion radical, but a cleavage reaction (see b) is mainly observed when R S02 " is a fairly good leaving group. Consequently, the two main classes of electroactive sulphones may react differently with aromatic sulphones, ArS02—R, cleavage is strongly favoured, while with unsaturated sulphones ... 

As a hypothesis to explain his observations, Krugerl3 proposed (as is shown in scheme 3 along with other hypothesis) that the derived 3-ketonitrosamine underwent a cleavage reaction to yield a methylalkylnitrosamine and a carboxylic acid derivative, in analogy to fatty acid metabolism. 

A surprising exception has been reported with evidence for a cleavage reaction in the case of divinyl sulphone. In non-aqueous and slightly acidic media, the behaviour of a., ji-unsaturated aromatic sulphones is also complex (see Table 7) since the cleavage and the saturation may compete. Strongly electrophilic double bonds undergo Michael additions in aprotic solvents by slowly protonated anions. Transfer of labile hydrogen may also lead to unactivated bases. It is noteworthy that in numerous cases (Table 6) the saturation is the preferred route. 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

Entry 11 illustrates a cleavage reaction using an acylating agent in conjunction with a Lewis acid. 

The compounds described herein were prepared by three methods. The first route involves deprotonation/substitution at the N-H sites of 1, the second consists of a cleavage reaction of an Si-N derivative of 1 with PhBCI2, and the third route is a transamination reaction between a bis(dimethylamino)boryl derivative of 1 and an aliphatic diamine. In the first approach, compound 1 is deprotonated by treatment with one equivalent of n-BuLi. Quenching of the resulting anion with various electrophiles produces the monosubstituted products 2-4 (eq 3). 

The allyl 2-nitrophenyl sulphoxide is apparently not an intermediate in this reaction as the MS of the latter compound is dominated by a cleavage reaction giving rise to C3H5+, whereas the ions corresponding to a loss of the HSC>2 radical apparently are of low intensity86. Thus, it was concluded that the double oxygen transfer to sulphur most probably should be formulated as a concerted reaction86. 

A vitamin whose pyrophosphorylated form is an essential coenzyme in so-called a condensation and a cleavage reactions, whose cardinal feature is that the scissile bond always lies immediately adjacent to the carbonyl group. 

Under oxidizing or chlorinating conditions, TPs sometimes undergo a cleavage reaction, e.g., from... 

Type I initiators are compounds that upon irradiation undergo a cleavage reaction (a- or 3-cleavage) to generate two radicals ... 

For assessment of the aminolytic effectiveness of the polymer amines, we have used22 a cleavage reaction commonly employed by enzymologists interested in hydrolytic enzymes ... 

Recently, Yoneda et al. [52] have reported an interesting electrochemical fluori-native a-cleavage reaction of cyclic ketones as a route to fluorocarboxylic acid esters. The reaction is carried out in the presence of Et3N 5HF as electrolyte, when the 2,2-disubstituted cyclic ketone is subject to selective cleavage of the C-C bond between the carbonyl carbon and the substituted a-carbon. 

Another complication of the reversible case may be that the reduction product A reacts chemically and is thus not available for reoxidation on the reverse scan, so only a small or no anodic peak is seen. In the usual electrochemical nomenclature, an electron-transfer reaction is called E and a chemical follow-up reaction C. The process in question would thus be an EC reaction the chemical step would after a reduction in most cases be a reaction with an electrophile, including protons, a cleavage reaction, where a nucleophile is expelled, or a dimerization for oxidation reactions with a nucleophile, loss of a proton or dimerization would be the most common follow-up reactions. 

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