Double bond cleavage. Table

Although the distribution among these products varies, depending on the reaction conditions, particularly on the solvent used, it is evident from the data in Table II that the latter type of products in which the double bond has been immunized towards further ozone attack comprises in each case a significant part (between 22 and 30% ) of the total product mixture. This is perhaps the most remarkable result of the present investigation. In contrast to the ozonolysis of hydrocarbon olefins, the ozonolysis of this dibromosubstituted double bond cannot be viewed primarily as a double bond cleavage reaction. 

Table 26.1 Double bond cleavage raethods Ozonolysis with reductive work-up...

While we must use polarity inversion to add the a-carbonyl cation synthon (1) 10 an enolate ion, no such tricks are needed to add synthon (3> ailyl halides are easily made Chapter 24) and are reactive in 8 2 reactions. The conversion of (4) to (2) by oxidative cleavage of the C=C double bond provides another route to ,4-dicarbonyl compounds. The various methods of double bond cleavage are summarised in Table 26,1. 

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

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. 

Oxidative cleavage of monocyclic and bicyclic allylic alcohols to keto acids and di-acids respectively is effected by RuClj/aq. Na(IO )/CCl -CH3CN thus trans-verbenol gave (+)-c/x-pinononic acid and (+)-frani-pinocarveol yielded (-)-cis-pinic acid (Table 3.6) [237]. The double bond in diphenylcholene was cleaved by RuOj/aq. Na(IO )/CCl to aldehyde and acid (Table 3.4) and the adjoining phenyl rings destroyed [238]. 

Aromatic and aliphatic aldehydes can be oxidized after careful and individual optimization of the reaction conditions to carboxylic acids (Eq. (7), Table 12). With aromatic aldehydes yields are excellent, with aliphatic aldehydes good to satisfactory. The electrolyte has to be less alkaline than normal to suppress the aldol condensation. 2-Phenylpropanol is best oxidized at low temperatures to render the cleavage to benzoic acid more difficult, at 70 °C benzoic acid becomes main product (47 %). Double bonds in y,8- or even a,P-position are not touched in the oxidation. 

In work reported elsewhere (31) we have shown that the oxidation of styrene under mild conditions is promoted by many group VIII metal complexes. The product profile depends on the nature of the metal center and often differs from that observed when radical initiators are used (Table IX). Substantial quantities of styrene oxide are found in some cases but not in others (31). The epoxide which is formed, however, seems to arise via the co-oxidation of styrene and formaldehyde which is formed by oxidative cleavage of the double bond. Formaldehyde may be oxidized to performic acid or formylperoxy radicals which are efficient epoxidizing agents. Reactions of styrene with oxygen in the presence of group VIII complexes exhibit induction periods and are severely retarded by radical inhibitors (31). Thus, the initial step in the oxidation of styrene in the presence of the Ir(I),Rh(I), Ru(II), and Os(II) com-... 

One reason alkenes and alkynes react more readily than alkanes is because the carbon-carbon bonds of a multiple bond are individually weaker than normal carbon-carbon single bonds. Consider the bond energies involved. According to Table 4-3, the strengths of carbon-carbon single, double, and triple bonds are 83, 146, and 200 kcal, respectively. From these values we can calculate that cleavage of one-half of a carbon-carbon double bond should require 63 kcal and cleavage of one-third of a carbon-carbon triple bond should require 54 kcal ... 

The cyclization of 2-cycloalkeneacetic acids leads to ris-fused y-lactones arising from anti addition across the double bond (equation 6 and Table 3). This type of reaction, effected with a wide variety of electrophiles and substrate substitution patterns, has been used both for generation of y-lactone target molecules and selective introduction of a hydroxy group onto the original ring by cleavage of the lactone... 

Table 7.10 presents results for this alkenetype. Carboxidation of norbornylene (entry 1) leads to the formation of both a ketone and an aldehyde in approximately equal amounts, indicating that a significant cleavage of the double bonds occurs. There is also a set of other reaction products as a consequence of the bond cleavage. 

However, such a cleavage only occurs if fluorine atoms are branched onto the double bond in the contrary case, the diol is obtained as Chambers [24] and Husain [25] showed (Table 1). 

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