Cinnamic acid, oxidation

The most common unsaturated aromatic carboxylic acid is cinnamic acid (8-73). In cinnamon and other spices the (E)-isomer predominates. Cinnamic acid oxidation products are 4-hydroxycinnamic acid (also known as 4-coumaric or p-coumaric acid) and 3,4-dihydroxycinnamic acids, which is called caffeic acid. A 3-methoxy derivative of caffeic acid is feruhc acid and the 3,5-dimethoxy derivative is known as sinapic acid. 

CgHgO, PhCH = CHCOiH. Colourless crystals. Decarboxylales on prolonged heating. Oxidized by nitric acid to benzoic acid. Ordinary cinnamic acid is the trans-isomer, m.p. 135-136 C on irradiation with u.v. light it can be isomerized to the less stable cis-isomer, m.p. 42" C. 

Oxidation, (a) Unsaturation test. Dissolve about o-i g. of cinnamic acid or of a soluble cinnamate in about 5 ml. of 10% NajCOg solution. To the cold solution add 1% aqueous KMn04 drop by drop. Immediate decolorisation denotes unsaturation. (Note. Many easily oxidisable substances, e.g.y formic acid, acetaldehyde, etc.y also rapidly decolorise alkaline permanganate. Cinnamates, however, do not reduce Fehling s solution.)... 

Use 01 g. of the platinum oxide catalyst and 11 4 g, of pure cinnamic acid dissolved in 100 ml. of absolute alcohol. The theoretical volume of hydrogen is absorbed after 7-8 hours. Filter off the platinum, and evaporate the filtrate on a water bath. The resulting oil solidifies on cooling to a colourless acid, m.p. 47-48° (11-2 g.). Upon recrystallisation from light petroleum, b.p. 60-80°, pure dihydrocinnamic acid, m.p. 48-49°, is obtained. 

Hydrocinnamic acid may also be prepared by the reduction of cinnamic acid with sodium and alcohol or with sodium amalgam or with hydrogen in the presence of Adams platinum oxide catalyst (Section 111,150) ... 

Pyrolytic Decomposition. The pyrolytic decomposition at 350—460°C of castor oil or the methyl ester of ricinoleic acid spHts the ricinoleate molecule at the hydroxyl group forming heptaldehyde and undecylenic acids. Heptaldehyde, used in the manufacture of synthetic flavors and fragrances (see Elavors and spices Perfumes) may also be converted to heptanoic acid by various oxidation techniques and to heptyl alcohol by catalytic hydrogenation. When heptaldehyde reacts with benzaldehyde, amyl cinnamic aldehyde is produced (see Cinnamic acid, cinnamaldehyde, and cinnamyl... 

Selective oxidation of either the aromatic ring or the side chain can also be accompHshed. For example, epoxidation of the double bond of cinnamic acid is effected in excellent yield by treatment with potassium hydrogen persulfate (10). 

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). 

When heated in the presence of a carboxyHc acid, cinnamyl alcohol is converted to the corresponding ester. Oxidation to cinnamaldehyde is readily accompHshed under Oppenauer conditions with furfural as a hydrogen acceptor in the presence of aluminum isopropoxide (44). Cinnamic acid is produced directly with strong oxidants such as chromic acid and nickel peroxide. The use of t-butyl hydroperoxide with vanadium pentoxide catalysis offers a selective method for epoxidation of the olefinic double bond of cinnamyl alcohol (45). 

Cinnamic alcohol forms a phenyl-urethane, melting at 90° to 91°, and a diphenyl-urethane, melting at 97° to 98°. On oxidation it yields cinnamic acid, melting at 133°, and by more thorough oxidation, benzoic acid, melting at 120°. 

Cinnamic acid Natural balsam, cocoa leaves, cinnamol oil Carlina oxide Carline aeaulis L. 

For cinnamic acid at 9.6 °C, a = 0.107, b = 1.25 and k = 0.69 l.mole .sec E = 26.7 0.5 kcal.mole" and AS = 34.5 eu. Identification of products of oxidation of a number of acids indicates two concurrent mechanisms. Predominating is direct attack on the double bond to give, ultimately, cleavage products, e.g. benzaldehyde from cinnamic acid (some phenylacetaldehyde is also found, indicating oxidative decarboxylation to occur) and also acetophenone from 3-phenylcrotonic acid. 

A few data exist on the oxidation by ferricyanide. This is simple second-order (in oxidant and neutral hydrazine), and leads to quantitative production of nitrogen in accordance with scheme (71)-(74) with A 4 k and A i. No scrambling occurs during oxidation of N-labelled N2H4 indicating that all N2 is formed via di-imine" Di-imine so prepared is capable of hydrogenating added unsaturated compounds, for example, phenylpropiolic acid gives m-cinnamic acid" " . 

Acrylic acid (AA) and methacrylic acid (MAA) (purchased from Merck) are freed from inhibitor on a neutral aluminium oxid column and distilled. Acrylamide (AM) from Kebo, Stockholm, is recrystallized once from chloroform solution before use. Other monomers of analytical grade were purchased from Merck and used as received crotonic acid (CA), tiglic acid (TA), 3-methyl crotonic acid (3-MCA), and a-methyl cinnamic acid (oi-MCia) (Table 1). Benzophenone (analytical grade, Kebo) and acetone (spectroscope grade, Merck) were used as supplied. 

Ranjit KT, Willner I, Bossmann SH, Braun AM (2001) Lanthanide oxide doped titanium dioxide photocatalysts Effective photocatalysts for the enhanced degradation of salicylic acid and t-cinnamic acid. J Catal 204 305-313... 

Recently, a series of cycloadducts possessing unusual flipping modes have been isolated from the 1,3-dipolar cycloaddition of 3,4,5,6-tetrahydropyridine IV -oxide to piperidides of cinnamic acid and para-substituted cinnamic acids (791). 

Figure 6. [3+2] transition state for the oxidation of cinnamic acid by permanganate [40]. 

Experimental studies of the oxidative cleavage of cinnamic acid by acidic permanganate [35] resulted in secondary kinetic isotope effects, kn/kp, of 0.77 (a) and 0.75 (P), while another paper from the same group on the same reaction with quaternary ammonium permanganates [36] reported very different isotope effects of 1.0 (a) and 0.91 - 0.94 (P) depending on the counterion. Different mechanisms were discussed in the literature [37, 38] to explain the variety of experimental results available, but the mechanistic issues were unresolved. The reported activation energy for the oxidation of... 

It can be concluded that the [3+2] pathway seems to be the only feasible reaction pathway for the dihydroxylation by permanganate. The study on the free activation energies for the oxidation of a. P unsaturated carboxylic acids by permanganate shows that the [3+2] mechanism is in better agreement with experimental data than the [2+2] pathway. Experimentally determined kinetic isotope effects for cinnamic acid are in good agreement with calculated isotope effects for the [3+2] pathway, therefore it can be concluded that a pathway via an oxetane intermediate is not feasible. 

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

Smith, D. G., and A. C. Neish Alkaline Oxidation of C-Labellcd Protolignin Formed from Cinnamic Acid in Spruce and Aspen Twigs. Phytochem. 3, 609—616 (1964). 

Kinetics of the reaction of diazodiphenylmethane (92) in a wide range of alcohols with pyridine and pyridine-A -oxide 3- and 4-carboxylic acids (84)-(87), 4-substituted benzoic acids (88)," cw -substituted cinnamic acids (89), 2-(4-phenyl-substituted)cyclohex-l-enyl carboxylic acids (90), and 4 -substituted-biphenyl-2-carboxylic acids (91)" have been reported. Comparison of the new results for 4-substituted benzoic acids with the published results of data for 3-substituted benzoic acids was made, " and it was concluded that the most important solvent property influencing the rate of reaction appears to be the polarity of the alkyl group expressed as Taft s polar constant a. Transmission coefficients in the cinnamic acids (89) were compared with those in the bicyclic acids (90) and... 

A study of the regioselectivity of the 1,3-dipolar cycloaddition of aliphatic nitrile oxides with cinnamic acid esters has been published. AMI MO studies on the gas-phase 1,3-dipolar cycloaddition of 1,2,4-triazepine and formonitrile oxide show that the mechanism leading to the most stable adduct is concerted. An ab initio study of the regiochemistry of 1,3-dipolar cycloadditions of diazomethane and formonitrile oxide with ethene, propene, and methyl vinyl ether has been presented. The 1,3-dipolar cycloaddition of mesitonitrile oxide with 4,7-phenanthroline yields both mono-and bis-adducts. Alkynyl(phenyl)iodonium triflates undergo 2 - - 3-cycloaddition with ethyl diazoacetate, Ai-f-butyl-a-phenyl nitrone and f-butyl nitrile oxide to produce substituted pyrroles, dihydroisoxazoles, and isoxazoles respectively." 2/3-Vinyl-franwoctahydro-l,3-benzoxazine (43) undergoes 1,3-dipolar cycloaddition with nitrile oxides with high diastereoselectivity (90% de) (Scheme IS)." " ... 

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