Phenols decarbonylation

Diphenol carbonate is produced by the reaction of phosgene and phenol. A new approach to diphenol carbonate and non-phosgene route is by the reaction of CO and methyl nitrite using Pd/alumina. Dimethyl carbonate is formed which is further reacted with phenol in presence of tetraphenox titanium catalyst. Decarbonylation in the liquid phase yields diphenyl carbonate. 

Harano and colleagues [48] found that the reactivity of the Diels-Alder reaction of cyclopentadienones with unactivated olefins is enhanced in phenolic solvents. Scheme 6.28 gives some examples of the cycloadditions of 2,5-bis-(methoxycar-bonyl)-3,4-diphenylcyclopentadienone 45 with styrene and cyclohexene in p-chlorophenol (PCP). Notice the result of the cycloaddition of cyclohexene which is known to be a very unreactive dienophile in PCP at 80 °C the reaction works, while no Diels-Alder adduct was obtained in benzene. PCP also favors the decarbonylation of the adduct, generating a new conjugated dienic system, and therefore a subsequent Diels-Alder reaction is possible. Thus, the thermolysis at 170 °C for 10 h of Diels-Alder adduct 47, which comes from the cycloaddition of 45 with 1,5-octadiene 46 (Scheme 6.29), gives the multiple Diels-Alder adduct 49 via decarbonylated adduct 48. In PCP, the reaction occurs at a temperature about 50 °C lower than when performed without solvent, and product 49 is obtained by a one-pot procedure in good yield. 

Phenolic compounds were confirmed to be very stable against thermal treatment. Diphenyl methanol and benzophenone were stable against decomposition but hydrogenated to form diphenyl -methane quantitatively. Phenyl benzyl ketone was found to be partially hydrogenated or decarbonylated to form diphenyl alkanes. 

In an alternative strategy functionalized phenols, such as iodophenol, were involved in palladium-catalyzed carbonylation of alkynes or allenes, producing coumarin or chromone derivatives (Scheme 23) [130-133]. After oxidative addition of the iodoarene to the Pd(0) catalyst the order of insertion of either CO or the unsaturated substrate mainly depends on the nature of the substrate. In fact, Alper et al. reported that CO insertion occurs prior to allene insertion leading to methylene- or vinyl-benzopyranone derivatives [130]. On the contrary, insertion of alkynes precedes insertion of CO, affording couma-rine derivatives, as reported by Larock et al. According to the authors, this unusual selectivity can be explained by the inability of the acyl palladium species to further react with the alkyne, hence the decarbonylation step occurs preferentially [131-133]. 

Enthalpy barriers for the decarbonylation and dethiocarboxylation of y-thiobutyrolactone (364) have been calculated as 378 and 404 kJ moP respectively, which accords with the experimental results which saw CO as the major and COS as the minor thermal degradation products.Phenyl and 4-nitrophenyl chlorothionoformates (365 X = H, NO2) reacted with phenolates in aqueous dioxane with /3 uc = 0.55 and 0.47, respectively, from which it was concluded that a concerted mechanism prevailed. 

It has been mentioned that phenol is formed via Path A, by diffusion of radicals from the solvent cage and hydrogen abstraction from the solvent. This process is undoubtedly favored (and the yield of phenol is increased) when the phenoxy radical 65 already loses its counterpart in the solvent cage, i.e., when it loses the acyl radical 68 as a consequence of its decarbonylation. From the hitherto reported results it can be assumed that decarbonylation is significant and proceeds very readily under two conditions. It occurs (1) if the acyl radical formed possesses excess energy ( hot radical) due to excitation of high energy, e.g., by y-radiolysis,41,46 and (2) if the alkyl or aryl radicals formed by the decarbonylation of the acyl radical are exceptionally stable.61... 

The aryl esters of formic and oxalic acid (exclusively) undergo very efficient decarbonylation to form phenol.5 It is noteworthy that aryl esters of formic acid, when irradiated in the presence of olefins, are added to the latter in the same way as phenols.83... 

Phenols. Presumably they arise exclusively via dissociative Path A, subsequent radical diffusion from the solvent cage, and abstraction of a hydrogen from the solvent (65 -> 74). The yields are (1) increased with decreasing viscosity of the reaction medium (2) higher in nonpolar and lower in polar solvents (3) practically independent of the hydrogen-donating ability of the solvent and (4) increased if a radical counterpart of a phenoxy radical, i.e., an acyl radical, decarbonylates in the solvent cage for structural reasons. 

Benzoic acid is an important chemical intermediate which can also be used as a phenol precursor by decarbonylation in the presence of copper catalysts (Lummus process). It is produced industrially by oxidation of toluene by air in the presence of cobalt catalysts (Dow and Amoco processes equation 240). The reaction can be carried out without solvent, or in an acetic acid solvent. The oxidation of toluene without solvent uses a cobalt octoate catalyst and operates at higher temperature (180-200 CC). Yields of benzoic acid are about 80% for ca. 50% toluene conversion.361 In an acetic acid solution and in the presence of cobalt acetate, the reaction occurs at lower temperature conditions (110-120 °C) and gives higher yields in benzoic acid (90%).83,84... 

Decarbonylations useful for preparative work are also observed with some aromatic hydroxy compounds. It is reasonable to assume a mechanism which involves the keto form 23). Phenol, which shows little tendency to tautomerize, reacts mainly by forming benzene and to a small extent cyclopentadiene 14). For... 

For samples photolyzed on ZSM-5 zeolite, the product distributions of 31 and 32 are dramatically different from those photolyzed in homogeneous solutions. First, the rearrangement products were totally suppressed. Second, diphenylethane 39 resulted from coupling of benzyl radical was not found. Only phenol 38 and toluene were detected. In contrast, photolyses of 33 and 34 on ZSM-5 follow strikingly different pathways. Both photo-Fries rearrangement 36 and 37 and decarbonylation products 35 and 39 were formed. These results can be understood from consideration of size- and shape-selective sorption combined with restriction on the mobility of the substrates and reaction intermediates imposed by the pentasil pore system. 

Mesylates are used for Ni-catalysed reactions. Arenediazodium salts 2 are very reactive pseudohalides undergoing facile oxidative addition to Pd(0). They are more easily available than aryl iodides or triflates. Also, acyl (aroyl) halides 4 and aroyl anhydrides 5 behave as pseudohalides after decarbonylation under certain conditions. Sulfonyl chlorides 6 react with evolution of SO2. Allylic halides are reactive, but their reactions via 7t-allyl complexes are treated in Chapter 4. Based on the reactions of those pseudohalides, several benzene derivatives such as aniline, phenol, benzoic acid and benzenesulfonic acid can be used for the reaction, in addition to phenyl halides. In Scheme 3.1, reactions of benzene as a parent ring compound are summarized. Needless to say, the reactions can be extended to various aromatic compounds including heteroaromatic compounds whenever their halides and pseudohalides are available. 

The reaction of 3,4-dimethoxyfuran with 2,3-diphenylcyclopropenone in refluxing toluene gave 2,3-dimethoxy-5,6-diphenylphenol (6 a) in 24% yield. Similarly, the reaction of 3,4-dimethoxyfuran with 2-methyl-3-phenylcyclopropenone produced 2,3-dimethoxy-5-methyl-6-phenylphenol (6b) in 9% yield. These phenols probably arise via the Diels-Alder 1 1 addition products followed by decarbonylation and rearrangement. The yields of these phenols were greatly improved to 51 and 69%, respectively, by applying 8-10 kbar to the reaction mixtures. ... 

As a minor pathway, L3 units are formed competitively to photo-Fries rearrangements some radicals formed in CO-O bond scissions may decarbonylate or decarboxylate before further radical recombination or hydrogen abstraction. This leads to the formation of hydroxy- and dihydroxybiphenyl units, aromatic ether structures, and phenol as end groups that further photolyzed into a mixture of species (in a convoluted absorption) that produces the yellowing of the PC film without any defined structure. 

Substitution reactions. Benzylation of phenols by benzyl methyl carbonates with Pd catalysis proceeds via transesterification and decarbonylation. Triarylmethanes are obtained from a reaction of benzhydryl carbonates with arylboronic acids. ... 

Furan is produced by the catalytic decarbonylation of 2-furaldehyde or by decarboxylation of 2-fiiroic acid with copper powder in quinoline. Furan is a colourless, water-insoluble liquid of pleasant odour, bp 32°C. Addition of hydroquinone or other phenols inhibits polymerization, which occurs slowly at room temperature. 

For example, the acyl-aryloxo bond cleavage (type b) is shown by the reaction of Ni(cod)2 with phenyl propionate in the presence of PPhs (Scheme 3.34) or 2,2 -bipyridine [65]. The reaction products are ethylene, phenol, and (car-bonyl)nickel complex. Formation of these products is conveniently understood by initial oxidative addition of EtC(0)-0Ph followed by decarbonylation, )S-hydrogen elimination and reductive elimination, though (acyl)(aryloxo)nickel(II) intermediate is not isolated. However, such an intermediate is isolated by the selective insertion of CO into the (alkyl)(aryloxo)nickel (or palladium) complexes, which smoothly affords esters by reductive elimination promoted by electron deficient olehns. The results suggest that the oxidative addition involving C-0 bond cleavage is essentially reversible. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

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