Decarbonylation benzaldehyde

At temperatures above ca. 200°C, the decarbonylation reaction can be driven catalytically (1,4,14, 20). Scheme I illustrates the proposed catalytic reaction scheme (15,16). This catalytic reaction is slow (activity for benzaldehyde decarbonylation at 178°C is 10 turnovers hr-1) presumably because the oxidative addition of RCOX to RhCl(CO)(PPh3)2 is difficult (7, 21, 22). Consistent with this, the rate is significantly greater when IrCl(CO)(PPh3)2 is used as the catalyst (benzaldehyde, 178°C, activity is 66 turnovers hr-1) (23). Oxidative addition to iridium complexes is well known to be more facile than with their rhodium analogues. 

Figure 11.2. Plot of fcobs vs. [PhCHO] for benzaldehyde decarbonylation using [Rh(dppp)2]BF4 as catalyst at 150°C.

Decarbonylation of Benzaldehyde Using Cationic Diphosphine Complexes of Rhodium (I)... 

Table I. Catalytic Decarbonylation of Benzaldehyde into Benzene and CO...

The mono(diphosphine) complexes, [Rh(dppp)]BF4 and RhCl-(dppp), are less effective than [Rh(dppp)2] + but are still more active than RhCl(PPh3)3. The mono(diphosphine) catalysts also decompose slowly under the reaction conditions, which renders them less useful than the bis(diphosphine) catalysts. The slower rate of decarbonyla-tion observed with the mono(diphosphine) catalysts compared with the bis(diphosphine) catalysts presumably is due to the lower basicity of the former which retards the rate of oxidative addition (vide infra). Consistent with this is the observation that [Rh(COD)(dppp)]BF4 (COD = 1,5-cyclooctadiene) shows a higher rate for catalytic de-carbonylation of benzaldehyde than does [Rh(dppp)]BF4 (22). An additional observation is that the type of anion, Cl or BF4 , has no apparent effect on decarbonylation rates for the bis(diphosphine) catalysts however, for the mono(diphosphine) complexes the chloride salts show slightly lower rates than their tetrafluoroborate analogues. 

Reaction Steps 3a and 3b also can be used to rationalize the observed para-substituent effects presented in Table III the more electron-releasing, para-substituted benzaldehydes retard the rate of oxidative addition (18) for RhCl(PPh3)3. Therefore, p-methyl- and p-methoxybenzaldehyde are expected to be decarbonylated slower than the unsubstituted benzaldehyde, as is observed in Table III. (This argument requires that Reaction 3a be saturated to the right, which is expected, in neat aldehyde solvent with electron-releasing, para-substituted benzaldehydes.) The unexpected slower rate for p-chloro-benzaldehyde could be accounted for ifK for this aldehyde is small and saturation of equilibrium in Equation 3a is not achieved. Note that fcobs is a function of K and k (see Equation 4b) under this condition. It is also possible that the rate-determining step is different for this aldehyde. Present research includes a careful kinetic analysis using several aldehydes so that K and k can be determined independently. 

In light of the previous comments it is possible to construct an overall mechanistic scheme for the decarbonylation of benzaldehyde (see Scheme II). Recall that various reagents retarded the decarbonyla-... 

Benzoylchloride presents a special case since its presence completely stops the decarbonylation of benzaldehyde and no chlorobenzene is produced. Analysis of this reaction has shown that PhCOCl reacts with [Rh(dppp)2]Cl to produce Rh(Cl)2(PhCO)(dppp). This rhodium(III) complex is inert to migration and reductive elimination and therefore no decarbonylation products are produced (26). 

Plasma reactions which seem especially interesting for preparative work are eliminations. In many plasma reactions atoms or small groups are eliminated without destroying the rest of the molecule (Table 2). Thus aldehydes easily decarbonylate to the corresponding hydrocarbons 20T The product obtained from benzaldehyde is mainly benzene and to a lesser extent biphenyl ... 

The 4,5-corane (84) is obtained in SOX yield on photo-decarbonylation of the pentacyclic ketone (85). Photochemical decomposition of the carbonate (86), by the loss of carbon dioxide, affords a mixture of products containing oxirane. styrene oxide, bibenzyl and phenylacetaldehyde. Triplet sensitized irradiation yields products solely from benzyl radicals. - An earlier study of the irradiation (at 254 nm) of the carbonate (87) reported that benzaldehyde, phenyl carbene, and carbon dioxide were produced. A reinvestigation of the irradiation of this compound (at 254 nm in acetonitrile) has provided evidence that the cis- and trans-stilbene oxides (88) and (89) are formed as well as deoxybenzoin and smaller amounts of diphenylacetaldehyde and bibenzyl. When methanol is used as the solvent the same products are produced accompanied by benzylmethyl ether, 1,2-diphenylethanol, and 2,2-diphenylethanol. These authors suggest that the oxiranes (88) and (89) are formed by way of... 

Chelation-assisted additions of formyl C-H bonds to olefins and dienes have been reported by Jun et al. [120]. In the case of the reaction of 8-quinolinecar-boxaldehyde, they proposed that the formation of the stable 5-membered met-allacyclic complex [121] suppressed the undesired decarbonylation reaction (Eq. 53) [120]. The intermolecular hydro acylation of 1-alkene with 2-(diphenyl-phosphino)benzaldehyde by rhodium(I) catalyst has been conducted on the basis of this working hypothesis [122]. 

Decarbonylation and Decarboxylation - The kinetics of the decarbonylation of benzaldehyde into benzene and carbon monoxide has been reported in detail. The m-di-/-butylcyclopropanone (62, R = H) has been synthesised and photochemically this readily undergoes decarbonylation to yield the corresponding cis-... 

Lock (1933) found long ago that two classes of polychlorinated benzalde-hydes exist, distinguished by their behaviour toward ethanolic potassium hydroxide (1) benzaldehydes that undergo the Cannizzaro reaction and (2) benzaldehydes that decarbonylate. In the second class, the formyl group is invariably flanked by two o-chlorines. These results, which are analogous to the dealkylations, indicate likewise that, when the Cannizzaro reaction— which is known to be initiated by hydroxide-ion addition to the formyl... 

Table 4. Catalytic DANIEL H. DOUGHTY AND LOUIS H. PIGNOLET Decarbonylation of Benzaldehyde Using Rhodium Catalysts ... 

Table 7. Inhibition in the Rate ofCataiytic Decarbonylation of Benzaldehyde ...

In principle, the relative magnitudes of the specific rate constants can be determined by complete kinetic analyses for the dppe and dppp systems using several pam-substituted benzaldehydes. Providing that all of these systems exhibit the same mechanism, this series of experiments should permit determinations of the specific rate and/or equilibrium constants of Equation 23. Although the observed para-substituent effect on kobs for the decarbonylation using [Rh(dppp)2] (Table 5) can be rationalized by either mechanism, a detailed comparison of the specific rate constants and equilibrium constants should permit a mechanistic distinction. These experiments have not yet been carried out. 

Catalytic decarbonylation of benzaldehyde using several iridium complexes has also been examined. Results of these experiments are shown in Table 8. The main points to be made here are (i) [Ir(P-P)2] catalysts have activities that are ca. twenty times lower than their Rh analogs (ii) the iridium mono-diphosphine catalysts are better than the 6w-diphosphine Ir catalysts (opposite trend noted using rhodium, see Table 5) and (iii) IrCl(CO)(PPh3)2 is a much better catalyst than RhCl(CO)(PPh3)2 and is also better than most of the iridium diphosphine catalysts. The results for the [M(P-P)2] catalysts may be explained in terms of the proposed mechanistic scheme in Figure 11.3. Since Ir-P bonds should be stronger than Rh-P bonds, the value of ki will be smaller for the Ir catalysts, thus... 

The rates of catalytic decarbonylation of benzaldehyde using mono-diphosphine complexes of Rh and Ir provide an interesting comparison. First of all, the mono-diphosphine complexes of Rh and Ir are not robust under the conditions of the catalytic reaction and therefore are of little practical use. However, they do provide useful data for mechanistic arguments. With Rh, the bis-diphosphine catalysts [Rh(P-P)2] are always more active than their mono-diphosphine analog [Rh(P-P)] when neat aldehyde is used as solvent. Although Rh-P bond rupture is not necessary with the coordinatively unsaturated mono-diphosphine complexes, the rhodium may not be electron-rich enough to promote facile oxidative addition. In support of this argument, the presence of the diolefin cod in the coordination core, [Rh(cod)(dppp)]", increased the activity of decarbonylation by a factor of 6 compared with [Rh(dppp)]. With Ir... 

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