Decarbonylated product

The reduction of acyl halides with hydrogen to form aldehydes using Pd catalyst is well known as the Rosenmund reduction[756]. Some acyl chlorides give decarbonyiation products rather than aldehydes under Rosenmund conditions. The diene 890 was obtained by decarbonyiation in an attempted Rosenmund reduction of acetyloleanolic acid chloride (889)[757], Rosenmund reduction of sterically hindered acyl chlorides such as diphenyl- and tnpheny-lacetyl chloride (891) gives the decarbonylated products 892[758],... 

Analogously, the tetrabromotetrapropylporphycene 9 furnishes the corresponding isocorroie-carbaldehyde 10 in good yields, but in this case no decarbonylated product was observed. The mechanism of these interesting ring contractions of porphycenes into isocorroles still needs to be determined. 

Chloroacylation of terminal aryl, alkyl or alkenyl alkynes [Le. the addition of RC(=0)-C1 across the CC triple bond] with aromatic acyl chlorides was catalysed by [IrCl(cod)(lPr)] (5 mol%) in good conversions (70-94%) in toluene (90°C, 20 h). Z-addition products were observed only, hitemal alkynes were umeactive. Surprisingly, a phosphine/[lr(p-Cl)(l,5-cod)]2 system under the same conditions provides decarbonylation products (Scheme 2.34) [117]. 

Afonso, Gois, and co-workers followed this report with an unexpected decarbonylation of diazo-acetamides (Scheme 9.14) using 43-NHC (Fig. 9.8) [58]. The reaction generated three different products, with low selectivity for the decarbonylated product. The authors tested other substrates with different R groups and bulk at the amine position but found no correlation to the amount of decarbonylation product formed. However, 43-IPr was more selective than 43-SIPr for the decarbonylation product. The authors attributed the decarbonylation to the axial coordination of the NHC ligand to the dirhodium (11) complexes. 

In previous sections of this chapter we have seen many examples of type I cleavage reactions in which loss of carbon monoxide was not an important process. In the examples given above, however, decarbonylation is important, as evidenced by the high yields of decarbonylated products. Factors which facilitate decarbonylation include the presence of a suitably located cyclo-... 

Thermolysis also produced the decarbonylation product plus a dimer, structure C. 

Under free-radical conditions, the reaction of (TMS)3SiH with acid chlorides, RC(0)C1, gives the corresponding aldehydes and/or the decarbonylation products depending on the nature of substituent R [42]. The reduction of 1-adamantanecarbonyl chloride is given in Reaction (4.19). 

The photoreactions of a-dicarbonyl compounds are quite different in the vapor and condensed phases. In the vapor phase, carbon-carbon bond cleavage is the preferred mode of reaction but in the condensed phase, many of the observed reactions can be rationalized by a mechanism involving hydrogen abstraction. Internal hydrogen abstraction, when possible, is generally preferred over abstraction from the solvent. With the exception of diethyl oxalate, which undergoes photoreactions typical of an ester, only those compounds that are reasonably strained or can yield reasonably stable free radicals give decarbonylation products. In the presence of suitable substrates, cycloaddition reactions have also been observed. 

Analogously, the tricyclic oxo lactone 9 gave, on direct irradiation in diethyl ether, the cyclobu-tanone 10 and its decarbonylation product 11.69... 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Carboxylic acid chlorides and chloroformate esters add to tetrakis(triphenylphosphine)palladium(0) to form acylpalladium derivatives (equation 42).102 On heating, the acylpalladium complexes can lose carbon monoxide (reversibly). Attempts to employ acid halides in vinylic acylations, therefore, often result in obtaining decarbonylated products (see below). However, there are some exceptions. Acylation may occur when the alkenes are highly reactive and/or in cases where the acylpalladium complexes are resistant to decarbonylation and in situations where intramolecular reactions can form five-membered rings. 

An important characteristic of the bis(diphosphine) catalysts is the remarkable selectivity observed in the decarbonylation products. Recall that 1-heptanal is converted into 86% hexane and 14% 1-hexene by RhCl(PPh3)3 (25°C, stoichiometric reaction) (4). In contrast, using [Rh(dppp)2]+ as the catalyst, the only volatile product is hexane,... 

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). 

Racemization reactions due to successive exchange of ligand roles and successive Walden inversions were discussed in Sections 3.1 and 3.2. The decarbonylation reaction seems to be stereospecific, yet, as mentioned in Section 3.3, subsequent to CO elimination there is a racemization of the decarbonylation products. 

Hostettler has examined the photochemistry of several 3-substituted derivatives of 2,2,4,4-tetramethylcyclobutanones (Table 4).31> In solvents containing proton donors, two major products were observed, the rearranged cyclic acetal 22 and the decarbonylation product 23. In inert solvents such as ether, cyclohexane and benzene, formation of 22 did not occur. 

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. 

Triply bridging orf/io-phenylene complexes sometimes arise in curious ways. In the reaction of Os3(CO)10(MeCN)2 with salicylaldehyde benzy-limine one would naively expect that coordination would be through N and O atoms as with other metal systems. The first formed compound (40, Scheme 6) results from ortho metallation while, of the three isomeric decarbonylation products (41 to 43), compound 43 contains only Os—C bonds to the ligand and is a ring-substituted version of Os3H2(C6H4)(CO)9 (Scheme 6) (237). 

Insertion of the alkyne into the chromium carbene bond in intermediate B affords vinyl carbene complex D, in which the C=C double bond may be either (Z) or (E). A putative chromacydobutene intermediate resulting from a [2+2] cydoaddition of the alkyne across the metal-carbene bond on the way to chromium vinylcarbene D, as was sometimes suggested in early mechanistic discussions, has been characterized as a high energy spedes on the basis of theoretical calculations [9c]. Its formation and ring-opening cannot compete with the direct insertion path of the alkyne into the chromium-carbene bond. An example of an (E)-D alkyne insertion product has been isolated as the decarbonylation product of a tetracarbonyl chromahexatriene (4, Scheme 4) [14], and has been characterized by NMR spectroscopy and X-ray analysis. 

Very recently, a new strategy for the hydroesterification and hydroamidation of olefins was reported by Chang and coworkers [83]. They used a chelation-assisted protocol for the hydroesterification of olefins. The reaction of 2-pyridylmethyl formate with 1-hexene in the presence of a Ru3(CO)12 catalyst gave the hydroesterification product in 98% yield as a mixture of linear and branched isomers (Eq. 54). The chain length of the methylene tether is important for a successful reaction. Thus, the reaction of 2-pyridyl formate (n=0) afforded 2-hydroxypyridine, a decarbonylation product, and the reaction of 2-pyridiylethyl formate (n=2) resulted in a low conversion (7% conversion) of the starting formate. From these results, the formation of a six-membered ruthenacycle intermediate is crucial for this chelation-assisted hydroesterification. 

The decarbonylation product cyclopropane 23 was not analyzed quantitatively under the conditions of photolysis... 

Klemm (16) and Lee and coworkers (17) have examined the effect of various solvents on the photochemistry of cyclobutanone. By monitoring the quantum yields for formation of ethylene (B-cleavage product) and cyclopropane (decarbonylation product) in different solvents, they were able to demonstrate a significant reduction in the quantum yields for product formation in methanol as compared to other hydrocarbon solvents. Whereas the quantum yield of ethylene formation was found to be essentially solvent insensitive, that for cyclopropane formation was found to be somewhat solvent sensitive. This suggested that B-cleavage and decarbonylation do not result from the same immediate precursor. Since ring-expansion derivatives have not been isolated from photolyses carried out in saturated hydrocarbon solvents, the importance of this process under these conditions remains to be determined. Irradiation of cyclobutanone in the presence of 1,3-penta-diene (17,59) or 1,3-cyclohexadiene (16) did not appear to affect the quantum yields for ketone disappearance or product appearance. 

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