Decarbonylation of acid chlorides

The mechanism that has been proposed for the decarbonylation of acid chlorides by chlorotris (triphenylphosphine) rhodium (I) involves acyl- and alkyl- or arylrhodium complexes (2). Three of the intermediates in the catalytic cycle, II, III, and IV (see Reaction 3), can be isolated in this unique reaction. Both acyl complexes (II) and alkyl... 

There is general agreement on the mechanism for the stoichiometric decarbonylation of acid chlorides (9,14,15,16). The overall mechanism is shown by Equation set 2 where X = Cl. The stoichiometric decarbonylation reaction results from initial oxidative addition of the acid chloride to RhCl(PPh3)2 (Equation 2b, X = Cl). RhCl(PPh3)2 is a very reactive, low-concentration intermediate which is likely to be solvated (see Equation 2a) (17). 

Homoallylic alcohols. Allylsamarium reagents prepared in tetrahydropyran are quite stable (in THF Wurtz coupling occurs, therefore the Barbier technique must be employed). The sequential procedure made possible by change of solvent gives cleaner products as side reactions such as pinacol formation from ketones and decarbonylation of acid chlorides are avoided. Because of the low basicity allylsamarium reagents can be used to allylate allyl acetoacetate. 

A considerable amount of work has been performed over the last 15 years to determine the mechanism of acid chloride decarbonylation with RhCl(PPh3)3/ " Although the discovery of aldehyde decarbonylation preceded that of acid chlorides/ much more time has been spent on the acid chloride system because it is more easily studied. Many intermediates have been isolated and characterized (see Table 1). Even though the mechanism of the catalytic reaction is not well understood, the mechanism for the stoichiometric decarbonylation of acid chlorides has been proposed. However, the generally accepted mechanism has recently been challenged/ In this section, we will first review the stoichiometric decarbonylation mechanism for acid chlorides, followed by the stoichiometric decarbonylation of aldehydes. Finally, the mechanism of catalytic decarbonlyation of acid chlorides and aldehydes will be discussed. 

Table 1. Isolated or Observed Intermediates during the Stoichiometric Decarbonylation of Acid Chlorides Using RhCliPPhsIs... 

The results discussed above lead to the conclusion that the mechanism of decarbonylation of aldehydes is very similar to that postulated for the decarbonylation of acid chlorides. However, kinetic studies of the reaction show that a different rate -limiting step is operative with aldehydes. With acid chlorides, the rate-limiting step is thought to be migration or reductive elimination, depending on the R-group. " A detailed kinetic study on the stoichiometric decarbonylation of aldehydes with RhCl(PPh3)3 has... 

A plausible, but speculative, reaction scheme for the catalytic decarbonylation of acid chlorides (and aldehydes) that involves phosphine dissociation from one of the Intermediates has been postulated. It is clear, though, that there are presently not enough facts to substantiate this hypothesis. Future work on the mechanism of catalytic decarbonylation using RhCl(CO)(PPh3)2 and other catalysts which investigates phosphine inhibition could be very informative. 

Rhodium-Catalyzed Regioselective C-H Functionalization via Decarbonylation of Acid Chlorides and C-H Bond Activation Under Phosphine-Free Conditions... 

Acyl halides are intermediates of the carbonylations of alkenes and organic-halides. Decarbonylation of acyl halides as a reversible process of the carbo-nylation is possible with Pd catalyst. The decarbonylation of aliphatic acid chlorides proceeds with Pd(0) catalyst, such as Pd on carbon or PdC, at around 200 °C[109,753]. The product is a mixture of isomeric internal alkenes. For example, when decanoyl chloride is heated with PdCF at 200 C in a distillation flask, rapid evolution of CO and HCl stops after I h, during which time a mixture of nonene isomers was distilled off in a high yield. The decarbonylation of phenylpropionyl chloride (883) affords styrene (53%). In addition, l,5-diphenyl-l-penten-3-one (884) is obtained as a byproduct (10%). formed by the insertion of styrene into the acyl chloride. Formation of the latter supports the formation of acylpalladium species as an intermediate of the decarbonylation. Decarbonylation of the benzoyl chloride 885 can be carried out in good yields at 360 with Pd on carbon as a catalyst, yielding the aryl chloride 886[754]. 

The reduction of acid chlorides may proceed at lower temperatures in the presence of a tertiary amine or sodium acetate. Peters and van Bekkum improved the method of Sakurai and Tanabe,260 using ethyldiisopropylamine, instead of A(7V-dimethylaniline, as a HC1 acceptor.261 Ethyldiisopropylamine had the advantage of forming an acetone soluble hydrochloride, and workup of the reaction mixture was easier when acetone was used as solvent. Reductions in the presence of these basic substances have been found to be especially effective when the acid chlorides are labile to decarbonylation. Examples of the use of base are shown in eqs. 13.139261 and 13.140.262 When the original procedure of the Rosenmund reduction was applied to 1 -/-bulylcyclohcxanc-carbonyl chloride, f-butylcyclohexanc was the sole product, compared to greater than 95% yield of the corresponding aldehyde in the presence of ethyldiisopropylamine or sodium acetate.261... 

Carboxylic acid halides present interesting alternative processes (Scheme 5). Oxidative addition of acid chlorides to Pd occurs under mild conditions, and the resulting acyl-Pd complex will undergo decarbonylation upon heating. The decarbonylated product, an organo-Pd species, can undergo... 

The acylation is limited to the use of acid chlorides due to their unique ability to oxidatively add palla-dium(0) other acylating agents are not generally useful in this context. It was also known that acid chlorides do not react with organotins without Lewis acid catalysis and more importantly organotins do not generally react with the expected product, the ketone, except under very strong Lewis acidic conditions. Even diacid chlorides may be utilized in this process (equation 77) however, oxalyl chloride cannot be used due to the indicated decarbonylation of the intermediate acid chloride (equation 78). ... 

Stille, J. K., Huang, F., Regan, M. T. Mechanism of acid chloride decarbonylation with chlotetris(triphenylphosphine)rhodium(l). Stereochemistry and direction of elimination. J. Am. Chem. Soc. 1974, 96,1518-1522. 

Decarbonylation and reduction of acid chlorides proceed with CoH(N.)(PPh3)3 to give the corresponding hydrocarbons (eq (14)) [18]. 

The stoichiometric decarbonylation reaction begins with the oxidative addition of acid chloride to RhCl(PPh3)2 (Equation 7b), which is presumably a solvent-stabilized, very reactive intermediate/ Tolman " and Halpern " have presented kinetic evidence for the importance of RhCl(PPh3)2 in the catalytic hydrogenation of olefins by RhCl(PPh3)3. In addition, the solvated species, RhCl(S)(PPh3)2 (where S = DMF, acetonitrile), was observed in the stoichiometric decarbonylation of aldehydes vide infra). 

Chlorides can be prepared under mild, essentially neutral, conditions from hydroxyl compounds with triphenylphosphine in CCI4 Also acid chlorides can be prepared by this method Ar. chlorides in particular can be obtained in high yield at relatively low temp, by decarbonylation of aroyl chlorides with chlorotris-(triphenylphosphine) rhodium 99%-Formic acid has been found to be an excellent medium for chloromethylations... 

The mechanism (Scheme 48) ° is expected to proceed through the acylpalladium species much as in the Rosenmund reduction. Indeed, the acyl complex 56 from oxidative addition of vinyl chloride with Pd(CO)(Ph3P)3 was isolated (Scheme 49). " The reaction of acid chlorides with the same catalyst provides aldehydes. However, aliphatic acid chlorides do not reduce effectively. The phosphine ligands present in the Heck acylpalladium intermediate are thought to be the cause, allowing decarbonylation and elimination to occur. Interestingly, the formylation will not occur with the Rosenmund catalyst. 

Iridium complexes have been shown to catalyse intermolecular additions of acid chlorides R COCl to terminal alkynes R C=CH producing (Z)- -chloro-a, -unsaturated ketones R C(Cl)=CHCOR. Ligands play a key role in this reaction NHC is efficient for the addition of aroyl chlorides, whereas dicyclohexyl(2-methylphenyl)phosphine (PCy2(o-Tol) is indispensable for the reaction of aliphatic acid chlorides. Among the salient features of these transformations is the suppression of decarbonylation and j6-hydrogen elimination. Some mechanistic insight has been obtained from stoichiometric experiments. ... 

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],... 

The product described here, 4-(4-chlorophenyl)butan-2-one, was previously prepared in the following ways a) by reduction of the corresponding benzalacetone, b) by catalyzed decarbonylation of 4-chlorophenylacetaldehyde by HFeiCO) in the presence of 2,4-pentanedione, - c) by reaction of 4-chlorobenzyl chloride with 2,4-pentanedione under basic catalysis (K2CO3 in EtOH), d) by reaction of 4-chlorobenzyl chloride with ethyl 3-oxobutanoate under basic catalysis (LiOH), - and e) by reaction of 3-(4-chlorophenyl )-propanoic acid with methyl lithium. - ... 

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). 

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