Decarbonylation acid chlorides

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. 

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. 

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 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],... 

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). 

Perhaps the most industrially feasible approach has been developed by Rich and co-workers at General Electric, a palladium-catalyzed silylative decarbonylation reaction of aromatic acid chlorides with disilanes [Eq. (35)].97 One of the silicon centers from the disilane is transferred to the arene whereas the other acts as a chloride acceptor to produce the chlorosi-... 

Tihe decarbonylation reaction utilizing chlorotris (triphenylphosphine)-- rhodium(I) is a potentially useful method for converting acid chlorides to olefins or alkyl chlorides. Acid chlorides which contain a f3 hydrogen produce olefins (Reaction 1) while an alkyl chloride is the product when no / hydrogen is present (J, 2, 3, 4, 5, 6) (Reaction 2). 

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... 

In an attempt to resolve this question of stereochemistry and also to determine whether or not the decarbonylation of an acid chloride containing a f3 hydrogen takes place stereospecifically, erythro- (XI) and fhreo-2,3-diphenylbutanoyl chlorides (XII), obtained by the reaction of the known acids (13, 14) with oxalyl chloride, were synthesized. The reaction of these acid chlorides (see Reaction 8) with chlorotris( triphenyl-phosphine) rhodium gave the corresponding acyl complexes of type lib [R = C6H5CH(CH3)CH(C6H5)]. Decarbonylation of the erythro- cy complex in benzene at 30 °C gave a 90% yield of frans-a-methylstilbene while decarbonylation of the threo-acyl complex under similar reaction... 

Scheme 2 shows a similar mechanistic pathway for a Heck reaction taking place on a Pd octahedral comer. This mechanism is based on that established for soluble Pd catalysts (ref. 5). Adsorption of the aryl halide (or aryl acid chloride after decarbonylation) gives the aryl Pd halide, 15, by way of the adsorbed intermediate, 14. Vinyl ether adsorption, as in 16, takes place as described in Scheme 1. Aryl insertion gives the halometalalkyl, 17, which on f) elimination to the available 4dxy orbital gives the aryl enol ether, 2 (or 1 depending on which hydrogen is eliminated in 17). The resulting halo palladium hydride, 18, then reacts with the tertiary amine to give the amine hydrochloride and regenerates the octahedral comer for further reaction.

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. 

Aromatic acid chlorides are decarbonylated to aryl chlorides when they are heated to 300-360 C with palladium on carbon. The reaction proceeds by way of an aroylpalladium chloride, then to an arylpalla-dium chloride and finally through a reductive elimination to the aryl chloride. If the reaction is conducted in the presence of a reactive alkene under mild conditions the aroylpalladium chloride intermediate will sometimes acylate the alkene, as noted in Section 4.3.5.3.I. More usually, however, decarboxylation is more rapid than acylation, especially at higher temperatures (>100 C), and decarbonylation occurs. The... 

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). 

Under certain conditions, aroyl chlorides are converted to arylsilanes by the reaction with disilanes. The oxidative addition of aroyl chloride and decarbonylation are followed by transmetallation and reductive elimination to give aryl silanes. Neat trimellitic anhydride acid chloride (377) reacts with dichlorotetramethyldisilane (376) at 145 °C to generate 378, which affords 4-chlorodimethylsilylphthalic anhydride (379) by reductive elimination. Finally it was converted to 380 and used for polyimide formation [185], Biphenyltetracarboxylic anhydride 381 is obtained at a higher... 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94],... 

The second method, referred to as the decarbonylation reaction, concerns a base-catalyzed elimination of HC1 and CO from an acid chloride.31 The amide moiety was converted into the amino acid 15 by reaction with Na202 in water.32 Alternatively, amides can be converted into the corresponding carboxylic acid by treatment with concentrated HC1.33 However, this method produced lower yields as a result of some decomposition. The carboxylic acid 15 was treated with the Vilsmeier reagent and triethylamine furnishing the imine 16 via decarbonylation. The imine 16 was then converted, in a similar manner to the retro-Strecker method, to the unprotected homoall-ylamine 17 in 75% overall yield and an ee of 98%. 

In addition to alkyl, aryl and vinylcobalt/radical species, acid chlorides may be converted into acylcobalt species by reaction with the cobalt(I) salens. These generate acyl radicals under photolysis, which participate in similar reactions to the alkyl radicals in most cases (equations 184 and 185). Acylcobalt(III) species bearing an a-aryl or vinyl substituent, on the other hand, undergo concomitant decarbonylation to afford a benzylic or allyl radical, which then may undergo a number of bond-forming processes, including homocoupling361. 

The Stille coupling may be combined with carbonylation in two ways. Acid chlorides may be used as substrates for the reaction with vinyl or aryl stannanes. However, an atmosphere of carbon monoxide is frequently required to prevent decarbonylation after the oxidative addition step. 

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 oxidative addition of carboxylic acid chlorides to Pd(0) and Pt(0) forms the trans-acyl complexes, which are stable toward decarbonylation at RT. The acyl chlorides are formed more rapidly as well owing to the greater facility with which acyl halides undergo nucleophilic attack. The Ni(0) analogs are more prone to undergo decarbonylation . 

A unique alkoxymethylation reaction can be acconplished by treatment of a-alkoxycarboxylic acid chlorides with ketones in the presence of Sml2 (equation 28). llie reaction is postulated to proceed by a reductive decarbonylation process, leading to a relatively stable a-alkoxy radical. Addition ctf this radical to the Sm -activated carbonyl and further reduction and hydrolysis provides the observed product. An... 

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). ... 

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