Rhodium catalysts acylation

Arylmetallic compounds have various, but not very widely used, applications in organic synthesis. Examples are acyl-de-metallation reactions using either dicobalt octacarbonyl in tetrahydrofuran (Seyferth and Spohn, 1969 Scheme 10-92), or carbon monoxide and a rhodium catalyst (Larock and Hershberger, 1980). 

The use of rhodium catalysts for the synthesis of a-amino acids by asymmetric hydrogenation of V-acyl dehydro amino acids, frequently in combination with the use of a biocatalyst to upgrade the enantioselectivity and cleave the acyl group which acts as a secondary binding site for the catalyst, has been well-documented. While DuPhos and BPE derived catalysts are suitable for a broad array of dehydroamino acid substrates, a particular challenge posed by a hydrogenation approach to 3,3-diphenylalanine is that the olefin substrate is tetra-substituted and therefore would be expected to have a much lower activity compared to substrates which have been previously examined. 

The carbonylation step that is involved in both hydroformylation and the Fischer-Tropsch reaction can be reversible. Under appropriate conditions, rhodium catalyst can be used for the decarbonylation of aldehydes285 and acyl chlorides.286... 

In the presence of a ruthenium catalyst, 3-diazochroman-2,4-dione 716 undergoes insertion into the O-H bond of alcohols to yield 3-alkyloxy-4-hydroxycoumarins 717 (Equation 285) <2002TL3637>. In the presence of a rhodium catalyst, 3-diazochroman-2,4-dione 716 can undergo insertion into the C-H bond of arenes to yield 3-aryl-4-hydroxy-coumarins (Equation 286) <2005SL927>. In the presence of [Rh(OAc)2]2, 3-diazochroman-2,4-dione 716 can react with acyl or benzyl halides to afford to 3-halo-4-substituted coumarins (Equation 287) <2003T9333> and also with terminal alkynes to give a mixture of 477-furo[3,2-f]chromen-4-ones and 4/7-furo[2,3-3]chromen-4-ones (Equation 288) <2001S735>. 

The reaction is first order in rhodium catalyst concentration, first order in dihydrogen pressure and has an order of minus one in carbon monoxide pressure. In our Scheme 6.1 this would be in accord with a rate-determining step at the end of the reaction sequence, e.g. reaction 6. Since the reaction order in substrate is zero, the rhodium catalyst under the reaction conditions predominates as the alkyl or acyl species any appreciable amount of rhodium hydride occurring under fast pre-equilibria conditions would give rise to a positive depence of the rate of product formation on the alkene concentration. The minus one order in CO suggests that the acyl species rather than the alkyl species is dominant under the reaction conditions. The negative order in CO is explained [20] by equilibrium... 

In the system described by Williams a rhodium catalyst was used, in the presence of an inorganic base and phenanthroline, and one equivalent of acetophenone, for the racemization, and a Pseudomonas sp. lipase for the acylation using vinyl acetate as the acyl donor (Fig. 9.3). 

Similar reactions have been carried out on acetylene.In an interesting variation, thiocarbonates add to aUcynes in the presence of a palladium catalyst to give a p-phenylthio a,p-unsaturated ester.Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones.In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopentanone with a rhodium-complex catalyst. An intramolecular acyl addition to an alkyne was reported using silyl ketones, acetic aid and a rhodium catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. 

Primary alcohols RCH2OH can be directly oxidized to acyl fluorides RCOF with cesium fluoroxysulfate. Lactones can be prepared by oxidizing diols in which at least one OH is primary, and addition of a chiral additive, such as sparteine, leads to lactones with high asymmetric induction. 2-(3-Hydroxypropyl)anihne was oxidized to an acyl derivative that cyclized to give a lactam when heated with a rhodium catalyst. ... 

Systematic studies on the isomerization of W-allylamides 24 and -imides to aliphatic enamides 25 were carried out with iron, rhodium, and ruthenium complexes as catalysts, Eq. (8). Regrettably, no prochiral substrate was applied for the rhodium catalyst bearing polymer-anchored DIOP [33]. In the framework of a study on the conjugative interaction in the isomerization of 1-azabicyc-lo[3.2.2]non-2-ene 26 to orthogonal enamine 27, catalyzed by either f-BuOK or RuH(NO)(PPh3)3, the enamine formation was calculated to be favored by 4 kcal mob, Eq. (9) [34]. Recently, the palladium-catalyzed isomerization of the N-acyl-2,5-dihydropyrroles 28 to N-formyl-2,3-dihydropyrroles 29 was reported, Eq. (10) [35]. 

Any discussion of mechanisms should take into account the early work on cobalt-catalysed hydroformylation. A monometal mechanism was proposed that has become the generally accepted pathway for both cobalt and rhodium catalysts. A more speculative mechanism was also suggested involving an inter-molecular hydride transfer from [CoH(CO)4] to [Co(acyl)(CO)4.]. Elimination... 

The HSAB concept can also be applied to the related hydrocarboxylation reaction, in which carboxylic acids are produced from olefins, CO, water, and small amounts of hydrogen. With hard cobalt/tert-amine catalysts, the products are the hard carboxylic acids, whereas rhodium catalysts give mainly aldehydes. Rhodiiun makes the intermediate acyl complexes softer, and in the subsequent elimination step H2, which is softer than H2O, gives aldehyde as product. 

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