Catalysts in carbonylations

The remarkable activity of copper catalysts in carbonyl hydrogenation and alcohol dehydrogenation prompts their use also for the racemization of chiral secondary alcohols. Actually, since the first report on chemoenzymatic dynamic kinetic resolution [68], racemization of alcohols via the corresponding ketone has attracted considerably attention, owing to its role as backbone in this resolution [69, 70]. 

In Chapter 12 pyridine was often used as a catalyst in carbonyl substitution reactions. It can act in two ways. In making esters from acid chlorides or anhydrides pyridine can act as a nucleophile as well as a convenient solvent. It is a better nucleophile than the alcohol and this nucleophilic catalysis is discussed in Chapter 12 (p. 282). But nonnucleophilic bases also catalyse these reactions. For example, acetate ion catalyses ester formation from acetic anhydride and alcohols. 

Immobilized copper-zeolite Y (Cu-HY) bis(oxazolines) were employed as heterogeneous catalysts in carbonyl-ene and imino-ene reactions, allowing the synthesis of a-hydroxy and a-amino carbonyl compounds 163 from 161 and 162 in satisfactory yields and high enantioselection <04AG(E)1685>. The use of a new, insoluble polystyrene-bound Box ligand (IPB-BOX) was also described with good activity (85-95% ee) <04TA3233>. 

The complex [RUL3] (LH = (-I-)-3-acetylcamphor) can be reduced by treatment with Na/Hg for 40 h, or oxidized by controlled potential electrolysis to the corresponding Ru and Ru species respectively with retention of configuration. " The Ru diastereomers are found to isomerize at 170°C. Electron transfer and reduction of derivatives of [Ru(acac)3] by Ti " have been investigated, the electron transfer in the ketoenolato bridged Ru—L—Ti binuclear intermediate occurring with a rate constant k = 2 s . [Ru(acac)j] has been found to be an active catalyst in carbonylation and homologation reactions, ... 

Nickel(II) phosphine complexes have been reported to he efficient catalysts in carbonylation reactions. To investigate this reaction mechanism, we have studied the reaction of CO on the related Ni(II) complexes NiX2(PMes)n (n = 2,3) and [NiX(PMes)m]BFj> (m = 3,4). Pentacoordinate carbonyl nickel(II) species (without reduction of Ni(II) to Ni(0)) were isolated (1) by direct substitution of PMcs by CO in the pentacoordinate complex and (2) by addition of CO on the trans square-planar tetracoordinate complex. These compounds are trigonal-bipyramidal complexes with CO in equatorial position. The Ni-CO distance (1.73 A) is the shortest reported Ni-CO distance. Since these carbonylation reactions can be viewed as substitution of an equatorial PMes by CO in a TBP, they can be related to the substitution reactions in square-planar d metal complexes. 

An industrially important example is the activation of [Co2(CO)8] with hydrogen (Eq. 2-49), the resulting complex being the active catalyst in carbonylation reactions. 

In Chapter 10 we used pyridine as a catalyst in carbonyl substitution reactions, even though it is only a weak base. Catalysis by pyridine involves two mechanisms, and is discussed on p. 200.Acetate ion is another weak base which can catalyse the formation of esters from anhydrides ... 

Compared with metathesis [6], the ability of ruthenium catalysts in carbonylation is also impressive. 

In summary, we have summarized representative examples of transition-metal-catalyzed carbonylative domino reactions. In the area of carbonylations, palladium, rhodium, and cobalt are still the main actors. The abihty of palladium catalysts in carbonylative cross-coupling, rhodium catalysts in carbonylative C-H activation, and cobalt catalyst in carbonylative reactions with unsaturated bonds is impressive. 

In the future, cheap catalysts such as iron and copper are expected to be explored and applied. In the case of noble metals, their reaction eflBciency and selectivity should be improved. The use of nickel catalysts in carbonylation is potentially accompanied with the formation of Ni(CO)4, which is highly dangerous for the operators. Therefore, methods for stabilizing Ni must be developed before Ni can be used in catalytic reactions. 

The formation of metal-oxo species which are not reduced by CO under the reaction conditions, or are only very slowly so, probably accounts for the fact that these reductions are not catalytic in the metal. In our experience [74], several molybdenum, vanadium and rhenium complexes that may act through a sequence of reactions like the one in eqs. 22 and 23, gave only negative results when used as catalysts in carbonylation reactions of nitroarenes. It appears that reaction 22 is fest in many cases, but reaction 23 is very slow, even when thermodynamically favourable, and its rate is too low to allow for an efficient catalytic cycle even at high temperature and under CO pressure. 

Mlither K, Oestreich M (2011) Self-regeneration of a silylium ion catalyst in carbonyl reduction. Chem Commun 47 334... 

---