Hydrocarbonyl complexes

Often the aldehyde is hydrogenated to the corresponding alcohol. In general, addition of carbon monoxide to a substrate is referred to as carbonylation, but when the substrate is an olefin it is also known as hydroformylation. The eady work on the 0x0 synthesis was done with cobalt hydrocarbonyl complexes, but in 1976 a low pressure rhodium-cataly2ed process was commerciali2ed that gave greater selectivity to linear aldehydes and fewer coproducts. 

Piacenti et al. suggested that the different results at low and high carbon monoxide pressure were due to different catalytic intermediates (A and B) under the two sets of conditions. Thus at low pressures A caused a rapid olefin isomerization and the formation of similar product distributions of aldehydes from 1- and 2-pentene. At high pressures little olefin isomerization occurred and 1-olefin yielded significantly more straight-chain aldehyde than 2-olefin. This would seem consistent with Heck and Breslow s mechanism (62) if A were an acylcobalt tricarbonyl in equilibrium with isomeric olefin-cobalt hydrocarbonyl complexes and B were an acylcobalt tetracarbonyl. 

Originally, Piacenti et al. explained the formation of isomeric products in terms of an equilibrium of alkylcobalt carbonyls with olefin-hydrocarbonyl complexes as in the Oxo reaction. More recently, however, they have noted that the conditions under which n-propyl orthoformate gave no isomeric products (below 150° C, carbon monoxide pressure 10 atm) are conditions under which isomerization occurs readily in the hydroformylation of olefins (115). Since alkylcobalt carbonyls were formed in both reactions they dismissed the possibility that this isomerization was due to alkyl- or acylcobalt carbonyls. The fact that Takegami et al. have found that branched-chain acylcobalt tetracarbonyls isomerize more readily than straight-chain acylcobalt tetracarbonyls would seem to fit in quite well with the results of Piacenti et al., however, and suggests that the two findings may not be so irreconcilable as might at first appear (see Section II, B,2). 

Structural types of agostic alkyl and unsaturated hydrocarbonyl complexes. 

Example 5.2. Hydroformylation of propene [2]. Hydroformylation converts an olefin to an aldehyde of next higher carbon number by addition of carbon monoxide and hydrogen. The reaction is catalyzed by dissolved hydrocarbonyl complexes of transition-metal ions such as cobalt, rhodium, or rhenium. The carbon atom of the carbon monoxide can attach itself to the carbon atom on either side of the olefinic double bond, so that two aldehyde isomers are formed. If the catalyst also has hydrogenation activity, the aldehydes are converted to alcohols and paraffin is formed as by-product. For propene and such a catalyst the (simplified) network is ... 

M. Orchin University of Cincinnati) Some very recent work done in our laboratory by Lawrence Kirch strongly suggests that an olefin-hydrocarbonyl complex is the important intermediate in the oxo synthesis. This new evidence was made possible by the experimental technique of quenching the hot, pressured autoclave in dry ice and releasing the gases below — 50°. The results of this work (1) show that (a) dicobalt octacarbonyl is rapidly converted to cobalt hydrocarbonyl (6) the hydrocarbonyl is rapidly complexed by olefin (c) when the olefin is consumed by normal oxo reaction, the cobalt again appears as the hydrocarbonyl (d) the extent of conversion of dicobalt octacarbonyl to cobalt hydrocarbonyl is dependent on the hydrogen partial pressure. 

Heck and Breslow have postulated equilibria between the acylcobalt-carbonyls, the alkylcobaltcarbonyls and the olefin-hydrocarbonyl complexes [35] and Takegami et aL have demonstrated in quite a number of experiments that acylcarbonyls can be readily isomerized [58, 70]. Heck and Breslow [35] explain the significant change in the isomer distribution mentioned above by the different thermal stability of the organometallic... 

More recently, systems based on polypyridine coordination compounds of ruthenium(II) [46-49], rhodium(I) [50a] and iridium(I) [50] have been shown to efficiently catalyse the thermal WGSR. An important effect of the substituent ortho to the nitrogen atom of the ligand has been demonstrated in the case of Ir(I) leading to one of the most efficient catalysts known today [50b]. [Ru(bpy)2(CO)Cl] has also been studied and all of the possible intermediates within the catalytic cycle (hydrocarbonyl complex, metal hydride, aquo species) have been isolated and characterized [48]. 

Ruthenium complexes have been used in the hydrocarbonylation of simple esters to produce the corresponding homologous esters (50). The hydrocarbonylation affects the alkyl moiety rather than the carboxylate group ... 

Hydroformylation, or the 0X0 process, is the reaction of olefins with CO and H9 to make aldehydes, which may subsequently be converted to higher alcohols. The catalyst base is cobalt naph-thenate, which transforms to cobalt hydrocarbonyl in place. A rhodium complex that is more stable and mnctions at a lower temperature is also used. 

Carbonylation reactions have been observed using both Pd(II)-alkene complexes and CT-bonded Pd(II) species formed by oxidative addition. Under reductive conditions, the double bond can be hydrocarbonylated, resulting in the formation of a carboxylic acid or ester.238 In nucleophilic solvents, the intermediate formed by solvopalladation is intercepted by carbonylation and addition of nucleophilic solvent. In both types of reactions, regioisomeric products are possible. 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

Cobalt hydroformylation of butadiene produced low yields (24%) of an equimolar mixture of n- and isovaleraldehyde (40). It has been established that the cobalt hydrocarbonyl adds to form a stable 7r-allyl complex (93, 94). 

R1 = ph R2 = Me) with the double hydrocarbonyl-bridged Cp2Zr(p-C=CPh)(p-CPh—CMeJAlMe 2 complex exhibiting a planar-tetracoordinate carbon atom within the central metallacyclic ring system. Adapted by the authors. 

Recently, a variety of reactions according to the general synthetic route shown in Scheme 7.32 were carried out, which facilitated a study of hydrocarbonyl-bridged gallium/zirco-nium compounds [173,174]. Two representative examples of these unusually structured R2Ga(g-R1,g-R2)ZrCp2 complexes will be described and discussed with regard to their structural properties [175]. 

Rhodium (I) complexes of chiral phosphines have been the archetypical catalysts for the hydrocarbonylation of 1-alkenes, with platinum complexes such as (61) making an impact also in the early 1990s[1461. More recently, rhodium(I)-chiral bisphosphites and phosphine phosphinites have been investigated. Quite remarkable results have been obtained with Rh(I)-BINAPHOS (62), with excellent ee s being obtained for aldehydes derived for a wide variety of substrates1 471. For example, hydroformylation of styrene gave a high yield of (R)-2-phenylpropanal (94% ee). The same catalyst system promoted the conversion of Z-but-2-ene into (5)-2-methylbutanal (82% ee). 

The activity of the Rh-catalysts is strongly promoted by phosphine oxides, which phenomenon is attributed to the generation of anionic Rh species which are responsible for the hydrocarbonylation of the ester, yielding 1,1-diesters as intermediates. Hydrogenolysis of these intermediates is catalyzed by the Ru species under mild conditions and may proceed with high selectivity. This compares very favourably with Co- or Ru-catalyzed homologation, which requires direct hydrogenation of intermediate acyl complexes and calls for severe reaction conditions. 

Table 14 Hydrocarbonylation of styrene 86a and its derivatives 86b and 86c catalyzed by chiral Pd(ii) complexes...

Reaction of 3,3-disubstituted-l,4-pentadiene 92 with a primary amine under cyclohydrocarbonylation conditions yielded cyclopenta[. ]pyrrole 96 as the predominant product accompanied by a small amount of cyclopentanone 95 (Scheme 15). This unique reaction is proposed to proceed through a cascade hydrocarbonylation-carbonylation process. The first hydrocarbonylation of 92 and the subsequent carbocyclization formed cyclopentanoylmethyl-Rh complex 93. If 93 immediately reacts with molecular hydrogen, 2-methylcyclopentanone 95 is formed. However, if CO insertion takes place faster than the hydrogenolysis, cyclopentanoylacetyl-Rh complex 94 is generated, which undergoes the Paal-Knorr condensation with a primary amine to yield cyclopenta[. ]pyrrole 96. ... 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

The reaction of alkenes (and alkynes) with synthesis gas (CO + H2) to produce aldehydes, catalyzed by a number of transition metal complexes, is most often referred to as a hydroformylation reaction or the oxo process. The discovery was made using a cobalt catalyst, and although rhodium-based catalysts have received increased attention because of their increased selectivity under mild reaction conditions, cobalt is still the most used catalyst on an industrial basis. The most industrially important hydrocarbonylation reaction is the synthesis of n-butanal from propene (equation 3). Some of the butanal is hydrogenated to butanol, but most is converted to 2-ethylhexanol via aldol and hydrogenation sequences. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

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