Olefins isomerization during hydroformylation

Reversible formation of a cobalt-alkyl complex from HCo(CO) and olefin has been established. The reversible formation of a cobalt alkyl is required to explain observations of alkene isomerization during hydroformylation by HCo(CO) and scrambling of deuterium in substrates, such as CD3(CHj)3CH=CH2, mider relatively low pressures of the mixture of CO and (Equation 17.4). Little deuterium is lost during transformation of the alkene to product, but the deuterium atoms are distributed nearly statistically along the chain. ... 

The isomerization of the olefin prior to its hydroformylation has been the explanation of this question (3) and the formation of isomeric aldehydes was related to the presence of isomeric free olefins during the hydroformylation. This explanation, however, is being questioned in the literature. The formation of (+) (S) -4-methylhexanal with an optical yield of more than 98% by hydroformylation of (+) (S)-3-methyl-l-pentene (2, 6) is inconsistent with the olefin isomerization explanation. Another inconsistency has been the constance of the hydroformylation product composition and the contemporary absence of isomeric olefins throughout the whole reaction in hydroformylation experiments carried out with 4-methyl-1-pentene and 1-pentene under high carbon monoxide partial pressure. The data reported in Ref. 4 on the isomeric composition of the hydroformylation products of 1-pentene under high carbon monoxide pressure at different olefin conversions have recently been checked. The ratio of n-hexanal 2-methylpentanal 2-ethylbutanal was constant throughout the reaction and equal to 82 15.5 2.5 at 100°C and 90 atm carbon monoxide. 

In the case of aliphatic or alicyclic olefins only norbornene hydroformylation with Rh/(—VDIOP does not fit into the picture. The contrasting results between (Z)-2-butene and bicyclo[2.2.2]oct-2-ene obtained with Co/(—)-DIOP and Ru/ (—)-DIOP catalytic systems have been attributed to extensive isomerization of (Z)-2-butene to (E)-2-butene during hydroformylation16>. In the case of the only phenyl-substituted substrate investigated, the prediction of the unsaturated carbon atom preferentially formylated is not correct. This type of exceptions is found also for styrene, as it will be discussed later. 

This conclusion was challenged by Johnson (72), who followed the formation of isomeric aldehydes and isomeric olefins during the hydroformylation of 4-methyl-1-pentene. He found that 2-olefin was formed very rapidly under Oxo conditions and suggested that this did not affect the preferential formation of terminal aldehyde significantly, due to the fact that 2-olefin was so much less reactive to hydroformylation. These results have now been checked by Piacenti et al. (115), who found that 2-olefin was only rapidly formed if the rate of the reaction was sufficiently high that the solution was no longer saturated with carbon monoxide. By using a lower catalyst concentration in benzene solvent they were able to avoid this deficiency and found very little olefin isomerization. 

Cyclooctadiene-1,5 is also partly isomerized during the reaction to cyclo6ctadiene-l,3 (VIII). Only the mono-aldehyde (VII) or monool (II) is obtained from (VIII), since conjugated dienes are hydrogenated rapidly to olefins (IX) in the hydroformylation reaction [10, 251]. (VIII) reacts practically quantitively through (IX) to (VII). 

Diels-Alder reactions, 133, 135 epoxidation, 69-72, 516 grafting on polyethylene, 462 hydroformylation, 44 hydrogenation, 41, 42 isomerization catalysts, 133, 484 isomerization during polymerizations, 484 isomerization kinetics, 484 isopropyl alcohol radical reaction, 207 MA copolymerization, 532, 534, 541 Michael reactions, 63-66 nitrone adducts, 224, 225 olefin copolymerization, 288 olefin ene reactions, 162 phenanthrene adducts, 181 plasticizers use, 14 production—synthesis, 14, 78-81 radical copolymerization, 270, 275-277, 307, 315, 317, 333, 345, 365, 379 radical polymerization, 239, 264, 287 reaction with allyl alcohol, 46 reaction with sodium bisulfite, 53 styrene copolymerization, 365, 483 tetraalkyl methylenediphosphonate adduct, 66 transesterification, 46 /7-xylylene copolymerization, 359 dialkyl stannyl, PVC stabilizer, 275 diaryl, synthesis from MA, 80 pyridinium, betaine intermediate, 216... 

A major interest for those practicing hydroformylation syntheses is the selectivity to the product desired. The factors which affect the yield of a specific aldehyde are (1) the structure of the olefinic substrate (a-olefin or internal olefin, branching, cyclic), (2) the isomers formed during the reaction (directly, with concomitant isomerization), (3) the effects of functional groups, and (4) the subsequent reactions of the product aldehyde. 

The fact that terminal olefin is recovered indicates that, although doublebond isomerization occurs at a rate which is significant compared with the rate of hydroformylation, it is considerably slower than hydroformylation kz > ki). The hydroformylation of terminal olefins is faster than that of, internal olefins, and there is an accumulation of internal olefins during the reaction. 

Reactions of internal olefins can even generate terminal alkylnitriles by a pathway that involves isomerization of intermediate cyanometal-alkyl complexes. Tlus isomerization is similar to the isomerization that occurs during the hydroformylation of internal olefins discussed in Chapter 17. In fact, the nickel catalyst rapidly isomerizes hexene to the equilibrium ratio of olefins faster than it adds HCN to the C=C bond. Thus, internal hexenes generate the terminal alkane nitrile. 

With nonfunctionalized terminal olefins, the formation of internal olefins is favored. Less than 5% of the terminal olefins may be present in thermodynamic equilibrium. Slow isomerization in comparison to the subsequent hydroformylation may lead to a continuous erosion of the regioselectivity during the reaction [18]. -01efins are more stable than Z-isomers, therefore, double bond migration can commence with a Z/ -isomerization step [19]. 

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