Aldehydes isomers

Alkyl substituents stabilize a carbonyl group m much the same way that they sta bilize carbon-carbon double bonds and carbocations—by releasing electrons to sp hybridized carbon Thus as then heats of combustion reveal the ketone 2 butanone is more stable than its aldehyde isomer butanal... 

If a double-bond shift occurs, the number of aldehyde isomers is increased. 

The commercially important normal to branched aldehyde isomer ratio is critically dependent on CO partial pressure which, in propylene hydroformylation, determines the rate of interconversion of the -butyryl and isobutyryl cobalt tetracarbonyl intermediates (11). 

Ru(1PP)2(00)2, at 2000 ppm mthenium and 1-hexene as substrate, gives only an 86% conversion and a 2.4 1 linear-to-branched aldehyde isomer ratio. At higher temperatures reduced conversions occur. High hydrogen partial pressures increase the reaction rate, but at the expense of increased hydrogenation to hexane. Excess triphenylphosphine improves the selectivity to linear aldehyde, but at the expense of a drastic decrease in rate. 

The principal commercial source of 1-butanol is -butyraldehyde [123-72-8] obtained from the Oxo reaction of propylene. A mixture of n- and isobutyraldehyde [78-84-2] is obtained in this process this mixture is either separated initially and the individual aldehyde isomers hydrogenated, or the mixture of isomeric aldehydes is hydrogenated direcdy and the n- and isobutyl alcohol product mix separated by distillation. Typically, the hydrogenation is carried out in the vapor phase over a heterogeneous catalyst. For example, passing a mixture of n- and isobutyraldehyde with 60 40 H2 N2 over a CuO—ZnO—NiO catalyst at 25—196°C and 0.7 MPa proceeds in 99.95% efficiency to the corresponding alcohols at 98.6% conversion (7,8) (see Butyraldehydes Oxo process). 

The Oxo process uses synthesis gas with a H2 to C(0 ratios of 1 1 to 2 1, Hydro for mylation in the Oxo reactor takes place at moderate temperatures, 212-400°F, but very high pressures, 3000-5000 psi. The effluent from the reactor is cleaned up to remove light gas by-products to recover the catalyst for recycling and to recycle any unreacted olefin. Ac this point, the balance between the aldehyde isomers can be adjusted by fractionating out the... 

The halophosphite ligands show the same relationship between activity and the preference for the more linear aldehyde isomer as a function of ligand concentration. A series of bench unit studies utilizing halophosphite catalysts were conducted in which propylene was allowed to react to form butyraldehyde. Table 1 presents bench unit data on the effects of the ligand to rhodium molar ratios. 

A rhodium catalyst derived from the 6-DPPon ligand 1 displayed behavior typical of a bidentate ligand upon hydroformylation of terminal alkenes [9]. Thus, excellent regioselectivity in favor ofthe linear aldehyde isomer was noted for hydroformylation of a range of functionalized terminal alkenes (Table 2.1). Among them even those... 

The oxo reaction (31) is carried out in the liquid phase at high pressure using a cobalt catalyst. A mixture of aldehyde isomers is always produced, each isomer being one carbon number higher than the starting olefin. As a group the oxygenated products of the hydrocarbon synthesis (Fischer-Tropsch) process and the oxo process are primary compounds and thus (except, of course, the methyl and ethyl derivatives) differ fundamentally from the products based on alcohols made by the hydration of olefins, which are always secondary or tertiary in structure. 

Effect of the Stereochemistry. With rhodium, the geometry of molecules such as (E)- and (Z)-2-pheny 1-2-butenes influences both the distribution of diastereoisomers formed by addition of CHO group on the ft carbon and the distribution of aldehydic isomers. Depending on the starting (E) or (Z) stereoisomer, one of the two diastereoisomers predominates this implies some degree of stereospecificity in the hydroformylation reaction. On the other hand, when Co2(CO)8 is used, no stereospecificity in the hydroformyaltion reaction is observed. It is thus possible to get information about the stereochemistry of the hydro-... 

The other feature of hydroformylation of ( )- and (Z)-2-pheny 1-2-butenes concerns the distribution of aldehydic isomers 26 and 27. With Co2(CO)8, whatever the stereochemistry of the olefin, the distribution of these two aldehydes is exactly the same. On the other hand, with Rh/Al203 the reaction is more or less oriented towards the formation of one of the aldehydic isomers. Moreover, the phenomenon is more striking with (Z) than with (E) stereoisomer. 

Other approaches that have been suggested include catalytic asymmetric hydroformylation of 2-methoxy-6-vinylnaphthalene (6) using a rhodium catalyst on BINAPHOS ligand followed by oxidation of the resultant aldehyde 7 to yield 5-naproxen (Scheme 6.3).22 However, the tendency of the aldehyde to racemize and the co-generation of the linear aldehyde isomer make the process less attractive. Other modifications related to this process include catalytic asymmetric hydroesterification,23 hydrocarboxylation,24 and hydrocyanation.25... 

Hadena moths use Silene flowers (Caryophyllaceae) as both nectar sources and as host plants for depositing larvae. The floral scent of Silene latifolia is characterized by the presence of several fatty acid derivatives, benzenoids, and monoterpenoids, while the most abundant compounds are the lilac aldehyde isomers ((2 S, 2 S, 5 S), (2R, 2 S, 5 V), (2S, 2 R, 5 S), and (2R, 2 R, 5 S )) (23), veratrole, and benzyl acetate.93 However, several chemotypes of these dominant components are found in S. latifolia, suggesting variability in attracting pollinators. Wind tunnel experiments have revealed that Hadena bicruris is strongly attracted by lilac aldehyde isomers although the moths respond electrophysiologically to all eight stereoisomers of lilac aldehyde, only four are present in the floral scent.94... 

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 ... 

Consider as a prototype the network 11.13 of n-heptene hydroformylation, keeping in mind that die arrows represent multistep pathways and that the reactions of higher straight-chain olefins involve still more parallel pathways of internal olefin isomers to aldehyde isomers and on to alcohol isomers. In such networks, all but one of the aldehyde-to-alcohol conversions involve the reaction of an aldehyde group on a secondary carbon atom, so that all these pathways can be assumed to involve essentially the same rate coefficients of their steps. Only the conversion of the straight-chain aldehyde (n-octanal to n-octanol in network 11.13) must be expected to occur with somewhat different rate coefficients. Likewise, all con-... 

The synthesis of eight isomers of maurenone 116 proceeded by coupling of each of the four aldehyde isomers 110, ewt-llO, 112 and ent-112 with the two ketone isomers 113 and 114. Attempts to couple the titanium enolate (TiCU, i-Pr2NEt) of ketone 113 with aldehyde 110 proved troublesome, because of desilylation taking place during enolization of ketone 113. Better results were achieved by means of lithium enolates, on treatment of ethyl ketone 113 with LiHMDS and subsequent addition of aldehyde 110, yielding the isomer 115 with an acceptable diastereoselectivity of 74% ds (equation 34). Separation of the isomers and further transformations gave (—)-maurenone (116), which NMR data showed to be an almost perfect correlation with all the peaks reported for the natural product maurenone. ... 

Mild reaction conditions that lead to significant energy conservation High selectivity towards desired linear aldehyde isomer Very low loss of precious metal catalyst... 

The ratio between enol and aldehyde isomers is about 1.4. The chain-transfer constant in eq 49 (Cc = 700) is an order of magnitude less than that of MMA. One may conclude that this value reflects steric obstruction of the methylene group by the OH group and that there is no significant enthalpy gain in the enol structure shown in eq 50 relative to a PMMA terminal double bond. 

On the other hand, the photoreaction of 2-cyanofuran in adduct of a cyclopropenyl aldehyde isomer (115) U4) ... 

Similarly, the equilibrium between cw-2-vinylcyclopropanecarboxaldehydes and dihydrooxe-pines strongly favors the aldehyde isomer. ... 

In the 1960s, Shell commercialized a one-step process using a phosphine modified cobalt catalyst. This process operates at a much lower pressure, provides a high selectivity for the normal aldehyde isomer, and carries out the hydroformylation and hydrogenation sequentially without isolating the aldehyde. 

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