Conversion to aldehydes

Chromium trioxide (obtained from J. T. Baker Chemical Company) is stored in a vacuum desiccator over phosphorus pentoxide prior to use. Six-mole equivalents of oxidant is required for rapid, complete conversion to aldehyde. With less than the 6 1 molar ratio, a second, extremely slow oxidation step occurs (see reference 7). 

Llewellyn, Green, and Cowley isolated the Co-H complex [CoH(CO)3(IMes)] 26, a relatively stable complex under inert conditions [31], The authors examined the hydroformylation activity of 1-octene with Co-hydride complex 26. With 8 atm of syngas (H /CO) at 50°C for 17 h and 1 mol% 26, the conversion to aldehyde products was 47% with a l b of 0.78. However, 83% of the product was the internal aldehyde 2-methyl-octanal, indicating isomerisation competed with hydroformylation and the rate of isomerisation occurred faster than hydroformylation. 

Conditions 6.20 mmol styrene in benzene, 0.0031 mmol catalyst, 20 bar ( Oi I. (1 1), 60 °C, 12 hours, conversions to aldehydes >99%. b Hexane as solvent.c Lower cross-linking degree. d Higher cross-linking... 

Somewhat better results were obtained by the use of (Ph3P)2PtI2 in a polar solvent such as dimethylformamide (122). At 180°C and 250 atm H2/CO, an 89% conversion to aldehydes with an isomer ratio of 4.3 1 was obtained in 1 hour. Relatively high concentrations of catalyst (2500 ppm as Pt metal) were required. Palladium, as (Ph3P)2PdI2, was less effective and also produced considerable amounts of lactones. 

The ejfect of water on the conversion and selectivity of cohalt-catalyzed hydroformylations has long been noticed in industry [7,85,86], A systematic study [87] of this effect in hydroformylation of 1-octene with [Co2(CO)s] with and without P Bu3 revealed that addition of water, and especially when it formed a separate aqueous phase, significantly inaeased the hydrogenation activity of the phosphine-modified catalyst Under the same reaction conditions (190 °C, 56 bar CO H2 1 1, P Co 3 1), approximately 40 % nonanols were formed instead of 5 % observed with water-free solutions. No clear explanation could be given for this phenomenon, although the possible participation of water itself in the hydroformylation reaction through the water gas shift was mentioned. It was also established, that the [Co2(CO)g]-catalyzed hydroformylation was severly retarded in the presence of water. Under the conditions above, 95 % conversion was observed in 15 hour with no added water, while only 10 % conversion to aldehydes (no alcohols) was found in an aqueous/organic biphasic reaction. 

Tire very limited set of data in Table 4.4 does not allow extensive generalizations. The most obvious conclusion is that with analogous pairs of ligands (NAPHOS/44, CBD/37, BDPP/36) lower enantioselectivities are obtained in water than in organic solvents. Conversion to aldehydes can be higher in aqueous systems, although in several reactions increased hydrogenation of the product aldehydes to alcohols was also observed [102]. 

Both the conversion to aldehydes and the selectivity to normal aldehydes observed in the hydroformylation of 1-hexene by these complexes were markedly ligand dependent. A linear relationship between the electron density on the nitrogen atom and the normal/branched aldehyde ratio was found, indicating that for these aminophosphines the ratio is largely controlled by electronic factors.332... 

The same observation also applies to the Pt/DIOP catalytic system, the contradictory results obtained in preliminary experiments being due to an extensive isomerization of the substrates when high conversion to aldehydes was achieved (see Sect. 2.1.3.) 44>. 

Conversion to aldehydes by a PPh3-mediated reaction of alkyl bromide with the [Fe(CO)4] anion occurs. Oxidative addition of RBr is suggested to form the alkyl anion, the latter undergoing a PPh3-promoted insertion of CO ... 

The 5//-pyrido[4,3-f ]benzo[/]indole ring system (e.g., 192) represents another type of isoellipticine, and its synthesis has been explored by Bisagni and co-workers (95) (Scheme 33). Azaindole 188 was elaborated by means of conventional lithiation methodology to alcohol 189. A sequence of dehydration, hydrogenation, and chlorination gave 190. Either Vilsmeier-Haack conversion to aldehyde 191 and polyphosphoric acid (PPA) cyclization to the desired ring system 192 or direct cyclization to 192 completed the synthesis. The side chain amines were introduced by heating the components neat to provide 193-195. The methoxyl derivatives 196 and 197 were also synthesized (95). 

The realization that high yields can be obtained under the driving conditions of the steam distillations described in this paper suggests that other and equally simple methods for aldehyde removal may achieve the same high yields. Research in this direction could prove rewarding, particularly where the aldehyde products are not steam distillable or where the ozonolysis intermediates may require special conditions for their conversion to aldehydes. Even more intriguing for future research, however, is the verification or disproof of the mechanisms proposed here. 

In addition, this synthetic scheme offers a mild reaction route for the degradation of a-hydroxy acids to the corresponding aldehydes containing one less carbon atom or for their conversion to aldehydes with the same number of carbon atoms. For example, reaction of 1,4-diphenylthio-semicarbazide with a-acetoxyacyl chlorides yields 5-(a-hydroxyalkyl)-1,2,4-triazolium salts, which, upon sodium hydride reduction and acid treatment, give benzaldehyde along with 138 (R = H) (Scheme 58) (75TL1889). Moreover, the 5-(a-hydroxyalkyl)-l,2,4-triazolium salts can be easily converted to the 5-alkyl compounds (Scheme 58) and borohydride reduction of the latter followed by acid hydrolysis provides a method for the transformation of a-hydroxy acids to aldehydes with the... 

A most satisfactory comparison of the activities of supported and unsupported catalysts of silver, copper, and nickel may be obtained from the results of experiments made by Faith and Keyes,803 who worked with methanol and ethanol. The fact that uniform methods of catalyst preparation, support, and size, and uniform methods of operations were used adds much to the value of the results. In all cases the catalyst mass was 45 mm. long by 12 mm. in diameter, the alcohol saturator was maintained at 45° C. for ethanol and at 36° C. for methanol, and the temperature of the hottest point in the catalyst mass was determined with a thermocouple embedded in the mass. In the case of ethanol oxidation the highest conversions to acetaldehyde per pass were obtained under the following conditions (1) silver gauze catalyst flow rate 0.57 liters per minute catalyst temperature 515° C., 80.6 per cent conversion to aldehyde 13.3 per cent conversion to carbon dioxide and 3.2 per cent conversion to add, (2) silver oxide supported on asbestos flow rate 0.37 liters per minute catalyst temperature 595° C. (conversions as above) 72.3 per cent 14.5 per cent 2.9 per cent, (3) copper turnings catalyst flow rate 0.62 liters per minute catalyst temperature 512° C. (conversions) 78.0... 

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