By F.G.A. Stone and Robert West (Eds.)
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Extra resources for Catalysis and Organic Syntheses
The products obtained are listed in Table XXVI. These products are rationalized by the following sequence: The initial products formed were n-aldehyde ( l,l-diacetoxy-3-formylpropane, ca. 60%), isoaldehyde (1,1-diacetoxy-2-formylpropane, 5-10%) and propionaldehyde diacetate, ca. 5%. In the alcohol solvent, the aldehydes were converted to the corresponding acetals. A portion of the n-aldehyde was converted to 2,5-diethoxytetrahydrofuranby acid catalysis, and the isoaldehyde was thermally decomposed to 2-methyl-3-acetoxyacrolein.
Any loss, by trace contamination of product, leakage, or otherwise, of an amount of rhodium equivalent to 1-2 parts per million (ppm) of aldehyde product, would be economically severe. The criticalness of this feature has contributed to some pessimism regarding the use of rhodium in large hydroformylation plants (63). However, recent successful commercialization of rhodium-catalyzed processes has proved that with relatively simple process schemes losses are not a significant economic factor (103, 104).
These substrates respond most dramatically to variations in reaction conditions with cobalt and with rhodium catalysis; both a- and p-isomers may be obtained in very high specificity. y Hs CH,=C-COOCH, + CO + yH3 CH,C-COC€H3 H,- + I CHO (Y FH3 CH,-CH-COOCH3 I C HO ( 29) -isomer 0 -isomer (9) (10) One of the most interesting observations is that the a-isomer (9) contains a quaternary carbon atom attached to the formyl group, which is a violation of the rule of Keulemans (49). It must be that electronic effects are dominant for this type of substrate, particularly when the effects of phosphine-modified rhodium systems are considered (vide infra).