Dienes, hydroformylation conjugated

A wide variety of substances can be hydroformylated. Conjugated dienes may give a number of products including monohydrogenated monoaldehydes. The use of chelating phosphines allows selective hydroformylation of butadiene to pentanal. The mechanism differs from that for monoalkenes since addition of M—H to conjugated dienes leads to allylic species, which may be present as tj1- or V-allyls ... 

The chemo- and regioselectivities of hydroformylation reactions of open chain, conjugated dienes using the usual catalyst are, in most cases, rather low [36]. The rhodium/ mesitylene co-condensate (catalyst A), in the presence of bis(diphenylphosphino)ethane, DPPE, catalyses the hydroformylation of 1,3-butadiene, isoprene, and E,Z)-, 3-pentadiene to the corresponding p,y-unsaturated monoaldehydes, with unusually high chemo- and regioselectivities (Scheme 17). 

Table 5. Hydroformylation of open-chain conjugated dienes by rhodium/mesitylene co-condensate, catalyst A . Substrate...

Butadiene, the simplest conjugated diene, appears to be a promising substrate for the hydroformylation reaction, because the expected product (hexanedial) could be easily converted to useful products. Oxidation would produce adipic acid, hydrogenation hexanediol, and reductive amination hexamethylenediamine. However, this objective has been difficult to realize. 

Since conjugated dienes form stable rc-allyl complexes with [Co(CO)4]2, they undergo hydroformylation very slowly to give saturated monoaldehydes in low yields.8 Mixtures of mono- and dialdehydes are usually formed in rhodium-catalyzed hydroformylations.71 72 Saturated monoaldehydes were isolated, however, when 1,3-butadiene and 1,3-pentadiene were hydroformylated in the presence of rhodium dioxide.70... 

Cyanide-containing cobalt catalysts, particularly potassium pentacyanocobalta-te(II) K3[Co(CN)5], are used in the reduction of activated alkenes (conjugated dienes).26,31 [Co(CO)4]2 is best known as a hydroformylation catalyst, but hydrogenation is also possible under specific conditions. Phosphine-substituted analogs are more successful. 

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. 

Asymmetric hydroformylation of conjugated dienes has been almost unexplored. Recently, however, promising results were reported when Rh(acac)(CO)2/(E,5)-BINAPHOS (6) is used as the catalyst. Reactions give the corresponding p,y-unsaturated aldehydes with high enan-tiopurity (<96% ee) and regioselectivity (78-95% branched) (Scheme 7.5) [20,81],... 

Foca CM, Barros HJV, dos Santos EN, Gusevskaya EV, Carles Bayon J (2003) Hydroformylation of myreene metal and ligand effects in the hydroformylation of conjugated dienes. New J Chem 27(3) 533-539... 

A wide variety of unsaturated substance can be hydroformylated by cobalt or rhodium catalysts but conjugated alkenes (e.g., butadiene) may give a number of products including hydrogenated monoaldehydes. The mechanism is different, since addition of M H to dienes leads to alicyclic species which may be present as a-bonded intermediate or as h3allyls. 

Asymmetric hydroformylation." Optically active aldehydes can be obtained by hydroformylation (CO/H2 = 1 1) of conjugated dienes with HRhfCOjCPfCaHsljjj-(-)-DIOP as catalyst. The highest optical yield (32%) was obtained in the hydroformylation of isoprene to give 3-methylpentanal. 

While hydroformylation of 1,3-butadiene has been intensively studied for the purpose of obtaining adipic acid [84,85], asymmetric hydroformylation of conjugated dienes has remained almost unexplored. A highly selective asymmetric hydroformylation of 1,3-dienes was reported using Rh(acac)[(i ,S)-BINAPHOS] as catalyst to give optically active p,y-unsaturated aldehydes (Scheme 9) [86,87]. 

Based on these results, asymmetric induction in the hydroformylation of conjugated dienes is interpreted to arise from secondary hydrogenation of the primary achiral unsaturated aldehyde. Thus (at least in the above case), enantioface-discriminating hydrogenation of an unsaturated aldehyde (which is easily formed) and not enantiofacc discriminating hydroformylation of the primary diene or an intermediate alkene is responsible for asymmetric induction18-45. 

The hydroformylation of conjugated dienes such as 1,3-butadiene, isoprene, and 1,3-pentadiene gives mixtures of regioisomers, isomerized aldehydes, and dialdehydes depending on the conditions and catalysts used. The reaction of 1,3-butadiene provides 1,6-hexanedial and has relevance to nylon produc-The reaction of 1,3-cyclohexadiene catalyzed by a rhodium complex... 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

The regioselective hydroformylation of conjugated and nonconjugated dienes has been reported. One example is the low-pressure 6w-hydroformylation of 1,3-butadiene 14 to adipic aldehyde The reaction yielded adipic aldehyde with 50% selectivity over 11 other possible products. The structure of the bisphosphite ligand had the largest effect on the selectivity of this reaction. 

The hydroformylation of compounds which have a triple bond conjugated with a double bond (199) takes place, contrary to what might be expected, in the triple bond to give formyl dienes [114, 115]. The catalytic system [RhH(PPh3)3] provides a mixture of diene 200 and cyclopentanone. 

---