Rhodium trans-2-hexene

The hydroformylation of trflns-3-octene at room temperature using the (non-encapsulated) rhodium catalyst based on tris(weta-pyridyl)phosphine afforded 2-ethylheptana] and 2-propylhexanal in exactly a 1 1 ratio. The encapsulated catalyst provided an unprecedented selectivity for 2-propylhexanal of 75% (Scheme 8.3). Again the selectivity is largely retained at 40 °C whereas at 80 °C the isomerization side reaction prohibits the selective formation of aldehydes. Similar regioselectivities were obtained in the hydroformylation of frflns-2-hexene, trans-2-nonene and trans-3-nonene at 25 °C. 

The oldest syntheses of chrysanthemates are those starting from 2,5-dimethyl-2,4-hexadiene (238). There have been more papers on the use of rhodium or antimony to catalyze the addition of diazoacetate and chiral copper complexes to create asymmetry during the addition (see Vol. 4, p. 482, Refs. 219-222). The problem with this route is to avoid the use of diazo compounds. An old synthesis of Corey and Jautelat used the ylide addition of a sulfurane to a suitable precursor (in this case a C3 unit was added to methyl 5-methyl-2,4-hexadienoate, 239), and a recent paper gives details about the addition of ethyl dimethylsulfuranylideneacetate to 2,5-dimethyl-4-hexen-3-one (240). This led exclusively to the tran -isomer 241, from which ethyl trans-chrysanthemate (185, R = Et) was made. Other ylide additions are mentioned below. 

The search is on for catalysts to replace those containing toxic heavy metals. The addition of hydrogen chloride to acetylene to form vinyl chloride is catalyzed by mercuric chloride. Rhodium (III) chloride on activated carbon works just as well and is much less toxic 97 It should be tried also in other addition reactions of acetylene as well as in trans-esteriflcation reactions of vinyl acetate. The reduction of 2 ethyl-2-hexenal to 2-ethylhexanol can be catalyzed by a mixture of copper, zinc, manganese, and aluminum oxides in 100% yield.98 This is said to be a replacement for carcinogenic copper chromite. In Reaction 4.15, the amount of toxic chromium(II) chloride has been reduced from stoichiometric to catalytic (9-15 mol% chromium(II) chloride) by the addition of manganese metal.99... 

Cobley CJ, Ellis DD, Guy Orpen A, Pringle PG. Rhodium complexes of robust phosphites derived from calix[4]arenes and their apphcation in the hydroformylation of 1-hexene. J Chem Soc Dalton Trans 2000 7 1109-12. 

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