Rhodium-Biphephos-Catalyzed Tandem Isomerization–Hydroformylation of Oleonitrile
Tandem isomerization-hydroformylation of oleonitrile (an 18-carbon nitrile with a remote internal (9-)C=C bond) has been studied using Rh-bisphosphite catalyst systems, targeting formation of the linear aldehyde. The best compromise between regioselectivity (l/b = 58:42) and chemoselectivity (60%) was obtained at 120 • C and 10 bar CO/H 2 (1:1) with a catalyst based on Biphephos at a 0.5 mol % catalyst load and a low ligand excess (2 equiv. versus Rh). These values stand among the better
... g the better reported ones for the tandem isomerization-hydroformylation of long chain olefins with a single-component catalyst system. Catalysts 2018, 8, 21 2 of 9 to a 26% maximum yield in linear aldehydes (70% conversion, l/b = 40:60)  . In 2015, Pandey and Chikkali explored the hydroformylation of the same substrate with a Rh-bisphosphite catalyst, reaching regioselectivities of up to l/b = 75:25 but only at 32% conversion (48 h, 1 mol % Rh) as experiments with >90% conversion resulted in l/b < 5:95, yet in all cases with unspecified chemoselectivity  . The authors also reported the tandem isomerization-hydroformylation of cashew nut shell liquid that contains a fatty mono-olefin (13 C) with C=C in the middle chain position capped by a phenol ring  . In 2013, Nozaki and coworkers combined tandem isomerization-hydroformylation of methyl oleate with hydrogenation of the resulting aldehydes in alcohols for a high final l/b = 81:19 and yet 29% of direct alkene hydrogenation  . Very recently, Behr and coworkers developed an efficient combination of Pd and Rh catalysts to convert methyl oleate into the corresponding linear aldehydes: HPdBr(P(tBu) 3 ) 2 catalyzes the dynamic isomerization of the C=C bond and a Rh-Biphephos catalyst converts the terminal olefin into aldehydes with a l/b of up to 91:9 [26,27]. It is noteworthy that, in all of the above examples, harsh conditions as well as a long reaction time favor the desired isomerization-hydroformylation and also all side reactions, especially the hydrogenation of C=C bond(s) and the direct hydroformylation of internal alkenes. In line with our previous reports on the hydroformylation of 10-undecenitrile [28, 29] , we here report on the tandem isomerization-hydroformylation of oleonitrile (1, an 18 C-nitrile with a remote internal (9-)C=C bond) using simple Rh-bisphosphite catalyst systems (Scheme 1).