Bulky Diphosphite-Modified Rhodium Catalysts: Hydroformylation and Characterization

The rhodium-catalyzed hydroformylation with the diphosphites P[O(2,2‘-(4-X-6-Y-C 6 H 2 ) 2 O][O(2,2‘-(4-X-6-Y-C 6 H 2 ) 2 )OP][O(4-Z-C 6 H 4 ] 2 (X = Y = tert -butyl, Z = H ( 1 ), Z = OMe ( 2 ), Z = C 6 H 5 ( 3 ), Z = Cl ( 4 ); X = OMe, Y = tert -butyl, Z = H ( 5 )), [PO(2,2‘-(4,6-( tert -C 4 H 9 ) 2 C 6 H 2 ) 2 )O] 2 R [R = O(CH 2 ) 2 O ( 6 ), R = O(CH 2 ) 3 O ( 7 )], [PO(2,2‘-(C 6 H 4 ) 2 )O] 2 R, R = O(2,2‘-(4-MeO-6- tert -C 4 H 9 C 6 H 2 ) 2 O ( 8 ), R = O(2,2‘(4,6- tert -C 4 H 9 ) 2 C 6 H 2 ) 2 O ( 9 )], and [PO(2,2‘-(4,6-( tert -C 4 H 9 ) 2 C 6 H 2 ) 2 )O] 2 [O(2,2‘-(C 6 H 4 ) 2 )O] ( 10 ) as ligands is studied with oct-1-ene and styrene as substrates. For oct-1-ene the highest normal to branched ratio obtained is 48 ( 5 ). For styrene the product selectivity depends strongly on the reaction temperature; a branched to normal ratio of 19 is found for 7 when T = 40 °C vs a branched to normal ratio of 0.19 for 1 when T = 120 °C. A bulky and bisequatorially (ee) coordinating diphosphite is required to obtain a high regioselectivity for linear aldehydes, while flexible diphosphites or equatorially−axially (ea) coordinating diphosphites lead to an enhancement of the formation of branched aldehydes. The hydroformylation of oct-1-ene has a first-order dependency in the oct-1-ene concentration, the order in CO is approximately −0.65, and the order in H 2 is approximately 0.2. This is consistent with a kinetic scheme in which alkene addition is the rate-determining step. The crystal structures of RhAcac( 4 ) and RhH(CO) 2 ( 4 ) ( 11 ), are presented. The hydrido ligand in 11 could not be located. The structure reveals a distorted TBP with 4 ee coordinated, the P(1)−Rh−P(2) angle being 115.95(9)°. The distortion is indicative of a crowded rhodium center which explains the obtained high linearity of the hydroformylation products.