Einfluß von (Ether‐Phosphan)‐Liganden bei der Alkylwanderung in Rhodium‐Komplexen

Abstract

Novel Basic Ligands for the Homogeneous Catalytic Carbonylation of Methanol, XXIII. - The Influence of (Ether-Phosphane) Ligands on Alkyl Migration in Rhodium Complexes The cationic complexes [Rh(COD)(P∼O)2[SbF6] (4a, b) are obtained from [Rh(COD)THF)2SbF6] (2) and the ligands 3a, b [P∼O = Pr2PCH2CH2OCH3 (3a), Ph2PCH2CH2OCH3 (3b)] (P∼O = 2=P coordinated; PO = η2-O,P coordinated). In 4a, b COD is displaced by CO to form [trans-(P∼O)2Rh(CO)3][SbF6] (5a, b). 5a, b readily eliminate carbon monoxide to form the complexes [trans -(P∼O)(P∼O)Rh(CO)[[SbF6](7a, b_. The reaction is reversible. Oxidative addition of CH3I to 7a, b affords the cyl complexes [cis -(P∼O)(P∼O)Rh(I)(C)(O)CH3)][SbF6] (8b) and [cis-(P∼O)2Rh(I)(C(O)CH3)](SbF6] (6a) containing one or two Rh–O bonds. In the case of 7a the 31P{1H}-NMR spectrum shows the reactive intermediate [trans -(P∼O)-(P∼O)Rh(I)(CH3)(CO)][SbF6] (9a). Similarly, with 13CH3I9′a and 6′a were formed. In the presence of CO and H2O 8b and 6a Yield CH3CO2H with cleavage of one or two Rh–O bonds and back reaction to 5a, b. The opening and closing mechanism” of the (ether-phosphane) ligands 3a, b promotes the formation of the complexes within the reaction cycle. On using the phospanes PPrR2 (10a, b), oxidative addition of CH3I by way of [trans (R2PrP)2Rh(CO)3][Sbf6] (12a) and [trans -(R2PrP)2Rh(CO)(THF)][Sbf6] (15a) also occurs, but no acyl complexes are formed. At 20°C 15a forms [trans -[R2PrP)2Rh(I)(CO)] (14a), which in turn reacts with AgSbF6 and CO to re-form 12a. Complex 13b does not react with CH3I. Even when THF is replaced by CO in 15a, migration of the methyl group does not occur, thus the product is [trans -(R2PrP)2Rh-cis-(CO)2(I)-(CH3)](SbF6] (16a). 13CO and 13CH3I were used for the structural investigation of the complexes 12′a, 13′a, 15′a and 16′a by 31P{1H}- and 13C{1H}-NMR spectroscopy. 13a reacts with EtI, PrI, and iPrI to form 14a, oxidative addition not being observed.