The Rh catalysed asymmetric transfer hydrogenation of imines; a mechanistic study

• Main Author: Grace, Jake
• Published: University of York 2007
• Subjects: 541.39
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485090

In a study of the mechanism of Catalytic Asymmetric Transfer Hydrogenation (CATHy), we report the synthesis and characterisation of the monotosylated diamine. ligands (R,R and S,S)-p-fluorobenzenesulfonyl-l,2diphenylethylenediamine (FsDPEN) and (R,R and S,S)-p-lbutylbenzenesulfonyll, 2-diphenylethylenediamine (BusDPEN) and the corresponding Cp*Rh .complexes Cp*RhO(TsNCHPhCHPh.NH2) (i), Cp*RhO(FsNCHPhCHPhNlli) (2) and Cp*RhCI(BusNGIPhCHPhNlU) (3). (1) has been shown to react with NaOMe to form Cp*Rh(H)(TsNCH:I'hCHPhNH2) (5) as the)najor product,.as evidenced by NMR spectroscopy, and also a small amount of Cp*Rh(TsNCHPhCHPhNH) (4), as -evidenced byUV-Vis spectroscopy. The monotosylated diamine ligand p-toluenesulfonyl-o-phenylenediamine (TsOPDA) has also been synthesised and characterised, and the 16-electron catalytic intermediate Cp*Rh(TSNC6H4NH) (6) has been isolated and characterised crystallographically. An important product of catalyst deactivation [(Cp*Rh)2(Il-H)(1l-O)(1l-HC02)][BPh4] (8), has been isolated and . characterised crystallographically. The reactivity of (1)~ (2) and (3) with CO, LiBr, KI, AgOTf and IBuNC has been studied and the complexes [Cp*Rh(CO)(OCNTsCHPhCHPhNlU)]<' (9), Cp*RhBr(TsNCHPhCHPhNILY (11), Cp*RhI(TsNCHPhCHPhNlU) (12), [Cp*Rh(FsNCHPhCHPhNlli)][OTf] .. (13) and [Cp*Rh(tBuNC)(TsNCHPhCHPhNlli)][PF6] (14b) have all bee? synthesised and characterised. Crystal structures are reported for all complexes other than (9) and (14b). Whilst (11) and (12) crystallise in chiral space groups, (13) crystallises as a mixture of enantiomers in an achiral space group.. The CATHy of 6,7-dimethoXY-l-methYI-3,4-dihYdroisoquinoline (Sla) with all the complexes described above (apart from (9» has been reported. A preliminary kinetic study of the reduction of (Sla) With the [Cp*RhCh]2 / TsDPEN system and TEAF as hydrogen donor has shown the reaction to be first order in catalyst and zero order in substrate. The TOF of the system has been determined as 8100 ± 180 mol product mol catalysr1 hour:,l. Comparative rates have been measured for complexes [Cp*RhCh]2 / TsDPEN, (I),. (2), (3) and (6) as catalysts, they are 1.00, 1.10, 1.21, 0.96 and 0.09 respectively.