The Mechanism of Efficient Asymmetric Transfer Hydrogenation of Acetophenone Using an Iron(II) Complex Containing an (S,S)-Ph2PCH2CH=NCHPhCHPhN=CHCH2PPh2 Ligand: Partial Ligand Reduction Is the Key

On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of a highly, active system generated from the precatalyst trans-[Fe(CO)(Br)(Ph2PCH2CH=N-((S,S)-C(Ph)H-C(Ph)H)-N=CHCH2PPh2)][BPh4] (2) for the asymmetric transfer hydrogenation of acetophenone in basic isopropanol. An induction period for catalyst activation is observed before the catalytic production of 1-phenethanol. The activation step is proposed to involve a rapid reaction of 2 with excess base to give an ene-amido complex [Fe(CO)(Ph2PCH2CH=N-((S,S)-C(Ph)H-C(Ph)H)-NCH=CHPPh2)](+) (Fe-p) and a bis(enamido) complex Fe(CO)(Ph2PCH=CH-N-(S,S-CH-(Ph)CH(Ph))-N-CH=CHPPh2) (5); 5 was partially Characterized: The slow step in,the catalyst activation is thought to be the reaction of Fe-p with isopropoxide to give the catalytically active amido-(erie-amido) Complex. Fe-a With a half reduced, deprotoriated PNNP ligand This can be trapped by reaction with HCl m ether to give, after isolation With NaBPh4, [Fe(CO)(Cl)(Ph2PCH2CH2N(H)-((S,S)-CH(Ph)CH(Ph))-N=CHCH2PPh2)][BPh4]. (7) Which was characterized: using multinuclear MAR and high resolution mass spectrometry. When compound 7 is treated with base,. it directly enters the catalytic cycle with no induction period. A precatalyst with the fully reduced P-NH-NH-P ligand was prepared and characterized.. by single crystal X-ray diffraction. It was found to be much less active than 2 or 7. Reaction profiles obtained by varying the initial concentrations of acetophenone, precatalyst, base, and acetone and by varying the temperature were fit to the kinetic model corresponding to the proposed mechanism by numerical simulation to obtain a unique set of rate constants and thermodynamic parameters.