Alkyl Group Isomerization in the Oxidative Addition of Acyl Chlorides to Iridium(I) Complexes

Abstract

Isomeric pairs of normal and α-branched acyl chlorides RCOCl [R=(CH2)2CH3 or CH(CH3)2, (CH2)3CH3 or CH(CH3)C2H5, (CH2)4CH3 or CH(C2H5)2, and CH2CH2Ph or CH(CH3)Ph] oxidatively add to IrCl(PPh3)n (n=2 or 3) to give the same n-alkyliridium(III) complexes RIrCl2(CO)(PPh3)2 [R=(CH2)2CH3, (CH2)3CH3, (CH2)4CH3, and CH2CH2Ph respectively], which contain trans-triphenylphosphine ligands. The α-branched acyl chlorides give IrCl(CO)-(PPh3)2 and IrHCl2(PPh3)3 as by-products. It is suggested that an initially formed sec-alkyl complex undergoes rapid β-hydride migration to form an olefin hydride which can either re-form the sterically more favorable n-alkyl or can decompose irreversibly to IrCl(CO)(PPh3)2, olefin, and HCl. In agreement, n-butanoyl chloride and 2-methylpropanoyl chloride add to the sterically less crowded cyclooctene complex [IrCl(CO)(C8H14)2]2 to give respectively dimeric n- and isopropyliridium(III) complexes [RIrCl2(CO)2]2 [R=(CH2)2CH3, CH(CH3)2] which interconvert in hot benzene to give an approximately 3:2 mixture of n and iso complexes. The equilibrium between the similarly prepared n- and sec-butyl complexes is established rapidly even at 34 °C and favors the n-butyl complex. The alkyl complexes [RIrCl2(CO)2]2 are formed from [IrCl(CO)-(C8H14)2]2 via acyls [RCOIrCl2(CO)(C8H14)]2 which can be isolated [R=CH3, CH(CH3)2]. Owing to reversible β-elimination, the ethyl complexes obtained from 2-dideuteropropanoyl chloride, CH3CD2COCl, and IrCl(PPh3)2 or [IrCl(CO)-(CgH14)2]2 have their deuterium atoms scrambled between methyl and methylene carbon atoms to an extent which depends on the reaction time. The oxidative additions of acyl chlorides to IrCl(PPh3)3 and RhCl(PPh3)3 are compared, and the alkyl group isomerizations observed in other reactions promoted or catalyzed by transition metal complexes are briefly discussed. In the complexes CH3(CH2)nIrCl2(CO)(PPh3)2 (n>3) the protons on the third and fourth carbon atoms from the metal are abnormally shielded, as also are the phenethyl aromatic protons in PhCH2CH2IrCl2(CO)(PPh3)2. These protons probably lie in the shielding zone of the aromatic rings of the trans-triphenylphosphine ligands. Show more