Reductive coupling reactions samarium complexes

Fluoro- and chloro-substituted tricarbonyl chromium arene complexes can be reductively coupled with ketones, in the presence of samarium iodide, to give tertiary alcohols (Scheme 109). A related reaction of a tethered alkoxyimine furnished a tricyclic compound (Scheme 110). Samarium also promotes the formation of a benzyhc ketyl... 

Scheme 60 illustrates the proposed catalytic cycles for a samarium(II) silylamide precursor. The first step in both reactions is supposed to be the formation of a bimetallic samarium bis(amidinate) species originating from the reductive coupling of carbodiimide promoted by the Ln(II) complex. The active species is proposed to be a samarium guanidinate and a lanthanide amidinate [70]. 

The chromium-complexed benzylic radicals are also known and were employed in a variety of useful organic reactions. One electron reductive coupling of the tricarbonylchromium complexes of o-substituted benzaldehydes and benzaldi-mines with samarium iodide gave 1,2-diols and diamines, respectively, in good yields with extremely high threo diastereoselectivity (Eq. 7) [7]. Therefore, enantiomerically pure 1,2-diols and diamines were easily obtained by using the planar chiral chromium complexes of benzaldehyde and benzaldimine. 

The reaction of Sc2-naph with excess pyridine (Scheme 3B) led to the isolation of a rare, reductively 4,4 -C-C-coupled diamide linker that bridges the two scandium ions in the complex [(NN )Sc(NC5H5)]2[p-(NC5H5-C5H5N)]. A similar reduction was reported in the reactions of divalent thulium complexes (Fedushkin et al., 2003 Jaroschik et al., 2007b) and, recently, samarium(ll) (LabouiUe et al., 2012) with pyridine however, the reaction of a rare earth arene complex with pyridine had not been reported previously. 

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