X-ray, ¹³C NMR, and DFT Studies on a Ruthenium(IV) Allyl Complex. Explanation for the Observed Control of Regioselectivity in Allylic Alkylation Chemistry

Abstract

X-ray, 13 C NMR, and DFT studies on the cationic Ru(IV) allyl complex Ru(Cp*)Cl(CH 3 CN)(η 3 -PhCHCHCH 2 ), as a PF 6 salt, have revealed a marked asymmetry in the bonding of the allyl ligand, which can be interpreted as arising from differences in π-bonding from the metal center to the two terminal allyl carbons. This asymmetry in the bonding is offered as an explanation for the observed control of regioselectivity in the Rucatalyzed allylic alkylation reaction.

Pentamethylcyclopentadienyl–Ruthenium Catalysts for Regio‐ and Enantioselective Allylation of Nucleophiles

“…However, additional species believed to arise from endo-exo isomerism have been detected in acetone. [18] The substitution of the acetonitrile ligand by the more sterically demanding Ph 2 POMe phosphorus ligand enhanced stereoselectivity as only one species was detected in the case of complexes 3 a and 3 b. In all cases, the allylic complexes contain a stereogenic ruthenium centre.…”

“…A subsequent intramolecular oxidative addition step completes the formation of h 3 -allyl-ruthenium(iv) complexes. In this way, not only the cationic [RuA C H T U N G T R E N N U N G (Cp*)A C H T U N G T R E N N U N G (MeCN) 3 ]PF 6 ruthenium(ii) precursor bearing very labile acetonitrile ligands has conveniently allowed the synthesis of cationic 6 complexes, [17,18] but also precursors such as 6 , which have led to monocationic and dicationic h 3 -allyl-ruthenium(iv) complexes, respectively (Scheme 3). [19,20] Complexes 2 a and 2 b resulted from oxidative addition of the allylic halide to 6 , and complex 4 is a rare example of a characterised h 3 -allyl-ruthenium(iv) complex arising from oxidative addition of a cinnamyl carbonate.…”

“…However, results from DFT calculations were more consistent with attack by a nucleophile at the less negative CHR carbon atom of a both geometrically and electronically distorded h 3 -allyl ligand. [18] From the chirality point of view, stereospecific nucleophilic substitution starting from optically active substrates has been reported in the presence of 6 as catalyst. [12,37] However, it is very difficult to control the ste-A C H T U N G T R E N N U N G reochemistry of the ruthenium centre and enantioselective catalytic processes have been achieved only with catalysts bearing an optically pure ligand.…”

Ruthenium catalysts for selective nucleophilic allylic substitution

“…The CH-CH 2 and CH-CHR bond lengths are close as might be assumed for a true η 3 -allyl ligand. Results from density functional theory (DFT) calculations showed that the less negative carbon atom of the allylic moiety, thus more favorable to nucleophilic attack, was located at the substituted CHR terminus [17]. The structural analyses and calculations reveal that geometrically and electronically distorted η 3 -allylic ligands are present in these complexes.…”

“…In the same reaction, it was noticed that the ancillary ligand plays a crucial role in the regioselectivity as the analog [CpRu(CH 3 CN) 3 ][PF 6 ] precursor mainly led to the linear product. Recently, Pregosin and colleagues have also shown that [RuCp*] complexes containing either acetonitrile or carbonate ligands were very effective and selective catalysts for the formation of branched allylic compounds [17][18][19][20]. …”