Asymmetric Diels–Alder and Ficini Reactions with Alkylidene β-Ketoesters Catalyzed by Chiral Ruthenium PNNP Complexes: Mechanistic Insight

Hydride abstraction from the β-position of the
enolato ligand
of the previously reported complex [Ru­(3a–H)­(PNNP)]­PF6 (5a; 3a–H is the enolate
of 2-tert-butoxycarbonylcyclopentanone) with (Ph3C)­PF6 gives
the dicationic complex [Ru­(6a)­(PNNP)]2+ (7a) as a single diastereoisomer, which contains the unsaturated
β-ketoester 2-tert-butoxycarbonyl-2-cyclopenten-1-one
(6a) as a chelating ligand. The methyl analogue 2-methoxycarbonylcyclopentanone
(3b) gives [Ru­(3b–H)­(PNNP)]­PF6 as a mixture of noninterconverting diastereoisomers (ester
group of 3b trans to P, 5b; or to
N, 5c), which were separated by column chromatography.
Hydride abstraction from 5b (or 5c) yields
diastereomerically pure [Ru­(6b)­(PNNP)]2+ (7b or 7c). Complexes 7b and 7c do not interconvert at room temperature in CD2Cl2 and form opposite enantiomers of the Diels–Alder
adduct upon reaction with Dane’s diene (1 equiv). X-ray studies
of 7a, 5b, and 5c give insight
into the origin of enantioselection and the sense of asymmetric induction
in the previously reported asymmetric Diels–Alder and Ficini
cycloaddition reactions with 2,3-disubstituted butadienes and ynamides,
respectively. Stoichiometric reactions (substrate coordination, cycloaddition,
and product displacement) between [Ru­(OEt2)2(PNNP)]2+ (2), 6b (or 6a), and Dane’s diene (15, to give estrone
derivatives) or N-benzyl-N-(cyclohexylethynyl)-4-methylbenzenesulfonamide
(17, to give cyclobutenamides) suggest that product displacement
from the catalyst is turnover limiting.