Chiral Phosphinooxazoline−Ruthenium(II) and −Osmium(II) Complexes as Catalysts in Diels−Alder Reactions

Abstract

The synthesis and characterization of optically active phosphinooxazoline chloride complexes (S M and R M )-[(η 6 -p-MeC 6 H 4 iPr)MCl(PN)]A (M ) Ru, Os; PN ) phosphinooxazoline ligand; A ) counteranion) and the derived aqua complexes (R M and S M )-[(η 6 -p-MeC 6 H 4 iPr)M(PN)(H 2 O)](A) 2 are reported. The OPOF 2 -containing compounds (R M and S M )-[(η 6 -p-MeC 6 H 4 iPr)M(OPOF 2 )(PNiPr)][PF 6 ] (M ) Ru, Os; PNiPr ) (4S)-2-(2-diphenylphosphinophenyl)-4-isopropyl-1,3-oxazoline) have been also prepared and characterized. The molecular structures of (S M )-[(η 6 -p-MeC 6 H 4 iPr)MCl( PNiPrand that of the OPOF 2 -containing compounds (R Ru and S Ru )-[(η 6 -p-MeC 6 H 4 iPr)Ru(OPOF 2 )(PNiPr)][PF 6 ] have been determined by X-ray diffractometric methods. Dichloromethane solutions of the aqua complexes [(η 6p-MeC 6 H 4 iPr)M(PN)(H 2 O)][SbF 6 ] 2 are active catalysts for the Diels-Alder reaction between methacrolein and cyclopentadiene. The reaction occurs rapidly at room temperature with good exo:endo selectivity (from 85:15 to 96:4) and moderate enantioselectivity (up to 47%). The intermediate Lewis acid-dienophile compound (R Ru and S Ru )-[(η 6 -p-MeC 6 H 4 iPr)Ru(PNInd)(methacrolein)][SbF 6 ] 2 was isolated, and the molecular structure of the S epimer was determined by diffractometric means. The osmium complexes (S Os and R Os )-[(η 6 -p-MeC 6 H 4 iPr)Os(PN)(H 2 O)][A] 2 (PN ) PNiPr, A ) SbF 6 , BF 4 ; PN ) PNInd, A ) SbF 6 ) evolve to the phenyl-containing compounds (S Os and R Os )-[(η 6 -p-MeC 6 H 4 iPr)OsPh(PN′)][SbF 6 ] (PN′ ) (4S)-2-(2-hydroxyphenylphosphinophenyl)-4-isopropyl-1,3-oxazoline (PNOHiPr), PN′ ) (3aS,8aR)-2-(2-hydroxyphenylphosphinophenyl)-3a,8a-dihydroindane[1,2d]oxazole] (PNOHInd)) and (S Os and R Os )-[(η 6 -p-MeC 6 H 4 iPr)OsPh(PNFiPr)][BF 4 ] (PNFiPr ) (4S)-2-(2-fluorophenylphosphinophenyl)-4-isopropyl-1,3-oxazoline), respectively, in which the phosphinooxazoline ligand incorporates a hydroxy or fluoro functionality. On the basis of spectroscopic and crystallographic observations, a common pathway for these reactions is proposed.

Half-sandwich organometallic complexes with stereogenic metal centres: Synthesis and characterization of diastereomeric [(ηn-ring)M(Aa)X] (Aa = α-amino carboxylate) compounds

“…All attempts to separate the diastereomers by column chromatography or/and fractional crystallization were unsuccessful. The prolinate ruthenium complex 11c was an exception: the major diastereomer could be obtained pure by recrystallization (CH 3 [48,50,65,66]. A perspective view and the numbering scheme for each molecule are presented in Fig.…”

Arene-Osmium(II) Complexes in Homogeneous Catalysis

“…Osmium-catalyzed cyclopropanation of isobutyl vinyl ether with 3-tert-butyldiazoindene. or TfO -) in the Diels-Alder reaction of cyclopentadiene with methacrolein was additionally explored by Carmona, Lamata, and co-workers [89]. They led to the expected cycloadduct product in high yields (73-93%) and with a good exo:endo selectivity (dr = 90-94%), but with a low enantioselectivity (ee values up to 39%).…”

Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

“…The stereochemically fluid nature of such molecules at room temperature is illustrated by the fluxional shifts that they undergo [ 1 , 2 , 3 ]. These systems perform a significant role in increasing enantioselectivity in asymmetric synthesis [ 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. CpRu((R)-BINOP-F)(H 2 O)][SbF 6 ] has been used as a catalyst in the Diels–Alder reaction of methacrolein and cyclopentadiene to produce a [4+2] cycloadduct with enantioselectivity of 92% ee (exo) [ 9 ].…”