Publication Date:
2016-07-13
Description:
Acrolein, methacrolein, methylvinylketone, ethylvinylketone, 3-methyl-3-en-2-one, and divinylketone were coordinated to a cationic cyclopentadienyl ruthenium(II) Lewis acid incorporating the electron-poor bidentate BIPHOP–F ligand. Analysis by NOESY and ROESY NMR techniques allowed the determination of conformations of enals and enones present in solution in CD 2 Cl 2 . The results were compared to solid state structures and to the facial selectivities of catalytic asymmetric Diels–Alder reactions with cyclopentadiene. X-Ray structures of four Ru-enal and Ru-enone complexes show the α , β -unsaturated carbonyl compounds to adopt an anti-s-trans conformation. In solution, enals assume both anti-s-trans and anti-s-cis conformations. An additional conformation, syn-s-trans , is present in enone complexes. Enantioface selectivity in the cycloaddition reactions differs for enals and enones. Reaction products indicate enals to react exclusively in the anti-s-trans conformation whereas with enones, the major product results from the syn-s-trans conformation. The alkene in s-cis conformations, while present in solution, is shielded and cannot undergo cycloaddition. A syn-s-trans conformation is found in the solid state of the bulky 6,6-dimethyl cyclohexanone-Ru(II) complex. The X-ray structure of divinyl ketone is unique in that the Ru(II) center binds the enone via a η 2 bond to one of the alkene moieties. In solution coordination to Ru-carbonyl oxygen is adopted. A comparison of facial preference is also made to the corresponding indenyl Lewis acids. This article is protected by copyright. All rights reserved.
Print ISSN:
0018-019X
Electronic ISSN:
1522-2675
Topics:
Chemistry and Pharmacology
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