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Tuning the Dissociation of the Fe–PPh2(OR) Bond in Chiral-at-Metal Complexes [CpFe(Prophos)PPh2(OR)]PF6 (R = Me, Et, iPr, tBu). The Preparative Trick of N2 Bubbling

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journal contribution
posted on 18.02.2016, 22:42 by Henri Brunner, Takaki Kurosawa, Manfred Muschiol, Takashi Tsuno, Gábor Balázs, Michael Bodensteiner
The compounds (RFe,RC)-/(SFe,RC)-[CpFe­(Prophos)­PPh­(OMe)2]­PF6 and (RFe,RC)-/(SFe,RC)-[CpFe­(Prophos)­PPh2(OR)]­PF6 (R = Me, Et, iPr, tBu) were synthesized, starting from (RFe,RC)-/(SFe,RC)-[CpFe­(Prophos)­NCMe]­PF6, and characterized, including X-ray analyses. Bubbling a stream of N2 through the solution speeded up the slow substitution reactions by removing the acetonitrile formed in the rate-determining cleavage of the Fe–NCMe bond. Due to their acceptor ligands P­(OMe)3 and PPh­(OMe)2 the complexes [CpFe­(Prophos)­P­(OMe)3]­PF6 and [CpFe­(Prophos)­PPh­(OMe)2]­PF6 are configurationally stable at the metal atom. In contrast, [CpFe­(Prophos)­PPh3]­PF6 does not form, due to the large cone angle of the ligand PPh3. Ideally, the electronic and steric effects of the ligands PPh2(OR) (R = Me, Et, iPr, tBu) are such that they tend to dissociate from the congested complexes (RFe,RC)- and (SFe,RC)-[CpFe­(Prophos)­PPh2(OR)]­PF6. In the series (RFe,RC)- and (SFe,RC)-[CpFe­(Prophos)­PPh2(OR)]­PF6, (R = Me, Et, iPr, tBu) electron donation and the cone angle of the ligands increase. Thus, the rates of the ligand exchange with P­(OMe)3 increased, initiated by the slow dissociation of the Fe–PPh2(OR) bond. For the RFe,RC/SFe,RC diastereomers of [CpFe­(Prophos)­PPh2(OR)]­PF6 the half-lives of the first-order reactions were 125/350 h (R = Me), 75/275 h (R = Et), and 12/34 h (R = iPr) in CDCl3 at 60 °C.

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