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α-β Alkenyl isomerisation at diiron centres

Lookup NU author(s): Emeritus Professor Bill Clegg, Dr Simon Doherty, Dr Mark Elsegood

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Abstract

α-Substituted alkenyl complexes [Fe2(CO)6(μ-RC=CH2)(μ-PPh2 )] 1a-1e (R = Ph, Me, Prn, Bun or But) have been prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)7(μ-PPh2)]. Thermolysis in toluene results in isomerisation to the β-substituted complexes [Fe2(CO)6(μ-HC=CHR)(μ-PPh2)] 2a-2e. Isomerisation is accelerated in the presence of a range of tertiary phosphines yielding [Fe2(CO)5(PR′3)(μ-HC=CHR)(μ -PPh2)] 3a-3j in which the phosphine is coordinated to the σ-bound metal centre and lies trans to the phosphido-bridge. Addition of trimethyl phosphite to [Fe2(CO)6(μ-PhC=CH2)(μ-PPh 2)] la at 60-70 °C affords mono- and di-substituted α-alkenyl complexes [Fe2(CO)5{P(OMe)3}(μ-PhC=CH2 )(μ-PPh2)] 4 and [Fe2(CO)4{P(OMe)3}2(μ-PhC =CH2)(μ-PPh2)] 5 respectively. Above 100 °C, conversion into the β-substituted isomers [Fe2(CO)5{P(OMe)3}(μ-HC=CHPh)(μ-P Ph2)] 6 and [Fe2(CO)4{P(OMe)3}2(μ-HC= CHPh)(μ-PPh2)] 7 occurs cleanly. Crystallographic studies have been carried out on la, [Fe2(CO)6(μ-PrnC=CH2) (μ-PPh2)] 1c, [Fe2(CO)6(μ-HC=CHPh)(μ-PPh2)] 2a, [Fe2(CO)5(PPh3)(μ-HC=CHPh) (μ-PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between isomers, β isomers being characterised by a longer Feπ-Cβ interaction and a more obtuse Feσ-Cα-Cβ angle. The mechanisms of alkyne addition to [Fe2H(CO)7(μ-PPh2)] and alkenyl isomerisation have been probed using PhC2D. Hydrodimetallation results in an equal distribution of the deuterium over both β sites in 1a, and since they do not in exchange on the NMR timescale suggests that a radical process is operating. Thermolysis of this mixture leads to a 3 : 1 mixture of [Fe2(CO)6(μ-DC=CHPh)(μ-PPh2)] 2a-d1α and [Fe2(CO)6(μ-HC=CDPh)(μ-PPh2)] 2a-d1β. A mechanism for α-β alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) β-protons to terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition. © The Royal Society of Chemistry 2001.


Publication metadata

Author(s): Kamal Anwar M, Hogarth G, Senturk OS, Clegg W, Doherty S, Elsegood MRJ

Publication type: Article

Publication status: Published

Journal: Journal of the Chemical Society, Dalton Transactions

Year: 2001

Issue: 3

Pages: 341-352

Print publication date: 01/01/2001

ISSN (print): 1472-7773

ISSN (electronic): 1364-5447

URL: http://dx.doi.org/10.1039/b007977o

DOI: 10.1039/b007977o


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