alpha-beta Alkenyl isomerisation at diiron centres


Anwar M., HOGARTH G., Senturk O. S. , CLEGG W., DOHERTY S., ELSEGOOD M.

JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS, no.3, pp.341-352, 2001 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Publication Date: 2001
  • Doi Number: 10.1039/b007977o
  • Title of Journal : JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS
  • Page Numbers: pp.341-352

Abstract

alpha -Substituted alkenyl complexes [Fe-2(CO)(6)(mu -RC=CH2)(mu -PPh2)] 1a-1e (R = Ph, Me, Pr-n, Bu-n or Bu-t) have been prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)(7)(mu -PPh2)]. Thermolysis in toluene results in isomerisation to the beta -substituted complexes [Fe-2(CO)(6)(mu -HC=CHR)(mu -PPh2)] 2a-2e. Isomerisation is accelerated in the presence of a range of tertiary phosphines yielding [Fe-2(CO)(5)(PR'(3))(mu -HC=CHR)(mu -PPh2)] 3a-3j in which the phosphine is coordinated to the sigma -bound metal centre and lies trans to the phosphido-bridge. Addition of trimethyl phosphite to [Fe-2(CO)(6)(mu -PhC=CH2)(mu -PPh2)] 1a at 60-70 degreesC affords mono- and di-substituted alpha -alkenyl complexes [Fe-2(CO)(5){P(OMe)(3)}(mu -PhC=CH2)(mu -PPh2)] 4 and [Fe-2(CO)(4){P(OMe)(3)}(2)(mu -PhC=CH2)(mu -PPh2)] 5 respectively. Above 100 degreesC, conversion into the beta -substituted isomers [Fe-2(CO)(5){P(OMe)(3)}(mu -HC=CHPh)(mu -PPh2)] 6 and [Fe-2(CO)(4){P(OMe)(3)}(2)(mu -HC=CHPh)(mu -PPh2)] 7 occurs cleanly. Crystallographic studies have been carried out on 1a, [Fe-2(CO)(6)(mu -(PrC)-C-n=CH2)(mu -PPh2)] 1c, [Fe-2(CO)(6)(mu -HC=CHPh)(mu -PPh2)] 2a, [Fe-2(CO)(5)(PPh3)(mu -HC=CHPh)(mu -PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between isomers, beta isomers being characterised by a longer Fe-pi-C-beta interaction and a more obtuse Fe-sigma-C-alpha-C-beta angle. The mechanisms of alkyne addition to [Fe2H(CO)(7)(mu -PPh2)] and alkenyl isomerisation have been probed using PhC2D. Hydrodimetallation results in an equal distribution of the deuterium over both beta 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 [Fe-2(CO)(6)(mu -DC=CHPh)(mu -PPh2)] 2a-d(alpha)(1) and [Fe-2(CO)(6)(mu -HC=CDPh)(mu -PPh2)] 2a-d(beta)(1). A mechanism for alpha-beta alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) beta -protons to terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition.