alpha-beta Alkenyl isomerisation at diiron centres

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)]. Thermolys is 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 t he 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 t he 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 -(P rC)-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 additi on to [Fe2H(CO)(7)(mu -PPh2)] and alkenyl isomerisation have been probed us ing PhC2D. Hydrodimetallation results in an equal distribution of the deute rium over both beta sites in 1a, and since they do not in exchange on the N MR timescale suggests that a radical process is operating. Thermolysis of t his 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 oxidati ve additions of the trans (exo) and cis (endo) beta -protons to terminal si tes on the diiron centre, giving a parallel alkyne intermediate, are in com petition.