Conversion of sigma,eta-allenyl group into a sigma,sigma-(diphenylphosphino)allyl and a hexa-1,3,5-triene-2,6-diyl ligand at a diiron center

Thermolysis of a toluene solution of [Fe-2(CO)(5)(PPh3)(mu -PPh2){mu-eta (1 ):eta (2)(alpha,beta)-(H)C-alpha=C-beta=CgammaR2}] (R = H, 4a; R = Me, 4b), obtained via CO substitution in [Fe-2(CO)(6)(mu -PPh2){mu-eta (1):eta (2)( alpha,beta)-(H)-C-alpha=CbetaCgammaR2}] (R = H, la; R = Me, Ib), results in regiospecific 1,2-migration of the phosphido bridge to the alpha -carbon o f the sigma,eta -allenyl group to give [Fe-2(CO)(5)(PPh3){mu-eta (1):eta (3 )-(Ph2P)-HCCCR2}] (R = H, 5a; R = Me, 5b) and, in the case of R = H, [Fe-2( CO)(4)(PPh3)(2){mu-eta (1):eta (3)- (PPh2)HCCCH2}] (R = H, 6).: Single-crys tal X-ray analysis of 6 revealed that the Ph2PC(H)C= CH2 fragment can be co nsidered as a 1diphenylphosphino-substituted propene-1,2-diyl group which f unctions as a six-electron donor. Thermolysis of a toluene solution of [Fe- 2(CO)(6)(mu -PPh2){mu-eta (1):eta (2)(alpha,beta)-(H)C-alpha=C-beta=CgammaM e2}] (Ib) results in a similar transformation to give [Fe-2(CO)(6){mu-eta ( 1):eta (3)-(Ph2P)HCCCMe2}] (7). In contrast, in the absence of PPh3, thermo lysis of a toluene solution of [Fe-2(CO)(6)(mu -PPh2){mu-eta (1):eta (2)(al pha,beta)-(H)C-alpha=C-beta=CgammaH2}] (1a) results in intermolecular carbo n-carbon coupling of two mu-eta (1):eta (2)-allenyl groups to give [Fe-4(CO )(11)(mu -PPh2)(2){mu-eta (1):eta (2):eta (1):eta (2):eta (2)-(H)C=CHCHCHC= CH2}] (8). Deuterium labeling studies have been used to identify the most l ikely regioselectivity of carbon-carbon coupling and hydrogen migration. A single-crystal X-ray study of 8 revealed a polyunsaturated hexa-1,3,5-trien -2,6-diyl chain, coordinated to one bimetallic unit as a mu-eta (1):eta (2) -alkenyl group and to the other as a mu-eta (1):eta (2): eta (2)-butadien-2 -yl group. Solution NMR studies reveal that both 4a and 4b exist as rapidly interconverting mixtures of two isomers (4a(1) + 4a(2) and 4b(1) + 4b(2)). The free energy of activation associated with this exchange (DeltaG double dagger = 15.9 +/- 0.2 kcal mol(-1), 4a; DeltaG double dagger = 16.1 +/- 0. 2 kcal mol(-1), 4b) is consistent with a flipping motion of the allenyl lig and that interchanges the isomer with the triphenylphosphine attached to th e sigma -bonded iron with that in which the phosphine is attached to the et a (2)-bonded iron. A value of 1.2 kcal mol(-1) has been estimated for the d ifference in free energy between isomers 4a(1) and 4a(2). Full structural d etails of the compounds 4a(1), 4b(2), and 6-8 are reported.