Hydride addition reactions of the olefin complexes [Fe(η5-C5H5)-(CO)2(olefin)][BF4]; formyl formation as the kinetically preferred process; regio- and stereo-selectivity during addition to the co-ordinated olefins
Reactions of the complexes [Fe(cp)(CO)2(olefin)]+(cp =η5-C5H5; olefin = CH2CH2, CH2CHMe, 1-hexene, or methylenecyclohexane) with hydride donors under a variety of conditions normally yield the alkyl products of hydride addition to the olefin, and the hydride [Fe(cp)(CO)2H], a product which has not been previously noted. Detailed examination of the addition reactions of the complexes of CH2CHMe, 1-hexene, and methylenecyclohexane show that the reactions exhibit no recognizable patterns of regioselectivity, in contrast to many other metal–olefin systems. Addition of deuteride to [Fe(cp)(CO)2(C6H10)]+(C6H10= cyclohexene) gives only the trans isomer of the 2-deuteriocyclohexyl complex, consistent with exo attack by the nucleophile on the co-ordinated olefin. Reaction of [Fe(cp)(CO)2(CH2CH2)]+ with NaBH4 in acetone at low temperature gives an unstable formyl compound, presumably [Fe(cp)(CO)(C2H4)(CHO)], as the kinetic product. On warming, the formyl compound converts smoothly to the above-mentioned hydride complex rather than to the corresponding ethyl complex. The mechanistic implications of these results are discussed.