Dearomatization of Furan: Elementary Transformations of η2-Coordinated Furan Complexes of Pentaammineosmium(II)
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Abstract
A series of complexes has been prepared of the form [Os(NH3)5(4,5-η2-L)]2+ where L = furan and various alkylated furans. These heterocycle complexes were surveyed for their reactivity with various electrophiles (e.g., acids, silyl triflate, N-methylacetonitrilium triflate, acetals, aldehydes, and Michael acceptors). Electrophilic addition to C(3) results in an unstable reaction intermediate, a 4,5-η2-3H-furanium species, that leads to complexes of 4,5-η2-3-substituted furans, 2,3-dihydrofurans, C(5)-substituted 2-oxoalkenes, cyclic vinyl ethers, or osmium carbyne complexes. The specific reactivity displayed by the η2-furan complexes strongly depends on the presence and location of alkyl substituents and their ability to stabilize η2-vinyl cation intermediates resulting from C(5)−O bond cleavage. In all cases investigated, the addition of an electrophile at C3 occurs anti to osmium coordination.
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