Regioselectivity, Stereoselectivity, and Isotopically Sensitive Branching in the Fe(I)-Mediated Dehydrogenation of Octane-1,8-diol in the Gas Phase

Article abstract of DOI:10.1002/hlca.19910740522

Regio- and stereospecific labeling experiments are conducted to unravel the mechanistic features of the Fe⁺-induced dehydrogenation of octane-1,8-diol in the gas phase.With regard to the regioselectivity, ca. 20percent of molecular hydrogen originates from C(3)/C(4) or the equivalent C(5)/C(6) postions.The remaining 80percent are provided by the C(4)/C(5) methylene units.The steps, preceding the reductive elimination of hydrogen, are irreversible, and the overall reaction follows a 1,2-elimination mode.The loss of HD from C(3)/C(4) is associated with a kinetic isotope effect k_H2/k_HD = 1.68.Formation of D2 from the positions C(4)/C(5) has an isotope effect of k_H2/k_D2 = 4.7; this figure is slightly dependent on the configuration at C(4)/C(5).Most interesting is the finding that the configuration at C(4)/C5) in <4,5-D2>octane-1,8-diol, i.e. 5c vs. 5d, plays a pivotal role in the dehydrogenation of the central C(4)/C(5) part.This unexpected and unprecedented result is explained in terms of conformational analysis.A staggered-like conformation serves as a precursor to generate a trans-fused bicyclic intermediate 6.It is this very intermediate from which most of the molecular hydrogen is eliminated.Of minor importance is the cis-fused chelate 7, which is formed from an eclipsed-like conformation of the octane-1,8-diol/Fe⁺ complex.The contribution of 6 and 7 to the product formation is controlled by the relative configuration at the labeled positions C(4)/C(5).For the D,L-form 5c, we estimate a ratio of ca. 9:1 for the contribution of 6 vs. 7; due to an isotope effect, this ratio drops to 1.85:1 for the meso-form 5d.This finding constitutes the first example for the existence of isotopically sensitive branching ('metabolic switching') in gas-phase organometallic chemistry.