Functional Group Tolerance and Substrate Scope in Bis(imino)pyridine Iron Catalyzed Alkene Hydrogenation

Abstract

The functional group tolerance and substrate scope for bis(imino)pyridine iron catalyzed olefin hydrogenation have been examined. Allyl amines, R 2 NCH 2 CH═CH 2 (R = H, Me), are hydrogenated in the presence of ( iPr PDI)Fe(N 2 ) 2 ( iPr PDI = 2,6-(2,6- i Pr 2 -C 6 H 3 N═CMe) 2 C 5 H 3 N, 1-(N 2 ) 2 ) and the observed turnover frequencies increase with steric protection of the nitrogen donor. Likewise, ether-substituted olefins such as ethyl vinyl ether, ethyl allyl ether, and allyl ether are hydrogenated with turnover frequencies indistinguishable from the analogous α-olefins. For carbonyl-substituted alkenes, the hydrogenation activity is exquisitely sensitive to the position and type of the functional group. Esters such as dimethyl itaconate and trans -methyl cinnamate are effectively hydrogenated at 23 °C while ketones such as 5-hexen-2-one require heating to 65 °C for efficient turnover. In contrast, conjugated α,β-unsaturated ketones induce decomposition of the iron compound. Stoichiometric experiments were conducted to probe the iron substrate interaction and to gauge the coordination affinity of the functional group relative to the alkene. In addition, several new bis(imino)pyridine iron amine and ketone complexes were synthesized and the molecular and electronic structures probed by NMR spectroscopy, Mössbauer spectroscopy, and in one case, X-ray diffraction.