Mechanistic Investigation of Iron-Catalyzed Coupling Reactions

摘要

The mechanism of the iron-catalyzed cross-coupling of aryl electrophiles with alkyl Grignard reagents is studied by a combination of GC monitoring, Hammett competition experiments, and DFT calculations. The reaction follows a pathway where an Fe~I complex, formed in situ, reacts in a rate-limiting oxidative addition with the aryi electrophile. A rapid thermoneutral transmetalation from a Grignard reagent occurs either before or after the oxidative addition, with little to differentiate be- tween the two pathways. A reductive elimination of the resulting alkyl aryl Fe~(III) complex closes the catalytic cycle. Iron in lower oxidation states can act as a competent precatalyst by oxidation into the Fe~I-Fe~(III) cycle. Fe~(II) complexes can give Fe~I catalysts through reductive elimination of a bimetallic complex. Added tigands, dilution, and powerful aryl electrophiles all serve to increase the stability of the active catalyst, presumably by counteracting oligomerization of low-valent iron.