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 FeI complex, formed in situ, reacts in a rate‐limiting oxidative addition with the aryl electrophile. A rapid thermoneutral transmetalation from a Grignard reagent occurs either before or after the oxidative addition, with little to differentiate between the two pathways. A reductive elimination of the resulting alkyl aryl FeIII complex closes the catalytic cycle. Iron in lower oxidation states can act as a competent precatalyst by oxidation into the FeI–FeIII cycle. FeII complexes can give FeI catalysts through reductive elimination of a bimetallic complex. Added ligands, dilution, and powerful aryl electrophiles all serve to increase the stability of the active catalyst, presumably by counteracting oligomerization of low‐valent iron.

The radical nature of iron-catalyzed cross-coupling between Grignard reagents and alkyl halides has been studied by using a combination of competitive kinetic experiments and DFT calculations. In contrast to the corresponding coupling with aryl halides, which commences through a classical two-electron oxidative addition/reductive elimination sequence, the presented data suggest that alkyl halides react through an atom-transfer-initiated radical pathway. Furthermore, a general iodine-based quenching methodology was developed to enable the determination of highly accurate concentrations of Grignard reagents, a capability that facilitates and increases the information output of kinetic investigations based on these substrates.

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Abstract:The title reaction has been studied under low temperature conditions. Coupling with active substrates can be done even at dry ice temperature. Initial rate studies at À25 8C indicate that high concentrations of any reagent can lead to either complete or partial catalyst deactivation. Under strongly reducing conditions, iron seems to form less active complexes that only slowly re-enter the catalytic cycle, possibly through bimolecular coupling of iron(II) complexes. Computational studies support the experimental observations, and indicate that oxidation states below + I cannot be reached by reductive elimination.

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Although diisopropyl­zinc has attracted considerable inter­est as a useful organometallic reagent over the past ten years, diisopropyl­zinc complexes are still absent in the present version of the Cambridge Structural Database. In the title compound, a tmeda (tmeda is N,N,N′,N′‐tetra­methyl­ethylene­diamine) adduct, [Zn(C3H7)2(C6H16N2)], the ZnII atom, which lies on a crystallographic twofold rotation axis, is in a distorted tetra­hedral geometry. The mol­ecules are held together by dispersion forces, without any contacts within the sum of the van der Waals radii.

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Di-sec-butylzinc has been used to synthesize [Zn(s-Bu) 2 L 2 ] complexes, where L denotes amine ligands: e.g., pyridine (py), 2-picoline (2-pic), and 3-picoline (3-pic). The crystal structures of [Zn(s-Bu) 2 (py) 2 ] (1), [Zn(s-Bu) 2 (2-pic) 2 ] (2), and [Zn(s-Bu) 2 (3-pic) 2 ] (3) were determined. Reaction of di-sec-butylzinc with the bidentate amine ligands N,N,N 0 ,N'-tetramethylethylenediamine (TMEDA) and N,N,N 0 ,N'-tetraethylethylenediamine (TEEDA) afforded the chiral complexes [Zn(s-Bu) 2 -(tmeda)] ( 4) and [Zn(s-Bu) 2 (teeda)] (5); their crystal structures were determined. Since they coordinate two s-Bu groups each, complexes 1-5 may exist both as chiral and meso molecules in solution, but the crystalline complexes were found to be exclusively chiral. The enantiomerization rate of 1-5 in benzene was found to be slow on the NMR time scale. Chiral crystalline complexes containing the Zn(s-Bu) 2 moiety are of particular interest, since they display reactive chirogenic R-carbon atoms. The identification of a Zn(s-Bu) 2 complex such as 1-5, which crystallized as a conglomerate of chiral crystals, would afford a simple and cheap way to optical resolution of a chiral organometallic reagent, without the need to use enantiopure ligands. Using preferential crystallization, it should be possible to perform total spontaneous resolution, which may be regarded as a form of absolute asymmetric synthesis. However, all complexes isolated so far, including the dinuclear alkylzinc amide [Zn(s-Bu)(Me 2 N(CH 2 ) 2 NH)] 2 (6), crystallize in centrosymmetric space groups. As a remedy, recrystallization of Zn(s-Bu) 2 complexes from different solvents was studied: in at least two cases, viz. [Zn(s-Bu) 2 (3-pic) 2 ] 3 C 7 H 8 ( 7) and [Zn(s-Bu) 2 (3,5-lut) 2 ] 3 C 7 H 8 (8), it was found that recrystallization from toluene gave rise to clathrates.