Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in CH Activation and CC Bond Formation

Abstract

The alkylation of complexes 2 and 7 with Grignard reagents containing β-hydrogen atoms is a process of considerable relevance for the understanding of C-H activation as well as C-C bond formation mediated by low-valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis-ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis-alkyne complex 6 as an unprecedented representation of a cycloaddition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1 equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14-electron count, which has no less than four β-H atoms but is nevertheless stable at low temperature against β-hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C-H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an η(3) -allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron-mediated C-H activation cascade can be coaxed to induce a stereoselective CC bond formation. The structures of all relevant new iron complexes in the solid state are presented.

Two Exceptional Homoleptic Iron(IV) Tetraalkyl Complexes

“…[9] TheFe IV center adopts aquasitetrahedral coordination sphere; [26] it entertains remarkably short bonds of almost uniform length to the neighboring C1atoms (1.931(1)/1.928(1) )that reflect the small radius of the high-valent iron atom. [23] The a-H atoms could be located on ad ifference Fourier map and their positions refined:t he Fe···H distances (2.249/ 2.281 )aswell as the Fe1-C-H angles with an average of no more than 968 8 seem to indicate an a-agostic interaction. [23] The a-H atoms could be located on ad ifference Fourier map and their positions refined:t he Fe···H distances (2.249/ 2.281 )aswell as the Fe1-C-H angles with an average of no more than 968 8 seem to indicate an a-agostic interaction.…”

Iron‐ or Palladium‐Catalyzed Reaction Cascades Merging Cycloisomerization and Cross‐Coupling Chemistry

“…The catalyst loading could be lowered to 5 mol % without compromising the outcome, except that the reaction proceeded more slowly; the same was noticed on replacement of Fe(acac) 3 by FeBr 2 . It is mechanistically relevant that the structurally well‐defined Fe 0 complex [(dippp)Fe(C 2 H 4 ) 2 ] was also found to be catalytically competent (entry 9) . Since Grignard reagents readily reduce the analogous Fe I complex [(dippp)FeBr(C 2 H 4 )] to the Fe 0 species, this precatalyst was similarly effective (entry 10).…”

“…Our group has recently shown that the reaction of FeX n ( n =2, 3) with cyclohexylmagnesium chloride is a highly involved process, in which disproportionation of the transient organoiron species and/or allylic C−H activation processes will eventually interfere . It was for this reason that this particular nucleophile had been chosen for reaction optimization.…”

An Iron‐Catalyzed Bond‐Making/Bond‐Breaking Cascade Merges Cycloisomerization and Cross‐Coupling Chemistry

“…[9][10][11][12][13] Apriori, however,t he present reaction manifold could also be explained by an alternative mechanism (Scheme 4). [9][10][11][12][13] Apriori, however,t he present reaction manifold could also be explained by an alternative mechanism (Scheme 4).…”

“…Under the proviso that many such reactions proceed via metallacyclic intermediates, [9][10][11][12][13] we envisaged an unorthodox reaction mode (Scheme 1). Thus,i ts eems unlikely that ar eactive intermediate of type B comprising ap otential leaving group Zw ithin the bicyclicc ore will evolve by canonical b-hydride elimination, since the fairly rigid framework prevents efficient orbital overlap with the hydrogen atom at the bridgehead position from occurring.…”