Iron Catalysis in Organic Synthesis

Transition metal catalysts, particularly those derived from the group VIII−X metals, display remarkable efficiency for the formation of carbon−carbon and carbon−heteroatom bonds through the reactions of suitable nucleophiles with organic electrophilic partners. Within this subset of the periodic table, palladium and nickel complexes offer the broadest utility, while additionally providing the deepest mechanistic insight into thus-termed “cross-coupling reactions”. The mammoth effort devoted to palladium and nickel catalysts over the past 30 years has somewhat obscured reports of alternative metal complexes in this arena. As cross-coupling reactions have evolved into a critical support for modern synthetic chemistry, the search for alternative catalysts has been taken up with renewed vigor. When the current generation of synthetic chemists reflects back to the origins of cross coupling for inspiration, the well-documented effect of iron salts on the reactivity of Grignard reagents with organic electrophile...

The Promise and Challenge of Iron-Catalyzed Cross Coupling

Metal-catalyzed cycloisomerization reactions of 1,n-enynes have appeared as conceptually and chemically highly attractive processes as they contribute to the highly demanded search for atom economy and allow the discovery of new reactions. Since the pioneering studies with palladium by the research group of Barry Trost in the mid-1980s, several other metals have been identified as excellent catalysts for the rearrangement of enyne skeletons. Moreover, the behavior of 1,n-enynes may be influenced by other functional groups such as alcohols, aldehydes, ethers, alkenes, or alkynes, thus enhancing the molecular complexity of the synthesized products. Apart from the intrinsic rearrangements of 1,n-enynes, several tandem reactions incorporating intramolecular trapping agents or intermolecular partners have been discovered. This Review aims to highlight the main contributions in this field of catalysis and to propose and comment on the mechanistic insights of the recent discoveries.

Cycloisomerization of 1,n-enynes: challenging metal-catalyzed rearrangements and mechanistic insights.

A series of unprecedented organoiron complexes of the formal oxidation states −2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo β-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me4Fe)(MeLi)][Li(OEt2)]2 (3) and [Ph4Fe][Li(Et2O)2][Li(1,4-dioxane)] (5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to β-hydride elimination reduce FeXn (n = 2, 3) to clusters of...

Preparation, Structure, and Reactivity of Nonstabilized Organoiron Compounds. Implications for Iron-Catalyzed Cross Coupling Reactions

Transition-metal-catalyzed C H functionalization for the atomand step-economical synthesis of functional molecules has attracted tremendous efforts in both academia and industry. The advantages of this method are the high efficiency, low cost, and minimal environmental impact. Heteroatom-containing molecules are abundant in natural products, pharmaceuticals, and materials. It is highly desirable to synthesize these molecules and their derivatives by direct C H functionalization. Metal carbene insertion into a C H bond adjacent to an oxygen atom presents an efficient synthetic approach to ether derivatives (Scheme 1, Meth-

Iron-catalyzed C-C bond formation by direct functionalization of C-H bonds adjacent to heteroatoms.

The 18-electron half-sandwich iron(0) complex [CpFe(C2H4)2] [Li(tmeda)] (1a), which is readily available in multigram quantities from inexpensive starting materials (ferrocene, ethylene, Li sand), is shown to be an efficient catalyst for the Alder-ene reaction of various 1,6(7)-enynes. Thereby, the presence of the labile alkene ligands in the ferrate catalyst is essential since the analogous complex [CpFe(CO)2]Na is catalytically incompetent. The cycloisomerizations catalyzed by 1a are compatible with various functional groups and turned out to be highly diastereoselective with regard to the configuration of the newly formed alkenes as well as relative stereochemistry at the ring junction. The alkyne moiety in the substrates may be terminal, silylated, or substituted with various groups, including cyclopropane rings. Likewise, the alkene substructure can be varied to a large extent, with cycloalkenes of ring sizes ≥7 being particularly suitable.

Cycloisomerization of Enynes Catalyzed by Iron(0)–Ate Complexes

A Cheap Metal for a “Noble” Task: Preparative and Mechanistic Aspects of Cycloisomerization and Cycloaddition Reactions Catalyzed by Low‐Valent Iron Complexes.

Cycloisomerization of Enynes Catalyzed by Iron(0)—Ate Complexes.

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Cycloisomerization of Enynes Catalyzed by Iron(0)–Ate Complexes

A Cheap Metal for a “Noble” Task: Preparative and Mechanistic Aspects of Cycloisomerization and Cycloaddition Reactions Catalyzed by Low‐Valent Iron Complexes.

Cycloisomerization of Enynes Catalyzed by Iron(0)—Ate Complexes.