A new iron(III) halide-promoted aza-Prins cyclization between γ,δ-unsaturated tosylamines and aldehydes provides six-membered azacycles in good to excellent yields. The process is based on the consecutive generation of γ-unsaturated-iminium ion and further nucleophilic attack by the unsaturated carbon−carbon bond. Homoallyl tosylamine leads to trans-2-alkyl-4-halo-1-tosylpiperidine as the major isomer. In addition, the alkyne aza-Prins cyclization between homopropargyl tosylamine and aldehydes gives 2-alkyl-4-halo-1-tosyl-1,2,5,6-tetrahydropyridines as the only cyclic products. The piperidine ring is widely distributed throughout Nature, e.g., in alkaloids,1 and is an important scaffold for drug discovery, being the core of many pharmaceutically significant compounds.2,3 The syntheses of these type of compounds have been extensively studied in the development of new drugs containing six-membered-ring heterocycles.4 Reactions between N-acyliminium ions and nucleophiles, also described as amidoalkylation or Mannich-type condensations, have been frequently used to introduce substituents at the R-carbon of an amine.5 There are several examples that involve an intramolecular attack of a nucleophilic olefin into an iminium cation for the construction of a heterocyclic ring system.6 Traditionally, the use of hemiaminals or their derivatives as precursors of N-acyliminium intermediates has been a common two-step strategy in these reactions.6a Among this type of cyclization is the aza-Prins cyclization,7 which uses alkenes as intramolecular nucleophile. However, cy† X-ray analysis. E-mail address: malopez@ull.es. (1) (a) Fodor, G. B.; Colasanti, B. Alkaloids: Chemical and Biological PerspectiVes; Pelletier, S. W., Ed.; Wiley: New York, 1985; Vol. 23, pp 1-90. (b) Baliah, V.; Jeyarama, R.; Chandrasekaran, L. Chem. ReV. 1983, 83, 379-423. (2) Watson, P. S.; Jiang, B.; Scott, B. Org. Lett. 2000, 2, 3679-3681. (3) Horton, D. A.; Bourne, G. T.; Smythe, M. L. Chem. ReV. 2003, 103, 893-930. (4) Buffat, M. G. P. Tetrahedron 2004, 60, 1701-1729 and references therein. (5) Speckamp, W. N.; Moolenaar, M. J. Tetrahedron 2000, 56, 3187- 3856 and references therein. (6) (a) Hiemstra, H.; Speckamp, W. N. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, O., Heathcock, C. H., Eds.; Pergamon: New York, 1991; Vol. 2, pp 1047-1081. (b) Speckamp, W. N.; Hiemstra, H. Tetrahedron 1985, 41, 4367-4416. (7) (a) Dobbs, A. P.; Guesne, S. J. J.; Hursthouse, M. B.; Coles, S. J. Synlett 2003, 11, 1740-1742. (b) Dobbs, A. P.; Guesne, S. J. J.; Martinove, S.; Coles, S. J.; Hursthouse, M. B. J. Org. Chem. 2003, 68, 7880-7883. (c) Hanessian, S.; Tremblay, M.; Petersen, F. W. J. Am. Chem. Soc. 2004, 126, 6064-6071 and references therein. (d) Dobbs, A. P.; Guesne, S. J. Synlett 2005, 13, 2101-2103. ORGANIC

Iron-Catalyzed Reactions in Organic Synthesis

Reduction of the five-coordinate iron(II) dihalide complexes (iPrPDI)FeX2 (iPrPDI = ((2,6-CHMe2)2C6H3NCMe)2C5H3N; X = Cl, Br) with sodium amalgam under 1 atm of dinitrogen afforded the square pyramidal, high spin iron(0) bis(dinitrogen) complex (iPrPDI)Fe(N2)2. In solution, (iPrPDI)Fe(N2)2 loses 1 equiv of N2 to afford the mono(dinitrogen) adduct (iPrPDI)Fe(N2). Both dinitrogen compounds serve as effective precatalysts for the hydrogenation and hydrosilation of olefins and alkynes. Effecient catalytic reactions are observed with low catalyst loadings (≤0.3 mol %) at ambient temperature in nonpolar media. The catalytic hydrosilations are selective in forming the anti-Markovnikov product. Structural characterization of a high spin iron(0) alkyne and a bis(silane) σ-complex has also been accomplished and in combination with isotopic labeling studies provides insight into the mechanism of both catalytic C−H and catalytic C−Si bond formation.

Preparation and molecular and electronic structures of iron(0) dinitrogen and silane complexes and their application to catalytic hydrogenation and hydrosilation.

ConspectusThe hydrogenation of alkenes is one of the most impactful reactions catalyzed by homogeneous transition metal complexes finding application in the pharmaceutical, agrochemical, and commodity chemical industries. For decades, catalyst technology has relied on precious metal catalysts supported by strong field ligands to enable highly predictable two-electron redox chemistry that constitutes key bond breaking and forming steps during turnover. Alternative catalysts based on earth abundant transition metals such as iron and cobalt not only offer potential environmental and economic advantages but also provide an opportunity to explore catalysis in a new chemical space. The kinetically and thermodynamically accessible oxidation and spin states may enable new mechanistic pathways, unique substrate scope, or altogether new reactivity. This Account describes my group’s efforts over the past decade to develop iron and cobalt catalysts for alkene hydrogenation. Particular emphasis is devoted to the inter...

Iron- and Cobalt-Catalyzed Alkene Hydrogenation: Catalysis with Both Redox-Active and Strong Field Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to “traditional” precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal–ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands.

The addition of a silicon-hydrogen or a boron-hydrogen bond across a carbon-carbon multiple bonds is a well-established method for the introduction of versatile silane and borane functional groups to base hydrocarbon feedstocks. Transition metal catalysis, historically with precious second- and third- row transition metals, has been used to broaden the scope of the hydrofunctionalization reaction, improve reaction rate and enhance selectivity. The anti-Markovnikov selectivity of platinum-catalyzed hydrosilylation of alkenes, for example, is an enabling synthetic technology in the multibillion-dollar silicones industry. Increased emphasis on sustainable catalytic methods and more economic processes has shifted focus to catalysis with more earth-abundant transition metals such as iron, cobalt and nickel. This review describes contemporary approaches and offers a contextual analysis of catalytic alkene hydrosilylation and hydroboration reactions using first-row transition metals. Emphasis is placed on defining advances in the field, what constitutes catalyst cost, safety, and important design features to enable precious metal-like reactivity, as well as new chemistry that is unique to first-row transition metals.

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Earth-Abundant Transition Metal Catalysts for Alkene Hydrosilylation and Hydroboration: Opportunities and Assessments.

Pentacarbonyliron(0) Photocatalyzed Reactions of Trialkylsilanes with Alkenes.

: Mechanistic studies of Fe(CO)5 photocatalyzed alkene isomerization are reported along with characterization of the photocatalyzed hydrogenation of olefins under mild conditions using Fe(CO)5 as the photocatalyst. The key repeating unit is shown to be a Fe(CO)3 species.

Mark A. Schroeder

Papers

Pentacarbonyliron(0) Photocatalyzed Reactions of Trialkylsilanes with Alkenes.

Pentacarbonyliron(0) photocatalyzed hydrogenation and isomerization of olefins

Chromium carbonyl photocatalyzed 1,4-hydrosilation of 1,3-dienes. Synthesis of allylsilanes

Structure and reactivity relations in chromium carbonyl photoassisted hydrogenation of 1,3-dienes

Tris(acetonitrile)tricarbonylchromium(0) catalyzed 1,4-addition of hydrogen to 1,3-dienes