Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex.

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-MeCNC)Fe(N) (3,5-MeCNC = 2,6-(2,4,6-Me-CH-imidazol-2-ylidene)-3,5-Me-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-MeCNC)Fe(Ph)(N)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl.

Direct Observation of Transmetalation from a Neutral Boronate Ester to a Pyridine(diimine) Iron Alkoxide.

Transmetallation of the neutral boronate esters, (2-benzofuranyl)BPin and (2-benzofuranyl)BNeo (Pin = pinacolato, Neo = neopentylglycolato) to a representative pyridine(diimine) iron alkoxide complex, (PDI)FeOEt (PDI = 2,6-(2,6-Pr2-CHN=CMe)CHN; R = Me, Et, SiMe), to yield the corresponding iron benzofuranyl derivative was studied. Synthesis of the requisite iron alkoxide complexes was accomplished either by salt metathesis between (PDI)FeCl and NaOR (R = Me, Et, SiMe) or by protonation of the iron alkyl, (PDI)FeCH2SiMe3, by the free alcohol R'OH (R' = Me, Et). A combination of magnetic measurements, X-ray diffraction, NMR and Mössbauer spectroscopies and DFT calculations identified each (PDI)FeOR compound as an essentially planar, high-spin, = 3/2 compound engaged in antiferromagnetic coupling with a radical anion on the chelate ( = 3/2; = 2, = 1/2).

Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes.

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CHSiMe), have been developed. The first route consists of addition of two equivalents of LiCHSiMe to the iron dihalide complex, (CNC)FeBr, while the second relies on addition of the free CNC ligand to readily-prepared (py)Fe(CHSiMe) (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( CNC)Fe(CHSiMe)(N) ( CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site to the pyridine of the CNC-chelate.

Oxidative Addition of Dihydrogen, Boron Compounds, and Aryl Halides to a Cobalt(I) Cation Supported by a Strong-Field Pincer Ligand.

Cationic cobalt(I) dinitrogen complexes with a strong-field tridentate pincer ligand were prepared and the oxidative addition of polar and non-polar bonds was studied. Addition of H to [(PNP)Co(N)] (PNP = 2,6-bis((diisopropylphosphaneyl)methyl)pyridine) in THF- resulted in rapid oxidative addition and formation of the -Co(III) dihydride complex, -[(PNP)Co(H)L] where L = THF or N. The addition of H was reversible as evidenced by the dynamics observed by variable temperature H NMR spectroscopy and the regeneration of [(PNP)Co(N)] upon exposure to dinitrogen.

Iron-Mediated Coupling of Carbon Dioxide and Ethylene: Macrocyclic Metallalactones Enable Access to Various Carboxylates.

Treatment of (PDI)Fe(N) (PDI, 2,6-(2,6-iPrCHN═CMe)CHN) with CO and ethylene resulted in the formation of a homologous series of saturated and unsaturated iron carboxylate products, (PDI)Fe(OCR), the distribution of which depends on the ratio of the reagents. The solid-state and electronic structures of a saturated product, (PDI)Fe(OCCH), were elucidated. Product distributions, deuterium labeling studies, and stoichiometric experiments support initial formation of a five-membered metallalactone intermediate, which undergoes subsequent ethylene insertions to generate macrocyclic metallalactones.

Half-Sandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-Hydrogenation of Internal Alkynes.

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent.

Two Enabling Strategies for the Stereoselective Conversion of Internal Alkynes into Trisubstituted Alkenes.

An expedient method for the C-methylation of alkenylstannanes with formation of trisubstituted alkenes is described, which relies on the use of MeI in combination with copper thiophene-2-carboxylate (CuTC) as promotor and tetra-n-butylammonium diphenylphosphinate as an effective tin scavenger; in some cases, it proved beneficial to further supplement the mixture with catalytic amounts of Pd(PPh ) . Under these conditions, the reaction is robust, high yielding, and compatible with many functional groups that might not subsist under more traditional conditions used to C-alkylate organotin derivatives. A qualitative analysis of the reaction profile suggested that the in situ formation of a reactive organocopper intermediate and its interception by MeI is only barely faster than O-methylation of the phosphinate additive by the same alkylating agent.

Enantioselective Iridium-Catalyzed Allylic Cyclizations.

A method for the enantioselective synthesis of carbo- and heterocyclic ring systems enabled through the combination of Lewis acid activation and iridium-catalyzed allylic substitution is described. The reaction proceeds with branched, allylic alcohols and carbon nucleophiles as well as heteronucleophiles to give a diverse set of ring systems in good yields and with high enantioselectivities. The utility of the method is highlighted by the asymmetric syntheses of erythrococcamides A and B.

Hydroxy-Directed Ruthenium-Catalyzed Alkene/Alkyne Coupling: Increased Scope, Stereochemical Implications, and Mechanistic Rationale.

The recognition of the dual binding mode of propargyl and allyl alcohols to [Cp*Ru] fragments fostered the development of a highly regioselective intermolecular Alder-ene-type reaction of alkynes with 1,2-disubstituted alkenes. The increased substrate scope opens new perspectives in stereochemical terms. As the loaded catalyst is chiral-at-metal, stereochemical information is efficiently relayed from the propargylic site to the emerging C-C bond.

Selective Formation of a Trisubstituted Alkene Motif by trans-Hydrostannation/Stille Coupling: Application to the Total Synthesis and Late-Stage Modification of 5,6-Dihydrocineromycin B.

Countless natural products of polyketide origin have an E-configured 2-methyl-but-2-en-1-ol substructure. An unconventional entry into this important motif was developed as part of a concise total synthesis of 5,6-dihydrocineromycin B. The choice of this particular target was inspired by a recent study, which suggested that the cineromycin family of antibiotics might have overlooked lead qualities, although our biodata do not necessarily support this view.

Interligand Interactions Dictate the Regioselectivity of trans-Hydrometalations and Related Reactions Catalyzed by [Cp*RuCl]. Hydrogen Bonding to a Chloride Ligand as a Steering Principle in Catalysis.

Reactions of internal alkynes with R3M-H (M = Si, Ge, Sn) follow an unconventional trans-addition mode in the presence of [Cp*Ru(MeCN)3]PF6 (1) as the catalyst; however, the regioselectivity is often poor with unsymmetrical substrates. This problem can be solved upon switching to a catalyst comprising a [Ru-Cl] bond, provided that the acetylene derivative carries a protic functional group. The R3M unit is then delivered with high selectivity to the alkyne-C atom proximal to this steering substituent.

Selective catalytic behavior of a phosphine-tagged metal-organic framework organocatalyst.

Steric hindrance by a metal-organic framework (MOF) is shown to influence the outcome of a catalytic reaction by controlling the orientation of its intermediates. This is demonstrated using an organocatalyst, phosphine MOF LSK-3, which is evaluated with the aid of molecular modeling and NMR spectroscopy techniques. This report is the first application of phosphine MOFs in organocatalysis and explores the potential of a framework steric hindrance to impose selectivity on a catalytic reaction.

Ruthenium-catalyzed trans-selective hydrostannation of alkynes.

In contrast to all other transition-metal-catalyzed hydrostannation reactions documented in the literature, the addition of Bu3SnH across various types of alkynes proceeds with excellent trans selectivity, provided the reaction is catalyzed by [Cp*Ru]-based complexes. This method is distinguished by a broad substrate scope and a remarkable compatibility with functional groups, including various substituents that would neither survive under the conditions of established Lewis acid mediated trans hydrostannations nor withstand free-radical reactions. In case of unsymmetrical alkynes, a cooperative effect between the proper catalyst and protic functionality in the substrate allows outstanding levels of regioselectivity to be secured as well.

Synthesis of water-soluble phosphine oxides by Pd/C-catalyzed P-C coupling in water.

Cross-coupling between diphenylphosphine oxide and halogenated benzoic acids catalyzed by Pd/C in water is a green, simple, and fast protocol to obtain water-soluble tertiary phosphine oxides without the addition of ligands and additives. Low reaction times and microwave irradiation make this method general and excellent for laboratory and large-scale synthesis without the need to use organic solvents in reactions and workup.