Efficient synthetic methods required to assemble complex molecular arrays include reactions that are both selective (chemo-, regio-, diastereo-, and enantio-) and economical in atom count (maximum number of atoms of reactants appearing in the products). Methods that involve simply combining two or more building blocks with any other reactant needed only catalytically constitute the highest degree of atom economy. Transition metal-catalyzed methods that are both selective and economical for formation of cyclic structures, of great interest for biological purposes, represent an important starting point for this long-term goal. The limited availability of raw materials, combined with environmental concerns, require the highlighting of these goals.

http://science.sciencemag.org/content/sci/254/5037/1471.full.pdf

The atom economy--a search for synthetic efficiency

The Cationminus signpi Interaction.

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

The reactions of RuCl2(PPh3)3 with a number of diazoalkanes were surveyed, and alkylidene transfer to give RuCl2(CHR)(PPh3)2 (R = Me (1), Et (2)) and RuCl2(CH-p-C6H4X)(PPh3)2 (X = H (3), NMe2 (4), OMe (5), Me (6), F (7), Cl (8), NO2 (9)) was observed for alkyl diazoalkanes RCHN2 and various para-substituted aryl diazoalkanes p-C6H4XCHN2. Kinetic studies on the living ring-opening metathesis polymerization (ROMP) of norbornene using complexes 3−9 as catalysts have shown that initiation is in all cases faster than propagation (ki/kp = 9 for 3) and that the electronic effect of X on the metathesis activity of 3−9 is relatively small. Phosphine exchange in 3−9 with tricyclohexylphosphine leads to RuCl2(CH-p-C6H4X)(PCy3)2 10−16, which are efficient catalysts for ROMP of cyclooctene (PDI = 1.51−1.63) and 1,5-cyclooctadiene (PDI = 1.56−1.67). The crystal structure of RuCl2(CH-p-C6H4Cl)(PCy3)2 (15) indicated a distorted square-pyramidal geometry, in which the two phosphines are trans to each other, and the alkyli...

Synthesis and Applications of RuCl2(CHR‘)(PR3)2: The Influence of the Alkylidene Moiety on Metathesis Activity

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Cascade reactions in total synthesis.

The tremendous potential of cyclopropenes and methylene-cyclopropanes as multifunctional reagents in organic syntheses has only been recognized in the last decade. The use of transition metal catalysts allows an effective control of their transformations enabling highly selecitive syntheses ranging from three- to sevenmembered carbocycles. The activities in this field are summarized in this review for the first time with an emphasis on the preparative aspects of this work.

Cyclopropenes and methylenecylopropanes as multifunctional reagents in transition metal catalyzed reactions

Organophosphorus compounds have been applied in two ways in chemical synthesis. They can either be used as a reagent in a step of the synthesis (for example, in the Wittig reaction) or they can be incorporated directly into the target molecule. This second application, in particular, has expanded greatly in the last few years with the preparation of low-coordination phosphorus compounds. These include the phosphaalkynes, which are of great interest to organic and inorganic chemists. Phosphaalkynes have been employed in the synthesis of heterocyclic compounds, phosphaarenes and their valence isomers, and polycyclic compounds. Further applications have been the use of phosphaalkynes as new ligand systems in complex chemistry and their cyclooligomerization with organometallic reagents. While the chemical properties of phosphaalkynes have little in common with those of nitriles, they are in many ways very similar to those of the isoelectronic acetylenes.

Phosphaalkynes—Syntheses, Reactions, Coordination Behavior [New Synthetic Methods (73)]†

Organophosphorverbindungen werden auf zweierlei Art in der Synthesechemie eingesetzt: Entweder dienen sie als Hilfsreagentien wie etwa bei der Wittig-Reaktion, oder sie werden selbst in die Zielmolekule eingebaut. Gerade das zweite Anwendungsfeld hat sich durch die Herstellung niederkoordinierter Phosphorverbindungen in den letzten Jahren enorm erweitert. Zu den niederkoordinierten Phosphorverbindungen zahlen auch Phosphaalkine, die im Mittelpunkt dieses Beitrags stehen und viel Interesse sowohl bei Organikern als auch bei Anorganikern gefunden haben. Der Einbau der PCR-Einheit in Heterocyclen, in Phosphaarene und deren Valenzisomere sowie in polycyclische Verbindungen, die Verwendung als neue Ligandensysteme in der Komplexchemie und schlieslich die Cyclooligomerisierung mit Organometallreagentien belegen dies auf eindrucksvolle Weise. Phosphaalkine haben bezuglich ihrer Reaktivitat wenig mit Nitrilen gemeinsam, zeigen aber um so mehr Verwandtschaft mit den isoelektronischen Alkinen.

Phosphaalkine – Synthesen, Reaktionen, Koordinationsverhalten

Die Reaktion des Zirconocendichlorids mit zwei Aquivalenten n-Butyllithium und Trimethylphosphan liefert (1-Buten)(trimethylphosphan)zirconocen (1) in 83 proz. Ausbeute. Komplex 1 ist ein ausgezeichnetes Ausgansmaterial zur Darstellung weiterer (Alken)- oder (Alkin)zirconocen-Komplexe; so erhalt man durch Verdrangung des 1-Butens mit Ethylen, Styrol oder Tolan die Zirconocen-Komplexe 2, 3 oder 7 in hohen Ausbeuten. Eine weitere Darstellungsmethode fur (1-Alken)(trimethylphosphan)zirconocen-Komplexe ist die Umsetzung von Zirconocendichlorid mit 1-Alkylmagnesiumhalogeniden in Gegenwart von Trimethylphosphan bei Raumtemperatur; z. B. sind so der 1-Propen-Komplex 9 und der 1-Penten-Komplex 10 gut zuganglich. Spektroskopische Untersuchungen (1H-, 13C- und 31P-NMR) zeigen, das alle dargestellten (1-Alken)(trimethylphosphan)zirconocen-Komplexe in Losung bei 0°C als getrennt beobachtbare Rotamerenpaare 1a, b, 3a, b, 9a, b und 10a, b im Verhaltnis ca. 4:1 vorliegen. Die Strukturen des Ethylen-Komplexes 2 und des Styrol-Komplexes 3a wurden zusatzlich durch Kristallstrukturanalysen ermittelt.



1-Alkene)trimethylphosphine)zirconocenes: Preparation, Characterization, and Reactivity



(1-Butene)(trimethylphosphine)zirconocene (1) is prepared in 83% yield by subsequent reaction of zirconocene dichloride with two equivalents of n-butyllithium and trimethylphosphine. Reaction of complex 1 with other alkenes or alkynes, e.g. ethylene, styrene, or tolane indicates that this compound is an ideal starting material for synthezising other (alkene)- or (alkyne)zirconocene complexes, e.g. 2, 3, or 7. Another route to (1-alkene)(trimethylphosphine)zirconocenes, e.g. the 1-propene complex 9 or the 1-pentene complex 10, is the reaction of zirconocene dichloride with 1-alkylmagnesium halide in the presence of trimethylphosphine at room temperature. Spectroscopic investigations 1H-, 13C-, and 31P-NMR spectroscopy) confirm that in solution, complex 1 as well as the other 1-alkene complexes 3, 9, and 10 exist as a mixture of two rotamers 1a, b, 3a, b, 9a, b, and 10a, b in a ca. 4:1 ratio. The structures of the ethylene complex 2 and the styrene complex 3a are also determined by X-ray analyses.

Paul Binger

Papers

Cyclopropenes and methylenecylopropanes as multifunctional reagents in transition metal catalyzed reactions

Phosphaalkynes—Syntheses, Reactions, Coordination Behavior [New Synthetic Methods (73)]†

Phosphaalkine – Synthesen, Reaktionen, Koordinationsverhalten

(1‐Alken)(trimethylphosphan)zirconocen‐Komplexe: Darstellung, Charakterisierung und Reaktivität

1,3‐Diphosphacyclobutadienecobalt(I) Complexes from Phosphaalkynes