Günter Seidel

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

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 beta-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 [(Me 4Fe)(MeLi)][Li(OEt 2)] 2 ( 3) and [Ph 4Fe][Li(Et 2O) 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 beta-hydride elimination reduce FeX n ( n = 2, 3) to clusters of the formal composition [Fe(MgX) 2] n . The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C 2H 4) 4][Li(tmeda)] 2 ( 8), [Fe(cod) 2][Li(dme)] 2 ( 9), [CpFe(C 2H 4) 2][Li(tmeda)] ( 7), [CpFe(cod)][Li(dme)] ( 11), or [Cp*Fe(C 2H 4) 2][Li(tmeda)] ( 14). Such electron-rich complexes, which are distinguished by short intermetallic Fe-Li bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such C-C bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(-2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively.

Elementary Steps of Gold Catalysis: NMR Spectroscopy Reveals the Highly Cationic Character of a "Gold Carbenoid"

What are you? Even though the metal-induced ring opening of 3,3-disubstituted cyclopropenes is known to serve as a genuine carbene generator, the use of Au(I) in this reaction leads to a reactive intermediate with highly cationic character. This result has important implications for gold catalysis in general, which in the past has been commonly attributed to the intervention of gold carbenes.

Indenylidene Complexes of Ruthenium: Optimized Synthesis, Structure Elucidation, and Performance as Catalysts for Olefin Metathesis—Application to the Synthesis of the ADE-Ring System of Nakadomarin A

An optimized and large scale adaptable synthesis of the ruthenium phenylindenylidene complex 3 is described which employs commercially available diphenyl propargyl alcohol 5 as a stable and convenient carbene source. Previous ambiguities as to the actual structure of the complex have been ruled out by a full analysis of its NMR spectra. A series of applications to ring closing metathesis (RCM) reactions shows that complex 3 is as good as or even superior to the classical Grubbs carbene 1 in terms of yield, reaction rate, and tolerance towards polar functional groups. Complex 3 turns out to be the catalyst of choice for the synthesis of the enantiopure core segment 77 of the marine alkaloid nakadomarin A 60 comprising the ADE rings of this target. Together with a series of other examples, this particular application illustrates that catalyst 3 is particularly well suited for the cyclization of medium-sized rings by RCM. Other key steps en route to nakadomarin A are a highly selective intramolecular Michael addition setting the quaternary center at the juncture of the A and D rings and a Takai-Nozaki olefination of aldehyde 73 with CH2I2, Ti(OiPr)4 and activated zinc dust.

Ring-Closing Metathesis of Functionalized Acetylene Derivatives: A New Entry into Cycloalkynes

A tungsten alkylidyne catalyst is the key to achieving ring-closing metatheses of diynes to form functionalized macrocycles [Eq. (a)]. Partial reduction of the cycloalkynes provides stereoselective Z-configured cycloalkenes, which are currently inaccessible by conventional alkene metathesis.

Structure of a Reactive Gold Carbenoid

A formal gold-for-chromium transmetalation allowed the gold carbenoid species [Cy3 PAuCAr2 ]NTf2 (11) (Ar=pMeOC6 H4 -) to be obtained in crystalline form. The structure in the solid state suggests that there is only little back donation of electron density from gold to the carbene center of 11 and hence very modest AuC double-bond character; rather, it is the organic ligand framework that is responsible for stabilizing this species by resonance delocalization of the accumulated positive charge. Because 11 is capable of cyclopropanating p-methoxystyrene even at low temperature, the discussion of its structure is deemed relevant for a better understanding of the mechanisms of π-acid catalysis in general.

Palladium-catalyzed arylation of polar organometallics mediated by 9-methoxy-9-borabicyclo[3.3.1]nonane: Suzuki reactions of extended scope

Abstract An alternative way for performing Suzuki reactions is presented. The necessary borate which is the actual nucleophile in these palladium catalyzed C-C-bond formations is prepared from 9-methoxy-9-bora-bicyclo[3.3.1]nonane (9-OMe-9-BBN) and a polar organometallic reagent RM, and not as usually from a borane and a base. This approach allows cross couplings of aryl halides with e.g. alkynyl-, methyl-, or TMSCH 2 -groups, which were beyond the scope of the conventional Suzuki reaction. The method is highly chemoselective and turned out to be compatible with aldehyde-, amide-, ketone-, ester- and cyano functions as well as with basic nitrogen atoms in the substrates. It was applied to the synthesis of the acetylenic natural products junipal ( 9a ) and eutypine methyl ether (10). Since 11 B NMR studies revealed that the 9-OMe-9-BBN only serves as a shuttle for delivering the RM reagent but remains unchanged during the course of the reaction, it has been possible to device the first Suzuki-type reaction sub-stoichiometric in boron. This “catalytic” protocol was used to prepare compound 8 which is highly valuable for its chemoluminescence properties.

Macrocycles by ring-closing-metathesis, XI: Syntheses of (R)-(+)-lasiodiplodin, zeranol and truncated salicylihalamides☆

Abstract Concise, flexible and high yielding approaches to the orsellinic acid type macrolides lasiodiplodin 1 and zeranol 3 are described which involve only metal-assisted or metal-catalyzed CC-bond formations. Key steps are the efficient allylation of aryl triflates 14 and 25 either by Stille or by modified Suzuki coupling reactions, and the high yielding ring closure of the macrocyclic rings by RCM using the ruthenium carbene 18 as the catalyst. One of the synthesis intermediates, i. e. cycloalkene 16, can also be regarded as a truncated analogue of the potent anti-tumor agent salicylihalamide A 7. From the in-vitro cytotoxicity data of 16 it is possible to deduce first insights into the structure/activity relationship of salicylihalamide.

Ringschlußmetathese von funktionalisierten Acetylen-Derivaten: ein neuer Weg zu Cycloalkinen

Ein Wolfram-Alkylidin-Katalysator ermoglichte die ersten Ringschlusmetathesen von Diinen zu funktionalisierten Makrocyclen [Gl. (a)]. Durch Reduktion der entstehenden Cycloalkine erhalt man Cycloalkene mit (Z)-konfigurierter Doppelbindung, die durch Alkenmetathese zur Zeit nicht stereoselektiv synthetisiert werden konnen.

Ring closing alkyne metathesis: stereoselective synthesis of civetone

Abstract A concise and stereoselective synthesis of the macrocyclic musk civetone 6 is reported starting from readily available 9-undecynol. The key steps comprise a ring closing metathesis of diyne 4 followed by Lindlar reduction of the resulting cycloalkyne 5 . The cyclization can be effected either by using catalytic amounts of the Schrock alkylidyne complex ( t -BuO) 3 WCCMe 3 or by means of an in situ catalyst mixture generated from Mo(CO) 6 and p -trifluoromethylphenol. Both catalyst systems turned out to be compatible with the unprotected ketone function of the substrate.

Unprecedented McMurry Reactions with Acylsilanes: Enedisilane Formation versus Brook Rearrangement

Abstract The first inter- and intramolecular McMurry reactions of aroyltrimethylsilanes to substituted 1,2-bis(trimethylsilyl)ethene derivatives 2a-c and 7 are described. A low-valent titanium reagent prepared by the reduction of TiCl3 with Na on inorganic supports (Al2O3, NaCl, TiO2) turned out to be best suited. Depending on the reaction conditions and on the particular substitution patterns of the substrates, Brook rearrangements of the intermediate 1,2-disilylated titanium-1,2-diolates leading to the formation of C,O-disilyl-enol ethers may compete with the McMurry deoxygenation pathway.