Sigrid Holle

Exploring the Reactivity of Carbon(0)/Borane-Based Frustrated Lewis Pairs

DOI 10.1002/anie.201002119 In a series of papers predating the current frustrated Lewis pair (FLP) terminology, the hydrosilation of carbonyl[1] and imine[2] functions as well as the silation of alcohols[3] has been achieved. Strong kinetic arguments point towards a Si H bond activation via a FLP-type mechanism in these processes. The authors would like to thank Prof. W. E. Piers for bringing this precedent to their attention.

Synthesis, Structure, and Reactivity of Carbene-Stabilized Phosphorus(III)-Centered Trications [L3P]3+

Carbene-stabilized [L(3)P](+3) cations have been synthesized for the first time by a reaction between 1-chloro-2,3-bis(dialkylamino)cyclopropenium salts and P(SiMe(3))(3). In addition, the first structural characterization of such an entity is reported. Consistent with the X-ray data, density functional calculations indicate that these P-centered cations, despite their high positive charge, still feature a nonbonding electron pair on the P-atom (HOMO) and a very low-lying LUMO depicting them as poor σ-donors and excellent π-acceptors.

Metal-Free Hydrogenation of Electron-Poor Allenes and Alkenes

Since their discovery, the chemistry of frustrated Lewis pairs (FLPs) has flourished, and they show remarkable reactivity towards the activation of small molecules. Thus, it has been reported in recent years that bonds such as C O, C H, B H, S S, C C, or Si H can be activated by using this elegant concept. In spite of this, their arguably most remarkable application is still the heterolytic cleavage of H2, [7] and the subsequent development of metal-free catalytic hydrogenations of a number or organic polar substrates, such as imines, enamines, nitrogenated heterocycles, or silyl enol ethers by employing H2 rather than Hantzsch esters. 13] Surprisingly, despite all of these achievements, the FLP-promoted catalytic hydrogenation of electron-poor unsaturated systems is still underdeveloped. Herein, in an attempt to address this clear limitation, we describe our efforts towards the catalytic reduction of allenes. These studies indicate that electron-deficient allenes are the most adequate substrates for hydrogenation, probably following a Michael-type hydride addition to the allene. Moreover, we have also extended this method to the hydrogenation of electron-poor alkenes, such as alkylidene malonates. At first glance it seemed to us that allenes, owing to the higher reactivity derived from their two adjacent double bonds, could be appropriate substrates for a preliminary screening of reaction conditions. Thus, tetraphenylallene 1 was exposed to mixtures of PhNMe2 or Ph2NMe/B(C6F5)3 (15 mol %) and H2 (60 bar). [15] Interestingly, consumption of 1 was observed, and two new products 2 and 3 could be isolated from the reaction mixtures after column chromatography (Scheme 1). While the formation of alkene 3 clearly indicates that reduction of allenes is possible by FLP chemistry, the detection of 2 suggests the existence of a competing reaction pathway. Thus, it can be envisaged that first 1 is protonated at the central carbon atom, and subsequent hydride transfer to the transient cation produces

Transition metal allyls. VI: The stoichiometric reaction of 1,3-dienes with ligand modified zerovalent-nickel systems

Abstract Butadiene and methyl substituted 1,3-dienes react with zerovalent-nickel-ligand complexes in a stoichiometric manner to give octadienediylnickel-ligand complexes. The structure, rearrangement and reactions with CO and P-donor ligands of these species have been studied with the help of 1 H and 13 C NMR spectroscopy. The results provide an insight into the mechanism of the nickel-catalysed cyclodimerization of 1,3-dienes.

Synthesis and reactivity of electron poor allenes: formation of completely organic frustrated Lewis pairs

The synthesis of several electron poor allenes bearing electron withdrawing substituents is described and their use as Lewis acids in the field of frustrated Lewis pair (FLP) chemistry reported. At room temperature the combination of N-heterocyclic carbenes (NHC) with the allenes under study invariably afforded the corresponding Lewis adducts; however, at -78 °C this reaction is in most of the cases inhibited and kinetically induced organic FLPs are formed. Under these conditions the activation of S-S bonds in disulfides has been achieved in excellent yields.

The preparation of bis(η3-allyl)iron(II) complexes and their reactions with 1,3-dienes

Abstract The title compounds were prepared by treating Fe(PR3)2Cl2 or Fe(R2P(CH2)nPR2)Cl2 with allylmagnesium chloride. Some of them were also made by reaction of Fe(PR3)2Cl2 with isopropylmagnesium chloride, or with active-magnesium and allyl chloride, or by reaction between triS(η3-allyl)iron and bis(dimethylphosphino)ethane. The new compounds were characterized by NMR spectroscopy and shown to be formed as a mixture of two isomers that differ in the mutual arrangement of the two allyl groups. The reactions of the bis(allyl)iron compounds with 1,3-dienes were studied: complexes containing η4-1,3-C4H6, η4-1-C3H5C5H5 or η5- 1-ethylcyclohexadienyl groups were isolated from the reaction with 1,3-butadiene. The reaction between (η3-C3H5)2Fe(iPr2PC2H4PiPr2) and 1,3-cyclohexadiene gave (η5-cyclohexadienyl)Fe(iPr2PC2H4Pi Pr2)H.

The preparation and reactions of (η3-allyl)2Ru[Pr2iP(CH2)nPPr2i] complexes

Abstract The title compounds have been prepared either by reacting (cod)(η3-2-MeC3H4)2Ru with the appropriate bisphosphine or Ru(Pr2iP(CH2)nPPr2i)Cl2 with allylmagnesium chloride. In contrast to the analogous iron compounds, the η3-allyl-Ru compounds react with 1,5-hexadiene to give (η5-1-hexadienyl)RuH complexes irrespective of the length of the methylene chain joining the two P atoms in the bidentate ligand. The η3-C3H5-containing compounds react more selectively than the η3-2-MeC3H4-containing compounds and the product of the reaction of the former with acetic acid has been shown by X-ray crystallography to be the ionic complex [Ru(Pr2iPC2H4PPr2i)2OAc]+OAc−.

Amino-substituted cyclopentadienyl cobalt complexes

Abstract Reduction of the (η 1 ,η 5 -R 2 NC 2 H 4 C 5 Me 4 )CoCl complexes (R=Me, R 2 =cyclo-C 4 H 8 ), prepared by treating CoCl 2 with 2-dimethylaminoethyl- or 2-pyrrolidinylethyl-tetramethylcyclopentadienyl-lithium, with active-Mg in the presence of ethylene or dienes leads to the formation of (η 5 -R 2 NC 2 H 4 C 5 Me 4 )(CH 2 :CH 2 ) 2 Co(I) or (η 5 -R 2 NC 2 H 4 C 5 Me 4 )(diene)Co(I) complexes. These react with ethereal HBF 4 with protonation of the amine followed, in the case of the diene complexes, by transfer of the proton to the diene to give (η 1 ,η 5 -R 2 NC 2 H 4 C 5 Me 4 )(η 3 -allyl)Co(III)-salts.

On the reactivity of tetrakis(trifluoromethyl)cyclopentadienone towards carbon-based Lewis bases.

The reactivitiy of tetrakis(trifluoromethyl)cyclopentadienone towards different C-based Lewis bases, such as N-heterocyclic carbenes (NHC), ylides and isonitriles, are reported. While sterically not hindered carbenes were found to yield kinetic adducts by regiospecific nucleophilic attack at the position adjacent to the carbonyl group of the ketone, bulkier nucleophiles afforded the thermodynamically more stable O-bridged zwitterions. Interestingly, isonitriles were found to dimerize and trimerize under the same reaction conditions, forming bicyclic products that evolve differently depending on the nature of the substituents.

η6-Arene–cobalt(I) complexes

Abstract ( η 6 -Arene)Co(I) complexes stabilized by bisphosphines, e.g. [( η 6 -MeC 6 H 5 )Co(Pr 2 i PC 2 H 4 PPr 2 i )] + BF 4 − , have been prepared by reacting ( η 3 -cyclooctenyl)Co(bisphosphine) species with HBF 4 in the presence of an arene. The ( η 6 -C 6 H 6 )Co(I) compounds can also be prepared by hydrogen abstraction from the corresponding ( η 5 -cyclohexadienyl)Co(I) complex or by hydrogenation of ( η 3 -cyclooctenyl)Co(II) species in the presence of benzene. Facile arene-exchange occurs upon treatment of these compounds with a second arene. In contrast, ( η 3 -cyclohexenyl)Co(I) and ( η 5 -cycloheptadienyl)Co(I) complexes are oxidized by HBF 4 in the presence of an alkene to give ( η 3 -cyclohexenyl)Co(II) and ( η 5 -cycloheptadienyl)Co(II) species: the former have been characterized as their diamagnetic NO adducts and the latter by a crystal structure determination.