Stephan Enthaler

A General Palladium‐Catalyzed Amination of Aryl Halides with Ammonia

A new robust palladium/phosphine catalyst system for the selective monoarylation of ammonia with different aryl bromides and chlorides has been developed. The active catalyst is formed in situ from [Pd(OAc)(2)] and air- and moisture-stable phosphines as easy-to-handle pre-catalysts. The productivity of the catalyst system is comparable to that of competitive Pd/phosphine systems; full conversion is achieved with most substrates with 1-2 mol % of Pd source and a fourfold excess of ligand (L).

Bis(silylenyl)‐ and Bis(germylenyl)‐Substituted Ferrocenes: Synthesis, Structure, and Catalytic Applications of Bidentate Silicon(II)–Cobalt Complexes

The chemistry of stable silylenes has received wide interest since the first isolation of Nheterocyclic silylenes (NHSis) by Denk and West et al. While stable silylenes have received a lot of attention, the chemistry of bis(silylenes), compounds with two divalent silicon sites in a single molecule, is much less developed. To date, bis(silylenes) have been limited to two types of compounds, which can be defined as follows: 1) “interconnected bis(silylenes)” in which the two divalent silicon atoms are adjacent to each other and connected by a central single bond (I–III), and 2) “spacer-separated bis(silylenes)” in which the divalent silicon atoms are separated by a spacer (A–E; Scheme 1). Recently, interconnected bis(silylenes) I–III have been the subject of active research because of their distinct reactivity in comparison to isoelectronic disilynes. For example, compounds I, which bear amidinate ligands, were independently synthesized by Roesky and co-workers, and Jones et al. Furthermore, bis(silylenes) of type II and III, which are stabilized by an intramolecular phosphine donor and an N-heterocyclic carbene (NHC), respectively, were also reported. On the other hand, spacer-separated bis(silylenes) are intriguing as novel bidentate s-donor ligands for transition metals because of their unique structure and their coordination ability. The first spacer-separated bis(silylene) A was reported by Lappert and co-workers in 2005. Bis(silylene) B was synthesized by the reduction of dichlorosilaimine NHC·Cl2Si = NR (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene, R = 2,6-bis(2,4,6-triisopropylphenyl)-phenyl) through initial formation of the elusive silaisocyanide intermediate. In addition, compound C was prepared by the reaction of bis(silylene) I (R = tBu, R = Ph) with phenylacetylene PhC CH. Very recently, we described the synthesis of the first isolatable oxygen-bridged bis(silylene) D and pincer-type bis(silylene) E. 11] This development underlines that silylenes are no longer laboratory curiosities and may provide access to new silicon(II)based functional groups in coordination chemistry toward transition metals. In general, complexes of silylenes and transition metals have received much attention because they can play a key role as intermediates in transition-metalcatalyzed transformation of silicon compounds. Very recently, bis(silylenes) D and E have been employed as new silicon(II)-based donor ligands to stabilize unusually electron-rich complexes of silylenes and Group 10 metals. In order to gain access to other new bis(silylenes) as potential bidentate s-donor ligands, we set out to investigate a novel Scheme 1. Bis(silylenes) I–III and spacer-separated bis(silylenes) A–E.

Electron-Rich N-Heterocyclic Silylene (NHSi)–Iron Complexes: Synthesis, Structures, and Catalytic Ability of an Isolable Hydridosilylene–Iron Complex

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(←:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(←:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(←:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.

A Practical and Benign Synthesis of Primary Amines through Ruthenium‐Catalyzed Reduction of Nitriles

The catalytic hydrogenation of nitriles represents an atom-economic and valuable route to amines. In the present study, the ruthenium-catalyzed hydrogenation of various organic nitriles to give primary amines has been examined in detail. Straightforward ruthenium complexes modified by cheap and widely available triphenylphosphine allow for the efficient and general reduction of various aryl, alkyl, and heterocyclic nitriles. By using a practical in situ catalyst composed of [Ru(cod)(methylallyl(2))] and PPh(3), excellent yields and chemoselectivity were achieved. Moreover, the catalyst system displays broad functional group tolerance.

High Efficiency in Catalytic Hydrosilylation of Ketones with Zinc‐Based Precatalysts Featuring Hard and Soft Tridentate O,S,O‐Ligands

The diprotic, tridentate O,S,O‐ligands LH2 {[RC(=O)CH2]2S; R=tBu (3 a) or Ph (3 b)}, comprising hard (O) and soft (S) donor atoms, have been employed for the first time in zinc‐mediated hydrosilylation of various ketones, giving, after protolytic workup, the corresponding alcohols. The respective precatalysts used are novel thiobis(enolato) zinc complexes, [LZn(tmeda)] [L=3 a−2H+ (4 a) or 3 b−2H+ (4 b); tmeda= N,N,N′,N′‐tetramethylethylenediamine], [LZn(bipy)] [L=3 a−2H+ (5 a); bipy=2,2′‐bipyridine], [LZn(phen)] [L=3 a−2H+ (6 a); phen=1,10‐phenanthroline], and [LZn(dabco)] [L=[3 a−2H+] (7 a); dabco=1,4‐diazabicyclo[2.2.2]octane]. These complexes are accessible by simple Brønsted acid–base reaction of 3 a or 3 b with dimethylzinc in a 1:1 molar ratio in the presence of tmeda, bipy, phen, or dabco as auxiliary ligands. The first four complexes are isolated as yellow or colorless crystals in 76 % (4 a), 53 % (4 b), 58 % (5 a), and 61 % (6 a) yields, whereas 7 a (74 % yield) is isolated as colorless powder. The zinc center in 4 a, 4 b, 5 a, and 6 a has trigonal bipyramidal coordination, as proven by single‐crystal X‐ray diffraction analysis. Remarkably, 4 a shows the highest catalytic activity hitherto reported for zinc‐catalyzed hydrosilylation over a wide range of substrates with a turnover frequency of 970 h−1. Furthermore, a catalytic mechanism is proposed.

Design of and mechanistic studies on a biomimetic iron-imidazole catalyst system for epoxidation of olefins with hydrogen peroxide.

Novel iron catalysts, both defined and in situ generated, for the epoxidation of aromatic and aliphatic olefins with hydrogen peroxide as terminal oxidant are described. Our catalyst approach is based on bio-inspired 1-aryl-substituted imidazoles in combination with cheap and abundant iron trichloride hexahydrate. We show that the free 2-position of the imidazole ligand motif plays a key role for catalytic activity, as substitution leads to a dramatic depletion of yield and conversion. X-ray studies, UV/Vis titrations, and NMR studies were carried out to clarify the mechanism.

Highly Selective Iron-Catalyzed Synthesis of Alkenes by the Reduction of Alkynes

Herein, the iron-catalyzed reduction of a variety of alkynes with silanes as a reductant has been examined. With a straightforward catalyst system composed of diiron nonacarbonyl and tributyl phosphane, excellent yields and chemoselectivities (>99%) were obtained for the formation of the corresponding alkenes. After studying the reaction conditions, and the scope and limitations of the reaction, several attempts were undertaken to shed light on the reaction mechanism.

Hydrosilylation of alkynes by Ni(CO)3-stabilized silicon(II) hydride.

Not copy and paste: Although β-diketiminato ligands have been employed for the stabilization of Ge(II) and Sn(II) hydrides, the corresponding Si(II) hydride is not accessible. However, coordination of silicon(II) to a {Ni(CO)(3)} fragment allowed the isolation of the first Si(II) hydride metal complex 1. This complex was used for the first silicon(II)-based and Ni(0)-mediated, stereoselective hydrosilylation of alkynes. R = phenyl, tolyl.

Straightforward Iron-Catalyzed Synthesis of Vinylboronates by the Hydroboration of Alkynes

Olefins have a wide range of applications including, as building blocks for bulk chemicals, pharmaceuticals, agrochemicals, polymers, in the syntheses of natural products, and as key intermediates in organic syntheses. During the last decades, a number of methodologies have been established to access alkenes; of these transformations, the reduction of alkynes to produce alkenes is of relevance, because of the availability and straightforward synthesis of the starting materials. In this regard, the application of transition metal catalysts has been demonstrated as a useful tool to access olefins through, for example, hydrogenation, hydrosilylation, or hydroboration. Especially, the construction of functionalized olefins, such as vinylsilanes or vinylboranes, allow for further straightforward transformations, such as coupling reactions (Scheme 1). Up to now, manifold sys-

Synthesis of Secondary Amines by Iron-Catalyzed Reductive Amination

The development of more sustainable, efficient, and selective procedures to access organic compounds with higher values is one of the fundamental research goals in modern chemistry. In the case of complex molecules, the common practice is the step-by-step construction or transformation of molecular bonds. However, in terms of “green chemistry” it would be more efficient if several transformations could be performed in one sequence without isolation of the intermediates. In consequence, ecological and economical advantages arise by reduction of waste, energy, and time. 3] The combination of this concept with catalysis creates further advancements; for example, high atom efficiency and selectivity are possible. 4] Indeed, during the last decades a number of methodologies have been established based on both concepts. Among the applied strategies, reduction processes are of great interest, because of the large availability and low cost of starting materials such as aldehydes, ketones, amines, hydrogen, and hydrogen sources. In this regard, the reduction of imines, which can be formed in situ by the reaction of ketones or aldehydes with amines, offers an attractive access to higher functionalized amines, which are of great interest for industrial applications. To date, several reductive amination protocols have been reported. Many catalysts rely on precious metals such as rhodium, ruthenium, or iridium. However, due to the high price and toxicity of such metals, less expensive and less toxic metals are highly sought. Thus the use of iron is of great interest. However, only few reductive aminations based on iron have been reported to date, and long reaction times, high iron loadings or reaction temperatures are necessary for the success of such procedures. 9] Hence it is a challenging task to develop a robust and easy-to-adopt iron-based catalyst for these reactions. Herein we report the efficient and selective synthesis of secondary amines by iron-catalyzed reductive amination. Initial studies on the influence of the reaction conditions were carried out with benzaldehyde 1 and aniline 2 as the model substrates by using 5.0 mol % iron precursor and poly(methylhydrosiloxane) (PMHS) as reducing reagent in THF at 60 8C (Table 1). The best performances for the formation of N-benzyl N-phenyl amine 4 were found for FeCl3, FeCl2, and Fe2(SO4)3 with yields up to 40 % after 1 h. Elongation of the reaction time gave almost quantitative yield after 6 h (90 %). Remarkably, the iron-catalyzed reductive amination is highly selective since no reduction of benzaldehyde to benzyl alcohol (after work-up) or over-alkylation were observed. In the absence of iron salts, only traces of product were detected (<3 %). The reaction was also performed with Cu(OAc)2, Cu(acac)2 (acac = acetylacetonate), and Cu(OTf)2 instead of FeCl3 as catalysts (conditions as for Table 1, entry 5). In this case, test result (8 % yield of 4) was obtained with Cu(OTf)2. Therefore, we assume that the activity of FeCl3 as catalyst originates from iron and not from copper impurities, although it was reported that traces of copper act as catalyst in other reactions. Various silanes were then tested with FeCl3 as catalyst based on its excellent performance (Table 2, entries 1–6). In this case, the best results for the formation of 4 were obtained with phenylsilane and PMHS, whereas alkyl or higher substituted silanes gave lower yields. Following studies were performed with PMHS, because of its easy availability and low price. Increasing the reaction temperature towards 80 8C caused no improvement in the reaction outcome. Notably, the catalytic activity was retained when the temperature was decreased to 25 8C, leading to a 25 % yield of 4 after one day (Table 2, entry 9). Besides, the reduction of the catalyst loading to 1 mol % resulted Table 1. Influence of the iron source in the reductive amination of benzaldehyde 1 with aniline 2.