Jean-Baptiste Sortais

Efficient and selective N -alkylation of amines with alcohols catalysed by manganese pincer complexes

Borrowing hydrogen (or hydrogen autotransfer) reactions represent straightforward and sustainable C–N bond-forming processes. In general, precious metal-based catalysts are employed for this effective transformation. In recent years, the use of earth abundant and cheap non-noble metal catalysts for this process attracted considerable attention in the scientific community. Here we show that the selective N-alkylation of amines with alcohols can be catalysed by defined PNP manganese pincer complexes. A variety of substituted anilines are monoalkylated with different (hetero)aromatic and aliphatic alcohols even in the presence of other sensitive reducible functional groups. As a special highlight, we report the chemoselective monomethylation of primary amines using methanol under mild conditions.

Well‐Defined Cyclopentadienyl NHC Iron Complex as the Catalyst for Efficient Hydrosilylation of Amides to Amines and Nitriles

The development of synthetic strategies toward anilines and amines is attracting continuous interest of chemists, because these compounds constitute an important class of products in agrochemical, pharmaceutical, and natural product areas. Among the numerous synthetic methods to obtain such amine derivatives, the reduction of imines or amides is one of the well-documented strategies that use a stoichiometric amount of reactive alkali hydrides, such as lithium aluminum hydride or boron hydrides. In the last decades, transition-metal-catalyzed reduction methodologies have been widely developed to obtain amines. Numerous transition metals have been used as catalysts in the hydrosilylation of amides. Among various catalytic systems, ruthenium, rhodium, palladium, platinum, indium, and titanium have shown their efficiency. Moreover, iron has emerged as an interesting potential replacement for precious transition metals in catalytic processes because of its high natural abundance, benign environmental impact, and low cost. The last decade has seen a rise in its use as a catalyst, and efficient processes are now able to compete with other metal-catalyzed processes. In the area of iron-catalyzed reductions, and more especially in hydrosilylation, 12] important breakthroughs have been made over the last decade. For the hydrosilylation of amides, some pioneering reports were published simultaneously by Beller and Nagashima. They described original iron-catalyzed reduction of secondary and tertiary amides that yielded amines by using [Fe3(CO)12] or [Fe(CO)5] as precatalysts. Furthermore, under hydrosilylation conditions, Beller et al. demonstrated that [Fe2(CO)9] , and mainly [Et3NH][HFe3(CO)11] , catalyzed the dehydration of primary amides into the corresponding nitriles in moderate to high yields. As part of our current interest in iron catalysis, and especially in hydrosilylation reactions using well-defined complexes as catalysts for the reduction of aldehydes, ketones, or imines, we were interested in studying the activity of cyclopentadienyl NHC iron complexes (NHC = N-heterocyclic carbene) in the hydrosilylation of amides. To start our investigation, we chose the well-defined iron complex (1) shown in Scheme 1, which contained the N-heterocyclic carbene ligand IMes [IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] , for the hydrosilylation of amides, and N,N-dimethylbenzamide (2) as the model substrate to identify optimal conditions (Scheme 1, Table 1). When performing the reaction in toluene at 70 8C in the presence of 2 equiv of diphenylsilane under visible light activation, very low conversion (9 %) was observed after 16 h (Table 1, entry 1). Interestingly, by using 2 equiv of phenylsilane under the same conditions, a promising 50 % conversion was obtained (Table 1, entry 2). All reactions performed in THF at 70 8C or in cyclopentylmethyl ether (CPME) or toluene at 100 8C did not yield full conversion (up to 70 %, Table 1, entries 3–6). Notably, when performing the reaction under solvent-free conditions with 2 equiv of phenylsilane in the presScheme 1. Reduction of amides by using the [CpFe(CO)2(IMes)]I complex as the catalyst.

Selective Reduction of Esters to Aldehydes under the Catalysis of Well-Defined NHC–Iron Complexes†

A simple methodol. for the chemoselective redn. of esters to aldehydes with a N-heterocyclic-carbene-iron complex, such as [(IMes)-Fe(CO)4], as the catalyst (1 mol%) in the presence of a secondary silane (diethylsilane or diphenylsilane) as the reducing agent. has been developed. This reaction occurs at room temp. under UV irradn. with both arom. and aliph. esters. Notably, this catalytic system also permitted the efficient and selective redn. of lactones to lactols. Exptl. evidence indicated that the hydrosilylation occurs by oxidative addn. of the hydrosilane to an unsatd. NHC-Fe species to yield a silyl iron hydride complex. [on SciFinder(R)]

Iron-Catalyzed C–H Borylation of Arenes

Well-defined iron bis(diphosphine) complexes are active catalysts for the dehydrogenative C-H borylation of aromatic and heteroaromatic derivatives with pinacolborane. The corresponding borylated compounds were isolated in moderate to good yields (25-73%) with a 5 mol% catalyst loading under UV irradiation (350 nm) at room temperature. Stoichiometric reactivity studies and isolation of an original trans-hydrido(boryl)iron complex, Fe(H)(Bpin)(dmpe)2, allowed us to propose a mechanism showing the role of some key catalytic species.

When iron met phosphines: a happy marriage for reduction catalysis

The last two decades have seen a huge development of well-defined organophosphorus (mainly phosphine) iron complexes or in situ generated systems used in homogeneous catalysed reduction (hydrogenation, hydrogen transfer and hydrosilylation). Emerging sustainability concepts emphasize the use of environmentally benign, earth abundant and inexpensive first row transition metals such as iron instead of the traditional second and third row transition metals. In the reduction area, this is also an important goal and for this purpose, achiral and chiral organophosphorus ligands permitted versatile modifications of the architecture of iron complexes to finely tune both the activity and stereoselectivity. Besides the classical goals such as aldehydes, ketones and imines, more challenging carboxylic substrates such as carboxylic acids, esters and ureas can be reduced efficiently, chemoselectively, and even enantioselectively using well designed iron–phosphine catalytic systems. The topical reduction of carbon dioxide and nitrogen is also an exciting area of research in which iron has started to show a promising performance.

1,2-Olefin addition of a frustrated amine–borane Lewis pair

Addition of B(C(6)F(5))(3) to an alpha-dimethylamino-o-vinylferrocenophane system gives a frustrated Lewis pair that undergoes intramolecular 1,2-N-B addition to the alkene.

[(NHC)Fe(CO)4] Efficient Pre‐catalyst for Selective Hydroboration of Alkenes

[(IMes)Fe(CO)4] [IMes=1,3‐bis(2,4,6‐trimethylphenyl) imidazol‐2‐ylidene] complex was found to be an efficient pre‐catalyst for the hydroboration of functional alkenes in the presence of pinacolborane at room temperature. Notably, UV irradiation (350 nm) is important to promote this catalytic transformation. Interestingly, high chemo‐ and regioselectivities were observed as only anti‐Markovnikov boronate derivatives were obtained and various functional groups can be tolerated.

Unexpected selectivity in ruthenium-catalyzed hydrosilylation of primary amides: synthesis of secondary amines

Selective ruthenium-catalyzed reductive coupling of primary amides under hydrosilylation conditions is achieved using an one pot procedure. Using 3 equiv. of phenylsilane and [RuCl2(mesitylene)]2 (1-2 mol%) as the catalyst at 100 °C under neat conditions, secondary symmetric amines were obtained in good yields and with high chemoselectivities.

A convenient nickel-catalysed hydrosilylation of carbonyl derivatives

Hydrosilylation of aldehydes and ketones catalysed by nickel acetate and tricyclohexylphosphine as the catalytic system was demonstrated using polymethylhydrosiloxane as a cheap reducing reagent.

Nickel‐Catalysed Reductive Amination with Hydrosilanes

Amines are central building blocks in organic synthesis for the preparation of natural products, pharmaceutical and agronomical compounds. Among the various synthetic methods to prepare amines, the catalytic reduction of imines is one of the most efficient methods developed. The stability of some imines is, however, rather limited, and their synthesis and purification can be tedious. On the other hand, direct reductive amination is an elegant and straightforward alternative to the synthesis of substituted amines, as the imine is generated in situ and reduced directly to the corresponding amines. The reducing agent is usually a borohydride derivative, although several methods based on transition metal-catalysed hydrogenation, hydrogen transfer and hydrosilylation have been also developed. Hydrosilanes are versatile reducing agents that allow the use of mild conditions and good chemoselectivities. In addition, the use of inexpensive silanes such as polymethylhydrosiloxane (PMHS), an abundant and non-toxic by-product of the silicone industry, or tetramethyldisiloxane (TMDS) offer useful alternatives for large-scale hydrogenations. The use of earth abundant transition metals has become an important goal in catalysis. In the field of reduction, particularly in hydrosilylation, many efforts have been devoted to using inexpensive earth abundant metals such as iron, 10] zinc, titanium or copper. Compared to the former metals of the first row of the periodic table, nickel has been employed much less. Notably, only two examples of imine reduction have been reported, one by hydrogen transfer reaction and the other by hydrosilylation. In the field of reductive amination, catalytic reactions involving nickel are extremely scarce and mainly nickel nanoparticles have been involved in the hydrogen transfer reductive amination of aldehydes. In 2012 and 2013, we have focused our attention on developing an efficient catalytic system for the reduction of carbonyl derivatives with nickel. We have found that the simple salt Ni(OAc)2 in the presence of tricyclohexylphosphine could be an efficient catalytic system for the reduction of aldehydes and ketones with PMHS as the silane. Here, we report the reductive amination of aldehydes by hydrosilylation by using an in situ-generated catalytic system from inexpensive nickel acetate and tricyclohexylphosphine. To start our investigation, we selected benzaldehyde (1 equiv.) and p-methoxyaniline (1.5 equiv.) as the model substrates and the combination of Ni(OAc)2 (5 mol %) and PCy3 (10 mol %) as the catalytic system in the presence of 4 molecular sieves (Scheme 1, Table 1). 21] At 100 8C in toluene with 4 equiv. of PMHS, the reaction led to 80 % conversion into the amine 3 and 20 % of benzyl alcohol 1 resulting from the