Jianxia Zheng

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)]

[(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.

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