Catalytic Activation of Diazo Compounds Using Electron‐Rich, Defined Iron Complexes for Carbene‐Transfer Reactions

Abstract

The activation of diazo compounds by nitrogen extrusion in the presence of transition-metal complexes is a convenient method for the transfer of a carbene to an organic substrate in, for example, [2+1] cycloadditions with olefins [1] or carbonyl compound [2] to give three-membered-ring compounds, or in reactions with electron-rich heteroatoms like sulfur, [3] phosphorus, [4] and nitrogen [5] to generate ylides. [6] In general, these types of reactions are catalyzed by oxidized transition-metal complexes based upon copper, [1, 7] rhodium, [5a-d] ruthenium, [8] palladium, [9] cobalt, [10] iron, [4,11] and silver. [12] In the initial step the metal center is reduced by nucleophilic addition of the diazo compound. The subsequent back-donation of an electron from the metal to the carbon atom with concomittant release of nitrogen results in the formation of the metal-carbon double bond [Eq. (1) in Figure 1].In contrast, the corresponding activation of stabilized, electron-poor diazo compounds by nucleophilic and hence electron-rich transition-metal complexes is unprecented. From a mechanistic point of view the metal center would add into the C = N bond of the diazo compounds followed by an electron back-donation with release of nitrogen and formation of the metal-carbon double bond [Eq.(2), Figure 1]. [13] With respect to the oxidation state of the metal center both mechanistic manifolds are redox-neutral; how-ever, the latter mechanism might favor the undesired carbene dimerization to the corresponding olefins.We were wondering whether the electron-rich ferrate complex Bu 4 N[Fe(CO) 3 (NO)] (TBAFe), in which the metal center has a formal oxidation state of ÀII and which has been investigated in detail by us and others over the past years, [14] might be able to activate diazo compounds for carbenetransfer reactions. Herein we report the successful realization of this concept in various catalytic transformations including Wittig-type olefinations, [15] the insertion into heteroatomhydrogen bonds, [16] and the Doyle-Kirmse reaction. [17] Since to the best of our knowledge the activation of diazo compounds by reduced metal catalysts has not been reported, we initiated our studies by investigating the carbene transfer using ethyl diazoacetate (6, EDA) as the electrophilic carbene precursor in different model reactions. Apart from its electronic properties, which could result in an increased reactivity toward electron-rich metal complexes, this diazo compound is stable and commercially available in pure form. Gratifyingly, a system consisting of catalytic amounts of TBAFe and stoichiometric amounts of 6 showed good reactivity in 1,2-dichloroethane (Scheme 1).Both the insertion into SÀH and NÀH bonds as well as the transfer of the carbenoide to electron-rich heteroatoms with intermediate formation of an ylide (olefination [18,19] and Doyle-Kirmse reaction) were accomplished under almost Figure 1. Mechanistic model for the decomposition of diazo compounds by electrophilic or nucleophilic metal catalysts.Scheme 1. TBAFe-cat...

Iron‐porphyrin Catalyzed Carbene Transfer Reactions – an Evolution from Biomimetic Catalysis towards Chemistry‐inspired Non‐natural Reactivities of Enzymes

“…Similarly, sigmatropic rearrangement reactions can be conducted as initially reported by van Vranken using TMS‐diazomethane or ethyl diazoacetate, allylic sulfides and the Fe(II)Cl 2 dppe complex as catalyst . Further important advances were reported in the years after by van Vranken, and Plietker using Fe(TBAFe) catalyst . Gross and Aviv described the application of Fe(III) corrole and porphyrin complexes in this transformation .…”

“…Another important catalyst in this context, is the TBA[Fe] (Bu 4 N[Fe(CO) 3 (NO)]) complex that was introduced by Plietker and co‐workers. This particular complex features among iron‐based carbene transfer catalyst an NO ligand that can act as a non‐innocent ligand and thus influence the electronic properties of the metal‐carbene complex (Figure ) …”

Sulfur-Based Ylides in Transition-Metal-Catalysed Processes

“…tolerance. When using TBAFe as a catalyst, the reaction was performed chemoselectively in the presence of boronates, allyl ethers, secondary anilines, disulfides and free alcohols (Scheme 33) [92]. Interestingly, and unusually for such catalysts, the metal complex used in this reaction is electron-rich and decomposes the diazo compound via a nucleophilic attack.…”

The Evolution of Fe‐catalyzed Nucleophilic Activation of Acceptor‐substituted Vinyl‐ and Arylcyclopropanes

“…Within recent years, our group has been interested in exploring the catalytic activity of the shelf‐stable and readily accessible electron‐rich iron complex Bu 4 [Fe(CO 3 )(NO)] (TBA[Fe]), which is prepared on a multigram‐scale starting from Fe(CO) 5 , NaNO 2 , and Bu 4 NBr in a water‐dichloromethane mixture. We have reported a variety of TBA[Fe]‐catalyzed reactions, such as hydrosilylations, transesterifications, carbene‐transfer reactions, C(sp 2 )‐H amination, and in particular, allylic substitutions . Within the context of the last reaction type, we became interested in extending the scope of allyl‐surrogates from the established allylic carbonates, in which a C−O bond is being cleaved in the ionization step, towards the use of labile allylic C−C bonds.…”