Gerhard Erker

Frustrated Lewis Pair Chemistry: Development and Perspectives

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Reversible Metal-Free Carbon Dioxide Binding by Frustrated Lewis Pairs†

Frustrated Lewis pairs comprising phosphine and borane react to reversibly bind and release CO2, offering a rare example of metal-free CO2 sequestration. The mechanism of formation of CO2 derivatives by almost simultaneous P-C and O-B bond formation was characterized by quantum chemical calculations.

The Mechanism of Dihydrogen Activation by Frustrated Lewis Pairs Revisited

Activation of dihydrogen is typically a domain of transition metal chemistry. Even nature uses metal-centered reactions to split the dihydrogen molecule in hydrogenase enzymes. A recent development is the use of metal-free systems for H2 activation: Stephan, Erker et al. have described frustrated Lewis pairs (FLP), that is, pairs of Lewis acids and bases that do not fully quench each other owing to the steric bulk of their substituents; these pairs heterolytically split the H2 molecule (Scheme 1). Phosphane/borane pairs, such as 1 or 3 (and an increasing number of related systems that have appeared in the literature), react rapidly and effectively with H2 to yield the corresponding phosphonium cation/hydridoborate anion pairs (here 2 and 4, respectively). These systems have been used as active metal-free hydrogenation catalysts. P pai et al. have presented the notion that the H H bond is cleaved by such systems in an almost linear P-H-H-B arrangement in the transition state (TS). We have now found that this proposal is probably a gross oversimplification of the mechanistic course taken, because the theoretical treatment used did not adequately take into account the interaction between the large substituents that are specifically used. We wish however to point out that the authors in Ref. [7a] pointed out correctly for the first time the importance of secondary, non-covalent C6F5···tBu interactions. Herein, we present the results of our state-of-the-art calculations for this problem, which has resulted in a more realistic description of the TS involved, which features a non-linear P-H-H-B unit. Furthermore, we present an even simpler mechanistic picture of the basic activation step that emphasizes on the polarization of H2 induced by the electric field of the FLP inside its cavity that can explain important (and hitherto unclear) experimental findings. One of the first and very basic questions involves the structure of the TS and in particular in how far the proposed linear P-H-H-B arrangement is required. For the intramolecular system 3 and the similar case of Sumerin et al., a linear TS is geometrically not possible, although these systems also efficiently activate H2 at ambient temperatures. For molecules 1–4, we performed high-level quantum chemical calculations at wavefunction (WF)-based levels (SCS-MP2 and MP2, extrapolated to the complete basis set [CBS] limit) and by state-of-the-art dispersion-corrected density functional theory (DFT-D) using the B97-D functional (for details, see the Supporting Information). For 1/H2, we first computed a relaxed two-dimensional potential energy surface (PES) with a fixed linear P-H-H-B unit with the most important H H and P B distances as variables. Full TS optimizations were then performed for both 1/H2 and 3/H2. It should be noted that all our computations refer to isolated molecule conditions; although this makes direct comparisons with experimental observations difficult, we think that it is very important to first pinpoint the mechanism and the associated energies without solvent effects to be on solid ground for further, rather ambitious theoretical condensed-phase treatments. The two-dimensional PES based on B97-D/TZVPP optimizations is shown in Figure 1. Furthermore, singlepoint computations at the MP2/CBS and SCS-MP2/CBS levels, which provide very similar results (Supporting Information), have been performed. The most striking result is that the PESs from all three methods lack the previously observed TS: on the contrary, when the P B distance decreases, there is Scheme 1. Investigated FLPs that split the H2 molecule.

Linear Alkane Polymerization on a Gold Surface

The confining channel geometry of a gold surface induces selective end-to-end linking of hydrocarbon chains. In contrast to the many methods of selectively coupling olefins, few protocols catenate saturated hydrocarbons in a predictable manner. We report here the highly selective carbon-hydrogen (C–H) activation and subsequent dehydrogenative C–C coupling reaction of long-chain (>C20) linear alkanes on an anisotropic gold(110) surface, which undergoes an appropriate reconstruction by adsorption of the molecules and subsequent mild annealing, resulting in nanometer-sized channels (1.22 nanometers in width). Owing to the orientational constraint of the reactant molecules in these one-dimensional channels, the reaction takes place exclusively at specific sites (terminal CH3 or penultimate CH2 groups) in the chains at intermediate temperatures (420 to 470 kelvin) and selects for aliphatic over aromatic C–H activation.

Reactions of an intramolecular frustrated Lewis pair with unsaturated substrates: evidence for a concerted olefin addition reaction.

The intramolecular frustrated Lewis pair (mesityl)(2)P-CH(2)-CH(2)-B(C(6)F(5))(2) was generated in situ by HB(C(6)F(5))(2) hydroboration of dimesitylvinylphosphine. The compound reacts with 1-pentyne by C-H bond cleavage. It undergoes a 1,2-addition to the carbonyl group of trans-cinnamic aldehyde to yield a zwitterionic six-membered heterocycle by B-O and P-C bond formation. The Lewis pair regioselectively adds to the electron-rich C=C double bond of ethyl vinyl ether, and it selectively undergoes an exo-cis-2,3-addition to norbornene. A combined experimental/theoretical study suggests that this reaction takes place in an asynchronous concerted fashion with the B-C bond being formed in slight preference to the P-C bond. The addition products were characterized by X-ray crystal structure analyses.

Frustrated Lewis pair chemistry of carbon, nitrogen and sulfur oxides

Frustrated Lewis pairs have been used to activate a variety of small molecules. In this review we focus on the recent chemistry of FLPs with CO2, CO, N2O, NO and SO2. While FLP capture of these small molecule is achieved in all of these cases, subsequent applications of the products include stoichiometric and catalytic reductions of CO2, C–O bond scission of CO and use of FLP–NO radicals in polymerization.

Capture of NO by a Frustrated Lewis Pair: A New Type of Persistent N-Oxyl Radical**

Frustrated Lewis Pairs (FLPs) show a rapidly increasing spectrum of interesting chemical reactions. They have been reported to activate dihydrogen under mild conditions and to serve as metal free hydrogenation catalysts toward specific organic substrates. FLPs add to numerous unsaturated substrates such as alkenes and alkynes, carbonyl compounds, azides and even CO2 or N2O. [3,4] For instance, the intramolecular P/B-FLP 1 adds to nitrosobenzene to form the sixmembered heterocycle 2. Nitric oxide (NO) is an important messenger molecule and regulator in biological systems. We find that the reactive frustrated Lewis pair 1 cleanly and rapidly reacts with this essential small molecule to form the persistent heterocyclicNoxyl radical “P/B-FLP-NOC” (3). Compound 3 represents a novel type of N-oxyl radical related to the ubiquitous TEMPO radical (4) and its congeners (e.g. 5 (PINO) and 6, see Scheme 1). Herein we describe the synthesis, characterization, and O-based reactivity of this novel type of N-oxyl radical derived from a frustrated Lewis pair and nitric oxide. Treatment of a yellow solution of the FLP Mes2PCH2CH2B(C6F5)2 (1) [10] (in situ generated from Mes2PCH=CH2 and Piers borane [HB(C6F5)2] ) in fluorobenzene with 1 equiv NOgas gave rise to an intense green solution fromwhich blue crystals of the P/B-FLP-NOC product 3 were isolated in 58% yield by precipitation with pentane (Scheme 1). X-ray crystal structure analysis of 3 (Figure 1)

N,N-addition of frustrated Lewis pairs to nitric oxide: an easy entry to a unique family of aminoxyl radicals.

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes(2)P-C(6)H(10)-B(C(6)F(5))(2)] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NO-CH(2)Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes(2)P-CHR(1)-CHR(2)-B(C(6)F(5))(2)] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NO-CHMePh 6b derived from 1-bromoethylbenzene undergoes NO-C bond cleavage at 120 °C with an activation energy of E(a) = 35(2) kcal/mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by N,N-cycloaddition of C(2)-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.

Carbon-carbon bond activation by 1,1-carboboration of internal alkynes.

Internal alkynes undergo 1,1-carboboration reactions upon treatment with boranes RB(C(6)F(5))(2) (R = C(6)F(5), CH(3)) to yield trisubstituted alkenylboranes. These products can be used as substrates in Pd-catalyzed cross-coupling reactions.

The reaction of intermediate zirconocene-aryne complexes with CH bonds in the thermolysis of diarylzirconocenes

Abstract Upon thermolysis in aromatic hydrocarbon solvents diarylzirconocenes undergo successive replacement of their σ-bonded ligands by aryl groups from the solvent. Analysis of the product mixture by photochemical degradation to biphenyls reveals that intermediate aryne zirconocene complexes are formed which then react with the solvent with CH bond fission and formation of new ZrC bonds. A β-hydride elimination process can be excluded for the formation of the aryne complexes and it is suggested that they are formed via abstraction of an ortho proton from one of the aryl ligands by the other σ-bonded group.