Gerald Kehr

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.

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.

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.

Reactions of phosphorus/boron frustrated Lewis pairs with SO2

The frustrated Lewis pair tBu3P/B(C6F5)3 (1) readily adds SO2 to yield the zwitterionic adduct tBu3P+–S(O)–OB−(C6F5)3 (3). A series of intramolecular vicinal P/B FLPs Mes2P–(X)–B(C6F5)2 [X = –CH2–CH2– (2a), –CHMe–CH2– (2b), cyclo-C6H10 (5)] add SO2 at −78 °C to yield the corresponding six-membered addition products 4a, 4b, 6. The adducts contain a chiral sulfur center. The [B]–O–(O)S–[P] addition products 3, 4b and 6 were characterized by X-ray diffraction.

Facile Carbon Monoxide Reduction at Intramolecular Frustrated Phosphane/Borane Lewis Pair Templates†

Finding novel pathways for the reduction of carbon oxides is important for the ongoing search for new systematic entries to hydrocarbon feedstocks. The CO to formyl conversion with readily available hydrides represents an important step along this way. Using boranes as the reducing reagents is desirable and potentially useful. Reactions of trialkylboranes with carbon monoxide are synthetically established. H. C. Brown had shown that R3B systems readily react with CO at 100 to 125 8C at normal pressure to yield the respective tertiary alcohols after oxidative workup. They modified this procedure to achieve the synthesis of ketones and aldehydes. In the latter case lithium aluminium hydride reagents were added. However, carbon monoxide is surprisingly reluctant to be reduced with [B]H boranes. Carbon monoxide is reported to react with B2H6 at 100 8C and 20 atm to give “borane carbonyl” [H3B-CO], a gas (b.p. 64 8C) that dissociates at atmospheric pressure. We have now found that B H borane reduction of carbon monoxide can be carried out with a suitable borane at a frustrated phosphane/borane Lewis pair (FLP) template. We stirred a mixture of the bulky cyclopentenylphosphane 1 with the hydroboration reagent [HB(C6F5)2] [9] for about 15 minutes at RT and then subjected the resulting mixture to an atmosphere of carbon monoxide (2 bar). Workup after 12 h at RT eventually gave the reduction product 2 as a colorless solid in 63% yield (see Scheme 1). Single crystals of 2 were obtained from dichloromethane/n-pentane. In the crystal compound 2 contains a “h-formyl” B(C6F5)2 subunit that is bonded through the acyl carbon atom (C1) to the phosphorus atom and through the acyl oxygen atom (O1) to the boron center (B1) of the FLP framework (see Figure 1 and Table 1). The resulting six-membered heterocycle fea-

1,1-Carboboration of 1-Alkynes: A Conceptual Alternative to the Hydroboration Reaction

Strongly electrophilic boranes R-B(C(6)F(5))(2) react readily with a variety of 1-alkynes by means of a 1,1-carboboration reaction to yield alkenylborane products, which can subsequently be used as reagents in metal catalyzed cross-coupling reactions.

Cyclizations via Frustrated Lewis Pairs: Lewis Acid Induced Intramolecular Additions of Amines to Olefins and Alkynes

An emerging strategy for the activation and reaction of small molecules is based on the concept of “frustrated Lewis pairs” (FLPs). Such systems exploit steric congestion that frustrates classical Lewis acid–base adduct formation. In this fashion the unquenched Lewis acidity and basicity is available for reaction with a third component. This concept was first employed to effect the heterolytic cleavage of H2 by sterically frustrated combinations of phosphines and boranes. This reactivity has subsequently been applied to develop a “metal-free” approach to hydrogenation catalysis of imines, enamines, and silylenol ethers. This strategy of FLP activation of H2 is not limited to P/B combinations. Indeed, suitably sterically hindered carbenes, 13] amines, 15] and pyridines have been shown to generate FLPs that react with H2. Moreover the nature of the Lewis acid can be adjusted so as to impart reversibility to the H2 uptake. 11] FLPs have also been exploited to effect the activation of a variety of small molecules including olefins, dienes, alkynes, 20] B H bonds, disulfides, PhNCO, CO2, [25] and N2O. [26] In the case of olefins and alkynes, Stephan and co-workers 19,20] showed that sterically encumbered phosphines and boranes or alanes, can effect interand intramolecular additions affording zwitterionic phosphonium borates. DFT studies have suggested that such additions to olefins occur via an antarafacial asynchronous concerted addition. For the reactions of alkynes, the trans-olefinic products suggest nucleophilic attack on a Lewis acid activated alkyne. Employing more basic phosphines, C H activation of terminal alkynes affords phosphonium alkynyl borate salts. Recently, the Erker group has shown that the linked phosphine borane (C6H2Me3)2PCH2CH2B ACHTUNGTRENNUNG(C6F5)2, reacts with pentyne to give a related C H activation product. However, this P/B species also reacts with a vinyl ether and norbornene to give the cyclized zwitterionic phosphonium borates. In a very recent example of similar reactivity, Erker and co-workers have also described the addition of an amine to an intramolecular olefinic residue in the presence of B ACHTUNGTRENNUNG(C6F5)3, which affords a thermally unstable zwitterionic ferrocene ammonium borate derivative. Herein, we describe reactions of sterically encumbered amines with intramolecular olefin or acetylene fragments in the presence of a Lewis acid. The resulting fiveand six-membered heterocyclic ammonium borate species demonstrate that FLP reactivity provides a facile route to intramolecular cyclizations. B ACHTUNGTRENNUNG(C6F5)3 was added to a solution of o-(2-propenyl)-N,Ndimethylaniline in CH2Cl2 and the mixture was stored at 32 8C for 48 h. Subsequent solvent removal from the supernatant gave a white powder (1) in 87 % yield. The B NMR spectrum of 1 showed a single resonance at d= 14.3 ppm. The corresponding F NMR resonances were observed at d= 132.0, 162.2 and 166.0 ppm. These data are consistent with the quaternization of B and thus a borate anion. The H NMR spectrum reveals inequivalent methyl resonances at d= 3.25 and 3.00 ppm in addition to broad resonances attributable to methylene protons at d= 2.25 and 1.91 ppm. The latter are consistent with B C bond formation. These data together with C, H/H-COSY, H/C-HSQC, and H/C-HMBC data were consistent with the formulation of the product as a reduced indole derivative C6H4ACHTUNGTRENNUNG(NMe2)ACHTUNGTRENNUNG(CH2CH ACHTUNGTRENNUNG(CH2B ACHTUNGTRENNUNG(C6F5)3) (Figure 1). This zwitterionic formulation of 1, comprising a five-membered, cyclic ammonium fragment with a pendant methylene borate group, was subsequently confirmed by X-ray crystallography. The metric parameters were unexceptional. [a] T. Voss, Dr. C. Chen, Dr. G. Kehr, E. Nauha, Prof. Dr. G. Erker Organisch-Chemisches Institut, Westf lische Wilhelms-Universit t 48149 M nster, Correnstrasse 40 (Germany) E-mail : erker@uni-muenster.de [b] T. Voss, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George Street, Toronto, ON, M5S 3H6 (Canada) E-mail : dstephan@chem.utoronto.ca [c] E. Nauha Nanoscience Center, P.O. Box 35 40014 University of Jyv skyl (Finland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200903483.