Jean Bouffard

Selective Synthesis of [12]Cycloparaphenylene

Shape-persistent, nanosized macrocycles that are composed of only spand sp-hydridized carbon atoms on their perimeter (conjugated molecular loops and belts) have attracted significant attention because of their potential applications in materials science and supramolecular chemistry. 2] Particularly interesting and challenging among these are aromatic belts and rings as exemplified by the V gtle belts, cyclophenacenes, 6] cycloparaphenylenes, and cyclacenes [10, 11] . Adding to their sheer aesthetic appeal, these unusual hydrocarbons represent structural models for carbon nanotube segments, and can be envisioned as potential precursors in the preparation of structurally uniform armchair or zigzag carbon nanotubes (Scheme 1). However, despite extensive trials the synthesis of these molecules remains a formidable challenge. In 2005, as a part of our research program exploring new synthetic methods and properties of oligoarenes and nanocarbons, we initiated a synthetic study of these aromatic belts/rings that aims at contributing to a bottom-up organic synthesis of structurally uniform single-walled carbon nanotubes. We selected cycloparaphenylene as a first target in view of a comparatively straightforward approach to their synthesis through aryl–aryl bond formation. Despite its structural simplicity, no successful synthesis had been reported at the inception of our work. Very recently, Bertozzi and co-workers have accomplished the elegant first synthesis of [9]-, [12]-, and [18]cycloparaphenylenes and coined them as “carbon nanohoops”. Herein, we report a selective synthesis of [12]cycloparaphenylene (1) through stepwise palladiumcatalyzed coupling reactions.

Crystalline 1H-1,2,3-triazol-5-ylidenes: new stable mesoionic carbenes (MICs).

In 2001, Crabtree and co-workers first reported complex A, which features an imidazole ring bound at the C5 position (III), and not at C2 as commonly observed.[7] More recently, Huynh and co-workers[8] and Albrecht and co-workers[9a] showed that pyrazolium and 1,2,3-triazolium salts can serve as precursors to metal complexes of type B and C, which feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V as the ligand, respectively. As a consequence of their lineage, these have also been referred to as N-heterocyclic carbenes (NHCs). However, as no reasonable canonical resonance forms containing a carbene can be drawn for free ligands III–V without additional charges (see V′), these ligands have been described as abnormal or remote carbenes (aNHCs or rNHCs, respectively).[10] As they are, in fact, mesoionic compounds,[11] we suggest naming this family of compounds mesoionic carbenes (MICs). There have been no reported dimerizations of MICs III and IV, which suggests that the Wanzlick equilibrium pathway for classical carbenes is disfavored;[12] this observation should lead to relaxed steric requirements for their isolation. Moreover, experimental and theoretical data suggest that MICs III–V are even stronger electron-donating species than NHCs I and II, which opens up interesting perspectives for their applications.[10] Our recent success in the isolation of a free imidazol-5-ylidene III[13] and pyrazolin-4-ylidenes IV (cyclic bent allenes),[14,15] prompted us to investigate the possibility of preparing new types of stable neutral compounds that feature a lone pair of electrons on the carbon atom.[16] Preliminary calculations (B3LYP, 6–311G(d,p); for details, see the Supporting Information) predicted that the parent MIC V is located at an energy minimum, about 32 kcalmol−1 above the regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore, parent V is predicted to exhibit an appreciably large singlet–triplet band gap (56 kcalmol−1), which is a good predictor of carbene stability and thus of possible isolation. Herein, we report the preparation, isolation, and characterization of two free 1,2,3-triazol-5-ylidenes of type V. By analogy with the synthetic route used for preparing NHCs and the related species III and IV, 1,2,3-triazolium salts (2a,b) were targeted as precursors for the desired 1,2,3-triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl substituent (2,6-diisopropylphenyl, Dipp) was selected to provide kinetic stabilization to the ensuing free ligand. 1,2,3-Triazole 1 was obtained in 83% yield from the copper-catalyzed azide–alkyne cycloaddition (CuAAC, click chemistry) of 2,6-diisopropylphenyl azide and phenylacetylene.[17] The one-pot conversion of aniline into the desired aryl azide, followed in situ by CuAAC as reported by Moses and co-workers[18] was found to be especially convenient for the synthesis of 1. Alkylation of 1 with methyl or isopropyl trifluoromethanesulfonate afforded the corresponding tri-azolium salts in moderate to excellent yields (2a and 2b, respectively; Scheme 2). Scheme 2 Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b. Potassium bases have been identified as the reagents of choice for the depronation of carbene precursors, as they avoid the formation of stable carbene–alkali-metal adducts that are commonly encountered when lithium bases are used.[12,13,14a,19] Gratifyingly, triazolium salts 2a,b were cleanly deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide in ethereal solvents to afford the corresponding MICs Va and Vb in 55 and 39% yield, respectively. Deprotonation was evidenced by the disappearance of the triazolium CH signal in their 1H NMR spectra (2a: δ =8.62 ppm; 2b: δ =8.85 ppm) and the appearance of a signal at low field in the 13C NMR spectrum (Va: δ = 202.1 ppm; Vb: δ =198.3 ppm). The structure of Va was unambiguously confirmed by X-ray crystallography (Figure 1).[20] In the solid state, Va contains a planar heterocycle, characterized by bond lengths that are intermediate between those of single and double bonds; both of these features are indicative of electronic delocalization. Upon deprotonation, the C5 carbon bond angle becomes more acute (2a: 106°; Va: 100°), which is consistent with an increased s character in the σ lone pair orbital of Va compared to the C–H bonding orbital of the precursor 2a. This is in agreement with the generally observed trend for carbenes and their conjugate acids.[5] Figure 1 Molecular views (thermal ellipsoids set at 50 % probability) of 2a (top) and Va (bottom) in the solid state. For clarity, counter ions, solvent molecules, and H atoms are omitted, except for the ring hydrogen of 2 a. Selected bond lengths [A] ... In the solid state, with the exclusion of oxygen and moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–52°C decomp.) remained stable for several days at −30°C and for a few hours at room temperature. By contrast, Vb (m.p. 110–112°C) was significantly more stable, showing no sign of decomposition after three days at room temperature in the solid state. Upon heating in a benzene solution for 12 hours at 50°C, Va decomposed to give, among other products, triazole 3 (Scheme 3; for details, see the Supporting Information). We surmise that the latter product results from a nucleophilic attack of the carbon lone pair of Va on the methyl group of a second molecule of Va, giving rise to heterocycles 4 and 5, which react together to afford the observed product 3. This apparent rearrangement is reminiscent of that recently observed in the formation of imidazol-2-ylidenes of type I from imidazol-5-ylidenes of type III that contain an electro-philic Y group.[21] In agreement with this hypothesis, MIC Vb, which contains the less-electrophilic isopropyl group at the N3 position, appears much more robust with respect to this decomposition pathway. Scheme 3 Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy with the rearrangement of III into I. To evaluate the donor properties of 1,2,3-triazol-5-yli-denes, the [(Va)Ir(CO)2Cl] complex was prepared by addition of Va to [{Ir(cod)Cl}2] (cod = 1,5-cyclooctadiene), followed by treatment with an excess of carbon monoxide. The CO vibration frequencies (ν =2061 and 1977 cm−1; νavg = 2019 cm−1) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (νavg = 2021 cm−1),[9a] and are indicative of donor properties that are superior to those of NHCs I and II (νavg = 2022–2031 cm−1),[22] but inferior to those of MICs III (νavg = 2003–2006 cm−1)[23] and IV (νavg = 2002 cm−1).[14b] Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by compounds Va,b, possess an ensemble of properties that portend to their utility. The synthesis of their precursors is short and efficient, from readily available starting materials, yet is modular and thus amenable to a wide variety of potential analogues. As with other mesoionic carbenes III and IV, the dimerization of MICs of type V has not been observed; therefore, the preparation of comparatively unhindered MICs is predicted to be viable. Their donor properties are greater than those of NHCs of type I and II, but they are nonetheless available by deprotonation using mild bases (e.g. alkoxides), thus signaling their potential for applications, such as nucleophilic organocatalysis. Free triazolylidenes V complement the rapidly growing numbers of neutral carbon-based κ1C ligands that are now available. We predict that many other classes of MICs, that are derived from a variety of heteroaromatic scaffolds, can be isolated. This endeavor is currently the object of ongoing efforts in our laboratory.

Polydiacetylene-Based Colorimetric and Fluorescent Chemosensor for the Detection of Carbon Dioxide

We developed a colorimetric and fluorescent turn-on carbon dioxide sensor that relies on a polydiacetylene, PDA-1, functionalized with amines and imidazolium groups. The pendant amines react with CO2 under basic conditions to form carbamoate anions, which partially neutralize the polymers positive charges, inducing a phase transition. PDA-1 allows for the selective sensing of CO2 with high sensitivity, down to atmospheric concentrations. Naked-eye detection of CO2 is accomplished either in water solutions of PDA-1 or in the solid state with electrospun coatings of PDA-1 nanofibers.

Rhodium-Catalyzed C–H Bond Arylation of Arenes

A review is presented of synthetic methods for the preparation of biaryls by the rhodium-catalyzed C-H bond arylation of arenes with aryl halides (C-H/ C-X couplings), arylmetal reagents (C-H/C-M couplings) and arenes (C-H/C-H couplings), with an emphasis on postulated mechanisms and their implications on reactivity, selectivity and substrate scope.

Protonolysis of a Ruthenium-Carbene Bond and Applications in Olefin Metathesis

The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Brønsted acid results in protonolysis of the Ru-MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicated a rate-determining protonation step in the generation of the metathesis-active species. The activity of the NHC/MIC catalyst was found to exceed those of current commercial ruthenium catalysts.

A bench-stable Pd catalyst for the hydroarylation of fullerene with boronic acids.

A Pd(II) catalyst for the hydroarylation of fullerene with boronic acids is presented. Treatment of C60 with an arylboronic acid in the presence of a catalytic amount of Pd(2-PyCH=NPh)(OCOC6F5)2 in H2O/1,2-Cl2C6H4 at room temperature furnishes the hydroarylation product (Ar-C60-H) in good yield with high selectivity. This complex possesses high catalytic activity paired with bench stability in the solid state.

A Nickel Catalyst for the Addition of Organoboronate Esters to Ketones and Aldehydes

A Ni(cod)(2)/IPr catalyst promotes the intermolecular 1,2-addition of arylboronate esters to unactivated aldehydes and ketones. Diaryl, alkyl aryl, and dialkyl ketones show good reactivity under mild reaction conditions (< or = 80 degrees C, nonpolar solvents, no strong base or acid additives). A dramatic ligand effect favors either carbonyl addition (IPr) or C-OR cross-coupling (PCy(3)) with aryl ether substrates. A Ni(0)/Ni(II) catalytic cycle initiated by the oxidative cyclization of the carbonyl substrate is proposed.

A Stable Acyclic Ligand Equivalent of an Unstable 1,3-Dithiol-5-ylidene

During the last two decades, N-heterocyclic carbenes (NHCs), such as A (Scheme 1), have played a prominent role as ligands for transition-metal catalysts.[1] Their popularity is mainly due to their strong σ-donor properties and the robustness of the corresponding complexes. These two features result from the presence of the electropositive carbon center and the strength of the carbon–metal bond. Therefore, other types of carbon-based L ligands are highly desirable. The simplest method for the preparation of metal complexes featuring a given L ligand is by ligand substitution at the metal center; however, the availability of stable compounds with a lone pair of electrons at a carbon center is very limited.[2] It has recently been shown that mesoionic carbenes (MICs)[3–5] B–D can be isolated as free species.[6] In contrast to “normal carbenes”, no obvious dimerization pathway can be foreseen for MICs. Consequently, a variety of these unusual carbenes should in principle be available without the need for kinetic protection. No derivatives of the 1,3-dithiol-2-ylidene F are known owing to their dimerization into derivatives of tetrathiafulvalene G.[7] Herein, we report our attempts to prepare a free MIC isomer by carbenes of type F, namely, a 1,3-dithiol-5-ylidene E. We show that this compound is unstable owing to spontaneous ring opening to form the corresponding ethynylcarbamodithioate. Importantly, the latter reacts with a variety of metals to give 1,3-dithiol-5-ylidene–metal complexes and therefore is a ligand equivalent of E. Scheme 1 Structural framework of classical NHCs (A), types of MIC that have been isolated previously (B–D), the targeted 1,3-dithiol-5-ylidenes (E), and their unknown 1,3-dithiol-2-ylidene isomers (F), which dimerize to tetrathiafulvalenes of type G. In analogy with the classical synthetic route used to prepare NHCs and MICs, we chose the readily available dithiolium tetrafluoroborate salt 1a as a precursor (Scheme 2).[8] Deprotonation with potassium bis(trimethylsilyl) amide proceeded cleanly, as shown by the disappearance of the signal for the dithiolium-ring proton in the 1H NMR spectrum. The 13C NMR spectrum displayed a signal at δ = 81.5 ppm: significantly further upfield than those observed for other MICs (B: δ = 200 ppm, C: δ = 115 ppm, D: δ = 200 ppm).[6] However, the 13C NMR chemical shift for carbenes is unpredictable[2b] (ranging from δ = 77 ppm[9] to δ = 326 ppm[10]). Therefore, in the hope of confirming quickly the MIC structure of the product, we added trifluoromethanesulfonic acid. We were pleased to observe the quantitative formation of the dithiolium triflate salt 1b (Figure 1).[11] However, when single crystals of the deprotonation product of dithiolium salt 1a were obtained, an X-ray diffraction study revealed that it was not the expected cyclic 1,3-dithiol-5-ylidene 3, but the acyclic ethynylcarbamodithioate 2 (see the Supporting Information). The true identity of 2 rationalizes the 13C NMR spectroscopic data; furthermore, the infrared spectrum shows a band at 2160 cm−1 characteristic of a C≡C triple bond. The formation of 2 is reminiscent of the ring-opening reaction observed in the deprotonation of isoxazolium[12] and isothiazolium salts.[13] Monitoring of the addition of potassium bis(trimethylsilyl)amide to 1a by NMR spectroscopy showed, even at −60 °C, the instantaneous formation of 2. Note that the deprotonation/ring-opening process might be concerted and therefore does not necessarily imply the transient formation of MIC 3. Figure 1 Molecular structures of 1b (top left), 5 (top right), 6 (bottom left), and 9 (bottom right) in the solid state (hydrogen atoms are omitted for clarity). Scheme 2 Deprotonation of the dithiolium salt 1a did not enable the isolation of MIC 3, but led to ethynylcarbamodithioate 2. The addition of trifluoromethanesulfonic acid to 2 induced ring closure to afford the dithiolium salt 1b. Tipp = 2,4,6-triisopropylphenyl, ... The proton-induced cyclization of 2 into 1 prompted us to study the reactivity of the ethynylcarbamodithioate 2 with gold(I) complexes, which are well-known alkynophilic π acids.[14] We were particularly interested in the apparent suitability of compound 2 as a precursor for the formation of stable vinyl–gold complexes [(R1R2C=CR3)AuL][15] (Scheme 3), which are still rare, although they are believed to be key intermediates in gold-catalyzed alkyne activation.[16] The reaction of 2 with (tetrahydrothiophene)gold chloride in THF proceeded cleanly, but did not afford the expected complex 4, in which the heterocycle acts as an X ligand as in vinyl–gold complexes. Instead, the MIC–gold(I) complex 5 was isolated in 68% yield. The 13C NMR spectrum of 5 showed a signal at δ = 146.9 ppm: a chemical shift comparable to that observed for the [(MIC B)AuCl] complex (δ = 153.7 ppm)[6a] and at significantly higher field than those of vinyl–gold complexes (δ = 178–199 ppm).[14] Similarly, an X-ray diffraction study of 5 (Figure 1) revealed that the gold–carbon bond distance (1.978(4)A) is similar to that found in [(MIC B)AuCl] (1.98A)[6a] and [(NHC)AuCl] (1.94–2.00A),[17] and slightly shorter than that in vinyl–gold complexes (2.04–2.06A).[15]. Scheme 3 The gold-induced cyclization of 2 did not afford the expected vinyl–gold complex 4 but the 1,3-dithiol-5-ylidene complex 5. THT = tetrahydrothiophene. These results show that with a gold(I) complex, ethynylcarbamodithioate 2 acts as a ligand equivalent of 1,3-dithiol-5-ylidene 3. To test the scope of this finding, we treated compound 2 with the less electrophilic complexes [{PdCl(allyl)}2] and [{RuCl2 (p-cym)}2] (Scheme 4). MIC complexes 6 (Figure 1) and 7 were isolated in 69 and 83%yield, respectively. To evaluate the donor properties of the 1,3-dithiol-5-ylidene ligand 3, we prepared the corresponding rhodium(I) dicarbonyl chloride complex 9 (Figure 1) by the addition of half an equivalent of [{RhCl-(cod)}2] to 2, followed by treatment with excess carbon monoxide. The CO vibration frequencies for 9 (νav = 2030.8 cm−1) indicate that 3 is a stronger electron donor than classical NHCs (νav = 2039–2041 cm−1)[18] and cyclic (alkyl)(amino)carbenes (CAACs; νav = 2036 cm−1),[19] but is weaker than other MICs (νav=2016–2025 cm−1).[2a] Scheme 4 The MIC–palladium, ruthenium, and rhodium complexes 6–9 were readily prepared. Thus, acyclic ethynylcarbamodithioate 2 is a ligand equivalent of MIC 3. p-cym = para-cymene, cod = 1,5-cyclooctadiene. The 1,3-dithiol-5-ylidene–metal complexes reported herein are thermally robust (m.p. = 272 (5), 219 (6), 217 (7), 194 (8), 186 °C (9)) and not air-sensitive. The precursor, namely, the acyclic ethynylcarbamodithioate 2, can be prepared in gram-scale quantities within a day and is stable for several weeks in the solid state under an inert atmosphere and in solution for up to 1 h at 140°C. These results suggest that the variety of isolable free mesoionic carbenes will be limited by their propensity to undergo ring-opening reactions. However, the reverse process, triggered by transition metals, should be of broad applicability. Since many different analogues of ethynylcarbamodithioate 2 (R-C≡C-X-C(Y)R′, in which X and Y are heteroatoms with a lone pair of electrons) can readily be prepared, numerous MIC complexes will be available. The catalytic study of metal complexes supported by MIC 3 is a subject of current investigations in our laboratory.

Anionic 1,2,3-Triazole-4,5-diylidene: A 1,2-Dihapto Ligand for the Construction of Bimetallic Complexes

A super pyrazolate: deprotonation of the flanking hydrogen of metal complexes of mesoionic carbenes (MICs) offers a simple and general route for the preparation of bimetallic complexes of a 1,2-dihapto anionic dicarbene ligand that is isoelectronic with widely used pyrazolate ligands, while conferring greater electron donation and stronger M-L bonds.

Instantaneous Colorimetric and Fluorogenic Detection of Phosgene with a Meso-Oxime-BODIPY

The meso-oxime-substituted-1,3,5,7-tetramethyl BODIPY (1-oxime) was developed into a colorimetric and fluorogenic probe to selectively detect and quantify phosgene. The fast (<10 s) and sensitive (LOD = 0.09 ppb) phosgene detection is achieved by the conversion of the meso-oxime to the meso-nitrile, resulting in a large fluorescence turn-on response. The utility of 1-oxime was established for the visual detection of phosgene in solution and in a practical solid-state platform, making it a suitable candidate for on-site monitoring of phosgene gas exposure in the workplace.