A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa-arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base-free strontium-perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa-arene yielded the highly Lewis acidic, dicationic strontium ansa-arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂-reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.     

Synthesis,Structure, and Reactivity of Borole‐Functionalized Ferrocenes

Herein, we report on the synthesis of ferrocenylborole [Fc(BC₄Ph₄)₂] featuring two borole moieties in the 1,1′‐positions. The results of NMR and UV/Vis spectroscopy and X‐ray diffraction studies provided conclusive evidence for the enhanced Lewis acidity of the boron centers resulting from the conjugation of two borole fragments. This finding was further validated by the reaction of [Fc(BC₄Ph₄)₂] and the 4‐Me‐NC₅H₄ adduct of monoborole [Fc(BC₄Ph₄)], which led to quantitative transfer of the Lewis base. The coordination chemistry of ferrocenylboroles was further studied by examining their reactivity towards several pyridine bases. Accordingly, the strong Lewis acidity of boroles in general was nicely demonstrated by the reaction of [Fc(BC₄Ph₄)] with 4,4′‐bipyridine. Unlike common borane derivatives such as [FcBMe₂], which only forms a 2:1 adduct, we also succeeded in the isolation of a 1:1 Lewis acid/base adduct, with one nitrogen donor of 4,4′‐bipyridine remaining uncoordinated. In addition, the reduction chemistry of ferrocenylboroles [Fc(BC₄Ph₄)] and [Fc(BC₄Ph₄)₂] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [Fc(BC₄Ph₄)] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either η⁵‐coordinated C₅H₄ or η⁵‐coordinated BC₄Ph₄ moieties. In contrast, no evidence for borole migration was observed during reduction of bisborole [Fc(BC₄Ph₄)₂], which readily resulted in the formation of the corresponding tetraanion. Finally, our efforts to further enhance the borole ratio in ferrocenylboroles aiming at the synthesis of [Fc(BC₄Ph₄)₄] failed and, instead, generated an uncommon ansa‐ferrocene containing two borole fragments in the 1,1′‐positions and a B₂C₄ ansa‐bridge.     

Triarylimidazo[1,2-a]pyridine-8-carbonitriles: solvent-free synthesis and their anti-cancer evaluation

In this study, we have presented the synthesis of novel 2,5,7-triarylimidazo[1,2-a]pyridine-8-carbonitriles from 2-amino-4,6-diarlypyridine-3-carbonitrile and nitrostyrene using FeCl₃ (20?mol%) as Lewis acid under solvent-free conditions. A library of compounds with diverse substitutions have also been synthesized and screened for in-vitro anti-cancer activity against various cell lines such as A549 (Lung), HCT-116 (Colon), SW-620 (Colon), and MIAPACA (Pancreas).     

One‐pot Synthesis of Benzamidonaphtho[2,1‐b]furans and Benzamidobenzo[b]furans as Novel Polycyclic Heteroaromatic Compounds

An operationally simple, green and efficient procedure for one‐pot synthesis of novel polycyclic heteroaromatic compounds such as benzamidonaphtho[2,1‐b]furans and benzamidobenzo[b]furans has been developed from the reaction of arylglyoxals, benzamide, and phenols. The reactions were mediated with low amounts of yttrium nitrate hexahydrate as a suitable Lewis acid catalyst without using solvent.     

A Catalytic Diastereoselective Formal [5+2] Cycloaddition Approach to Azepino[1,2‐a]indoles: Putative Donor–Acceptor Cyclobutanes as Reactive Intermediates

A catalytic formal [5+2] cycloaddition approach to the diastereoselective synthesis of azepino[1,2‐a]indoles is reported. The reaction presumably proceeds through a Lewis acid catalyzed formal [2+2] cycloaddition of an alkene with an N‐indolyl alkylidene β‐amide ester to form a donor–acceptor cyclobutane intermediate, which subsequently undergoes an intramolecular ring‐opening cyclization. Azepine products are formed in up to 92 % yield with high degrees of diastereoselectivity (up to 34:1 d.r.).     

Reactivity of Xantphos-Type Rhodium Complexes Towards SF4: SF3 Versus SF2 Complex Generation

S−F-bond activation of sulfur tetrafluoride at [Rh(Cl)(^tBuxanPOP)] ( 1 ; ^tBuxanPOP=9,9-dimethyl-4,5-bis-(di-tert-butylphosphino)-xanthene) led to the formation of the cationic complex [Rh(F)(Cl)(SF₂)(^tBuxanPOP)][SF₅] ( 2 a ) together with trans-[Rh(Cl)(F)₂(^tBuxanPOP)] ( 3 ) and cis-[Rh(Cl)₂(F)(^tBuxanPOP)] ( 4 ) which both could also be obtained by the reaction of SF₅Cl with 1 . In contrast to that, the conversion of SF₄ at the methyl complex [Rh(Me)(^tBuxanPOP)] ( 5 ) gave the isolable and room-temperature stable cationic λ⁴-trifluorosulfanyl complex [Rh(Me)(SF₃)(^tBuxanPOP)][SF₅] ( 6 ). Treatment of 6 with the Lewis acids BF₃ or AsF₅ produced the dicationic difluorosulfanyl complex [Rh(Me)(SF₂)(^tBuxanPOP)][BF₄]₂ ( 8 a ) or [Rh(Me)(SF₂)(^tBuxanPOP)][AsF₆]₂ ( 8 b ), respectively. Refluorination of 8 a was possible with the use of dimethylamine giving [Rh(Me)(SF₃)(^tBuxanPOP)][BF₄] ( 9 ). A reaction of 6 with trichloroisocyanuric acid (TClCA) gave the fluorido complex [Rh(F)(Cl)(SF₂)(^tBuxanPOP)][Cl] ( 2 b ) together with chloromethane and SF₅Cl.     

Microwave‐Assisted Expedite Synthesis of 2‐Phenylimidazo[1,2‐a]pyridylquinoxalin‐2(1H)‐ones

An efficient methodology has been developed for the synthesis of quinoxalin‐2(1H)‐one derivatives of 2‐phenylimidazo[1,2‐a]pyridines by microwave‐irradiated Hinsberg heterocyclization between 2‐phenylimidazo[1,2‐a]pyridine‐3‐glyoxalates and o‐phenylenediamine using either montmorillonite K‐10 or Yb(OTf)₃ as catalysts. Montmorillonite K‐10 was proven to be an efficient catalyst for the heterocyclization reaction between sterically hindered glyoxalate and o‐phenylenediamine only under microwave conditions. The use of Yb(OTf)₃/tetrahydrofuran was also found to be an effective catalyst for the above chemical transformation among a series of Lewis acids screened under microwave conditions; however, comparatively lesser yields were obtained as compared with the use of montmorillonite K‐10.     

Oxidative Addition of Diphenyl Disulfide/Thiophenol to [Mn(CO)5]−: Crystal Structure of cis-[Mn(CO)4(SPh)2]−

Oxidative addition of diphenyl disulfide to the coordinatively unsaturated [Mn(CO)₅]^? led to the formation of low-spin, six-coordinate cis-[Mn(CO)₄(SPh)₂]^?. The complex cis-[PPN][Mn(CO)₄(SPh)₂] crystallized in monoclinic space group P2₁/c with a = 9.965(2) Å, b = 24.604(5) Å, c = 19.291(4) Å, β = 100.05(2)°, V = 4657(2)ų, and Z = 4; final R = 0.036 and R_w = 0.039. Thermal transformation of cis-[Mn(CO)₄(SPh)₂]^? to [(CO)₃Mn(μ-SPh)₃Mn(CO)₃]^? was completed overnight in THF at room temperature. Additionally, reaction of [Mn(CO)₅]^? and PhSH in 1:2 mole ratio also led to cis-[PPN](Mn(CO)₄(SPh)₂]. Presumably, oxidative addition of PhSH to [Mn(CO)₄]^? was followed by a Lewis acid-base reaction to form cis-[Mn(CO)₄(SPh)₂]^? with evolution of H₂.     

Reactions of Rhenium and Technetium Nitrido Complexes with ER3 Fragments (E = B,Ga, Al,C, P; R = H,Alkyl, Aryl)

Terminal ‘N^3—’ ligands in rhenium and technetium nitrido complexes are sufficiently nucleophilic to react with Lewis acids under formation of nitrido‐bridged compounds. The reactivity of the nucleophilic centre and the nature of the formed compounds are strongly dependent on the Lewis acid and the composition of the metal complex used. Air‐stable compounds with Re≡N‐ER₃ bridges are formed when ER₃ is BR₃ (R = H, Cl, Br, Ethyl, Phenyl, C₆F₅), BCl₂Ph, GaCl₃, CPh₃⁺, or PPh₃. The six‐co‐ordinate rhenium(V) complexes [ReNX₂(PMe₂Ph)₃] (X = Cl, Br), [ReN(X)(Et₂dtc)(PMe₂Ph)₂] (Et₂dtc^— = diethyldithiocarbamate) and [ReN(Et₂dtc)₂(PMe₂Ph)] have been proved to be excellent starting materials for this type of reactions, whereas the five‐co‐ordinate precursors [ReNCl₂(PPh₃)₂], [ReN(Et₂dtc)₂], [ReN{Ph₂P(S)NP(S)Ph₂}₂] or [ReNCl₄]^— only react with the most reactive Lewis bases of the examples mentioned above such as BCl₂Ph or B(C₆F₅)₃. The rhenium‐nitrido bond lengths remain almost unchanged by the adduct formation, whereas a significant decrease of the trans‐influence of the nitrido complexes has been observed as can be seen by a shortening of the corresponding bond lengths or dimerization of five‐co‐ordinate precursors. Electrophilic attack of the Lewis acid to a donor atom of the equatorial co‐ordination sphere of the rhenium complex results in the formation of ‘underco‐ordinate’ metal centres which resemble to di‐, tri or tetrameric units with asymmetric nitrido bridges between each two rhenium atoms. EPR spectroscopy is an excellent tool to reflect the formation of nitrido bridges at the paramagnetic (d¹) [ReNX₄]^— core (X = F, Cl, Br, NCS). The spectral parameters derived for the products of reactions of [ReNCl₄]^— with various boron compounds indicate an increase of the covalency of the equatorial Re‐L bonds as a consequence of the formation of a nitrido bridge. The tendency for the formation of nitrido bridges with Lewis acids is significantly lower for technetium compounds compared to their rhenium analogues. Only a few examples with BH₃ and BPhCl₂ have been established.     

Synthesis of novel 1,2,3,4-tetrahydrobenzo[c]-1,5-naphthyridines from pyrrolo[1,2-b]isoquinolines

1,2,3,4-Tetrahydrobenzo[c]-1,5-naphthyridine ( 5a ) was prepared by a novel synthetic route involving the rearrangement of (±)-(Z)-1,10a-dihydropyrrolo[1,2-b]isoquinoline-3,10(2H,5H)-dione oxime to afford 1,4-dihydrobenzo[c]-1,5-naphthyridin-2(3H)-one, which was reduced to 5a. The cholinomimetic activity observed with 5a prompted the synthesis and biological evaluation of additional analogues.     

Pentacyclic and Hexacyclic Osmaarynes and Their Derivatives

Although osmabenzyne, osmanaphthalyne, osmaphenanthryne, and osmaanthracyne have been previously reported, the synthesis of polycyclic osmaarynes is still a challenge. Herein, we report the successful synthesis of the first pentacyclic osmaarynes (pyreno[b]osmabenzynes 1 a and 2 a ) and hexacyclic osmaaryne (peryleno[b]osmabenzyne 3 a ). Nucleophilic reaction of osmaarynes was used to obtain the corresponding pyreno[b]osmium complexes ( 1 and 2 ) and peryleno[b] osmium complex ( 3 ), which exhibited near-infrared luminescence and aggregation-induced emission (AIE) properties. Complexes 2 and 3 are resistant to photodegradation, and complex 2 has better photothermal conversion properties than 3 .     

Indole and quinoline derivatives of 2,3-dihydro-l-benzothiepin-4(5H) one

By application of the Friedlander synthesis on 2,3-dihydro-l-benzothiepin-4(5H) one (4), the corresponding [4,5-b]quinoline derivatives 5a and 5b were obtained. Starting from the ketone (4) and by application of the Fischer indole synthesis, 1-benzolhiepino[4,5-b ]indole (6) and 1-benzothiepino[4,5-b]benzo[g]indole (7) were obtained. When β-naphthylhydrazine was used in the indolisation reaction, a mixture of 1-benzothiepino[4,5-b]benzo[e]indole ( 8 ) and 1-benzothiepino[4,3-b]benzo[e] indole (9) was obtained.     

Diethyldisulfoammonium chlorometallates as heterogeneous Brønsted–Lewis acidic catalysts for one‐pot synthesis of 14‐aryl‐7‐(N‐phenyl)‐14H‐dibenzo[a,j]acridines

A new series of Brønsted–Lewis acidic diethyldisulfoammonium chlorometallates, [DEDSA][FeCl₄] and [DEDSA]₂[Zn₂Cl₆], were synthesized as solid materials from the reaction of [(Et)₂N(SO₃H)₂][Cl] ionic liquid with transition metal chlorides (FeCl₃ and ZnCl₂) at 80 °C in neat condition for 2 h. The chlorometallates were fully characterized using various spectroscopic and analytical techniques such as Fourier transform infrared, UV–visible and Raman spectroscopies, powder X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray and thermogravimetric analyses, Hammett acidity and elemental analyses. Their catalytic activity was studied as reusable heterogeneous catalysts for the three‐component synthesis of novel 14‐aryl‐7‐(N‐phenyl)‐14H‐dibenzo[a,j]acridines under solvent‐free conditions at 100 °C.     

A revision of the synthesis of some polycyclic systems related to benzoheteroquinolizidines

The preparation of 3-methoxycarbonyl-N-2-(3-benzo[b]thienyl)ethylpyridinium bromide, 5-methoxy-carbonyl-N-2-(3-benzo[b]thienyl)ethyl-1,2,3,4- tetrahydropyridine and (1RS,12bRS)-1-methoxycarbonyl(benzo-[b]thienyl[2,3-a]quinolizidine is described. Some observations dealing with different methods discussed in the literature for the synthesis of such polycyclic systems are reported.     

The synthesis of benzo[b]phenanthro[d]thiophenes and anthra[b]benzo[d]thiophenes

The synthesis of benzo[b]phenanthro[2, 3-d]thiophene ( 5 ), benzo[b]phenanthro[4, 3-d]thiophene ( 6 ), benzo-[b]phenanthro[2, 1-d]thiophene ( 9 ), benzo[b]phenanthro[3, 2-d]thiophene ( 14a ), anthra[1, 2-b]benzo[d]thiophene ( 24 ), anthra[2, 3-b]benzo[d]thiophene ( 29 ) and anthra[2, 1-b]benzo[d]thiophene ( 30 ) is described as well as the preparation of 13-methylbenzo[b]phenanthro[3, 2-d]thiophene ( 14b ).     

Reactivity of Bridged Pentelidene Complexes with Isonitriles: A New Way to Pentel‐Containing Heterocycles

The reaction of [Cp*E{W(CO)₅}₂] (E=P ( 1 a ), As ( 1 b ); Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) with isonitriles RNC (R=tBu, cyclohexyl (Cy), nBu) depends on the steric demand of the substituent at the isonitrile as well as on the stoichiometry of the starting materials. With tBuNC only the Lewis acid/base adducts [Cp*E{W(CO)₅}₂(CNtBu)] (E=P ( 2 a ), As ( 2 b )) are formed. The use of Cy and n‐butylisonitrile leads first to the formation of the Lewis acid/base adduct, but only at low temperatures. At ambient temperatures, a rearrangement occurs and bicyclo[3.2.0]heptane derivatives of the type [{C(Me)C(CH₂)C(Me)C(Me)C(Me)}C(NR)‐ E{W(CO)₅}₂] (E=P, As; R=Cy, nBu) ( 3 a‐Cy , 3 b‐Cy , 3 a‐nBu and 3 b‐nBu ) are obtained. The use of a further equivalent of isonitrile results in products revealing two new structural motifs, the four‐membered ring derivatives [C(Cp*)N(R)C(NR)E{W(CO)₅}₂] ( 4 : E=P, As; R=Cy, nBu) and the bicyclic complexes [[{C(Me)C‐ (CH₂)C(Me)C(Me)C(Me)}C(NR)₂‐ E{W(CO)₅}₂] ( 5 : E=As; R=Cy). The reaction pathway depends on the substituent at the isonitrile. By treatment of 1 a with two equivalents of CyNC only a 2H‐1,3‐azaphosphet complex 4 a‐Cy (E=P; R=Cy) is formed. Treatment of 1 b with two equivalents of CyNC exclusively leads to the complex 5 b‐Cy (E=As; R=Cy). Treatment of 1 a with two equivalents of nBuNC results in a mixture of complexes, the 2H‐1,3‐azaphosphet 4 a‐nBu (E=P; R=nBu) and the bicyclic complex 5 a‐nBu (E=P; R=nBu). For the arsenidene complex 1 b a mixture of the 2H‐1,3‐azarsete complex 4 b‐nBu (E=As; R=nBu) and the bicyclic complex 5 b‐nBu (E=P, As; R=Cy, nBu) is obtained. Complex 4 b‐nBu is the first example of a 2H‐1,3‐azarsete complex. All products have been characterized by using mass spectrometry, NMR spectroscopy, and X‐ray diffraction analysis.     

Ansa‐Bridged Bis(benzene) Titanium Complexes

Taking advantage of an improved synthesis of [Ti(η⁶‐C₆H₆)₂], we report here the first examples of ansa‐bridged bis(benzene) titanium complexes. Deprotonation of [Ti(η⁶‐C₆H₆)₂] with nBuLi in the presence of N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (pmdta) leads to the corresponding 1,1′‐dilithio salt [Ti(η⁶‐C₆H₅Li)₂] ? pmdta that enables the preparation of the first one‐ and two‐atom‐bridged complexes by simple salt metathesis. The ansa complexes were fully characterized (NMR spectroscopy, UV/Vis spectroscopy, elemental analysis, and X‐ray crystallography) and further studied electrochemically and computationally. Moreover, [Ti(η⁶‐C₆H₆)₂] is found to react with the Lewis base 1,3‐dimethylimidazole‐2‐ylidene (IMe) to give the bent sandwich complex [Ti(η⁶‐C₆H₆)₂(IMe)].     

Sc(OTf)3 catalysed multicomponent synthesis of chromeno[2,3-d]pyrimidinetriones under solvent-free condition

An efficient, simple, and environmentally friendly synthesis of a series of chromeno[2,3-d]pyrimidine-trione derivatives has been accomplished via the three-component reaction of a barbituric acid, dimedone/cyclohexane-1,3-dione, and aromatic aldehydes using Sc(OTf)₃ as a recyclable catalyst under solvent-free condition. This method exploits the use of Sc(OTf)₃ as a Lewis acid catalyst in organic synthesis and offers many rewards such as excellent product yield and easy work-up procedure. Harmless reaction conditions, as well as the absence of side-products, are another green aspects of this protocol.     

Tetrakis[(4S)-4-phenyloxazolidin-2-one]dirhodium(II) and Its Catalytic Applications for Metal Carbene Transformations

The synthesis and X-ray structure of the binuclear complex tetrakis[(4S)-4-phenyloxazolidin-2-one]-dirhodium(II) ([Rh₂{(4S)-phox}₄]) are reported. Structure-selectivity comparisons are made for typical metal carbene transformations, such as inter- and intramolecular cyclopropane formation, intermolecular cyclopropene formation and intramolecular C–H insertions of diazoacetates and diazoacetamides. The enantioselectivity achieved in the [Rh₂{(4S)-phox}₄]-catalyzed reactions is intermediate between that of [Rh₂{(5S)-mepy}₄] and [Rh₂{(4R)-bnox}₄], which were described previously (mepy = methyl 5-oxopyrrolidine-2-carboxylate; bnox = 4-benzyloxazolidin-2-one). In contrast to other catalyzed intermolecular cyclopropane formations, those using [Rh₂{(4S)-phox}₄] result preferentially in formation of the cis-cyclopropane.     

Synthesis and Antibacterial Activity of Certain Fused Azoles and Oxazinones

2‐Phenyl‐4‐benzylidene‐5(4H)‐oxazolones ( 1a‐e) were used as versatile starting materials for the synthesis of fused compounds such as: 4H‐3,1‐benoxazin‐4‐ones (2a,b), imidazo[4,3‐b]benzimidazoles (3,4), imidazo[1,5‐b]1,2,4‐triazoles (5a,b), and 7 .