Regioselective Alkyne Cyclotrimerization with an In Situ-Generated [Fe(II)H(salen)]·Bpin Catalyst

Synthetic and Mechanistic Studies into the Reductive Functionalization of Nitro Compounds Catalyzed by an Iron(salen) Complex

We report on the use of a simple, bench-stable [Fe(salen)2]-μ-oxo precatalyst in the reduction of nitro compounds. The reaction proceeds at room temperature across a range of substrates, including nitro aromatics and aliphatics. By changing the reducing agent from pinacol borane (HBpin) to phenyl silane (H3SiPh), we can chemoselectively reduce nitro compounds while retaining carbonyl functionality. Our mechanistic studies, which include kinetics, electron paramagnetic resonance (EPR), mass spectrometry, and quantum chemistry, indicate the presence of a nitroso intermediate and the generation of an on-cycle iron hydride as a key catalytic intermediate. Based on this mechanistic insight, we were able to extend the chemistry to hydroamination and identified a simple substrate feature (alkene lowest unoccupied molecular orbital (LUMO) energy) that could be used to predict which alkenes would result in productive catalysis.

Air-Stable Iron(III) Salen Complexes for Selective Hydroboration of Ketones and Unactivated Imines without Base Activation

Herein, we designed and synthesized a series of air-stable, cost-effective, and readily synthesizable iron(III) salen complexes (Fe-1 and Fe-2) for facilitating the selective hydroboration of ketones and unactivated imines with pinacolborane in the absence of any additive. These catalyst systems exhibited good yields, chemoselectivity, high atom economy, and a broad substrate scope under mild reaction conditions with a minimal catalyst loading of 0.2 mol %. The catalytic efficiency of Fe-1 has been demonstrated through the hydroboration of diverse aromatic, aliphatic, and heterocyclic ketones and imines with a turnover number of up to 1000, highlighting its broad substrate scope. Ketones are chemoselectively hydroborated over other functional groups such as imines, alkenes, esters, nitriles, acids, and alcohols. Besides, the synthetic utility of this strategy has also been showcased by the construction of a natural chiral monoterpenoid carveol. This protocol can be readily scaled up for gram-scale synthesis of alcohols, which underscores the potential industrial applicability of our catalyst system in the synthesis of secondary alcohols on a larger scale.

A Versatile, Functional Group-Tolerant, and Bench-Stable Iron Precatalyst for Building Arene and Triazine Rings by [2+2+2] Cycloadditions

We report an efficient iron-catalyzed cycloaddition procedure leading to the construction of (hetero)aromatic rings by alkyne [2+2+2] cycloisomerization. This method relies on the use of an air-stable (N,N)Fe(II) precursor easily prepared from a commercially available ligand derived from 1,10-phenanthroline, reduced in situ into a catalytically active non-innocent (N,N ⋅-)2Fe(II) species. This system displays a large scope application, operates under mild conditions and at low catalytic charges (25 cycloadducts formed, up to 1.5 mol% catalyst). Moreover, this method also enables access to 29 cycloadducts by cross-cycloisomerization between 1,6- or 1,7-diynes and alkynes in near-equimolar conditions. 1,3,5-Triazines can also be prepared with this procedure starting from the corresponding cyanamides. Scale-up reactions and post-functionalization of several cycloadducts also show that this [2+2+2] cycloaddition can be used in multistep sequences.

Palladium-Copper-Catalyzed Oxidative Intermolecular [2 + 2 + 2] Coupling-Annulation: Regioselective Synthesis of Multiaryl-Substituted Benzenes

A palladium-catalyzed intermolecular [2 + 2 + 2] oxidative coupling-annulation of terminal alkenes and alkynes using copper(II) as the oxidant has been developed through direct C-C bond formation. These reactions provide effective access to multiaryl-substituted benzenes with high regioselectivity in the absence of any ligands. The features of this protocol are broad substrate scope, and high atom and step economy. The aggregation-induced emission properties of selected products were further investigated. These synthesized multiaryl-substituted benzenes may be worth exploring for further applications in the fields of advanced functional materials or drugs.

The Complex Reactivity of [(salen)Fe]2(μ-O) with HBpin and Its Implications in Catalysis

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase here an example of self-activation by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-Xyl₂C₆H₃)₂] (1, Xyl = 2,6-Me₂C₆H₃). This behavior was observed for aryl acetylenes in their regioselective cyclotrimerization to 1,2,4-(aryl)-benzenes. Two kinetically distinct regimes are observed dependent upon the substrate-to-catalyst ratio ([RC≡CH]₀/[1]₀), referred to as the low ([RC≡CH]₀/[1]₀ < 40) and high ([RC≡CH]₀/[1]₀ > 40) regimes. Both showed sigmoidal kinetic response, with positive Hill indices of 1.85 and 3.62, respectively, and nonlinear Lineweaver-Burk replots with an upward curvature, which supports positive substrate cooperativity. Moreover, two alkyne molecules participate in the low regime, whereas up to four are involved in the high regime. The second-order rate dependence on 1 indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the high one being 6 times faster than that involved in the low one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the low regime than in the high one, but with a more ordered transition state in the high regime than in the low one.

An Iron-Catalyzed Route to Dewar 1,3,5-Triphosphabenzene and Subsequent Reactivity

The application of an alkyne cyclotrimerization regime with an [Fe(salen)]2 -μ-oxo (1) catalyst to triphenylmethylphosphaalkyne (2) yields gram-scale quantities of 2,4,6-tris(triphenylmethyl)-Dewar-1,3,5-triphosphabenzene (3). Bulky lithium salt LiHMDS facilitates a rearrangement of 3 to the 1,3,5-triphosphabenzene valence isomer (3'), which subsequently undergoes an intriguing phosphorus migration reaction to form the ring-contracted species (3''). Density functional theory calculations provide a plausible mechanism for this rearrangement. Given the stability of 3, a diverse array of unprecedented transformations was investigated. We report novel crystallographically characterized products of successful nucleophilic/electrophilic addition and protonation/oxidation reactions.

Crystal structure of diiodido-bis(6,6′-dimethoxy-2,2′-(ethane-1,2-diylbis(nitrilomethanylylidene)) diphenolato)tricadmium(II), C36H36Cd3I2N4O8

C36H36Cd3I2N4O8, orthorhombic, Pbca (no. 61), a = 18.909(4) Å, b = 16.149(3) Å, c = 26.050(5) Å, V = 7955(3) Å3, Z = 8, Rgt (F) = 0.0439, wRref (F 2) = 0.0752, T = 293(2) K.

Assembly of unsymmetrical 1,3,5-triarylbenzenes via tandem reaction of β-arylethenesulfonyl fluorides and α-cyano-β-methylenones

A transition-metal-free tandem reaction of β-arylethenesulfonyl fluorides and α-cyano-β-methylenones has been revealed. In the presence of cesium carbonate, 2-arylethenesulfonyl fluorides react with α-cyano-β-methylenones through a tandem Diels-Alder cycloaddition/sulfur (VI) fluoride...

Regioselectively switchable alkyne cyclotrimerization catalyzed by the system of Ni(II)/bidentate P-ligand/Zn with ZnI2 as additive

A simple catalyst system of Ni(PPh3)2Cl2/dppb or dppm/Zn can effect an efficient and regioselectively controlled alkyne cyclotrimerization to form 1,2,4- (with no additive) or 1,3,5-regioisomers (with the use of ZnI2...

H-BPin/KOtBu Promoted Activation of Cobalt Salt to a Heterotopic Catalyst for Highly Selective Cyclotrimerization of Alkynes

A mixture of HBPin with KO^tBu was found to activate cobalt salt to form a heterotopic cobalt species that is highly active for catalytic intermolecular trimerization of alkynes. This protocol affords 1,2,4-regioisomers in good yields with high regioselectivities under mild conditions. These salient features, together with the operational simplicity and high efficiency, as well as obviating the use of any costly and/or air sensitive ligands, renders the protocol promising for practical applications.

Hydroboration of aldehydes, ketones and CO2 under mild conditions mediated by iron(iii) salen complexes

The hydroboration of aldehydes, ketones and CO₂ is demonstrated using a cheap and air stable [Fe(salen)]₂-μ-oxo pre-catalyst with pinacolborane (HBpin) as the reductant under mild conditions. This catalyst system chemoselectively hydroborates aldehydes over ketones and ketones over alkenes. In addition, the [Fe(salen)₂]-μ-oxo pre-catalyst shows good efficacy at reducing "wet" CO₂ with HBpin at room temperature.

Hydrogen-Bonding Controlled Nickel-Catalyzed Regioselective Cyclotrimerization of Terminal Alkynes

Herein we report a hydrogen-bonding controlled nickel-catalyzed regioselective cyclotrimerization of terminal alkynes in moderate to excellent yields with high regioselectivities toward 1,3,5-trisubstituted benzenes. This method features a cheap catalyst, mild reaction conditions, and excellent functional group compatibility. The Ni-B(OH)₂ complex in situ generated from NiCl₂·DME and tetrahydroxydiboron might act as an active catalyst. After three consecutive cis-additions of terminal alkynes, internal migratory insertion cyclization, and β-boron elimination induced aromatization, 1,3,5-trisubstituted benzenes were selectively established.