A simple metal–ligand cooperative approach for the dehydrogenative functionalization of alcohols to various substituted quinolines and quinazolin-4(3H)-ones under relatively mild reaction conditions (≤90 °C) is reported. Simple and easy-to-prepare air-stable Cu(II) complexes featuring redox-active azo-aromatic scaffolds, 2-arylazo-(1,10-phenanthroline) (L 1,2 ), are used as catalyst. A wide variety of substituted quinolines and quinazolin-4(3H)-ones were synthesized in moderate to good isolated yields via dehydrogenative coupling reactions of various inexpensive and easily available starting materials under aerobic conditions. A few control experiments and deuterium labeling studies were carried out to understand the mechanism of the dehydrogenative coupling reactions, which indicate that both copper and the coordinated azo-aromatic ligand participate in a cooperative manner during the catalytic cycle.
Here, we delineate a maiden example of a diiron(III) dication diradical porphyrin dimer as a competent catalyst for the oxa-Diels–Alder type reaction of aldehydes with 1,3-dienes, which is a cardinal reaction in the syntheses of natural products. This catalyzed process does not demand the use of electron-deficient aldehydes such as glyoxylic acid derivatives or activated electron-rich 1,3-dienes such as Danishefsky’s, Brassard’s, or Rawal’s diene. The robust catalyst exhibited high functional group tolerance. The computational studies corroborated the detailed spectroscopic investigation, which focused on the pivotal roles played by the metal ion as the Lewis acidic center in combination with counteranions and also enabled us to delve deeper into the reaction mechanism. Previously developed methodologies invariably require dissociation of the iron-axial bond of the catalyst; on the contrary, the axial ligand of the catalyst remains intact during the catalysis reported here. The use of a dication diradical iron(III)porphyrin as the Lewis acid catalyst facilitates activation of the aldehyde via coordination whose formation has also been confirmed experimentally. Moreover, the counteranion has a considerable effect on the reaction pathway; its coordination to the metal inhibits the coordination of the substrate to form the product. The efficacy of employing such a diheme catalyst over a monoheme analogue is manifested in the cooperative effect, which resulted in a lower catalyst loading with excellent yields.
We have described copper(II)‐iron(III) and copper(II)‐manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV‐visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV‐visible spectroscopic studies show that upon 2e‐oxidation of the heterobimetallic complexes only ring‐centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low‐energy NIR band obtained for the 2e‐oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π‐cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π‐cation radical. Moreover, the coupling between two π‐cation radicals are much stronger in the 2e‐oxidized complexes of dimanganese(III) and copper(II)‐manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)‐iron(III) analogs. Furthermore, coupling between the unpaired spins of a π‐cation radical and copper(II) is much stronger in the 2e‐oxidized complex of copper(II)‐iron(III) porphyrin dimer as compared to its copper(II)‐manganese(III) analog. The Mulliken spin density distributions in 2e‐oxidized homo‐ and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e‐oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.
Diheme cytochromes, the simplest members in the multiheme family, play substantial biochemical roles in enzymatic catalysis as well as in electron transfer. A series of diiron(III) porphyrin dimers have been synthesized as active site analogues of diheme cytochromes. The complexes contain six-coordinated iron(III) having thiophenol and imidazole at the fifth and sixth coordination sites, respectively. The iron centers in the complexes have been found to be in a low-spin state, as confirmed through solid-state Mössbauer and electron paramagnetic resonance (EPR) spectroscopic investigations. Mössbauer quadrupole splitting of complexes having mixed ligands is substantially larger than that observed when both axial ligands are the same. Rhombic types of EPR spectra with narrow separation between g x , g y , and g z clearly distinguish heme thiolate coordination compared to bis(imidazole)-ligated low-spin heme centers. The redox potential in diheme cytochromes has been found to be tuned by interheme interactions along with the nature of axial ligands. The effect of mixed-axial ligation within the diiron(III) porphyrin dimers is demonstrated by a positive shift in the Fe(III)/Fe(II) redox couple upon thiophenolate coordination compared to their bis(imidazole) analogues. The pK a of the imidazole also decides the extent of the shift for the Fe(III)/Fe(II) couple, while the potential of the mixed-ligated diiron(III) porphyrin dimer is more positive compared to their monomeric analogue. A variation of around 1.1 V for the Fe(III)/Fe(II) redox potential in the diiron(III) porphyrin dimer can be achieved with the combined effect of axial ligation and a metal spin state, while such a large variation in the redox potential, compared to their monomeric analogues, is attributed to the heme–heme interactions observed in dihemes. Moreover, theoretical calculations also support the experimental shifts in the redox potential values.
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