Instead of using aryl halides, aryl sulfides, typically poisonous to transition-metal catalysts, were found to serve as aryl electrophiles in the catalytic α-arylation of ketimines, a class of carbonyl derivatives. Low catalyst loadings (down to 0.5 mol %) of a palladium–NHC complex are sufficient for efficient arylation. α-Arylated ketimine products are useful for the synthesis of various azaarenes, including 2,3-diarylpyrroles, an indole, and pyrrolediones.

A reaction sequence of 2-(diphenylsilyl)phenylation by Negishi coupling and intramolecular sila-Friedel–Crafts reaction has been explored for the synthesis of mono-triphenylsilane-fused porphyrins 5 M and 6 M (M= Ni, Zn) and bis-triphenylsilane-fused porphyrins 7 M and 8 Ni. A triply linked triphenylsilane-fused Ni^II porphyrin, 13 Ni, was synthesized in a stepwise manner involving the above reaction sequence and a final Pd-catalyzed C−H activating arylative cyclization. The silicon atom in 13 Ni takes a distorted planarized structure with an almost perpendicular Si-phenyl group, causing an electronic effect due to effective σ*–π* interaction.

Mono- and bis(diphenylborane)-fused porphyrins were synthesized from the corresponding β-(2-trimethylsilylphenyl)-substituted porphyrins through the sequence of Si–B exchange reaction, intramolecular bora-Friedel–Crafts reaction, and ring-closing Si–B exchange reaction. Effective electronic interactions of the empty p-orbital of the boron atom with the porphyrin π-circuit lead to red-shifted absorption spectra and substantially decreased LUMO energy levels. Pyridine adds at the boron center to cause disruption of the electronic interaction of the boron atom with large association constants (1.9–17×10⁴ m⁻¹) depending on the central metal at the porphyrin. The Zn^II complex behaved as a hetero-dinuclear Lewis acid, exhibiting regioselective binding of pyridines at the boron or the zinc center.

Mono- and bis(diphenylborane)-fused porphyrins were synthesized from the corresponding β-(2-trimethylsilylphenyl)-substituted porphyrins through the sequence of Si–B exchange reaction, intramolecular bora-Friedel–Crafts reaction, and ring-closing Si–B exchange reaction. Effective electronic interactions of the empty p-orbital of the boron atom with the porphyrin π-circuit lead to red-shifted absorption spectra and substantially decreased LUMO energy levels. Pyridine adds at the boron center to cause disruption of the electronic interaction of the boron atom with large association constants (1.9–17×10⁴ m⁻¹) depending on the central metal at the porphyrin. The Zn^II complex behaved as a hetero-dinuclear Lewis acid, exhibiting regioselective binding of pyridines at the boron or the zinc center.

Instead of using aryl halides, aryl sulfides, typically poisonous to transition-metal catalysts, were found to serve as aryl electrophiles in the catalytic α-arylation of ketimines, a class of carbonyl derivatives. Low catalyst loadings (down to 0.5 mol %) of a palladium–NHC complex are sufficient for efficient arylation. α-Arylated ketimine products are useful for the synthesis of various azaarenes, including 2,3-diarylpyrroles, an indole, and pyrrolediones.

Conditions for the palladium–NHC-catalyzed amination of aryl sulfides with aliphatic as well as aromatic amines were established. The KHMDS-mediated amination of heteroaryl sulfides could proceed without palladium. Based on the distinct difference in reactivity of C–Br and C–S bonds, a sequential amination of bromothioanisole can take place to install two different alkylamino groups onto the aromatic ring in one pot.

The chlorination of β-halo or β,β-dihaloporphyrins with 2-chloro-1,3-bis(methoxycarbonyl)guanidine (Palau′Chlor) proceeded selectively at the neighboring unsubstituted meso position to afford meso,β-dihalo or meso,β,β-trihaloporphyrins. Such oligohaloporphyrins are useful platforms for constructing more-elaborate porphyrin-based extended π systems. For example, meso-chloro-β,β-diiodoporphyrin participated in an efficient single-step synthesis of a diphenylamine-fused porphyrin. In addition, meso-chloro-β-iodoporphyrin was transformed in stepwise fashion into an efficiently conjugated meso-to-meso,β-to-β doubly butadiyne-linked porphyrin dimer, a system which was previously difficult to access without such haloporphyrin precursors.

The chlorination of β-halo or β,β-dihaloporphyrins with 2-chloro-1,3-bis(methoxycarbonyl)guanidine (Palau′Chlor) proceeded selectively at the neighboring unsubstituted meso position to afford meso,β-dihalo or meso,β,β-trihaloporphyrins. Such oligohaloporphyrins are useful platforms for constructing more-elaborate porphyrin-based extended π systems. For example, meso-chloro-β,β-diiodoporphyrin participated in an efficient single-step synthesis of a diphenylamine-fused porphyrin. In addition, meso-chloro-β-iodoporphyrin was transformed in stepwise fashion into an efficiently conjugated meso-to-meso,β-to-β doubly butadiyne-linked porphyrin dimer, a system which was previously difficult to access without such haloporphyrin precursors.

Silylation of peripherally lithiated porphyrins with silyl electrophiles has realized the first synthesis of a series of directly silyl-substituted porphyrins. The meso-silyl group underwent facile protodesilylation, whereas the β-silyl group was entirely compatible with standard work-up and purification on silica gel. The meso-silyl group caused larger substituent effects to the porphyrin compared with the β-silyl group. Silylation of β-lithiated porphyrins with 1,2-dichlorodisilane furnished β-to-β disilane-bridged porphyrin dimers. A doubly β-to-β disilane-bridged Ni^II-porphyrin dimer was also synthesized from a β,β-dilithiated Ni^II-porphyrin and characterized by X-ray crystallographic analysis to take a steplike structure favorable for interporphyrinic interaction. Denickelation of β-silylporphyrins was achieved upon treatment with a 4-tolylmagnesium bromide to yield the corresponding freebase porphyrins.

CH arylation of polyfluoroarenes and heteroarenes with aryl sulfides proceeds smoothly with the aid of a palladium–N-heterocyclic carbene catalyst. A bulky zinc amide, TMPZnCl⋅LiCl, plays a key role as an effective base to generate the corresponding arylzinc species in situ. This arylation protocol is practically much easier to perform than our previous method, which necessitates preparation of the arylzinc reagents in advance from the corresponding aryl halides. Aryl sulfides that are prepared through sulfur-specific reactions, such as S_NAr sulfanylation and extended Pummerer reactions, undergo this direct arylation, offering interesting transformations that are otherwise difficult to achieve with conventional halogen-based organic synthesis.

Two new palladium-catalyzed reactions of aromatic sulfur compounds enabled the conversion of dibenzothiophenes into triphenylenes in four steps. This transformation of one aromatic framework into another consists of 1) 4-chlorobutylation of the dibenzothiophene to form the corresponding sulfonium salt, 2) palladium-catalyzed arylative ring opening of the sulfonium salt with a sodium tetraarylborate, 3) an intramolecular S_N2 reaction to form a teraryl sulfonium salt, and 4) palladium-catalyzed intramolecular CS/CH coupling through electrophilic palladation. Symmetrical as well as unsymmetrical triphenylenes of interest were synthesized in a tailor-made fashion in satisfactory overall yields.

Two new palladium-catalyzed reactions of aromatic sulfur compounds enabled the conversion of dibenzothiophenes into triphenylenes in four steps. This transformation of one aromatic framework into another consists of 1) 4-chlorobutylation of the dibenzothiophene to form the corresponding sulfonium salt, 2) palladium-catalyzed arylative ring opening of the sulfonium salt with a sodium tetraarylborate, 3) an intramolecular S_N2 reaction to form a teraryl sulfonium salt, and 4) palladium-catalyzed intramolecular CS/CH coupling through electrophilic palladation. Symmetrical as well as unsymmetrical triphenylenes of interest were synthesized in a tailor-made fashion in satisfactory overall yields.

As the most nucleophilic porphyrins, meso- or β-lithiated porphyrins were generated by iodine–lithium exchange reactions of the corresponding iodoporphyrins with n-butyllithium at −98 °C. Porphyrinyllithiums thus prepared were used for synthesis of dimesitylporphyrinylboranes through reactions with fluorodimesitylborane. The boryl groups proved to serve as an electron-accepting unit to alter the photophysical and electrochemical properties. In addition, 5-diarylamino-15-dimesitylboryl-substituted donor–accepter porphyrins showed increased intramolecular charge-transfer character in the S₁ state. Furthermore, the reaction of β-lithiated porphyrin with dichloromesitylborane provided a boron-bridged porphyrin dimer, which exhibited a conjugative interaction between two porphyrin units through the vacant p-orbital on the boron center.

Dibenzothiophene dioxides, which are readily prepared through oxidation of the parent dibenzothiophenes, undergo nucleophilic aromatic substitution with anilines intermolecularly and then intramolecularly to yield the corresponding carbazoles in a single operation. The “aromatic metamorphosis” of dibenzothiophenes into carbazoles does not require any heavy metals. This strategy is also applicable to the synthesis of indoles. Since electron-deficient thiaarene dioxides exhibit interesting reactivity, which is not observed for that the corresponding electron-rich azaarenes, a combination of a thiaarene-dioxide-specific reaction with the S_NAr-based aromatic metamorphosis allows transition-metal-free construction of difficult-to-prepare carbazoles.

Dibenzothiophene dioxides, which are readily prepared through oxidation of the parent dibenzothiophenes, undergo nucleophilic aromatic substitution with anilines intermolecularly and then intramolecularly to yield the corresponding carbazoles in a single operation. The “aromatic metamorphosis” of dibenzothiophenes into carbazoles does not require any heavy metals. This strategy is also applicable to the synthesis of indoles. Since electron-deficient thiaarene dioxides exhibit interesting reactivity, which is not observed for that the corresponding electron-rich azaarenes, a combination of a thiaarene-dioxide-specific reaction with the S_NAr-based aromatic metamorphosis allows transition-metal-free construction of difficult-to-prepare carbazoles.

Iodine–magnesium exchange between iodoporphyrins and iPrMgCl·LiCl proceeded successfully without decomposition of the porphyrin core. The resulting porphyrinyl Grignard reagents are nucleophilic enough to react with various carbonyl compounds, such as aldehydes, ketones, and amides. Furthermore, the porphyrinyl Grignard reagents underwent transmetalation to afford porphyrinyl copper and zinc species of mild and unique reactivity. These could be engaged in 1,4-addition and Negishi coupling, respectively.

7,8-Dehydropurpurin has attracted much attention owing to the dual 18π- and 20π-electron circuits in its macrocyclic conjugation. The two-fold Pd-catalyzed [3+2] annulation of meso-bromoporphyrin with 1,4-diphenylbutadiyne furnished 7,8-dehydropurpurin dimers. The 8^a,8^a-linked dimer displays a red-shifted and enhanced absorption band in the NIR region and a small electrochemical HOMO–LUMO band gap as a consequence of efficient conjugation between the two coplanar 7,8-dehydropurpurin units. Treatment of this dimer with N-bromosuccinimide in chloroform and ethanol gave β-to-β vinylene-bridged porphyrin dimers. Owing to the highly constrained conformations, these dimers exhibit perturbed absorption spectra, small Stokes shifts, and high fluorescence quantum yields.

2,18-Bis(diphenylphosphino)porphyrins undergo peripheral cyclometalation with group 10 transition-metal salts to afford the corresponding porphyrin-based PCP pincer complexes. The porphyrinic plane and the PCP-pincer unit are apparently coplanar, with small strain. The catalytic activities of the porphyrin-based pincer complexes at the periphery were investigated in the allylation of benzaldehyde with allylstannane and in the 1,4-reduction of chalcone to discover the electronic interplay between the inner metal and the outer metal in catalysis.

Operationally simple, efficient, and widely applicable Pummerer annulations of simple phenols with ketene dithioacetal monoxides, with the aid of trifluoroacetic anhydride, have been shown to provide a variety of benzofurans having a methylthio group at the 2-position. Subsequent and newly developed nickel-catalyzed arylation at the methylthio group culminates in diversity-oriented synthesis of multisubstituted benzofurans. Our extended Pummerer annulation/cross-coupling sequence is powerful enough to synthesize biologically active natural products as well as highly fluorescent benzofuran derivatives.

Operationally simple, efficient, and widely applicable Pummerer annulations of simple phenols with ketene dithioacetal monoxides, with the aid of trifluoroacetic anhydride, have been shown to provide a variety of benzofurans having a methylthio group at the 2-position. Subsequent and newly developed nickel-catalyzed arylation at the methylthio group culminates in diversity-oriented synthesis of multisubstituted benzofurans. Our extended Pummerer annulation/cross-coupling sequence is powerful enough to synthesize biologically active natural products as well as highly fluorescent benzofuran derivatives.

A combination of a palladium–NHC catalyst and potassium hexamethyldisilazide enables the amination of aryl sulfides with anilines to afford a wide variety of diarylamines. The reaction conditions are versatile enough for the reaction of even bulky ortho-substituted aryl sulfides. This amination can be applied to the modular synthesis of N-aryl carbazoles from the corresponding ortho-bromothioanisoles. As aryl sulfoxides undergo extended Pummerer reactions to afford ortho-substituted aryl sulfides, the Pummerer products are thus useful substrates for the amination to culminate in efficient syntheses of a 2-anilinobenzothiophene and an indole as proof-of-principle of the utility of the extended Pummerer reaction/amination cascade.

2,18-Bis(diphenylphosphino)porphyrins undergo peripheral cyclometalation with group 10 transition-metal salts to afford the corresponding porphyrin-based PCP pincer complexes. The porphyrinic plane and the PCP-pincer unit are apparently coplanar, with small strain. The catalytic activities of the porphyrin-based pincer complexes at the periphery were investigated in the allylation of benzaldehyde with allylstannane and in the 1,4-reduction of chalcone to discover the electronic interplay between the inner metal and the outer metal in catalysis.

7,8-Dehydropurpurin has attracted much attention owing to the dual 18π- and 20π-electron circuits in its macrocyclic conjugation. The two-fold Pd-catalyzed [3+2] annulation of meso-bromoporphyrin with 1,4-diphenylbutadiyne furnished 7,8-dehydropurpurin dimers. The 8^a,8^a-linked dimer displays a red-shifted and enhanced absorption band in the NIR region and a small electrochemical HOMO–LUMO band gap as a consequence of efficient conjugation between the two coplanar 7,8-dehydropurpurin units. Treatment of this dimer with N-bromosuccinimide in chloroform and ethanol gave β-to-β vinylene-bridged porphyrin dimers. Owing to the highly constrained conformations, these dimers exhibit perturbed absorption spectra, small Stokes shifts, and high fluorescence quantum yields.

Cross-coupling of general aryl alkyl sulfides with arylzinc reagents proceeds smoothly, even at room temperature or below, with a palladium–N-heterocyclic carbene (NHC) catalyst. When combined with reactions that are unique to organosulfurs, that is, the S_NAr sulfanylation or Pummerer reaction, the cross-coupling offers interesting transformations that are otherwise difficult to achieve. An alkylsulfanyl group is preferentially converted whilst leaving the tosyloxy and chloro intact, which expands the variety of orthogonal cross-coupling.

Palladium-catalyzed direct β-diarylation of the nickel complexes of 5,10,15-triarylporphyrins with π-extended aryl bromides, such as 9-bromoanthracene, is described. The reaction proceeded in N,N-dimethylacetamide in the presence of potassium carbonate, pivalic acid, and a catalytic amount of palladium acetate without any additional ligands. The absorption and emission properties as well as X-ray crystallographic analysis of the arylated porphyrins are also discussed.

Porphyrins play vital roles in living systems as well as in advanced organic materials science. Efficient synthesis of porphyrins and creation of novel porphyrins are thus important. Recently, organometallic chemistry has become more powerful and is now sophisticated enough to modify porphyrin CH bonds directly, efficiently, and selectively. This Focus Review discusses methods for modifying peripheral porphyrin CH bonds, specifically direct metalations and CC bond formations.

Palladium-catalyzed aminations of bulky aryl halides such as meso-bromoporphyrins and 9-haloanthracenes with diarylamines provided efficient syntheses of meso-aminoporphyrins and 9-aminoanthracenes. Commercially available and air-stable Pd-PEPPSI complexes that have N-heterocyclic carbene (NHC) as ligands have the highest catalytic activities. The scope of diarylamines is wide enough to use diphenylamine, phenoxazine, phenothiazine, 9,10-dihydroacrydine, and carbazole.

The first examples of metal-catalyzed extended Pummerer reactions through the activation of sulfoxides are described. The copper-catalyzed reactions of ketene dithioacetal monoxides with alkynyl sulfides and ynamides provided a wide variety of γ,γ-disulfanyl-β,γ-unsaturated carbonyl compounds with an accompanying oxygen rearrangement. The products can be easily converted into 1,4-dicarbonyl compounds and substituted heteroaromatics. DFT calculations and mechanistic experiments revealed a new interesting stepwise addition/oxygen rearrangement mechanism.

Triisopropylsilyltetrathiafulvalene (TIPS-TTF) has been devised as a promising platform for the synthesis of low-symmetry TTF derivatives. The bulky TIPS group allows TIPS-TTF to undergo palladium-catalyzed direct diarylation as well as LDA-mediated dilithiation exclusively on the roomier dithiole ring. Subsequent fluoride-mediated protodesilylation provided vicinally difunctionalized TTF, which could undergo further functionalization.

A library of 2-aryl-3-trifluoromethylnaphthofurans was synthesized with high efficiency from simple naphthols. In this synthesis, the Pummerer-type annulation of naphthols with 3-(2,2,2-trifluoroethylidene)-2,4-dithiapentane 2-oxide was followed by a cross-coupling of the resulting 2-methylthio-3-trifluoromethylnaphthofurans with a variety of arylzinc reagents. A palladium complex, Pd-PEPPSI-IPr, was the most efficient catalyst for the arylation step, which represents the first cross-coupling of aryl sulfides by using an N-heterocyclic-carbene-ligated palladium complex. This library consists of new π-expanded molecules, all of which are fluorescent in the solid state as well as in solution. Their photophysical properties, such as absorption and emission, fluorescence quantum yields, and fluorescence lifetimes, were thoroughly investigated. This library was also useful to identify acidochromic molecules.