Ludmiła Szterenberg

Palladium Vacataporphyrin Reveals Conformational Rearrangements Involving Hückel and Möbius Macrocyclic Topologies

5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid) which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The metal ion of square-planar coordination geometry is firmly held via three pyrrolic nitrogen atoms where the fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin. The macrocycle reveals the unique structural flexibility triggered by coordination of palladium. The structural rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between two pyrrolic rings of vacataporphyrin albeit the significant ruffling of the tripyrrolic block is also of importance. Two fundamental modes of interactions between the palladium ion and annulene moiety have been recognized. The first one resembles an eta(2)-type interaction and involves the C(2)C(3) unit of the butadiene part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires Hückel or extremely rare Möbius topologies readily reflected by spectroscopic properties. The palladium vacataporphyrin complexes reveal Hückel aromaticity or Möbius antiaromaticity of [18]annulene applying the butadiene fragment of vacataporphyrin as a topology selector. The properties of specific conformers were determined using (1)H NMR and density functional theory calculations.

A Facile Palladium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Palladium(II) p‐Benziporphyrin

The contraction of benzene and its derivatives to form a cyclopentadiene ring has rarely been reported. Pioneering studies on the photooxidation of benzene led to the conclusion that cyclopentadienecarboxyaldehyde was formed in this reaction. Since then, research on photoinduced reactions of hydroxyand dihydroxybenzene revealed interesting mechanistic features, including ring contraction from benzene to cyclopentadiene. A similar structural motif was detected in the course of thermal decomposition of anisole or dihydroxybenzene. The oxidation of phenol with dioxygen in the presence of metallic copper resulted in the aromatic ring contraction to afford substituted cyclopentenes. Carbocycle contraction to benzvalene followed by opening of the ring to form benzene was postulated in theoretical studies on the high-temperature intramolecular topomerization of [1,2C2]benzene to [1,3C2]and [1,4C2]benzene. [5] In more general terms, the benzene contraction belongs to an exclusive group of reactions where the cleavage of aromatic structures is of fundamental importance. Significantly, oxidative ring cleavage is a key metabolic step in the biodegradation of aromatic compounds by bacteria. The common metabolic pathway is a ring fission by catechol dioxygenases that contain a nonheme iron(II) center in the active site. The representative examples where such a challenge has been chemically addressed include cleavage of the aromatic rings with formation of metallacyclopentadiene complexes according to a retro-alkyne cyclotrimerization mechanism, a reductive silylation of silylsubstituted arenes, or insertion of tungsten into unstrained aromatic rings. Recently, an impressive room-temperature C C bond fission of an arene by a metallacarborane was reported. Porphyrinoids (including carbaporphyrinoids) provide a unique macrocyclic platform that is suitable for exploring organometallic chemistry confined to a particular macrocyclic environment. Often C H or C C bonds are held close to the metal center, thus enforcing an unusual coordination geometry and unique reactivity. Herein we report the contraction of the benzene ring embedded in palladium(II) p-benziporphyrin 1. This process affords palladium(II) 21formyl-21-carbaporphyrin 4 and palladium(II) 21-carbaporphyrin 5, and proceeds via palladium(II) 22-hydroxycyclohexadieneporphyrin 3 as a spectroscopically detectable intermediate. Reaction of palladium(II) chloride with p-benziporphyrin 1 in acetonitrile results in the formation of the four-coordinate palladium(II) p-benziporphyrin 2 (Scheme 1). The geometry

Phosphorus Complexes of N-Fused Porphyrin and Its Reduced Derivatives : New Isomers of Porphyrin Stabilized via Coordination

N-fused isophlorin 3 and its tautomeric phlorin forms 4 and 5, the new constitutional isomers of porphyrin which preserve the basic skeleton of their maternal N-fused porphyrin, have been identified in the course of investigation of phosphorus insertion into N-fused porphyrin 2. N-fused porphyrin reacts with PCl3 in toluene yielding phosphorus(V) N-fused isophlorin 3-P wherein the macrocycle acts as a trianionic tridentate ligand. The identical product has been formed in the reaction of N-confused porphyrin 1 and POCl3 or PCl3. The coordinating environment of phosphorus(V) in 3-P as determined by X-ray crystallography resembles a distorted trigonal pyramid with the nitrogen atoms occupying equatorial positions with the oxygen atom lying at the unique apex. Phosphorus(V) is significantly displaced by 0.732(1) A from the N3 plane. The P-N distances are as follows P-N(22) 1.664(2), P-N(23) 1.645(2), and P-N(24) 1.672(2). All P-N(pyrrolic) bond lengths are markedly shorter than the P-N distances in phosphorus porphyrins. 3-P is susceptible to proton addition at the inner C(9) carbon atom, yielding aromatic 4-P. The modified macrocycle acts as a dianionic ligand and allows the efficient 18 pi-electron delocalization pathway. Two stereoisomers affording the syn (4-P syn) and anti (4-P anti) location of the H(9) atom with respect to the oxygen atom of the PO unit have been identified by (1)H NMR. A regioselective reduction of free base N-fused porphyrin 2 with NaBH4 yielded a nonaromatic isomer of 4, that is, N-fused phlorin 5 due to an addition of a hydride to the C(15) carbon and a proton to one of the pyrrolic nitrogens. The isomer 5 reacts with PCl 3 yielding phosphorus(V) fused isophlorin 3-P. Density functional theory has been applied to model the molecular and electronic structure of porphyrin isomers 3, 4, and 5 and their phosphorus(V) complexes.

Phenanthriporphyrin: An Antiaromatic Aceneporphyrinoid as a Ligand for a Hypervalent Organophosphorus(V) Moiety

The incorporation of a phenanthrene moiety into a porphyrin framework results in the formation of a hybrid macrocycle—phenanthriporphyrin—merging the structural features of polycyclic aromatic hydrocarbons and porphyrins. An antiaromatic aceneporphyrinoid, adopting the trianionic {CCNN} core, is suitable for the incorporation of a phosphorus(V) center to form a hypervalent organophosphorus(V) derivative.

A Flexible Porphyrin–Annulene Hybrid: A Nonporphyrin Conformation formeso-Tetraaryldivacataporphyrin

An annulene-porphyrin hybrid, the diaaza-deficient porphyrin 5,10,15,20-tetraaryl-21,23-divacataporphyrin, has been synthesized by an extrusion of tellurium atom(s) from 5,10,15,20-tetraaryl-21,23-ditelluraporphyrin under treatment with HCl. In addition, a monoaza-deficient 5,10,15,20-tetraaryl-21-tellura-23-vacataporphyrin was formed in the same reaction. The two new members of the vacataporphyrin family were characterized by X-ray crystallography, as well as UV/Vis and NMR spectroscopy. These aromatic molecules preserve the fundamental structural and spectroscopic features of the parent tetraarylporphyrin. The X-ray crystal structures of 21,23-divacataporphyrin and 21-tellura-23-vacataporphyrin show typical porphyrin patterns. The molecules are not strictly planar and show distortion of the annulene moieties. The N22⋅⋅⋅N24 distances (5.23 and 5.09 Å) are considerably longer than in regular porphyrins. For 21,23-divacataporphyrin, variable-temperature (1)H NMR spectroscopy data allowed the identification of divacataporphyrin stereoisomers differentiated by the geometry of the butadiene bridges. The forms remain in thermodynamic equilibrium.

Organocopper(II) Complex of 21-Diphenylphosphoryl-Carbaporpholactone Hybrid: A Side-On Coordination Mode of Copper(II)

The diphenylphosphoryl-carbaporpholactone hybrid is a new aromatic porphyrinoid, which preserves the essential features of the carbaporpholactone frame and provides a suitable environment allowing stabilization of the organocopper(II) complex affording a peculiar side-on coordination of copper(II).

Gold(III)‐Mediated Contraction of Benzene to Cyclopentadiene: From p‐Benziporphyrin to Gold(III) True Tetraarylcarbaporphyrin

The reaction of p-benziporphyrin, sodium tetrachloroaurate(III) dihydrate, and potassium carbonate in dichloromethane yielded gold(III) 5,10,15,20-tetraaryl-21-carbaporphyrin owing to the contraction of p-phenylene to cyclopentadiene. This molecule is the very first representative of a true 5,10,15,20-tetraaryl-21-carbaporphyrin complex where four trigonal donor atoms are involved in equatorial coordination. The contraction adds an unprecedented route to numerous organic transformations of aromatic compounds catalyzed by simple gold(III) compounds. p-Benziporphyrin provided the unique environment to alter the fundamental reactivity of the benzene unit facilitating its contraction to cyclopentadiene.

Synthesis and Switching the Aromatic Character of Oxatriphyrins(2.1.1)

Triangularly shaped, contracted porphyrinoids belong to a group of molecules where the geometry significantly modifies the observed electronic properties. The need for a controllable, effective, and widely applicable approach to triphyrins drives extensive research towards macrocyclic materials that act as potential controlling motifs by switching their aromaticity. Two isomeric thiophene-fused triphyrins(2.1.1) were synthesized by applying an innovative approach. Spectroscopic techniques (NMR, UV/Vis) show that both macrocycles are aromatic and quantitatively convert into anti-aromatic structures after reduction with a zinc amalgam. The reduced forms were stabilized through boron(III) coordination, thereby allowing the observation of anti-aromatic 16 π delocalization within a contracted porphyrin.

Towards True Carbaporphyrinoids: Synthesis of 21‐Carba‐23‐thiaporphyrin

In the search for porphyrinoids with a built-in cyclopentadienyl moiety (true carbaporphyrins), a rational synthesis of carbathiaporphyrin, the synthons, has been elaborated. The donors (C,N,S,N) in the porphyrinic core of carbathiaporphyrinoids are potentially of fundamental importance for generating organometallic complexes, as exemplified through formation of the palladium(II) complex.

Transformations of N-confused porphyrin triggered by insertion of silicon(IV).

N-confused porphyrin, 5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin, dissolved in triethylamine reacts with dichloromethylsilane yielding the methylsilicon(IV) complex of 5,10,15,20-tetraaryl-2-aza-21-hydroxy-21-carbaporphyrin. Addition of aldehydes or ketones (acetone, acetaldehyde acetophenone, butanone, propanal, benzaldehyde, p-methylbenzaldehyde, p-methoxybenzaldehyde, terephthaldehyde) into the insertion mixture triggered the profound transformation of N-confused porphyrin to form the methylsilicon(IV) complex of N-fused porphyrin derivative substituted at the inner C(9) position by a hydroxyalkyl moiety derived from aldehyde or ketone. The macrocyle is structurally related to an aromatic N-fused inner phlorin while the coordination polyhedron of bound silicon resembles the trigonal bipyramid. The macrocyclic ligand coordinates in the facial mode as the three pyrrolic nitrogen donors lie at the vertices of the single trigonal face. The meridional positions of the trigonal bipyramid are occupied by two pyrrolic nitrogen donors and a sigma-methyl ligand. The coordination sphere is completed by apical coordination of the alkoxy oxygen atom derived from alkanal or alkonone. The incorporation of aldehydes and ketones is stereoselective. Acidic desililation of alkanal compounds yields two aromatic N-confused porphyrin derivatives, that is, 3-(1-hydroxyalkyl)-5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin and its oxidation product 3-alkanoyl-5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin. The acid triggered desililation of ketone derivatives produces the equimolar amounts of N-confused porphyrin and ketone. The first spectroscopically identified step involves the protonation of the C(7) position affording the non-aromatic silicon(IV) complex. The density functional theory (DFT) has been applied to model the molecular and electronic structure of all species identified in the course of silicon insertion into the N-confused and N-fused porphyrin.