Bartosz Szyszko

Three-level topology switching in a molecular Möbius band.

Möbius pi-conjugation in cyclic molecules leads to the reversal of Huckel aromaticity rules and affects the electronic and magnetic properties of these systems. We found the first example of a medium-sized macrocyclic structure that is sufficiently flexible to switch between three distinct pi-conjugation topologies, planar (T0), Möbius (T1), and twisted Huckel (T2), without changing its oxidation level. The switching is under thermodynamic and kinetic control and can be realized in a three- or four-step cycle. On titration with trifluoroacetic acid (TFAH) or dichloroacetic acid (DCAH), the Möbius free base (T1-H(2)), which is the preferred structure in dichlorofluoromethane at 150 K, undergoes a series of acid-base reactions involving changes of the pi-conjugation topology. The forms observed in the course of titration involve a Möbius aromatic monocation ([T1-H(3)](+)), an antiaromatic twisted Huckel species ([T2-H(4)(A)](+)) containing a coordinated carboxylate anion (A = TFA, DCA), and two additional Möbius forms ([T1-H(4)(A)(HA)(n)](+) (n = 1, 2)), containing complex carboxylate anions. The protonated forms undergo a thermally activated ring planarization to yield an antiaromatic quasi-planar dication [T0-H(4)](2+), characterized in the solid state as a TFA salt. The corresponding free base (T0-H(2)) is metastable but can be trapped by addition of triethylamine at low temperatures.

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

Steric Control in the Synthesis of p-Benziporphyrins. Formation of a Doubly N-Confused Benzihexaphyrin Macrocycle

The use of 1,4-phenylene-containing tripyrrane analogs provides a general route to expanded p-benziporphyrins. The course of macrocyclization shows a striking dependence on the steric bulk of meso substituents.

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.

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.

Hückel and Möbius Expanded para‐Benziporphyrins: Synthesis and Aromaticity Switching

The four expanded p-benziporphyrins A,C-di-p-benzi[24]pentaphyrin(1.1.1.1.1), N-fused A-p-benzi[24]pentaphyrin, A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1), and A,C-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) were obtained in three-component Lindsey-type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY-like difluoroboron complex was obtained from the A,C-di-p-benzi[24]pentaphyrin, whereas A,C-di-p-benzi[28]hexaphyrin yielded a Möbius-aromatic Pd(II) complex containing fused pyrrole and phenylene subunits. Conformational behavior, tautomerism, and acid-base chemistry of the new macrocycles were characterized by means of NMR spectroscopy and DFT calculations. Free base N-fused A-p-benzi[24]pentaphyrin showed temperature-dependent Hückel-Möbius aromaticity switching, whereas the A,C-di-p-benzi[28]hexaphyrin formed a Möbius-aromatic dication.

Incorporation of a Phenanthrene Subunit into a Sapphyrin Framework: Synthesis of Expanded Aceneporphyrinoids.

32-Hetero-5,6-dimethoxyphenanthrisapphyrins-macrocycles that link structural features of polycylic aromatic hydrocarbons and expanded porphyrins-were obtained in a straightforward [3+1] condensation reaction of dimethoxyphenanthritripyrrane and 2,5-bis(arylhydroxymethyl)heterocyclopentadienes. The highly folded conformation of formally 4 n π-electron macrocycles causes them to manifest only limited macrocyclic π conjugation as explored by means of NMR spectroscopic and X-ray structural analyses, and supported by DFT calculations. Although protonation does not change their π-conjugation characteristics, the cleavage of ether groups at the phenanthrenylene moiety yields nonaromatic 32-hetero-5,6-dioxophenanthrisapphyrins.

Incorporation of the 1,5-naphthalene subunit into heteroporphyrin structure: toward helical aceneporphyrinoids.

5,10,15,20-Tetraaryl-22-hetero-1,5-naphthiporphyrins, which contain a 1,5-naphthylene moiety instead of one pyrrole embedded in the macrocyclic framework of heteroporphyrins, were obtained by the [3 + 1] approach using the 1,5-naphthylene analogue of tripyrrane (1,5-bis(phenyl(2-pyrolyl)methyl)naphthalene) and 2,5-bis(arylhydroxymethyl)heterocyclopentadiene (heterocyclopentadiene: thiophene, selenophene, tellurophene). The steric constraints, imposed by the substitution mode of the 1,5-naphthylene building block, resulted in the specific helical conformation of 22-hetero-1,5-naphthiporphyrins. The spectroscopic and structural properties of these aceneporphyrinoids indicate a lack of macrocycle aromaticity. Their protonation yielded solely dicationic species.

Aromaticity control via modifications of a macrocyclic frame: 5,6-dimethoxyphenanthriporphyrin and 5,6-dioxophenanthriporphyrin

The incorporation of a 9,10-dimethoxyphenanthrene moiety into a porphyrin framework results in the formation of a hybrid macrocycle – 5,6-dimethoxyphenanthriporphyrin 1, fusing the structural features of polycyclic aromatic hydrocarbons and porphyrins. Simple transformations of antiaromatic 1 led to two macrocycles incorporating phenanthrene and phenanthrenequinone units: isophenanthriporphyrin and 5,6-dioxophenanthriporphyrin. The reversible protonation of 1 at the central meso-carbon atom stabilizes its constitutional isomer, i.e. the Cs-symmetric isophenanthriporphyrin in its dicationic form 1-A-H22+. The addition of an acid to nonaromatic 5,6-dioxophenanthriporphyrin 2 yielded the aromatic tricationic form protonated at the carbonyl oxygen atoms. In the presence of tetrafluoroboric acid, etherate, 2 underwent borylation at carbonyl oxygen atoms forming the aromatic BF2-derivative.