Biomedical subjects

Matthew K Kiesewetter

Publications and source records attributed to Matthew K Kiesewetter.

10 recordsLinked to original sources

PubMed2006

Reactions involving di-trans-[12]annulenes.

The low temperature complete dehydrohalogenation of pentabromocyclododecene (C12H17Br5) with potassium tert-butoxide in THF followed by exposure to potassium metal leads to the formation of the anion radical of 1,5-di-trans-[12]annulene, which loses hydrogen and undergoes ring closure to form the anion radical of 11,12-dihydro-[8]annuleno-[6]annulene. This product can, in turn, be isolated as its neutral molecule via reoxidation with iodine. A [12]annulene obtained via the dimerization of 1,5-hexadiyne in the presence of 18-crown-6 and potassium tert-butoxide undergoes ring closure, with concomitant loss of hydrogen, to yield the heptalene anion radical. It follows that the heptalene anion radical precursor was the 1,7-di-trans isomer of [12]annulene.

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PubMed2006

Selective intercalation of Cs+ in the "V"-shaped cavity of a bichromophoric anion radical: Cs+ assisted pi-s-pi-delocalization of an electron.

EPR studies in tetrahydrofuran, reveal that the one electron reduction of 1-(9-methyl-9H-fluoren-9-yl)-4-methylbenzene via electron transfer from cesium metal produces an anion radical that has a large affinity for the cesium cation. The affinity of this anion radical for Cs+ is so great that it will actually "suck" the Cs+ (but not Na+ or K+) right out of the grasp of 18-crown-6, leading to a cation-assisted pi-stacked complex, where the s-orbital of the metal cation is simultaneously overlapped with the pi-clouds of the phenyl and fluorenyl moieties. At ambient temperature, proton- and cesium-electron coupling constants are rapidly (on the EPR time scale) modulated as a result of the simultaneous existence of two interconverting conformers having an averaged cesium splitting (a(Cs)) of about 1.6 G. The pi-s-pi-electronic coupling can be turned on or off via the addition or removal of cesium cations. Analogous pi-s-pi-electronic coupling is observed in the 1,4-bis(9-methyl-9H-fluoren-9-yl)benzene-cesium system.

Anions↗

PubMed2006

Intramolecular electron transfer in cofacially pi-stacked fluorenes: evidence of tunneling.

The one-electron reduction of neutral pi-stacked di- and trifluorenes (F-2 and F-3) in HMPA, where ion association is absent, results in the formation of anion radicals in which the odd electron resides predominantly on just one of the external fluorene moieties, as established by EPR spectroscopy. However, in the case of tetrafluorene, introduction of a single electron leads to a kinetically controlled anion radical F-4(int)*- in which the odd electron undergoes rapid exchange between two central fluorene rings, where the anionic charge is partially shielded from solvation due to the presence of external fluorene rings. On a time scale of minutes, anion radical F-4(int)*- converts to a thermodynamically stabilized anion radical F-4(ext)*-, with the electron exhibiting coupling from the protons on an external fluorene moiety. The charge and spin residing on an external moiety allow efficient solvation of the anionic charge. A similar fast exchange of a single electron (probably with the involvement of quantum mechanical tunneling) among three and four internal fluorene moieties is initially observed via EPR spectroscopy in the penta- and hexafluorene derivatives, F-5 and F-6, respectively.

Electrons↗

PubMed2005

[12]Annulynes.

Only one isomer of o-benzyne ([6]annulyne or 1,2-didehydrobenzene) exists, but the dehydro analogue of the "ring-opened double benzene", [12]annulyne, was generated in several isomeric forms. 1,5-Hexadiyne undergoes self-condensation in the presence of potassium tert-butoxide to yield two isomers of [12]annulyne (3,11-di-trans-[12]annulyne and 5,9-di-trans-[12]annulyne), both of which exhibit a weak paratropic ring current in their 1H NMR spectra and are oxygen sensitive. They can be reduced to their respective dianions, which are diatropic. A third isomer (3,9-di-trans-[12]annulyne) was generated via the complete dehydrohalogenation of hexabromocyclododecene and found to be much less stable but can be tamed via one- or two-electron reduction. A tight association of the cation (K+) with the p(y)-orbitals within the alkyne moiety results in an unusually low-field resonance for an adjacent external proton.

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PubMed2005

The capture of sym-[8]annuldiyne: the cyclooctadienyne-eta2-ynyl potassium zwitterionic radical.

[reaction: see text] Reaction of 1,4-dibromo-[8]annulene (C(8)H(6)Br(2)) with potassium tert-butoxide in THF followed by exposure to potassium metal leads to the formation of the anion radical of sym-[8]annuldiyne. The rapid interchange of Jahn-Teller-induced alternating bond angle conformers of sym-[8]annuldiyne is halted by ion association with a metal-crown ether complex forming the cyclooctadienyne-eta(2)-ynyl potassium zwitterionic radical, rendering all four protons nonequivalent. Neutral sym-[8]annuldiyne can form the [2 + 2] polymer, which is not soluble in the THF solution.

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PubMed2005

Intramolecular C-H/C-D exchange in cofacially stacked polyfluorenes via electron-induced bond activation.

EPR studies in hexamethylphosphoramide, along with DFT studies, reveal that the one-electron reduction of pi-stacked polyfluorenes containing two, three, and four fluorene units leads to the corresponding anion radicals where the odd electron is located exclusively on an outside fluorene moiety. These anion radicals disproportionate to form small concentrations of diamagnetic dianions, wherein there is a nonclassical pz overlap that crosses the C2 axis. The presence of the two extra electrons activates the C-H bonds in the ortho positions, and concomitant overlap of the p-orbitals involving adjacent fluorene moieties results in intramolecular hydrogen exchange at temperatures as low as 90 K.

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PubMed2005

The second cyclopropannulene: cycloprop-[8]annulene.

Reacting (at 0 degrees C) a mixture of CH2Cl2 and monobromo[8]annulene (C8H7Br) with potassium tert-butoxide in hexamethylphosphoramide (HMPA) and following with exposure to potassium metal led to the formation of the anion radical of an HMPA-[6.1.0]bicyclononatetraene condensation product, in which two HMPA fragments are geminal and attached to the number 9 carbon. When the reaction sequence is carried out in THF, the dianion of cycloprop[8]annulene is predominantly formed. Neutral cycloprop[8]annulene can be isolated via the I2 oxidation of the THF solution. The NMR analysis reveals that the eight-membered ring is nearly planar, and the three-membered ring is more like a dimethylenecyclopropane than it is like a cyclopropene. Further, the chemical shifts due to the protons on the eight-membered ring are nearly 2 ppm further upfield than are those for [8]annulene itself, suggesting a paratropic ring current.

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PubMed2004

The second triannulenylene: tri-[8]annulenylene.

Room-temperature dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with potassium tert-butoxide followed (after a couple of minutes) by alkali metal reduction was used to generate the anion radical of tri-[8]annulenylene [(C8H6*-)3] in HMPA. EPR analysis reveals that the odd electron is primarily located in one of the three eight-membered ring systems, which is rendered planar. Excellent agreement was obtained between spin densities predicted by B3LYP/6-31G* calculations and those observed. The neutral tri-[8]annulenylene system has a propensity toward polymerization, but it can be isolated for NMR and mass spectral analysis via the I2 oxidation of the anion radical. The NMR analysis reveals that two of the eight-membered rings are bent above the plane of the benzene ring and the other is bent below. Tri-[6]annulenylene (triphenylene) is the only other known member of the triannulenylenes.

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PubMed2003

Single-electron entrapment of [8]annulyne, biannulenylenes, and an annulenoannulene.

The low-temperature (-100 degrees C) dehydrohalogenation of bromocyclooctatetraene followed by immediate electron-transfer yields a stable solution of the [8]annulyne anion radical. If the unstable [8]annulyne is reacted with itself, cyclobutadiene, or benzyne, the respective bi-[8]annulenylene, [6]annuleno[8]annulene, or [6]-[8]annulenylene can be trapped as their anion radicals via one-electron transfer. These condensation products were all obtained from simple [2 + 2] cycloaddition reactions. B3LYP/6-31G geometry optimizations were carried out, and the calculated spin densities were compared to the EPR spectral results obtained for the anion radicals of [6]annuleno[8]annulene, [8]annulyne, bi[8]annulenylene, and [6]-[8]annulenylene, and excellent agreement has been realized. This simple "one-pot" approach should be applicable to a wide range of such systems.

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PubMed2003

The cyclooctatriene-eta2-ynyl potassium zwitterionic radical: evidence for a potassium organometallic.

Low-temperature (-120 degrees C) dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with either sodium or potassium tert-butoxide followed by alkali metal reduction was used to generate the anion radical of [8]annulyne (C8H6*-) in tetrahydrofuran. EPR analysis at -120 degrees C reveals an extraordinarily large metal splitting when K or Cs (aK of 0.214 G and aCs of 3.26 G) serves as the reducing agent. The large aM is due to the metal cation interacting with the p-orbitals, within the alkyne moiety, that are in the plane of the ring system. The ionic radius of K+ is 1.33 A, which is larger than the B3LYP predicted distance between carbons 1 and 2 (1.23 A). However, the ionic radius of Na+ is only 0.95 A, and it is too small to simultaneously interact with both p-orbitals. Hence, no aM is observed when Na (ordinarily aNa > aK) or Li serves as the reducing agent. After the addition of 18-crown-6 to either the K or the Cs reduced system, two anion radicals are present. One is the system where the 18-crown-6 encapsulated metal complex is normally ion paired over the face of the ring system and aM = 0. The other is the cyclooctatriene-eta2-ynyl 18-crown-6 encapsulated metal zwitterion radical exhibiting a large aM. The ion pair to organometallic equilibrium constant is 1.6 +/- 0.1 and 3.5 +/- 0.1 for the K and Cs systems, respectively.

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