Lothar Knothe

Proximate, syn‐Periplanar Bisdiazene Skeletons: Syntheses, Structures, Homoconjugate Reactivity and Photochemistry

Rigid NN/NN (diazene/diazene) systems (F) consisting of more or less alkylated DBH and DBO chromophoric units (1, 2, X-ray structures), with very short π,π distances [d = 2.849 (1a, av.), 2.822 A (2)] and almost perfect syn-periplanar π,π alignments [ω = 168.6 (1a), 174.2° (2)] as well as the more flexible, less “proximate” metathesis isomers (3a,c, 27a,c, d >4.6 A, ω = 90-100°) have been synthesized. Homoconjugate π,π interaction (in 1, 2, not in 3, 27) is deduced from UV spectroscopic measurements [π π* maxima at 239 (234) nm (sh, 260)], while PE analyses furnished only small interaction parameters (1a: <0.3 eV). The potential of the novel syn-periplanar NN/NN motif in 1 and 2 for the synthesis of somewhat exotic polyheterocycles has been explored by calculation (B3LYP) as well as experimentally: i.a. kinetically stabilized, all-cis-peralkylated tetrazolidines (38, 44) and perhydro-1,2,4,5-tetrazines (41, 47) have become accessible (i.a. via novel azomethine/diazene and azomethine/azomethine cycloadditions). In 1a with its unreactive DBO chromophoric subunits, in the “buttressed” derivatives 1b-d, as well as in the DBH/DBO combination 2, and likewise in more ‘distant’ 27 (differently from the analogous CC/CC and NN/CC systems), irrespective of the excitation conditions employed (light of λ ≥≥ 280, 254 nm, low temperature matrix irradiation, acetone sensitization) no [2+2]photocycloaddition was observed. Instead exclusively N2-elimination took place. It is argued that unproductive NN/NN photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.

Secododecahedradienes - Syntheses, Reactivity, in-Plane Homoconjugated 3C/2e Cations, 4C/3e Radical Cations, andσ-Bishomoaromatic 4C/2e Dications?

Secodecahedradiene 2a, featuring very proximate, perfectly syn-periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in-plane(σ)-homoconjugational electron delocalization, starting from the available pagodane 15b. The response of 2a (and in part its diester 2b) – in π,π-distance (average 3.08 A), olefinic pyramidalization (average 26.9°), and π,π-split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16-dodecahedradiene 3a – to selected 4π-reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in-plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in-plane homoconjugated 4C/3e-species 2a.+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two-electron oxidation in SbF5/SO2ClF did not disclose the σ-bis-homoaromatic dication 4C/2e (see 2a2+), but a bis-allylic dication 75 as persistent species. In support of 2a2+ as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H-cage σ-homoconjugated cations.

Proximate, syn‐Periplanar, Rigid Imine(Nitrone)/Ene‐, and Diazene(Diazeneoxy)/Ene Systems: Syntheses, Homoconjugate Reactivity and Photochemistry

The chances for intramolecular imine/ene ( azetidines), diazene/ene ( 1,2-diazetidines), diazeneoxy/ene ( 1,2-diazetidine oxides) and diazenedioxiene ( 1,2-diazetidine dioxides) [2+2]photocycloadditions and for the isolation of the respective photoproducts, have been probed with specifically designed substrates. Upon direct or sensitized excitation, [2+2]cycloaddition was found to be the exclusive or at least dominant chemical process for the CN/CC, NN/CC and ONN(O)/CC systems featuring very small π,π-distances of 2.8-3.0 A and large π,π-interorbital angles of 160-170° (7 51, 17 55, 33 58 (competing N2 elimination), 22 62). This is not the case, however, in ONNO/CC (23, where electron transfer is a possibility), or in the more flexible, less “proximate” CN/CN (57) and CNO/CN (63) systems (π,π-distances of >3.8 A). While the corseted 1,2-diazetidine photoadducts (55, 58) proved to be thermally stable, their N-oxides (62, 65) were thermally too labile to be directly observable above -65 °C. For the latters only fleeting existence, electronic rather than strain effects are held responsible (B3LYP/6-31G* calculations). Very facile CNO/CC (12 13) and NNO/CC (22 24) [3+2]cycloadditions, homoconjugate addition of H2 and of dienophiles ([2+2+2]) to the diazene/ene 17 ( 39, 41, 45) are manifestations of “proximity” in these bichromophoric skeletons.

Intercyclic [π 2+σ2+σ2]-and π2+ π2]-cycloaddition reactions in sterically fixed norbornadiene-/quadricyclane-systems

The stereoelectronic prerequisites for unusual “zipper-type” [π 2+σ 2+σ 2]- and [π 2+π 2]-reaction sequences, e.g. in the still unknown C7H6-pentamers (1)/(2), are being studied using model compounds with systematically varied geometrical parameters. In line with expectations based on model considerations, in the [3]-series ((6), (7)) intracyclic, in the [5]-series ((10), (11)) intercyclic processes clearly predominate.

Proximate, syn-Periplanar Diazene/Diazene(di)oxy, Diazeneoxy/Diazene(di)oxy, and Diazenedioxy/Diazenedioxy Skeletons: Syntheses, [2+2]Photocycloadditions, Metathesis

Of two very proximate syn-periplanar bisdiazenes (1,2) mono-, di-, tri- and tetra-N-oxides were prepared, representing six combinations of the individual NN/NNO/ONNO chromophores. According to DFT calculations (B3LYP/6-31G*), [2+2]photocycloaddition to the respective oxidized tetrazetidines is significantly to moderately endothermic. The metathesis isomerization of the oxidized tetrazetidines is generally highly exothermic and kinetically increasingly favorable with increasing oxidation state. In practice, four out of the six bichromophoric combinations undergo selectively, in competition with N2 elimination from a DBH unit (13) still partially, metathesis isomerization upon π π* excitation (monochromatic 254 nm light). In the case of the syn-NNO/NNO combinations (5/6, 14), the photoaddition is thermally reversed. For a ONNO/NN combination (30), internal electron transfer is responsible for a complex reaction pattern. The preparative value of the metathesis reactions, though, is limited: The metathesis-derived bis[diazene mono(di)oxides] undergo relatively fast secondary photoreactions, while the tri(tetra)oxides undergo rapid thermal transformations. For the NN/NNO systems (12), of three potential pathways for its metathesis isomerization, the one that takes place via σ-symmetric intermediates (63, 64) is excluded by virtue of the retention of optical purity in the photometathesis of a highly enriched enantiomer [(−)-12]. Matrix irradiation experiments (12 K, IR control) with 12 result in the appearance of a kinetically highly labile transient. Supported by DFT calculations it is concluded that in the metathesis reactions, the respective tetrazetidine oxides (increasingly destabilized by interactions between oxygen lone pairs and NNσ* orbitals) function as vibrationally excited transients. That thermal reversion of these transients might be a general, nonproductive competition, is suggested by the experimental verification of a “reversed photometathesis” (51 15) and by the generally low rates in product formation upon irradiation. The question remains to be answered why in structurally analogous molecular skeletons, [2+2]photocycloaddition occurs in the CC/NN and variously oxidized NN/NN, and not, however, in the parent NN/NN combinations.

Synthesis and 20π-electrocyclisation of heptahendecafulvadiene - An unusual sequence of pericyclic processes

Abstract Upon thermal activation the newly synthesised heptahendecafulvadiene E ( 3) is isomerised into the pentacyclic hydrocarbons 9/11 (xylene,t1/2(150°C) ca.5min), the formation of which is explained by initial conrotatory 20-electron electrocyclisation followed by a cascade of 10π/6π-pericyclic processes.

The 18-electron electrocyclisation of vinylogous fidecene. an unusual sequence of pericyclic processes

Abstract Upon thermal activation the vinylogous fidecene 1 is isomerised into the pentacyclic indenoid hydrocarbon 13 (benzene, t 1 2 (150°C) ca. 5 min), the formation of which is explained by an initial conrotatory (“symmetry-forbidden”) 18-electron electrocyclisation followed by two fast hydrogen migrations ([1.9], [1.5]) and 14π-electrocyclisation.

Electrochemical oxidation of [1.1.1.1]pagodanes, bisseco-, seco- and dodecahedra(di)enes: stability of caged 4c/3e radical cations and 4c/2e dications

The anodic oxidation of variously functionalized[1.1.1.1]pagodanes A and bissecododecahedradienes B(π,π-distance, ca. 2.8 A) has been studied by cyclic voltammetry (CV). For nine out ot ten pagodanes A, independent of their functionalization and of the scan rate, an irreversible two-electron oxidation wave (Ep= 1.20–2.23 V) was observed in line with the sequence A→A˙+→B˙+→B2+(ECE) in which the chemical step C is very fast on the CV timescale. For seven of the corresponding valence isomeric dienes B, the first one-electron oxidation wave (B→B˙+), was found to be reversible(Eo′= 0.66–1.39 V) whilst the second one-electron oxidation wave (B˙+→B2+) was found to the irreversible (Ep= 1.20–1.80 V). The standard half-wave potentials allowed a thermodynamically meaningful estimate of the conjugative stabilization of the unusually persistent caged, non-classical 4c/3e radical cations B˙+(ΔEo′= 0.91 V for the parent diene B and the monoene reference) and of the destabilization by various functionalization patterns (0.40–0.73 V). For the secododecahedradienes D(π,π-distance ca. 2.85/3.2 A) and the dodecahedradienes E(π,π-distance ca. 3.5 A), only irreversible one-electron oxidation waves were recorded [Ep=(0.8)–1.46 V and (1.0)–1.39 V, respectively]. The degree of conjugative stabilization in the increasingly spherical, yet still non-classical 4c/3e radical cations D˙+ and E˙+ is approximated by ΔEp= 0.62 and 0.39 V, respectively.

“Heptahendecafulvalene” : Synthesis-x-ray structure analysis-cycloaddition reactions ([18+2]) ?

Abstract An efficient synthesis for an 18π-heptahendecafulvalene (6), its X-ray structure analysis and its cycloaddition reaction with TCNE - yielding i.a. a potential [π18a+π2s]-adduct - are reported.

The 16-electron electrocyclisation of vinylogous heptafulvalene

Abstract The vinylogous heptafulvalene (“heptafulvadiene”) (5) cleanly undergoes the antarafacial 16-electron electrocyclisation to (11) upon thermal activation (benzene, Ea = 22.1 kcal/mol; log A = 10.8).