Jens-Peter Steffen

TWO NOVEL THERMAL BIRADICAL CYCLIZATIONS OF ENYNE-KETENIMINES : THEORY, EXPERIMENT, AND SYNTHETIC POTENTIAL

Both benzocarbazoles and quinolines can be synthesized from enyne ketenimines 1 generated in situ via biradical intermediates (see reaction below). Which of the heterocyclic ring systems is formed depends on the choice of the substituent R1 at the alkyne terminus.

Two Novel Thermal Biradical Cyclizations in Theory and Experiment: New Synthetic Routes to 6H‐Indolo[2,3‐b]quinolines and 2‐Aminoquinolines from Enyne‐Carbodiimides

The regioselectivity of the biradical cyclization of enyne-carbodiimides 1 can easily be controlled by variation of R1 at the alkyne terminus. Attachment of a hydrogen atom (R1 =H) leads to C2 -C7 cyclization and formation of biradical 2, whereas C2 -C6 cyclization to provide biradical 3 is observed with R1 =Me3 Si or Ph.

Thermal C2−C6 Cyclization of Enyne−Carbodiimides: Experimental Evidence Contradicts a Diradical and Suggests a Carbene Intermediate

Mechanistic details of the thermal C2-C6 cyclization of enyne-carbodiimides are investigated. A variety of product and kinetic studies on solvent and substituent effects open the way for a deeper mechanistic understanding. Nonlinear Hammett correlations suggest that a change of mechanism takes place: the thermal C2-C6 cyclization of enyne-carbodiimides with electron-withdrawing substituents may be best described as a coarctate cyclization to a carbene and with electron-donating substituents as a polar cyclization to a carbene with strong zwitterionic character. Theoretical investigations had originally suggested a diradical intermediate. DFT computations and NBO analysis for the parent diazafulvenediyl are in agreement with a carbene intermediate. While any intermolecular trapping of the intermediate failed, the formation of the C-H insertion product 19 strongly supports the carbene hypothesis.

Ring Strain Effects in Enyne-Allene Thermolysis: Switch from the Myers-Saito Reaction to the C2-C6 Biradical Cyclization

The mode of the thermal cyclization of enyne-allenes 1 depends on ring strain effects: when the ene functionality is part of a benzene, cyclohexene or cycloheptene ring the novel C2-C6 biradical cyclization is observed, while when it is part of a cyclopentene ring the Myers-Saito cycloaromatization is registered.

Zwei neue thermische Diradikal‐Cyclisierungen in Theorie und Experiment: neue Syntheserouten zu 6H‐Indolo[2,3‐b]chinolinen und 2‐Aminochinolinen aus Enincarbodiimiden

Die Regioselektivitat der Diradikal-Cyclisierung von Enincarbodiimiden 1 last sich leicht durch Variation von R1 am Alkinterminus steuern. Mit R1 = H entsteht in einer C2-C7-Cyclisierung das Diradikal 2, mit R1 = Me3Si oder Ph dagegen durch C2-C6-Cyclisierung das Diradikal 3.

Ring size effects in the C2–C6 biradical cyclisation of enyne–allenes and the relevance for neocarzinostatin

The regioselectivity of the thermal cyclisations of enyne–allenes 1 can be toggled as a function of the ring size of the cycloalkene. With a cyclopentene as the ene moiety the Myers–Saito (C2–C7) cycloaromatisation product is formed, whereas with six- and seven-membered cycloalkenes the novel C2–C6 cyclisation is observed. DFT calculations are used to rationalise these changes. The implications of these findings for alternative thermal biradical cyclisations of neocarzinostatin are discussed.

Synthesis and Photochemical Cyclization of a Novel Enyne-Carbodiimide

The triplet sensitized cyclization of enyne-carbodiimide 4 leads to efficient formation of indoloquinoline 5 with concomittant loss of a methyl group. The efficient loss of the methyl group was explained using AM1 semiempirical calculations.

One-electron oxidation of enol phosphates, enol phosphites and enol phosphinates. Evidence for an unprecedented P–O bond cleavage in phosphoenol radical cations in solution1

For the first time phosphoenol radical cations are generated in solution and monitored by cyclic voltammetry and EPR; the sterically congested radical cations undergo an unprecedented P–O bond cleavage, the kinetics of which are determined.