Ingeborg Heise

The photo-Nazarov cyclization of 1-cyclohexenyl phenyl ketone revisited. Observation of intermediates

Abstract The photochemical transformation of 1-cyclohexenyl phenyl ketone ( 1 ) to the hexahydrofluorenone 2 (“photo-Nazarov cyclization”) was reinvestigated on the level of conventional preparative chemistry backed up by nuclear magnetic resonance (NMR) spectroscopy and deuteration experiments. Hitherto unreported intermediates were detected. These include the enol 8 and, in non-protic media, the crystalline dimers 3 and 4 . Dimer 3 , which was characterized by single-crystal X-ray analysis, is a kinetically stable non-conjugated enol which readily isomerizes to 4 on acid-base catalysis. Dimer 4 is photolysed to give ultimately 2 . The enol 8 is sufficiently long-lived to be seen in the 1 H NMR spectra of freshly irradiated solutions of 1 in CD 3 OD and of 4 in CD 3 OD or CD 3 CN before it isomerizes to 2 . Enol 3 resuls from trapping of the oxyallyl 7 , the presumed precursor of the enol 8 , by 8 in a novel type of reaction. Irradiation of 1 in the presence of 1,3-cyclopentadiene (CPD) gives adducts 10–16 . Adducts 13–16 appear to be derived from 7 , whereas 10 , 11 and 12 possess the structures of Diels-Alder adducts of CPD to the trans -cyclohexene isomer of 1 ( 6 ). The intermediacy of 6 as the primary photoproduct is also suggested by futher circumstantial evidence.

Photochemistry of 1,1-dicyano-1-alkenes: The olefin-to-cyclopropane rearrangement

Abstract 1,1-Dicyano-1-alkenes (DCNA) that lack further unsaturation undergo formation of 1,1-dicyano-cyclopropanes via 1,2-migration of either hydrogen or methyl/alkyl from C-3 to C-2 in their lowest exited singlet state. Quantum yields for this “olefin-to-cyclopropane photorearrangement” (OCPR) were found to span a wide range (≤0.1) and to depend characteristically on alkyl substituents on C-3 and C-2. OCPR occurs preferentially via such 1,2-migration that leaves behind the more alkylated C-3 atom. 1,2-Migration was found to occur suprafacially (i.e. to follow maximum orbital overlap), but to be rather tolerant towards unfavorable orientation of the migrant in the starting DCNA. The ring-closure that completed OCPR was found to be devoid of any intrinsic stereoselectivity; thus, in cyclohexane, each of the two epimeric 2-[(2-methyl-cyclohexyl)-methylene]-malononitriles ( 3 and 4 ) yielded the same approximately 1:1 mixture of the two epimeric 4-methyl-spiro[2.5]octane-1,1-dicarbonitriles ( 28 and 29 ). OCPR proceeded via an intermediate that also led to isomeric DCNA and to 1,1-dicyano-3-alkenes as minor by-products. Some deprotonation at C-3 of the photoexcited DCNA was noticeable in methanol, but not in hexane. Supplementary experiments included preparative and kinetic investigations of thermolyses of 1,1-dicyano-cyclopropanes. The combined evidence allowed the deduction of the following reaction path for OCPR. In their lowest excited singlet state, a ππ ∗ state, DCNA exhibit cationic reactivity of their C-2 atoms (presumably in the perpendicular conformation of C-2 relative to C-1, according to Salem’s seminal concept). This reactivity triggers the 1,2 (Wagner–Meerwein type) migration to yield, still on the excited hypersurface, a 1,3 dipole. This 1,3-dipole achieves an almost complete conformational equilibration in cyclohexane (though less so in more polar solvents) before it decays to the electronic ground-state thereby becoming a 1,3-diradical. This 1,3-diradical undergoes three competing terminating reactions: ring closure to cyclopropane (the major path); 1,2-back migration of hydrogen to form starting or isomeric DCNA; 1,4-hydrogen shift to produce 1,1-dicyano-3-alkenes. The 1,3-dipole is too short-lived to undergo a potentially favorable Wagner–Meerwein rearrangement. Like the reactive excited DCNA singlet state, the 1,3-dipole is not trapped to any significant extent by nucleophilic addition of the solvents tert -butanol or methanol to its cationic center. The reason for this failure appears to be the excited-state nature of this species, which bars the formation of a ground-state adduct in an adiabatic reaction.

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

The thermal [2+2] cyclodimerisation of (E,Z)-cycloocta-1,3-diene (9), which is known to afford the cyclobutane dimers 11, 12, and 13, has been investigated in the presence of the nitroxyls 17 and 18 and of atmospheric dioxygen, all of which are known to be efficient trapping agents for carbon-centred free radicals. The nitroxyls have been found to divert the reaction from formation of the dimers to formation of 2:2 adducts of two molecules of 9 and two molecules of nitroxyl. The rate constant for the formation of the overall sum of the dimers plus the 2:2 adducts in the presence of nitroxyl has been found to equal the rate constant for the formation of dimers in the absence of nitroxyl. This and the molecular structures of the 2:2 adducts prove that two molecules of 9 combine irreversibly to produce the two epimeric bis(allylic) 1,4-diradicals 14 and 15 (meso and rac, respectively) which undergo two competing reactions: ring-closure to dimers 11, 12, and 13, and trapping by nitroxyl to form the 2:2 adducts. Dioxygen, too, was found to trap 14 and 15 efficiently. From the kinetics of the latter trapping reaction, studied at six temperatures between 5 and 55 °C, the heights of the activation barriers separating 14 from 11 and 15 from 12 + 13 were estimated at 11.1 ± 1.5 and 10.2 ± 1.5 kcal·mol−1, respectively, corresponding to diradical lifetimes of ca. 0.5 μs. These unexpectedly high barriers have been verified by MM3 force-field calculations and by an investigation of the kinetics of the gas-phase thermolysis of 12 (to give 13 and 16 which is an epimer of 12 and 13) and of 13 (to give 12 and 16). When the cyclodimerisation of 9 was carried out in the presence of spin = 1/2 transition metal complexes, no trapping was observed, but a shift in the 12/13 ratio, resulting from a catalysed conversion of 15 from its singlet to its triplet spin state, was seen. Since nitroxyl also catalysed the singlet-to-triplet conversion (dioxygen did not) in competition with trapping, the kinetics of trapping by nitroxyl was complex, but it did show that 14 and 15 were jeopardised to trapping twice in their lifetimes, meaning that 14 and 15 were generated in their anti conformations, which had to change to the gauche conformations by crossing the barriers referred to above before they could ring-close to dimers; all the anti and the triplet gauche conformations are trappable, but the singlet gauche conformations are not. The same conclusion was reached for 15 from independent experimental evidence. In addition, there is a minor path from 9 to dimers 11, 12, and 13, which is not subject to trapping and which involves the direct formation of the singlet gauche conformers of 14 and 15 from 9. The gauche conformer formed in the smallest amount along this minor pathway is the one giving rise to 12, which is the one with its two radical p-orbitals pointing towards each other most strongly, thus causing adverse Woodward−Hoffmann effects. The direct formation of the principal gauche conformer (which gives rise to 13) from 9 requires 2.75 ± 0.44 and 2.57 ± 0.42 kcal·mol−1 more activation energy than required for the formation of the anti conformers of 14 and 15, respectively.

The photo-Nazarov cyclisation of 1-cyclohexenyl- phenyl-methanone revisited II: Reaction between primary and secondary key intermediates

Abstract Trans-1-cyclohexenyl-phenyl-methanone (2) and enol 4, both key intermediates in the title reaction, react with each other in a Michael-type addition to form predominantly enol 10. This enol, kinetically stable but too reactive to be isolated, reveals its presence in the irradiated solutions by formation of the isomeric ketone 11 on acid catalysis, and by formation of the oxidation product 9 on exposure to atmospheric oxygen. In the absence of acid, formation of 10 competes significantly with the title reaction of cis-1-cyclohexenyl-phenyl-methanone (1). In a secondary photoreaction of 10, 1,6-hydrogen abstraction by the excited carbonyl group and cyclisation afford 13 and 14. Enols 4, 10, and 13, in striking contrast to enol ethers and to thermodynamically stable enols, are unstable towards atmospheric oxygen. Thus, 4 autoxidises to form five compounds (Ox-1 through Ox-5), 10 to form 9, and 13 to form 15.

The photochemistry of an 1,1-dicyano-3-oxa-1-alkene and of its 3-aza analogue

Abstract The photoreactions of two 3-alkoxy-1,1,-dicyano-1-alkenes, viz. 2-(2,2-dimethyl-tetrahydrofuran-3-ylidene)-malononitrile (1) and 2-(2-methoxy-2-methyl-propylidene)-malononitrile (6), and of the analogue of 1 bearing methylamino in place of oxygen, 8, on direct irradiation has been investigated. Compounds 1 and 6 rearrange to 2-alkoxy-1,1-dicyano-cyclopropanes via 1,2-migration of methyl followed by ring closure. The cyclopropanes are thermally unstable but when the irradiation of 1 was carried out in methanol, the product 4 resulting from ring opening addition of methanol to the cyclopropane was obtained in high yield. In contrast to 1, 8 was photostable due to efficient intramolecular quenching of the excited dicyanoalkene chromophore by the tertiary amine nitrogen atom. A 1,3-transposition product of 1, 3-isopropylidene-tetrahydrofuran-2,2-dicarbonitrile (3), is formed as a by-product of 4. Its probable mode of formation is discussed.

The photochemistry of 1-cyclohexenyl-2,4,6-trimethylphenyl-methanone (1-mesitoyl-cyclohexene)

Abstract The photochemistry (direct irradiation in solution, λ>300 nm ) of the title compound ( 8 ) has been investigated. Whereas, in contrast to the methyl-free analog of 8 , only undefined material of higher molecular weight was obtained upon irradiation without additives in acetonitrile and toluene, this situation changed dramatically in the presence of co-reactants. Thus, in ethyl vinyl ether, [4+2] adducts 11a and 11b were obtained in good combined yield. In acetonitrile, in the presence of acid which catalyses a skeletal rearrangement, the hexahydrofluorenone 14 was the main product. The results point to the intermediacy of the trans -cyclohexene isomer of 8 , viz. 9 , and to the 2-oxyallyl 10 as consecutive intermediates, in analogy to the methyl-free system. The molecular structures of some unprecedented minor by-products such as 15a – c even more clearly reveal their origin from 10 . Photoreactions of 8 in the presence of either water or phenol as co-reactants proceeded less cleanly but did give defined products such as 16 – 18 in moderate yields.