Direct and indirect single electron transfer (SET)-photochemical approaches for the preparation of novel phthalimide and naphthalimide-based lariat-type crown ethers

• Dae Won Cho,
• Patrick S. Mariano and
• Ung Chan Yoon

Beilstein J. Org. Chem. 2014, 10, 514–527, doi:10.3762/bjoc.10.47

• processes form zwitterionic biradical intermediates 3, in which a proton transfer and an α-heterolytic fragmentation proceeds, produce biradicals 7 that are precursors of the heterocyclic products 6 (Scheme 2). Photochemical reactions of naphthalimides with electron donors have also been intensively studied

• the low oxidation potentials and known reactivity profile of cation radicals arising from α-trialkylsilyl substituted n-electron donors, it is feasible to design substrates that undergo sequential SET-desilylation processes to produce radicals or biradical intermediates in a highly regioselective and

• states of the imide acceptors occurs to generate rapidly interconverting zwitterionic radicals 26 and 27, each of which can undergo secondary reactions to generate biradical intermediates 28 and 29 that serve as precursors of cyclic products 30 and 31. The results of exploratory studies revealed that the

Tuning the interactions between electron spins in fullerene-based triad systems

• Maria A. Lebedeva,
• Thomas W. Chamberlain,
• E. Stephen Davies,
• Bradley E. Thomas,
• Martin Schröder and
• Andrei N. Khlobystov

Beilstein J. Org. Chem. 2014, 10, 332–343, doi:10.3762/bjoc.10.31

• the majority of the molecules exist as two independent doublet (S = 1/2) radicals and suggesting a small singlet-triplet energy gap [29]. The intramolecular triplet biradical (S = 1) of 42− is also present with a zero-field splitting parameter (D) of 27.8 G (Figure 4a,b). This value is in the same

• − molecules in the frozen solution however we do not exclude other possible assignments [31]. The frozen solution EPR spectrum of 12− displays a central feature at g = 2.0003, consistent with that of a doublet biradical (Figure 4c) that is flanked on each side by broad “wings” that we assign to the presence

• assignment to an intramolecular triplet must be excluded. Hence, by changing the linker from oxalate in 42− to terephthalate in 12− we have either reduced the interaction of the spin centres or significantly perturbed the formation of an intramolecular triplet biradical. The EPR spectra of the C70 containing

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

• Alexander O. Terent'ev,
• Dmitry A. Borisov,
• Vera A. Vil’ and
• Valery M. Dembitsky

Beilstein J. Org. Chem. 2014, 10, 34–114, doi:10.3762/bjoc.10.6

• (DCA) as the sensitizer [242]. After irradiation of diazene 34 in an argon matrix at 10 K, biradical 35 was detected by IR spectroscopy and the reaction of the latter with oxygen at 10 K proceeded regioselectively to give dioxolane 36 (Scheme 12) [243]. Bicyclic peroxide 2-heptyl-3,4-dioxabicyclo[3.3.0

Thermochemistry and photochemistry of spiroketals derived from indan-2-one: Stepwise processes versus coarctate fragmentations

• Götz Bucher,
• Gernot Heitmann and
• Rainer Herges

Beilstein J. Org. Chem. 2013, 9, 1668–1676, doi:10.3762/bjoc.9.191

• C–C cleavage (pathway A, more favourable) or a C–O cleavage (pathways B or C, less favourable). In principle, a chelotropic elimination of 1,3-dioxol-2-ylidene is also conceivable (pathway D). Scheme 4 shows a possible mechanistic scenario. While it is questionable whether the biradical

• facile decay mechanism for 1. The primarily formed benzyl-dialkoxymethyl biradical 4 should undergo a very facile ring-opening reaction to yield an ester biradical 7, which can either cleave into ethylene, carbon dioxide and o-xylylene (XY), or eliminate acetaldehyde (AA) to yield an acyl-benzyl

• formaldehyde in the pyrolysis of 2 is readily explained by the fact that the ester biradical 13 formed can lose one equivalent of ethene and formaldehyde to yield the acyl-benzyl type biradical 15 (Scheme 5). In the FVP of 1 and 2, CO is formed in large excess over CO2. This excess is far less pronounced in

4-Pyridylnitrene and 2-pyrazinylcarbene

• Curt Wentrup,
• Ales Reisinger and
• David Kvaskoff

Beilstein J. Org. Chem. 2013, 9, 754–760, doi:10.3762/bjoc.9.85

• shift finally converts the nitrile ylide 9 to the open-chain ketenimine 10 (Scheme 2). 3-Pyridylcarbene undergoes analogous Type I ylidic ring opening to an ethynylvinylnitrile imine [6]. Type II ring opening is diradicaloid and proceeds via an open-chain vinylnitrene or biradical 13 in nitrenes such as

Complete σ* intramolecular aromatic hydroxylation mechanism through O₂ activation by a Schiff base macrocyclic dicopper(I) complex

• Albert Poater and
• Miquel Solà

Beilstein J. Org. Chem. 2013, 9, 585–593, doi:10.3762/bjoc.9.63

• methodology, while for the closed-shell singlet states the restricted formalism was used. Theoretical treatment of biradical singlet species requires multiconfigurational or multireference methods due to strong static electron correlation. Unfortunately, these methods can only be applied to relatively small

• systems because computationally they are extremely demanding. As an alternative, we have used the unrestricted UB3LYP method in broken symmetry (BS, using GUESS = MIX) [64]. This method improves the modeling of biradical singlet states at the expense of introducing some spin contamination from higher spin

• kcal·mol−1, evolving to the μ-η1:η2-peroxo isomer c with an energetic stabilization of 12.9 kcal·mol−1 with respect to the preceding complex b. Furthermore, this step also requires change to a biradical singlet ground state, although the triplet state is only 1 kcal·mol−1 higher as a result of the long

Photoreactions of cyclic sulfite esters: Evidence for diradical intermediates

• Rick C. White,
• Benny E. Arney Jr. and
• Heiko Ihmels

Beilstein J. Org. Chem. 2012, 8, 1208–1212, doi:10.3762/bjoc.8.134

• lack of oxirane formation may be explained by initial cleavage of the C–O bond to give the biradical BR1, whose rotation about the C–C bond to give the relaxed biradical BR1rot is significantly faster than the loss of sulfur dioxide (Scheme 3). As a consequence, the elimination of SO2 results in a

• biradical BR2 with a conformation that is unfavorable for oxirane formation, but enables efficient hydrogen migration to result in aldehyde 3a. Theoretical calculations have confirmed that acetaldehyde derivatives may be formed upon rearrangement of the 1,3-oxyethandiyl [20][21] and that a subsequent

• . Such as in the case of the sulfite derivative 8, the products 3b, 5, and 6 may be formed by photoinduced elimination of SO2 through a biradical intermediate (Scheme 3). Furthermore, no oxirane photoproduct could be detected, presumably due to the relatively slow elimination of SO2, thus resembling the

Synthesis and photooxidation of styrene copolymer bearing camphorquinone pendant groups

• Branislav Husár,
• Norbert Moszner and
• Ivan Lukáč

Beilstein J. Org. Chem. 2012, 8, 337–343, doi:10.3762/bjoc.8.37

• the formation of 11 can oxidize another molecule of CQ to 11. It is likely that common biradical intermediates are responsible for the formation of lactones 12 and 13 in solution [10] and acids 14a and 14b formed in the PS matrix [21]. The intramolecular recombination of biradical intermediates is

• intermediate is most probably not formed. Similar to BZ, the n→π* triplet state of the CQ structure may also add molecular oxygen to form a 1,4-biradical. In the case of the BZ structures, formation of the 1,4-biradical is followed by BP formation. In comparison, CQ structures react with oxygen forming 1,4

Photochemical and thermal intramolecular 1,3-dipolar cycloaddition reactions of new o-stilbene-methylene-3-sydnones and their synthesis

• Kristina Butković,
• Željko Marinić,
• Krešimir Molčanov,
• Biserka Kojić-Prodić and
• Marija Šindler-Kulyk

Beilstein J. Org. Chem. 2011, 7, 1663–1670, doi:10.3762/bjoc.7.196

• , lose CO2 under the reaction conditions to afford intermediates 14A and 14 B, respectively. Owing to the favourable conformation in the case of biradical 15A, the 1,3-H abstraction and formation of the C–N double bond in product 15 is possible. In the biradical 14A the intramolecular hydrogen

Complete transfer of chirality in an intramolecular, thermal [2 + 2] cycloaddition of allene-ynes to form non-racemic spirooxindoles

• Kay M. Brummond and
• Joshua M. Osbourn

Beilstein J. Org. Chem. 2011, 7, 601–605, doi:10.3762/bjoc.7.70

• variant of this reaction under thermal conditions [1][2]. This thermally forbidden process is believed to proceed via a biradical intermediate mechanism, a conclusion supported by both computational and experimental studies [3]. Recently, the scope of this method has been expanded to the synthesis of

• biradical intermediate, but the tert-butyl group hinders rotation around the carbon–carbon bond as shown in Figure 4, thus slowing racemization of the resulting radical containing carbon. This hypothesis is supported by a report by Pasto, where transfer of chiral information was incomplete in a thermal

• expand the scope of this chirality transfer to other allenyl systems possessing less bulky and/or traceless groups. Chiral NMR shift analysis of propargyl acetate 7. Chiral NMR shift analysis of allenyloxindole 8. Chiral NMR shift analysis of spirooxindole 9. Thermally generated biradical intermediate 10

The arene–alkene photocycloaddition

• Ursula Streit and
• Christian G. Bochet

Beilstein J. Org. Chem. 2011, 7, 525–542, doi:10.3762/bjoc.7.61

• site [28]. On the other hand, it was also proposed that a biradical intermediate is involved in this photocycloaddition mechanism. Nevertheless, all attempts to trap a biradical or a zwitterionic intermediate have so far been unsuccessful. A very clever plan to test whether a biradical intermediate is

• indeed involved was carried out by Reedich and Sheridan [29]: By incorporating a diazo group into the last formed bond of the cyclopropyl ring in the meta photocycloadduct [30], they would be able to see whether the biradical formed by the extrusion of nitrogen gave the same products as the meta

• photocycloaddition (Scheme 6). This was indeed the case, as very similar ratios of the two distinct stereoisomers C and D were found to be formed from the diazo compounds A and B as well as from the photocycloaddition of o-xylene to cyclopentadiene. This finding seems to indicate that a biradical structure is indeed

Synthesis of 5-(2-methoxy-1-naphthyl)- and 5-[2-(methoxymethyl)-1-naphthyl]-11H-benzo[b]fluorene as 2,2'-disubstituted 1,1'-binaphthyls via benzannulated enyne–allenes

• Yu-Hsuan Wang,
• Joshua F. Bailey,
• Jeffrey L. Petersen and
• Kung K. Wang

Beilstein J. Org. Chem. 2011, 7, 496–502, doi:10.3762/bjoc.7.58

• -protropic rearrangement to form the corresponding benzannulated enyne–allene 10a. A Schmittel cyclization reaction [1][2][3][4] generates biradical 11a, which then undergoes an intramolecular radical–radical coupling to afford 12a. This is followed by a second prototropic rearrangement to restore the

An efficient and practical entry to 2-amido-dienes and 3-amido-trienes from allenamides through stereoselective 1,3-hydrogen shifts

• Ryuji Hayashi,
• John B. Feltenberger,
• Andrew G. Lohse,
• Mary C. Walton and
• Richard P. Hsung

Beilstein J. Org. Chem. 2011, 7, 410–420, doi:10.3762/bjoc.7.53

• significantly lowered thermal activation barrier of 1,3-hydrogen-shifts of allenamides is that the nitrogen atom can serve to stabilize the biradical intermediate [2][4][5][6][7][8][9][10][11][12] (for another leading reference on related radical intermediates see [78]) which are presumed to be electron

• deficient. Based on the model in Figure 2 (left side), stabilization of the biradical intermediate is direct when isomerizations proceed from the α-position, whereas the isomerization from the γ-position is “vinylogous”, or remotely stabilized through the olefin. Therefore, thermal isomerizations at the α

• -position should be faster than at the γ-position. While under thermal conditions, a biradical intermediate is at play [2][4][24], under acidic conditions, the isomerization clearly proceeds through an N-acyl iminium intermediate via protonation of the allenamide (Figure 2, center). Consequently, a similar

Effects of anion complexation on the photoreactivity of bisureido- and bisthioureido-substituted dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 278–289, doi:10.3762/bjoc.7.37

• benzophenone, a triplet-state di-π-methane rearrangement is induced. Thus, in the initial reaction step connection between one vinyl and one benzo carbon atom takes place, i.e., a so called vinyl–benzo bridging, that leads to the intermediate biradical BR1a [29]. Subsequent rearomatization with the formation

Photoinduced homolytic C–H activation in N-(4-homoadamantyl)phthalimide

• Nikola Cindro,
• Margareta Horvat,
• Kata Mlinarić-Majerski,
• Axel G. Griesbeck and
• Nikola Basarić

Beilstein J. Org. Chem. 2011, 7, 270–277, doi:10.3762/bjoc.7.36

• of the 1,5-biradical. Minor products 7 were formed by photoinduced γ H-abstractions, followed by ring closure to azetidinols and ring enlargement to azepinediones. The observed selectivity to exo-alcohol 6 was explained by the conformation of 5 and the best orientation and the availability of the δ-H

• readily abstracted [7][13][14], the conformation of the molecule is probably the most important factor that directs the selective δ-abstraction. The δ-abstraction gives rise to a 1,5-biradical (1,5BR) that undergoes stereoselective cyclization to furnish the major product, exo-alcohol 6. The observed

• gives a 1,4-biradical (1,4BR) that cyclizes to azetidinol intermediates AZT1 and AZT2. Azetidinols undergo subsequent ring enlargement to furnish products anti-7 and syn-7, respectively. The ratio of the isolated compounds 7 is 5:1. However, no assignment of their stereochemistry was made from their

Formation of macrocyclic lactones in the Paternò–Büchi dimerization reaction

• Junya Arimura,
• Tsutomu Mizuta,
• Yoshikazu Hiraga and
• Manabu Abe

Beilstein J. Org. Chem. 2011, 7, 265–269, doi:10.3762/bjoc.7.35

• intermediary triplet biradicals, BR versus BR’, and also by the relative nucleophilicity of the furan-ring carbons, i.e., C1 versus C2 (Scheme 1) [14][15][16][17][18]. Biradical BR, in principle, possess two resonance forms, i.e., 1,4-biradical form and 1,6-biradical form. The 1,4-biradical form affords

• oxetane 2OX after the intersystem crossing (ISC). Alternatively, 2,7-dioxabicyclo[2.2.1]hept-5-ene OBH would be formed from the 1,6-biradical form. The regioisomeric oxetane 3OX should be formed via the regioisomeric biradical BR’. Biradical BR is energetically more stable than BR’, because BR can undergo

• radical delocalization. The electrophilic oxygen of the excited carbonyl should preferably interact with more nucleophilic C1 carbon to give selectively the biradical BR. Thus, only the 2-alkoxyoxetane 2OX has been observed in the Paternò–Büchi reactions reported so far [19][20][21][22][23][24][25][26][27

Photocycloaddition of aromatic and aliphatic aldehydes to isoxazoles: Cycloaddition reactivity and stability studies

• Axel G. Griesbeck,
• Marco Franke,
• Jörg Neudörfl and
• Hidehiro Kotaka

Beilstein J. Org. Chem. 2011, 7, 127–134, doi:10.3762/bjoc.7.18

• -controlled formation of the corresponding triplet 1,4-biradical and high stereocontrol due to SOC-controlled crossing from the triplet to the singlet surface [4][5]. Reaction behavior of the photoproducts 9a–c All bicyclic oxetanes obtained in the analytical photochemical experiments as well as in

Synthesis of cationic dibenzosemibullvalene-based phase-transfer catalysts by di-π-methane rearrangements of pyrrolinium-annelated dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 119–126, doi:10.3762/bjoc.7.17

• according to the mechanism shown in Scheme 1 through biradical intermediates BR1 and BR2 and leads to the formation of the corresponding dibenzosemibullvalene (DBS) [6][7]. Notably, the photoreactivity of the dibenzobarrelene system is multiplicity-dependent: Upon photoinduced triplet sensitization in the

Synthesis of spiropyrans: H-abstractions in 3-cycloalkenyloxybenzopyrans

• Satish C. Gupta,
• Mandeep Thakur,
• Somesh Sharma,
• Urmila Berar,
• Surinder Berar and
• Ramesh C. Kamboj

Beilstein J. Org. Chem. 2007, 3, No. 14, doi:10.1186/1860-5397-3-14

• can be visualized as having occurred through an initial abstraction of the O-methine proton of cycloalkenyl group by the excited carbonyl of the pyrone moiety to produce 1,4-biradical 8. The photoproduct 5 (a, b, c, d), is then formed through bond formation between the alkoxy radical and furan (8