The chemical behavior of terminally tert-butylated polyolefins

• Dagmar Klein,
• Henning Hopf,
• Peter G. Jones,
• Ina Dix and
• Ralf Hänel

Beilstein J. Org. Chem. 2015, 11, 1246–1258, doi:10.3762/bjoc.11.139

• formation of 52 must be explained by a different mechanism. One alternative could be the photochemical generation of a diradical from the conjugated oligoene 19 and interception of the former by the oxygen present in the reaction solution. Interestingly, when the solution is degassed before irradiation and

Recent applications of the divinylcyclopropane–cycloheptadiene rearrangement in organic synthesis

• Sebastian Krüger and
• Tanja Gaich

Beilstein J. Org. Chem. 2014, 10, 163–193, doi:10.3762/bjoc.10.14

• isomerization to the desired cis-isomer 9 at elevated temperature (≈200 °C [1][2], lowered for more conjugated systems). The isomerization pathways have been suggested to proceed either via the formation of intermediate diradical-species (pathway A, Scheme 3) [16][20][26][27] or through one-center epimerization

• (pathway B) [28][29]. Following pathway A, the C1–C2 bond of 15 is cleaved homolytically to give diradical 16. The two radicals are stabilized as allylic radicals (depicted as 16'), rotation around the C1–C3 bond takes place (16' to 16'') followed by radical recombination to give cis-divinylcyclopropane (9

• ). Pathway B proceeds through the formation of planar allylic anion 17, which undergoes inversion to give cis-divinylcyclopropane (9). An alternative reaction pathway of the trans-divinylcyclopropane (15) to yield the cycloheptadiene product is the direct formation of the seven membered ring from diradical

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

• diradical 9 [27]. The latter can then either undergo ring closure to form indan-2-one (IN), or decarbonylate to give o-xylylene (XY). The equilibrium of XY and benzocyclobutene (BC) is established in the literature [28], as well as the formation of styrene ST from BC [29]. An alternative mechanism, the

Substituent effect on the energy barrier for σ-bond formation from π-single-bonded species, singlet 2,2-dialkoxycyclopentane-1,3-diyls

• Jianhuai Ye,
• Yoshihisa Fujiwara and
• Manabu Abe

Beilstein J. Org. Chem. 2013, 9, 925–933, doi:10.3762/bjoc.9.106

• formation process) were determined by the temperature-dependent change of the lifetime. The energy barrier was found to be largely dependent upon the substituents Ar and Ar’. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl, OCH2Ar’ = OCH2(3,5-dimethoxyphenyl)) was the longest-lived, τ293 = 5394 ± 59 ns

• , among the diradicals studied here. The lifetime of the parent diradical DR (Ar = Ph, OCH2Ar’ = OCH3) was 299 ± 2 ns at 293 K. Conclusion: The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar’ at the C(1)–C(3) positions. Both the enthalpy and

• -diyls [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and cyclopentane-1,3-diyls [17][21][22][23][24][25][26]. Detailed experimental study of singlet diradical chemistry is thus now possible using the long-lived localized singlet diradicals. So far, we have studied singlet diradical

Polar reactions of acyclic conjugated bisallenes

• Reiner Stamm and
• Henning Hopf

Beilstein J. Org. Chem. 2013, 9, 36–48, doi:10.3762/bjoc.9.5

• previously by Pasto [26]. In the first step, oxygen attacks the substrate under formation of the diradical 42. This, in turn, closes from its resonance structure 43 to yield the isolated product 44. Had the oxidation taken place at the unsubstituted allene moiety of 3, the tert-butyl substituent would have

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

• -shift to provide the diradical intermediate 90. In the next step the ring is split to furnish derivative 91, which, by another TMS-shift, isomerizes to the propargylallene 92. The sequence ends with still another TMS-relocation to provide the final product 88. Both, 91 and 92 are pyrolysis products

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

• cyclic carbonates 1a and 1b (Scheme 1) generate products derived from 1,3-diradical intermediates [14]. Interestingly, the photoinduced fragmentation reactions of nitrogen- and oxygen-containing functionalities have been studied intensively, whereas reports on photoextrusion reactions of sulfur

• glycol sulfite (8) and the hydrobenzoin sulfite (9); and we herein demonstrate that diradical intermediates are generated during the photoextrusion processes. Results and Discussion Styrene glycol sulfite 8 was prepared from thionyl chloride and styrene glycol in the presence of triethylamine [19]. The

• decarbonylation results in an alkyl radical. The latter reaction yields bibenzyl (4) and toluene (7) in the photoreaction of 8. It should be noted that the diradical BR2 may be formed also upon photoinduced ring-opening reaction of an intermediate oxirane 2a, which is not detectable in the reaction mixture

Valence isomerization of cyclohepta-1,3,5-triene and its heteroelement analogues

• Helen Jansen,
• J. Chris Slootweg and
• Koop Lammertsma

Beilstein J. Org. Chem. 2011, 7, 1713–1721, doi:10.3762/bjoc.7.201

• -triene [14]. Besides the 1–2 interconversion, the C7H8 system is rich in rearrangements (Scheme 3). In 1957, Woods found that bicyclo[2.2.1]hepta-2,5-diene (12) converts to cycloheptatriene (1), which was postulated to proceed via diradical 11 and norcaradiene (2) [28]. Instead, pyrolysis of 1 yielded

• toluene, presumably through a [1,3]-H shift of the diradical [29]. Norcaradiene (2) can also undergo a [1,5]-carbon circumambulatory rearrangement (“walk”), as was discovered by Berson and Willcott in 1965 [30][31]. Although, this process should proceed with retention of the configuration according to the

Fine-tuning alkyne cycloadditions: Insights into photochemistry responsible for the double-strand DNA cleavage via structural perturbations in diaryl alkyne conjugates

• Wang-Yong Yang,
• Samantha A. Marrone,
• Nalisha Minors,
• Diego A. R. Zorio and
• Igor V. Alabugin

Beilstein J. Org. Chem. 2011, 7, 813–823, doi:10.3762/bjoc.7.93

• ]. The mechanism of triplet photocycloaddition involves a sequence of radical closures initiated by the formation of a triplet 1,4-diradical via the reaction of 1,4-CHD and the alkyne π,π*-triplet state. Although several plausible mechanistic pathways converge at the same homoquadricyclane product in

• upon the addition of a drop of water suggest that benzoxazines are also the intermediate products in our case but are rapidly hydrolyzed during work-up and purification. Although one can suggest the intermediacy of the triplet diradical in the photocyclization of o-amido acetylene 6, this

Intraannular photoreactions in pseudo-geminally substituted [2.2]paracyclophanes

• Henning Hopf,
• Vitaly Raev and
• Peter G. Jones

Beilstein J. Org. Chem. 2011, 7, 658–667, doi:10.3762/bjoc.7.78

• -diastereomer. If this process took place, it would involve the diradical 20, which could isomerize to 21 with release of strain. The process could also occur a second time to provide a pseudo-geminally substituted [2.2]paracyclophane, now carrying two cyclopentenyl substituents. Should these ring-enlarged

• paracyclophanes not be observed, this would not necessarily constitute a proof against diradical(oid) intermediates in these reactions. However, if derivatives such as 21 were among the photoproducts the involvement of radicals in the photoisomerizations would be indicated. We therefore reacted the bis-aldehyde 9

• diradical intermediate of type 17, its lifetime is evidently too short to allow ring-expansion as depicted in Scheme 7. Whether this process might be induced thermally (vinylcyclopropane→cyclopentene rearrangement; [23]) is an open question. Conclusion Although the detailed mechanisms of the

Rh-Catalyzed rearrangement of vinylcyclopropane to 1,3-diene units attached to N-heterocycles

• Franca M. Cordero,
• Carolina Vurchio,
• Stefano Cicchi,
• Armin de Meijere and
• Alberto Brandi

Beilstein J. Org. Chem. 2011, 7, 298–303, doi:10.3762/bjoc.7.39

• shift in the cyclopropanated 1,6-diradical intermediate, which in this case is probably facilitated by the enhanced mobility of the benzylic hydrogen and by the formation of the conjugationally stabilized imine 9 [11]. The 1,3-dipolar cycloaddition/thermal rearrangement domino reaction of BCP (2) with

Heavy atom effects in the Paternò–Büchi reaction of pyrimidine derivatives with 4,4’-disubstituted benzophenones

• Feng-Feng Kong,
• Jian-Bo Wang and
• Qin-Hua Song

Beilstein J. Org. Chem. 2011, 7, 113–118, doi:10.3762/bjoc.7.16

• 1c) is enthalpy–entropy controlled. A heavy atom effect is suggested to be responsible for these unusual phenomena based on the triplet-diradical mechanism of the Paternò–Büchi reaction. Keywords: benzophenone; heavy atom effect; Paternò–Büchi reaction; regioselectivity; triplet diradical

• (singlet or triplet or both) on a ground-state olefin. For aromatic carbonyl compounds, the reaction is a triplet cycloaddition, that is, a triplet-excited carbonyl compound adding to an olefin to yield a triplet 1,4-diradical intermediate, which undergoes intersystem crossing (ISC) to produce a singlet

• 1,4-diradical. Ring-close of the latter gives an oxetane. The higher selectivity observed for the triplet reaction is rationalized by the optional conformation of the intermediate 2-oxabutane-1,4-diyl for ISC to the singlet diradical, which is preferentially controlled by spin-orbit coupling, thus

En route to photoaffinity labeling of the bacterial lectin FimH

• Thisbe K. Lindhorst,
• Michaela Märten,
• Andreas Fuchs and
• Stefan D. Knight

Beilstein J. Org. Chem. 2010, 6, 810–822, doi:10.3762/bjoc.6.91

• of the benzophenone 3, on the other hand, delivers a reactive triplet diradical. In addition, in order to combine a photoactive functional group with an affinity label within the same mannoside, the orthogonally protected glycoamino acid scaffold 4 was synthesized and used for the preparation of the