Damage of polyesters by the atmospheric free radical oxidant NO₃^•: a product study involving model systems

• Catrin Goeschen and
• Uta Wille

Beilstein J. Org. Chem. 2013, 9, 1907–1916, doi:10.3762/bjoc.9.225

• these systems. Thus, CAN photolysis generates NO3• in the presence of excess NO3−, which acts as Brønsted base and mediates deprotonation of the initial radical cation 3•+ to give benzyl radical 7 [23], followed by transformation to the products 4–6. In the NO2•/O3 system, on the other hand, [NO3−] = [3

• intermediate radical cation has a lifetime on the nanosecond time scale [23]. It was further demonstrated that deprotonation of arylradical cations is accelerated by nitrate (NO3−) that is present in the reaction system as ‘byproduct’ of the oxidation process and as ligand in CAN, and which acts as a Brønsted

• base [23]. It is important to note that the formation of radical intermediate 7 could principally also occur in one step through NO3•-induced benzylic HAT in 3 (not shown). However, it appears from the outcome of the reactions with the neopentyl derivatives of 1 and 2 that HAT by NO3• is not

Synthesis of the reported structure of piperazirum using a nitro-Mannich reaction as the key stereochemical determining step

• James C. Anderson,
• Andreas S. Kalogirou,
• Michael J. Porter and
• Graham J. Tizzard

Beilstein J. Org. Chem. 2013, 9, 1737–1744, doi:10.3762/bjoc.9.200

• . Enantioselective reactions have been controlled by asymmetric metal-centred Lewis acids; chiral hydrogen bond donors, in particular by the use of asymmetric thiourea organocatalysts, chiral Brønsted acids, phase-transfer catalysts and Brønsted base catalysts [3][15][25][26][27][28][29][30][31][32][33][34][35][36

Organocatalytic asymmetric Michael addition of unprotected 3-substituted oxindoles to 1,4-naphthoquinone

• Jin-Sheng Yu,
• Feng Zhou,
• Yun-Lin Liu and
• Jian Zhou

Beilstein J. Org. Chem. 2012, 8, 1360–1365, doi:10.3762/bjoc.8.157

• TLC and NMR analysis of the crude reaction mixture. While the simple quinine and quinidine as catalysts could deliver product 3a in 59% ee (Table 1, entry 2), all other bifunctional catalysts turned out to be much less enantioselective (Table 1, entries 3–5). However, the dinuclear Brønsted base

Organocatalysis

• Benjamin List

Beilstein J. Org. Chem. 2012, 8, 1358–1359, doi:10.3762/bjoc.8.156

• -molecular-weight organic compounds, in which a metal is not part of the active principle. Organocatalysts donate or remove electrons or protons as their activation mode, hence defining four distinct subareas: Lewis base and Lewis acid catalysis on the one hand, and Brønsted base and Brønsted acid catalysis

Control over molecular motion using the cis–trans photoisomerization of the azo group

• Estíbaliz Merino and
• María Ribagorda

Beilstein J. Org. Chem. 2012, 8, 1071–1090, doi:10.3762/bjoc.8.119

• (ortho or meta to N=N) by irradiation with light causes a determined geometric, conformational and rigid change, inducing a specific phototropism as in the stem of a sunflower. Photoactive Brønsted base The conformation of a molecule can have direct implications for its reactivity. Thus, the control of

• ] designed a Brønsted base whose pKa changes with light. The study focuses on the azobenzene 22, which possesses, in an aromatic ring, a spirocyclic lactone fused to a conformationally restricted piperidine (Figure 19). In the structure of the trans isomer 22, the pair of unshared electrons of nitrogen is

• inaccessible. The trans–cis photoisomerization process changes the disposition of the aromatic rings, unlocking access to the basic center of the piperidine. This switch, of Brønsted base type, has been tested in the Henry reaction between p-nitrobenzaldehyde and nitroethane ensuring that only the cis isomer

C₂-symmetric bisamidines: Chiral Brønsted bases catalysing the Diels- Alder reaction of anthrones

• Deniz Akalay,
• Gerd Dürner,
• Jan W. Bats and
• Michael W. Göbel

Beilstein J. Org. Chem. 2008, 4, No. 28, doi:10.3762/bjoc.4.28

• a cycloaddition step stereoselectively controlled by the chiral ion pair. Keywords: Asymmetric Catalysis; Bisamidines; Brønsted base; Diels-Alder reaction; Organocatalysis; Introduction The cycloadditions of anthrones 1 and N-substituted maleimides 2 are prominent examples of asymmetric catalysis

• temperature. In the absence of catalyst, no product could be observed after 4 days. 5 mol% of the bisamidinium salt 8a·H+ with tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (TFPB-) as weakly coordinating anion resulted in 7% yield of 3a after 4 h. In contrast, only 1 mol% of the free Brønsted base 8a led to

• -symmetric bisamidines were shown to be potent chiral Brønsted base catalysts for the Diels-Alder reaction of N-substituted maleimides and anthrones. Compared to bisoxazolines 7, much shorter reaction times under comparable conditions were sufficient with the more basic bisamidine catalysts 8 (~50-fold [5