The application of a monolithic triphenylphosphine reagent for conducting Appel reactions in flow microreactors

• Kimberley A. Roper,
• Heiko Lange,
• Anastasios Polyzos,
• Malcolm B. Berry,
• Ian R. Baxendale and
• Steven V. Ley

Beilstein J. Org. Chem. 2011, 7, 1648–1655, doi:10.3762/bjoc.7.194

• initiator as it was found to be soluble in the polymerisation mixture at the stock solution temperature of 50 °C, giving a homogeneous mixture. The slower initiation rate compared to azo-based initiators also ensured the entire polymerisation mixture was at the target temperature before precipitation of

Natural product biosyntheses in cyanobacteria: A treasure trove of unique enzymes

• Jan-Christoph Kehr,
• Douglas Gatte Picchi and
• Elke Dittmann

Beilstein J. Org. Chem. 2011, 7, 1622–1635, doi:10.3762/bjoc.7.191

• integrated formyl transferase domain in the initiation module and NAD-dependent halogenase. The formation of Ahp remains to be analyzed. Aeruginosin Aeruginosins are specific inhibitors of serine type proteases and produced by different genera of freshwater cyanobacteria. The strain Planktothrix agardhii

• product. The initiation module of curacin biosynthesis contains a GCN5-related N-acetyltransferase (GNAT) domain. These enzymes typically catalyze acyl transfer to a primary amine. The curacin GNAT, however, was shown to be bifunctional and to exhibit decarboxylase/S-acetyltransferase activities [45]. The

• development of cancer therapeutics. The corresponding biosynthetic pathway comprises three PKS and two NRPS-type enzymes [57]. The chain-initiation module of the enzyme CrpA closely resembles the loading module of microcystin biosynthesis where phenylpropanoic acids are activated and finally phenylacetate is

The catalytic performance of Ru–NHC alkylidene complexes: PCy₃ versus pyridine as the dissociating ligand

• Stefan Krehl,
• Diana Geißler,
• Sylvia Hauke,
• Oliver Kunz,
• Lucia Staude and
• Bernd Schmidt

Beilstein J. Org. Chem. 2010, 6, 1188–1198, doi:10.3762/bjoc.6.136

• the results summarized in Figure 2, catalyst H gave only 34% conversion in this solvent after one hour, suggesting that initiation was rather slow. In acetic acid, the analogous phosphine containing indenylidene catalyst E displayed a slightly reduced conversion of 89%, which might be attributed to

• slow catalyst deactivation, whereas pyridine complex H showed significantly enhanced activity in acetic acid, with a conversion of 68% after one hour. This result suggests that by switching from toluene to acetic acid the balance of catalyst deactivation and enhanced initiation is shifted to the

• deactivation side for phosphine catalyst E, and to the enhanced initiation side for pyridine catalyst H. The most remarkable result from this set of experiments, however, is a collapse of conversion if benzylidene catalyst D is used in acetic acid. With D, a reproducible conversion of only 9% to the

New library of aminosulfonyl-tagged Hoveyda–Grubbs type complexes: Synthesis, kinetic studies and activity in olefin metathesis transformations

• Etienne Borré,
• Frederic Caijo,
• Christophe Crévisy and
• Marc Mauduit

Beilstein J. Org. Chem. 2010, 6, 1159–1166, doi:10.3762/bjoc.6.132

• studies. Comparison between catalysts was done at the initiation step. Moreover, conversions were compared over a reaction time of one hour. The 2-allyl-2-methallylmalonate 8 is usually used as benchmark substrate for ring-closing metathesis, inasmuch it shows significant differences between an activated

• Hoveyda-like complexes. Complexes 4a, 4b and 4f can be considered to be activated catalysts, while catalyst 4g bearing a more sterically demanding NHC (SIPr) ligand shows a faster initiation compared to its SIMes analogue catalyst 4b. Additionally, 4g gave the best conversion over a reaction time of one

About the activity and selectivity of less well-known metathesis catalysts during ADMET polymerizations

• Hatice Mutlu,
• Lucas Montero de Espinosa,
• Oĝuz Türünç and
• Michael A. R. Meier

Beilstein J. Org. Chem. 2010, 6, 1149–1158, doi:10.3762/bjoc.6.131

• complex (C1) [41]. High temperatures allow for better ligand dissociation, and hence for a higher initiation rate of C1 in RCM [33][35]. Moreover, good activities were obtained in the self-metathesis reaction of undecylenic aldehyde, a renewable building block derived from castor oil cracking [42

• other hand, when the temperature is raised to 120 ºC, the catalysts degradation could be compensated by an extremely fast initiation and short-term propagation promoted by the high temperature, giving as a result high molecular weight polymers before degradation of the catalysts occurs. This

Miniemulsion polymerization as a versatile tool for the synthesis of functionalized polymers

• Daniel Crespy and
• Katharina Landfester

Beilstein J. Org. Chem. 2010, 6, 1132–1148, doi:10.3762/bjoc.6.130

• distribution when the monomers were polymerized by γ-rays compared to initiation with potassium peroxodisulfate. The presence of carboxylic acid groups at the surface of the particles was confirmed by X-ray photoelectron and FT-IR spectroscopy. Finally, graft hybrid copolymers of polyurethane and polymers from

Backbone tuning in indenylidene–ruthenium complexes bearing an unsaturated N-heterocyclic carbene

• César A. Urbina-Blanco,
• Xavier Bantreil,
• Hervé Clavier,
• Alexandra M. Z. Slawin and
• Steven P. Nolan

Beilstein J. Org. Chem. 2010, 6, 1120–1126, doi:10.3762/bjoc.6.128

• catalytic activity, the possibility of fine-tuning of NHC steric and electronic properties has been exploited. Bulkier and more electron-donating NHCs allow faster initiation with usually a concurrent increase in reaction rate when the olefin substrate is of low steric hindrance [13][14][15][16][17]. Less

• -electron active species and allow better catalytic activity, the faster initiation is also related to faster catalyst decomposition; at 80 °C, this deactivation contributes considerably to the catalytic outcome. In conclusion, we suggest that 6d represents the most advantageous catalyst owing to its

• improved stability, which is attributed to reduced initiation from poorer electron-donating ability of the NHC ligand. Conclusion The effects of modulating the nature of substituents on the backbone (C4 and C5) positions of the IMes ligand have permitted a quantification of the electronic and steric

Light-induced olefin metathesis

• Yuval Vidavsky and
• N. Gabriel Lemcoff

Beilstein J. Org. Chem. 2010, 6, 1106–1119, doi:10.3762/bjoc.6.127

• leads to the active species. The mechanism for initiation of the sandwich compounds was proposed to proceed through gradual photodissociation of the arene ligand, followed by solvation of the photochemically excited molecule (Scheme 3). Both the intermediate half-sandwich species [(η6-arene)Ru(solvent)3

• complexes are proposed to be activated by a similar mechanism where several species of the type [Ru(RCN)6−x(H2O)x] are responsible for the polymerisation initiation. Disadvantages of these catalysts are the overall moderate activities achieved and that they are only soluble in polar solvents, due to their

Halide exchanged Hoveyda-type complexes in olefin metathesis

• Julia Wappel,
• César A. Urbina-Blanco,
• Mudassar Abbas,
• Jörg H. Albering,
• Robert Saf,
• Steven P. Nolan and
• Christian Slugovc

Beilstein J. Org. Chem. 2010, 6, 1091–1098, doi:10.3762/bjoc.6.125

• ] and pseudohalides [14] as well as halides other than chloride [15][16][17][18][19]. An early study dealing with the change of reactivity upon exchanging the chloride ligands in G2 for bromides and iodides showed increasing initiation rates (phosphine dissociation is facilitated), but decreasing

• ratio of initiation rate to propagation rate (ki/kp) of a given initiator and monomer combination [30]. Polymers made with M2 and M31 were used for further comparison. M2 (ki/kp ≈ 1–0.01) is a typical initiator, producing in most cases polymers with high Mn values and high polydispersity indices (PDI

• propagation rate constant decreases with increasing steric demand of the halo ligands (i.e., kp(1) > kp(2) >> kp(3)) and the ratio of initiation rate to propagation rate increases on going from 1 to 2 (i.e., ki/kp(1) < ki/kp(2) ≈ ki/kp(3) > 1) but remains of the same order of magnitude. By studying the

Hybrid biofunctional nanostructures as stimuli-responsive catalytic systems

• Gernot U. Marten,
• Thorsten Gelbrich and
• Annette M. Schmidt

Beilstein J. Org. Chem. 2010, 6, 922–931, doi:10.3762/bjoc.6.98

• chlorine groups for subsequent initiation of atom transfer radical polymerization (ATRP). Oligo(ethylene glycol) methyl ether methacrylates with different length of the hydrophilic side chain are used as the main monomer to generate a hydrophilic polymer shell with tunable critical solution behavior in

Synthesis and crystal structures of multifunctional tosylates as basis for star-shaped poly(2-ethyl-2-oxazoline)s

• Richard Hoogenboom,
• Martin W. M. Fijten,
• Guido Kickelbick and
• Ulrich S. Schubert

Beilstein J. Org. Chem. 2010, 6, 773–783, doi:10.3762/bjoc.6.96

• reported. The use of tetra- and hexa-tosylates, based on (di)pentaerythritol as initiators for the CROP of 2-ethyl-2-oxazoline, resulted in very slow initiation and ill-defined polymers, which is most likely caused by steric hindrance in these initiators. As a consequence, a porphyrin-cored tetra-tosylate

• introduction of specific end-groups by initiation and termination [4]. Moreover, variation of the 2-substituent of the 2-oxazoline results in a variety of the amidic side chains of the poly(2-oxazoline)s [5][6], which strongly influences the properties of the resulting polymers: ranging from hydrophilic to

• initiators as well as their use for the initiation of the CROP of 2-ethyl-2-oxazoline for the formation of star-shaped poly(2-oxazoline)s. Results and Discussion Multi-tosylate preparation The preparation of tosylates from alcohols is a straightforward synthetic procedure using p-toluenesulfonic acid

Aromatic and heterocyclic perfluoroalkyl sulfides. Methods of preparation

• Vladimir N. Boiko

Beilstein J. Org. Chem. 2010, 6, 880–921, doi:10.3762/bjoc.6.88

• disulfides with Li/liquid NH3 (Table 8), in much the same way as the described above for RFI [158][159]. 4.1.5. Other methods of initiating From the knowledge that the reaction mechanism is a single-electron transfer process involving RF• radicals, alternative methods to photochemical initiation have been

Poly(glycolide) multi-arm star polymers: Improved solubility via limited arm length

• Florian K. Wolf,
• Anna M. Fischer and
• Holger Frey

Beilstein J. Org. Chem. 2010, 6, No. 67, doi:10.3762/bjoc.6.67

• controlled synthesis of the multi-arm star polymers in order to avoid initiation by trace amounts of water, which leads to concurrent glycolide homopolymerization and thus an undesired mixture of linear and star-like PGAs. In order to prevent possible precipitation from solution, the polymerization was

• elution volume (Figure 2) which is in line with expectations. These apparent impurities could be caused by compounds capable of co-initiation such as water or other hydroxy functionalities. Table 1 illustrates the correlation between theoretical molecular weight and values obtained from SEC measurements

• homopolymer [38]. It can be observed for samples that exhibit an additional mode in SEC. This therefore supports the assumed formation of PGA homopolymer by co-initiation with water. Despite careful drying of the hygroscopic PG macroinitiator in vacuo, contamination with water could obviously not be fully

Synthesis and crossover reaction of TEMPO containing block copolymer via ROMP

• Olubummo Adekunle,
• Susanne Tanner and
• Wolfgang H. Binder

Beilstein J. Org. Chem. 2010, 6, No. 59, doi:10.3762/bjoc.6.59

• molybdenum catalysts [7][8][9]. Often, the polydispersity indices of the resulting polymers obtained with initiator G1 are large with values ranging between 1.3 and 1.5 arising from an unfavorable rate of initiation (ki) relative to propagation (kp) as well as considerable secondary metathesis (backbiting

• ). Grubbs’ second generation catalyst G2 displays an activity comparable to the Schrock type initiators. It exhibits a higher functional group tolerance than G1, but initiation by catalyst G2 is often slow as a result of the slow dissociation of the phosphine group, sometimes limiting its application in

• polymer synthesis. Alternatively, Grubbs’ third generation catalyst G3 introduced by Grubbs et al. [10] has an ultrafast initiating ruthenium benzylidene. The rate of reaction of G3 with ethyl vinyl ether thus is six orders of magnitude higher than for G2 [10], leading to a faster initiation and often

From discovery to production: Scale- out of continuous flow meso reactors

• Peter Styring and
• Ana I. R. Parracho

Beilstein J. Org. Chem. 2009, 5, No. 29, doi:10.3762/bjoc.5.29

• product so the possibility arises that the catalyst takes an alternative pathway (route B) once the catalyst initiation has occurred. As the cross-coupled product is observed as the highest yielding product, it was proposed that the rate constant for route A is smaller than for route B. The proposed

Radical cascades using enantioenriched 7-azabenzonorbornenes and their applications in synthesis

• David M. Hodgson and
• Leonard H. Winning

Beilstein J. Org. Chem. 2008, 4, No. 38, doi:10.3762/bjoc.4.38

• under conditions [refluxing toluene, thermal initiation by AIBN] similar to those we had previously reported for the tandem deoxygenation–rearrangement–reduction of 7-azabenzonorbornenyl xanthates (e.g. 5 to 8, Scheme 2), although the work in our laboratory had employed a syringe pump in order to

Stereoselective synthesis of (2S,3S,4Z)-4-fluoro- 1,3-dihydroxy- 2-(octadecanoylamino)octadec- 4-ene, [(Z)-4-fluoroceramide], and its phase behavior at the air/water interface

• Gergana S. Nikolova and
• Günter Haufe

Beilstein J. Org. Chem. 2008, 4, No. 12, doi:10.3762/bjoc.4.12

• [12][13]. The specific initiation of apoptosis by suitable derivatives of these signal molecules is discussed as a new method for treatment of numerous diseases [1][14][15], and of cancer in particular [16][17][18]. A few years ago Herdewijn et al. showed that fluorinated ceramide and dihydroceramide

Part 3. Triethylborane- air: a suitable initiator for intermolecular radical additions of S-2-oxoalkyl- thionocarbonates (S-xanthates) to olefins

• Jean Boivin and
• Van Tai Nguyen

Beilstein J. Org. Chem. 2007, 3, No. 47, doi:10.1186/1860-5397-3-47

• viewpoint, the results reported herein may be rationalised as follows (Scheme 2). The initiation of the process is governed by interaction of dioxygen with Et3B to give Et•. This reaction occurs within a wide range of temperatures. The reaction of ethyl radical with the highly radicophilic species A leads