Side-product Formation

The NOBS system undergoes an additional reaction that forms a diacyl peroxide as a result of the nucleophilic attack of the peracid anion on the NOBS precursor as shown in equation 21. This undesirable side reaction can be minimized by the use of an excess molar quantity of hydrogen peroxide (91,96) or by the use of shorter dialkyl chain acid derivatives. However, the use of these acid derivatives also appears to result in less efficient bleaching. The dependence of the acid group on the side product formation is apparentiy the result of the proximity of the newly formed peracid to unreacted NOBS in the micellar environment (91). A variety of other peracid precursor stmctures can be found (97—118). 

Complexes of the type 48-53 (Scheme 2.7) have been targeted as pre-catalysts for the hydrosilylation of alkenes [44]. For example, in the hydrosilylation of 1-octene with (Me3SiO)2Si(Me)H, which was studied in detail as a model reaction, the activity of complexes 48-49 with alkyl substituted NHC ligands, is inferior to that of the Karstedt s system. However, selectivity and conversions are dramatically improved due to the suppression of side-product formation. In this reaction... 

Most of the side product formation was caused by the oxidation of the alcohol function, as expected. 

R Dolling, M Beyermann, J Haenel, F Kemchen, E Krause, P Franke, M Brudel, M Bienert. Piperidine-mediated side product formation for Asp(OtBu)-containing peptides. J Chem Soc Chem Commun 853, 1994. 

Scheme 4 Proposed Mechanism of Side-Product Formation during the Coupling of Gly-OPac or Pro-OPac with Boc Amino Acid...

In addition to the mentioned side products, formation of the piperidyl amide of the linear precursors has been observed as deriving from the piperidine-mediated cleavage of the Fmoc/OFm groups used for intermediate protection of the functionalities involved in the cyclization. In these cases, a brief washing with 0.4% (v/v) concentrated aqueous HC1 in DMFt375l or with tertiary amines, e.g. DIPEA/369 prior to cyclization is recommended in order to displace the piperidine. Moreover, both these extra washings were found to shorten the cyclization time to 2 hours with BOP and 0.5 hours with HATU. 

Due to side product formation the 1 b ratio could not be calculated, but <5% of the product mixture consisted of branched aldehyde. 

Performing this reaction at higher concentrations (i.e., in 1-2 L solvent) results in significantly increased reaction times, incomplete reaction, and increased side product formation. 

Primary amines were readily obtained in 60-83% yield after 15 min of microwave irradiation under solvent-free conditions, when using ammonium chloride as the amine source (R=H). When substituted amine hydrochlorides were used, the reaction failed under solvent-free conditions however, when performed in ethanol once more high yields were obtained (80-83%). In both cases, no traces of side-product formation were found. 

Freshly prepared NaBH4-alumina (1.13 g, 3.0 mmol of NaBH4) is thoroughly mixed with neat acetophenone Id (0.36 g, 3.0 mmol) in a test tube and placed in an alumina bath inside the microwave oven and irradiated (30 s). Upon completion of the reaction, monitored on TLC (hexane-EtOAc, 8 2, v/v), the product is extracted into ethylene chloride (2x15 mL). Removal of solvent under reduced pressure essentially provides pure sec-phene thy 1 alcohol 2d in 87% yield. No side product formation is observed in any of the reactions investigated and no reaction takes place in the absence of alumina. 

With the key pyrrole 27 in hand, the team was in a position to complete the synthesis. Hydrolysis and decarboxylation of the tert-butyl ester was initially attempted using the conditions employed in the discovery route for the conversion of 14 to 15. While these reaction conditions effected the hydrolysis and decarboxylation of 27 to 28 in good yield, the formation of impurities resulting from dimerization of the pyrrole was also observed. After screening various acids, the team eventually found that side product formation could be completely suppressed using 1 M H2S04 in 3 1 Me0H/H20 at 65 °C to afford the pyrrole 28 in quantitative yield. 

Evaluate the effect of temperature on side-product formation rate. Can an increase in temperature be beneficial or should one decrease the temperature ... 

Both batch and continuous stirred tank reactors are suitable for reactions that exhibit pseudo-zero-order kinetics with respect to the substrate concentration. In other words, under operating conditions the rate is more or less independent of the concentration of the substrate. However, for reactions where pseudo-first-order kinetics with respect to the concentrations of the substrates prevail, a batch tank reactor is preferred. Batch tank reactors are also ideally suited when there is a likelihood of the reactant slowly deactivating the catalyst or if there is a possibility of side product formation through a parallel reaction pathway. 

The rhodium process has been in commercial use since 1976. In the interim the increasing importance ot environmental, health and safety considerations in industrial processes make the use of less-extreme reaction conditions, catalyst longevity and the minimization of side-product formation of crucial importance. 

The methanol hydrocarbonylaiion is usually carried out in the liquid phase using discontinuous batch reactors and continuous liquid phase processes have also been examined. It was claimed that in these processes, side-product formation can be suppressed and ethanol yields can be improved [23, 50]. Continuous... 

Since a large number of polycarbonates possessing the desired structural features can be prepared from a variety of diols, it is useful to develop simple methods to predict their behavior as resist materials. To this effect, we have devised spectroscopic methods to follow the degradation of the polycarbonates under a variety of thermolysis or acidolysis conditions. For example, the thermolysis of the solid polymers can be followed conveniently by gas-chromato-graphy-mass spectrometry. The thermolysis is a very clean reaction which proceeds as shown in Scheme III without side-product formation. Figure 4 shows the gas chromatographic trace obtained when polymer II is subjected to thermolysis near 250 the products analyzed by the mass spectrometer have the expected structures as shown in Figure 4. 

A synthesis of quinolines from reaction of 2-isopropenylaniline hydrochloride with cyclic ketones was described. The method employs a hydrothermal process with no organic solvents involved <030L1605>. The authors suggest this as an environment-friendly process. The product yields and side product formations are heavily dependent upon reaction temperature. 

Story, S. C. and Aldrich, J. V. (1994) Side-product formation during cyclization with HBTU on a solid support. Int. J. Pept. Protein Res. 43, 292-296. 

Quibell, M., Owen, D., Packman, L. C., and Johnson, T. (1994) Suppression of piperidine-mediated side product formation for Asp(OBut)-containing peptides by the use of N-(2-hydroxy-4-methoxybenzyl)(Hmb) backbone amide protection. J. Chem. Soc. Chem. Commun. 20, 2343—2344. 

The trans-Xo-ana and the reverse quantum yields of 40 were determined to be 0.60 and 0.10, respectively. The coloration/decoloration cycle could be repeated more than 500 times. The quantum yield of side product formation was also determined to be 5 X 10 which agrees with the fatigue resistance of this molecule [41]. Vari-... 

Thermodynamical the skeletal isomerization of alkenes is fiivoured at low tenq>eratures and the rec rocal tenqrerature increases with increating carbon number. The equ rhim concentration of isobutene in the fraction of butenes decreases from ca. 50 % at 200°C to 37% at 500°C [149]. Thus, the convertion of n-butoies into isobutene at these temperatures will be limited by thermodynamic constraints. The skeletal isomerization of the alkenes with more than 4 carbon atoms is a relatively dle reaction step, vdiich is carried out at ca. 290°C over H-Ferrierite [150] or at 340 C over ZSM-5 [151]. This reaction proceeds via the skeletal rearrangement of a carbenium ion yielding a secondary carbenhun ion. The angular reaction meclumism indicates that side product formation can be minimized. Even the skeletal isomerization of C5- and C5-alkanes over Pt-Mordenite, vtiiich is thought to proceed... 

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