- The influence of conventional heating and microwave irradiation on the resolution of (RS)-sec-butylamine catalyzed by free or immobilized lipases
-
The lipases CAL-B, PSL, PSL-C, PSL-D, and A. niger lipase, free or immobilized in starch (obtained from two types of yam, known in Brazil as cara? (Discorea alata L.) and inhame (Colocasia esculenta (L.) Schott) or gelatin films, were used in the acylation of (RS)-sec-butylamine with different acyl donors in various organic solvents applying conventional heating (CH) or microwave (MW) irradiation. In the case of free A. niger lipase, the conversion degrees were three times higher using MW irradiation when compared to conventional heating at 35 °C. Using free A. niger lipase, the (R)-amide was obtained with a conversion degree of 21percent, resulting in eep > 99percent and E-value (enantioselectivity value) > 200, in 1 min of reaction under MW irradiation. When the A. niger lipase was immobilized in yam starch films, the (R)-amide was obtained in moderate conversions of 8-25percent after 3 or 5 min of reaction under MW irradiation, but with higher selectivity (eep > 99percent and E > 200) in comparison with the free form (conversion degree of 45percent, eep 81percent and E value of 18).
- Pilissa?o, Cristiane,De Oliveira Carvalho, Patri?cia,Da Grac?a Nascimento, Maria
-
p. 1688 - 1697
(2013/01/15)
-
- Resolution of chiral aliphatic and arylalkyl amines using immobilized Candida antarctica lipase and isolation of their R- and S-enantiomers
-
The resolution of chiral aliphatic and arylalkyl amines in high enantiomeric excess (up to 97.5% ee for the R-enantiomers and up to 99.9% ee for the S-enantiomers) and good yield (50-80%) using immobilized Candida antarctica lipase and ethyl acetate as acyl donor has been demonstrated. A second resolution on the Ramine increased the enantiomeric excess to more than 99.5% (up to 99.9%).
- Davis,Durden
-
p. 569 - 578
(2007/10/03)
-
- Ester Aminolysis: New Reaction Series for the Quantitative Measurement of Steric Effects
-
Further development of theoretical methods for computing steric effects on chemical reactivity requires a large body of new reliable quantitative data for calibration and for testing.We report here on design criteria for reaction series suitable for obtaining these data and on a successful implementation that shows promise of providing access to a particularly broad range of steric hindrance and which additionally has shown a new form of steric hindrance.The series examined in the present study is ester aminolysis in the form RCOOC6H4NO2-p + R'NH2 in acetonitrile solution.A primary purpose of the examination has been to ascertain whether aminolysis may be a useful general series or whether known or unexpected complications might render it unsuitable.We have measured rate constants for a matrix of reactions using five different R groups and four different R' groups, each reaction at a series of concentrations of amine.This is the first systematic study of the sumultaneous action of steric hindrance effects in both the acylating agent and the entering nucleophile.The reactions showed both a second-order term k2 and a relatively less important third-order term k32.The Taft equation was applied to subsets of the rate constants.For each amine there were data for a set of esters for which the R group was the variable.The slopes ρs for these sets were nearly unity.For each ester there were corresponding data for a series of amine reactions in which R' was the variable.These sets also gave good correlations, but the slopes ρs' were considerably larger, about 2.3.This unusually large difference in response to structural effects in the acylating agent and in the nucleophile is unexpected and appears to arise from a new type of steric hindrance.An obvious explanation based on bond lengths proves to be quantitatively insufficient; that explanation postulates that there is greater hindrance for the amine because the C-N bond is short in comparison with the C-C bond.The difference may be due instead to a requirement for special orientation within the transition state, a matter currently under theoretical investigation.The k2 and the k3 sets gave similar correlations, an important finding in at least two respects.It means that steric effects are well-defined in this example of ester aminolysis, and it means also that the extra molecule of amine is far enough from the reaction center so that no additional steric hindrance results.The reactions observed in the present study cover a range of 5 powers of 10 in relative rate constants.Preliminary studies with other examples of aminolysis suggest that a range of relative rate constants covering well in excess of 12 powers of 10 should be observable.
- DeTar, DeLos F.,Delahunty, Claire
-
p. 2734 - 2739
(2007/10/02)
-
- OXIDATION-REDUCTION MECHANISMS - INNER-SPHERE AND OUTER-SPHERE ELECTRON TRANSFER IN THE REDUCTION OF IRON(III), RUTHENIUM(III), AND OSMIUM(III) COMPLEXES BY ALKYL RADICALS. MECHANISMS -
-
Alkyl radicals are readily oxidized by the tris(phenanthroline) and tris(bipyridine) complexes ML//3**3** plus of iron(III), ruthenium(III), and osmium(III) in acetonitrile solution, the second-order rate constants easily exceeding 10**6 M** minus **1s** minus **1 at 25 degree C. Two oxidative processes are identified as (a) ligand substitution on the coordinated 1,10-phenanthroline to yield various alkylphenanthrolines and (b) cation formation to afford alkenes and N-alkylacetamides (after hydrolysis). Cation formation is characterized by extensive skeletal rearrangement of neopentyl, isobutyl, and n-propyl groups, whereas ligand substitution by the same alkyl radicals occurs without any rearrangement.
- Rollick,Kochi
-
p. 1319 - 1330
(2007/10/02)
-
- Alkylation of Nitriles with Gaseous Carbenium Ions. The Ritter Reaction in the Dilute Gas State
-
Radiolytically formed carbenium ions, such as sec-C3H7+, sec-C4H9+, and t-C4H9+, react in the gas phase with model aliphatic and aromatic nitriles yielding the corresponding nitrilium ions.The latter undergo efficient condensation with water that eventually leads to the formation of the corresponding N-alkylamides.The mechanism is analogous to the Ritter reaction in solution.The reactivity and selectivity of the gas-phase electrophilic attack on nitriles has been deduced from competition experiments under conditions that largely exclude the effects of solvation, ion pairing, etc., which complicate the interpretation of solution-chemistry measurements.
- Cacace, Fulvio,Ciranni, Giovanna,Giacomello, Pierluigi
-
p. 2258 - 2261
(2007/10/02)
-
- The Effect of Electrochemically Generated Positive Bromine Species in Acetonitrile on the Cleavage of C-Br and C-Cl Bonds.
-
The fate of bromine formed from C-Br cleavage during the course of anodic oxidation of alkyl bromides in acetonitrile on platinum has been investigated potentiostatically.It is suggested that positive bromine species are formed and they are potentially reactive towards alkyl bromides, yielding similar products to those obtained by direct anodic oxidation of the same bromides.Furthermore, whereas alkyl chlorides do not undergo C-Cl fission by direct anodic oxidation it is shown that positive bromine species are energetically sufficient to break C-Cl bonds, although not very efficiently.We suggest that a possible structure for the complex between acetonitrile and positive bromine species is mainly +Br3- and the mechanism for its formation is discussed.The spectrum of this species found identical to that of Br3- (269 nm) in acetonitrile.
- Becker, James Y.,Zemach, Dvora
-
p. 336 - 340
(2007/10/02)
-