Valentim Emilio Uberti Costa

Steric effects on carbon‐13 NMR shifts: carbon–hydrogen bond polarization contributions

The shielding observed in the chemical shifts of carbon atoms subjected to steric compression was originally attributed to carbon–hydrogen bond polarization. There is increasing evidence that this is not universally the case and theoretical studies reveal that changes in dihedral angles may be responsible for many steric effects. Hydrogen shifts, bond lengths and charge distributions were used as probes for these effects in methylnorbornanes and similar tetracyclic norbornyl hydrocarbons. Carbon–hydrogen bond polarization can make a significant contribution to shielding and can be distinguished from effects caused by changes in dihedral angles due to steric congestion.

Frações de fósforo em estrumes e sua eficiência como adubo fosfatado

Manures used as phosphorus sources may have different efficiencies from those of commercial soluble fertilizers. These efficiencies may vary according to the manure origin and to the proportions of its total P in different chemical fractions. A greenhouse experiment was conducted in order to estimate the efficiencies of dairy cattle and chicken manures as phosphorus source, in comparison to triple superphosphate (TSP), as well as to relate these efficiencies to their P fractions. The treatments were: control, without fertilizer or manure; three rates of P, applied as TSP (6.6, 13.2 and 19.7 mg kg-1 soil); and a single phosphorus application rate of 15.3 mg kg-1 soil, from 11 samples of both cattle and chicken manures. Treatment responses were evaluated cultivating millet during 27 days after P application, in pots filled with a Kandiudult soil. Manure P fractions were determined by sequential extractions. The Efficiency Index (EI), estimated by the TSP equivalence, was 0.84 ± 0.071 for chicken manure, significantly lower than that for cattle manure, 0.94 ± 0.095. There were no statistical differences in EI among manures samples of the same animal species. Considering all samples, EI was positively correlated with proportions of labile P (extracted with resin and NaHCO3) and total inorganic P and negatively correlated with proportions extracted with NaOH and HCl and with total organic P. There was no association between EI and the total P content in the manures.

O emprego de lipases como agentes de resolução cinética de enantiômeros em síntese orgânica: aspectos gerais sobre a influência do solvente

In organic synthesis, lipases are the most frequently used biocatalysts. They are efficient stereoselective catalysts in the kinetic resolution of a wide variety of chiral compounds. The discovery that enzymes possess catalytic activity in organic solvents has made it possible to address the question of reaction medium influence on enzymatic specificity. Perhaps the most exciting and significant development in this emerging area is the discovery that enzyme specificity, in particular enantioselectivity, can be affected by changing from one organic solvent to another. This article discusses the scope and possible mechanistic models of this phenomenon in hydrolases, specially lipases, as well as directions of future research in the area.

MIXTURES OF CHIRAL AND ACHIRAL CHEMICAL SHIFT REAGENTS OF EUROPIUM AND THEIR BENEFITS FOR THE DETERMINATION OF ENANTIOMERIC PURITY

For complex compounds, it is usually difficult to identify the signals in the 1H NMR spectra of enantiomeric mixtures. To solve this problem, a combination of chiral and achiral chemical shift reagents of europium is proposed.

An efficient synthesis of enantiopure (+)- and (−)-3-exo-amino-7,7-dimethoxynorbornan-2-exo-ols

Abstract We describe the synthesis of new amino alcohols (+)- and (−)-3- exo -amino-7,7-dimethoxynorbonan-2- exo -ols. The (+)- or (−)-7,7-dimethoxy-1,4,5,6-tetrachlorobicyclo[2.2.1]hept-5-en-2- endo -ol, obtained from the enzyme catalysed transesterification of the racemate, was reduced and dechlorinated (Na/NH 3 ; ethanol), followed by pyridinium chlorochromate oxidation of the resultant alcohols to the corresponding ketones. After treatment with t -BuOK/BuONO, in a nitrosation reaction, α-keto oximes were obtained. Reduction over two steps with NaBH 4 and NaBH 4 /NiCl 2 ·6H 2 O followed by in situ acetylation furnished the corresponding acetamido esters, which were hydrolysed with CH 3 OH/Na to produce the enantiopure amino alcohols in good yields.

Conformational effects on NMR chemical shifts of half-cage alcohols calculated by GIAO-DFT

Abstract Half-cage compounds have played an important role in the investigation of the way steric compression affects physical and chemical properties of organic molecules. Recent theoretical studies of half-cage alcohols have also shown that rotation around the carbon–oxygen bond of the hydroxyl group leads to low-energy conformers in which hyperconjugation affects bond lengths, bond angles, and charge distribution on carbon and hydrogen atoms in its vicinity while charge distribution is also affected by electrostatic effects. Chemical shifts are also sensitive to such variations, but we found that in smaller model systems steric effects may strongly attenuate those due to hyperconjugation so we optimized geometries for low energy rotamers of ‘outside’ and ‘inside’ half-cage alcohols, where these effects can be separated, and calculated their respective hydrogen and carbon-13 chemical shifts by gauge-independent atomic orbital (GIAO) methods at the B3LYP/6-31G(d) level. Results are compared to those obtained for the corresponding norbornyl alcohols as well as for the half-cage hydrocarbon. Carbon-13 chemical shifts respond more strongly to effects owing to hyperconjugation while hydrogen chemical shifts are more sensitive to electrostatic effects due to the proximity of the hydroxyl group.

Evaluation of lipase from Candida rugosa in the resolution of endo-(±)-1,8,9,10,11,11-hexachloropentacyclo[6.2.1.13,6.02,7.05,9]dodecan-4-alcohol

Abstract endo -(±)-1,8,9,10,11,11-Hexachloropentacyclo[6.2.1.1 3,6 .0 2,7 .0 5,9 ]dodecan-4-ol (±)- 7 and exo -(±)-1,8,9,10,11,11-hexachloropentacyclo[6.2.1.1 3,6 .0 2,7 .0 5,9 ]dodecan-4-ol (±)- 4 have been prepared and the enantiomeric enrichment capacity of the lipase from Candida rugosa in the transesterification with vinyl acetate of these compounds was evaluated. It was verified that the lipase recognize only the alcohol (±)- 7 , producing endo -(+)-1,8,9,10,11,11-hexachloropentacyclo[6.2.1.1 3,6 .0 2,7 .0 5,9 ]dodecan-4-yl acetate (+)- 8 with ee >95% and conversion of 44% as the only product.

Structures of half-cage pentacyclododecane derivatives: skeletal deformations due to steric compression-decompression

Abstract Hexachlorinated half-cage isomer acetates may be obtained from isodrin in acetic acid under the proper conditions and separated in highly pure form. The X-ray crystal structure of the “inside” acetate shows that it is still more sterically congested than the previously reported “outside” one. Bond lengths and angles for both isomers reveal large deviations from those normally found in saturated systems providing interesting models for the investigation of steric effects on chemical bonding.

Conformational effects on properties of half-cage compounds

Abstract Derivatives of pentacyclo [6.2.1.1 3,6 .0 2,7 .0 4,10 ] dodecane (or half-cage hydrocarbon) were investigated at the 6-31G* level. Bond lengths and angles as well as charge distributions in conformers of the “outside” alcohol reveal the importance of hyperconjugation of the lone pair on oxygen with suitably oriented neighboring carbon or hydrogen atoms. These alcohols may serve as sensitive probes for electrostatic effects owing to hydroxyl groups and thus for lone pair directionality.

Skeletal and chlorine effects on 13C-NMR chemical shifts of chlorinated polycyclic systems

Com o objetivo de realizar uma analise comparativa do efeito de compressao anelar e do Cl no deslocamento quimico dos carbonos clorados tencionados, foram analisados os deslocamentos quimicos de 27 estruturas policiclicas cloradas e nao cloradas derivadas dos inseticidas “aldrin” (5) e “isodrin” (14). Os compostos foram divididos em quatro grupos de acordo com a conformacao e numero de aneis: tetraciclododecanos endo-exo, tetraciclododecanos endo-endo, pentaciclododecanos e hexaciclododecanos. A comparacao do deslocamento quimico de carbono-13 entre compostos clorados e nao-clorados mostrou que, quando C-9 e C-10 sao carbonos olefinicos, ocorre um deslocamento para baixa frequecia de 0.5-2.4 ppm para os tetraciclododecanos endo-endo e de 4.7-7.6 ppm para os tetraciclododecanos endo-exo. Para C-11, a variacao do deslocamento quimico atinge valores de 49-53 ppm para os tetraciclicos endo-exo e endo-endo, de 54 ppm para o pentaciclico e de 56-59 ppm para os hexaciclicos. A partir desses dados foi possivel observar a influencia da compressao anelar no deslocamento quimico. In order to establish a comparative analysis of chemical shifts caused by ring compression effects or by the presence of a chlorine atom on strained chlorinated carbons, a series of the chlorinated and dechlorinated polycyclic structures derived from “aldrin” (5) and “isodrin” (14) was studied. Compounds were classified in four different groups, according to their conformation and number of ring such as: endo-exo and endo-endo tetracyclics, pentacyclics and hexacyclics. The 13 C chemical shift comparison between the chlorinated and dechlorinated compounds showed that when C-9 and C-10 are olefinic carbons, it occurs a shielding of 0.5-2.4 ppm for endo-endo tetracyclics and of 4.7-7.6 ppm for endo-exo tetracyclic. The chemical shift variation for C-11 reaches 49-53 ppm for endo-exo and endo-endo tetracyclics, 54 ppm for pentacyclic and 56-59 ppm for hexacyclic compounds. From these data, it was possible to observe the influence of ring compression on the chemical shifts.