Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H2 Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton-Transfer Dynamics of the Boroalkoxide Intermediate

2017 ◽  
Vol 23 (38) ◽  
pp. 9098-9113 ◽  
Author(s):  
Mojgan Heshmat ◽  
Timofei Privalov
Keyword(s):  
Carbon Atom ◽  
Proton Transfer ◽  
Lewis Acid ◽  
Transition States ◽  
Carbonyl Carbon Atom ◽  
Carbonyl Carbon

Argon Matrix Infrared Spectroscopic Evidence for the Generation of Pentatetraenone by Flash Pyrolysis of Suitable Precursors

1988 ◽  
Vol 41 (2) ◽  
pp. 225 ◽  
Author(s):  
RFC Brown ◽  
KJ Coulston ◽  
FW Eastwood ◽  
MJ Irvine ◽  
ADE Pullin
Keyword(s):  
Carbon Atom ◽  
Infrared Spectra ◽  
Spectral Region ◽  
Narrow Band ◽  
Broad Band ◽  
Carbonyl Carbon Atom ◽  
Carbonyl Carbon ◽  
Argon Matrix ◽  
Spectroscopic Evidence ◽  
Argon Mixture

Five compounds were investigated as precursors for the pyrolytic generation of pentatetraenone, H2C=C=C=C=C=O. These were (1)-(4): 3- ethenylidenebicyclo [2.2.1]hept-5-ene with the following 2,2 substituents : H, COOCOCF3 (1); H, 13COOCOCF3 (1′); (COOCOCF3)2 (2); (COO)2C(CH3)(OCH3) (3); (COO)2Si(CH3)2 (4) and 5-(3′- methylenebicyclo [2.2.1]hept-5′-en-2′-ylidene)-2,2-dimethyl-1,3-dioxan-4,6-dione (5). The five precursors were pyrolysed in a stream of argon at temperatures in the range 350-725°C and the pyrolysate -argon mixture condensed on a CsI plate at c. 10 K. Infrared spectra were obtained between 4000 and 250 cm-1. All five precursors gave two strong bands in the spectral region 2070-2250 cm-1, possibly attributable to pentatetraenone. At lower pyrolytic temperatures the more intense of the two bands was a broad band centred at c. 2128 cm-1 [precursors (1)- (4)] or at c. 2094 cm-1 [precursor (5)]. At higher pyrolytic temperatures these bands were diminished in intensity and replaced by a narrow band at 2207 cm-1 for all five precursors. Bands due to the expected other products for each pyrolysis reaction to form pentatetraenone were observed. H2C413CO ( pentatetraenone substituted by 13C at the carbonyl carbon atom) was prepared by pyrolysis of precursor (1′). We assign the broad bands at c. 2128 cm-1 [precursors (1)-(4)] and at c. 2094 [precursor (5)] to incompletely pyrolysed precursor in which cyclopentadiene has been retained but decomposition in the rest of the molecule has resulted in formation of a =C=C=O group. Bands at 2207, 2068 and 1726 cm-1 we assign to v2-v4 of pentatetraenone. Corresponding bands at 2168, 2056 and 1720 cm-1 are observed in the spectrum of H2C413CO.

Aril Azines. II. Hydrogenation of p-Tolil Monoazine

1975 ◽  
Vol 53 (5) ◽  
pp. 748-752 ◽  
Author(s):  
Peter Yates ◽  
E. M. Levi
Keyword(s):  
Hydrogen Atom ◽  
Carbon Atom ◽  
Sodium Borohydride ◽  
Sodium Methoxide ◽  
Major Product ◽  
Lead Tetraacetate ◽  
Ring Closure ◽  
Carbonyl Carbon Atom ◽  
Carbonyl Carbon ◽  
Independent Synthesis

Hydrogenation of p-tolil monoazine (1b) over palladium-on-charcoal gives as the major product 4,5-dihydro-5-(p-toluyl)-3,4,5-tri-(p-tolyl)-1H-pyrazol-4-ol (2b), which has previously been obtained by treatment of 1b with sodium methoxide. Several minor products are formed, which include p-tolualdehyde, p-toluic acid, and p-toluamide, p-tolunitrile, p-tolualazine, and 3,4,5-tri-(p-tolyl)-4H-pyrazo-4-ol (9). The structure of the last compound, which is also formed on reduction of 1b with sodium borohydride, was established by its independent synthesis from 1,2,3-tri-(p-tolyl)-1,3-propanedione by oxidation with lead tetraacetate followed by treatment with hydrazine. It is suggested that 2b arises via reduction of a C=N bond of 1b and aldol ring closure. The minor hydrogenation products are of interest in that their formation involves C—C hydrogenolysis; it is suggested that this is initiated by addition of a hydrogen atom to a carbonyl carbon atom of 1b.

scholarly journals Mechanism of acyl transfer by the class A serine β-lactamase of Streptomyces albus G

1991 ◽  
Vol 279 (1) ◽  
pp. 213-221 ◽  
Author(s):  
J Lamotte-Brasseur ◽  
G Dive ◽  
O Dideberg ◽  
P Charlier ◽  
J M Frère ◽  
...  
Keyword(s):  
Carbon Atom ◽  
Amide Bond ◽  
Carbonyl Carbon Atom ◽  
Cephalosporin C ◽  
Beta Lactamase ◽  
Beta Lactam ◽  
Carbonyl Carbon ◽  
Class A ◽  
Streptomyces Albus ◽  
Hydrolysis Of

Optimization by energy minimization of stable complexes occurring along the pathway of hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase has highlighted a proton shuttle that may explain the catalytic mechanism of the beta-lactamases of class A. Five residues, S70, S130, N132, T235 and A237, are involved in ligand binding. The gamma-OH group of T235 and, in the case of benzylpenicillin, the gamma-OH group of S130 interact with the carboxylate group, on one side of the ligand molecule. The side-chain NH2 group of N132 and the carbonyl backbone of A237 interact with the exocyclic CONH amide bond, on the other side of the ligand. The backbone NH groups of S70 and A237 polarize the carbonyl group of the scissile beta-lactam amide bond. Four residues, S70, K73, S130 and E166, and two water molecules, W1 and W2, perform hydrolysis of the bound beta-lactam compound. E166, via W1, abstracts the proton from the gamma-OH group of S70. While losing its proton, the O-gamma atom of S70 attacks the carbonyl carbon atom of the beta-lactam ring and, concomitantly, the proton is delivered back to the adjacent nitrogen atom via W2, K73 and S130, thus achieving formation of the acyl-enzyme. Subsequently, E166 abstracts a proton from W1. While losing its proton, W1 attacks the carbonyl carbon atom of the S70 ester-linked acyl-enzyme and, concomitantly, re-entry of a water molecule W'1 replacing W1 allows E166 to deliver the proton back to the same carbonyl carbon atom, thus achieving hydrolysis of the beta-lactam compound and enzyme recovery. The model well explains the differences found in the kcat. values for hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase. It also explains the effects caused by site-directed mutagenesis of the Bacillus cereus beta-lactamase I [Gibson, Christensen & Waley (1990) Biochem J. 272, 613-619].

Correlation of rates of uncatalyzed and hydroxide-ion catalyzed ketene hydration. A mechanistic application and solvent isotope effects on the uncatalyzed reaction

2000 ◽  
Vol 78 (4) ◽  
pp. 508-515
Author(s):  
John Andraos ◽  
A Jerry Kresge
Keyword(s):  
Carbon Atom ◽  
Rate Constants ◽  
Isotope Effects ◽  
Nucleophilic Attack ◽  
Carbonyl Carbon Atom ◽  
Basic Solution ◽  
Carbonyl Carbon ◽  
Hydroxide Ion ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

Rates of hydration of a number of ketenes were measured in neutral and basic solution using flash photolytic techniques, and rate constants for their uncatalyzed, kuc, and hydroxide-ion catalyzed, kHO, reactions were determined. These results, plus additional data from the literature, were found to provide the remarkably good correlation log kuc = -3.21 + 1.14 log kHO, which spans 10 orders of magnitude in reactivity and includes 31 ketenes. This good correlation implies that uncatalyzed and hydroxide-ion catalyzed ketene hydraton occur by similar reaction mechanisms, which for the hydroxide-ion catalyzed process is known to involve nucleophilic attack on the carbonyl carbon atom of the ketene. Rate constants for phenylhydroxyketene, on the other hand, do not fit this correlation, which suggests that the mechanistic assignment upon which these rate constants are based may not be correct. Solvent isotope effects on these uncatalyzed ketene hydrations are weak; most are less than kH/kD = 2. It is argued that these isotope effects are largely, if not entirely, secondary in nature and that they are consistent with both a reaction mechanism in which nucleophlic attack of a single water molecule on the ketene carbonyl carbon atom produces a zwitterionic intermediate and also a mechanism that avoids this intermediate by passing through a cyclic transition state involving several water molecules.Key words: ketene hydration, rate correlation, nucleophilic attack, solvent isotope effects, phenylhydroxyketene.

Vibrations of the scissile C—O bond in an acyl-chymotrypsin observed by resonance Raman spectroscopy

1983 ◽  
Vol 61 (11) ◽  
pp. 2590-2595 ◽  
Author(s):  
P. R. Carey ◽  
D. J. Phelps
Keyword(s):  
Raman Spectroscopy ◽  
Carbon Atom ◽  
Active Site ◽  
Resonance Raman Spectroscopy ◽  
Resonance Raman ◽  
Acyl Moiety ◽  
Unsaturated Ester ◽  
Carbonyl Carbon Atom ◽  
Carbonyl Carbon ◽  
Stretching Vibration

5-Methylthienylacryloyl chymotrypsin has been prepared with 13C at the carbonyl and, separately, at the α carbon atoms of the acyl moiety. The isotopically labelled species confirm the assignments of several of the bands in the resonance Raman (RR) spectrum of the acyl-enzyme and show that the feature occurring near 1260 cm−1 involves the motion of the carbonyl carbon atom. It is likely that the 1260 cm−1 band is associated with the stretching vibration of the C—O bond in the =C—C(=O)—O— chymotrypsin moiety, i.e. the vibration of the bond being cleaved in the active site. A moderately intense 1260 cm−1 feature occurs only in the RR spectrum of the native acyl-enzyme and possible reasons are given for its appearance. The labelled species also confirm that the bands appearing near 1715 cm−1 in the RR spectrum of the acyl-enzyme are associated with the stretching mode of the acyl carbonyl, νC=O. This frequency is that expected for an unperturbed α, β-unsaturated ester. Additionally, RR data are presented for 5-methylthienylacrylic acid in its —COOH and —COO− forms labelled with 13C at the C=O and α carbon positions.

Biosynthesis of chloramphenicol. Studies on the origin of the dichloroacetyl moiety

1978 ◽  
Vol 24 (2) ◽  
pp. 136-142 ◽  
Author(s):  
J. N. Simonsen ◽  
K. Paramasigamani ◽  
L. C. Vining ◽  
A. G. McInnes ◽  
J. A. Walter ◽  
...  
Keyword(s):  
Carbon Atom ◽  
Succinic Acid ◽  
Free Acid ◽  
Krebs Cycle ◽  
Dichloroacetic Acid ◽  
Carbonyl Carbon Atom ◽  
Carbonyl Carbon ◽  
Streptomyces Species ◽  
Random Combination ◽  
Covalently Bonded

Chloramphenicol produced by cultures of Streptomyces species 3022a supplemented with sodium [1,2-13C]acetate was labelled with 13C exclusively in the dichloromethine (2.6 ± 0.1%) and carbonyl (0.59 ± 0.05%) carbon atoms. Satellite signals from 13C–13C coupling between covalently bonded 13C-enriched carbon atoms were too intense to be attributed to random combination of labelled atoms at the average enrichments measured, but their intensity relative to those of the signals for uncoupled 13C atoms indicated that most of the precursor had been incorporated after 13C–13C bond fission. Since [2,3-13C]succinic acid enriched only the carbonyl carbon atom of chloramphenicol, these results suggest that neither acetate nor a Krebs cycle intermediate is a direct precursor of the dichloroacetyl group. Cultures supplemented with [2-3H]-or [2H2]-dichloroacetic acid incorporated negligible amounts of isotope into the antibiotic; on this evidence, the free acid is not an intermediate in chloramphenicol biosynthesis and the acylation step may precede chlorination.

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