Formation of a cyclic tetrahedral intermediate by the addition of water to 2-methyl-4H- 3,1-benzoxazine followed by ring opening to 2-aminobenzyl acetate and 2-acetylaminobenzyl alcohol; pH-dependence of rate of reaction and product ratio

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1035-1041 ◽  
Author(s):  
Wendy J Dixon ◽  
Frank Hibbert
Keyword(s):  
Ring Opening ◽  
Ph Dependence ◽  
Kinetic Studies ◽  
Ph Profile ◽  
Hydroxide Ion ◽  
Product Ratio ◽  
Tetrahedral Intermediate ◽  
Rate Of Reaction ◽  
Buffer Base ◽  
Carbonyl Addition

Kinetic studies have shown that addition of water to protonated 2-methyl-4H-3,1-benzoxazine occurs to give a cyclic tetrahedral carbonyl addition intermediate. At pH <5, the intermediate is protonated and reacts to 2-aminobenzyl acetate, whereas at pH >7.5, the unprotonated intermediate collapses to give 2-acetylaminobenzyl alcohol. The former reaction is catalysed by buffer base but the latter is uncatalysed. At pH 9-12, reaction of hydroxide ion with protonated 2-methyl-4H-3,1-benzoxazine to give 2-acetylaminobenzyl alcohol becomes important, and at pH >12, the same product is formed by reaction of hydroxide ion with unprotonated 2-methyl-4H-3,1-benzoxazine.Key words: mechanism, addition, tetrahedral intermediate, hydrolysis, pH profile.

Basic hydrolysis of some N-phenylcarbamates and basic methanolysis of some N-phenylacetamides containing an ortho nitro substituent

1984 ◽  
Vol 37 (10) ◽  
pp. 2005
Author(s):  
TJ Broxton
Keyword(s):  
Induction Period ◽  
Nitro Group ◽  
Resonance Effect ◽  
Kinetic Studies ◽  
Bond Breaking ◽  
Hydroxide Ion ◽  
Tetrahedral Intermediate ◽  
Group Reaction ◽  
Basic Hydrolysis ◽  
Hydrolysis Of

Kinetic studies of the basic methanolysis of N-(2-nitropheny1)acetamides indicate that unlike the 4-nitro isomer, no change of mechanism occurs on inclusion of an N-methyl group. Reaction occurs with rate-determining C-N bond breaking for both the N-H and N-methyl compounds. Basic hydrolysis of some methyl N-(2-nitropheny1)carbamates occurred by the BAC2 mechanism and the tetrahedral intermediate formed during the hydrolysis decomposed with preferential C-O bond breaking. This is in contrast to the basic hydrolysis of methyl N-methyl-N-4-nitrophenyl- carbamate, which has previously been shown to occur with preferential C-N bond breaking. For the hydrolysis of methyl N-methyl-N-(2-nitrophenyl)carbamate, an induction period in amine production was detected at 0.45 M hydroxide ion. This was interpreted to mean that the tetrahedral intermediate breaks down by loss of methoxide ion. At 0.93 M hydroxide ion, however, no induction period in amine production was observed. The possibility of reaction through a dianionic intermediate was raised to explain this observation. The amide ion (2-NO2C6H4NMe-) is a poorer leaving group than its 4-nitro isomer. This is explained by steric crowding in the 2-nitro compound, resulting in twisting of the nitro group out of the plane of the benzene ring and a consequent reduction in the electron-withdrawing resonance effect of the 2-nitro group compared to the 4-nitro group.

Kinetics of the Imidazolium Ring-Opening of mRNA 5'-cap Analogs in Aqueous Alkali

2006 ◽  
Vol 71 (4) ◽  
pp. 567-578 ◽  
Author(s):  
Alicja Stachelska ◽  
Zbigniew J. Wieczorek ◽  
Janusz Stępiński ◽  
Marzena Jankowska-Anyszka ◽  
Harri Lönnberg ◽  
...  
Keyword(s):  
Electrostatic Interaction ◽  
Ring Opening ◽  
Nucleophilic Attack ◽  
Amino Groups ◽  
Hydroxide Ion ◽  
Methyl Group ◽  
Structural Modifications ◽  
Alkyl Groups ◽  
Imidazolium Ring ◽  
Kinetics Of

Second-order rate constants for the hydroxide-ion-catalyzed imidazolium ring-opening of several mono- and dinucleosidic analogs of mRNA 5'-cap have been determined. Intramolecular stacking of the two nucleobases in the dinucleosidic analogs, m7GpppN (m7G = 7-methylguanosine, N = 5'-linked nucleoside), and electrostatic interaction between the N-alkylated imidazolium ring and phosphate moiety have been shown to shield the m7G moiety against the nucleophilic attack of hydroxide ion. In addition, the effect of methylation of the nucleobase amino groups and replacement of the 7-methyl group with other alkyl groups have been studied. The influence of all the structural modifications studied turned out to be modest, the cleavage rates of the most and least reactive analogs (with the exception of non-phosphorylated nucleosides) differing only by a factor of 5.

Kinetic studies of methylcyclopentane ring opening on EuroPt-1 (Pt/SiO2)

1999 ◽  
Vol 177 (2) ◽  
pp. 205-217 ◽  
Author(s):  
Yiping Zhuang ◽  
Alfred Frennet
Keyword(s):  
Ring Opening ◽  
Kinetic Studies

Reactivity of norbornene esters in ring-opening metathesis polymerization initiated by a N-chelating Hoveyda II type catalyst

RSC Advances ◽  
2016 ◽  
Vol 6 (7) ◽  
pp. 5177-5183 ◽  
Author(s):  
S. A. Kiselev ◽  
D. A. Lenev ◽  
A. A. Lyapkov ◽  
S. V. Semakin ◽  
G. Bozhenkova ◽  
...  
Keyword(s):  
Ring Opening ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Ring Opening Metathesis Polymerization ◽  
Metathesis Polymerization

The reactivity and activation parameters for the ROMP of eight norbornene esters in the presence of a N-chelating Hoveyda–Grubbs II type catalyst were determined. Kinetic studies prove that these parameters highly depend on the monomer structures.

Accelerated living cationic ring-opening polymerization of a methyl ester functionalized 2-oxazoline monomer

2015 ◽  
Vol 6 (4) ◽  
pp. 514-518 ◽  
Author(s):  
Petra J. M. Bouten ◽  
Dietmar Hertsen ◽  
Maarten Vergaelen ◽  
Bryn D. Monnery ◽  
Marcel A. Boerman ◽  
...  
Keyword(s):  
Methyl Ester ◽  
Ring Opening ◽  
Kinetic Studies ◽  
Ring Opening Polymerization ◽  
Cationic Ring Opening Polymerization

Kinetic studies on the homo- and copolymerization of 2-methoxycarboxyethyl-2-oxazoline with 2-methyl-2-oxazoline and 2-ethyl-2-oxazoline is discussed.

scholarly journals Mechanism of d-fructose isomerization by Arthrobacterd-xylose isomerase

1992 ◽  
Vol 283 (1) ◽  
pp. 223-233 ◽  
Author(s):  
M Rangarajan ◽  
B S Hartley
Keyword(s):  
Ring Opening ◽  
Ph Dependence ◽  
Xylose Isomerase ◽  
Competitive Inhibitor ◽  
Apparent Dissociation Constant ◽  
Ph Values ◽  
Rate Limiting Step ◽  
Competitive Inhibitors ◽  
Rate Limiting ◽  
Very High

The mechanism of D-fructose isomerization by Arthrobacter D-xylose isomerase suggested from X-ray-crystallographic studies was tested by detailed kinetic analysis of the enzyme with various metal ions at different pH values and temperatures. At D-fructose concentrations used in commercial processes Mg2+ is the best activator with an apparent dissociation constant of 63 microM; Co2+ and Mn2+ bind more strongly (apparent Kd 20 microM and 10 microM respectively) but give less activity (45% and 8% respectively). Ca2+ is a strict competitive inhibitor versus Mg2+ (Ki 3 microM) or Co2+ (Ki 105 microM). The kinetics show a compulsory order of binding; Co2+ binds first to Site 2 and then to Site 1; then D-fructose binds at Site 1. At normal concentrations Mg2+ binds at Site 1, then D-fructose and then Mg2+ at Site 2. At very high Mg2+ concentrations (greater than 10 mM) the order is Mg2+ at Site 1, Mg2+ at Site 2, then D-fructose. The turnover rate (kcat.) is controlled by ionization of a residue with apparent pKa at 30 degrees C of 6.0 +/- 0.07 (Mg2+) or 5.3 +/- 0.08 (Co2+) and delta H = 23.5 kJ/mol. This appears to be His-219, which is co-ordinated to M[2]; protonation destroys isomerization by displacing M[2]; Co2+ binds more strongly at Site 2 than Mg2+, so competes more strongly against H+. The inhibition constant (Ki) for the two competitive inhibitors 5-thio-alpha-D-glucopyranose and D-sorbitol is invariant with pH, but Km(app.) in the Mg[1]-enzyme is controlled by ionization of a group with pKa 6.8 +/- 0.07 and delta H = 27 kJ/mol, which appears to be His-53. This shows that Km(app.) is a complex constant that includes the rate of the ring-opening step catalysed by His-53, which explains the pH-dependence. In the Mg[1]Mg[2]-enzyme or Co[1]Co[2]-enzyme, the pKa is lower (6.2 +/- 0.1 or 5.6 +/- 0.08) because of the extra adjacent cation. Hence the results fit the previously proposed pathway, but show that the mechanisms differ for Mg2+ and Co2+ and that the rate-limiting step is isomerization and not ring-opening as previously postulated.

ESTIMATION OF MARSUPIAL RENIN USING MARSUPIAL RENIN-SUBSTRATE

1972 ◽  
Vol 53 (1) ◽  
pp. 125-130 ◽  
Author(s):  
PAMELA A. SIMPSON ◽  
J. R. BLAIR-WEST
Keyword(s):  
Substrate Concentration ◽  
Structural Difference ◽  
Plasma Renin ◽  
Angiotensin I ◽  
Ph Optimum ◽  
Ph Profile ◽  
Renin Substrate ◽  
Grey Kangaroo ◽  
Rate Of Reaction ◽  
Highly Correlated

SUMMARY Bilateral nephrectomy of an Eastern Grey kangaroo (Macropus giganteus) increased plasma renin-substrate concentration approximately tenfold when compared with intact kangaroos. A preparation made from this plasma had a renin-substrate concentration of 3000 ng/ml. A pH profile of rate of reaction with pig renin had an optimum at pH 5·39. By comparison, the pH optimum of sheep renin-substrate was pH 6·15. Estimates of plasma renin concentration for kangaroos, wombats and wallabies, using kangaroo renin-substrate or sheep renin-substrate were highly correlated. Results from incubation with sheep renin-substrate were greater and hence indicate the advantage in using this substrate for marsupial renin estimation. The consistently large difference between sheep and kangaroo renin-substrate when incubated with renin from marsupial and eutherian species appears to be due to a structural difference between the two substrates, probably near the C-terminal end of the angiotensin I molecule.

Hydrolysis of N-acetyl-p-benzoquinone imines: pH dependence of the partitioning of a tetrahedral intermediate

1989 ◽  
Vol 111 (12) ◽  
pp. 4447-4456 ◽  
Author(s):  
Michael Novak ◽  
Gayl A. Bonham ◽  
Julio J. Mulero ◽  
Maria Pelecanou ◽  
Joseph N. Zemis ◽  
...  
Keyword(s):  
Ph Dependence ◽  
Tetrahedral Intermediate ◽  
Hydrolysis Of

Mechanistic and Kinetic Studies of the Ring Opening Metathesis Polymerization of Norbornenyl Monomers by a Grubbs Third Generation Catalyst

2019 ◽  
Vol 141 (44) ◽  
pp. 17918-17925 ◽  
Author(s):  
Michael G. Hyatt ◽  
Dylan J. Walsh ◽  
Richard L. Lord ◽  
José G. Andino Martinez ◽  
Damien Guironnet
Keyword(s):  
Ring Opening ◽  
Kinetic Studies ◽  
Third Generation ◽  
Ring Opening Metathesis Polymerization ◽  
Metathesis Polymerization
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